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Species richness and aggregation effects on the productivity of ruderal plant communities under drought perturbation

Species richness and aggregation effects on the productivity of ruderal plant communities under... Volume 1 † Number 2 † June 2008 10.1093/biohorizons/hzn017 ......................................................................................................................................................................................................................................... Research article Species richness and aggregation effects on the productivity of ruderal plant communities under drought perturbation Nodoka Nakamura* School of Agriculture, Policy and Development, University of Reading, Earley Gate, Reading, PO Box 237, RG6 6AR, UK. * Corresponding author. Oxford University Centre for the Environment, South Parks Road, Oxford, OX1 3PG, UK. Tel: 01865 283933 Email: nodoka.nakamura@ouce.ox.ac.uk Supervisor: Dr Andrew Wilby, School of Agriculture, Policy and Development, University of Reading, Reading, UK. Present address: Department of Biological Sciences, Lancaster University, Lancaster, UK. ........................................................................................................................................................................................................................................ The effect of species richness and spatial aggregation on the stability of community productivity in response to drought perturbation was investigated with experimental plant communities. Communities comprising all single- and three-species combinations of the ruderal species, Capsella bursa-pastoris, Tripleurospermum inodorum, Poa annua,and Stellaria media, were established in glasshouse. Habitat patchi- ness was manipulated by applying different seed-sowing patterns, either aggregated or random. After the establishment of communities, 8 days of drought treatment was imposed. Followed by a week of recovery with a regular watering regime, aboveground biomass was har- vested. Community biomass was not affected by species richness or by aggregation, but was affected by perturbation. When multi-species community productivity was compared with monocultures in relative terms, species mixtures performed better in drought-induced con- ditions. This suggests that the positive effect of species richness may be enhanced under the perturbed condition. Sampling effects were evident under perturbation favouring the least productive species, P. annua and drought-tolerant S. media. All species except C. bursa-pas- toris showed reduced productivity in species mixtures, but this may be mitigated under perturbed environments by species complemen- tarity. Lack of clear responses to aggregation may suggest that the revealed diversity effect is not related to spatial structure. While competition predominates in communities in the resource-rich environment, drought perturbation enhance overall community pro- ductivity via a shift in relative significance of species interactions from competition to sampling and complementarity effects. Key words: biodiversity effects, ecosystem stability, drought perturbation, relative yield total. ........................................................................................................................................................................................................................................ be climatic, such as drought, heavy rain, frost or lack of sun- Introduction light or may be biotic; outbreak of diseases, pests or intense Ecosystem properties are regulated through interactions grazing which limit production of biomass or the life cycle of between abiotic factors, such as soil fertility and climate, individual species, plant communities and ecosystems. and the functional traits of individual organisms that occur Evidence supporting the view that different species express 1– 3 in a community. Two important ecological questions different response patterns to such changes in the environ- have been the focus of research: how do ecosystems ment suggests that an increase in diversity, species richness respond to changes in the environment, and which elements or functional diversity would lead to the extension of in the ecosystem are important determinants for maintaining range of possible responses by an ecosystem. Hence, diversity the function of ecosystems? Disturbance (changes in the con- would increase the stability of an ecosystem against environ- 1,6,7 ditions of the environment) can occur over various spatial mental perturbation. This idea is expressed in the and temporal scales with different intensity and selectivity ‘insurance hypothesis’ which proposes that a more diverse of impacts on ecosystems. For instance, disturbance may ecosystem has a higher potential of containing species ......................................................................................................................................................................................................................................... 2008 The Author(s). This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/2.0/uk/) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited. 128 Bioscience Horizons † Volume 1 † Number 2 † June 2008 Research article ......................................................................................................................................................................................................................................... 25 – 27 which are well adapted to a change in the environment and of complex responses by communities to perturbation. are able to compensate for the decline of the less adapted To clarify the link between biodiversity effects and ecosystem 8,9 species. stability, further empirical studies are needed to provide It is suggested that several explicit mechanisms, termed explicit evidence. ‘biodiversity effects’, underlie diversity – ecosystem functioning One area where experiments have failed to represent relationships and it is likely that these mechanisms also natural systems is spatial patchiness of communities. mediate the response to environmental perturbation. These Distributional and colonization patterns of organisms are consist of three components: the sampling effect, functional dependent upon the mobility and dispersability of species, complementarity and positive species interaction or facili- thus individual species tend to create aggregated populations 2, 10 – 12 28 tation. The sampling effect is the dominance of par- in some spatial scales. This leads to patchiness in a habitat ticular species. This component is suggested based on the occupied by different species and provides some possibilities theory that a highly diverse ecosystem is likely to contain for the coexistence of species through spatial niche differen- some superior species, which have strong effects on ecosystem tiation, i.e. resource partitioning. The heterogeneity in 12,13 functioning. Complementarity arises through niche parti- community structure has been given little attention in tioning between species, resulting in the alleviation of inter- biodiversity experiments. Aggregation increases intra- 1, 2, 11, 14 – 16 specific competition over limiting resources. specific competition between individuals, resulting in the Ecosystem stability should be correlated with biodiversity if formation of monoculture-like patches in a habitat. In resources limit the growth of species in a community and if such circumstances, complementarity is expected to be 29, 30 the resource partitioning allows for the increased efficiency reduced. in total resource use over temporal and spatial scales. A model developed by De Boeck et al. demonstrated that Facilitation ( positive interspecific interaction) occurs when a aggregation alters the relative significance of biodiversity certain species has the ability to mitigate the harsh environ- effects. Under high levels of clumping, resource-use efficiency mental conditions to which a community is exposed or both aboveground and belowground was reduced due to a when there is a supply flow of resources from one species to decrease in complementarity. The increase in intraspecific others which is beneficial to the survival of the recipients. competition between neighbouring individuals of the same Thus, productivity is enhanced with increased diversity if species results in a decrease in overall productivity. Stoll 1, 17, 18 30 facilitative interactions increase. If the effect of facili- and Prati found that the growth of a superior competitor tation was predominant under perturbed conditions, it was suppressed in an intraspecifically aggregated community, would lead to an increase in ecosystem stability. Individual whereas the performance of inferior species was stimulated. instances or combinations of these biodiversity effects have This can be interpreted as a mechanism to promote coexis- been shown in experimental work under controlled environ- tence of species, and hence to maintain species diversity. If ments. It might be expected that these effects would also be interspecific interactions in aggregated communities under observed in the perturbed environment. perturbation are altered to mitigate the dominance hierarchy There is still a lack of consistency among empirical studies in manner advantageous to weak competitors, it would lead 30 – 32 over the expression of diversity—function mechanisms under to enhanced ecosystem stability. While much work has perturbation. There is an evidence of complementarity in highlighted the significance of incorporating the spatial some studies, such as an increase in community water-use structure of communities in examining the functional efficiency under drought conditions. Another experiment importance of biodiversity, empirical evidence to support in a species-rich, semi-natural grassland revealed there was those proposed consequences is still limited. Assuming that an increase in belowground biomass production via shifts aggregation has impacts on competitive interactions, comple- in resource allocation from aboveground to belowground mentarity or other biodiversity effects and coexistence of in response to perturbation. Negative sampling effects species, it is also likely to affect the response of community have also been found in some empirical studies that biomass to perturbation. The effects of spatial aggregation showed perturbation selected for the least productive on diversity – stability relationships should be taken into 28, 33, 34 species, such as grasses, by allowing faster resource uptake account. which exceeded their inherent low resource use efficiency The aim of this study is to investigate three topics in 14, 15 and productivity. However, other studies have failed relation to biodiversity, ecosystem functioning and ecosystem 1, 20 – 22 to show such effects. Several studies have indicated stability using experimental glasshouse plant communities. that there is a strong dependency of stability on the traits Three hypotheses are tested: 23, 24 of dominant species present in a given ecosystem. In addition, the significance of species composition and identity (1) Multispecies communities have higher aboveground pro- has been highlighted in some experiments. The presence of a ductivity than had the monocultures. particular species, for instance, nitrogen-fixing legumes, (2) The relative performance of multi-species and monocul- seems to cause nutrient enrichment, leading to the expression tures is moderated by drought. ......................................................................................................................................................................................................................................... 129 Research article Bioscience Horizons † Volume 1 † Number 2 † June 2008 ......................................................................................................................................................................................................................................... (3) Aggregation affects the relative performance of multi- species compared with constituent monocultures. Materials and methods Four common ruderal plant species (three forbs and one grass) were used in the experiment: shepherd’s purse Capsella bursa-pastoris (Brassicaceae) denoted Cap, scentless mayweed Tripleurospermum inodorum (Asteraceae) denoted Tri, annual meadow-grass Poa annua (Poaceae) denoted Poa and chickweed Stellaria media (Caryphyllaceae) denoted Ste. These species were chosen for their reliable germination and high growth rate. All four species can proliferate on poor, disturbed soil and are adapted to complete their life cycle by rapid growth to maximize seed production. These species often co-occur in disturbed habitats, such as agricul- 35, 36 tural fallows, though all seeds were purchased from Herbiseed (Twyford, UK). The experiment was conducted in the glasshouse at the University of Reading in summer 2006 (from 5 July to 17 Figure 1. The experimental design. (A) The Paired plot which was divided into two sub-plots with a plastic divider. One of the pairs was under control August). Plant communities were established on compost- treatment and the other exposed to perturbation. (B) Plot with nine cells of filled plastic pots (37  21.5  5cm ) each divided by water- 5  5cm which shows an example of aggregated seed sowing pattern of tight barriers into two sub-plots (18.25  21.5  5cm ). Cap–Tri–Poa. In the random and single pots, seeds were broadcasted over Each main pot contained the same plant treatment and one 2 the 15  15 cm plot. sub-plot was exposed to drought and the other adjacent sub-plot received regular watering (Fig. 1A). The density of divided into nine cells of 5  5cm ; each species occupied seeds per pot was equalized, such that each plot contained a three cells in a Latin square pattern (Fig. 1B). For random total of 900 seeds with equivalent proportion of three treatment and for monocultures, seeds were sown randomly species for multi-species mixtures. Seeds were sown on 5 – 6 over the soil within 15  15 cm (Fig. 1A). July 2006 over compost-filled pots. A regular watering In sum, there were a total of 24 combinations per experi- regime was set at 400 ml once every 3 days. At 25 days mental block; 12 diversity and aggregation treatments after sowing (29 July), the drought perturbation treatment (4 monocultures þ 4 aggregated three-species mixtures þ 4 started. For the first 2 days and the 2 days before the cessation randomly sown three-species mixtures)  2 perturbation of drought, perturbed plots received half the amount of water treatments ( perturbed or control). Four such randomized controlled plots received (200 ml). Then, there was no water- blocks were sown in the glasshouse. ing on perturbed pots for 8 days (from 31 July to 7 August). Soil moisture content was monitored daily during and The original watering regime was maintained on control plots after the drought treatment period using ThetaProbe soil over the experimental period. Following the drought treat- moisture sensor (type ML 2x). Maximum and minimum ment, regular watering of 400 ml water per pot once every air temperatures were also monitored daily. Aboveground 3 days was performed across the plots until the end of the biomass was harvested on 17th August by clipping the experiment (15 August). shoots at ground level, and dried at 608C for 48 h. After In addition to the drought treatment, two other treatments drying, the total aboveground shoot weights of each were applied to the plots: plant diversity and species aggrega- species in a pot were recorded. tion treatments. For the diversity treatment, single- and multiple-species communities were compared. The total Data analysis number of species combinations was eight; four single species (Cap – Cap – Cap, Tri – Tri – Tri, Poa – Poa – Poa and In order to investigate the relative difference in yield between Ste – Ste – Ste) and all possible three-species mixtures (Cap – monoculture and species mixtures, the extent of overyielding Tri – Poa, Cap – Poa – Ste, Cap – Tri – Ste and Tri – Poa – Ste). was calculated by comparing the productivity of mono- The aggregation treatment was conducted only in multi- cultures with the expected productivity of the species mix- species communities by sowing seeds in aggregated cell pat- tures based on the biomass of their constituent species in 13, 17, 37 terns or randomly over the plot. In aggregated spatial monoculture. The proportion of biomass of species pattern, these sub-plots of 15  15 cm were further mixture relative to that of monoculture was used as a ......................................................................................................................................................................................................................................... 130 Bioscience Horizons † Volume 1 † Number 2 † June 2008 Research article ......................................................................................................................................................................................................................................... measure of overyielding in this study which is called relative yield total (RYT) ; RYT ¼ O=E ¼ O =ðY =3 þ Y =3 þ Y =3Þ ijk i j k ¼ observed biomass/expected biomass where Y is the biomass of component species i in monocul- ture and O the observed total yield of mixture including ijk species i, j and k. Expected biomass was calculated from the pot total weight of monoculture in the same block for observed biomass to take account of variation between blocks. For individual species, biomass of each species in species mixture was compared with the one-third of biomass of the same species in monocultures; Figure 2. Changes in mean soil moisture content during the experiment (+SE). Mean of each time point is derived from four randomly selected Species relative yield ¼ O =ðY =3Þ i i measurements from each block of either treatment, i.e. 4 measurements  4 blocks ¼ 16 measurements for each treatment at one time point. The where O is the observed biomass of species i in species shaded period from 31 July to 8 August indicates the period of drought. The control watering treatment is denoted by solid circles, and the droughted by mixture and Y the biomass of species i monoculture. open squares. The gaps are when the data were not obtained. The three response variables analysed were total above- ground biomass per pot, biomass of individual species, RYT and the relative yield (RY) of individual species in perturbation treatment at the greatest by 48.80% in species mixtures. Data were analysed with general linear C. bursa-pastoris monoculture, and at the least by 1.66% models (Type _ Sum of Squares). The effects of perturbation in three-species mixture of Tri–Poa–Ste, average reduction treatment, aggregation and diversity (species richness) and of aboveground productivity was 24.53%. their interactions on each response variable were tested. Perturbation exhibited significant effects on RYT Though biomass data usually require log-transformation, (F ¼ 4.50; P ¼ 0.038). The observed biomass of species- 1,57 distributions of residuals suggested that absolute above- mixture under the perturbation treatment was slightly ground biomass was more appropriate here for both pot greater than the expected biomass (on average by 7%). total individual species weights. RYT and species RY were There was an overall reduction of productivity of the log-transformed to achieve normality. observed biomass of controls by 7% on average. Hence, in terms of RY, perturbed communities performed better than control ones by 14% on average. The response of multi- Results species communities depended on the occurrence of pertur- During the drought treatment, soil moisture content of bation, relative biomass increased in perturbed, but the perturbed pots declined day by day (Fig. 2). When the decreased in the control treatment. This result shows that treatment finished, soil moisture content decreased approxi- negative interspecific interactions predominated in the 3 23 mately to 0.05 m m . Control plots showed moderately control environment, whereas positive interactions predomi- fluctuating patterns, but the soil moisture content was main- nated under drought stress. Aggregation did not induce 3 23 tained on an average of 0.2 m m . The soil moisture changes in RY (Fig. 4), indicating that the productivity of content was kept equivalent during the week of recovery multi-species communities was not affected by spatial struc- period. tural differences in any combination of species composition. Block effects (F ¼ 17.07; P, 0.001) and perturbation Perturbation caused a decrease in biomass production of 3,89 (F ¼ 52.89; P, 0.001) had a statistically significant three species; C. bursa-pastoris (F ¼ 10.31; P ¼ 0.002), 1,89 1,49 impact on total aboveground biomass; however, there was T. inodorum (F ¼ 7.66; P ¼ 0.008) and P. annua 1,49 no significant effect of species richness, aggregation treat- (F ¼ 4.27; P ¼ 0.044); however, there was no significant 1,49 ment or the interactions of these factors with perturbation effect of species richness or its interaction with perturbation (Fig. 3A and B). Variation in perturbed plots across the treatment. blocks was greater than control ones on average: 2.59 g Under perturbation, there was a reduction in biomass on (44.5% reduction) difference between maximum and average of 37.5% in C. bursa-pastoris (34.7% in species minimum biomasses in perturbed treatment, 1.43 g mixtures, 48.0% in monocultures), 25.2% in T. inodorum (21.9%) in control. Compared with the aboveground (23.9% and 28.2% in mixtures and monocultures, respect- biomass of the controls, productivity was decreased by the ively) 17.2% in P. annua (11.3% in mixtures, 32.8% in ......................................................................................................................................................................................................................................... 131 Research article Bioscience Horizons † Volume 1 † Number 2 † June 2008 ......................................................................................................................................................................................................................................... Figure 3. Differences in mean (+SE) aboveground community biomass between perturbed and control pots in relation to plot-level treatments of (A) species richness (three or one); (B) aggregation treatment of polyculture-aggregated, polyculture-random, monocultures. Coloured bars, control; open bars, droughted. Figure 4. Differences in mean (+SE) relative yield total of aboveground community biomass between perturbed and control pots in relation to aggregation treatment. Coloured bars, control; open bars, droughted. monocultures) (Fig. 5). In general, monocultures were more affected by drought, but also showed a greater variability. Productivity responses of S. media were different from the other species (Fig. 5D), although perturbation had no signifi- cant effect, aboveground biomass production was signifi- cantly lower in species mixture compared with monoculture (F ¼ 6.65; P ¼ 0.013). S. media was the 1,49 least affected by perturbation with average reduction of biomass production by 16.1% (14.1% and 19.9% in mix- tures and monocultures, respectively). When aboveground biomass of each species was com- pared with the monoculture productivity, responses of species differed considerably (Fig. 6). C. bursa-pastoris increased productivity in three species mixtures (71.4% increase in the perturbed treatment, 34.9% increase in Figure 5. Differences in mean (+SE) aboveground biomass of individual species between perturbed and control pots and how biomass differed control compared to biomass of monocultures), but there depending on species composition. (A) Capsella bursa-pastoris;(B) was general decline of productivity in T. inodorum in Tripleurospermum inodorum;(C) Poa annua;(D) Stellaria media. Species mixture (12.9% decrease in perturbed, 17.7% decrease in composition: ctp, Cap–Tri–Poa; cps, Cap–Poa–Ste; cts, Cap–Tri–Ste; tps, control compared with monocultures, but for P. annua Tri–Poa–Ste; singles are monocultures of each species. Coloured bars, there was 17.7% increase in the perturbed treatment and control; open bars, droughted. ......................................................................................................................................................................................................................................... 132 Bioscience Horizons † Volume 1 † Number 2 † June 2008 Research article ......................................................................................................................................................................................................................................... showed clear differences reflecting the effects of species identity. Several pieces of empirical work highlighted the idiosyncratic nature of species response to perturbation, which suggests that the underlying mechanisms of the diver- sity – stability relationship in real communities are more 1,15,19,26 complex than those predicted by theory. The exper- iment here provided one week with adequate watering after perturbation to investigate the recovery of communities. Quick recovery by ruderal species was observed with inflor- escences produced in some plants of C. bursa-pastoris and T. inodorum, and from the emergence of new seedlings Figure 6. Differences in mean (+SE) relative aboveground biomass yield shown in S. media. Hence, the lack of diversity effects in multi-species communities of each species under perturbed and control appears to be a consequence of over-riding species/commu- conditions. Species code: cap, Capsella bursa-pastoris; tri, Tripleurospermum nity composition effects, consistent with other previous inodorum; poa, Poa annua; ste, Stellaria media. Coloured bars, control; open experiments. Diversity response may therefore originate bars, droughted. from a combination of factors, some of which remain 1, 20, 23, 24 hidden. 10.9% decrease in the control. P. annua was the only species to show a response to the multispecies treatment that varied Relative yield with perturbation. When productivity of multi-species mixtures was compared with expectation based on the monoculture performance Discussion (RYT), it was clear that the positive effect of species-richness was enhanced under drought perturbation. Indeed the Absolute aboveground biomass species mixtures under-yielded in the control treatment but 14,15 The effect of species richness was exhibited on the absolute over-yielded in perturbed conditions (Fig. 4). This aboveground biomass of S. media as an individual species. shows that diversity effect was negative in control, but posi- Aboveground community biomass and biomass of all indi- tive in perturbed. This suggests that the emergence of diver- vidual species other than S. media were not affected by sity effects is dependent on resource availability. This is a key species richness. The selection of ruderal species may have finding, which proposes that the relationship between eco- led to functional similarity among species that might have system functioning and species diversity may be context blurred the biodiversity effect. High potential growth rate dependent, e.g. it may vary in sign depending on the avail- is one of the strategies of ruderal species to survive in ability of abiotic resources. It is suggested that under the per- environments with limited supply of resources and nutri- turbed condition, functional complementarity or facilitation ents. This allows individuals to alter the length of life occurred, promoting the greater relative resource uptake by 15, 37, 38 cycle according to the availability of resources found in the least productive species (P. annua), whereas the any environment. In this study, species response to the limit- drought-tolerant species S. media maintained its pro- ation in water may be similar, to encourage fast development ductivity. A combination of these effects would have led to to change generations in short period of time. This may have the enhanced relative community biomass. In the case of resulted in the overall similarity in community productivity S. media, relative biomass in species mixture did not out- irrespective of differences in species richness. Such response weigh the performance of monocultures in spite of its toler- 10, 20 was also found in experiments on the soil food web. ance to perturbation, indicating the limited magnitude of These studies showed that although there was high species effects of dominance (selection) and complementarity in 15,19 diversity, processes driven by each species were similar or this experiment. Relative productivity of individual partly compensatory to each other due to low specialization species did not increase in multi-species communities, but of functioning in decomposer ecosystems. Therefore in this that of community biomass increased under the perturbed study, it appears that all four ruderal species respond to environment. This suggests that complementary resource drought in the same way, shortening life cycles, and the simi- use or facilitation effectively mitigated the negative effects larity of functionality does not lead to biodiversity effects. of diversity and perturbation of the component species, The great variability in response pattern, depending on the resulting in the overall increase in community biomass. community, species or species composition observed, leads Aggregation to some difficulty in proposing explicit explanations regard- ing the interaction of perturbation with diversity-functioning Aggregation did not induce changes in response patterns mechanisms. Although there is a lack of evidence for biodi- of multi-species communities in either watering treatment. versity effects in this study, responses of individual species In terms of interaction of spatial structure with perturbation, ......................................................................................................................................................................................................................................... 133 Research article Bioscience Horizons † Volume 1 † Number 2 † June 2008 ......................................................................................................................................................................................................................................... random communities did not increase productivity in Acknowledgements response to perturbation. It was hypothesized that drought- I would like to express my sincere thanks to my supervisor, tolerant species would increase productivity in the randomly 22, 30, 32 Dr Andrew Wilby for his advice on the conduct of the experi- distributed treatment. However, such response was ment, support on analysis and writing-up and for always not observed in this experiment, which was probably due giving me positive feedback on my work. Special thanks go to a great reduction of productivity in all species except to Dr Gillian Fraser and Dr Julian Park for their help S. media under perturbed conditions. C. bursa-pastoris and during the experiment, my sister Miyabi for devoting much T. inodorum reached reproductive development even in per- time with me to watering and harvesting and Dr. Duncan turbed conditions, while P. annua in mixtures performed Vaughan and Suzi Heaton for their great help on manuscript. better, in relative terms, under perturbation. Such differences I would like to thank my family and Ryosuke for their endur- in strategies to overcome drought stress among species indi- ing support. cate that the component species are not spatially complemen- tary to each other. There were some probable limitations in manipulating spatial clumping patterns in this experiment such as differ- Funding ences in the timing of germination and the growth rate, This project was funded by the University of Reading. and limitation in spatial scales of the experimental plant communities. The time taken for germination after the sowing of seeds differed among the four species which References could have pre-set the hierarchy in resource uptake efficiency in favour of fast germinating or fast growing species. 1. Hooper DU, Chapin FS, Ewel JJ et al. (2005) Effects of biodiversity on Although explicit biomass measurements were not under- ecosystem functioning: a consensus of current knowledge. 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Ecol Lett 7: 574–583. ........................................................................................................................................................................................................................................ Submitted on 3 October 2007; accepted on 21 December 2007; advance access publication 24 April 2008 ......................................................................................................................................................................................................................................... http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Bioscience Horizons Oxford University Press

Species richness and aggregation effects on the productivity of ruderal plant communities under drought perturbation

Nakamura, Nodoka
Bioscience Horizons , Volume 1 (2) – Jun 24, 2008
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Volume 1 † Number 2 † June 2008 10.1093/biohorizons/hzn017 ......................................................................................................................................................................................................................................... Research article Species richness and aggregation effects on the productivity of ruderal plant communities under drought perturbation Nodoka Nakamura* School of Agriculture, Policy and Development, University of Reading, Earley Gate, Reading, PO Box 237, RG6 6AR, UK. * Corresponding author. Oxford University Centre for the Environment, South Parks Road, Oxford, OX1 3PG, UK. Tel: 01865 283933 Email: nodoka.nakamura@ouce.ox.ac.uk Supervisor: Dr Andrew Wilby, School of Agriculture, Policy and Development, University of Reading, Reading, UK. Present address: Department of Biological Sciences, Lancaster University, Lancaster, UK. ........................................................................................................................................................................................................................................ The effect of species richness and spatial aggregation on the stability of community productivity in response to drought perturbation was investigated with experimental plant communities. Communities comprising all single- and three-species combinations of the ruderal species, Capsella bursa-pastoris, Tripleurospermum inodorum, Poa annua,and Stellaria media, were established in glasshouse. Habitat patchi- ness was manipulated by applying different seed-sowing patterns, either aggregated or random. After the establishment of communities, 8 days of drought treatment was imposed. Followed by a week of recovery with a regular watering regime, aboveground biomass was har- vested. Community biomass was not affected by species richness or by aggregation, but was affected by perturbation. When multi-species community productivity was compared with monocultures in relative terms, species mixtures performed better in drought-induced con- ditions. This suggests that the positive effect of species richness may be enhanced under the perturbed condition. Sampling effects were evident under perturbation favouring the least productive species, P. annua and drought-tolerant S. media. All species except C. bursa-pas- toris showed reduced productivity in species mixtures, but this may be mitigated under perturbed environments by species complemen- tarity. Lack of clear responses to aggregation may suggest that the revealed diversity effect is not related to spatial structure. While competition predominates in communities in the resource-rich environment, drought perturbation enhance overall community pro- ductivity via a shift in relative significance of species interactions from competition to sampling and complementarity effects. Key words: biodiversity effects, ecosystem stability, drought perturbation, relative yield total. ........................................................................................................................................................................................................................................ be climatic, such as drought, heavy rain, frost or lack of sun- Introduction light or may be biotic; outbreak of diseases, pests or intense Ecosystem properties are regulated through interactions grazing which limit production of biomass or the life cycle of between abiotic factors, such as soil fertility and climate, individual species, plant communities and ecosystems. and the functional traits of individual organisms that occur Evidence supporting the view that different species express 1– 3 in a community. Two important ecological questions different response patterns to such changes in the environ- have been the focus of research: how do ecosystems ment suggests that an increase in diversity, species richness respond to changes in the environment, and which elements or functional diversity would lead to the extension of in the ecosystem are important determinants for maintaining range of possible responses by an ecosystem. Hence, diversity the function of ecosystems? Disturbance (changes in the con- would increase the stability of an ecosystem against environ- 1,6,7 ditions of the environment) can occur over various spatial mental perturbation. This idea is expressed in the and temporal scales with different intensity and selectivity ‘insurance hypothesis’ which proposes that a more diverse of impacts on ecosystems. For instance, disturbance may ecosystem has a higher potential of containing species ......................................................................................................................................................................................................................................... 2008 The Author(s). This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/2.0/uk/) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited. 128 Bioscience Horizons † Volume 1 † Number 2 † June 2008 Research article ......................................................................................................................................................................................................................................... 25 – 27 which are well adapted to a change in the environment and of complex responses by communities to perturbation. are able to compensate for the decline of the less adapted To clarify the link between biodiversity effects and ecosystem 8,9 species. stability, further empirical studies are needed to provide It is suggested that several explicit mechanisms, termed explicit evidence. ‘biodiversity effects’, underlie diversity – ecosystem functioning One area where experiments have failed to represent relationships and it is likely that these mechanisms also natural systems is spatial patchiness of communities. mediate the response to environmental perturbation. These Distributional and colonization patterns of organisms are consist of three components: the sampling effect, functional dependent upon the mobility and dispersability of species, complementarity and positive species interaction or facili- thus individual species tend to create aggregated populations 2, 10 – 12 28 tation. The sampling effect is the dominance of par- in some spatial scales. This leads to patchiness in a habitat ticular species. This component is suggested based on the occupied by different species and provides some possibilities theory that a highly diverse ecosystem is likely to contain for the coexistence of species through spatial niche differen- some superior species, which have strong effects on ecosystem tiation, i.e. resource partitioning. The heterogeneity in 12,13 functioning. Complementarity arises through niche parti- community structure has been given little attention in tioning between species, resulting in the alleviation of inter- biodiversity experiments. Aggregation increases intra- 1, 2, 11, 14 – 16 specific competition over limiting resources. specific competition between individuals, resulting in the Ecosystem stability should be correlated with biodiversity if formation of monoculture-like patches in a habitat. In resources limit the growth of species in a community and if such circumstances, complementarity is expected to be 29, 30 the resource partitioning allows for the increased efficiency reduced. in total resource use over temporal and spatial scales. A model developed by De Boeck et al. demonstrated that Facilitation ( positive interspecific interaction) occurs when a aggregation alters the relative significance of biodiversity certain species has the ability to mitigate the harsh environ- effects. Under high levels of clumping, resource-use efficiency mental conditions to which a community is exposed or both aboveground and belowground was reduced due to a when there is a supply flow of resources from one species to decrease in complementarity. The increase in intraspecific others which is beneficial to the survival of the recipients. competition between neighbouring individuals of the same Thus, productivity is enhanced with increased diversity if species results in a decrease in overall productivity. Stoll 1, 17, 18 30 facilitative interactions increase. If the effect of facili- and Prati found that the growth of a superior competitor tation was predominant under perturbed conditions, it was suppressed in an intraspecifically aggregated community, would lead to an increase in ecosystem stability. Individual whereas the performance of inferior species was stimulated. instances or combinations of these biodiversity effects have This can be interpreted as a mechanism to promote coexis- been shown in experimental work under controlled environ- tence of species, and hence to maintain species diversity. If ments. It might be expected that these effects would also be interspecific interactions in aggregated communities under observed in the perturbed environment. perturbation are altered to mitigate the dominance hierarchy There is still a lack of consistency among empirical studies in manner advantageous to weak competitors, it would lead 30 – 32 over the expression of diversity—function mechanisms under to enhanced ecosystem stability. While much work has perturbation. There is an evidence of complementarity in highlighted the significance of incorporating the spatial some studies, such as an increase in community water-use structure of communities in examining the functional efficiency under drought conditions. Another experiment importance of biodiversity, empirical evidence to support in a species-rich, semi-natural grassland revealed there was those proposed consequences is still limited. Assuming that an increase in belowground biomass production via shifts aggregation has impacts on competitive interactions, comple- in resource allocation from aboveground to belowground mentarity or other biodiversity effects and coexistence of in response to perturbation. Negative sampling effects species, it is also likely to affect the response of community have also been found in some empirical studies that biomass to perturbation. The effects of spatial aggregation showed perturbation selected for the least productive on diversity – stability relationships should be taken into 28, 33, 34 species, such as grasses, by allowing faster resource uptake account. which exceeded their inherent low resource use efficiency The aim of this study is to investigate three topics in 14, 15 and productivity. However, other studies have failed relation to biodiversity, ecosystem functioning and ecosystem 1, 20 – 22 to show such effects. Several studies have indicated stability using experimental glasshouse plant communities. that there is a strong dependency of stability on the traits Three hypotheses are tested: 23, 24 of dominant species present in a given ecosystem. In addition, the significance of species composition and identity (1) Multispecies communities have higher aboveground pro- has been highlighted in some experiments. The presence of a ductivity than had the monocultures. particular species, for instance, nitrogen-fixing legumes, (2) The relative performance of multi-species and monocul- seems to cause nutrient enrichment, leading to the expression tures is moderated by drought. ......................................................................................................................................................................................................................................... 129 Research article Bioscience Horizons † Volume 1 † Number 2 † June 2008 ......................................................................................................................................................................................................................................... (3) Aggregation affects the relative performance of multi- species compared with constituent monocultures. Materials and methods Four common ruderal plant species (three forbs and one grass) were used in the experiment: shepherd’s purse Capsella bursa-pastoris (Brassicaceae) denoted Cap, scentless mayweed Tripleurospermum inodorum (Asteraceae) denoted Tri, annual meadow-grass Poa annua (Poaceae) denoted Poa and chickweed Stellaria media (Caryphyllaceae) denoted Ste. These species were chosen for their reliable germination and high growth rate. All four species can proliferate on poor, disturbed soil and are adapted to complete their life cycle by rapid growth to maximize seed production. These species often co-occur in disturbed habitats, such as agricul- 35, 36 tural fallows, though all seeds were purchased from Herbiseed (Twyford, UK). The experiment was conducted in the glasshouse at the University of Reading in summer 2006 (from 5 July to 17 Figure 1. The experimental design. (A) The Paired plot which was divided into two sub-plots with a plastic divider. One of the pairs was under control August). Plant communities were established on compost- treatment and the other exposed to perturbation. (B) Plot with nine cells of filled plastic pots (37  21.5  5cm ) each divided by water- 5  5cm which shows an example of aggregated seed sowing pattern of tight barriers into two sub-plots (18.25  21.5  5cm ). Cap–Tri–Poa. In the random and single pots, seeds were broadcasted over Each main pot contained the same plant treatment and one 2 the 15  15 cm plot. sub-plot was exposed to drought and the other adjacent sub-plot received regular watering (Fig. 1A). The density of divided into nine cells of 5  5cm ; each species occupied seeds per pot was equalized, such that each plot contained a three cells in a Latin square pattern (Fig. 1B). For random total of 900 seeds with equivalent proportion of three treatment and for monocultures, seeds were sown randomly species for multi-species mixtures. Seeds were sown on 5 – 6 over the soil within 15  15 cm (Fig. 1A). July 2006 over compost-filled pots. A regular watering In sum, there were a total of 24 combinations per experi- regime was set at 400 ml once every 3 days. At 25 days mental block; 12 diversity and aggregation treatments after sowing (29 July), the drought perturbation treatment (4 monocultures þ 4 aggregated three-species mixtures þ 4 started. For the first 2 days and the 2 days before the cessation randomly sown three-species mixtures)  2 perturbation of drought, perturbed plots received half the amount of water treatments ( perturbed or control). Four such randomized controlled plots received (200 ml). Then, there was no water- blocks were sown in the glasshouse. ing on perturbed pots for 8 days (from 31 July to 7 August). Soil moisture content was monitored daily during and The original watering regime was maintained on control plots after the drought treatment period using ThetaProbe soil over the experimental period. Following the drought treat- moisture sensor (type ML 2x). Maximum and minimum ment, regular watering of 400 ml water per pot once every air temperatures were also monitored daily. Aboveground 3 days was performed across the plots until the end of the biomass was harvested on 17th August by clipping the experiment (15 August). shoots at ground level, and dried at 608C for 48 h. After In addition to the drought treatment, two other treatments drying, the total aboveground shoot weights of each were applied to the plots: plant diversity and species aggrega- species in a pot were recorded. tion treatments. For the diversity treatment, single- and multiple-species communities were compared. The total Data analysis number of species combinations was eight; four single species (Cap – Cap – Cap, Tri – Tri – Tri, Poa – Poa – Poa and In order to investigate the relative difference in yield between Ste – Ste – Ste) and all possible three-species mixtures (Cap – monoculture and species mixtures, the extent of overyielding Tri – Poa, Cap – Poa – Ste, Cap – Tri – Ste and Tri – Poa – Ste). was calculated by comparing the productivity of mono- The aggregation treatment was conducted only in multi- cultures with the expected productivity of the species mix- species communities by sowing seeds in aggregated cell pat- tures based on the biomass of their constituent species in 13, 17, 37 terns or randomly over the plot. In aggregated spatial monoculture. The proportion of biomass of species pattern, these sub-plots of 15  15 cm were further mixture relative to that of monoculture was used as a ......................................................................................................................................................................................................................................... 130 Bioscience Horizons † Volume 1 † Number 2 † June 2008 Research article ......................................................................................................................................................................................................................................... measure of overyielding in this study which is called relative yield total (RYT) ; RYT ¼ O=E ¼ O =ðY =3 þ Y =3 þ Y =3Þ ijk i j k ¼ observed biomass/expected biomass where Y is the biomass of component species i in monocul- ture and O the observed total yield of mixture including ijk species i, j and k. Expected biomass was calculated from the pot total weight of monoculture in the same block for observed biomass to take account of variation between blocks. For individual species, biomass of each species in species mixture was compared with the one-third of biomass of the same species in monocultures; Figure 2. Changes in mean soil moisture content during the experiment (+SE). Mean of each time point is derived from four randomly selected Species relative yield ¼ O =ðY =3Þ i i measurements from each block of either treatment, i.e. 4 measurements  4 blocks ¼ 16 measurements for each treatment at one time point. The where O is the observed biomass of species i in species shaded period from 31 July to 8 August indicates the period of drought. The control watering treatment is denoted by solid circles, and the droughted by mixture and Y the biomass of species i monoculture. open squares. The gaps are when the data were not obtained. The three response variables analysed were total above- ground biomass per pot, biomass of individual species, RYT and the relative yield (RY) of individual species in perturbation treatment at the greatest by 48.80% in species mixtures. Data were analysed with general linear C. bursa-pastoris monoculture, and at the least by 1.66% models (Type _ Sum of Squares). The effects of perturbation in three-species mixture of Tri–Poa–Ste, average reduction treatment, aggregation and diversity (species richness) and of aboveground productivity was 24.53%. their interactions on each response variable were tested. Perturbation exhibited significant effects on RYT Though biomass data usually require log-transformation, (F ¼ 4.50; P ¼ 0.038). The observed biomass of species- 1,57 distributions of residuals suggested that absolute above- mixture under the perturbation treatment was slightly ground biomass was more appropriate here for both pot greater than the expected biomass (on average by 7%). total individual species weights. RYT and species RY were There was an overall reduction of productivity of the log-transformed to achieve normality. observed biomass of controls by 7% on average. Hence, in terms of RY, perturbed communities performed better than control ones by 14% on average. The response of multi- Results species communities depended on the occurrence of pertur- During the drought treatment, soil moisture content of bation, relative biomass increased in perturbed, but the perturbed pots declined day by day (Fig. 2). When the decreased in the control treatment. This result shows that treatment finished, soil moisture content decreased approxi- negative interspecific interactions predominated in the 3 23 mately to 0.05 m m . Control plots showed moderately control environment, whereas positive interactions predomi- fluctuating patterns, but the soil moisture content was main- nated under drought stress. Aggregation did not induce 3 23 tained on an average of 0.2 m m . The soil moisture changes in RY (Fig. 4), indicating that the productivity of content was kept equivalent during the week of recovery multi-species communities was not affected by spatial struc- period. tural differences in any combination of species composition. Block effects (F ¼ 17.07; P, 0.001) and perturbation Perturbation caused a decrease in biomass production of 3,89 (F ¼ 52.89; P, 0.001) had a statistically significant three species; C. bursa-pastoris (F ¼ 10.31; P ¼ 0.002), 1,89 1,49 impact on total aboveground biomass; however, there was T. inodorum (F ¼ 7.66; P ¼ 0.008) and P. annua 1,49 no significant effect of species richness, aggregation treat- (F ¼ 4.27; P ¼ 0.044); however, there was no significant 1,49 ment or the interactions of these factors with perturbation effect of species richness or its interaction with perturbation (Fig. 3A and B). Variation in perturbed plots across the treatment. blocks was greater than control ones on average: 2.59 g Under perturbation, there was a reduction in biomass on (44.5% reduction) difference between maximum and average of 37.5% in C. bursa-pastoris (34.7% in species minimum biomasses in perturbed treatment, 1.43 g mixtures, 48.0% in monocultures), 25.2% in T. inodorum (21.9%) in control. Compared with the aboveground (23.9% and 28.2% in mixtures and monocultures, respect- biomass of the controls, productivity was decreased by the ively) 17.2% in P. annua (11.3% in mixtures, 32.8% in ......................................................................................................................................................................................................................................... 131 Research article Bioscience Horizons † Volume 1 † Number 2 † June 2008 ......................................................................................................................................................................................................................................... Figure 3. Differences in mean (+SE) aboveground community biomass between perturbed and control pots in relation to plot-level treatments of (A) species richness (three or one); (B) aggregation treatment of polyculture-aggregated, polyculture-random, monocultures. Coloured bars, control; open bars, droughted. Figure 4. Differences in mean (+SE) relative yield total of aboveground community biomass between perturbed and control pots in relation to aggregation treatment. Coloured bars, control; open bars, droughted. monocultures) (Fig. 5). In general, monocultures were more affected by drought, but also showed a greater variability. Productivity responses of S. media were different from the other species (Fig. 5D), although perturbation had no signifi- cant effect, aboveground biomass production was signifi- cantly lower in species mixture compared with monoculture (F ¼ 6.65; P ¼ 0.013). S. media was the 1,49 least affected by perturbation with average reduction of biomass production by 16.1% (14.1% and 19.9% in mix- tures and monocultures, respectively). When aboveground biomass of each species was com- pared with the monoculture productivity, responses of species differed considerably (Fig. 6). C. bursa-pastoris increased productivity in three species mixtures (71.4% increase in the perturbed treatment, 34.9% increase in Figure 5. Differences in mean (+SE) aboveground biomass of individual species between perturbed and control pots and how biomass differed control compared to biomass of monocultures), but there depending on species composition. (A) Capsella bursa-pastoris;(B) was general decline of productivity in T. inodorum in Tripleurospermum inodorum;(C) Poa annua;(D) Stellaria media. Species mixture (12.9% decrease in perturbed, 17.7% decrease in composition: ctp, Cap–Tri–Poa; cps, Cap–Poa–Ste; cts, Cap–Tri–Ste; tps, control compared with monocultures, but for P. annua Tri–Poa–Ste; singles are monocultures of each species. Coloured bars, there was 17.7% increase in the perturbed treatment and control; open bars, droughted. ......................................................................................................................................................................................................................................... 132 Bioscience Horizons † Volume 1 † Number 2 † June 2008 Research article ......................................................................................................................................................................................................................................... showed clear differences reflecting the effects of species identity. Several pieces of empirical work highlighted the idiosyncratic nature of species response to perturbation, which suggests that the underlying mechanisms of the diver- sity – stability relationship in real communities are more 1,15,19,26 complex than those predicted by theory. The exper- iment here provided one week with adequate watering after perturbation to investigate the recovery of communities. Quick recovery by ruderal species was observed with inflor- escences produced in some plants of C. bursa-pastoris and T. inodorum, and from the emergence of new seedlings Figure 6. Differences in mean (+SE) relative aboveground biomass yield shown in S. media. Hence, the lack of diversity effects in multi-species communities of each species under perturbed and control appears to be a consequence of over-riding species/commu- conditions. Species code: cap, Capsella bursa-pastoris; tri, Tripleurospermum nity composition effects, consistent with other previous inodorum; poa, Poa annua; ste, Stellaria media. Coloured bars, control; open experiments. Diversity response may therefore originate bars, droughted. from a combination of factors, some of which remain 1, 20, 23, 24 hidden. 10.9% decrease in the control. P. annua was the only species to show a response to the multispecies treatment that varied Relative yield with perturbation. When productivity of multi-species mixtures was compared with expectation based on the monoculture performance Discussion (RYT), it was clear that the positive effect of species-richness was enhanced under drought perturbation. Indeed the Absolute aboveground biomass species mixtures under-yielded in the control treatment but 14,15 The effect of species richness was exhibited on the absolute over-yielded in perturbed conditions (Fig. 4). This aboveground biomass of S. media as an individual species. shows that diversity effect was negative in control, but posi- Aboveground community biomass and biomass of all indi- tive in perturbed. This suggests that the emergence of diver- vidual species other than S. media were not affected by sity effects is dependent on resource availability. This is a key species richness. The selection of ruderal species may have finding, which proposes that the relationship between eco- led to functional similarity among species that might have system functioning and species diversity may be context blurred the biodiversity effect. High potential growth rate dependent, e.g. it may vary in sign depending on the avail- is one of the strategies of ruderal species to survive in ability of abiotic resources. It is suggested that under the per- environments with limited supply of resources and nutri- turbed condition, functional complementarity or facilitation ents. This allows individuals to alter the length of life occurred, promoting the greater relative resource uptake by 15, 37, 38 cycle according to the availability of resources found in the least productive species (P. annua), whereas the any environment. In this study, species response to the limit- drought-tolerant species S. media maintained its pro- ation in water may be similar, to encourage fast development ductivity. A combination of these effects would have led to to change generations in short period of time. This may have the enhanced relative community biomass. In the case of resulted in the overall similarity in community productivity S. media, relative biomass in species mixture did not out- irrespective of differences in species richness. Such response weigh the performance of monocultures in spite of its toler- 10, 20 was also found in experiments on the soil food web. ance to perturbation, indicating the limited magnitude of These studies showed that although there was high species effects of dominance (selection) and complementarity in 15,19 diversity, processes driven by each species were similar or this experiment. Relative productivity of individual partly compensatory to each other due to low specialization species did not increase in multi-species communities, but of functioning in decomposer ecosystems. Therefore in this that of community biomass increased under the perturbed study, it appears that all four ruderal species respond to environment. This suggests that complementary resource drought in the same way, shortening life cycles, and the simi- use or facilitation effectively mitigated the negative effects larity of functionality does not lead to biodiversity effects. of diversity and perturbation of the component species, The great variability in response pattern, depending on the resulting in the overall increase in community biomass. community, species or species composition observed, leads Aggregation to some difficulty in proposing explicit explanations regard- ing the interaction of perturbation with diversity-functioning Aggregation did not induce changes in response patterns mechanisms. Although there is a lack of evidence for biodi- of multi-species communities in either watering treatment. versity effects in this study, responses of individual species In terms of interaction of spatial structure with perturbation, ......................................................................................................................................................................................................................................... 133 Research article Bioscience Horizons † Volume 1 † Number 2 † June 2008 ......................................................................................................................................................................................................................................... random communities did not increase productivity in Acknowledgements response to perturbation. It was hypothesized that drought- I would like to express my sincere thanks to my supervisor, tolerant species would increase productivity in the randomly 22, 30, 32 Dr Andrew Wilby for his advice on the conduct of the experi- distributed treatment. However, such response was ment, support on analysis and writing-up and for always not observed in this experiment, which was probably due giving me positive feedback on my work. Special thanks go to a great reduction of productivity in all species except to Dr Gillian Fraser and Dr Julian Park for their help S. media under perturbed conditions. C. bursa-pastoris and during the experiment, my sister Miyabi for devoting much T. inodorum reached reproductive development even in per- time with me to watering and harvesting and Dr. Duncan turbed conditions, while P. annua in mixtures performed Vaughan and Suzi Heaton for their great help on manuscript. better, in relative terms, under perturbation. Such differences I would like to thank my family and Ryosuke for their endur- in strategies to overcome drought stress among species indi- ing support. cate that the component species are not spatially complemen- tary to each other. There were some probable limitations in manipulating spatial clumping patterns in this experiment such as differ- Funding ences in the timing of germination and the growth rate, This project was funded by the University of Reading. and limitation in spatial scales of the experimental plant communities. The time taken for germination after the sowing of seeds differed among the four species which References could have pre-set the hierarchy in resource uptake efficiency in favour of fast germinating or fast growing species. 1. Hooper DU, Chapin FS, Ewel JJ et al. (2005) Effects of biodiversity on Although explicit biomass measurements were not under- ecosystem functioning: a consensus of current knowledge. 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Ecol Lett 7: 574–583. ........................................................................................................................................................................................................................................ Submitted on 3 October 2007; accepted on 21 December 2007; advance access publication 24 April 2008 .........................................................................................................................................................................................................................................

Journal

Bioscience Horizons – Oxford University Press

Published: Jun 24, 2008

Keywords: Key words biodiversity effects ecosystem stability drought perturbation relative yield total

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