Austral Ecology

BELL, DAVID T.; KING, LAURA A.; PLUMMER, JULIE A.

doi: 10.1046/j.1442-9993.1999.00940.xpmid: N/A

Germination occurs usually in response to multiple environmental cues. Seeds with the ecophysiological ability to simultaneously sense the previous presence of fire and appropriate levels of temperature, light and soil nitrate could restrict germination to postfire, winter and competition‐free microhabitats, where the potential for seedling survival is enhanced. Germination responses of 16 species with a range of life forms, fire responses and seed weights were determined under controlled conditions of 15°C temperature, a 12 h light cycle, exposure to 1 g L−1 nitrate solution, and six conditions of light quality (white, blue, yellow, red, far‐red light and darkness). Germination in Oenothera stricta, a weedy naturalized ephemeral, and two small‐seeded indigenous Asteraceae species of mulga woodlands, Leucochrysum fitzgibbonii and Craspedia sp., were enhanced by white, yellow or red light compared with germination achieved in the dark, or under far‐red or blue light. In red light, KNO3 further enhanced germination of these positively photoblastic species. The germination response of Trachyandra divaricata, a naturalized herb of sandy, seaside locations, and several native jarrah forest legumes (four Acacia species, Bossiaea aquifolium, Gompholobium marginatum and Sphaerolobium vimineum) proved to be negatively photoblastic. Of these seven negatively photoblastic herb and shrub species, exposure to KNO3 overcame the inhibition of light only in the resprouter species, Acacia lateriticola. In the serotinous, negatively photoblastic tree species, Corymbia calophylla and Eucalyptus marginata, KNO3 seemed to be required before the negative response to light exposure was recorded. A dose–curve experiment on two positively photoblastic and three negatively photoblastic species indicated that although KNO3 exposure affected germination in all species, different concentrations of KNO3 (0, 0.5, 1, 2, and 5 g L−1) produced different levels of response. Detailed studies with additions of KNO3 (1 g L−1) and the growth hormone, gibberellic acid (GA3; 50 mg L−1), showed that increased germination percentages of the positively photoblastic species, Oenothera stricta, occurred in the light, but blocking endogenous gibberellic synthesis with paclobutrazol, or adding exogenous GA3 or KNO3 had no effect on the light‐induced germination levels. In the negatively photoblastic species Trachyandra divaricata, additions of KNO3 and GA3 had no influence on the germination inhibition induced by exposure to light nor did blocking endogenous GA synthesis. The 16 species growing naturally in Western Australia, Australia show a range of germination responses to environmental conditions, but depending on their natural habitat, the ecophysiology of each species appears to be optimized for subsequent seedling survival.

COATES, FIONA; KIRKPATRICK, J. B.; MINCHIN, P. R.

doi: 10.1046/j.1442-9993.1999.00941.xpmid: N/A

Spyridium microphyllum (F. Muell. ex Reissek) Druce and Spyridium obcordatum (Hook. f) W. M. Curtis are narrow endemic shrubs which are confined to small parts of Tasmania, Australia, and are rare within their present ranges. Analysis of quadrat data from transects through populations of both species showed that at most individual sites the rare taxa occur in floristically and environmentally distinct situations. However, sites supporting the same species differ floristically and environmentally, with the only communality being a lack of dense tall shrub or tree cover. Both species are absent from many areas with closely similar physical environments and vascular plant species composition to those in which they occur. Thus, their current distributions cannot be explained by limitation to a rare habitat. Spyridium microphyllum appears to be susceptible to local extinction because it is an obligate seeder, is killed by fire, does not survive in taller, dense vegetation, and is not adapted for dispersal over the distances between its potential habitats. While S. obcordatum recovers vegetatively after fire, it also does not survive in dense taller vegetation, has limited dispersal ability and highly disjunct potential habitat. Both species may have been more widespread and competitive during cooler and drier times.

SINGER, RUTH J.; BURGMAN, MARK A.

doi: 10.1046/j.1442-9993.1999.00942.xpmid: N/A

The spatial structure, age structure and population dynamics of the shrub Kunzea ericoides (A. Rich.) J. Thompson were investigated at Coranderrk Reserve, near Healesville, Victoria, Australia. The shrub is known to be invasive in many areas and, although indigenous to the reserve, has greatly increased its population size and distribution within the reserve in the past 30 years. Dendrochronology showed a constant relationship between age and stem diameter for K. ericoides in the reserve. The spatial structure of the variable stem diameter was investigated by the use of spatial correlograms and results suggested that K. ericoides is spreading via the formation of discrete clumps and gap‐phase regeneration. The two main populations in the reserve recruited continuously in time but patchily in space. A simple statistical model for size–frequency data showed that K. ericoides is expanding faster in some areas of the reserve than others. The trend of expansion of K. ericoides may be irreversible. Intensive management of Coranderrk Reserve will be required if K. ericoides is to be controlled and the conservation value of the reserve maintained.

COATES, MAURICIO; JAKSIC, FABIAN M.

doi: 10.1046/j.1442-9993.1999.00943.xpmid: N/A

It is widely believed that only precipitation levels (through increased primary production) determine irruptions of small mammals in semi‐arid areas of western South America. Nevertheless, density‐dependent factors may also drive population fluctuations. To test statistically these putative effects we analysed 11 years of population records on three sympatric species of small mammals at two different habitat types in north central Chile. We applied the classical diagnostic tools of time series analysis (the autocorrelation function: ACF) to the observed time series of three neotropical small mammals. We also used simple linear autoregressive time series models to reconstruct the endogenous dynamics of these populations. The analysis strongly suggests that population fluctuations of the three species have an important density‐dependent component, with the most irruptive species (Phyllotis darwini, Waterhouse 1837) displaying stronger second order population feedbacks than the other two (Akodon olivaceus, Waterhouse 1837 and Thylamys elegans, Waterhouse 1839). The latter two species showed direct density‐dependent feedbacks. We hypothesize that the frequent population outbreaks of P. darwini (and perhaps of other species) in semi‐arid regions of western South America, may be the result of population‐level (direct density‐ dependence) and community‐level processes (delayed density‐dependence), interacting with exogenous perturbations (rainfall and associated primary production).

SMITH, MARTIN A.; BELL, DAVID T.; LONERAGAN, WILLIAM A.

doi: 10.1046/j.1442-9993.1999.00944.xpmid: N/A

Austrostipa compressa, a native ephemeral of southwest Western Australia was stimulated to germinate under a range of temperatures, in the presence of light, and exposure to smoke‐water. This combination of environmental cues results in winter‐maximum germination in immediate postfire and disturbed‐soil environments of this Mediterranean‐type climate. In contrast, Ehrharta calycina, an introduced perennial grass from southern Africa that has invaded Banksia woodlands, germinated under a wide range of temperature and light conditions, but showed no promotive response to smoke‐water. Although A. compressa seeds tolerated heat shock better than E. calycina, the self‐burial mechanism of A. compressa seeds ensures protection from fire. High‐intensity fire could have a greater impact on E. calycina, as the seeds of this species tend to accumulate in the top of the soil profile where they are more susceptible to high temperatures. Although seeds of E. calycina are more susceptible to high temperatures, survival of mature individuals by postfire resprouting ensures continued survival in native woodlands. Estimates of soil seed bank densities showed extreme variability, but some recently burnt areas of the Yule Brook Botany Reserve contained up to 8000 seed m−2 of A. compressa and nearly 75 000 seeds m−2 of E. calycina. Viable soil seed bank densities of A. compressa are reduced with time‐since‐last fire, but areas of greater than 45 years since the last fire, still contained up to 119 seeds m−2. In both species, only about half their soil seed bank germinates following fire, thus ensuring the potential for later recruitment. Massive soil seed populations of E. calycina in native Banksia woodlands pose a major problem to management of this plant community type.

NOBBS, MADELEINE; McGUINNESS, KEITH A.

doi: 10.1046/j.1442-9993.1999.00945.xpmid: N/A

Counting the number of individuals emerging from burrows is the most practical method for estimating the apparent abundance of Australian Uca species living in mangrove habitats. Experiments were conducted to investigate the effect on counts of quadrat design, distance of observer, quadrat size, recovery time and observational technique. Significant differences in the apparent abundance of one species were found when the subjects were within 2 m of the observer, and when a conspicuous quadrat was used. The largest quadrat tested provided the least variability in counts but an intermediate size (0.56 m2) was more practical. Most Uca active within a 30‐min period emerged during the first 10 min regardless of site, species, sex or season. There was a linear correlation between scanning and continuous observation indicating that the former method could be useful when sampling time was limited. Temporal changes in the apparent abundance of Uca suggest that long‐term sampling and more detailed studies will be worthwhile.

WILLIAMS, R. J.; COOK, G. D.; GILL, A. M.; MOORE, P. H. R.

doi: 10.1046/j.1442-9993.1999.00946.xpmid: N/A

Dry season fires are a feature of the tropical savannas of northern Australia. As part of a landscape‐scale fire experiment, we examined the effects of fire regimes on tree survival in a tropical savanna in Kakadu National Park, northern Australia. The fire regimes were annual early dry season (June) fires, annual late dry season (September) fires, and, no fire (control). Prescriptive, experimental fires were lit annually, between 1990 and 1994, in replicate compartments, each 15–20 km2. In addition to the prescribed fires, however, one of the control compartments, which had been unburnt for seven years, was burnt by an unplanned, high intensity fire (~ 20 000 kW m−1) in September 1994. This provided an opportunity to compare the impacts on the tree stratum of frequent, prescribed burning at various intensities, and a single unplanned fire. In all fire regimes, stem survival was substantially lower than whole‐plant survival, and decreased linearly with increasing fire intensity. Significantly, stem death following the single, high intensity 20 000 kWm−1 fire (75%) was comparable to that of a regime of annual late dry season burning for five years, at an average intensity of c. 8000 kWm−1. In the high intensity unplanned fire, stem survival showed a non‐linear response to stem size, being least in the small (< 10 cm DBH) and large (> 40 cm DBH) size classes, and highest in the intermediate size classes. Stem survival was also species‐dependent, being higher in the dominant Eucalyptus miniata than in the subdominant, broad‐leaf deciduous trees. In the absence of fire for 5–10 years, the structure and composition of the tree stratum of these savannas tends to become more complex than in sites burnt more frequently, especially by high intensity fire. Such a long‐term absence of fire may be a conservation objective for some areas of savanna. However, build‐up of fuel to near maximal levels can occur in 2–4 years without fire. This may predispose the savannas to high‐intensity, late dry season fires. Whatever the fire‐management goal within a given patch of savanna, whether it be the prescribed use of fire on a biennial basis, or the exclusion of fire at a semidecadal scale, careful attention still needs to be given to the consequences of fuel build‐up in fire‐excluded sites.

WOINARSKI, J. C. Z.; HORNER, P.; FISHER, A.; BRENNAN, K.; LINDNER, D.; GAMBOLD, N.; CHATTO, R.; MORRIS, I.

doi: 10.1046/j.1442-9993.1999.00947.xpmid: N/A

Forty‐four species of terrestrial reptiles and eight species of frogs were recorded from 60 continental islands of the Wessel and English Company groups off northeastern Arnhem Land, Northern Territory. Two gecko species, Oedura rhombifer and Heteronotia binoei, were present on the most islands (34 and 31, respectively), and occurred on islands < 5 ha. In contrast, agamids, pygopodids and varanids were absent from islands < 18 ha, and snakes and frogs were not reported from islands < 240 ha. Island size explained 82% of the variation in species richness for terrestrial reptiles, and 84% of that for lizards. The relationship was less good for (i) groups with generally uncommon species (notably snakes), for which sampling effort explained more variation, and (ii) groups with species which had relatively specific habitat requirements (notably frogs), for which island size and isolation factors were not especially relevant. For most taxonomic groups considered, isolation factors added little to the relationship between species richness and island size. Across all reptiles, larger species were found on fewer islands, and had larger island size thresholds. This relationship broke down with analysis restricted to the single most species‐rich family, Scincidae. Only 6 of the 20 most frequently recorded species showed significant variation in abundance among 8 vegetation types sampled by 226 quadrats across 40 islands. The number of species (alpha‐diversity) and total abundance of herpetofauna within quadrats was generally unrelated to island size; however, (with analysis restricted to islands on which they occurred) six individual species were significantly more abundant on smaller islands than on larger islands, with no species showing the opposite pattern. The islands’ herpetofauna is largely a relatively depauperate subset of that of the far more complex sandstone massif and escarpment of western Arnhem Land, especially missing species associated with rugged sandstone gorges, riparian areas, open forests, swamps and clay soils. Patterns in species richness and composition are explained by greater range of environments on larger islands allowing better retention of species since isolation and/or richer tallies at the time of isolation. The evidence suggests that there has been relatively little colonization, although at least two gecko species and one varanid may have moved reasonably frequently.

MADSEN, THOMAS; SHINE, RICHARD

doi: 10.1046/j.1442-9993.1999.00948.xpmid: N/A

A five‐year mark–recapture study of dusky rats (Rattus colletti) on the Adelaide River floodplain, within the Australian wet–dry tropics, revealed substantial spatial and temporal variation in demographic characteristics (abundance, condition, and rates of survival, growth, and reproduction) of the rats. Our data suggest that annual variation in the intensity and timing of monsoonal rainfall during the ‘wet‐season’ is the main factor driving the demography of the rats. When total rainfall figures are modified to reflect the magnitude and duration of inundation of the floodplain each year, a link is evident between rainfall patterns and the rat population dynamics. Minor spatial variations in elevation (and hence, in the duration of inundation) across the floodplain engender large differences in rat growth rates, condition factors, survival rates, and the duration of reproductive activity each year. Because these rats have very high reproductive rates, small rain‐induced differences in the duration of their reproductive season (i.e. number of litters per year) can cause massive differences in subsequent rat abundances. Hence, rat numbers can be predicted from rainfall patterns during the preceding wet‐season. Similar links between rainfall, the duration of breeding, and fluctuations in abundance may typify many rodent populations in tropical and arid regions of the world.

GREEN, PETER T.; HART, ROGER; JANTAN, JAMIL BIN; METCALFE, DANIEL J.; O’DOWD, DENNIS J.; LAKE, P. S.

doi: 10.1046/j.1442-9993.1999.00950.xpmid: N/A

Red land crabs (Gecarcoidea natalis) are both abundant and widespread in intact rainforest on Christmas Island, Indian Ocean, occurring at densities of ~ 1 crab m–2. We used an on‐going exclusion experiment to test the hypothesis that through their activities as seed predators and soil disturbers, these omnivores are important determinants of the density and relative species abundance of seeds in the soil seed bank of undisturbed primary rainforest. After 6.5 y of continuous crab exclusion, there was no significant difference in the density of seeds in the top 3 cm of soil between the control (3671 ± 517 seeds/0.19 m2) and exclusion plots (3285 ± 801 seeds/0.19 m2), nor was there any significant treatment difference in relative species composition. There were also no significant treatment differences when Muntingia calabura, which accounted for 91.9% of all seeds in the seed bank, was excluded from the analyses. We attribute the lack of a significant impact of red crabs to their probable inability to handle the very small seeds which occurred in the seed bank, and the low level of soil disturbance at the study site. We conclude that the observed impact of red crabs on the abundance and diversity of successional species in natural light gaps on Christmas Island is due to their impact on germinating seedlings alone, rather than any additional impact on the soil seed bank.