Ecological Monographs

Iverson, Louis R.; Prasad, Anantha M.

doi: 10.1890/0012-9615%281998%29068%5B0465:PAOTSF%5D2.0.CO%3B2pmid: N/A

Projected climate warming will potentially have profound effects on the earth’’s biota, including a large redistribution of tree species. We developed models to evaluate potential shifts for 80 individual tree species in the eastern United States. First, environmental factors associated with current ranges of tree species were assessed using geographic information systems (GIS) in conjunction with regression tree analysis (RTA). The method was then extended to better understand the potential of species to survive and/or migrate under a changed climate. We collected, summarized, and analyzed data for climate, soils, land use, elevation, and species assemblages for >2100 counties east of the 100th meridian. Forest Inventory Analysis (FIA) data for >100000 forested plots in the East provided the tree species range and abundance information for the trees. RTA was used to devise prediction rules from current species––environment relationships, which were then used to replicate the current distribution as well as predict the future potential distributions under two scenarios of climate change with twofold increases in the level of atmospheric CO 2 . Validation measures prove the utility of the RTA modeling approach for mapping current tree importance values across large areas, leading to increased confidence in the predictions of potential future species distributions. With our analysis of potential effects, we show that roughly 30 species could expand their range and/or weighted importance at least 10%%, while an additional 30 species could decrease by at least 10%%, following equilibrium after a changed climate. Depending on the global change scenario used, 4––9 species would potentially move out of the United States to the north. Nearly half of the species assessed (36 out of 80) showed the potential for the ecological optima to shift at least 100 km to the north, including seven that could move >250 km. Given these potential future distributions, actual species redistributions will be controlled by migration rates possible through fragmented landscapes.

Iverson, Louis R.; Prasad, Anantha M.

doi: 10.1890/0012-9615(1998)068[0465:PAOTSF]2.0.CO;2pmid: N/A

Projected climate warming will potentially have profound effects on the earth's biota, including a large redistribution of tree species. We developed models to evaluate potential shifts for 80 individual tree species in the eastern United States. First, environmental factors associated with current ranges of tree species were assessed using geographic information systems (GIS) in conjunction with regression tree analysis (RTA). The method was then extended to better understand the potential of species to survive and/or migrate under a changed climate. We collected, summarized, and analyzed data for climate, soils, land use, elevation, and species assemblages for >2100 counties east of the 100th meridian. Forest Inventory Analysis (FIA) data for >100000 forested plots in the East provided the tree species range and abundance information for the trees. RTA was used to devise prediction rules from current species–environment relationships, which were then used to replicate the current distribution as well as predict the future potential distributions under two scenarios of climate change with twofold increases in the level of atmospheric CO2. Validation measures prove the utility of the RTA modeling approach for mapping current tree importance values across large areas, leading to increased confidence in the predictions of potential future species distributions.

Lichter, John

doi: 10.1890/0012-9615%281998%29068%5B0487:PSAFDO%5D2.0.CO%3B2pmid: N/A

Vegetation and soil properties were described across a well-dated sand-dune chronosequence bordering northern Lake Michigan to document patterns and rates of primary succession and forest ecosystem development, and to determine environmental constraints that potentially drive succession and regulate species diversity. The site experienced frequent and continuing formation of 72 shore-parallel dune ridges over the past 2375 yr. Across the chronosequence represented by the youngest 13 dune ridges aged 25––440 yr, there were clear patterns of species turnover and community convergence as well as successional changes in species diversity, aboveground biomass, aboveground litter production, net ecosystem production, nutrient pools, and nutrient cycling. Dune-building species were replaced by evergreen shrubs and bunchgrass within 100 yr, which in turn, were replaced by mixed pine forest within 345 yr. Plant-species richness increased to a peak in developing forest at 285 yr but thereafter decreased as early-successional species disappeared from the communities. Rates of species addition peaked between 95 and 145 yr as forest species invaded, whereas rates of species loss peaked between 345 and 440 yr as early-successional species were lost from the developing forest. Development of the forest ecosystem required ∼∼300 yr (i.e., 145––440 years). Total ecosystem carbon increased in a logistic manner to 128 Mg C/ha, with net ecosystem production peaking at 30 g C··m −−2 ··yr −−1 in developing forest. Aboveground biomass and O horizon mass increased to ∼∼137 Mg/ha and ∼∼79 Mg/ha, respectively, whereas aboveground litter production increased to 3.5 Mg··ha −−1 ··yr −−1 at 440 yr, but thereafter varied between 175 and 350 Mg··ha −−1 ··yr −−1 . Total carbon and total nitrogen in the upper 15 cm of mineral soil and O horizon accumulated to ∼∼42 Mg/ha and ∼∼1.36 Mg/ha, respectively. Estimated average rates of carbon and nitrogen accumulation over 440 yr of ecosystem aggradation were 23.2 g··m −−2 ··yr −−1 for carbon and 0.38 g··m −−2 ··yr −−1 for nitrogen. Because nitrogen-fixing plants are rare on the upland dune ridges, ecosystem aggradation depends largely on atmospheric nitrogen inputs. Following colonization by conifers, soil acidification resulted in rapid leaching losses of calcium and magnesium, whereas phosphorus and potassium were cycled more tightly. The dune chronosequence represents a complex gradient of changing environmental constraints that differentially reduce the survival, growth, and reproduction of plant species. Young dune ridges near the lake shore are characterized by strong winds, sand burial and erosion, high insolation, high rates of evaporation, and low availability of nitrogen and phosphorus. These conditions ameliorate with increasing dune age as wind velocities and sand movement diminish with distance from the lake, as accumulating organic matter improves the moisture-holding capacity and nitrogen availability of the soil, and as mineral weathering mobilizes soil phosphorus. However, in developing forest, light and cationic nutrients may become limiting, and decreased light availability, cool soil temperatures, and accumulation of a thick forest floor may limit recruitment from seed for many species. These numerous potential environmental constraints suggest a considerable complexity in this ostensibly simple ecosystem.

Lichter, John

doi: 10.1890/0012-9615(1998)068[0487:PSAFDO]2.0.CO;2pmid: N/A

Vegetation and soil properties were described across a well‐dated sand‐dune chronosequence bordering northern Lake Michigan to document patterns and rates of primary succession and forest ecosystem development, and to determine environmental constraints that potentially drive succession and regulate species diversity. The site experienced frequent and continuing formation of 72 shore‐parallel dune ridges over the past 2375 yr. Across the chronosequence represented by the youngest 13 dune ridges aged 25–440 yr, there were clear patterns of species turnover and community convergence as well as successional changes in species diversity, aboveground biomass, aboveground litter production, net ecosystem production, nutrient pools, and nutrient cycling. Dune‐building species were replaced by evergreen shrubs and bunchgrass within 100 yr, which in turn, were replaced by mixed pine forest within 345 yr. Plant‐species richness increased to a peak in developing forest at 285 yr but thereafter decreased as early‐successional species disappeared from the communities. Rates of species addition peaked between 95 and 145 yr as forest species invaded, whereas rates of species loss peaked between 345 and 440 yr as early‐successional species were lost from the developing forest.

Herrera, Carlos M.

doi: 10.1890/0012-9615(1998)068[0511:LTDOMF]2.0.CO;2pmid: N/A

The relationship between fleshy‐fruited plants and their vertebrate seed dispersal agents often has been depicted as subject to important interannual variation, but no study has thus far documented such variation on a long‐term basis. This paper presents the results of a 12‐yr investigation on fleshy‐fruited plants and avian frugivores in a Mediterranean montane locality of southeastern Spain. The main objective was to document patterns and correlates of long‐term variation in the composition and abundance of fruits and birds, with particular reference to seed dispersers. During October–December (“autumn” period) 1978–1990, abundance of ripe fruits and birds was assessed in a 4‐ha plot in dense, well‐preserved sclerophyllous scrub, by means of counts in permanent plots and mist‐netting, respectively. Diet composition and fruit preference patterns of Erithacus rubecula and Sylvia atricapilla, the two most abundant seed dispersers, were also investigated over the same period, using fecal sample analyses. Possible consequences to the birds of annual variation in fruit supply and diet composition were investigated using data on fat deposition levels and recapture rates of mist‐netted individuals.

Herrera, Carlos M.

doi: 10.1890/0012-9615%281998%29068%5B0511:LTDOMF%5D2.0.CO%3B2pmid: N/A

The relationship between fleshy-fruited plants and their vertebrate seed dispersal agents often has been depicted as subject to important interannual variation, but no study has thus far documented such variation on a long-term basis. This paper presents the results of a 12-yr investigation on fleshy-fruited plants and avian frugivores in a Mediterranean montane locality of southeastern Spain. The main objective was to document patterns and correlates of long-term variation in the composition and abundance of fruits and birds, with particular reference to seed dispersers. During October––December (““autumn”” period) 1978––1990, abundance of ripe fruits and birds was assessed in a 4-ha plot in dense, well-preserved sclerophyllous scrub, by means of counts in permanent plots and mist-netting, respectively. Diet composition and fruit preference patterns of Erithacus rubecula and Sylvia atricapilla, the two most abundant seed dispersers, were also investigated over the same period, using fecal sample analyses. Possible consequences to the birds of annual variation in fruit supply and diet composition were investigated using data on fat deposition levels and recapture rates of mist-netted individuals. Total fruit abundance (i.e., mean ripe fruit density of all species combined) fluctuated among years between 5.4 ±± 11.1 fruits/m 2 (mean ±± 1 sd ; 1986) and 77.1 ±± 78.0 fruits/m 2 (1989) and was positively related to the amount of rainfall in the preceding spring. Not all fruiting species bore ripe fruits every year, and among those species that did, fruit density fluctuated asynchronously and to variable degrees. Seven out of 13 species exhibited significant supra-annual periodicity in fruit abundance, with fluctuation periods ranging from 2 to 6 yr. Variation in the abundance of each of the six most abundant fruit species was unrelated to annual variation in rainfall. The autumn bird assemblage at the study site was made up of year-round resident species (54.1%% of captures, all years combined) that were largely fruit predators (feeding on pulp or seeds without performing dispersal) and of autumn––winter resident species (45.5%% of captures) that were largely seed dispersers. Bird abundance, all species combined, ranged between 27.1 and 61.5 captures/100 net-hours for 1987 and 1986, respectively. Depending on year, seed dispersers made up 25.6––75.4%% of the total captures, and fruit predators made up 20.9––69.7%%. The relative importance of nonfrugivores was always negligible (1.1––9.9%%). No correlation existed across years between total fruit abundance and the capture rates of all bird species combined, seed dispersers, or fruit predators. Annual variation in the abundance of seed dispersers was positively related to November mean maximum temperature. At the individual species level, S. atricapilla capture rates were correlated with the abundance of the fruits of Phillyrea latifolia, a species exhibiting extreme annual fluctuations. Annual variation in the importance of fruits in the diet of S. atricapilla and E. rubecula was not significantly related to changes in fruit abundance. Composition of the fruit diet of these species fluctuated markedly among years, and there was little agreement between composition of the diet and of the fruit supply. Certain fruit species were significantly preferred, and others avoided, by both S. atricapilla and E. rubecula. Ranking of interspecific fruit preferences remained consistent among years and was related to differences in carbohydrate and lipid content of fruit pulp. Fat deposition levels of S. atricapilla and E. rubecula did not vary among years and were not significantly related to fruit abundance, percentage of fruit volume in the diet, or contribution of lipid-rich fruits to the diet. Return rates of individuals of these species to the study locality in successive wintering periods were very low, and not significantly related to diet composition or fruit abundance. Prevalence of abiotic over biotic determinants of annual variations, extensive decoupling of the long-term temporal dynamics of fruits and dispersers, and the remarkable ““indifference”” of frugivores to variations in the fruit supply all point to the non-equilibrial nature of this assemblage of fleshy-fruited plants and their avian dispersers.

Wiegand, Thorsten; Naves, Javier; Stephan, Thomas; Fernandez, Alberto

doi: 10.1890/0012-9615%281998%29068%5B0539:ATROEF%5D2.0.CO%3B2pmid: N/A

The status of the brown bear ( Ursus arctos ) in Spain has suffered a dramatic decline during the last centuries, both in area and numbers. Current relict populations are suspected to be under immediate risk of extinction. The aim of our model is to attain an understanding of the main processes and mechanisms determining population dynamics in the Cordillera Cantabrica. We compile the knowledge available about brown bears in the Cordillera Cantabrica, northern Spain, and perform a population viability analysis (PVA) to diagnose the current state of the population and to support current management. The specially constructed simulation model, based on long-term field investigations on the western brown bear population in the Cordillera Cantabrica, includes detailed life history data and information on environmental variations in food abundance. The method of individual-based modeling is employed to simulate the fate of individual bears. Reproduction, family breakup, and mortalities are modeled in annual time steps under the influence of environmental variations in food abundance, mortality rates, and reproductive parameters. In parallel, we develop an analytical model that describes the mean behavior of the population and that enables us to perform a detailed sensitivity analysis. We determine current population parameters by iterating the model with plausible values and compare simulation results with the 1982––1995 time pattern of observed number of females with cubs of the year. Our results indicate that the population suffered a mean annual decrease of ∼∼4––5%% during the study period, 1982––1995. This decrease could be explained by a coincidence of high poaching pressure with a series of climatically unfavorable years during the period 1982––1988. Thereafter, population size probably stabilized. We estimate that the population currently consists of 25 or 26 independent females and a total of 50––60 individuals. However, our viability analysis shows that the population does not satisfy the criterion of a minimum viable population if mortalities remain at the level of the last few years of 1988––1995. The ““salvation”” of at least one independent female every three years is required. The population retains relatively high reproductive parameters, indicating good nutritive conditions of the habitat, but mortality rates are higher than those known in other brown bear populations. The most sensitive parameters, adult and subadult mortality of females, form the principal management target. Our model shows that the series of females with cubs contains valuable information on the state of the population. We recommend monitoring of females with cubs as the most important management action, both for collecting data and for safeguarding the most sensitive part of the population.

Wiegand, Thorsten; Naves, Javier; Stephan, Thomas; Fernandez, Alberto

doi: 10.1890/0012-9615(1998)068[0539:ATROEF]2.0.CO;2pmid: N/A

The status of the brown bear (Ursus arctos) in Spain has suffered a dramatic decline during the last centuries, both in area and numbers. Current relict populations are suspected to be under immediate risk of extinction. The aim of our model is to attain an understanding of the main processes and mechanisms determining population dynamics in the Cordillera Cantabrica. We compile the knowledge available about brown bears in the Cordillera Cantabrica, northern Spain, and perform a population viability analysis (PVA) to diagnose the current state of the population and to support current management.

Lewellen, Ruth H.; Vessey, Stephen H.

doi: 10.1890/0012-9615(1998)068[0571:TEODDA]2.0.CO;2pmid: N/A

The identification of what factors determine the population dynamics of polyvoltine species has been a difficult problem in ecology because population dynamics can contain intra‐ and interannual variability, and because the time scale at which factors affect the population is often unknown. We created a comprehensive population model to determine how density dependence (linear, nonlinear, and time‐delayed) and weather affected the rate of population growth of white‐footed mice (Peromyscus leucopus) in an isolated woodlot. We studied this nonoutbreak, polyvoltine species using a 257‐mo data set spanning 23 yr, which incorporated both detailed intra‐annual and long‐term dynamics, and we used this model to forecast future population size. We then evaluated whether 3‐yr spans of monthly data or a 22‐yr span of annual data were better able to identify the key determinants that drive population dynamics, and we identified which data type created more accurate forecasts. The 257‐mo comprehensive model determined that the intra‐annual cycle was caused by seasonally varying intrinsic growth rates and density dependence on a 1–2 mo scale and indicated that peak population size in one year did not affect the population in the subsequent year. Interannual variability in peak and trough density was caused by the effect of weather on monthly rate of growth with a 0–2 mo time delay, with the exception of two droughts. These droughts negatively affected the population for 9 mo; the effects were probably mediated through reduced seed crop. This model explained 81% of the variability in density. Because weather determined interannual variability in density, forecasts that did not use known weather data during the forecast period were poor. When weather data were used, forecasts were accurate within 1–3 animals (10%) of observed densities up to 8 mo in the future but were inaccurate beyond 8 mo. We found that short‐term monthly data detected more factors affecting the population and created more accurate forecasts than long‐term annual data, because all factors affecting the population (except droughts) occurred on a monthly scale. The annual model did not detect any weather effects except droughts and detected annual density dependence, which represents time‐delayed density dependence in polyvoltine species. We argue that this annual relationship is spurious and caused by studying this polyvoltine species on an inappropriate time scale. Our work suggests that the time scale of the analysis may affect the conclusions drawn about which types of factors determine population size and with what time lag. It also suggests that, even when population fluctuations can be explained, accurately predicting future densities may be impossible when fluctuations are driven by weather.

Lewellen, Ruth H.; Vessey, Stephen H.

doi: 10.1890/0012-9615%281998%29068%5B0571:TEODDA%5D2.0.CO%3B2pmid: N/A

The identification of what factors determine the population dynamics of polyvoltine species has been a difficult problem in ecology because population dynamics can contain intra- and interannual variability, and because the time scale at which factors affect the population is often unknown. We created a comprehensive population model to determine how density dependence (linear, nonlinear, and time-delayed) and weather affected the rate of population growth of white-footed mice ( Peromyscus leucopus ) in an isolated woodlot. We studied this nonoutbreak, polyvoltine species using a 257-mo data set spanning 23 yr, which incorporated both detailed intra-annual and long-term dynamics, and we used this model to forecast future population size. We then evaluated whether 3-yr spans of monthly data or a 22-yr span of annual data were better able to identify the key determinants that drive population dynamics, and we identified which data type created more accurate forecasts. The 257-mo comprehensive model determined that the intra-annual cycle was caused by seasonally varying intrinsic growth rates and density dependence on a 1––2 mo scale and indicated that peak population size in one year did not affect the population in the subsequent year. Interannual variability in peak and trough density was caused by the effect of weather on monthly rate of growth with a 0––2 mo time delay, with the exception of two droughts. These droughts negatively affected the population for 9 mo; the effects were probably mediated through reduced seed crop. This model explained 81%% of the variability in density. Because weather determined interannual variability in density, forecasts that did not use known weather data during the forecast period were poor. When weather data were used, forecasts were accurate within 1––3 animals (10%%) of observed densities up to 8 mo in the future but were inaccurate beyond 8 mo. We found that short-term monthly data detected more factors affecting the population and created more accurate forecasts than long-term annual data, because all factors affecting the population (except droughts) occurred on a monthly scale. The annual model did not detect any weather effects except droughts and detected annual density dependence, which represents time-delayed density dependence in polyvoltine species. We argue that this annual relationship is spurious and caused by studying this polyvoltine species on an inappropriate time scale. Our work suggests that the time scale of the analysis may affect the conclusions drawn about which types of factors determine population size and with what time lag. It also suggests that, even when population fluctuations can be explained, accurately predicting future densities may be impossible when fluctuations are driven by weather.