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We modeled the dynamics of a metapopulation of the land snail Arianta arbustorum in north-eastern Switzerland to investigate the effect of population subdivision on the persistence of a land snail metapopulation and to analyze the interaction between spatial factors, population subdivision, and catastrophes. We developed a spatially structured, stochastic, age-structured metapopulation model with field data from previous studies on the metapopulation in Switzerland and experimental and meteorological data. The model incorporated distance-dependent dispersal through stream banks, correlated environmental fluctuations, and catastrophes resulting from heavy rains. The results point to various complex interactions among factors involved in metapopulation dynamics and suggest that in some cases population subdivision may act to decrease threats from environmental fluctuations and catastrophes.

*Key Centre for Tropical Wildlife Management, Northern Territory University, Darwin, Northern Territory 0909, Australia†School of Botany, University of Melbourne, Parkville, Victoria 3052, Australia‡Applied Biomathematics, 100 North Country Road, Setauket, NY 11733, U.S.A.§Key Centre for Biodiversity and Bioresources, Macquarie University, New South Wales 2109, Australia

Stochastic models to estimate extinction risks and recovery probabilities offer an effective alternative to the mostly qualitative or deterministic methods used in wildlife population modeling and viability analysis. The risk analysis approach is demonstrated by three models developed for single and multiple populations (metapopulations). These models simulate population dynamics with age, stage, and spatial structure. They are used to study the dynamics of rare or endangered species, to design nature reserves, to evaluate wildlife management practices, and to assess human impact on natural populations. The age- and stage-structured models can take into account species-specific demographic information, environmental and demographic stochasticity, and density dependence. The metapopulation model uses these types of data for each population in a metapopulation, and connects the populations by incorporating information on the number, size and geographic configuration of habitat patches, and dispersal phenomena. The models use Monte Carlo methods to simulate future population trajectories and compute risks of population extinction and chances of recovery from a disturbance. The models, implemented as interactive microcomputer programs, move population modeling and viability analysis from an academic domain to an operational one for conservation biologists and wildlife managers.

The California least tern (Sterna antillarum browni) is federally listed as an endangered species. Its nesting habitat has been degraded, and many colony sites are vulnerable to predation and human disturbance. We examined a metapopulation model for the California least tern that can be used to predict persistence of populations along the Pacific coast and the effects of various management actions. We used the model to estimate the effect of reducing predation impact, an important source of reduced fecundity, in various populations. Apart from restricting human access to nesting sites, most management efforts have concentrated on predation. In the model, each cluster of nearby colonies is defined as a population. Within each population, the model includes age-structure, year-to-year changes in survival and fecundity, regional catastrophes (strong El Nino/Southern Oscillation [ENSO] events), and local catastrophes (reproductive failure due to predation). The model predicted a continuing population increase and a low risk of substantial decline over the next 50 years. However, this prediction was sensitive to assumptions about survival and fecundity. Under a pessimistic scenario, the model predicted a high risk of decline, although a low risk of extinction. We simulated the effect of predator management by reducing the probability of reproductive failure due to predation. The improvement in viability ranged from 1% to 4% for single populations and up to 8% when all populations were included. Results indicated that the number and location of populations selected for focused management influenced the effectiveness of management efforts.

1. The California spotted owl Strix occidentalis occidentalis is found in the Sierra Nevada and in a series of isolated populations in the mountains of southern California. 2. We developed a model to assess the risk of decline of the southern California spotted owl metapopulation. We modelled the spatial structure of this metapopulation by incorporating distance-dependent dispersal and correlation among the population growth rates. 3. Demographic characteristics of the largest insular population were estimated from colour-ringing the majority of the territorial population. This owl population declined dramatically during the study period, 1987-93

Approaches to modeling the evolution of community structures are reviewed and criticized; a new approach is proposed. New species introduced to a model community are chosen as “ecologically continuous” mutants of their progenitors. Although competition is strong in every step of such a process, the community is shown to evolve towards non-interaction.

The implications of the use of models from three fields in biology, namely, island biogeography, population genetics, and population dynamics, for the management of wildlife populations and the conservation of species are reviewed. The theory of island biogeography has little to offer to conservation practice because the formulation of the model is questionable, empirical evidence does not support it, and the predictions of the theory do not address directly the central problem of species extinctions. Unlike island biogeography, models in population genetics and population dynamics utilise population-specific information and provide answers to problems in terms of extinction probabilities and the change in population parameters in response to changes in specific environmental conditions.

Population trends play a large role in species risk assessments and conservation planning, and species are often considered threatened if their recent rate of decline meets certain thresholds, regardless how large the population is. But how reliable an indicator of extinction risk is a single estimate of population trend? Given the integral role this decline-based approach has played in setting conservation priorities, it is surprising that it has undergone little empirical scrutiny. We compile an extensive global dataset of time series of abundance data for over 1300 vertebrate populations to provide the first major test of the predictability of population growth rates in nature. We divided each time series into assessment and response periods and examined the correlation between growth rates in the two time periods. In birds, population declines tended to be followed by further declines, but mammals, salmon, and other bony fishes showed the opposite pattern: past declines were associated with subsequent population increases, and vice versa. Furthermore, in these taxa subsequent growth rates were higher when initial declines were more severe. These patterns agreed with data simulated under a null model for a dynamically stable population experiencing density dependence. However, this type of result could also occur if conservation actions positively affected the population following initial declines—a scenario that our data were too limited to rigorously evaluate. This ambiguity emphasizes the importance of understanding the underlying causes of population trajectories in drawing inferences about rates of decline in abundance.

Several theoretical studies have suggested that the nature of interactions between species effect the structure of communities. Most of these studies were based on the analysis of an interaction matrix A, and its stability properties. The elements of this matrix describe the interactions between species: diagonal elements α ii describe the effect of the population of species i on itself (i.e., self-limitation as a result of density-dependent factors), and off-diagonal elements α ij describe the effect of species j on the growth rate of species i. The signs of the two corresponding elements of the matrix, α ij and α ji together define the type of interaction between species i and species j: (0,0) for no interaction, (−,−) for competition, (−,+) for predation/parasitism, (0,−) for amensalism, (0,+) for commensalism, and (+,+) for mutualism. The number of species (i.e., the dimension of the matrix), m, and the proportion of non-zero elements of the matrix (connectance), C, were used as measures of the complexity of the system (May, 1972).