Metapopulation Dynamics and Amphibian Conservation

Abstract: In many respects, amphibian spatial dynamics resemble classical metapopulation models, in which subpopulations in breeding ponds blink in and out of existence and extinction and colonization rates are functions of pond spatial arrangement. This "ponds-as-patches" view of amphibian spatial dynamics is useful in several respects. First, it highlights the importance of regional and landscape processes in determining local patterns of abundance. Second, it offers a straightforward, pond-based approach to monitoring and managing amphibian populations. For many species, however, the ponds-as-patches view may be an oversimplification and metapopulation structure may be more apparent than real. Changes in distribution may be caused by processes other than extinction and recolonization, and most extinctions probably result from deterministic factors, not stochastic processes. In addition, the effects of pond isolation appear to be important primarily in disturbed environments, and in many cases these isolation effects may be better explained by the distribution of terrestrial habitats than by the distribution of breeding ponds. These complications have important implications for both researchers and managers. For researchers, future efforts need to determine the mechanisms underlying patterns of abundance and distributional change and patterns in amphibian populations. For managers, effective conservation strategies must successfully balance metapopulation considerations with careful attention to local habitat quality. Furthermore, translocations and active management may be indispensable tools for conserving amphibians in landscapes containing multiple breeding ponds.     

Local Population Dynamics in Metapopulation Models: Implications for Conservation

Abstract: Habitat fragmentation and the division of populations into spatially separated units have led to the increasing use of metapopulation models to characterize these populations. One prominent model that has served as a heuristic tool was introduced by Levins and is based on a collection of simplifying assumptions that exclude information on the dynamics and spatial distribution of local populations. Levins's and similar models predict the proportion of occupied habitat patches at equilibrium and the conditions needed to avoid total extinction. There are many obvious concerns about using such models, including how realistic alterations might change the predictions and whether occupancy has any relationship to population-level processes. Although many of the assumptions of these simple models are known to be unrealistic, we do not know how the assumptions affect model predictions. We simulated a metapopulation, and our results show that assumptions such as homogeneity of habitat patches, random migration among patches, equivalent extinction probabilities in all patches, and a large number of patches can lead to large overestimations of habitat occupancy. But when we explicitly modeled the underlying population dynamics within each patch, we found (1) that there was a strong correlation between proportion of occupied patches and total metapopulation size and (2) that the distribution of individuals among patches was relatively insensitive to model assumptions. Thus, our results show that although realistic modifications will change model predictions for occupancy, occupancy and population trends will be correlated. These correlations between occupancy and population size suggest that occupancy models may have some utility in conservation applications.     

Applying Metapopulation Theory to Conservation of Migratory Birds

Abstract: Metapopulation theory has proven useful for understanding the population structure and dynamics of many species of conservation concern. The metapopulation concept has been applied almost exclusively to nonmigratory species, however, for which subpopulation demographic independence—a requirement for a classically defined metapopulation—is explicitly related to geographic distribution and dispersal probabilities. Defining the degree of demographic independence among subpopulations of migratory animals, and thus the applicability of metapopulation theory as a conceptual framework for understanding population dynamics, is much more difficult. Unlike nonmigratory species, subpopulations of migratory animals cannot be defined as synonymous with geographic areas. Groups of migratory birds that are geographically separate at one part of the annual cycle may occur together at others, but co-occurrence in time and space does not preclude the demographic independence of subpopulations. I suggest that metapopulation theory can be applied to migratory species but that understanding the degree of subpopulation independence may require information about both spatial distribution throughout the annual cycle and behavioral mechanisms that may lead to subpopulation demographic independence. The key for applying metapopulation theory to migratory animals lies in identifying demographically independent subpopulations, even as they move during the annual cycle and potentially co-occur with other subpopulations. Using examples of migratory bird species, I demonstrate that spatial and temporal modes of subpopulation independence can interact with behavioral mechanisms to create demographically independent subpopulations, including cases in which subpopulations are not spatially distinct in some parts of the annual cycle.     

Inferring Metapopulation Dynamics from Patch-Level Incidence of Florida Scrub Plants

Spatial structure and dynamics of multiple populations may explain species distribution patterns in patchy communities with heterogeneous disturbance regimes, especially when species have poor dispersal. The endemic-rich Florida (U.S.A.) rosemary scrub occupies about 4% of the west portion of Archbold Biological Station and occurs scattered within a matrix of less xeric vegetation. Longer fire-return times and higher frequency of open patches in rosemary scrub provide favorable habitat for many plant species. Occupancy of 123 species of vascular plants and ground lichens in 89 patches was determined by repeated site surveys. About two-thirds of the species occurring at more than 14 patches had a significant logistic regression of presence on time-since-fire, patch size, patch isolation, or their interactions. Species with presence related to the interaction between patch isolation and patch size were primarily herbs and small shrubs specializing in rosemary scrub. These results suggest the importance of spatial characteristics of the landscape for population turnover of these species. An incidence-based metapopulation model was used to predict extinction and colonization probabilities of those species with presence in rosemary scrub patches related to the studied spatial variables. This is the first attempt to apply incidence-based metapopulation models to plants. The results showed stronger effects of patch size and patch isolation on extinction probabilities of herbs than on those of woody species. Because of their effect on spatial heterogeneity and habitat availability, fire suppression and habitat destruction may decrease persistence probabilities for these rosemary scrub specialists, many of which are endangered species.     

Geometrical Factors and Metapopulation Dynamics of the Bush Cricket, Metrioptera bicolor Philippi (Orthoptera: Tettigoniidae)

This paper presents a metapopulation study of the bush cricket, Metrioptera bicolor , living in a recently fragmented landscape. The species inhabits grass and heathland patches of varying area and isolation. Analyses are made of how these geometrical factors affect local population size and density, distribution pattern, and the probability of local extinction and colonization. The proportion of available patches occupied varied between 72 and 79% during 1985–1990. Unoccupied patches were smaller and more isolated than those that were occupied. Patches where populations became extinct during this period were smaller than those with persisting populations. Since local population size was well correlated with patch area, it was clear that stochastic extinctions only occurred in small populations. Critical patch size for population extinction was approximately half a hectare. Colonized patches were less isolated than those that had not been colonized. Critical inter-patch distance for colonization was about 100 meters. The turnover was restricted to an identifiable share of the available patches. Only 33% of the patches were so small that extinction due to stochastic causes could be considered highly probable. This metapopulation will therefore most likely persist over a considerable period in its present spatial structure. There are apparent threats of further fragmentation, however, and nothing is known about the likelihood of large-scale extinctions resulting from extremely unfavorable weather conditions. Nevertheless, our results show that it is appropriate to include geometrical factors in metapopulation models.     

Conservation of Stream Fishes: Patterns of Diversity, Rarity, and Risk

Abstract: North America has a rich fauna of freshwater fishes which attains greatest diversity in the central and southeastern United States. Many stream fishes have limited ranges and are locally rare and patchily distributed. Local diversity increases downstream and total diversity follows typical species-area relationships. Drainages linked with the large Mississippi system support more species than those of comparable area flowing directly to the sea and rivers isolated by falls have notably few species. The between-drainage component of diversity is large. Threats to this fauna, which are not addressed by management focused on threatened species, include fragmentation of drainage networks by impoundments and homogenization of faunas by interbasin connections and introductions. Conservation efforts require a biogeographic perspective; they should focus on streams of intermediate size (orders 4–6) plus the upstream portion of each drainage and should attempt to maintain total diversity and, inclusively, populations of rare or threatened species.     

Cyclic Drought, Dispersal, and the Conservation of the Snail Kite in Florida: Lessons in Critical Habitat

Estudios realizados sobre el Halcón Caracolero de Florida, un ave en peligro de extinción que habita areas pantanosas y se alimenta de caracoles, ilustra los problemas asociados con la protección de hábitats de especies migratorias y nómadas que requieren de habitats disyuntos. Anteriomente a una sequía en el año 1985, la muyoría de halcones caracoleros se observaban en las areas extensas de la región pantanosa de los Everglades a donde nidificaban Sin embargo, durante la sequía el 60% de los halcones observados se localizaron en areas pantanosas más pequeñas ubicadas, en su mayoria eta la región de Palm Beach y et condados vecinos A pesar de que estas pequeñas areas sirven como importantes refugios de emergencia para los hálcones durante epocas de sequia, estas no estan designadas como "hábitat critico" y estan siendo rápidamente transformadas por las presiones de desarrollo. Se discuten problemas de incertidumbre, de efectosa cumulativos, y de transigencia en la evaluación de propuestas para el desarrollo y en la protección de pequeños hábitats aislados Hemos identificado areas de refugio, usadas por los hálcones durante la epoca de sequia y sugerimos maneras para conservar esta especie en Florida mitigando las grandes disminuciones de la población causadas por las sequias. Tambien se evalua la utilidad de una red de comunicación para el reporte de observaciones que sirva para la evaluación de datos de dispersión     

Combining Measures of Dispersal to Identify Conservation Strategies in Fragmented Landscapes

Abstract: Understanding the way in which habitat fragmentation disrupts animal dispersal is key to identifying effective and efficient conservation strategies. To differentiate the potential effectiveness of 2 frequently used strategies for increasing the connectivity of populations in fragmented landscapes—corridors and stepping stones—we combined 3 complimentary methods: behavioral studies at habitat edges, mark‐recapture, and genetic analyses. Each of these methods addresses different steps in the dispersal process that a single intensive study could not address. We applied the 3 methods to the case study of Atrytonopsis new species 1, a rare butterfly endemic to a partially urbanized stretch of barrier islands in North Carolina (U.S.A.). Results of behavioral analyses showed the butterfly flew into urban and forested areas, but not over open beach; mark‐recapture showed that the butterfly dispersed successfully through short stretches of urban areas (<500 m); and genetic studies showed that longer stretches of forest (>5 km) were a dispersal barrier, but shorter stretches of urban areas (≤5 km) were not. Although results from all 3 methods indicated natural features in the landscape, not urbanization, were barriers to dispersal, when we combined the results we could determine where barriers might arise: forests restricted dispersal for the butterfly only when there were long stretches with no habitat. Therefore, urban areas have the potential to become a dispersal barrier if their extent increases, a finding that may have gone unnoticed if we had used a single approach. Protection of stepping stones should be sufficient to maintain connectivity for Atrytonopsis new species 1 at current levels of urbanization. Our research highlights how the use of complementary approaches for studying animal dispersal in fragmented landscapes can help identify conservation strategies.     

Science, Power, and System Dynamics: the Political Economy of Conservation Biology

Abstract: Frustration with the lack of action on conservation issues by governments has sparked debate around the policy role of conservation biologists. We analyzed the political economy of conservation biology, that is, of the dynamics of the political and economic structures within which conservation biology operates, and we suggest more productive means for conservation biologists to achieve conservation goals. Within the modern state, conservation goals are marginalized because the growth needs of industrial capital have the highest priority. Environmental advocacy within this system largely addresses only proximate concerns and has limited success. Science is a product of modern society, but scientists now need to foster novel institutional arrangements in which humans can function within the limits of natural systems. This entails a larger recognition of the inherent contradictions residing within current institutions that themselves depend on unsustainably high resource flows. As one critical counterbalance to these institutions, we discuss community-based management and research as primary institutions through which sustainable use of natural resources might be achieved.     

The Population Dynamics and Conservation of Primate Populations

Abstract: Primates are among the most threatened taxa, with more than half of all species in jeopardy. In this paper we develop population models to use the kind of data on wild primates that primatologists actually collect. Our survey of recentprimate journals suggests that the average field study uses 1.5 years of data from 50 animals The models are based on the simple Leslie-Lefkovitch matrix. They suggest a simple method that allows assessment from a few years'data, of whether a population is collapsing and requires intervention To a good approximation, populations will collapse when adult survival, per inter-birth interval, is less than 70 percent.
Modifications of the basic model incorporate more realistic assumptions about social organization and densitydependent resource limitation. These allow us to identify population densities at which potential Allee effects operate, and permit more precise estimates of the minimum population sizes and compositions required for successful reintroductions to the wild The most important result is that populations of primates that live in small family groups may be more prone to "demographic" extinction than are more promiscuous species that live in more extended groups.     

Patch Occupancy and Potential Metapopulation Dynamics of Three Forest Mammals in Fragmented Afromontane Forest in South Africa

Abstract: We investigated the persistence of three medium-sized (2–9 kg), rare forest mammals in the fragmented mist-belt Podocarpus forests of the midlands of KwaZulu-Natal Province, South Africa. We recorded patch occupancy of blue duiker (   Philantomba monticola ), tree hyrax (   Dendrohyrax arboreus ), and samango monkey ( Cercopithecus mitis labiatus ) in 199 forest patches. Their rarity is ascribed to the fragmentation and destruction of their forest habitat. Incidence functions, derived from presence and absence data, were formulated as generalized linear models, and environmental effects were included in the fitted logistic models. The small and mostly solitary hyrax and duiker persisted in smaller patches than the large and social monkey. Although this result follows expectations based on relative home-range sizes of each species, the incidence probability of the samango monkey was invariant with increasing isolation, whereas a gradual decrease with increasing isolation was observed for the hyrax and duiker. Group dynamics may inhibit dispersal and increase the isolation effect in social species such as samango monkeys. A mainland-island metapopulation model adequately describes patterns of patch occupancy by the hyrax and duiker, but the monkeys' poor dispersal ability and obvious area-dependent extirpation suggest that they exist in transient, nonequilibrium (declining) metapopulations. Through identification of large forest patches for careful protection and management, the survival of all three species—especially the monkey—could be prolonged. Because no functional metapopulation may exist for the monkey, however, this is an emergency measure. For the duiker and hyrax, larger patches should form part of a network of smaller and closer patches in a natural matrix.     

A Comparison of Global Conservation Prioritization Models with Spatial Spending Patterns of Conservation Nongovernmental Organizations

Abstract: In recent decades, various conservation organizations have developed models to prioritize locations for conservation. Through a survey of the spending patterns of 281 conservation nongovernmental organizations (NGOs), we examined the relation between 2 such models and spatial patterns of spending by conservation NGOs in 44 countries in sub‐Saharan Africa. We tested whether, at the country level, the proportion of a country designated as a conservation priority was correlated with where NGOs spent money. For one model (the combination of Conservation International's hotspots and High Biodiversity Wilderness Areas, which are areas of high endemism with high or low levels of vegetation loss respectively), there was no relation between the proportion of a country designated as a priority and levels of NGO spending, including by the NGO associated with the model. In the second model (Global 200), the proportion of a country designated as a priority and the amount of money spent by NGOs were significantly and positively related. Less money was spent in countries in northern and western sub‐Saharan Africa than countries in southern and eastern Africa, relative to the proportion of the country designated as a conservation priority. We suggest that on the basis of our results some NGOs consider increasing their spending on the areas designated as of conservation priority which are currently relatively underfunded, although there are economic, political, cultural, historical, biological, and practical reasons why current spending patterns may not align with priority sites.     

Exploring Population Dynamics Patterns in a Rare Fish, Zingel asper, through Capture-Mark-Recapture Methods

Abstract:  We performed a capture-mark-recapture study on one of the last populations of Zingel asper , an endemic percid species of the Rhône River basin in France. The distribution of Z. asper has decreased dramatically during the last century. We sampled three sites in suitable habitats in the Beaume River. No impact of individual tagging on survival was found. The demography of the population was analyzed using capture-recapture methods that allow the estimation of survival, recruitment, and demographic growth rates. Annual survival rates were low (0.35–0.50). The level of transience was high (5% to 25%), suggesting that a significant number of individuals were highly mobile or shifted to suboptimal habitats. Seniority rates suggested random highly variable recruitment between years. The three sites had similar variation patterns in all demographic parameters, indicating broad spatial covariation in population dynamics. We found some local differences in demographic parameters, which could be linked to local habitat quality. Individual tagging allowed for the estimation of demographic parameters that improved our understanding of Z. asper population dynamics and revealed mechanisms that may affect population persistence, such as stochastic recruitment, low survival, and frequent dispersal. The fragmentation of habitat through river damming inhibits dispersal and represents a threat to the persistence of Z. asper in the Rhône basin. Our results offer evidence of the importance of dispersal in nonmigratory fishes and confirm the usefulness of individual tagging methods in rare fish demography.     

Long-Term Ecosystem Dynamics in the Serengeti: Lessons for Conservation

Abstract:  Data from long-term ecological studies further understanding of ecosystem dynamics and can guide evidence-based management. In a quasi-natural experiment we examined long-term monitoring data on different components of the Serengeti-Mara Ecosystem to trace the effects of disturbances and thus to elucidate cause-and-effect connections between them. The long-term data illustrated the role of food limitation in population regulation in mammals, particularly in migratory wildebeest and nonmigratory buffalo. Predation limited populations of smaller resident ungulates and small carnivores. Abiotic events, such as droughts and floods, created disturbances that affected survivorship of ungulates and birds. Such disturbances showed feedbacks between biotic and abiotic realms. Interactions between elephants and their food allowed savanna and grassland communities to co-occur. With increased woodland vegetation, predators' capture of prey increased. Anthropogenic disturbances had direct (hunting) and indirect (transfer of disease to wildlife) effects. Slow and rapid changes and multiple ecosystem states became apparent only over several decades and involved events at different spatial scales. Conservation efforts should accommodate both infrequent and unpredictable events and long-term trends. Management should plan on the time scale of those events and should not aim to maintain the status quo. Systems can be self-regulating through food availability and predator-prey interactions; thus, culling may not be required. Ecosystems can occur in multiple states; thus, there may be no a priori need to maintain one natural state. Finally, conservation efforts outside protected areas must distinguish between natural change and direct human-induced change. Protected areas can act as ecological baselines in which human-induced change is kept to a minimum     

Extirpation and Recolonization in a Metapopulation of an Endangered Fish, the Tidewater Goby

Abstract: The tidewater goby ( Eucyclogobius newberryi ), an endangered species in the United States, occurs in a series of isolated coastal wetlands in California. Using historical presence-absence data and our own surveys, we estimated annual rates of extirpation and recolonization for several populations of the goby in southern California. As predicted, large wetlands had lower rates of extirpation than small wetlands. There was a negative but statistically nonsignificant correlation between recolonization rate and distance to the nearest northerly source population. Populations at small sites were sensitive to drought, presumably because droughts can eliminate suitable habitat at small wetlands. Populations in small wetlands have declined over time, even after accounting for variation in stream flow, supporting the species' endangered status. Our study emphasizes the need to understand metapopulation dynamics for conserving species where the unit of conservation is a local population. It is also emphasizes the importance of not treating metapopulations as identical units. Finally, our results provide a means for describing the decline of a species that is complex in time and space and provide insight into how to target protection measures among metapopulations.