John F. McLaughlin

Climate change hastens population extinctions

Climate change is expected to alter the distribution and abundance of many species. Predictions of climate-induced population extinctions are supported by geographic range shifts that correspond to climatic warming, but few extinctions have been linked mechanistically to climate change. Here we show that extinctions of two populations of a checkerspot butterfly were hastened by increasing variability in precipitation, a phenomenon predicted by global climate models. We model checkerspot populations to show that changes in precipitation amplified population fluctuations, leading to rapid extinctions. As populations of checkerspots and other species become further isolated by habitat loss, climate change is likely to cause more extinctions, threatening both species diversity and critical ecosystem services.

The route to extinction: population dynamics of a threatened butterfly

Abstract. We compare results of field study and model analysis of two butterfly populations to evaluate the importance of alternative mechanisms causing changes in abundance. Although understanding and predicting population fluctuations is a central goal of population ecology, it is not often achieved because long-term abundance data are available for few populations in which mechanisms causing fluctuations also are known. Both kinds of information exist for two populations of the checkerspot butterfly, Euphydryas editha bayensis, which are matched in most ways except for habitat area and topography. We applied results from field study to make predictions about the dynamics of the two populations. Then we tested these predictions using nonlinear modeling of abundance data. Models included endogenous factors, exogenous effects of weather, or both. Results showed that the populations differed in variability and responses to endogenous and exogenous factors. The population in the more homogeneous habitat varied more widely, went extinct first, and fluctuated more severely with climate. Dynamics of the population occupying the topographically diverse habitat were more complex, containing damped oscillations and weaker influences of weather. We draw four main conclusions. First, the routes to extinction for E. e. bayensis populations in protected habitat were random walks driven by climatic variability. Climatic influences dominated both populations, but the timing and functional forms of climatic effects differed between populations. Second, topographic diversity reduced weather-induced population variability and increased persistence time. Third, one must explicitly consider both endogenous and exogenous components to fully understand population dynamics. Fourth, resolving the debate over population regulation requires integrating long-term population sampling, model analysis, and investigation of mechanisms in the field.

Avian Predation on Anolis Lizards in the Northeastern Caribbean: Inter-Island Contrast

This paper presents results of a field study of avian predation on Anolis lizards on islands in the northeastern Caribbean, addressing the question of what causes annual fluctuation in lizard abundance. The two main potential avian predators, the Pearly- eyed Thrasher (Margarops fuscatus) and the American Kestrel (Falco sparverius), were studied on three adjacent islands in the northern Lesser Antilles: Anguilla, St. Martin, and St. Eustatius. Anoles on Anguilla were found to be quantitatively unaffected by avian predation; thrashers there did not eat vertebrates, and the kestrel density was too low to have a significant impact on the lizard population. In contrast, thrashers in moist habitat on St. Martin ate vertebrates including anoles, and thrasher predation potentially accounted for the anole decline to the dry-season abundance low. Additionally, an unanticipated thrasher niche shift was discovered. Thrasher body size and diet appear to be related to the abundance of a congeneric species, M. fuscus. On Anguilla, where M. fuscus was absent, M. fuscatus was smaller and had a non-vertebrate diet. On St. Eustatius, M. fuscus was abundant while M. fuscatus was larger and preyed on anoles. M. fuscus was rare on St. Martin, where M. fuscatus had an intermediate body size and a diet comparable to that on St. Eustatius.

Predation across spatial scales in heterogeneous environments

Abstract A hierarchy of scales is introduced to the spatially heterogeneous Lotka-Volterra predator-prey diffusion model, and its effects on the models spatial and temporal behavior are studied. When predators move on a large scale relative to prey, local coupling of the predator-prey interaction is replaced by global coupling. Prey with low dispersal ability become narrowly confined to the most productive habitats, strongly amplifying the underlying spatial pattern of the environment. As prey diffusion rate increases, the prey distribution spreads out and predator abundance declines. The model retains neutrally stable Lotka-Volterra temporal dynamics: different scales of predator and prey dispersal do not stabilize the interaction. The model predicts that, for prey populations that are limited by widely ranging predators, species with low dispersal ability should be restricted to discrete high density patches, and those with greater mobility should be more uniformly distributed at lower density.

Pattern and stability in predator-prey communities: How diffusion in spatially variable environments affects the Lotka-Volterra model

Abstract Introducing a spatially heterogeneous environment to the Lotka-Volterra predator-prey diffusion model reveals that spatial pattern is amplified and the dynamics are stabilized by nonlinear environmental heterogeneity and differential species mobility. When predators diffuse and prey are sedentary, heterogeneity in the prey growth rate is amplified in the prey equilibrium distribution. When both species diffuse rapidly, patterns in their distributions are homogenized, but their total abundances are maximized. In contrast to the nonspatial constant Lotka-Volterra model, which features sustained oscillations, perturbations to the heterogeneous environment model decay to equilibrium exponentially fast. When prey are stationary, the rate of decay is proportional to the predator diffusion coefficient. The rate of approach to equilibrium slows as prey mobility increases. These results are interpreted as the creation of a prey refuge by density dependence implicit in the motion of nonuniformly distributed diffusing populations.

Do hypotheses from short-term studies hold in the long-term? An empirical test

Abstract. 1. A sequence of population estimates for two now‐extinct populations of Euphydryas editha bayensis is presented. After removing biased sampling days, estimates of demographic parameters from the long‐term data were used to test five hypotheses built from studies of shorter duration. Such tests of short‐term conclusions are rare.

Scrub Jay Predation on Starlings and Swallows: Attack and Interspecific Defense

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Forecasting Avian Responses to Elwha River Restoration

Riparian restoration increasingly employs dam removal, but with limited knowledge about post-removal ecological impacts. Comprehensive ecological study has occurred on few dam removals and rarely on associated terrestrial components. Research supporting the Elwha River restoration program, Washington, USA, helps fill this gap as the largest dam removal project to date. We developed forecasts of avian responses to post-removal forest restoration in Elwha reservoirs. We modeled abundance patterns for 8 species using avian inventory data and habitat variables at local, patch, and landscape scales. We evaluated models using Akaike’s Information Criterion and determined the relative importance of each variable and scale using multi-model inference. Local habitat variables best explained abundance patterns for most species. We forecasted avian responses to forest restoration in the drained Lake Mills reservoir with and without conifers. Planting conifers hastened forecasted avian responses. Forecasts suggest birds associated with deciduous forests and edge habitats will colonize restored habitats rapidly, but birds associated with conifers may not respond markedly for more than a century. Birds can provide a measure of early restoration progress, but comprehensive restoration of Elwha avifauna will take longer than the dams have been in place. This conclusion echoes a familiar theme in Pacific Northwest forest restoration: anthropogenic impacts occur rapidly, but full restoration is comparatively slow.

Engaging Birds in Vegetation Restoration after Elwha Dam Removal

Elwha River restoration is humanity’s largest dam removal project to date. Reservoir revegetation will be most effective if it is rapid and spatially extensive to stabilize residual sediment, impede invasive plants, and restore other riparian functions. I evaluated how birds could enhance Elwha restoration by dispersing native seeds to reservoir sediment deposits. I evaluated three factors affecting avian seed dispersal: (1) number of native woody plants with bird-dispersed seeds; (2) identity and abundance of seed-dispersing birds; and (3) distribution of avian seed deposition. (1) I sorted Elwha early seral woody plants into species dispersed by birds vs. other mechanisms. (2) I recorded avian flights from seed sources, and I estimated disperser abundances from Elwha avian inventory data. (3) I assessed patterns of avian seed dispersal by evaluating eight causal hypotheses, using fresh scat as a seed surrogate. I recorded scat density and habitat characteristics along random transects in Lake Mills reservoir delta and Geyser Valley floodplain. I evaluated models for each hypothesis using Akaike’s Information Criterion. Birds disperse seeds of most (59%) native woody plants likely to establish in the reservoirs. American robins (Turdus migratorius) accounted for most flights between seed sources and sediment deposits, with mean abundance >1 bird/ha. Most avian scats occurred on logs, and scat density increased with log volume. These results suggest birds can disperse native seeds throughout Elwha restoration sites. Revegetation programs associated with dam removal can leverage birds as restoration agents by retaining or placing large woody debris to recruit fruit-bearing plants.

Terrestrial Fauna are Agents and Endpoints in Ecosystem Restoration Following Dam Removal

Dam removal is an effective and increasingly applied river restoration strategy. This has led to heightened calls for research and monitoring aimed at understanding physical and ecological outcomes following dam removal. While such research programs have increased, roles of terrestrial fauna in the restoration process remain poorly understood, although wildlife and invertebrate fauna are key components of restored ecosystems. Wildlife play reciprocal roles in restoration: they benefit from restored habitats and their activities affect restoration trajectories. Dam removal exposes substrates on former reservoirs and reconnects river corridors, providing new habitat and food resources for terrestrial fauna. Conversely, many wildlife may influence the river restoration process, with both short-term and long-term consequences for community composition, nutrient transfer, and ecosystem function. We assert that considering terrestrial fauna more directly in river restoration research and planning can enhance restoration outcomes. We illustrate these concepts by describing short-term patterns and potential future processes expected from the recent removal of two large dams on the Elwha River in Washington State, the largest dam removal effort ever undertaken. We conclude that an ecosystem-level understanding of restoration following dam removal is critical to fully assessing the impacts and benefits of restoration. This includes measuring the roles and responses of terrestrial fauna to these ecologically and culturally significant restoration projects.