The rescue effect and inference from isolation-extinction relationships.

The rescue effect in metapopulations hypothesises that less isolated patches are unlikely to go extinct because recolonisation may occur between breeding seasons ('recolonisation rescue'), or immigrants may sufficiently bolster population size to prevent extinction altogether ('demographic rescue'). These mechanisms have rarely been demonstrated directly, and most evidence of the rescue effect is from relationships between isolation and extinction. We determined the frequency of recolonisation rescue for metapopulations of black rails (Laterallus jamaicensis) and Virginia rails (Rallus limicola) from occupancy surveys conducted during and between breeding seasons, and assessed the reliability of inferences about the occurrence of rescue drawn from isolation-extinction relationships, including autologistic isolation measures that corrected for unsurveyed patches and imperfect detection. Recolonisation rescue occurred at expected rates, but was elevated during periods of disturbance that resulted in non-equilibrium metapopulation dynamics. Inferences from extinction-isolation relationships were unreliable, particularly for autologistic measures and for the more vagile Virginia rail.

Integrating social and ecological data to model metapopulation dynamics in coupled human and natural systems.

Understanding how metapopulations persist in dynamic working landscapes requires assessing the behaviors of key actors that change patches as well as intrinsic factors driving turnover. Coupled human and natural systems (CHANS) research uses a multidisciplinary approach to identify the key actors, processes, and feedbacks that drive metapopulation and landscape dynamics. We describe a framework for modeling metapopulations in CHANS that integrates ecological and social data by coupling stochastic patch occupancy models of metapopulation dynamics with agent-based models of land-use change. We then apply this framework to metapopulations of the threatened black rail (Laterallus jamaicensis) and widespread Virginia rail (Rallus limicola) that inhabit patchy, irrigation-fed wetlands in the rangelands of the California Sierra Nevada foothills. We collected data from five diverse sources (rail occupancy surveys, land-use change mapping, a survey of landowner decision making, climate and reservoir databases, and mosquito trapping and West Nile virus testing) and integrated them into an agent-based stochastic patch occupancy model. We used the model to (1) quantify the drivers of metapopulation dynamics, and the potential interactions and feedbacks among them; (2) test predictions of the behavior of metapopulations in dynamic working landscapes; and (3) evaluate the impact of three policy options on metapopulation persistence (irrigation district water cutbacks during drought, incentives for landowners to create wetlands, and incentives for landowners to protect wetlands). Complex metapopulation dynamics emerged when landscapes functioned as CHANS, highlighting the importance of integrating human activities and other ecological processes into metapopulation models. Rail metapopulations were strongly top-down regulated by precipitation, and the black rail's decade-long decline was caused by the combination of West Nile virus and drought. Theoretical predictions of the two metapopulations' responses to dynamic landscapes and incentive programs were complicated by heterogeneity in patch quality and CHANS couplings, respectively. Irrigation cutbacks during drought posed a serious extinction risk that neither incentive policy effectively ameliorated.

Validating dispersal distances inferred from autoregressive occupancy models with genetic parentage assignments.

Dispersal distances are commonly inferred from occupancy data but have rarely been validated. Estimating dispersal from occupancy data is further complicated by imperfect detection and the presence of unsurveyed patches. We compared dispersal distances inferred from seven years of occupancy data for 212 wetlands in a metapopulation of the secretive and threatened California black rail (Laterallus jamaicensis coturniculus) to distances between parent-offspring dyads identified with 16 microsatellites. We used a novel autoregressive multi-season occupancy model that accounted for both unsurveyed patches and imperfect detection to quantify patch isolation using buffer radius (BRM) and incidence function (IFM) connectivity measures at 15 scales (1-10, 15, 20, 25, and 30 km). Connectivity measures were then fit as colonization covariates in occupancy models to estimate a model-averaged dispersal distance. As predicted, colonization was more strongly related to connectivity at small spatial scales (<10 km). AIC weights were greatest at 7 km for BRM and at 4 km for IFM. Model-averaged dispersal distances (BRM = 7.46 km; IFM = 5.48 km) showed good agreement with the mean M(±SE) dispersal distance from 23 parent-offspring dyads (5.58 ± 1.92 km), indicating reasonably accurate mean dispersal distances can be inferred from occupancy data when isolation strongly affects colonization.

Effects of chronic polybrominated diphenyl ether exposure on gonadal development in the northern leopard frog, Rana pipiens.

Polybrominated diphenyl ethers (PBDEs) are bioaccumulative, persistent organic pollutants used as flame retardants in consumer goods. Concentrations of PBDEs in North American wildlife have been increasing for decades and been shown to have estrogenic effects on sexual development. No studies, however, have examined the effects of PBDEs on the sexual development of North American frogs at ecologically relevant concentrations. This study examined the effects of five dietary concentrations of DE-71 (0, 1.1, 6.1, 71.4, and 634 ng ΣPBDEs/g diet), a technical PBDE mixture, on the gonadal development of the northern leopard frog, Rana pipiens. Tadpoles were exposed chronically from the time they became free-swimming until metamorphosis. Frogs were killed either at metamorphic climax or 10 weeks after completing metamorphosis, processed for histology, and examined for alterations in sexual development. The experimental group exposed to PBDEs at 1.1 ng/g had a significantly larger proportion of females compared with the expected 50:50 sex ratio. At 10 weeks post-metamorphosis, male frogs exposed to 6.1 and 71.4 ng/g had significantly smaller testes, but all other measure of gonadal development tested showed no effects. No intersex or increased incidence of gonadal abnormality were detected. These findings indicate that PBDEs may disrupt sexual differentiation in frogs at low, environmentally relevant concentrations.