No abstract

No abstract

Aim Ecological niche models are increasingly being used to aid in predicting the effects of future climate change on species distributions. Complex models that show high predictive performance on current distribution data may do a poor job of predicting new data due to overfitting. In addition, model performance is often evaluated using techniques that are sensitive to spatial sampling bias. Here, we explore the effects of model complexity and spatial sampling bias on niche models for 90 vertebrate taxa of conservation concern. Location California, USA. Methods We used Akaike information criterion (AICc) to select variables and tune Maxent's built‐in regularization parameter (β) to constrain model complexity. In addition, we incorporated several estimates of spatial sampling bias based on interpolations of target group data. Ensemble forecasts were developed for future conditions from two emission scenarios and three climate change models for the year 2050. Results Reducing the number of predictors and tuning β resulted in a reduction in the number of parameters in models built with sample sizes greater than approximately 10 occurrence points. Reducing the number of predictors had a substantially higher impact on the relative prioritization of different grid cells than did increasing regularization. There was little difference in prioritization of habitat when comparing models built using different spatial sampling bias estimates. Over half of the taxa were predicted to experience >80% reductions in environmental suitability in currently occupied cells, and this pattern was consistent across taxonomic groups. Main Conclusions Our results demonstrate that reducing the number of correlated predictor variables tends to decrease the breadth of models, while tuning regularization using AICc tends to increase it. These two strategies may provide a reasonable bracketing strategy for assessing climate change impacts.

Wind-borne pesticides have long been suggested as a cause of amphibian declines in areas without obvious habitat destruction. In California, the transport and deposition of pesticides from the agriculturally intensive Central Valley to the adjacent Sierra Nevada is well documented, and pesticides have been found in the bodies of Sierra frogs. Pesticides are therefore a plausible cause of declines, but to date no direct links have been found between pesticides and actual amphibian population declines. Using a geographic information system, we constructed maps of the spatial pattern of declines for eight declining California amphibian taxa, and compared the observed patterns of decline to those predicted by hypotheses of wind-borne pesticides, habitat destruction, ultraviolet radiation, and climate change. In four species, we found a strong positive association between declines and the amount of upwind agricultural land use, suggesting that wind-borne pesticides may be an important factor in declines. For two other species, declines were strongly associated with local urban and agricultural land use, consistent with the habitat-destruction hypothesis. The patterns of decline were not consistent with either the ultraviolet radiation or climate-change hypotheses for any of the species we examined.Resumen: Por mucho tiempo se ha sugerido que los pesticidas transportados por el viento son una causa de la declinación de anfibios en áreas sin destrucción de hábitat evidente. En California, el transporte y depósito de pesticidas provenientes del Valle Central, donde se practica la agricultura intensiva, hacia la Sierra Nevada adyacente está bien documentado y se han encontrado pesticidas en el cuerpo de ranas de la Sierra. Por lo tanto, los pesticidas son una causa verosímil de las declinaciones, pero a la fecha no se han encontrado relaciones directas entre los pesticidas y la declinación de anfibios. Construimos mapas de sistemas de información geográfica del patrón espacial de las declinaciones de ocho taxones de anfibios de California, y comparamos los patrones de declinación observados con los esperados por las hipótesis de pesticidas transportados por el viento, la destrucción del hábitat, la radiación ultravioleta y el cambio climático. En cuatro especies, encontramos una fuerte asociación positiva entre las declinaciones y la cantidad de tierras de uso agrícola en dirección contraria a los vientos, lo que sugiere que los pesticidas transportados por el viento pueden ser un factor importante en las declinaciones. Para otras dos especies, las declinaciones se asociaron contundentemente con el uso del suelo urbano y agrícola, lo cual es consistente con la hipótesis de la destrucción del hábitat. Los patrones de declinación no fueron consistentes con la hipótesis de la radiación ultravioleta ni la de cambio climático para ninguna de las especies examinadas.

We present a phylogenetic analysis of the Ambystoma tigrinum complex, based on approximately 840 base pairs of mitochondrial-DNA sequence from the rapidly evolving D-loop and an adjacent intron. Our samples include populations of the continentally distributed species, A. tigrinum, plus all described species of Mexican ambystomatids. Sequence divergence is low, ranging from 0-8.5%, and most phylogenetic groupings are weakly supported statistically. We identified eight reasonably well-defined clades from the United States and Mexico, with the geographically isolated A. californiense from California as the probable sister group to the remaining taxa. Our sequence data are not capable of resolving the relationships among these clades, although the pattern of transitional-site evolution suggests that these eight lineages diverged during a period of rapid cladogenesis. We roughly calibrate a molecular clock and identify a few lineages that significantly deviate from the slow, baseline rate of 0.5-0.75% per million years. Our data also suggest that species boundaries for several U.S. and Mexican species need to be altered and that the concept of a continentally distributed, polytypic tiger salamander is not valid.