The influence of species life history and distribution characteristics on species responses to habitat fragmentation in an urban landscape.

Fragmentation within urbanized environments often leads to a loss of native species diversity; however, variation exists in responses among-species and among-populations within species. We aimed to identify patterns in species biogeography in an urbanized landscape to understand anthropogenic effects on vertebrate communities and identify species that are more sensitive or resilient to landscape change. We investigated patterns in species richness and species responses to fragmentation in southern Californian small vertebrate communities using multispecies occupancy models and determined factors associated with overall commonness and sensitivity to patch size for 45 small vertebrate species both among and within remaining non-developed patches. In general, smaller patches had fewer species, with amphibian species richness being particularly sensitive to patch size effects. Mammals were generally more common, occurring both in a greater proportion of patches and a higher proportion of the sites within occupied patches. Alternatively, amphibians were generally restricted to larger patches but were more ubiquitous within smaller patches when occupied. Species range size was positively correlated with how common a species was across and within patches, even when controlling for only patches that fell within a species' range. We found sensitivity to patch size was greater for more fecund species and depended on where the patch occurred within a species' range. While all taxa were more likely to occur in patches in the warmer portions of their ranges, amphibians and mammals were more sensitive to fragmentation in these warmer areas as compared to the rest of their ranges. Similarly, amphibians occurred at a smaller proportion of sites within patches in drier portions of their ranges. Mammals occurred at a higher proportion of sites that were also in drier portions of their range while reptiles did not differ in their sensitivity to patch size by range position. We demonstrate that taxonomy, life history, range size and range position can predict commonness and sensitivity of species across this highly fragmented yet biodiverse landscape. The impacts of fragmentation on species communities within an urban landscape depend on scale, with differences emerging among and within species and populations.

Quantifying climate sensitivity and climate-driven change in North American amphibian communities.

Changing climate will impact species' ranges only when environmental variability directly impacts the demography of local populations. However, measurement of demographic responses to climate change has largely been limited to single species and locations. Here we show that amphibian communities are responsive to climatic variability, using >500,000 time-series observations for 81 species across 86 North American study areas. The effect of climate on local colonization and persistence probabilities varies among eco-regions and depends on local climate, species life-histories, and taxonomic classification. We found that local species richness is most sensitive to changes in water availability during breeding and changes in winter conditions. Based on the relationships we measure, recent changes in climate cannot explain why local species richness of North American amphibians has rapidly declined. However, changing climate does explain why some populations are declining faster than others. Our results provide important insights into how amphibians respond to climate and a general framework for measuring climate impacts on species richness.

Range position and climate sensitivity: The structure of among-population demographic responses to climatic variation.

Species' distributions will respond to climate change based on the relationship between local demographic processes and climate and how this relationship varies based on range position. A rarely tested demographic prediction is that populations at the extremes of a species' climate envelope (e.g., populations in areas with the highest mean annual temperature) will be most sensitive to local shifts in climate (i.e., warming). We tested this prediction using a dynamic species distribution model linking demographic rates to variation in temperature and precipitation for wood frogs (Lithobates sylvaticus) in North America. Using long-term monitoring data from 746 populations in 27 study areas, we determined how climatic variation affected population growth rates and how these relationships varied with respect to long-term climate. Some models supported the predicted pattern, with negative effects of extreme summer temperatures in hotter areas and positive effects on recruitment for summer water availability in drier areas. We also found evidence of interacting temperature and precipitation influencing population size, such as extreme heat having less of a negative effect in wetter areas. Other results were contrary to predictions, such as positive effects of summer water availability in wetter parts of the range and positive responses to winter warming especially in milder areas. In general, we found wood frogs were more sensitive to changes in temperature or temperature interacting with precipitation than to changes in precipitation alone. Our results suggest that sensitivity to changes in climate cannot be predicted simply by knowing locations within the species' climate envelope. Many climate processes did not affect population growth rates in the predicted direction based on range position. Processes such as species-interactions, local adaptation, and interactions with the physical landscape likely affect the responses we observed. Our work highlights the need to measure demographic responses to changing climate.

Quantitative evidence for the effects of multiple drivers on continental-scale amphibian declines.

Since amphibian declines were first proposed as a global phenomenon over a quarter century ago, the conservation community has made little progress in halting or reversing these trends. The early search for a "smoking gun" was replaced with the expectation that declines are caused by multiple drivers. While field observations and experiments have identified factors leading to increased local extinction risk, evidence for effects of these drivers is lacking at large spatial scales. Here, we use observations of 389 time-series of 83 species and complexes from 61 study areas across North America to test the effects of 4 of the major hypothesized drivers of declines. While we find that local amphibian populations are being lost from metapopulations at an average rate of 3.79% per year, these declines are not related to any particular threat at the continental scale; likewise the effect of each stressor is variable at regional scales. This result - that exposure to threats varies spatially, and populations vary in their response - provides little generality in the development of conservation strategies. Greater emphasis on local solutions to this globally shared phenomenon is needed.