Measured Effects of Anthropogenic Development on Vertebrate Wildlife Diversity

Abstract

A major driver of the declining biodiversity is landcover change leading to loss of habitat. Many studies have estimated large-scale declines in biodiversity, but loss of biodiversity at a local scale due to the immediate effects of development has been poorly studied. California, in particular, is a biodiversity hotspot and has rapidly developed; thus, it is important to understand the effects of development on wildlife in the State. Here, we conducted reconnaissance surveys—a type of survey often used by consulting biologists in support of environmental review of proposed projects—to measure changes in the relative abundance and richness of vertebrate species in response to urban development. We completed 2 reconnaissance surveys at each of 52 control sites that remained undeveloped at the times of both surveys, and at each of 26 impact sites that had been developed by the time of the second survey. We completed the surveys as part of a before–after, control–impact (BACI) experimental design. Our main interest was the interaction effect between the before–after phases and the control–impact treatment levels, or the impact of development. After controlling for survey duration, we also tested for the effects of the number of years intervening the surveys in the before and after phases, project area size, latitude, degree of connectedness to adjacent open space, and whether the site was a redevelopment site, infill, or not infill. After development, the average number of vertebrate wildlife species we detected declined by 48% within the project area, and by 66% within the bounds of the project sites. Further, the average number of vertebrate animals we counted declined by 90% within the project area, and 89% within the bounds of the project sites. Development impacts measured by the mean number of species detected per survey were greatest for amphibians (−100%), followed by mammals (−86%), grassland birds (−75%), raptors (−53%), special-status species (−49%), all birds as a group (−48%), non-native birds (−44%), and synanthropic birds (−28%). Our results indicated that urban development substantially reduced vertebrate species richness and numerical abundance, even after richness and abundance had likely already been depleted by the cumulative effects of loss, fragmentation, and degradation of habitat in the urbanizing environment. Monitoring is needed in and around urbanizing areas to measure the cumulative effects of urbanization, and so are conservation measures to mitigate the effects of urbanization.