The effects of urbanization on anuran communities

Effects of time since urbanization on anuran community composition in remnant urban ponds

 

Low-density residential development, the fastest growing land use in the USA, is increasingly occurring adjacent to protected areas and in areas of high biodiversity. Thus, determining the environmental impacts, including the cumulative impacts, of proposed residential developments is a pressing challenge. One of the most useful ways of determining the likely response of an environmental resource to change (in order to quantify a project’s cumulative impacts) is to evaluate the effects of past activities similar to those proposed. We thus investigated how anuran communities in remnant urban ponds vary in relation to the time elapsed since urbanization, measured as the mean age of residential developments surrounding ponds.

 

Age of residential development was a predictor in the best models describing the responses of four individual anuran species and total anuran relative abundance (see the results below and Gagné and Fahrig (2010) for more information). In particular, all of the best models of gray treefrog (Hyla versicolor) relative abundance included age of residential development as a predictor. Present-day anuran communities in remnant urban ponds are evidently responding to the effects of residential development that occurred up to 54 years in the past.

 

The relationships between (a) spring peeper (Pseudacris crucifer), (b) American toad (Anaxyrus americanus), (c) gray treefrog (Hyla versicolor), (d) green frog (Lithobates clamitans) and (e) total anuran relative abundance at 19 focal ponds and the mean age of residential development in surrounding landscapes. A linear trend line is shown for each    relationship except in the case of the gray treefrog, for which a second-order polynomial line of best fit is shown.

anuran best fit

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Effect of landscape context on anuran communities in breeding ponds in the National Capital Region, Canada

 

Land cover change during the past 50 years is now recognized as the primary driver of biodiversity loss in terrestrial ecosystems. Worldwide, the dominant land cover change has been the conversion of natural land cover to agricultural use, resulting in croplands and pastures now covering 40% of the Earth’s land surface. Intensifying human land use   is generally accompanied by urbanization, or the conversion of rural land use to urban land use. Although urban areas only represent a small portion of the Earth’s land surface (2%), the effects associated with urbanization are widespread.

 

In light of the above trends and considering that one third of amphibian species are now thought to be globally threatened, it is not surprising that researchers have begun studying the effects of urban and agricultural land uses on anuran communities. However, few have, as yet, directly compared these effects in a single study.

 

We evaluated the relative effects of urban and agricultural landscapes, as compared to landscapes dominated by forested habitat, on anuran species richness and the abundance of individual anuran species found at breeding ponds.

 

We found that anuran species richness was significantly lower in breeding ponds in urban landscapes compared to forested and agricultural landscapes, which exhibited no significant difference in species richness (see below and Gagné and Fahrig (2007) and Gagné et al. (2006) for more information). The abundances of individual anuran species were also consistently lower in urban landscapes for all species except one, which exhibited no response to landscape type. Three species had their highest abundances in ponds in forested landscapes, whereas two species had their highest abundances in ponds in agricultural landscapes.

 

We conclude that ponds embedded in urban landscapes support lower biodiversity than ponds in agricultural settings. We suggest that landscapes composed of a mosaic of forest and open habitats surrounding wetlands would hold the highest biodiversity of these species.

 

The mean number of anuran species (±95% confidence interval) recorded in ponds surrounded by primarily urban (U), agricultural (A), and forested (F) landscapes in the National Capital Region, Canada. Lowercase letters indicate significant differences between pairs of landscapes.

mean number anuran

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Mean abundance (±95% confidence interval) of six anuran species in ponds surrounded by primarily urban (U), agricultural (A), and forested (F) landscapes in the National Capital Region, Canada. Lowercase letters indicate significant differences between pairs of landscapes.

mean abundance anuran

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