Effects of exurbanization on avian and macroinvertebrate diversity in Sewanee, Tennessee
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Subjectexurban gradient; conservation; avifauna; invertebrates; biodiversity; urbanization; Cumberland Plateau
AbstractThe fastest growing form of development in North America is exurbanization, defined as low density housing development beyond suburban areas. The effects of exurbanization on biodiversity are complex and depend on the degree of habitat modification. Many studies have concluded that avian diversity increases with moderate levels of exurbanization. However, native species richness declines and exotics proliferate with high levels of exurbanization. The mechanisms that cause changes in bird diversity are relatively unknown. We investigated the relationship between bird diversity and macroinvertebrate food abundance over an exurban gradient. Macroinvertebrates serve as valuable bio-indicators due to their prominence in terrestrial ecosystems, but there are few studies that examine macroinvertebrate diversity in reference to exurbanization. We used the Cumberland Plateau, a biodiversity hotspot in North America, in Sewanee, Tennessee as a case study to assess how birds and macroinvertebrates vary over an exurban gradient. Much of the land on the Cumberland Plateau is privately owned without zoning restrictions, so the potential for unrestricted exurban growth is high. To determine avian diversity, we used point counts with visual and acoustic detections at fifteen exurban and fifteen forested points during the summer of 2008. To assess macroinvertebrate diversity, we used sweep netting, soil core samples, pitfall traps, and caterpillar frass samples at the same sites. Bird species richness, bird abundance, and macroinvertebrate taxa richness had significant negative correlations with canopy cover. Conversely, bird species richness, bird abundance, and macroinvertebrate taxa richness had significant positive correlations with impervious surface cover. Overall, avian and macroinvertebrate diversity increased over an exurban gradient. There was a weak negative correlation between insectivorous bird diversity and macroinvertebrate abundance. Detrended correspondence analyses (DCA) revealed high correlation between avian community composition and exurbanization but little correlation between community compositi on of macroinvertebrates and exurbanization. According to a stepwise linear regression, bird abundance is best modeled by macroinvertebrate abundance from pitfall traps and habitat type. Thus we can conclude that exurbanization has a multidimensional impact on biodiversity. While avian diversity demonstrates an unambiguous increase with low-density urbanization, macroinvertebrate diversity varies depending on site-specific factors.
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