Stream-breeding amphibian responses to land use distrubances
Type of Degreedissertation
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Conversion of land from an undeveloped state into agricultural or urban areas is widespread. Urban areas in particular are growing both in size and number globally. Such land use changes can potentially have negative consequences for organisms such as amphibians, many of which require aquatic and terrestrial environments to complete their life cycle. I conducted herpetofaunal richness surveys of several watersheds in western Georgia, USA, subject to varying degrees of urbanization or pasture land uses, which revealed amphibians are particularly sensitive to urban areas, but not pasture lands. Reptiles showed the opposite response (i.e., more reptile species were found in urban areas). The results of this survey prompted a more detailed study of the stream-breeding salamander, Eurycea cirrigera (two-lined salamander). I found that this species has a reproductive output in urban streams equal to conspecifics in reference environments, but that survivorship of larvae to metamorphosis was much lower in urban streams. Path analysis of potential environmental factors contributing to this putative decline in survivorship revealed frequent and intense flooding in urban environments is the most probable cause for observed declines. To validate this finding, I created a series of experimental streams (flumes), in which water flow and substrate were manipulated. I found salamanders were more likely to be eroded from experimental flumes at lower water velocities when flumes contained sandy substrates without rocky cover, a streambed condition common in urban streams. Taken together, field and experimental data strongly suggest an altered hydrology in urban areas is one of the leading factors causing stream-breeding salamander decline in urban habitats. In addition, I followed my studies on hydrological effects by evaluating growth of the two-lined salamander across an urban – forest gradient. I found that larvae in urban streams grew faster than larvae in forested streams. The benefit of faster larval growth could explain the persistence of the two-lined salamander in urbanized watersheds. Finally, I described shifts in consumed prey by two-lined salamanders that accompany urbanization. While these shifts were not dramatic, the descriptions I offer provide a foundation for describing food web dynamics in urban habitats. In total, urbanization dramatically alters herpetofaunal assemblages in and around streams. Those species that are not extirpated apparently suffer survivorship costs and shifts in growth and diet. Information from assemblage-wide and species-specific perspectives, provided here, is needed to increase our ability to ameliorate effects of urbanization on stream-dwelling amphibian species.
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