Human land use and patterns of parasitism in amphibian hosts

Depending on the type of land use and the life histories of parasites and hosts, parasitism may increase or decrease in response to changes in land use patterns. For amphibians, the interactions between parasites and pathogens, the environment, and host immunity are becoming key issues in understanding threats to amphibian welfare. The conversion of natural habitats to agricultural or urbanized landscapes can influence both water quality in amphibian habitats and the surrounding faunal communities.  These changes can result in shifts in the abundance of hosts for complex life cycle parasites, direct effects on free-living parasitic stages in the environment, and/or impacts on amphibian immune systems. By comparing several studies of amphibian parasite responses to land use disturbances, this study presents a synthesis that informs the overall impacts on amphibians. Trophically transmitted parasites that use amphibians as intermediate hosts will often respond to land use-imposed changes mediated by the response of the definitive host vertebrates (e.g. birds, mammals, and reptiles that prey on amphibians) and intermediate hosts (e.g. molluscs and insects). In highly disturbed sites devoid of vertebrate activity, these parasites are absent, but amphibians may have to contend with poor water quality and toxicants. Alternatively, parasites that use amphibians as definitive hosts respond to land use-imposed changes mediated by the response of intermediate hosts. In this case, parasites can be significantly more abundant in agricultural, nutrient-rich water bodies. Overall, land use disturbances can create parasite "hotspots", challenge amphibian immune systems with toxicants, or allow both sets of problems to act concurrently. 

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