DISSERTATION RESEARCH: Impacts of human activities on aquatic insect emergence and effects on terrestrial food webs

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Leaves falling into streams provide food for many aquatic organisms. Likewise, streams contribute materials to the terrestrial environment. Aquatic insects spend most of their life in the water as larvae, but emerge as adults and fly into forests, providing important food for terrestrial predators such as spiders, lizards, birds, and bats. Streams and their adjacent forests are thus intimately connected. Human activities may affect this reciprocal movement of energy and materials between these ecosystems. We have long recognized the effect of human activities on the movement of materials from the landscape to streams. For instance, intense agricultural activities are known to change the community composition of aquatic insects in streams by decreasing sensitive aquatic insects and favoring insects tolerant to pollution, increased nutrients, and the low oxygen conditions resulting from intense agriculture. These land-use-associated changes to aquatic insect communities could be reflected in the kinds and amount of insects emerging and their movement to land, with consequences for terrestrial predators and their prey. Little is known, however, about how land use change could affect the transport of materials from streams to land. What?s more, most of our knowledge on the movement of aquatic insects to land comes from research in temperate, forested areas, leaving a gap in our understanding of how the movement of materials from streams to land affects terrestrial food webs in human-altered systems, particularly in the tropics. The proposed research integrates extensive undergraduate training and mentoring, teacher training through the NSF GK-12 program, community outreach, and support for a Latin American graduate student.Initial research in Costa Rica suggests that aquatic insect density and biomass, and streamside terrestrial spider density increase with the intensity of agricultural land-use, and the community composition of aquatic insects changes across a land use gradient. Using a field experiment, this project aims to understand the mechanisms behind these changes, and the consequences for terrestrial insect density and community composition. The planned experiment will manipulate the movement of aquatic insects to land at four forested streams and four streams with intensive agricultural land use, and quantify the response of spiders and their terrestrial prey. The approach will test whether land use effects on aquatic insects is the mechanism for the observed increase in riparian spiders with increasing agricultural land use, and will test whether land-use effects on aquatic insects propagate through the terrestrial food web to affect terrestrial insect prey of spiders.