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Top-down and bottom-up interactions in freshwater ecosystems: emerging complexities

Taylor, Jason M., Vanni, Michael J., Flecker, Alexander S.
Trophic Ecology: Bottom-Up and Top-Down Interactions across Aquatic and Terrestrial Systems 2015 ch.3 pp. 55-85
limnology, freshwater, food chain, aquatic organisms, energy, invasive species, nutrients, freshwater ecosystems, ecosystem engineers, eutrophication, aquatic habitat, biodiversity, case studies, biogeochemical cycles
Understanding the relative role of resource (bottom-up) and consumer (top-down) controls on ecosystem structure and function has been a central question in ecology for some time. In more recent times, ecologist have come to recognize that the two processes do not act independently and that interactions between bottom-up (BU) and top-down (TD) forces likely facilitate complex food webs. Moreover, accounting for BU-TD interactions in applied aspects of ecology may help managers predict ecosystem consequences of biodiversity loss, invasive species, and cultural eutrophication. In this chapter we explore four emerging concepts that integrate BU and TD interactions within aquatic ecosystems. These concepts include: 1) nutrient ratios, not just nutrient supply regulate productivity in aquatic ecosystems; 2) animals integrate BU and TD processes by re-mineralizing nutrients and making them available to primary producers; 3) ecosystem subsidies of energy, nutrients, and organisms that move across the landscape can enhance or stabilize local food webs, but may also be modulated by consumers; and 4) ecosystem engineers, or organisms that physically modify their environment, are widespread and their activities can alter habitat and resources availability which can have strong effects on BU and TD processes. These four integrative concepts represent emerging research areas that illustrate the complex ways in which BU and TD interactions are manifested, and demonstrate how ecologists' view of BU-TD interactions has changed from simple linear food chains to complex food webs that include multiple feedbacks. Additionally, through several case studies, we demonstrate that BU-TD interactions are involved with several current threats to aquatic ecosystems, including biodiversity loss, invasive species, and eutrophication. For example, impacts of excess nutrients in aquatic ecosystems can be enhanced by consumer processes, and in some cases prolonged after external nutrient reductions. In addition to providing ecologists with a broader view of BU and TD processes, incorporating BU-TD interactions into applied ecological problems will help water resource managers better understand how aquatic ecosystems respond to different management strategies designed to protect or restore freshwater environments.