NCEAS Project 3061

Drought effects on riparian forests and reorganization of nested stream food webs

  • Alan P. Covich

ActivityDatesFurther Information
Sabbatical Fellow1st September 2000—31st August 2001Participant List  
Visitor1st June—31st August 2001Participant List  

Abstract
Drought intensity and frequency are important ecosystem driving forces. Both climatic changes (such as El Nino events) and societal demands for freshwater resources will likely cause drought impacts to be even more important in the future. An integrative analysis is needed to evaluate spatially specific impacts on stream food webs from drought-altered riparian forests and to determine how stream communities reorganize in response to drought disturbances within a drainage network. This project will bring together forest ecologists, stream ecologists, hydrologists, and climatologists to develop a conceptual framework that will examine linkages among drought frequency, riparian-forest litter fall, benthic community composition, and stream food webs. A review of published research will focus on long-term study plots at several temperate-zone and tropical sites where the drainage networks are spatially well defined and highly instrumented with a hierarchy of multiple stream gages. Digital elevation models will be used to define spatial boundaries of subsets within drainage networks and to examine pathways for multi-directional dispersal of consumer species up and down stream channels. Because leaf fall is widely used as a measurement of forest productivity, many previous studies have documented drought effects by recording changes in deposition rates of leaf litter. Trees often respond to drought conditions by rapidly dropping leaves to reduce water loss from evapotranspiration. Leaf litter, in turn, is the main source of energy to food webs in tributaries so that rapidly increased detrital inputs may alter species diversity of these headwater stream communities. The effects of extremely low flows on stream food webs in forested catchments are also widely recorded because stream flows are important for dilution of toxins and ecosystem integrity. Reduced stream flow can alter benthic detritivore distributions and further modify how energy flows through food webs. Simultaneous or sequential increases in primary production during droughts may also influence specific responses among omnivorous consumer species. Thus, stream food webs can reorganize themselves into different structural assemblages after a period of changing physical conditions. As habitats contract during the drying out of stream channels, some species' adaptations for surviving and feeding in low flows change relative competitive advantages among benthic species and increase exposure to fish predators at different locations within a nested sequence of food webs distributed within a drainage network. How food webs respond to reduced flow will likely depend on specific locations with the stream-channel hierarchy, hydrological conditions (e.g., time lags for subsurface groundwater inflows to enter the stream), stream-channel geomorphology (especially pool depths and catchment porosity) as well as riparian tree species composition and the starting composition of the benthic community. The project will identify gaps in knowledge as well as synthesize existing long-term data on species responses to drought stress within drainage networks.