Nov 6
John Sabo
Arizona State University
Hydroecology in desert riparian zones—water as currency for trophic dynamics and a catalyst of turnover in species pools
Abstract
A common caricature of the ecology of deserts is one of stressful conditions, intolerable variability, low production and insignificant biodiversity. I study the mechanisms by which rivers alter this caricature (true or not). Rivers in deserts provide a constant source of water—above and below ground—which alters microclimate, stimulates primary productivity, changes the physiognomy and composition of the plant community and fuels higher production of animals at higher trophic levels. From the animal perspective rivers are oases that provide higher resource availability and some protection from the harsh elements of the desert climate. Classic theories of diversity suggest that high productivity should lend to lower diversity but that moderate stress should compensate and increase diversity to some extent. In this talk I will explore the various roles water may play in determining species richness in desert riparian zones. First I will describe some recent work on species richness gradients. Riparian zones are recognized hotspots of diversity. Surprisingly, show that riparian zones are not generally more species rich than uplands across the globe, but instead harbor different species pools altogether. This result is upheld in a more controlled study on a single desert river when nine groups of taxa representing most of the plant and animal community are considered. Thus, despite strong gradients in productivity and stress in desert riparian zones, there is much stronger evidence for turnover in species pools than any gradients in richness per se. Second, I will discuss how riparian trees provide belowground supplies of water to animals in desert riparian habitats. This subsidy of groundwater fuels high abundance of arthropods at higher trophic levels and alters species interactions between riparian predators and their prey. The upshot from this work is that water—not energy—is a key limiting resource such that water flux—not energy flux (primary production)—is likely a better proxy for the effect of resources on diversity. Increased water consumption alleviates thermal stress and allows for a fauna dominated by riparian obligate species, rather than desert specialists more equipped with various defenses against desiccation.
Bio
I am a stream and riparian community ecologist. Typically I use experimental and comparative approaches in field settings to answer questions about how the land-water boundary in riparian zones mediates energy or water fluxes and between rivers and riparian habitats. I then examine how these fluxes alter species interactions in riparian food webs. More broadly, I am interested in advancing theory in community ecology by integrating insights from this field with tools and concepts from population biology and ecosystem ecology. I use large-scale experiments and stable isotopic tracers to understand connections between ecosystem level processes such as energy flow and water fluxes and the structure of food webs in rivers and riparian forests.
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