From an ecosystem perspective, desert streams represent a dynamic mixture of autochthonous (algae and diatom-based) and allochthonous (detritus-based) basal resources. These two energy pathways feed the aquatic food web. While in situ production is cited as the dominant pathway in many aquatic ecosystems, predators, can couple these two pathways. We took a food-web approach to see how species richness relates to ecosystem structure (food-chain length) and function (energy flow) of desert stream food webs. We hypothesized that communities with higher richness (functional and taxonomic) would have longer food chains and that energy flow pathways would differ among stream sites because of prey richness differences. In streams with low species richness we expected broad isotopic niche widths compared to stream reaches with high species richness.
We quantified the isotopic niche of the top predator, predaceous giant water bug, Abedus herberti across multiple levels of population structure (metapopulation, population, and subpopulation). We also tested whether season affects trophic position, energy flow, niche size, and food-chain length. We used stable isotope analyses (δ13C and δ15N) to measure the trophic niche size and determine contribution of allochthonous material to the diet of the top predator. We found food-chain length did not vary substantially across a wide taxonomic and functional richness gradient. The isotopic niche of A. herberti was highly conserved across seasons and stream sites, and >80% of the carbon in tissues came from allochthonous carbon sources. Richness of the prey base did not influence the food-chain length. This result is contrary to proposed theoretical mechanisms stating more resources and or higher functional diversity may lead to longer food chains. Stable isotope ratios suggest that giant water bugs in our system occupy similar feeding niches in spring and autumn, despite strong seasonal changes in physical stream condition, leaf litter inputs, and invertebrate community structure. Human activities and climate-driven alteration of flow regimes that reduce habitat are likely to have negative impacts on aquatic–terrestrial linkages in these systems.