Terrestrial arthropod community composition in relation to latitudinal and longitudinal gradients in a desert stream

Background/Question/Methods

The importance of water in mediating species trophic interactions has been increasingly recognized. These effects have been particularly well-demonstrated in arid habitats in proximity to perennial streams and rivers. However, community-wide impacts on whole food webs remain poorly understood. The Coachella Valley within southern California’s Colorado Desert is a high-biodiversity wilderness area home to many endemic species of plants and animals, including some currently under threat of extinction. At the west edge of the Coachella Valley, a series of 12 transects were established perpendicular to a semi-perennial desert stream. Transects are ~100 m apart with collection plots at 0, 4, 8, 16 and 32 m from the desert stream channel to assess arthropod community composition in relation to longitudinal position and latitudinal distance from the stream. Pitfall and pan traps were deployed for 24 hours bimonthly and arthropod samples identified to morphospecies.

Results/Conclusions

Nearly 300 distinct morphospecies from 20 orders were collected. While only separated by ~1 km in space, transects at perennially flowing stream reaches collected distinct arthropod assemblages from transects at dry (ephemeral) reaches. Transects at perennially flowing sites also collected more arthropods in total. Similarly, plots nearer stream banks collected more arthropods and slightly different assemblages than plots further away. Greater species turnover in longitudinal than latitudinal gradients is not surprising, given a recent meta-analysis determining the “biological” width of a stream is significantly greater than it’s topographical width. The presence of this desert stream and the “water subsidy” it provides significantly enhance the richness and complexity of the terrestrial arthropod community. Consideration of entire landscapes, rather than target habitats, is critical for effective conservation of communities and species interactions.