Crayfish Impact Desert River Ecosystem Function and Litter-Dwelling Invertebrate Communities through Association with Novel Detrital Resources

Eric K Moody,John L Sabo,Francesco De Bello

doi:10.1371/journal.pone.0063274

Abstract

Shifts in plant species distributions due to global change are increasing the availability of novel resources in a variety of ecosystems worldwide. In semiarid riparian areas, hydric pioneer tree species are being replaced by drought-tolerant plant species as water availability decreases. Additionally, introduced omnivorous crayfish, which feed upon primary producers, allochthonous detritus, and benthic invertebrates, can impact communities at multiple levels through both direct and indirect effects mediated by drought-tolerant plants. We tested the impact of both virile crayfish (Orconectes virilis) and litter type on benthic invertebrates and the effect of crayfish on detrital resources across a gradient of riparian vegetation drought-tolerance using field cages with leaf litter bags in the San Pedro River in Southeastern Arizona. Virile crayfish increased breakdown rate of novel drought-tolerant saltcedar (Tamarix ramosissima), but did not impact breakdown of drought-tolerant seepwillow (Baccharis salicifolia) or hydric Fremont cottonwood (Populus fremontii) and Gooding's willow (Salix goodingii). Effects on invertebrate diversity were observed at the litter bag scale, but no effects were found at the cage scale. Crayfish decreased alpha diversity of colonizing macroinvertebrates, but did not affect beta diversity. In contrast, the drought-tolerant litter treatment decreased beta diversity relative to hydric litter. As drought-tolerant species become more abundant in riparian zones, their litter will become a larger component of the organic matter budget of desert streams which may serve to homogenize the litter-dwelling community and support elevated populations of virile crayfish. Through impacts at multiple trophic levels, crayfish have a significant effect on desert stream ecosystems.

Highlights

Ecosystems around the world are threatened by anthropogenic global change, the spread of introduced species [1]
Several examples of novel consumers relying on novel resources have been reported (e.g., [39,40]), we investigated whether such a situation may occur in desert streams
Virile crayfish presence did have a significant effect on breakdown rates across species (Tukey-Kramer post-hoc test: p,0.01); direct comparisons revealed that virile crayfish significantly increased the breakdown rate only of saltcedar (Tukey-Kramer post-hoc test: p,0.01; Figure 2)

Summary

Introduction

Ecosystems around the world are threatened by anthropogenic global change, the spread of introduced species [1]. In most ecological communities there are multiple, interacting sources of novel species that form what have been called novel ecosystems [3]. Arid and semiarid streams are impacted by introduced aquatic and riparian species as well as declining precipitation and water tables [5,6,7]. Changes to flow regime can alter macroconsumer effects on stream ecosystem function, competition between native and introduced species, as well as macroinvertebrate community composition [13,14,15,16]. Altered flow regimes can have indirect effects on aquatic benthic communities through effects on riparian flora

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Results

Discussion

Conclusion