Energy pathways modulate the resilience of stream invertebrate communities to drought.

While climate change is altering ecosystems on a global scale, not all ecosystems are responding in the same way. The resilience of ecological communities may depend on whether food webs are producer- or detritus-based (i.e. 'green' or 'brown' food webs, respectively), or both (i.e. 'multi-channel' food web). Food web theory suggests that the presence of multiple energy pathways can enhance community stability and resilience and may modulate the responses of ecological communities to disturbances such as climate change. Despite important advances in food web theory, few studies have empirically investigated the resilience of ecological communities to climate change stressors in ecosystems with different primary energy channels. We conducted a factorial experiment using outdoor stream mesocosms to investigate the independent and interactive effects of warming and drought on invertebrate communities in food webs with different energy channel configurations. Warming had little effect on invertebrates, but stream drying negatively impacted total invertebrate abundance, biomass, richness and diversity. Although resistance to drying did not differ among energy channel treatments, recovery and overall resilience were higher in green mesocosms than in mixed and brown mesocosms. Resilience to drying also varied widely among taxa, with larger predatory taxa exhibiting lower resilience. Our results suggest that the effects of drought on stream communities may vary regionally and depend on whether food webs are fuelled by autochthonous or allochthonous basal resources. Communities inhabiting streams with large amounts of organic matter and more complex substrates that provide refugia may be more resilient to the loss of surface water than communities inhabiting streams with simpler, more homogeneous substrates.

Dispersal in the acanthocephalan Acanthocephalus dirus.

In acanthocephalans, dispersal typically occurs when eggs that have been released in the intestines of definitive hosts are expelled with the feces. We examined whether the acanthocephalan Acanthocephalus dirus adopts a strategy of dispersal in which eggs are carried into the environment by gravid females. Using a combination of field surveys and lab-based experiments, we showed that the A. dirus female retained eggs as they passed out of the intestines and that these eggs could develop in intermediate hosts (sediment-dwelling isopods). Lab-based behavioral experiments revealed that the bodies of gravid females were attractive to foraging isopods. We propose that a strategy of egg dispersal could occur in A. dirus in which eggs are carried into the environment by females. This strategy could increase transmission success by dispersing eggs closer to the sediment, rather than in the water column, and by directing the feeding behavior of target hosts.