Biological invasions alter consumer-stress relationships along an estuarine gradient.

Estuaries represent steep stress gradients for aquatic organisms, with abiotic stress due to temperature and salinity typically increasing with distance into estuary. Invertebrate communities and their predators are strongly influenced by these stress gradients. The environmental stress model predicts that the importance of predation in structuring communities decreases with increasing environmental stress. Estuaries contain a stress gradient for marine organisms this includes salinity, temperature, and other abiotic properties. Additionally, estuaries are hotspots for biological invasions; increased stress tolerance among non-native species could change the predictions of the environmental stress model. In this study, we investigate how introduced species alter the predictions of the environmental stress model by examining the effects of predators on sessile invertebrates across an estuarine gradient. To do this, we deployed recruitment plates across the estuarine gradient of Tomales Bay, California, USA using various caging treatments over the summer of 2019. We found that the effect of predation changed across sites, with the mid-estuary site experiencing the greatest reductions in prey abundance and prey species richness when exposed to predators. This was likely to be due to higher proportions of non-native prey and predator taxa mid-estuary, including solitary ascidians, which are highly susceptible to predation. Overall, predation did not follow the predictions of the environmental stress model, but rather followed the abundance of functional groups with non-native species, whose distribution could be mediated by environmental stress gradients. We suggest that this may be a general result and that communities subject to large numbers of stress-tolerant invaders may have high rates of consumption in high stress areas, contrasting predictions by previous models.