project3_ZAKE
Energy limitation or sensitive predators? Trophic and non-trophic impacts of wastewater pollution on stream food webs
Wastewater pollution severely impacted predator richness, likely through a combination of energy constraints (reduction in the amount and diversity of resources) and increased sensitivity to pollution. As longer-lived organisms, predators tend to be more exposed to pollution (Table 2) while showing relatively higher energy demands than primary consumers (e.g., Brose et al. 2017). Although the effects of stress and disturbance on predators has often focused on resource limitation (McHugh et al. 2010), food web theory suggests that consumer diversity decreases risk of local extirpation of predators via the stabilizing effects of weak trophic links (McCann et al. 1998). Our results further support that wastewater-driven reductions in consumer richness may increase predator vulnerability indirectly, beyond what would be expected from their pollution sensitivity alone.
Physical and chemical stressors generally reduce species richness and diversity in stream ecosystems, and often have trophic-level specific effects (Sabater et al. 2018). Notably, their impacts can be magnified along the food web, and bottom-up and top-down mechanisms can operate in concert to redirect energy flows. This study shows that chemical stress can reduce community richness, truncating or compressing food webs via both a reduction of energy transfer efficiency from basal to consumers, and the extinction of larger predators. However, some caveats exist. First, stream invertebrates undergo fluctuations in size–abundance relationships, which can modify CSS slopes (Dossena et al. 2012). Our study took place in spring, when Mediterranean rivers peak in richness, but patterns could differ seasonally. Second, energy flows and food-web structure (as measured by CSS) can provide complementary insights.
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