The importance of functional responses among competing predators for avian nesting success

Kristen S. Ellis, Randy T. Larsen, David N. Koons

Predation plays a fundamental role in shaping most ecosystems. An important starting point for assessing predator-prey relationships is to evaluate rates of prey consumption across a range of prey abundances, because this can provide insights into mechanisms of predation. Yet these relationships are more complicated when there are multiple predator species competing for shared prey. Predator species may affect prey differently and are often forced to interact with one another through competition. Characterizing the roles of specific predators and combinations of competing predators is important for predicting how predators drive changes in prey populations.

Predation on nests is the main source of reproductive failure for many bird species, yet little research has been done on competing nest predators. Management and conservation plans that aim to increase avian reproductive output often include removing nest predators, or altering habitats that may support certain nest predators. However, if predation by one predator is reduced through management actions, overall nest predation could remain constant if predation shifts to other predator species (i.e., compensatory effects). Alternatively, reductions in certain predator species may indeed decrease nest predation if nest predation by other species is independent (i.e., additive effects). To address these questions, we compared how specific predators responded to snowy plover (Charadrius nivosus) nest abundance in Utah, USA between 2011 and 2017. We then estimated the correlation between rates of nest consumption by specific predators to assess their combined effects on overall nest predation.

We found that nest consumption by gulls Larus spp.decreased with increasing nest abundance and gulls became satiated at high nest abundance. Alternatively, nest consumption by foxes Vulpes spp. and ravens Corvus corax initially increased with nest abundance, indicating that dietary switching may have occurred. Nest consumption by foxes and coyotes Canis latrans was negatively correlated, suggesting that these predators may have been limited by competition. Similarly, nest consumption by avian and mammalian nest predators was negatively correlated, suggesting that these predator groups vary in their foraging efficiencies. Our study shows that nest abundance influenced rates of consumption differently among predator species, and strategies for increasing nest success should consider if compensatory effects of predation exist so that funding for management methods can be most effective. 

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