Available at: https://digitalcommons.calpoly.edu/theses/2000
Date of Award
MS in Biological Sciences
Dean Wendt & Lisa Needles
Humans have caused grave ecological and economic damage worldwide through the introduction of invasive species. Understanding the factors that influence community susceptibility to invasion are important for controlling further spread of invasive species. Predators have been found to provide biotic resistance to invasion in both terrestrial and marine systems. However, predators can also have the opposite effect, and facilitate invasion. Therefore, recovery or expansion of native predators could facilitate the spread of invasive species. Needles et al. (2015) demonstrated that the threatened southern sea otter (Enhydra lutris nereis) facilitated the invasion of an exotic bryozoan, Watersipora subatra. However, the underlying mechanism was not fully understood. We tested the hypothesis that sea otter predation on Romaleon antennarium crabs indirectly facilitated the abundance of W. subatra. To do this, we collected weekly data on sea otter foraging and quantified the abundance of crabs in the sea otter diet. We also conducted a caging experiment, where we experimentally manipulated crab densities and limited otter access using exclusion cages on pier pilings in Morro Bay, CA. We used photoQuad image processing software to calculate the abundance of W. subatra on PVC panels within each treatment group. We found that crabs were the second most abundant prey item in Morro Bay, comprising 25.1% of the otter diet. Through the caging experiment, we found that W. subatra abundance significantly increased as crab densities decreased. Our results indicated that sea otters indirectly facilitated the invasion of W. subatra by reducing R. antennarium crab densities and sizes. Removal of crabs may release W. subatra from the disturbance caused by crab foraging behavior. Understanding the impacts of top predators in invaded ecosystems has important management implications, as recovery of predator populations could unintentionally benefit some non-native species. Therefore, management should focus first on prevention and second on early detection and eradication of invasive species likely to benefit from predator recovery.