Evaluating consumptive and nonconsumptive predator effects on prey density using field time-series data.

Determining the degree to which predation affects prey abundance in natural communities constitutes a key goal of ecological research. Predators can affect prey through both consumptive effects (CEs) and nonconsumptive effects (NCEs), although the contributions of each mechanism to the density of prey populations remain largely hypothetical in most systems. Common statistical methods applied to time-series data cannot elucidate the mechanisms responsible for hypothesized predator effects on prey density (e.g., differentiate CEs from NCEs), nor can they provide parameters for predictive models. State-space models (SSMs) applied to time-series data offer a way to meet these goals. Here, we employ SSMs to assess effects of an invasive predatory zooplankter, Bythotrephes longimanus, on an important prey species, Daphnia mendotae, in Lake Michigan. We fit mechanistic models in an SSM framework to seasonal time series (1994-2012) using a recently developed, maximum-likelihood-based optimization method, iterated filtering, which can overcome challenges in ecological data (e.g., nonlinearities, measurement error, and irregular sampling intervals). Our results indicate that B. longimanus strongly influences D. mendotae dynamics, with mean annual peak densities of B. longimanus observed in Lake Michigan estimated to cause a 61% reduction in D. mendotae population growth rate and a 59% reduction in peak biomass density. Further, the observed B. longimanus effect is most consistent with an NCE via reduced birth rates. The SSM approach also provided estimates for key biological parameters (e.g., demographic rates) and the contribution of dynamic stochasticity and measurement error. Our study therefore provides evidence derived directly from survey data that the invasive zooplankter B. longimanus is affecting zooplankton demographics and offer parameter estimates needed to inform predictive models that explore the effect of B. longimanus under different scenarios, such as climate change.

Histology of herniations through the body wall and cuticle of zooplankton from the Laurentian Great Lakes.

Zooplankton of the Laurentian Great Lakes developed hernial protrusions whose gross appearance matches those on zooplankton described elsewhere in the world. We have carried out a histologic and cytologic analysis of the protrusions and found that they are composed of apparently degenerating or necrotic tissue(s) that has been expressed from the organism through the process of herniation. At their base the protrusions are continuous with viable tissue(s) within the organism through a fissure in the exoskeleton. Our observations lead us to suspect that these hernial protrusions are lethal. The development of such protrusions in zooplankton may be a worldwide phenomenon, but the cause of the herniation remains a mystery.