How detectable is predation in stage-structured populations? Insights from a simulation testing analysis

Background/Question/Methods

The potential of predation to structure marine food webs is widely acknowledged. However, available tools to detect the influence of predators on prey population dynamics are limited, partly because available population data often aggregate a population’s age structure into a single biomass or abundance metric. Additionally, many food webs are relatively complex, with prey species subject to different assemblages of predators throughout their ontogeny. This creates challenges when accounting for predator-prey interactions in management of exploited populations. Furthermore, while many analyses compare the abundances of predators and prey, predator abundance in fact directly influences prey productivity, making productivity a potentially better response variable. Here we evaluated the extent to which stage-structured predation could be reliably detected from time series of total biomass of predators and prey. We simulated age-structured populations of four mid-trophic level fish with distinct life history traits, exposed them to variable predation at different life stages, and fit production models to resulting population biomass to determine how reliably the effects of predators could be detected.

Results/Conclusions

Predation targeting early life history and juvenile life stages generally led to larger fluctuations in annual production, and was therefore more detectable. However, realistic levels of observation error and environmental stochasticity masked most predator signals. Including a second predator in the simulation and statistical models sharply decreased the detectability of both predators, even when the signal was easily detected for a single predator. We conclude an absence of detectable species interactions from biomass time series may be partly due to the interactive effects of environmental variability and complex food web linkages and life histories. We also note that predation signals are most robust for predator-prey systems where predators primarily act on mortality of sub-mature life history stages.