Substitutes, compliments, edge effects, and decoupled consumption of transient and resident prey

Background/Question/Methods

Some populations are maintained by trophic subsidies.  If spatially and temporally transient prey species predominate in the diet of a resident predator, they may constitute subsidies to the local predator population.  Understanding the effects of subsidies on other prey populations is important for forecasting the likely effects of changes in target populations.  Because subsidies are by definition from without a defined habitat, there is an associated edge where a consumer has potential access to resources that are both from without (transient) and from within (resident).  An important consideration for how an influx of transient prey may influence other prey types is if they are directly substitutable for resident prey or if they are complimentary.  In this case prey compliments co-occur in the diet because they are found in the same time and place, whereas substitutes do not, whether from a shift in foraging behavior or encounter rate.  However, both of these conditions imply a linkage of some kind with respect to foraging rates.  A third possibility is that there is no linkage and in this case resident and transient prey types are de-coupled.

Results/Conclusions

Lingcod Ophiodon elongatus are a large relatively site-attached generalist predator on reefs in the northeastern Pacific.  In this study, lingcod diets were extremely generalized with 21 species of fish and invertebrates in 14 ecologically different prey categories identified from 186 lingcod stomachs containing food.  The most frequently consumed prey species were Pacific herring Clupea pallasii and Pacific sand-lance Ammodytes hexapterus, both transient species that together comprised 46% of prey items by number.  Variation in consumption of transient prey was the primary influence on inter-annual trends in consumption of all prey types and transient prey appeared to substantially subsidize the local lingcod population.  However, resident prey provided a less variable resource over time as there was always some consumption of resident prey among actively feeding lingcod.  Preliminary analyses of lingcod foraging behavior during the summer season over three years find little evidence of complementarity or direct prey substitution.  Rather, the two general prey categories (resident and transient) appear to be decoupled.  Large fluctuations in the presence of transient prey had no clear effect on the consumption rates, either specifically or generally, of resident prey. This pattern suggests that changes in the distributions of transient prey species are likely to disproportionally affect this resident predator.