The typical way to infer the “importance” of a generalist predator’s prey species is to calculate the (proportional) occurrence in the average predator individual’s stomach contents, either by frequency counts, volume or mass. These are indices of importance.
What we’d really like to know, however, is how important a particular prey item is in terms of the rate at which predator individuals consume them, either in terms of prey numbers per time or prey mass per time. (Clearly, if you know both prey counts per time and the mass of an average prey individual you can also estimate prey mass per time.)
In this paper we show how that can be done: it requires knowing how long a prey item remains identifiable (i.e. present, detectable, and identifiable to an observer) in a predator individual’s stomach. Because prey items are either identifiable or not (i.e. the data are binary) and the transition point between the two cannot be directly observed (i.e. because it occurs inside the fish!), we use Survival models to estimate detection times.
In this paper we use experiments on Reticulate sculpins (Cottus perplexus) to show that detection times can be estimated and that they vary with prey identity, predator and prey size, and temperature (similar to what we’d shown previously for intertidal whelks).
We also show that, for sculpins, detection times can vary across prey species by over an order-of-magnitude! That means that prior inferences of prey “importance” have likely been off by over an order-of-magnitude as well!!!!
Preston, Henderson, Falke & Novak (2017) Using Survival Models to Estimate Invertebrate Prey Identification Times in a Generalist Stream Fish. Transactions of the American Fisheries Society Vol. 146(6): 1303-1314.