Methods to identify the prey of invertebrate predators in terrestrial field studies

Abstract

Predation is an interaction during which an organism kills and feeds on another organism. Past and current interest in studying predation in terrestrial habitats has yielded a number of methods to assess invertebrate predation events in terrestrial ecosystems. We provide a decision tree to select appropriate methods for individual studies. For each method, we then present a short introduction, key examples for applications, advantages and disadvantages, and an outlook to future refinements. Video and, to a lesser extent, live observations are recommended in studies that address behavioral aspects of predator-prey interactions or focus on per capita predation rates. Cage studies are only appropriate for small predator species, but often suffer from a bias via cage effects. The use of prey baits or analyses of prey remains are cheaper than other methods and have the potential to provide per capita predation estimates. These advantages often come at the cost of low taxonomic specificity. Molecular methods provide reliable estimates at a fine level of taxonomic resolution and are free of observer bias for predator species of any size. However, the current PCR-based methods lack the ability to estimate predation rates for individual predators and are more expensive than other methods. Molecular and stable isotope analyses are best suited to address systems that include a range of predator and prey species. Our review of methods strongly suggests that while in many cases individual methods are sufficient to study specific questions, combinations of methods hold a high potential to provide more holistic insights into predation events. This review presents an overview of methods to researchers that are new to the field or to particular aspects of predation ecology and provides recommendations toward the subset of suitable methods to identify the prey of invertebrate predators in terrestrial field research.

Keywords: cage experiments; fatty acid analysis; field observations; molecular gut content analysis; prey baits; stable isotope analysis.

Figure 1

Decision tree leading to the major domains of methods to measure predation in terrestrial field studies. Decision #1 addresses whether the focus is a single or very few predator species or a whole community of predators. Decision #2 addresses whether the whole prey community is of interest or whether predation on a single species is assessed. Decision #3 addresses the need for qualitative (link between prey present or absent) or quantitative data on predation. The table then illustrates the suitability of each method for that particular domain with +, suitable in most cases; ○, suitable in some cases and – unsuitable in most cases. After a subset of methods is selected from this figure, please refer to Table 1 for additional suitability criteria of individual methods for different body size and mobility traits of predators

Figure 2

Examples of different methods to assess predation events in the field with (a) field observations, (b) exclosure barrier, (c) enclosure cage, and (d) sentinel prey (darkling beetle on a string)