Spatial and seasonal variability in the trophic role of aquatic insects: An assessment of functional feeding group applicability

Abstract

Abstract Functional feeding groups (FFGs) have been used over recent decades to classify aquatic insects according to their trophic resources and the mechanisms used to acquire them. Nonetheless, the classification of a taxon into an FFG is often based on its membership of a genus or family rather than on species level information. This extrapolation of the FFG membership of one taxon to the remaining taxa within a group has led to misconceptions and erroneous conclusions. This study aimed to determine the amount of variation in FFG membership within species among seasons and locations and whether this was related to resource availability. For this purpose, we sampled over two seasons (autumn and spring) at three altitudes in each of four basins, measuring the main trophic resources available for aquatic insects and collecting representatives of taxa traditionally assigned to each of the main FFGs. Based on analyses of the diet of multiple individuals in each location and season, we calculated the percentage belonging to each FFG. Diets differed widely among taxa, and within taxa they often varied significantly between sites in the same season and at the same site between different seasons. Species' diets could also differ between different river basins when specimens were collected at the same altitude and season. In some cases, although the primary food source remained the same, the contribution from a secondary source varied between sites. Although availability of the three main trophic resources (epilithic algae, fine and coarse particulate organic matter) differed among sites, we found no association overall between the availability of these resources and insect diets. Our results agree with several recent studies that show that the diets of stream invertebrates vary widely in space and time, potentially indicative of widespread omnivory/opportunistic feeding. This makes classifying taxa into particular FFGs without collection of dietary data, or by assigning one FFG to a taxon across all places and times, fraught with potential errors. These have possible repercussions for ecological studies related to matter cycles or food webs, amongst other issues. Some stream insects are consistently predaceous or have piercing mouthparts that constrain their diet. However, it appears increasingly likely that the majority of taxa, formerly labelled as scraper, shredder, and collector–gatherer or filterer, should be regarded as feeding on a range of sources and not routinely assigned into distinct FFGs (in the absence of local dietary data). We suggest researchers classify stream insect taxa as omnivores unless there are local data that show otherwise, or the taxa are obligate predators, feed by piercing or are otherwise physiologically constrained to a particular diet.