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

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.

Lake browning impacts community structure and essential fatty acid content of littoral invertebrates in boreal lakes

“…We hypothesized that different habitats contain different food sources, leading to differences in the FA compositions of invertebrates. Most invertebrate taxa are considered opportunistic regarding their food sources (Tierno de Figueroa et al, 2019) and can utilize several types of food (Friberg & Jacobsen, 1994), which might lead to differences in their FA composition. It is possible that the shore types in our study contained similar food sources, and thus clear differences in the FA compositions were not detectable.…”

Thermal traits of freshwater macroinvertebrates vary with feeding group and phylogeny

“…An additional factor that is important to consider in the application of these data is the flexibility that organisms can have in their actual feeding strategies. Tierno de Figueroa et al (2019) demonstrate that organisms can have diets that differ from their commonly assigned FFG , that also are variable in space and time. The use of a “Generalist’ category helps account for this, but variation in diets still represents uncertainty in our analysis, and may explain some of the relatively low R 2 values we found in parts of our analysis.…”

“…We used the classification of FFGs from this dataset listed under the trait values for “Primary functional feeding group abbreviated” and “Primary feeding mode.” We recoded the values of these traits to fit into the six FFGs (i.e., “Herbivore”, “Collector‐gatherer”, “Predator”, “Shredder”, “Filterer” and “Generalist”). The diet of some taxa changes throughout their life cycle, and across seasons or space (Tierno de Figueroa et al, 2019), resulting in variation that may be represented by conflicting records in the database. We assigned taxa that had conflicting records for their FFG assignment to the “Generalist” category.…”

Do Neotropical adult dobsonflies (Megaloptera, Corydalidae) feed on solid material in the nature? Information from a gut content study

“…A compound microscope Zeiss Primo Star© equipped with ocular micrometer was used, if needed, to estimate the % absolute gut content (% of the whole digestive tract occupied by content) at 40× magnification and the relative abundance of food items in the gut content (% of the total gut content occupied by each component) at 400×. The employed methodology was a modification of that proposed by Bello and Cabrera (1999), previously used in gut content studies of both adults and nymphs/larvae of aquatic insect orders (e.g., Tierno de Figueroa and Sánchez-Ortega 1999; López-Rodríguez et al 2009; Tierno de Figueroa et al 2019), including also Megaloptera belonging to the family Sialidae (Zamora-Muñoz et al 1999; Tierno de Figueroa and Palomino Morales 2002).…”