Terrestrial–aquatic transitions: Local abundances and movements of mature female caddisflies are related to oviposition habits but not flight capability

Abstract

Movement behaviours of adult aquatic insects can produce distinct spatial distribution patterns. Studies of adult abundance with distance away from water bodies are common and may invoke flight capability to explain species differences. In contrast, distribution patterns along river channels are poorly described, but are no less important for understanding population dynamics. Longitudinal patterns in adult abundance along short river lengths may differ between sexes and at different life stage transitions between aquatic and terrestrial environments, i.e. at emergence and oviposition. Flight capability is unlikely to influence longitudinal patterns created at emergence, but may influence local abundances of mature females seeking to lay eggs. We tested hypotheses about how local abundances of mature females might differ according to oviposition habits and flight capability. We surveyed abundances of mature female caddisflies at adjacent riffle–pool pairs along short river lengths with homogeneous riparian cover. Our survey included nine species in three families (Hydrobiosidae, Leptoceridae, Hydropsychidae), which encompassed multiple different oviposition habits and a range of wing sizes and shapes. Several of the species oviposit preferentially in riffles. Accordingly, we tested for differences in female abundance between channel units (adjacent riffle–pool pairs). We also tested whether females attained higher abundances in some places along channels than others (i.e. over larger spatial scales and regardless of channel unit) which imply movements along the channel and aggregation in some locations. Wing morphology was used as a proxy measure of flight capability and included measures of wing span, area, aspect ratio and the second moment of wing area. Three distinctly different distribution patterns of mature female caddisflies were identified. The abundance of three species varied over larger scales only (multiple channel units). Six species that oviposit preferentially in riffles had higher female abundances at riffles than pools, but for only one did abundances also vary over larger scales. There was no association between these different patterns and measures of wing morphology, after removing metrics that were correlated and that differed systematically between taxonomic families. However, we could not reject the hypothesis that some aspect of flight behaviour may have contributed to observed patterns. The diverse but distinct distributions of mature female caddisflies we observed along short channel lengths are novel and suggest that species differ in their propensity for movement along streams, which could have consequences for local densities of eggs and juveniles in the aquatic environment. The degree to which population sizes are coupled across the terrestrial‐to‐aquatic transition is rarely investigated in aquatic insects and may provide fresh insight into sources of spatial variation within populations. Similarly, a more nuanced approach to research on the flight of aquatic insects, in...

Hydrological controls on oviposition habitat are associated with egg‐laying phenology of some caddisflies

“…was egg mass abundance also correlated with ER numbers. In spatial surveys, egg mass and ER densities are often correlated (references above) and frequent movements of individuals between resource patches may underpin these close associations (Lancaster, Downes, & Dwyer, 2020; Peckarsky et al., 2000). However, monthly fluctuations in ER abundance were largely synchronous across the three rivers, suggesting that females were unlikely to locate many resource‐rich patches in this landscape during high discharge.…”

“…In addition, life‐history patterns are typically related to long‐term averages in environmental variables (Lytle, 2001) and thus insects may have limited capacity to respond to short‐term fluctuations in resources. Spatially, close associations between numbers of ERs and egg masses do occur within and between rivers (Lancaster et al., 2010) and are likely to arise when females can fly between resource‐poor and resource‐rich locations, thereby closely tracking resource abundance (Lancaster et al., 2020; Peckarsky et al., 2000). Temporally, however, such close tracking of resources is less likely because, at times, there may be no resource‐rich locations (e.g.…”

Habitat use and movements of a large dragonfly (Odonata: Anax imperator) in a pond network

“…Whether they represent dispersal events or local movements within the home range, distances moved by insects are very variable among species. They depend on intrinsic factors such as flight capability (Lancaster & Downes, 2017) and may differ according to age or sex at the intraspecific scale (Lancaster et al., 2020). These movements also depend on extrinsic factors such as land use, connectivity in the surrounding matrix (Maynou et al, 2017) and meteorological factors (e.g.…”

Coexistence of predatory caddisfly species may be facilitated by variations in the morphology of feeding apparatus and diet

“…Foreleg morphology of the genera Ethochorema and Taschorema are very similar and, taxonomically, these genera and some others are considered part of the Taschorema complex. Larvae of U. rubiconum are difficult to separate from Ulmerochorema membrum and Ulmerochorema stigma (Dean, 1997), but adults of this species group sampled in the study area have been overwhelmingly U. rubiconum (Lancaster et al., 2020), so larval samples are likely to be dominated by U. rubiconum also. In general, all seven species occurred in all sample locations and times.…”

“…50 m of channel length. The spatial scale of sample collection is appropriate to provide population‐level descriptions of diet for predators that typically occur at low densities (Reich & Downes, 2004), that are highly mobile and may range over large areas during larval life, and also disperse as terrestrial adults (Bovill et al., 2019; Lancaster et al., 2020). The study sites are cold‐water rocky streams with intact native vegetation.…”