Connected headwaters: Indelible field evidence of dispersal by a diverse caddisfly assemblage up stream valleys to dry catchment boundaries

Abstract

Abstract1. Integral to many ecological models is the notion that dispersal connects populations and communities in disparate locations. For stream insects, however, there is very little empirical information about which species and what proportion of the species in a community are likely to disperse, in what direction and how far they travel, and whether dispersal is successful in connecting populations and communities. This study examines whether species of caddisfly can disperse between catchments and potentially connect communities.2. We used a novel sampling design comprising five pairs of streams. Each stream in a pair flowed from the same boundary, but in opposite directions and into isolated catchments. We surveyed abundances of adult caddisflies on the catchment boundary (boundary species) where there was no water (Up location), and on two locations per stream (Middle, Down locations) where there was permanently flowing water (resident species). If caddisflies fly along the topographic features of stream valleys, then we expected to find adults on the ridge between catchments and, if dispersal is successful, stream communities in adjacent catchments should have similar species composition.3. Over 130 species of caddisfly (from 18 families) were collected. Approximately half of them occurred on at least one catchment boundary, and Up locations averaged almost 20 species. Boundary species included both the smallest‐ and largest‐bodied taxa and they were a near perfect subset of species in resident assemblages. However, boundary species were not simply the most common in resident assemblages. There was no evidence of sex‐biased dispersal. Unsurprisingly, assemblage composition varied across the landscape, within and among catchments. However, resident assemblages within stream pairs were significantly more similar to one another than unpaired streams in either the same or in different catchments.4. Our results suggest that a surprisingly diverse set of caddisflies disperse up to catchment boundaries and potentially connect streams on either side of catchment divides. Nevertheless, many other caddisfly species were never recorded at Up sites and hence may never or only rarely reach catchment boundaries.5. These outcomes suggest, firstly, that some caddisflies are capable of trans‐catchment dispersal but it is incorrect to assume that all caddisflies are good dispersers, as is often stated in the literature. Secondly, caddisfly assemblages in headwater streams may be less isolated than is often considered and the dendritic structure of stream networks may be less important for their distribution patterns than the proximity of tributaries in neighbouring catchments, in many landscapes. Trans‐catchment dispersal could ensure community connectivity and facilitate recovery after catchment‐wide extinctions.

Bigger is not necessarily better: empirical tests show that dispersal proxies misrepresent actual dispersal ability

“…Such work is logistically challenging but possible for multi-species assemblages [26,62]. The current study arose from a survey that exploited landscape structures to provide unequivocal evidence of dispersal in the field [26]. Other researchers have also exploited environmental features that allowed direct measurements of dispersal ability in field surveys [63][64][65], particularly when monitoring the recolonization of new or restored streams [66], and experimental tests of dispersal in the field are possible [67].…”

“…True dispersal requires that individuals travel away from natal locations and, for adults, that predominantly means travelling through the terrestrial environment to reach new aquatic habitats. To quantify actual dispersal, Lancaster et al [26] compared the assemblage of adult caddisflies on dry catchment boundaries that were far from permanent water ('boundary species') with caddisflies in the riparian zone of permanently flowing water a few kilometres away ('residents', which included both boundary and non-boundary species). Individuals on the boundary must have dispersed there because there was no water to support local populations.…”

“…A rare opportunity to test whether, or how, actual dispersal and dispersal proxies are related arose from a recent field survey that quantified instances of dispersal by stream-dwelling caddisflies [ 26 ]. Caddisflies have complex life cycles with larvae that are typically confined to aquatic environments and flying adults are the primary means of dispersal among distant or spatially isolated aquatic habitats [ 11 ].…”

“…The primary objective of this study was to test whether the wing morphology of caddisflies is related to empirical, field-based evidence of dispersal, as described above and reported by Lancaster et al [ 26 ]. Specifically, we tested whether boundary and non-boundary species differed in aspects of wing morphology known to reflect a capability for long-distance flight.…”

“…Specifically, we tested whether boundary and non-boundary species differed in aspects of wing morphology known to reflect a capability for long-distance flight. We used the relative abundance of species on catchment boundaries (from [ 26 ]) as a measure of relative dispersal ability and tested for associations with wing morphology. For wing morphology, we measured two gross parameters (wing span and area) and two shape parameters (wing aspect ratio (AR) and the second moment of wing area).…”