Itinerant, nomad or invader? A field experiment sheds light on the characteristics of successful dispersers and colonists

Abstract

Many hypotheses in ecology depend upon identifying species that can disperse across landscapes and colonise new habitat patches. Species differ in dispersal ability, yet some good dispersers reach new locations but fail to colonise them (“itinerants”). Additionally, successful colonists may comprise species that routinely disperse and maintain connectivity among locations (“nomads”) as well as species that can also take advantage of new situations (“invaders”). In a previously reported landscape‐scale, field experiment that boosted detrital resources in Hughes Ck (south‐eastern Australia), species diversity increased at manipulation sites due to successful dispersal and colonisation. The species composition at manipulation sites gradually converged on that seen at upstream sites, implying the latter locations supplied dispersers. Here, we ask (a) Did dispersers arise from within the same creek, and were drift and adult flight both implicated? (b) Were successful colonists (“responders”) clearly better dispersers (e.g., nomads or invaders) than those that did not colonise manipulation sites (“non‐responders”)? (c) Did traits commonly assumed to denote successful dispersal provide a reliable guide to species that were actually successful colonists? Simultaneous with benthic samples, we collected animals in the drift immediately upstream of control and manipulation sites, and trapped winged adults along the banks. We used PERMANOVA to contrast (a) the assemblages of species in the drift with those in the benthos and (b) the assemblages of adults, at different sites and times. For 54 common taxa (26 responders; 28 non‐responders), we used linear regression to test whether benthic densities were significantly related to drift numbers and, if so, whether the relationship was the same at manipulation and control sites (“nomads”) or delivered higher densities at manipulation sites (“invaders”). We compiled information on dispersal traits (presence in drift, voltinism, flight‐capable adults) to assess whether dispersal traits predicted species that successfully colonised manipulation sites. Drift assemblages were more similar to benthic assemblages at manipulation sites than to those at control sites. Adult assemblages did not differ between manipulation sites and controls, but adult assemblages at downstream sites converged on those seen upstream. Many dispersers arose from within Hughes Creek, with ˜60% of common taxa arriving via the drift. Among the common taxa, 4% were poor dispersers (rarely in the drift) and 33% were itinerants (drift and benthos were unrelated), with these categories equally represented among responders and non‐responders. In the remaining taxa, benthic densities were related to drift numbers. Most responders were invaders, whereas most non‐responders were nomads. Responders and non‐responders did not differ in traits that are commonly assumed to reflect a high potential for dispersal and colonisation. Responders were not demonstrably better dispersers than non‐responders but instead we...

Multiyear resource enrichment creates persistently higher species diversity in a landscape‐scale field experiment

“…The study site and experimental design have been described in detail elsewhere (Downes et al 2017, Downes and Lancaster 2018, so only a brief description is provided here.…”

“…Dispersal within the metacommunity facilitated rapid species responses (<1 year), with no evidence that new taxa colonized from outside the catchment. Dispersal was not a constraint for any species examined (Downes and Lancaster 2018). To enrich resources, we increased channel retention and this boosted detrital densities (leaves and small woody debris) over five years, which corresponds to at least five generations for invertebrates in that system.…”

Aquatic versus Terrestrial Insects: Real or Presumed Differences in Population Dynamics?

“…In contrast, drift was prevalent in driving benthic density increases during a 12-month landscape-scale experiment that boosted resources of food and living space at sites distributed along a stream gradient [156]. Drift was associated with benthic densities in ≈60% of common taxa, and some upstream species moved downstream by up to 20 km by using manipulation sites as stepping-stones [157]. Not all drifters successfully colonised experimental sites, however, and some of the unsuccessful species were closely related taxonomically to species that were successful.…”

“…In summary, dispersal by stream insects is still greatly under-studied. It is likely that for many species both drift and adult flight play roles in setting population structure [157], but there is little evidence that populations are characterised by frequent drift dispersal downstream followed by adult flight upstream, nor is this necessary for population persistence [162,163]. Even so, modelling studies continue to be published that assert that downstream drift is pervasive among stream insects, and results in long-distance dispersal that must be countered by upstream flight by adults [175,176,177] in a seeming triumph of myth over evidence.…”

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

“…Even quite basic information about local distribution patterns of adults is scarce, in sharp contrast to a plethora of studies on the distribution of larvae within water bodies. The local distributions of terrestrial adults and aquatic juveniles may be correlated in some circumstances (Downes & Lancaster, ), and such associations may have consequences for population dynamics in both terrestrial and aquatic environments. At the transition from aquatic juveniles to terrestrial adults, for example, variations in benthic densities of larvae and pupae may correspond to local variations in the abundance of emerging adults, and the abundance of their terrestrial predators (Gray, ; Iwata, ).…”