Terrestrial–Aquatic Transitions Freshwater Biology 2020 Accepted
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Accepted version of MS published in Freshwater Biology 2020 DOI: 10.1111/fwb.13472 Terrestrial–aquatic transitions: local abundances and movements of mature female caddisflies are related to oviposition habits but not flight capability Jill Lancaster1, Barbara J. Downes1 and Georgia K. Dwyer1,2 1 School of Geography, University of Melbourne, 221 Bouverie Street, Parkville, Victoria 3010, Australia 2 Centre for Regional and Rural Futures, Deakin University, Locked Bag 20000, Geelong, Victoria 3220, Australia Corresponding author: Jill Lancaster, School of Geography, University of Melbourne, 221 Bouverie Street, Parkville, Victoria 3010, Australia Email: [email protected] RUNNINGHEAD: Terrestrial-aquatic transitions KEYWORDS: aquatic insect, dispersal, flight, Trichoptera, wing morphology Summary 1. 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. 2. We surveyed abundances of mature female caddisflies at adjacent riffle–pool pairs along short river lengths with homogenous 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 1 channel and aggregation in some locations. Wing morphology was used as a proxy measure of flight capability and included measures on wing span, area, aspect ratio and the second moment of wing area. 3. 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. 4. 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, including age- and sex- specific phenomena, may provide greater insight into the diverse ecological functions and consequences of movement. 1. INTRODUCTION Key life history events for adult aquatic insects include emerging from the aquatic to terrestrial environments, mating, potentially dispersing between populations and, for females, depositing eggs or neonates back into the aquatic environment. These events are poorly understood for the vast majority of species. 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, 2018), 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, 1993; Iwata, 2007). Similarly, at the transition from terrestrial to aquatic habitats, adults may aggregate at places where eggs are laid and produce local aggregation of juveniles, with potential consequences for spatial heterogeneity in consumer-resource dynamics for aquatic juveniles (Harrison & Hildrew, 2001). Thus, understanding sex- and age-specific distribution patterns of adults and how they arise, especially when life-cycle and habitat transitions are concurrent, may provide insights into how life history events affect population size (Hildrew et al., 2004; Encalada & Peckarsky, 2012; Lancaster & Downes, 2014). 2 Local distribution patterns of adults are likely to differ between the two aquatic–terrestrial habitat transitions (i.e. at emergence and oviposition) because adults of most species move away from water during maturation. Indeed, the final instar larvae of some stoneflies may travel long distances terrestrially before metamorphosing into adults (Kuusela & Huusko, 1996). Adult movements may range from short-distance movements within the immediate surroundings (Southwood, 1962), e.g. to avoid predators or locate resting sites, food and mates, to long-distance dispersal or migration (Anderson, 2009; Buden, 2010). The total distance travelled between emergence and oviposition will vary between species; some will travel multiple kilometres (Baldwin, West & Gomery, 1975; Masters et al., 2007) and even short-lived adults can move 10s to 100s of metres (e.g. Maciel-De-Freitas, Codeco & Lourenco-De-Oliveira, 2007). For many species, most individuals may spend their adult life within 50 m riparian corridors (Svensson, 1974; Petersen et al., 2004; Bogan & Boersma, 2012) but collective, short distance movements are sufficient to reduce the likelihood that an individual will emerge from and oviposit in the same location. Regardless of where adult aquatic insects go or how far they fly, at some point females must locate water and find an oviposition site. Water bodies may be located via the polarization of light reflected off water surfaces (Kriska, Horváth & Andrikovics, 1998; Horváth & Varjú, 2004), whereas the suitability of a particular water body may depend on water-borne chemicals indicating the presence of predators or competitors (Resetarits Jr., 2001; Brodin, Johansson & Bergsten, 2006). Locating an oviposition site within a water body may be straightforward for species that broadcast eggs, but complex for those with specialized oviposition behaviours. Oviposition site selection can involve a hierarchy of decisions at multiple spatial scales (Hoffmann & Resh, 2003; Lancaster, Downes & Reich, 2003; Reich & Downes, 2003b) and this may involve a lot of flying. Thus, female distribution patterns may vary with oviposition habits, as well as their flight capability and behaviour. Many stream-dwelling insects, for example, oviposit primarily in particular channel units, e.g. riffles or pools (examples below), and thus must be capable and willing to fly along channels to locate a suitable riffle or pool. There is ample evidence that many aquatic insects fly along channels (Winterbourn et al., 2007; Graham, Storey & Smith, 2017), but how far individuals actually fly is largely unknown. Prolonged flight is risky, e.g. aerial predation can be significant (Nakano & Murakami, 2001; Fukui et al., 2006), so the odds of survival may be correlated with flight capability and behaviour, and related to species-specific movement patterns (Svensson, 1974). The general aims of this study were to test hypotheses about the spatial distribution patterns of mature female caddisflies along short lengths of stream channels to elucidate how their local abundance patterns might be influenced by oviposition habits, flight capability and behaviour. Typically, newly emerged female caddisflies move away from water, mate and remain away from water during maturation (generally several weeks), before returning to water to oviposit (Svensson, 1972; Svensson, 1974). We know that some caddisfly species oviposit preferentially in particular channel units. For example, some species lay eggs on submerged rocks in pools (Deutsch, 1984; Lancaster, Downes & Arnold, 2010b), whereas others oviposit in 3 riffles and primarily on rocks that protrude above the water surface, i.e. emergent rocks (Hoffmann & Resh, 2003; Reich & Downes, 2003a; Lancaster, Downes & Arnold, 2010b; Lancaster & Glaister, 2019). However, numbers of such emergent rocks vary between riffles and there is often a correlation of oviposition site densities