Life history variation in the Galaxias vulgaris complex: implications for interactions with invasive salmonids

Abstract

A proliferation of recent studies has highlighted the global prevalence of cryptic species-complexes, but life history study of these newly identified species has not kept pace. Given the dominant role that life histories can play in determining how species respond to extinction threats, this represents a serious knowledge gap in conservation biology. This thesis examines interspecific trait variation in a threatened species-complex of non-migratory galaxiid fish and assesses its influence in determining species’ vulnerability to a principal cause of their decline, i.e. negative interactions with invasive salmonids.

A suite of divergent life history strategies were identified across species within the Galaxias vulgaris complex which were associated with the contrasting habitat types they occupy. Species predominantly found in low productivity, but stable, headwater creeks showed larger egg sizes (mean diffs. of up to over 130%) and lower size-relative fecundities (mean diffs. of up to over 240%) than species occurring in higher productivity, but frequently disturbed, lower catchment streams. Headwater species also delayed reproduction by at least one or two years and showed lower reproductive effort compared to lower catchment species. Larvae of headwater species hatched large, with many morphological features well developed, and had better swimming ability; traits which are likely to be advantageous in the resource-scarce habitats they occupy. The lower catchment species had small and relatively altricial larvae with poor swimming ability; a strategy which promotes high fecundity in these resource-rich environments. Larval abundance was consistently low in streams inhabited by headwater species, and their larger, better-swimming larvae showed minimal downstream dispersal (<300 m) from natal spawning areas. Contrastingly, in lower catchment species, larval production was high and their small larvae dispersed large distances (up to over 12 km downstream). Analysis of a broad-scale presence-absence dataset revealed that headwater species showed minimal overlap with salmonids (<6% of reaches) whereas the lower catchment species regularly co-occurred with salmonids (>50% of reaches). While habitat differences explained some of the co-occurrence/exclusion patterns, species’ life histories were consistently a strong determinant of galaxiid-salmonid interactions. Low larval abundance and poor dispersal in headwater species mean they form isolated populations and are generally completely excluded from salmonid-invaded reaches. In contrast, the large numbers of dispersive larvae in lower catchment species support a source-sink metapopulation system (Woodford & McIntosh, 2010) whereby populations in salmonid-invaded reaches are sustained by immigration from upstream salmonid-free refugia. The ‘slow’ life histories exhibited by headwater species make them inherently vulnerable to salmonid pressures whereas the ‘fast’ life histories shown by lower catchment species may explain their continued presence in the often highly invaded streams they occupy. This thesis demonstrates the importance of detailed life history study in elucidating interspecific differences, which can have major consequences for conservation management. It shows that cryptic diversity can represent fundamentally different species, which can behave ecologically very differently, despite their similar adult appearance. Given the consequences of trait differences for population dynamics and the implications for how species respond to conservation threats, this highlights the importance of traits-based research, particularly concerning the early life history stages.