Abstract
1. Landscape-scale clines in environmental parameters can lead to life-history gradients. Study of interspecific life-history differences can provide evidence of evolutionary adaptations in response to environmental cues, whereas examination of intraspecific life-history variation can identify the ecological mechanisms driving trait evolution.
2. We examined egg size and fecundity variation in three species of closely related freshwater fish (Galaxiidae) which occur across a habitat gradient in South Island, New Zealand. Inter-population trait variation was assessed in the species with the widest distribution to investigate the ecological mechanisms driving egg size evolution.
3. Substantial interspecific egg size and fecundity differences were observed. Galaxias paucispondylus, which generally occurs in high altitude headwater streams, showed an average egg size an order of magnitude larger, and fecundity an order of magnitude lower than G. brevipinnis, a species which rears in pelagic environments. Galaxias vulgaris, which rears fluvial environments but is generally distributed in lower catchment reaches than G. paucispondylus, showed intermediate fecundity and egg size traits. This interspecific trait variation is likely to be driven by differences in the larval rearing habitat, i.e. rearing in cold high altitude locations requires large well-developed larvae (and therefore large eggs) but these come at the cost of maternal fecundity, whereas in rearing in warmer lower catchment watercourses, and pelagic habitats, does not require such large well-developed larvae. Hence, fitness is maximised by progressively increasing fecundity by reducing egg size across this habitat cline.
4. Life history-habitat trends were repeated when intraspecific patterns in the widely distributed G. vulgaris were assessed, with population egg size increasing, and fecundity decreasing, in relation to a suite of interrelated environmental variables up an approximate altitudinal cline.
5. The results highlight how inter-and intraspecific variation in egg size-fecundity trade-offs in closely related species can interact to drive the evolution of landscape-scale life-history gradients. Egg size differences are likely to be driven by differences in the quality of the larval rearing habitat, i.e. rearing in a challenging habitat requires a larger egg whereas in relatively benign larval rearing habitat, fecundity can be maximised by reducing egg size. The interspecific variation observed provides evidence of trait adaptation in response to an environmental cue, while the intraspecific inter-population differences indicate the potential ecological mechanisms driving trait evolution.