Trade-offs and dispersal in unpredictable environments: Generalist invertebrate persistence across spatially and temporally fragmented pond

Background/Question/Methods

Variable hydrology in lentic habitats poses a variety of life-history challenges for generalist species that persist and disperse across this range of habitats. Invertebrates in temporary habitats must develop quickly and disperse as adults, or have wide environmental tolerances. Conversely, in permanent habitats they must avoid a suite of vertebrate and invertebrate predators (e.g., fish and dragonflies). These diverse conditions result in life-history trade-offs that influence fitness, population dynamics and genetic structure. In addition, recruitment between habitats may be crucial to maintain generalist populations. This is particularly important under temporal fragmentation that results from unpredictable drying in temporary habitats and requires subsequent recolonization from permanent habitats. These metapopulation dynamics sustain regional populations, but may cause population bottlenecks during dry years when temporary habitat larvae are unable to complete development and do not contribute to regional gene pools. We examined Xanthocnemis damselfly and Sigara waterboatman populations in New Zealand lakes and irregularly drying ponds through field surveys, mesocosm experiments and microsatellite analyses to test whether: (1) generalists have flexible developmental and behavioral traits to survive different habitat conditions, and 2) terrestrially dispersing generalists have bet-hedging behaviors and oviposit in both habitat types to maximize reproductive potential.

Results/Conclusions

Both species developed faster in temporary habitats, and were more susceptible to fish than dragonfly predation. However, they used alternative life-history strategies to exploit habitats across the predator-permanence gradient. Xanthocnemis had longer development time and were consequently exposed to more biotic and abiotic stressors. This exposure was offset by behavioral flexibilities which enhanced predator avoidance in natal habitats and increased drying tolerance. In contrast, Sigara's rapid life cycle enabled them to exploit more temporary habitats, but they had little behavioral or developmental variation when exposed to predators and were intolerant of drying. Rapid adult colonization and oviposition by both species in temporary habitats was consistent with bet-hedging supporting our second hypothesis. Overall, our results indicate that although generalists have different life-history strategies, dispersal is critical to the maintenance of generalist populations across habitats with unpredictable disturbance regimes. While dispersal and life-history flexibilities allow generalists to exploit unpredictable habitats, our analysis of microsatellites indicates irregular disturbance of these habitats results in bottlenecks that probably limit genetic variation and speciation. Such knowledge of mechanisms allowing generalist species to persist across spatially and temporally fragmented habitats is important for understanding for how species may respond to further unpredictable habitat fragmentation due to altered hydrology under climate change.