The influence of drought on Neochanna apoda metapopulation persistence under global warming and land-use change.

Abstract

Population size is the primary criteria used globally to determine species extinction risk and prioritise conservation risks due to the widely documented positive scaling of population persistence with abundance. However, it is unknown how such scaling is affected by land-use change combined with climate change which, for many populations, is expected to decrease population growth rates and increase population variability. Moreover, it is unknown how such changes to scaling in sub-populations will impact the persistence of larger interconnected metapopulation networks. Using empirically-derived models of Neochanna apoda (brown mudfish) metapopulations, I investigated the interactive effects of land-use- (forest clear-felling) and climate-change (extreme drought frequency) on the scaling of population persistence with carrying capacity and quantified how changes in scaling controlled metapopulation persistence. The metapopulation matrix model was parameterised using data from a long-term mark-recapture study of over 70 brown mudfish sub-populations living in forest pools affected by frequent extreme droughts (including a 1/25 year extreme drought), historic clear-felling and wind disturbances. After correcting these data for climate driven uncertainty in capture probability using Cormack-Jolly-Seber models, I found that mudfish survival during droughts was high for populations occupying pools deeper than 139 mm, but declined steeply in shallower pools. This threshold was caused by an interaction between increasing population density and drought magnitude associated with decreasing habitat size, which acted synergistically to increase physiological stress and mortality. Pool depths were lowest in forests affected by historic clear-felling due to the absence of large trees to fall over and excavate deep pools. Consequently, the metapopulation matrix model parameterised by these data showed that the scaling of time-to-extinction with carrying capacity in sub-populations was driven by an interaction between land-use change (forest clear-felling) and increasing extreme drought frequency with global warming. Population persistence increased exponentially with carrying capacity in large stable habitats, but this relationship was asymptotic at small population sizes in shallow habitats contracted in size by forest logging. Metapopulation persistence in logged forests dropped by over 50 percent due to such asymptotic scaling and lost persistence of large populations. Thus even large populations are likely vulnerable in stochastic environments, with their loss having disproportionately large negative effects on metapopulation persistence in landscapes affected by human disturbances. These results confirm longstanding theory predicting asymptotic population size-persistence thresholds under environmental stochasticity, and by doing so, highlight the keystone role large populations play in mitigating the impacts of global warming and land-use change.