Fine‐scale refuges can buffer demographic and genetic processes against short‐term climatic variation and disturbance: a 22‐year case study of an arboreal marsupial

Abstract

Ecological disturbance and climate are key drivers of temporal dynamics in the demography and genetic diversity of natural populations. Microscale refuges are known to buffer species' persistence against environmental change, but the effects of such refuges on demographic and genetic patterns in response to short-term environmental variation are poorly understood. We quantified demographic and genetic responses of mountain brushtail possums (Trichosurus cunninghami) to rainfall variability (1992-2013) and to a major wildfire. We hypothesized that there would be underlying differences in demographic and genetic processes between an unburnt mesic refuge and a topographically exposed zone that was burnt in 2009. Fire caused a 2-year decrease in survival in the burnt zone, but the population grew after the fire due to immigration, leading to increased expected heterozygosity. We documented a fire-related behavioural shift, where the rate of movement by individuals in the unburnt refuge to the burnt zone decreased after fire. Irrespective of the fire, there were long-term differences in demographic and genetic parameters between the mesic/unburnt refuge and the nonmesic/burnt zone. Survival was high and unaffected by rainfall in the refuge, but lower and rainfall-dependent in the nonmesic zone. Net movement of individuals was directional, from the mesic refuge to the nonmesic zone, suggesting fine-scale source-sink dynamics. There were higher expected heterozygosity (HE ) and temporal genetic stability in the refuge, but lower HE and marked temporal genetic structure in the exposed habitat, consistent with reduced generational overlap caused by elevated mortality and immigration. Thus, fine-scale refuges can mediate the short-term demographic and genetic effects of climate and ecological disturbance.

Where do animals come from during post‐fire population recovery? Implications for ecological and genetic patterns in post‐fire landscapes

“…Two other mammals that occur in these forests serve as ‘bookends’ to illustrate the range of potential recovery patterns and their dependence on resource distribution. The tree hollow‐dependent but dietary generalist mountain brushtail possum Trichosurus cunninghami had no major limiting changes in abundance or resource distribution after the fire (Lindenmayer et al , Banks et al ). In contrast, the tree hollow‐dependent and folivorous greater glider Petauroides volans requires unburnt eucalypt forest canopy for foraging and is restricted to a coarse‐grained patchy distribution after this fire (Berry et al ), typically in patches of unburnt forest canopy in deep‐sheltered gullies (Leonard et al , Berry et al ).…”

“…low population density relative to habitat quality) in burnt areas (Lehtinen andGalatowitsch 2001, Jaquiery et al 2008), which may occur if fire reduces population abundance by directly killing individuals, instead of by limiting resource availability in the post-fire environment. However, post-fire resource limitation is likely to be common for species other than early successional specialists (Lindenmayer et al 2013, Nimmo et al 2014, and recently-burnt habitats are unlikely to be attractive resource-rich dispersal destinations for many species (Banks et al 2015).…”

“…However, these predictions are dependent on the scaling of disturbance size and dispersal distance, with population distribution and genetic diversity less sensitive to fire size or frequency when dispersal capacity is high relative to the size of fires (Davies et al 2016).Ecological disturbances such as wildland fires can have major impacts on the distribution and abundance of species and the genetic diversity of populations (Turner 2010, Banks et al 2013. While the abundance of some species is largely unaffected by disturbances such as fire (Banks et al 2015), disturbance events cause severe declines in abundance, or local extinctions, for many others (Lindenmayer et al 2013). For species negatively impacted by disturbance events, identifying the source of individuals for population recovery is an important research issue.…”

A comparison of regression methods for model selection in individual‐based landscape genetic analysis

“…For many species, persistence in a rapidly changing environment will require landscapes permeable to movement such that they can track suitable habitat over time (Rudnick et al., ). Permeable landscapes also serve to unite patchily distributed populations into larger aggregates, thereby improving genetic and demographic viability (Banks et al., ). Understanding spatial genetic patterns and identifying migration barriers is therefore critical to conservation efforts designed to mitigate current and future threats (e.g., construction of wildlife crossing structures and conservation of habitat in key connectivity corridors).…”

Effects of crowding due to habitat loss on species assemblage patterns

“…Crowding in refuges is assumed to be a minor disturbance, allowing populations to reorganize in the landscape and preventing impoverishment of genetic diversity (Banks et al. ). Planned faunal refuges allow for reconstitution of lost habitat and thus the return of animals to their original geographic areas.…”