Associate Editor, Bret Elderd discusses the ‘double-edged sword’ fire presents to endangered species, based around the recent article by Warchola et al, Balancing ecological costs and benefits of fire for population viability of disturbance-dependent butterflies. The article features in issue 55:2 of Journal of Applied Ecology.
In their recently published paper in Journal of Applied Ecology, Warchola et al. tackle a problem of conservation concern by constructing a mega-matrix model that accounts for both habitat dynamics and animal behaviour. Problems associated with habitat and behaviour are often difficult to tackle on an issue-by -issue basis and are rarely tackled in concert. For the US federally endangered, Fender’s blue butterfly (Plebejus icarioides), habitat dynamics are vitally important for it survival. The species is restricted to remnant native prairies of western Oregon, USA, where its two lupine host species occur. To maintain its habitat and assure that its population remains viable, the Fender’s blue needs fire. However, fire is a double-edged sword. At the time of the disturbance, fire can lead to an increase in larval mortality. But without fire, habitat begins to degrade, which can push these populations towards extinction. While there are short-term costs, Schultz et al. determine how to maximize the long-term benefits of disturbance given these costs.
By constructing a succession-based matrix model or a mega-matrix model, the authors examined a number of scenarios to ask how to best manage Fender’s blue populations. This included burns initiated by land managers versus the random burning of habitat, which is initiated naturally. They also investigated how butterfly dispersal within and between habitat patches may influence population dynamics. Schultz et al. found that directed burning of all habitats greater than three years old is the best strategy to ensure population viability. They also found local dispersal within the same successional stage adversely impacts populations to a greater extent than dispersing throughout the entire site, which contains a variety of successional stages. In terms of population dynamics, fire immediately decreases larval survival but results in an increase in fecundity during the two years post burn.
Thus, to maximize the benefits of fire for this endangered species, intermediate levels of disturbance are required within a patch. The results also show the importance of maintaining a pyrodiverse environment, measured as time since fire, across multiple habitat patches to ensure population survival. The work nicely demonstrates that considering the life history of the organism rather than focusing on specific life stages (e.g., larval survival alone) allows population demographers to understand how natural or managed disturbance impacts population growth and viability. These ideas and methods can also be applied across numerous species. In turn, the results stemming from these types of analyses can help direct management actions by identifying viable strategies for maintaining populations. As the authors point out, the use of simple population models holds incredible promise for assessing conservation priorities when managing populations of concern.