How do fire regimes and an invasive grass affect habitat occupancy of an Australian songbird?
Fire is a driving force of ecological change and nutrient cycling in tropical ecosystems. Fire regimes maintain vegetative heterogeneity and influence available habitat. Intensity and patterns of burns are dictated by biomass during the dry season. Invasive vegetation frequently has deleterious effects on biota in tropical grasslands as it alters the availability of that biomass. Andropogon gayanus (gamba grass) is a highly invasive species in northern Australia with biomass greater than four times that of native species. The rapid spread of gamba grass is a threat to human health and safety, as high intensity fires spread quickly through the canopy of tropical grasslands and burn down anthropogenic structures. Cool “controlled” burns are a management strategy used to reduce biomass before late dry season fires, yet in the long term, cool burns facilitate the spread of gamba grass. While many studies have investigated the influence of gamba on fire regimes, few have examined the how gamba grass affects native biota. This study explores the effect of gamba grass on ecosystem health by using a mid-trophic level passerine species as a proxy. Malurus melanocephalus (redbacked fairy-wrens) depend on grasses to feed, shelter, and breed, making them a suitable species to study the biotic consequences of gamba and fire. Our study took place on Coomalie Farm, Northern Territory Australia (13.0667° S, 131.0167°E) during the dry season of 2014. We performed line transect surveys to determine occupancy of redbacked fairy-wrens before and after a naturally intense wildfire. Vegetation surveys (n=224) were collected throughout the study site to quantify vegetation types at occupied and unoccupied sites. Surveys also measured fire intensity and severity after natural fire events.
Our results show that redbacked fairy-wrens preferentially use gamba grass, a habitat that burns at high intensity and decreases after fire events. Current fire strategies for management of gamba grass facilitate its spread, which leads to a long term decline for redbacked fairy-wren populations. To our knowledge, this is the first study to examine ecosystem impacts of gamba grass, and we put forth a framework for more effective invasive grass management.