Fire regime effects on annual grass seeds as food for threatened grass-finch

Background: The Gouldian finch (Erythrura gouldiae, Gould 1844) is a threatened grass finch (Estrildidae) endemic to the tropical savannas of northern Australia. Current fire regimes, consisting of frequent and extensive fire across these savanna grasslands, affect the type and availability of grass seed for granivores. Gouldian finches are particularly affected as they feed exclusively on grass seed and, unlike other finches, do not supplement their own or their hatchlings’ diet with other protein sources. Annual Sorghum spp. provides the main source of seed to Gouldian finches throughout the monsoonal dry season and concurrent breeding season, making it a critical resource throughout breeding habitat. This study examined the effects of fire regimes, including fire frequency, time since the last fire, seasonality of fire on plant density, seed production, and overall seed abundance of the annual grass Sorghum stipoideum (Ewart & White) C.A. Gardn. & C.E. Hubb. Monitoring of S. stipoideum took place across Gouldian finch breeding habitat over three consecutive years and these measures were used in conjunction with local fire history at these sites to test for effects of fire attributes on Sorghum spp. seed ecology.

Results: We found that seasonality of fire had the greatest impact on S. stipoideum plant density and overall seed abundance, with early dry season fires resulting in 25% higher plant and seed density compared to late dry season fires. Seed production per plant peaked at three years post fire but then declined. There was no significant influence of fire frequency in the analysis.

Conclusions: Although fire effects were detected, these were muted within current fire regimes experienced in the region, and it is unlikely that appreciable impacts would occur on S. stipoideum seed availability to Gouldian finches while breeding. However, reduced seed density resulting from repeated high intensity fires could lead to exacerbation of food shortages postulated for Gouldian finches in the late dry season, as seeds naturally become scarce at the soil surface where finches forage. Early dry season fires maximize Sorghum spp. seed abundance. These findings support the implementation of low intensity early dry season burning to promote optimal food and breeding resources for threatened finches.