The effects of fire and fragmentation upon two threatened coastal heath species, Acacia baueri ssp. baueri (Mimosaceae) and Blandfordia grandiflora (Blandfordiaceae)

Gabriel Conroy

doi:10.25907/00611

Anthropogenic threats to biodiversity in coastal heath ecosystems include direct habitat loss, fragmentation, and alterations to fire, nutrient and hydrological regimes. This study used two iconic taxa, Acacia baueri (vulnerable) and Blandfordia grandiflora (endangered) to investigate the impacts of fire and fragmentation on threatened herb layer species from the subtropical 'wallum' heathlands of southeast Queensland, Australia. A minimum of eight populations of each species were sampled from both fragmented and natural heathland zones to collect demographic, reproductive and fire history data. Genetic analysis was also undertaken using microsatellite markers to determine the population genetic structure of A. baueri. Population demographic, reproductive and genetic data were subsequently analysed for relationships with fire history and population isolation, and to determine any differences between fragmentation zones. To examine the influence of fire frequency, stage structured population viability analysis (PVA) models were constructed for each species to examine the potential outcomes of the following five fire management options; (1) Fire exclusion, (2) Wildfire only, (3) 3-yearly prescribed burn, (4) five-yearly prescribed burn, (5) 7-yearly prescribed burn. Fragmentation was found to have a pronounced effect on the demographic and reproductive characters of both species. For A. baueri, populations in the unfragmented zone were significantly larger (t=2.327, p<0.05) and with higher percentages of reproductive individuals (t=2.337, p<0.05) compared to fragmented zone populations. Blandfordia grandiflora had more flowers per reproductive individuals (t=5.377, p<0.01) and more seedpods per flower (t=4.414, p<0.05) in unfragmented zone populations. However, population isolation was not significantly (p>0.05) related to any demographic or reproductive characters for either species. Acacia baueri had low population level genetic diversity (A=1.64; %P=40), high levels of inbreeding (F=0.56) and low population differentiation (FST=0.177), with the majority of variation found within populations (FIS=0.470). Although 10 of the 12 private alleles were found in unfragmented zone populations, there were no other significant (p>0.05) differences among diversity and inbreeding measures between populations in either the unfragmented or fragmented zone. However, in accordance with the expectations of conservation genetic theory, significant positive relationships between increasing population size and genetic diversity (He, r=0.481, p<0.05; %P, r=0.479, p<0.05), and negative relationships between population isolation and diversity (Ap, r=-0.587, p<0.05), were apparent. Results of the PVA analysis indicated that both A. baueri and B. grandiflora benefit from a fire return frequency of 3-5 years which is substantially shorter than current management recommendations. PVA also predicted that longer fire intervals, the occurrence of only wildfires and fire exclusion would all lead to localised population extinction at relatively short time scales for both species (fire exclusionâ‰ˆ<20 years; 7-yearly firesâ‰ˆ20-40 years; wildfire onlyâ‰ˆ40-60 years). In terms of management implications, results from the fragmentation component of the study highlight the importance of conserving populations in protected areas, of maintaining larger populations and the need to maintain habitat connectivity and quality in the inter-remnant landscape. In the probable continued absence of natural fire regimes in the region, prescribed burning involving spatial and temporal variability (patch-mosaic-burning) may be an option to provide an equitable balance for both biodiversity maintenance and the persistence of more specialised threatened species. Overall, while habitat fragmentation appears to have had negative ecological and genetic consequences for the species, inappropriate burning regimes likely constitute the most immediate and prominent threat to the persistence of both A. baueri and B. grandiflora..

The effects of fire and fragmentation upon two threatened coastal heath species, Acacia baueri ssp. baueri (Mimosaceae) and Blandfordia grandiflora (Blandfordiaceae)

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