Effects of Red Imported Fire Ants (Solenopsis invicta) on Juvenile Houston Toads (Bufo houstonensis) in a Coastal Prairie Grassland
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The Houston toad (Bufo houstonensis) was first described in 1953 in Houston, Texas, but has since been extirpated from the area. Houston toad populations have been in a nearly continuous decline across their known distribution since discovery, primarily due to multiple stressors, including red imported fire ants (Solenopsis invicta; hereafter referred to as RIFA). In spite of the uncertainty of historical presence, the 1984 Recovery Plan attempted to reintroduce the Houston Toad into coastal prairie habitats. Although originally thought unsuccessful, the coastal prairie proved to be suitable habitat, even if only as dispersal habitat. In 2015, on Attwater Prairie Chicken National Wildlife Refuge (APCNWR), a total of forty-eight exclosures were placed in four prairie locations (12 exclosures per site) two of which were treated for RIFA and two prairie locations were used as untreated controls. Morphometric data (snout-urostyle length, head-width, and weight) were collected for all toadlets were detected on a weekly basis, slowing to bi-weekly after six weeks. A mixed-effects for repeated measures model was used in R to evaluate growth rates between treatment and control areas, which showed no difference in growth between treatments (F-value=1.747, df=42.7, 45, p=0.09) or density (t-value=-1.095, df=140.61, p>0.1). Program MARK was used to estimate survivorship and detection between treatments using a Cormack-Jolly Seiber (CJS) model. The model chosen, using ΔAICc, assumed that detection and survivorship changed through time but not between treatments. Because there was no difference in growth or survivorship, I fail to reject our null hypothesis that RIFA has a negative impact on the survival and growth of juvenile Houston Toads. A trend seen in the data comparing the exclosures in the open prairie to those within the drip line showed higher survival within the drip line, but much faster growth in the open prairie. This supports that connectivity of habitats is vital for the survival of juvenile Houston toads. However, because it has been shown that Houston toads are able to persist on the RIFA controlled prairies of APCNWR, the area of suitable Houston toad habitat can now be more explicitly delineated to include native grasslands, particularly for dispersal habitat. These landscape-connecting habitats are one of the most critical and least understood ecological aspects for Houston toad management. The results from this study also clearly assist with assessing new sites for reintroduction through propagation and population restoration efforts.