Shrub Encroachment Threatens Persistence of An Endemic Insular Wetland Rodent

Abstract

Shrub encroachment is altering the structure and species composition of freshwater wetlands across the globe. These changes are likely to be particularly detrimental for threatened wetland vertebrates. However, little research has focused on how shrub encroachment influences threatened wetland vertebrates. We sought to determine how vegetative structure and shrub cover influenced the occurrence of a threatened semi-aquatic mammal species, the Sanibel Island rice rat (Oryzomys palustris sanibeli). Using aerial photography from across the Sanibel Island rice rat’s range, we found a 5.5-fold increase in shrub cover over a 71-year period. To link these changes to the current distribution of the species, we evaluated the influence of vegetation and metrics of inundation on probability of occurrence and localized colonization and extinction rates over a three-year period. We found Sanibel Island rice rats on 18 (33.3%) of our 54 sites. Their occurrence was positively associated with elevated sand cordgrass (Spartina bakeri) cover and increased elevation, but negatively associated with elevated shrub cover. Their probability of colonization was negatively associated with increased shrub cover. Extinction probabilities for Sanibel Island rice rats were positively associated with increased rainfall totals in the preceding three months and increased shrub cover. Increases in shrub cover reduce sand cordgrass cover, which additively may be driving the decline of this once ubiquitous species. We conclude that shrub encroachment of freshwater wetlands requires greater attention due to its potential to imperil wetland endemic wildlife species.

Oryzomys palustris (Rodentia: Cricetidae)

“…Oryzomys palustris is classified as "Least Concern" (LC) and as a G5 (secure) species by the International Union for Climate change and its attendant sea-level rise will likely eliminate the island subspecies sanibeli and planirostris in southern Florida, due to the low elevation of the islands and their exposure to increasingly violent hurricanes. Based on the analysis of aerial photos of the 4,900-ha Sanibel Island taken over the past 70 years, increasing encroachment of shrubby vegetation is evident on the island, resulting in the loss of open wetland areas (Boone et al 2021). Continued loss of such prime habitat of the endemic O. p. sanibeli will threaten the viability of extant populations.…”