Collapse of invasive competitor expands distribution of endangered ecosystem engineer

Negative interactions among invasive predators, competitors and native species can often disrupt ecosystem services, particularly when keystone species are affected. The Key Largo woodrat (Neotoma floridana smalli) is an endangered ecosystem engineer, endemic to protected hammocks on the northern third of the island of Key Largo, FL, USA. Invasive predator control efforts have assisted in woodrat recovery, but less is known about how a potential competitor, the black rat (Rattus rattus), interacts with the woodrat. We conducted camera trap surveys at supplemental nest sites throughout the range of the woodrat and used multi-species occupancy models to investigate factors influencing woodrat and black rat co-occurrence. Supplemental nest sites were surveyed each year over a three-year period, during which the population of black rats within the protected hammock declined precipitously. Woodrats and black rats occurred at similar levels in the first survey with occupancy probabilities of 22.8% and 15.6%, respectively. Both species occurred at fewer sites in the second survey, but while woodrats later rebounded, black rats were only detected twice in the final survey. There was evidence of species avoidance between the rodents based on a species interaction term, and they exhibited opposing relationships with predictor variables of both detection and occupancy probabilities. As the occupancy of black rats decreased, the relationship between woodrats and distance from developed areas also weakened. Following the black rat decline, woodrats recolonized an area previously dominated by black rats, where they had been presumed extirpated for 2-3 decades. Our results indicate that black rat competition restricted occupancy of Key Largo woodrats, hindering recovery efforts that previously concentrated on invasive predator removal, demonstrating the need to consider influences of both invasive predators and competitors in species recovery programs.