Black rat (Rattus rattus) invasion shapes endemic small mammal communities and reduces the body weight of the threatened brown mouse lemur (Microcebus rufus)

Invasive species are one of the major drivers of biodiversity loss worldwide, with particularly severe negative impacts on island ecosystems where endemic species tend to have small populations and limited defense strategies towards new competitors. This study examines how invasive black rats (Rattus rattus) affect native small mammal communities, with a particular focus on the threatened brown mouse lemur (Microcebus rufus), in regenerating forests of southeastern Madagascar. We used a chronosequence of 25 regenerating and mature forest sites where we captured small mammals for one year. Community composition analyses (NMDS and PERMANOVA) showed that black rat abundance significantly shaped native small mammal communities, with no differential effects between regenerating stages. While black rat abundance did not have a significant effect on brown mouse lemur abundance, it did have a significant negative effect on its body mass, particularly in mid-successional forests. We identified a critical black rat density threshold of approximately 3.3 individuals per 100 trapping nights, beyond which mouse lemur body mass declined sharply. Our findings show that invasive species exert a stronger influence on native small mammal community recovery than forest succession. Suppressing black rat populations below the identified threshold and focusing management efforts on Mid-successional forests could substantially mitigate their negative impacts on some native mammals.