Determinants of population structure in the world's smallest primate, <i>Microcebus berthae</i>, across its global range in Menabe Central, Western Madagascar

Abstract

This study investigated the population ecology of the world s smallest primate, Microcebus berthae, which is locally endemic to Menabe Central, the largest remaining tract of dry deciduous forest in Western Madagascar. Madame Berthe s mouse lemur is a highly charismatic and therefore a promising flagship species for the protection of this dry forest remnant. The ecological specialist regionally co-occurs with a closely related generalist, the more widely distributed and abundant Microcebus murinus that is competitively superior to M. berthae. Interspecific coexistence cannot be explained by ecological differentiation, as the congeners considerably overlap in feeding niches and microhabitat utilization. The mouse lemurs co-occur with several other members of the family Cheirogaleidae (Cheirogaleus medius, Mirza coquereli, and Phaner pallescens). I examined the ecological structure within this species assemblage across M. berthae s global range in order to identify determinants of this species distribution. The spatial distribution and abundance of cheirogaleids was assessed by distance sampling and trapping on various spatial scales and analyzed on the population as well as on the individual level. Results show that M. berthae s population is largely limited to pristine habitats and sensitive to human frequentation. On a regional scale, the mouse lemurs were found in interspecific negative association and habitat was partitioned along anthropogenic disturbance gradients during the dry season. In non-degraded habitat, interactions with third agents regulated interspecific competition of Microcebus spp. to a level stabilizing their coexistence. Mirza coquereli represents an intraguild predator of Microcebus spp. and exerts predation pressure disproportionally on M. murinus. Consistent with this notion, M. coquereli negatively affected M. murinus populations, but was positively associated with M. berthae on a regional scale. The species interspecific distribution across spatial and temporal heterogeneities affirmed M. coquereli s stabilizing impact on the coexistence of mouse lemurs. In local co-occurrence, intense interspecific competition between mouse lemur individuals was not indicated. Mirza coquereli expelled M. murinus individuals from their immediate vicinity only during the dry season, but did not impact the distribution of M. berthae individuals. In the rainy season, C. medius operated complementarily on the local scale, as individuals were found in negative association with inferior competitor M. murinus, but were positively associated with M. berthae individuals. I conclude that interspecific coexistence of Microcebus spp. is stabilized by a complex agent-mediated spatial storage effect. Co-occurring cheirogaleid species create refuges from competition for M. berthae in productive habitat, whereas anthropogenic environments provide M. murinus with exclusive resources and an escape from intraguild predation. Persistence of M. berthae does therefore not only rely on the maintenance of forested area in Menabe Central, but also on the preservation of habitat content. Extinction thresholds may be exceeded by ongoing habitat degradation as it corrupts fundamental interspecific mechanisms or causes the loss of a keystone species that stabilizes mouse lemur coexistence.