Science Spotlight: Mouse Lemurs and Edge Effects in Madagascar

Hello and welcome to the second edition of Science Spotlight! This edition is going to be looking at a new paper by Malabet et al.(2024), who have been researching how edge effects impact the density and distribution of mouse Lemur population in Madagascar. This study examined two sites in Northwestern Madagascar (see map below). The northern site in the yellow square is the Mariarano classified forest (MCF) which is an OpWall study site, and the southern area in the purple square is Ambanjabe forest fragment site (AFFS) in Ankarafantsika national park. The study focused on two species of mouse lemur, the gray mouse lemur Microcebus murinus and the golden brown mouse lemur Microcebus ravelobensis. No points for guessing how they got those names!

Study sites – Malabet et al, 2023

Mouse Lemurs make up the genus Microcebus and are exclusively nocturnal, so they spend their days curled up in nest holes up in the canopy. As they are omnivorous they spend their nights foraging for insects, fruits, flowers and other plants. They live in small, female-dominated groups of around 15 individuals. There are more than 20 species of mouse lemur and, like all lemurs, they are completely endemic to the island of Madagascar. Due to their secretive, nocturnal habitats mouse lemurs are comparatively poorly-studied compared to many other lemurs; indeed, several species of mouse lemur have only been discovered in recent years! This is very unusual for primates and begs the question, how much do we really know about these species? As you might guess, mouse lemurs are very small, weighing between 28 and 120g and are small enough to comfortably fit in a teacup! Unfortunately, mouse lemurs face a number of threats (hence all the more reason for us to learn as much about them as possible!). Although in theory they are legally protected from hunting, in practice they are still sometimes captured for the exotic pet trade. However, the most significant threat comes through the loss of forest habitats – Madagascar as a whole having lost more of its original forest cover than almost anywhere else on the planet.

Photo by Buck Denton

Forest loss and habitat fragmentation create edge effects; very simply if one big forest is cut up into lots of smaller patches, then there is far more forest boundary and a significant loss to interior forest. This increased ratio of edge habitats is one of the most significant problems with habitat fragmentation and it is now estimated that around 70% of global forest cover lies within 1km of a forest edge. Edge effects come about when abiotic factors creep into forest habitats and alter natural habitat structure. Specific effects include; loss of shade, moisture, wind protection and warmer temperatures. These can have a profound impact on a species population and can fundamentally alter the structure and function of plant and animal communities. To make it even worse, these edge habitats often have very poor regenerative abilities. The impact of edge effects varies between species and can be positive, negative or neutral – often due to the availability of food between edge, interior and non-forest habitat.

Understanding edge effects are an extremely important part of conservation research,  allowing scientists to determine which species will be most affected by habitat destruction and fragmentation. Such research is particularly relevant to Madagascar given that such a large amount of the forest habitat has been lost to agricultural land and what forest remains is extremely fragmented. It was previously known that edge effects have had mixed responses from mouse lemur species, but there was a need to understand the effects they had on the species found in the north of the island.

In this paper the authors asked two questions:

1. “Do two species of mouse lemurs vary in their distribution and population density patterns in response to forest edge structure?”
2. “Do density edge responses vary between abrupt edges and ecotone edges – as represented by the two landscapes sampled in this project?”

To answer these questions they used a statistical approach called a spatially explicit capture recapture model (SECR). Capture-recapture is a method for estimating the population size of a species by capturing it, marking it and then capturing it again in the future. As you repeat the process, you begin to catch more animals that you have already caught than new ones, and then you can use a formula to estimate the overall population size. SECR combines this information with the location of the traps across the study area and applies something called a spatial detection function, which is essentially a formula that looks at how often each individual is caught in each trap and can determine where the clusters of populations are. This SECR model can also consider environmental information, which allowed the authors to estimate how population density changed in relation to the forest edge.

The resulting prediction was able to answer both of the authors’ questions. The results indicated that the two species did have varying responses to edge effects. in-fact, across the landscape as a whole, they had completely opposite responses. M. murinus showed increasing density towards edges and M. ravelobensis decreasing. However, M. murinus showed contrasting responses at the two different sites, possibly due to the difference in edge structure, thereby answering question 2.

Overall results show that ‘landscape context’ can have a big effect on predicted population densities and edge responses. In other words, there are a number of factors that affect lemur populations at both large and small spatial scales. The study is one of the first to use the SECR model with live trapping, and is an important contribution for improving our understanding of how forest loss and edge effects are affecting elusive or rare species. Finally, the study also showed that some lemurs can show ‘ecological flexibility’ in response to forest loss, in other words some species will be worse affected than others.

Photo by Jack Hague

Thank you for reading and I hope this was informative. If you want to see some incredible wildlife and learn about how conservationists study these problems, why not think about joining an OpWall expedition! Or, if you want to read more about these issues then check out the references below.

Malabet, F. M., Ramsay, M., Chell, C., Andriatsitohaina, B., Radespiel, U., & Lehman, S. (2024). Where the small things are: Modelling edge effects on mouse lemur population density and distribution in northwestern Madagascar. American Journal of Primatology, e23621. https://doi.org/10.1002/ajp.23621

Julian Nicholas G. Willmer, Thomas Püttker, Jayme Augusto Prevedello, Global impacts of edge effects on species richness, Biological Conservation, Volume 272, 2022, 109654, ISSN 0006-3207, https://doi.org/10.1016/j.biocon.2022.109654.

https://www.nationalgeographic.com/animals/mammals/facts/mouse-lemurs

Title photo by Michael Darling