Coupling field surveys, forest mapping, and simulation models to understand the future of a Madagascar corridor

In spite of their popularity in landscape management, understanding how corridors function has remained challenging – particularly at the very large spatial scales over which landscape-scale conservation often occurs. This challenge stems in large part from a simple lack of data – how taxonomically-diverse species respond to corridors – but also because we know little about how these responses will be affected by dynamic patterns of land use and habitat protection. Important questions revolve around not only how corridors work today, but how they will work if they are built, enlarged…or destroyed at time points in the future.

A new study by Tanjona Ramiadantsoa and colleagues confronts these challenges to understand the current and potential future functioning of a large forest corridor in Madagascar. This corridor is >200km long and 2-50km wide, connecting two large (311 and 416 km²) forested national parks (see map).  This region is experiencing active forest destruction; predicted patterns of forest cover are also shown. Several major findings come from this study.

First, the authors conclude that this is a high quality corridor, relative to the parks it connects. Using census data for 300 species of birds, amphibians, reptiles, small mammals, and lemurs, they find little difference in the diversity of vertebrate animals living in the corridor vs. parks. They suggest that this is likely due to the width of the corridor, which is similar to that of the parks.

Second, the authors employ a simulation model to understand which types of species will be most impacted by the predicted future destruction of this corridor. They find that passively-dispersing organisms that disperse long distances, such plants or fungi blown in the wind, will be most strongly impacted, and that actively-dispersing organisms will be least impacted. The reason for this is that, as the corridor is fragmented, long-distance, passively-dispersing species will increasingly disperse from forest (habitat) into areas of cleared forest (non-habitat). This finding reinforces recent calls for better consideration of passively-dispersed organisms in conservation, including in the face of habitat fragmentation and corridor implementation (Damschen et al. 2014, Teller et al. 2015).

Finally, the authors show that for a wide variety of species inhabiting the two national parks, there will be severe consequences of destroying the forest that currently forms this corridor. Based on current rates of forest clearance (>1% forest loss per year), this corridor will become severely broken by 2065. Even before this – by 2040, when the corridor is expected to be fragmented into five forest ‘stepping stones’ – they expect significant declines in gene flow for species between the two national parks. Clearly, this represents a call to arms for added protection of the forest in a remarkable corridor, to assist conservation of a world biodiversity hotspot.