What are Edge Effects?

Edges effects are characterized by dynamic, ecological changes that occur at habitat boundaries; these include changes in species composition, gradients of moisture, sunlight, soil and air temperature, wind speed, etc. Hypothetically, if edge effects penetrate 300 m into a 100 ha square-shaped forest fragment, then approximately only 16% of the total forest amount will be unaffected by edge effects. For example, the image to the right shows a hypothetical forest fragment. The brown area represents forest habitats influenced by edge effects. The green interior is the only part of the fragment unaffected by edges. Moreover, shape can have a major influence on edge penetration in forest fragments. A long, narrow forest fragment may be completely dominated by edge effects (see below for example).

Why are Edge Effects Relevant to Forests & Primates in Madagascar?

Forest habitats are becoming increasingly fragmented in most tropical regions of the world. One of the most significant consequences of forest fragmentation is an increase in the amount of habitat edge. Edge effects are particularly relevant to studies of forests and primates in Madagascar. Lemurs are one of the most threatened primate taxa in the world due to the loss of 80% - 90% of forest habitats in Madagascar. The remaining forest is highly fragmented and, therefore, may be prone to extreme edge effects. For example, the picture to the right shows how human pertubations have altered forest landscapes in SE Madagascar. Most forest destruction occurs as the result of slash and burn agriculture and logging. However, there are few data on how edge effects influence lemur and forest ecology in Madagascar.

How do Lemurs Respond to Edge Effects?

To date, our research team has focused on edge responses in four nocturnal lemur species (Avahi laniger, Cheirogaleus major, Lepilemur sp., & Microcebus rufus) and four diurnal lemur species (Eulemur fulvus rufus, Eulemur rubriventer, Hapalemur griseus griseus, & Propithecus edwardsi). Our research indicates that some lemurs--such as E. rubriventer (see photo to right)--exhibit a neutral edge response (no differences in densities between edge and interior habitats). Conversely, the smallest-bodied (M. rufus) and largest-bodied (P. edwardsi) species show a positive edge response (higher densities in edge habitats), which may be related to edge-related variations in food abundance. C. major tends to exhibit a negative edge response (lower densities in edge habitats), which we think may be due to spatial patterns of resource availability and predation pressures. The ecological correlates to these biogeographic patterns are complicated and are the subject of on-going research projects by members of the TREE program.