What are Edge Effects?

Edges effects are characterized by dynamic, ecological changes that occur at habitat boundaries; these include biotic factors--such as changes in species composition-- and abiotic factors--such as 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. Moreover, fragment shape, orientation, and altitude as well as matrix conditions can have a major influence on edge penetration in forest fragments. For example, a long, narrow forest fragment may be completely dominated by edge effects. Recent studies indicate that roads, such as the one seen in the photo to the left and even seemingly undistrubed habitats, such as riparian forests, create edges. Finally, some species exhibit only seasonal or successional responses to edges, which further complicates our understanding of dynamic edge processes.

Why are Edge Effects Relevant to Forests and 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 the forested regions of Madagascar, where anthropogenic disturbance over the last 50 years has resulted in the loss of half of the original forest cover. Forest conversion is due primarily to slash and burn agriculture and anthropogenic fires used to create cattle pasture. The remaining forest is highly fragmented and prone to extreme edge effects, as evidenced by a ground-breaking GIS study of change in forest loss and fragmentation over a period of fifty years (Harper et al., 2007). Of the few in situ studies of edge effects conducted to date, most have documented how plant and animal abundance covary with edge proximity in forest fragments. For example, trees in edge habitats are significantly shorter, have smaller diameters, and lower stem frequencies than trees in interior habitats in eastern humid forests. However, there are few data on the biological processes resulting from edge creation, particularly in the rare tropical dry forests of Madagascar.

How do Lemurs Respond to Edge Effects?

To date, our research team has focused on determining edge responses in nine lemur species that range into eastern humid forests (Vohibola III Classified Forest) and congenera that are found in western dry forests; specifically, those in and around Ankarafantsika National Park, NW Madagascar. In the humid forests, we found that some lemurs--such as Eulemur rubriventer (see photo to left)--exhibit a neutral edge response (no differences in densities between edge and interior habitats). Conversely, the smallest-bodied (Microcebus rufus) and largest-bodied (Propithecus edwardsi) species show a positive edge response (higher densities in edge habitats), which may be related to edge-related variations in food abundance. Cheirogaleus 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. We have now transitioned all our active fieldwork from Vohibola III to Ankarafantsika.

Near the Ampijoroa Field Station in Ankarafantsika, preliminary data indicate that Propithecus coquereli rarely exploit edge habitats. We are currently investigating ecological and anthropogenic correlates to this distribution pattern. Conversely, two species of mouse lemurs (Microcebus murinus and Microcebus ravelobensis) differentially exploit forest edges, which contain varying abundances of important insect resources. Little is known how the other lemur species, such as Eulemur fulvus fulvus and Eulemur mongoz (such as the handsome individual to the left), respond to edge effects. Thus, there are ample opportunities for students to collect basic but critical information on the distribution and abundance of all lemurs within the Park.