Agroforestry and Biodiversity Conservation in Tropical Landscapes

Chapter 2

Ecological Effects of

Habitat Fragmentation in the Tropics

William F. Laurance and Heraldo L. Vasconcelos

This chapter provides an overview of the ecological consequences of habitat
fragmentation on tropical biota. We begin by describing the demographic and
genetic effects of fragmentation on individual populations, then discuss key
landscape factors that affect fragmented populations, particularly area, edge,
matrix, and distance effects. We then consider briefly the interactions of habi-
tat fragmentation with other simultaneous environmental changes that often
occur in human-dominated tropical landscapes, such as fire, logging, and
overhunting. We conclude by proposing some ways in which the deleterious
effects of habitat fragmentation could be partially ameliorated by agroforestry
and reforestation.

Why Isolated Populations Are Vulnerable

Forest fragmentation proceeds as intact forest blocks are subdivided and
reduced in size. This also reduces and subdivides natural populations, often
greatly increasing the rate of local species extinction. Such losses occur for sev-
eral reasons. First, small populations are highly vulnerable to random demo-
graphic events (Shafer 1981). Consider, for example, the fate of a population
of 20 short-lived animals that, simply by chance, had two consecutive breed-
ing seasons in which few females were born into the population. The repro-
ductive capacity of the population would be drastically reduced, and it could
easily disappear. In large populations such chance events are of little impor-
tance, but simple random fluctuations in births and deaths can have dire
impacts on small populations.
Such events probably are important in nature. Many species appear to exist
in metapopulations, that is, a series of small subpopulations, each of which is
partially isolated from other such subpopulations (Hanski and Gilpin 1996).

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