These subpopulations may disappear frequently because of random demo-
graphic events but are generally reestablished by immigrants from nearby sub-
populations. Although no individual subpopulation is stable over the long
term, the overall metapopulation is likely to persist almost indefinitely (Smith
et al. 1978; Harrison 1989; Hecnar and M’Closkey 1996; Wahlberg et al.
1996). In fragmented habitats, however, immigration for most species is
halted or drastically reduced. Small populations may then falter and disappear,
never to be replenished.
Second, small, isolated populations are also vulnerable to inbreeding and
genetic drift. Inbreeding occurs because individuals are forced to breed with
close relatives, lowering genetic heterozygosity and often reducing fecundity
and offspring viability (Ralls et al. 1986). (Genetic heterozygosity is the degree
to which an organism has more than a single form of each gene; these variants
are called alleles; outbred individuals have greater heterozygosity than those
that are inbred and generally suffer fewer genetic problems.) As a result, inbred
populations may grow more slowly and be increasingly prone to random
demographic events (Mills and Smouse 1994). Genetic drift (the random loss
of alleles) is also amplified in small populations, and the resulting loss of
genetic variability may reduce a population’s resistance to new diseases or envi-
ronmental challenges (Nei et al. 1975; Allendorf and Leary 1986).
Finally, natural environmental variations and local catastrophes often com-
pound the effects of random demographic events and genetic problems (Leigh
1981). Environmental changes such as adverse weather conditions, increasing
densities of predators or competitors, or pathogen outbreaks may drive a small
population down to a critically low level. Once the population falls below a
certain threshold, the interacting and potentially reinforcing effects of random
demographic events, genetic problems, and environmental variations can
become a powerful driving force of extinction (Gilpin and Soulé 1986).
Fragmentation Effects on Tropical Biota
Forest fragmentation affects tropical species and ecosystems in many ways.
Here we describe its most important consequences and the interactions of
fragmentation with other simultaneous environmental changes (such as log-
ging, fires, and hunting) that commonly occur in human-dominated land-
scapes.
Area Effects
Large habitat fragments usually contain more species overall (greater species
richness) and a higher density of species per unit area than do smaller frag-
ments (Figure 2.1). This occurs for at least four reasons.
First, large fragments are less strongly influenced by sample effects. Simply
34 I. Conservation Biology and Landscape Ecology in the Tropics