EQUAL CHANCE AND DISPERSAL LIMITATION OF TROPICAL FOREST COMMUNITIES

The equal chance or null community hypothesis is based on the idea that all species are equivalent in terms of their habitat requirements and growth rates, and therefore that every species has an equal chance of inhabiting openings as they become available in the forest.

Basically this hypothesis suggests that local diversity is based on the number of species contributing seeds in the local area, and that tree replacement occurs via random chance. Recent modelling studies indicate that simple null models of tropical forest communities can retain very high species diversity over long periods of time.

Although species in such a null community eventually go extinct via a ‘random walk’ process, the time to extinction is sufficiently long that speciation may act to add new species to the system. For such a null community to maintain very high levels of diversity, one more kind of process is also generally required : namely "dispersal limitation", which refers to the fact that seeds of a given species do not germinate in all possible sites that could potentially be occupied by that species.

The result of dispersal limitation is that many species fail to encounter one another. If dispersal limitation is sufficiently strong, competitive exclusion can be avoided entirely. Recent studies have shown that the absence of plant species at a given site is in fact commonly due to a lack of seed dispersal rather than their inability to compete in that particular area.

TROPICAL DEFORESTATION | Conservation And Sustainable Management Of Tropical Forests

Greatly refined estimates of tropical deforestation have recently been obtained through analyses of changes in forest cover in satellite images. For example, analyses of Landsat imagery covering the Brazilian Amazon indicated an increase in deforested area of 78.000 km2 in 1978 to 230.000 km2 in 1988, or a loss of approximately 6% of the total forested area. Tropical deforestation rates vary greatly across geographic regions, and have shown marked swings over the last decades. Through the 1980s the highest deforestation rates were observed in southeast Asia, but more recently deforestation has shifted to the neotropics and Africa.

In addition to the outright removal of forest, tropical deforestation also acts to fragment landscapes, a pattern of great conservation concern. Tropical forest fragments offer an insufficient amount of habitat for many larger or wide-ranging species of animals, and forest fragments can be seriously degraded by decreased humidity and high wind exposure near edges.

The internal fragmentation of tropical forests caused by selective logging is also a major concern. Studies suggest that low-intensity logging can allow for recovery of primary forest conditions within a couple of decades; however, heavy logging requires a much longer recovery period, and some highly degraded forests may not be able to approach pre-harvest conditions even after hundreds of years.

In many regions construction of logging roads makes forested areas far more accessible to those interested in further exploitation such as subsistence farmers, hunters and fuelwood gatherers. For example, when a commercial logger leaves the concession, subsistence farmers are able to penetrate deeper into the forest than would have previously been the case. Post-logging forest use is becoming increasingly intense due to high population growth rates in many tropical countries.

One partial answer to these difficulties is development of sustainable forestry practices in combination with improved conservation of remaining tropical forests. "Natural forest management" in which gap phase dynamics is emulated by harvesting has been advocated as a means of mitigating losses of diversity and ecosystem function while allowing continued timber harvests. Alternative harvesting practices, such as planning of harvest areas and skid trails, tree marking and directional felling, can be used to reduce the residual impacts of the harvest. Recent studies suggest that such reduced-impact logging in tropical forests can dramatically reduce post-harvest tree mortality. This results in greater retention of forest biomass, increased long-term value of the forest in terms of timber commodities, and more rapid recovery of pre-harvest forest conditions.

GAP PHASE DYNAMICS IN TROPICAL FOREST

Gap phase dynamics has been hypothesized to play an important role in the maintenance of high diversity in the tropics. When one or a few trees die, an opening in the canopy occurs, resulting in increases in light levels and other plant resources. Seedlings and saplings grow rapidly in gaps, competing for these resources; only one canopy tree ultimately will be able to occupy the space relinquished by the original gap-forming tree.

For gap phase dynamics to contribute to species diversity, one must assume that there are differences in resources (light, nutrients, etc.) associated with different parts of the gap (i.e. gap edge or
centre), and that different species are adapted to these differences. Such a pattern is referred to as "gap partitioning".


Larger gaps are expected to contain greater resource heterogeneity than smaller gaps, and thus should show higher diversity of regenerating trees. This hypothesis was recently tested by Stephen Hubbell and co-workers using data from a 50-ha mapped forest plot on Barro Colorado Island in Panama. Although gap sites were found to have greater species diversity of saplings, this was due entirely to higher stem density in the gap sites. The number of species encountered per stem did not differ between gap and nongap sites. Thus, recent evidence suggests that gap phase dynamics may not be the major mechanism for maintaining diversity in tropical forests. There is, however, clear evidence for important niche differences in tropical trees related to soil types and forest hydrology.