No ecosystem is stable, and rainforests are no exceptions to this rule. Rainforests are subject to many types of natural disturbances which may further affect population sizes and proportions: fire, flooding, storms, winds, alterations in rainfall, and, in some places, volcanic or other seismic activities. Even where there is no apparent variation in conditions, the populations of many species will fluctuate over time. This is true for plants as well as animals. In Panamanian research plots, the proportion of shrubs in plant populations varies from approximately 10% to 40%, depending upon conditions such as pest levels and the presence of pathogens (Wright, 1996). Insect population booms and busts are well known, and lizard populations in tropical forests appear highly variable. Nevertheless, in undisturbed forests, although there is continual flux in the abundance of any one species, the overall composition of the forest tends to remain virtually the same. In Peninsular Malaysia, several two-hectare plots of rainforest maintained their species compositions for many years with relatively little variation. Over the period of the studies (24-38 years), the researchers found a low percentage of local extinctions (17% or less) or immigration (17% or less), mainly of rare species. For the most part, species which were abundant as adults were also abundant as juveniles. Thus, the structure of the forest is expected to remain similar over time (Manokaran, 2002).
a. Fire: Catastrophic fire is a relatively rare event in most rainforests which are devoid of humans (in contrast to temperate forests). In Amazonia, for example, natural major fires occur in any given area approximately every 440-1550 years. This is so because, where there is a closed canopy forest, fires are inhibited by the humidity, which often reaches 65% and more. Nevertheless, charcoal deposits (indicating previous fires) which predate human occupation have been located in the Amazon forest.
The probability of fires occurring in rainforests is greatly increased by the opening of the canopy by logging. This decreases the relative humidity and increases the ambient temperature. Logging activities also leave behind “slash,” the remnants of trees and underbrush, which are dry and much more susceptible to burning than intact forest. A 50% reduction of the canopy cover can increase the average temperature of an area of forest by 10o C. and may reduce the humidity by 35% or more. While a primary forest will burn under conditions of drought (and even small clearings can reduce humidity sufficiently to facilitate burning), a selectively-logged forest can catch fire after less than a week without rain, and a secondary forest, after eight to ten rainless days. In a forest which has burned, fallen branches and other combustible materials cover the ground, and highly flammable weeds and grasses invade the open areas. All of these add to the combustible fuel in the forest; thus, previously-burned forests are more susceptible to recurring fires at frequent intervals than are intact forests (Cochrane, et al., 1999).
Forest trees are able to survive and resprout following low-intensity fires. The probability of severe tree damage is much greater after repetitive fire, and, with repeated burning, most trees will be killed. Even large trees will be destroyed (up to 98%, compared to 45% of large trees in the initial fire). If fires occur more frequently than every 90 years, many trees will be lost; if more often than every 20 years, the burned area may become deforested entirely. Currently, in some parts of the Amazon, fires recur within less than five years. These secondary fires often occur in forests adjacent to land being burned for agricultural purposes, whereupon the area may become entirely treeless and reduced to scrub or grassland, probably irreversibly (Cochrane, et al., 1999).
b. Wind: In some regions there are severe storms with high winds. The largest and tallest trees generally have the strongest root systems and are able to resist wind activity, while smaller trees may be blown over and killed. Usually the forest composition will not be much altered, since the downed trees and undergrowth will resprout, or be replaced by natural succession (See Section F).
c. Flooding: Flooding is a natural event in some lowland rainforests, as in the varzea forests of the Amazon Basin, where many riparian (riverside) trees and plants spend months partially or wholly submerged during the rainy season. These plants are adapted to the flood cycle and are undamaged by long periods of submersion. Many do not even lose their leaves. Unusual flooding (often caused by human activity), however, can destroy the forest along riverbanks, leading to succession.
d. Geological activity: Long ago, glaciation altered the mix of plants and animals in forested areas by changing the physical environment in which they lived. Today, earthquakes and volcanic activity occur in a number of tropical areas, and can destroy or disrupt considerable areas of forest. Some will eventually become reforested; if the damaged area is very large, perhaps not.
e. Invasions by “exotic” (non-native) organisms: A very serious problem in many forests is the invasion by exotic species introduced mainly by human activities. Neotropical forests have been invaded by Africanized bees, and the fruit fly Drosophila malerkotliana from India has also been introduced into many of these forests. Tropical forests seem to have little resistance to such invasions, and the newly-introduced species frequently supplant or threaten native species, as they have no local enemies to control their populations.