Rainforest Diversity


July 31, 2012

The size of a habitat is another factor in the great diversity of the rainforest. Area increases diversity because a larger plot is likely to have more habitats, hence niches, to support a greater variety of species. In addition, many species require a large range for adequate prey or seed forage. The basis for this idea was set forth by MacArthur and Wilson in The Theory of Island Biogeography (1967) using small islands in the Florida Keys. Soon after the work was published, research focused on whether island biogeography could be applied to fragments of habitat. Evidence for this concept was found in an experiment devised by Thomas Lovejoy in the late 1970s. The experiment was known as the Minimum Critical Size of Ecosystems Project and measured ecosystem decay in forest patches ranging in size from 2.5 acres (1 hectare) to 2,500 acres (1,000 hectares). During the late 1970s the Brazilian government was encouraging widespread clearing of rainforest by offering tax incentives to landowners. However, in an area known as the Manaus Free Zone, just north of the Amazonian city of Manaus, the government required that 50 percent of the forest on a developed area must be saved. Lovejoy used this stipulation for his experiment, convincing landowners to leave their required forest patches in neatly cut squares.

The experiment, today known as the Biological Dynamics of Forest Fragments Project, found that the most seriously degraded forest with the least diversity were the smallest, one- hectare reserves, while the reserves that retained the most diversity were the ones of the largest area. In the smaller reserves, drying winds reached the interior, affecting tree species and resulting in more tree falls. Gaps in the canopy allowed more sunlight to reach the forest floor, further altering the understory microclimate and causing changes in the makeup of resident species. Larger herbivores left the patches since the limited number of trees could not provide sustenance, soon followed by predators, which could not cope with the loss of prey. The loss of predators caused an imbalance in the food chain, and the populations of small herbivores and omnivores increased, adding pressure on forest seed banks and impairing the reproducing ability of forest trees. Troops of army ants could not be supported by meager forest patches and they too left, along with the bird, butterfly, and other insect species that depended on the troop. Shade- loving plants and animal species died off as more sunlight penetrated the diminished canopy, and "gap" species, like vines and certain bird and insect species, proliferated. These losses continued to set off a chain reaction that caused profound changes in the system, eventually resulting in its collapse.

Forest fragment in Sumatra, Indonesia. Photo by Rhett A. Butler
Forest fragment in Costa Rica. Photo by Rhett A. Butler

Similar experiments carried out around the world have yielded similar results (although in some cases diversity among certain groups may actually increase). The colonization of forest patches by forest- edge species, light-gap specialists, and savanna species can counter the loss of species less tolerant of the changed forest and maintain the diversity of the patch. In some cases, forest fragment diversity may hold steady, but overall (global) diversity declines as some unique species lost from the forest patch are not replaced. Floor- dwelling species appear more affected by forest fragmentation than canopy species. Declining biodiversity in accordance with decreasing land area is an important trend to consider for conservation (see section 10).

In global studies, larger forest patches lost fewer of their species. Diversity declined but at a rate and to a degree inversely proportional to the size of the patch. In other words, the larger the patch, the more organisms survived and were successful in reproducing. Thus these experiments demonstrated that the area of an ecosystem directly affects biodiversity.

Articles on fragmentation

'Ecological Armageddon': mammals vanish entirely from forest fragments after 25 years

(09/26/2013) As tropical forests worldwide are increasingly cut into smaller and smaller fragments, mammal extinctions may not be far behind, according to a new study in Science. Tracking native smalls mammals in Chiew Larn Reservoir, Thailand for over 25 years, scientists found a stunning and rapid decline in mammal populations, until most forests were almost completely emptied of native mammals.

Forest fragmentation leading to higher extinction rates

(08/13/2013) The world's species are in worse trouble than widely-assumed, according to a new paper in the Proceedings of the National Academy of Sciences (PNAS), which reevaluates how scientists estimate extinction rates. The new model takes into account the impact of forest fragmentation on extinction rates for the first time, filling in a gap in past estimates. Much of the world's tropical forests, which house the bulk of the world's species, have been whittled down to fragments: small forest islands that no longer connect to larger habitat. According to the paper, species confined to fragments have a higher likelihood of vanishing.

Key mammals dying off in rainforest fragments

(08/15/2012) When the Portuguese first arrived on the shores of what is now Brazil, a massive forest waited for them. Not the Amazon, but the Atlantic Forest, stretching for over 1.2 million kilometers. Here jaguars, the continent's apex predator, stalked peccaries, while tapirs waded in rivers and giant anteaters unearthed termites mounds. Here, also, the Tupi people numbered around a million people. Now, almost all of this gone: 93 percent of the Atlantic Forest has been converted to agriculture, pasture, and cities, the bulk of it lost since the 1940s. The Tupi people are largely vanished due to slavery and disease, and, according to a new study in the open access journal PLoS ONE, so are many of the forest's megafauna, from jaguars to giant anteaters.

Lessons from the world's longest study of rainforest fragments

(08/15/2011) For over 30 years, hundreds of scientists have scoured eleven forest fragments in the Amazon seeking answers to big questions: how do forest fragments' species and microclimate differ from their intact relatives? Will rainforest fragments provide a safe haven for imperiled species or are they last stand for the living dead? Should conservation focus on saving forest fragments or is it more important to focus the fight on big tropical landscapes? Are forest fragments capable of regrowth and expansion? Can a forest—once cut-off—heal itself? Such questions are increasingly important as forest fragments—patches of forest that are separated from larger forest landscapes due to expanding agriculture, pasture, or fire—increase worldwide along with the human footprint.

Satellites show fragmented rainforests significantly drier than intact forest

(10/13/2010) A new study in Biological Conservation has shown that edge forests and forest patches are more vulnerable to burning because they are drier than intact forests. Using eight years of satellite imagery over East Amazonia, the researchers found that desiccation (extreme dryness) penetrated anywhere from 1 to 3 kilometers into forests depending on the level of fragmentation.

Rainforest canopy in Peru. Photo by Rhett A. Butler

Review questions:

  • How does area impact biodiversity?
  • Does forest fragmentation reduce forest diversity?

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  • MacArthur and Wilson presented the idea that habitat size is correlated with the diversity of species in The Theory of Island Biogeography, Princeton, N.J.: Princeton University Press, 1967.
    The background for the Minimum Critical Size of Ecosystems Project (Biological Dynamics of Forest Fragments Project) is given in Lovejoy, T.E. et al., "Ecosystem Decay of Amazon Forest Remnants," in M.H. Nitecki, ed., Extinction, Chicago: University of Chicago Press, 1984; Lovejoy, T.E. et al., "Edges and other effects of isolation on Amazon Forest Fragments." in M.E. Soulè, ed., Conservation Biology: The Science of Scarcity and Diversity, Sunderland: Sinauer, 1986; Wilson, E.O., The Diversity of Life, Cambridge, Mass.: Belknap Press, 1992; Quammen, D., The Song of the Dodo, New York: Scribner, 1996; and Laurance, W.F. and R.O. Bierregaard, Jr, eds., Tropical Forest Remnants: Ecology, Management, and Conservation of Fragmented Communities, Chicago: University of Chicago Press, 1997.
  • Smaller fragments suffered greater disturbance through tree falls and suffered losses of biomass according to Laurance, W.F. and R.O. Bierregaard, Jr, eds., Tropical Forest Remnants: Ecology, Management, and Conservation of Fragmented Communities, Chicago: University of Chicago Press, 1997; and Laurance, W.F., "Biomass Collapse in Amazonian Forest Fragments," Science Vol. 278 (1117-1118), Nov. 1997. The work edited by Laurance and Bierregaard further surveys fragmented sites around the world coming to the conclusion that fragmentation reduces global biodiversity. A similar result is reached in Bawa, K.S. and Seidler, R., "Natural Forest Management and Conservation of Biodiversity in Tropical Forests," Conservation Biology Vol. 12 No. 1 (46-55), Feb 1998.
  • Island biogeography is discussed further in Williamson, M. (Island Populations, Oxford: Oxford University Press, 1981); Quammen, D. (The Song of the Dodo, New York: Scribner, 1996); Oosterzee, P. (Where Worlds Collide, New York: Cornell University Press, 1997); James H. Brown, J.H., and M.V. Lomolino (Biogeography (2nd edition), Sunderland: Sinauer Associates, 1998); and Whittaker, R.J. (Island Biogeography: Ecology, Evolution and Conservation, Oxford: Oxford University Press, 1999).