Rainforest Diversity


July 31, 2012


The hot and humid climate plays an important role in rainforest variety. As a general rule, diversity and ecosystem productivity increase with the amount of solar energy available to the system. Sunlight is captured in the leaves of canopy plants via photosynthesis, converted into simple sugars, and transferred throughout the forest energy system as the leaves and fruit are eaten or decomposed by various organisms. The primary measure of ecosystem net primary production is the fixation of carbon by plants. Tropical rainforests have the highest mean net primary production of any terrestrial ecosystem, meaning an acre of rainforest stores more carbon than an acre of any other vegetation type. The humid climate adds another ingredient essential to rich diversity: water.


The stable tropical rainforest environment promotes diversity by allowing plants and animals to interact all year round without needing to develop protection against cold or frost. In addition, because the sun shines all year long providing plants with the energy to manufacture food via photosynthesis there is no seasonal food shortage in the ecosystem. The abundant food source for plants (sunlight) is passed up through the system to herbivores, which consume the plant leaves, seeds, and fruits, to carnivores which consume the herbivores. Over the course of millions of years, with abundant food, rainforest species have adapted to take full advantages of all the available niches.

Millions of years of battle between predator and prey have resulted in an extensive array of defenses, weapons, and specializations. Camouflage, mimicry, specialized breeding and feeding habits, symbiotic relationships with other species, and other complex adaptations have allowed species to out-compete rivals by making use of resources not available to generalists. Virtually no niche in the rainforest is unfilled and many different species can coexist in a relatively small area, without encroaching on their neighbors. The evolutionary process continues and species are pushed into narrower and narrower niches until they are unbelievably specialized to their particular way of life.

An alternative theory of why rainforests are so diverse

In 2005, a group of scientists proposed a new theory to explain rainforest diversity. Arguing that "niche theory", as described in the body of the text to the left, fails to explain how ecological communities assemble themselves to share a limited space, the team instead suggests that community membership is determined by variations in birth rate and mortality rate between species. These birth and death rates depend on species density—more abundant species have lower birth rates and higher mortality rates, while rarer species will have higher birth rates and lower death rates. In effect, the forest species will regulate themselves to make room for each other if they follow community membership rules.

Feb 2014: Another theory

This evolutionary process ensures that no one well-adapted species (i.e. beetle) dominates the whole population of beetles because that one species cannot be possibly adapted to all the niches available in the forest. As a generalist, the species would be quickly out-competed by more specialized species. Generalists appear to thrive most under disturbed conditions, such as areas cleared for agriculture. Here these "weedy" species may be quite common. Furthermore, any species abundant in natural forest faces the threat that a predator would adapt to exploit its abundance. For example, the failure of rubber tree (Hevea brasiliensis) plantations in the Amazon is due to leaf blight. In the ordinary rainforest, rubber trees are widely dispersed so blight can never wipe out more than one individual tree at a time.

Tropical rainforests are markedly different from temperate forests. In temperate regions many plant and animal species have wide distributions, and a forest may consist of a half dozen or so tree species. In contrast, tropical species have evolved to fit narrow niches in a relatively constant environment, producing grandiose diversity. For example, more than 480 tree species have been identified in a single hectare of tropical rainforest.

Visitors to the rainforest are often disillusioned by what they see because they confuse the word "diversity" with "abundance." They visit the rainforest expecting to see ten jaguars, dozens of iguanas lying on the lodge patio, and large toucans waiting for them with breakfast. You will not encounter giant herds of wildebeest or zebra as on the African savanna. Nor will you find an eruption of flowers or even an abundance of colorful birds. Life in the rainforest is strikingly subtle.

Rainforests are diverse, in terms of numbers of species, but any one given species is not necessarily plentiful. Some rainforest species have populations that number in the millions, whereas others may consist of a handful of individuals. The biology of tropical rainforests is a biology of rare species. The reason for this occurrence is that the majority of rainforest species are scarce over the range of the forest and may be common in only a few small areas where they are particularly well adapted. A certain species may be quite common in one area but exceedingly rare only 500 yards away, where it is replaced by another similar, but distinct, species. There are a few common species found in scattered patches and a great number of rare species scattered throughout a forest. Some of these species are extremely rare and on the verge of extinction, especially where the forest has been disturbed. The reason for this pattern is that many species are highly specialized to fit a particular niche. Where that niche exists, that species may have a large population and constantly produce offspring that head off to colonize new areas. However, the colonizers almost always fail, because they cannot compete with the specialized species of other areas. Thus these colonizers are rare in the areas where they try to establish a foothold.

Rainforest tree with magenta flowers. Photo by Rhett A. Butler

Review questions:

  • Why does biodiversity generally increase towards the tropics?
  • Where does the rainforest ultimately get its energy?
  • Why are few species relatively abundant in the rainforest?

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Continued / Next: Canopy structure, soils, effect of area on biodiversity

  • The section on stability - especially on competition and evolutionary processes - is heavily influenced by E.O. Wilson, The Diversity of Life, Cambridge, Mass.: Belknap Press, 1992.