ROLE OF CLIMATE IN RAINFOREST BIODIVERSITY
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:
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
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
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 along the bank of the Tambopata river in Peru. (Photo by R. Butler)
- 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?
Other versions of this page
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.