Plants: Uniquely High Species
Richness
While looking around inside a Neotropical rain forest one
cannot help but wonder just how many species of plants
and animals inhabit that complex ecosystem (plate 9-
1). Life is obviously abundant and diverse, but so many
animal species are secretive and cryptic, and many of the
plants look pretty much alike. The term species richness,
or biodiversity, refers to the number of different species
of any given taxon inhabiting a specified area. Thus
we speak of the species richness of flowering plants in
Amazonia, or ferns in Costa Rican montane forests, or
birds in Belize, or mammals in Río Negro igapo forest, or
beetles in the canopy of a single Kapok Tree, or whatever.
High species richness among many different taxa is one of
the most distinctive features of tropical forests worldwide
and Neotropical lowland forests in particular.
In a temperate forest it is often possible to count the
number of tree species on the fingers of both hands
(though a toe or two may be needed). Even in the most
diverse North American forests, the lush southeastern
Appalachian cove forests, only about 30 species of trees
occur in a hectare (10,000 m^2 , or about 2.5 ac). In the
tropics, however, anywhere from 40 to 100 or more
species of trees are typically found per hectare. Indeed,
one site in the Peruvian Amazon surveyed by the late
and legendary botanist Alwyn Gentry (1988) was found
to contain approximately 300 tree species per hectare.
That’s a lot. Brazil has been estimated to have somewhere
around 55,000 flowering plant species. A recent survey
estimates that there are 352,000 flowering plant species
in the world. Therefore Brazil contains about 16% of the
world’s flowering plants. Gentry (1982) estimated that
approximately 85,000 species of flowering plants occur
in the Neotropics, very nearly 25% of the world’s total.
That estimate was made in 1982 and is likely higher now,
as more species are being discovered and described. In
any case, this is roughly double the richness of tropical
and subtropical Africa, about 1.7 times that of tropical
and subtropical Asia, and five times that of North
America.
British naturalist Alfred Russel Wallace, codiscoverer
with Charles Darwin of the theory of natural selection
(chapter 8), commented upon the difficulty of finding
two of the same species of tree near each other in the
tropics. Wallace (1895) stated:
If the traveller notices a particular species and wishes
to find more like it, he may often turn his eyes in vain
in every direction. Trees of varied forms, dimensions
and colour are around him, but he rarely sees any
one of them repeated. Time after time he goes
towards a tree which looks like the one he seeks, but
a closer examination proves it to be distinct.
As Wallace observed, though richness is high, the
number of individuals within a single species often
tends to be low, which is another way of saying that
rarity is usual among many species in the lowland
tropics. Though some plant species are abundant and
widespread (for example, Kapok Tree), many are not,
existing in small numbers over extensive areas. The
concept of identifying a forest type by its dominant
species, which works well in the temperate zone (e.g.,
eastern white pine forest, beech- maple forest, redwood
forest), is much less useful in the tropics. Exceptions do
occur. On the island of Trinidad one can visit a Mora
forest, where the canopy consists almost exclusively of a
single species, Mora excelsa, a tree that can reach heights
of 50 m (165 ft). The understory is also dominated by
Mora saplings. However, examples of such low- diversity
forests are extremely rare in the Neotropics.Patterns of Plant Species Richness
Within the Neotropics, plant species richness, though
high, is variable. In a classic 1975 study Dennis Knight,
working on Barro Colorado Island in Panama, found
an average of 57 tree species per 1,000 m^2 (0.25 ac) in
mature forest and 58 species in young forest. Knight
found that in the older forest, when he counted 500 trees
randomly, he encountered an average of 151 species.
This species richness is far higher than it would be at
higher latitudes. In the younger forest, he encountered
an average of 115 species in a survey of 500 trees.
Barro Colorado Island field station has long been a
bastion of plant demographic studies. Back in the early
1980s Stephen Hubbell and Robin Foster established
a 50 ha (about 125 ac) permanent study plot in old-
growth forest at BCI. They surveyed approximately
238,000 woody plants with a stem diameter at breast
height (dbh) of 1 cm (0.39 in) or greater and found 303
species: 58 shrub species, 60 understory tree species, 71
mid- story tree species, and 114 canopy and emergent
tree species.Chapter 9. Why Are There So Many Species?
134 why are there so many species?