Chapter 9 Why Are There So Many Species?

Plant species richness peaks in Amazonia. Alwyn

Gentry, working in upper Amazonia and Chocó

(Colombia), found between 155 and 283 species of trees

of greater than 10 cm (4 in) dbh in a single hectare (a

mere 2.47 acres). When he included lianas of greater than

10 cm dbh, he found that the total increased to between

165 and 300 species. In contrast, a research group under

the leadership of Ghillean Prance found 179 species of

greater than 15 cm (6 in) dbh in a 1 ha plot near Manaus,

on a terra firme forest characterized by poor soil and a

very strong dry season. The lower species richness was

likely a result of the seasonality and the soil quality.

An inventory compiled in 1990 by Foster and Hubbell

for BCI surveyed all vascular flora (trees, shrubs, herbs,

epiphytes, lianas, but excluding introduced weedy

species) and found 1,320 species from 118 families. By

comparison, the total number of vascular plant species

documented by Barry Hammel to occur at La Selva

Biological Station in nearby Costa Rica totaled 1,668

species from 121 families.

Let’s compare these totals, both from Central

America, with those from Amazonian rain forests.

A floodplain forest on rich soils at Cocha Cashu

Biological Station along the Río Manú, a whitewater

tributary of the vast Rio Madeira in southeastern Peru,

was found by Robin Foster to contain 1,856 species

(in 751 genera and 130 families) of vascular plants.

At Reserva Ducke, a forest reserve on poor soil near

Manaus, Brazil, in central Amazonia, Ghillean Prance

found a total of 825 species of vascular plants from 88

families. This lower diversity, as above, speaks to the

poor soils. The two contrasting Amazonian sites show

a correlation of diversity and soil quality.

Comparing Central American sites with Amazonian

sites reveals other important differences. Tree species

richness is greater in Amazonia but the richness of

epiphytes, herbs, and shrubs is greater in Central

America. Gary Hartshorn and Barry Hammel found

23% of all vascular plant species at La Selva to be

epiphytes, the highest percentage recorded among the

closely studied sites. The most species rich of the four

sites was Cocha Cashu, located on fertile várzea soils

in western Amazonia. A total of 29 plant families that

were present at BCI, La Selva, and Cocha Cashu were

Plate 9- 1. How many plant species, to say nothing of how many unseen animal species, are part of this panoramic view of an

Ecuadorian rain forest? Photo by John Kricher.

why are there so many species? 135