species richness. Many questions remain, however.
One is the question of how an abundance of plant
productivity translates into supporting high species
richness of animals. A simple measure of biomass may
be insufficient to explain some patterns.
Look around in a rain forest. How would you try
to ascertain just how many other taxa such as insects,
lizards, or birds are there because of the green around
you?
Perhaps high tropical plant productivity and an
abundance of plant biomass translate directly into more
space for more species. Daniel Janzen, in a provocative
1976 paper titled “Why Are There So Many Species of
Insects?” concluded by saying, “I think that there are
so many species of insects because the world contains
a very large amount of harvestable productivity that is
arranged in a sufficiently heterogeneous manner that it
can be partitioned among a large number of populations
of small organisms.” Janzen, a legendary tropical
ecologist and conservationist, was not restricting
his speculation to the tropics, but his remark fits the
tropics particularly well. There is indeed a tremendous
potential harvestable productivity, and there are lots
of spaces for small animals in the three- dimensionally
complex rain and cloud forests.
Bird species diversity often correlates with a measure
termed foliage height complexity in mid to high latitudes.
The more layers of foliage there are, the more bird
species. But in 1974 Thomas Lovejoy showed that in the
tropics, bird species diversity does not correlate closelywith foliage height diversity. This is because tropical
rain forests are more spatially complex than temperate
forests, offering resources not detected or measured
by simple structural analysis (which was based on
measuring horizontal strata from ground to canopy).
As James Karr first pointed out in 1975, tropical
forests offer unique resources for birds and, by
implication, for other kinds of animals. Let’s take a
look at these offerings:- Among tropical forests’ additional resources are large
numbers of vines, high epiphyte density, and large
dried leaves (which harbor many kinds of arthropods
that are food for birds; plate 9- 13), all of which add
space, complexity, and potential food resources.
Plate 9- 12. The Ringed Kingfisher (Megaceryle torquata) is common along major rivers but also can be found in streams. It is the
largest of the New World kingfishers (41 cm/16 in). This bird is female. Photo by John Kricher.Plate 9- 13. Spiders and other arthropods may utilize the cover
provided by large dried leaves and epiphyte growth and form
a unique food base for other animals. Photo by John Kricher.chapter 9 why are there so many species? 143