CASE 8 BIODIVERSITY HOTSPOTS: RAINFORESTS AND CORAL REFFS

When we compare biomes across the continents, the pattern just described stands out clearly: Species diversity generally peaks near the equator and declines toward the poles. What are causes of this latitudinal diversity gradient, and what are its consequences? Does this global pattern reflect the ecological processes that characterize populations?

Ideas abound on why the tropics contain so many species. One hypothesis, for which there is broad support, looks to the relative ages of tropical and higher-latitude biomes. Tropical biomes tend to be older, having evolved over tens of millions of years, whereas biomes at higher latitudes have been shaped by climatic change, and ultimately an ice age, within the past few million years. Because tropical habitats have existed much longer than temperate ones, biologists hypothesize that there has been more time for species to evolve and diversify at low latitudes. Several studies of diversification rate, in frogs for example, show comparable rates of diversification in temperate and tropical biomes, supporting the hypothesis that the higher tropical diversity reflects the longer interval through which diversification has taken place.

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Time, however, is just one of several factors that influence diversity. Another hypothesis is that temperate communities have fewer species because it is relatively difficult to adapt successfully to cold, dry winters and the range of weather typically experienced at higher latitudes. A tree growing in a temperate area like New England experiences a wide range of temperatures over a year (and sometimes even over a single day) as winds blow and snow falls, or heat waves move through the area. Because species living at high latitudes are adapted to a greater range of conditions on average than those nearer the equator, they tend to have larger geographic ranges than tropical species, which are adapted to the narrow range of environmental variation experienced in any particular place.

It has also been suggested that the many insects and fungi specialized to attack particular kinds of tree are generally more abundant in wet tropical forests, and so trees of a given species tend to be as far apart as they can be, thus reducing the chance of being found by their enemies. This means that there would be more room for different species in any given place. It has also been noted that there is more land surface area at low latitudes, so the species–area curve (Chapter 46) plays a role in the latitudinal diversity gradient.

Very likely, the latitudinal diversity gradient reflects all these influences and more not yet appreciated. What is clear, however, is that where there are more species of trees, there are also more species of animals and other consumers. Tropical temperatures and high humidity have permitted a wide range of plant species to evolve over the past 100 million years, creating many niches for the animals that depend on them.

The high diversity—and therefore the low population density—of many tropical forests prompts another question. What determines how far apart individual trees can be and still maintain a viable population? One important consideration goes back to mutualisms: Animal pollination ensures that pollen from one tree can reach another individual of the same species if it is within the foraging range of a pollinator carrying its pollen. Of course, if trees are farther apart than any single pollinator can visit within a foraging excursion, they will not be pollinated. Animal pollination, therefore, contributes to the diversity differences between tropical trees and higher-latitude species, which are more likely to be pollinated by wind.

Both on land and in the sea, biodiversity hotspots occur predominantly in tropical to subtropical environments. In part, this simply reflects the higher species diversity typical of lower latitudes, but it also highlights that tropical communities contain a relatively high proportion of endemic species, species found in one place and nowhere else. Continued speciation coupled with a limited ability to colonize new areas successfully underpins the distribution of hotspots. Because of the way that biodiversity hotspots are distributed, the burden of conservation efforts falls disproportionately on developing countries.

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Quick Check 5 From what you know about how species diversity changes with latitude, predict how species diversity changes with altitude.