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The tropical zones generally have a more congenial climate for life,
providing it with longer growing seasons, an even distribution of solar
energy, and freedom from freezing and other extreme, unpredictable, shortterm
changes in temperature. The rainforest, moreover, offers a humidity
regime and tree structure (that is, prevalence of broad, nearly horizontal
branches) favorable to epiphytes such as orchids and bromeliads. This
“elevated swampland” with its little pools of water and moist root masses
offers vast numbers of additional living sites for animals. The delicate
life cycles of the epiphytes and their co-evolved animal populations are
pre-eminently tropical. It is unlikely that the organisms could endure the
freezes of the Temperate Zone. The stability of the climate and the layering
of vegetation allows division of the ecosystem into large numbers of niches
and a corresponding number of plant and animal species, many bound
together by intricate and finely tuned symbioses. A small shift from one
part of a tree to another, or from one species of tree to another, or from
one elevation on a mountainside to another, opens an opportunity for the
evolution of yet another kind of animal or plant. The entirety of evolution
has built the equivalent of a house of cards: vast numbers of species propped
and leaning on one another and dependent on a steady environment to
avoid collapse. It used to be thought that diversity created stability; in
other words, the more species were locked together by co-evolution, the
less likely any one of them could be extirpated. This diversity-stability
hypothesis has gradually given way to its exact reverse, the stability-diversity
hypothesis, wherein external, climatic stability is thought to allow the
buildup of biodiversity. In the Temperate Zones, plant and animal species
must adapt to a more drastically and unpredictably shifting environment.
As a consequence, each Temperate Zone species is, on the average, likely to
occur in a greater range of habitats, elevation and so forth than individual
tropical species. In short, Temperate Zone species occupy a broader niche.
Fewer species can be fitted together, resulting in lower biodiversity in
temperate climates.
Destructive human activity, including habitat removal, pollution, and excessive
exploitation, have reduced large numbers of plant and animal species in the Temperate
Zones even though they are “tougher” in the sense of having wider ranges on the
B i o l o g i c a l
21 D i v e r s i t y