Visiting Maine at this time of year reminds me how winter is the limiting factor for life in the north. Every plant and animal which lives there has a complex and energy intensive survival mechanism for the cold: insulation, hibernation, migration, etc. Where I live now in California the limiting factor is the long, dry, summer. The adaptations here are less dramatic, but just as important.

Probably the places with the fewest environmental limits are the rain forests. There, the limit instead becomes all the other living things. This leads to an interesting pattern: the northern plants and animals have wide ranges–roughly all around the world in the northern latitudes. Once they’ve learned how to survive the winter, they face no major other competition. In the rain forest, many plants and animals have vanishingly small ranges. Some insects only live on certain trees. Sometimes there will be only one of those trees in a square mile. Where the limit is other life, evolution leads to hyper-specialization seeking any advantage. Where the limit is the environment, evolution leads to generality once the environment is survivable.

This is of course why the rain forest is so remarkably abundant yet so strangely fragile. The northern evergreens will still be here long after we’ve reduced the rainforest to national parks and zoos.

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