As we begin to take a general sweep through the Earth's biomes, the ever-changing hubs of natural history, the idea of biodiversity should be preeminent. In recent years, biodiversity has become an increasingly important term for ecologists, describing not only the entirety of organisms themselves, but also their behaviors, interactions and genetics. Think about each of these levels as we progress, coupled with the different lenses of ecology we discussed earlier. Hopefully, by the end, we will have a more satisfactory perspective on biodiversity and will be able to better define the term.
As with many systems of classification in science, there are different classifications of biomes; you may run across lists shorter or in excess of the one described in this series of posts. I will detail the most expansive terrestrial biomes here, though later we may delve into some of the more obscure classifications.
The tropical rain forest (tropical wet forest, tropical moist broadleaf forest) is probably the most well-known biome to the non-biologist, immediately recognizable for its garish displays of color, shape and texture. Evolution has produced a nearly unlimited array of plants, animals, fungi and microbes, all interwoven in an intricate organic tapestry.
The source of the density and richness of the tropical rain forest is two fold: temperatures are relatively constant throughout the year and these areas are drenched by rain almost daily. Rainfall can exceed 400 cm in a single year in these forests (temperate forests rarely exceed 100 cm of rain), draining the soil of nutrients and leaving it acidic.
Not all rain forests have soils that are poor in nutrients; those along rivers are consistently rejuvenated by flooding, while those rooted in the soil of young volcanoes have yet to be leached by steady rains.
Nutrients are rapidly recycled in a rain forest system. With as many as 300 tree species inhabiting a single hectare and as many as 1,000 species of insects on a single tree, not to mention the high density of other organisms, it is easy to see how the nutrient budget in the forest would be tight, and how organisms rely on one another to perpetuate. Bacteria, mites and other soil creatures quickly break down organic matter from dead organisms which is absorbed by other animals or by the shallow, twisted roots systems of trees and plants. Mutualistic fungi called mycorrhizae often assist these root systems in acquiring elusive elements like nitrogen and phosphorous. Termites break down tough cellulose with the help of microbes in their guts. Leaf cutting ants slice up plant material and cultivate a nutritious fungi for the colony that aids in decomposition. And, since the density of plant life within the forest almost negates the effects of wind, plants are almost completely reliant on insects for fertilization and frugivorous (fruit-eating) animals for the dispersal of seeds.
If you want to find most of the life in a rain forest, it might be best to look up. The canopy is home not only to countless species of birds, monkeys, amphibians and snakes, but also to thick draping vines called lianas. Epiphytes, plants that live on other plants, make homes out of the crooks and crannies of trees that can reach 80 meters in height.
Rain forests make up only about 2% of the Earth's terrestrial surface, but contain about half of the planet's land-loving species. On a macroscopic level, these forests are the taxonomist's last great frontier of discovery, especially in entomology; low estimates predict tens of millions of unclassified insects.
The thick web of life, the behavior guiding (sometimes forcing) interaction and the underlying genetics of it all that constitutes the tropical rain forest system is the quintessential representation of biodiversity. While other biomes on the planet may not by as dense, the biological relationships of each provide a fresh view of the diversity of life on Earth.
Some authors skip the tropical dry forest when listing biomes, but due to its peculiar (and extreme) double life of wet and dry seasons, we'll give it a nod, next time.