Why Are Tropical Forests So Diverse?
Tropical forests are more diverse than temperate ones, but why? Several hypotheses fall into two broad categories 1) complexity and diversity of available niches within the forest is greater in the tropics so more species will be able to reside together and 2) potential for speciation events is magnified in the tropics so more species will successfully evolve.The Neotropical forests, relative to the temperate forests, experience a more stable and warmer temperate throughout the year and, in general, receive more average (across the year) rainfall. Warm, moist areas facilitate the rapid growth of many plants including mosses, vines, and even many angiosperms, resulting in a diversity of plants that is very high. Neotropical soils are nutrient poor, but plants have evolved many strategies to allow for productive growth, including nitrogen-fixing in legumes and becoming epiphytic. Epiphytes grow on the surface of other plants (generally; they may grow on other structures as well) and include many orchids and bromeliads. This further expands not only the diversity but the surface area of plants within the Neotropics. With the increased surface area and diversity of plants comes a diversity of microhabitats for other organisms. Not only do these above factors provide a variety of places to feed, hide from predators, and reproduce, but they also provide a regime of slight differences in temperatures, humidity, exposure to sun through the canopy, etc. This has allowed other organisms, including many birds, arachnids and insects, mammals, and reptiles and amphibians, but also perhaps less obvious organisms such as other plants and the wide range of invertebrates (mollusks, nematodes, annelids, etc.) to adapt to a specific area within the available habitat to carry out their daily and life activities. There simply is more space available to hold more animals. Events, such as glaciers, have driven mobile organisms into tropical refugia more than once in history and certainly some have remained after the retreat of the glaciers. Furthermore, events such as the uprising of the Panamá Isthmus, have resulted in the interchange of animals and plants. Incredible diversity of habitat has allowed for some organisms to move in, survive, and perhaps radiate; even if initially one or more competitors are loss, the net result may be an increase in diversity. Adaptation, and ultimately, specialization to a niche can lead to speciation.