The role of niche conservatism in producing biodiversity gradients

Why do some places, like tropical forests, have so many species of plants and animals, while others, like deserts, have so few? Understanding the factors that create and maintain species diversity is extremely important, especially now that we are losing species rapidly due to habitat destruction, climate change, and disease. Across large areas on the globe, like continents and oceans, productivity of the environment is closely linked to high biodiversity. Productivity is the amount of sunlight energy that plants capture from the sun and translate into energy usable to sustain life both of plants and the organisms that eat plants, like humans, insects, and birds. For plants on land, productivity is highest where sunlight, warmth, and water are abundant. Such environments are typically tropical, places like Costa Rica, New Guinea, and the Congo. Places on the globe that are cold and lack water, like the arctic tundra, mountain tops, and deserts have much lower productivity and much lower biodiversity. Scientists have known of these patterns for decades, but the reasons for this relationship are still not understood. We still don’t know if current ecological factors, like plant abundances and species interactions at local sites, are driving these large-scale patterns, or whether much older trends, such as how species originally evolved over millions of years, may be underlying these patterns. Our group of scientists is exploring the latter, evolutionary trends. One new hypothesis for this productivity-diversity relationship is called the “climatic niche conservatism hypothesis”. The idea is based on the fact that long ago the majority of the globe was covered in tropical-like climates for millions of years. During that time many of the species that we know of today and their ancestors originated. When these species came into existence, their niches—the environmental and behavioral conditions within which a species can survive and reproduce—were based on the conditions of those historical environments. Because most of the earth was warm and wet—productive—most species niches were constrained to productive climatic conditions. As the earth became cooler over millions of years, the temperate, desert, and arctic climates became more prevalent at higher latitudes. It may be that fewer species were adapted to live in these environmental conditions of lower productivity. We are going to test these ideas by looking at the distributions of older groups of species that evolved in tropical conditions and the younger species that may have evolved in lower productivity environments. If a particular group of organisms has occurred in one climate for longer than another, it follows that there has been more time for species to evolve in that region and under those climatic conditions. We are going to look at many different types of organisms—oceanic plankton, terrestrial plants, salamanders, and bats—to see if these predictions are consistent with what we know about species distributions in various climates, ages of group, and evolutionary trends of speciation and niche construction.