13.1.2. Patterns of Biodiversity

The diversity of plants and animals is not uniform throughout the world but shows a rather uneven distribution. For many groups of animals or plants, there are interesting patterns in diversity, the most well-known being the latitudinal gradient in diversity. In general, species diversity decreases as we move away from the equator towards the poles. With very few exceptions, tropics (latitudinal range of 23.5° N to 23.5° S) harbor more species than temperate or polar areas.

Latitudinal gradients refer to the pattern that species diversity tends to be greater near the equator and decreases towards the poles. In practical terms, tropical regions, which lie between latitudes 23.5° N and 23.5° S, have a richer variety of species compared to temperate or polar regions. For example, Colombia, located near the equator, is home to about 1,400 bird species, whereas New York, which is further north at 41° N, has only 105 bird species, and Greenland at 71° N has just 56. This shows a clear trend of decreasing biodiversity as one moves away from the equator.

Think of the equator like a vibrant market filled with a variety of colorful fruits and vegetables. As you travel north or south to less tropical environments, you encounter markets with fewer options and variety, much like the biodiversity found in colder regions where plant and animal species are less abundant.

India, with much of its land area in the tropical latitudes, has more than 1,200 species of birds. A forest in a tropical region like Ecuador has up to 10 times as many species of vascular plants as a forest of equal area in a temperate region like the Midwest of the USA.

India's geographic location in the tropical latitudes contributes to its rich biodiversity. The country boasts over 1,200 bird species, indicating its dense wildlife. In a comparison, a tropical forest in Ecuador can have ten times the variety of vascular plants compared to a similarly-sized temperate forest in the USA, showcasing how tropical regions are biologically more productive and diverse.

Imagine a huge buffet where the tropical forest is a table overflowing with different dishes—each representing a species—while the temperate forest is a smaller table with just a handful of options. The variety and abundance of food (biodiversity) at the tropical table illustrate how these regions can support far more species.

What is so special about tropics that might account for their greater biological diversity? Ecologists and evolutionary biologists have proposed various hypotheses; some important ones are (a) Speciation is generally a function of time, unlike temperate regions subjected to frequent glaciations in the past, tropical latitudes have remained relatively undisturbed for millions of years and thus, had a long evolutionary time for species diversification, (b) Tropical environments, unlike temperate ones, are less seasonal, relatively more constant and predictable. Such constant environments promote niche specialization and lead to a greater species diversity.

Several reasons explain why tropical regions exhibit such immense biodiversity. Firstly, time plays a crucial role; tropical areas have been more stable over long periods, allowing species to evolve and diversify without interruptions from climatic changes, such as glaciations, seen in temperate regions. Secondly, the stable nature of tropical climates fosters niche specialization, where species adapt to specific roles in their environment, promoting the coexistence of a greater number of species.

You can think of this as an artist's studio. In a gradual and quiet environment with plenty of resources (tropical areas), the artist can create numerous pieces of diverse artwork. However, in a chaotic environment where conditions change quickly (temperate areas), the artist is less likely to produce a wide variety of styles, resulting in less overall diversity.

During his pioneering explorations in the South American jungles, the great naturalist Alexander von Humboldt observed that within a region, species richness increased with increasing explored area, but only up to a limit. In fact, the relation between species richness and area for a wide variety of taxa (angiosperm plants, birds, bats, freshwater fishes) turns out to be a rectangular hyperbola.

The species-area relationship is an important ecological principle observed by Alexander von Humboldt. He noted that as the size of an area increases, the number of species it can support generally increases, but this growth levels off after a certain point. This pattern can be represented mathematically as a rectangular hyperbola, which illustrates that while larger areas can host more species, there are eventually diminishing returns as area continues to increase.

Imagine a large piece of land divided into plots for a community garden. Initially, as more plots are added, the number of different vegetables grown increases. But after reaching a certain number of plots, adding more won’t significantly increase the diversity of vegetables; instead, there will be just more of the same types, illustrating that there is a limit to how much diversity can fit into a given space.