13.1.2.1. Latitudinal gradients

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.

Biodiversity varies across different regions of the world, and one prominent pattern is that there are more species near the equator, which is the hottest and most humid part of the planet. As we move towards the poles, whether to the north or south, the number of species tends to decrease. This means that tropical regions have a higher variety of living organisms compared to temperate and polar regions.

Think of the Earth like a big party. At the center, where the climate is warm (the equator), there are lots of guests (species) enjoying themselves and interacting with each other. As you go towards the edges of the party (the poles), the number of guests decreases until there are hardly any left. Just like that, the equatorial regions are bustling with species diversity.

With very few exceptions, tropics (latitudinal range of 23.5° N to 23.5° S) harbour more species than temperate or polar areas. Colombia located near the equator has nearly 1,400 species of birds while New York at 41° N has 105 species and Greenland at 71° N only 56 species. India, with much of its land area in the tropical latitudes, has more than 1,200 species of birds.

Specific examples illustrate how species count changes based on geography. Colombia, which is close to the equator, boasts an impressive 1,400 bird species compared to only 105 species in New York and a mere 56 species in Greenland. India, primarily situated in the tropical latitudes, also reflects this trend with over 1,200 species recorded. These numbers highlight the stark contrast in biodiversity influenced by latitude.

Imagine you are taking a trip; you will find plenty of different types of local dishes when you visit a bustling tropical marketplace (Colombia) compared to a quieter place like a northern café that offers only a few basic options (New York) or a desolate diner in the Arctic (Greenland). This illustrates how environments rich in warmth and resources support more biodiversity.

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. The largely tropical Amazonian rainforest in South America has the greatest biodiversity on earth- it is home to more than 40,000 species of plants, 3,000 of fishes, 1,300 of birds, 427 of mammals, 427 of amphibians, 378 of reptiles, and over 125,000 invertebrates.

The comparison between different forests shows the great disparity in biodiversity. For instance, a tropical forest in Ecuador can house 10 times more vascular plant species than an equally sized forest in a temperate region. The Amazon rainforest epitomizes this biodiversity, being an unparalleled hotspot with thousands of species across various categories.

Consider a well-stocked supermarket (the Amazon) filled with an immense variety of fresh fruits, vegetables, grains, and snacks compared to a small convenience store (temperate forests) that offers just a few basic items. The supermarket offers an abundance of options reflecting the rich biodiversity of tropical forests.

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 greater species diversity and (c) There is more solar energy available in the tropics, which contributes to higher productivity; this in turn might contribute indirectly to greater diversity.

Several reasons explain why tropics support greater biodiversity. First, the tropics have had a long time for species to evolve because they were not subjected to harsh glaciation events like temperate regions. Second, the stable conditions in the tropics allow species to adapt to specific niches, while third, the abundance of sunlight boosts productivity across ecosystems, enabling more species to thrive.

Imagine a long-term investment; if you have time to nurture a plant (the ecosystem), it can grow to be very tall and strong, just as species have had the time in the tropics to diversify. On the other hand, consider a garden that is frequently hit by harsh weather (glaciation); plants there struggle to survive and grow because they haven't had the chance to adapt well.

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