Why is biodiversity so high in the tropics? 

It is well known that there is higher biodiversity near the equator than there is at the poles. You may see the trend referred to as the LDG in some literature, which refers to the latitudinal diversity gradient. The complete explanation for this gradient is still debated, but some proposed reasons are outlined below: 

Stable climate 

In the tropics the climate is generally stable and seasons have much less of an effect, meaning that species don’t need to adapt to conditions that change throughout the year. Instead, selection pressures drive each species to occupy a very specific niche that they can capitalize on all year round. Because each species adapts in such a specific way, there are more niches available in the tropics.  

For example, in a temperate area a bird might specialize to eat seeds. The trees here change throughout the year along with the seasons – they may only produce seeds at certain points when the weather is suitable but at other times will stop producing seeds while they conserve energy and wait for the sunny weather to return. For this reason a bird here that eats seeds may need to adapt to eat lots of types of seeds from different tree species. 

This differs with the tropics, where the climate is stable and so generally the trees don’t need to conserve energy as much. They may produce seeds for a longer period of time than trees in a temperate forest. This means the seed-eating birds can have specific adaptations to specific tree species and as a result, there is more niche space available for species to fill. 

Photo by Matthew Minho

Higher levels of solar energy 

Due to the Earth’s orbit around the Sun and the angle at which it spins, the tropics receive a higher amount of solar energy than the poles. This increased level of solar radiation results in higher primary productivity. More energy in the system supports a higher number of plant species, which in turn supports more herbivores and so carnivores too. This higher productivity level can therefore result in a more biodiverse habitat. 

Warmer temperatures 

Higher temperature levels in the tropics is thought to increase metabolism. This results in faster growth rates and more rapid life history cycles, which is linked with higher reproductive rates. Species that have short life cycles and produce lots of offspring will generally be able to evolve faster as there will be a higher number of generations within which mutations can occur. This links well with the first point about niche space, as this logic suggests that species may evolve more rapidly than in species at the poles with slower life histories. 

Photo by Dean Ashton

Historical climate 

Diversity in the tropics could also be a delayed reaction to past climatic stresses. One theory is that the gradient is a “legacy” from glaciation events which would have covered higher latitudes and meant that parts of the Northern hemisphere would’ve been largely uninhabited. Since these events there hasn’t been enough time for species to disperse and colonize those temperate zones from the tropics. 

There are many theories as to why the observable pattern of generally increased biodiversity in the tropics may exist, and many of the suggestions link together well. This implies there are many factors contributing to the pattern. 

With a global changing climate it is important to understand the impact that more unpredictable weather patterns and increasingly warmer climates across the globe could have on our environment. We may start to observe tropical species moving their ranges closer to the poles as the temperature changes, potentially leaving species that are adapted to cooler environments with nowhere to go.  

Within the tropics, another observable phenomenon is the increased prevalence of infectious diseases due to the faster life histories and increased niche space among other influences. As the climate changes we could start to see a further increase of types of tropical diseases, and higher rates of infections as they spread across the globe with the other tropical species. This is yet another reason that we need to understand the tropics and why biodiversity is so high there, as it may help to predict the potential impacts of climate change. 

Photo by Achyuthan Srikanthan

Many of OpWall’s sites are in the tropics due to this diversity of species and the fragility of these ecosystems. Interested in learning more about the tropics and supporting research which will protect these ecosystems? Come along to one of our talks to find out more!  

Wondering why marine biodiversity is so high in the tropics too? Find out here!

Title photo by Glenn Shiach