As you move from the poles richness increases. It’s a pattern that’s been termed the ‘latitudinal biodiversity gradient’ by ecologists, and Victorian naturalist Alfred Russel Wallace was a pioneer in recognising and attempting to explain it.
Wallace attributed this abundance to time: specifically that land closer to the equator was the earliest formed and hadn’t been disturbed by glacial activity at higher latitudes. He theorised that the relative stability of the tropical climate through the year, and through the ages, allowed species to accumulate here.
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Almost 150 years later, understanding patterns of species richness is critical given today’s global biodiversity crisis, and theories are hotly debated. One avenue of research claims the larger amount of solar energy at the equator is responsible for increased productivity, meaning a greater abundance of resources and more opportunity to adapt and specialise.
Another argues that the lack of ‘catastrophic mortality’ from climate events (particularly winter freezes) means species focus on competition for resources, resulting in faster evolutionary responses. Other experts contest that the increased number of predators at low latitudes drives the diversity of prey species.
While Wallace reasoned that “uniformity and permanence” were the essential drivers for biodiversity, recent research suggests change in the environment could be a key component.
Researchers from the University of St Andrews, Scotland, monitored fish populations on an Amazonian floodplain in surveys spanning almost 20 years. The research took place in the Mamirauá Reserve, which extends over more than a million hectares, and is the largest area of flooded forest in Brazil.
During the high-water season, the forest can be flooded to a depth of up to 12m, with fish swimming between the branches of inundated trees. In the study, even the supremely adaptable red-bellied piranha and Amazon cichlid were unable to dominate in this fluctuating ecosystem, enabling more species (including rare ones) to coexist. The scientists concluded that this natural environmental change could therefore fuel biodiversity, as no one species was found to predictably exploit the largest fraction of local resources.
However, in the year following the final survey, the dry season exceeded all previous records. Researchers found that in higher water temperatures and lower water levels, omnivorous and bottom-feeding fish species flourished while others declined. How a changing climate may alter the tropics’ biodiversity is yet to be seen.
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