Scientists can’t agree on why the tropics are so diverse – but a flooded forest in Brazil (and some piranhas) may solve the mystery

As you move from the poles towards the equator, species richness increases. It’s a pattern that’s been termed the ‘latitudinal biodiversity gradient’ by ecologists, and 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 had not been disturbed by the 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. 

Almost 150 years later, understanding patterns of species richness remains critical and theories are still debated.

Why are the tropics so diverse?

One avenue of research claims the greater 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, new research suggests change in the environment could be a key component.

Researchers from the University of St Andrews, Scotland, and the Mamirauá Institute for Sustainable Development in the Brazilian Amazon 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 square 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 12 meters, with fish swimming between the branches of inundated trees.

Natural environmental change and biodiversity

In the study, published in Science Advances, even the supremely adaptable red-bellied piranha and Amazon cichlid were unable to consistently dominate 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 and thrive.

Mamirauá Reserve in Brazil, where the research took place. Credit: Anne Magurran/University of St Andrews

In the year following the final survey, the dry season exceeded all previous records, following the trend towards a hotter and drier climate in the region. In higher water temperatures and lower water levels, researchers found omnivorous and bottom-feeding fish species flourished while fish-eating specialists declined.

Anne Magurran, professor of ecology and evolution at the Centre for Biological Diversity at the University of St Andrews, said: “We sincerely hope that the biodiversity changes reported are within the natural range of ecological variation in the ecosystem rather than indicative of a step along the path to a different, and less diverse, state.”

Read the full paper here: Spatial and temporal heterogeneity foster high fish biodiversity in an Amazonian floodplain

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Top image: Mamirauá Reserve in Brazil. Credit: Anne Magurran/University of St Andrews

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