August 14, 2013

Is the Amazon still hurting over old extinctions?

Mineral nutrients are essential components of any ecosystem. Their absolute presence and relative representation in an environment help shape habitats and the communities that can be supported on them. These nutrients can be one of the most important limiting factors to an ecosystem's productivity.

The Amazon rainforest is one of the biologically richest environments on Earth. However, its soil is relatively poor, and much of the vegetation it supports lives on other plants (e.g. see previous posts Who's who up a tree and Kingdom in the canopy). One important limited nutrient in the Amazon Basin as a whole is phosphorous. Phosphorous (hereafter represented by its chemical symbol, P) is important for all living cells, with an active roll in energy transport and obtention in the form of ATP; as a core component of our DNA in the form of inorganic phosphate; and composing the living cell walls as phospholipids. Plants have the ability to take inorganic P from the soil and incorporate it into organic molecules that animals can use.

The major contribution of inorganic P to Amazonian soils comes from the activity of the pubescent  Andean mountain range (yes, still growing and cranky! – on a geologic timescale). This may influence the excessively high levels of biodiversity along the Tropical Andes and foothills. Incredibly, another P contributor to South America is the Sahara desert in Africa. However, today P in the Amazon is concentrated along white water rivers, with lower levels along black waters and floodplains.

Payamino River in the Ecuadorian Amazon.

A recent study lead by Christopher Doughty suggests that P distribution in Amazonia may not have always been so geographically restricted. Doughty and collaborators developed a mathematical formula to estimate the influence animals have on nutrient diffusion throughout environments. They applied their formula to P dispersion in a region of the Amazon.

Animals distribute nutrients such as P though dung and decay. Doughty's study highlights the importance of large animals in this process. The larger the animals (in this case mostly mammals, since in recent geologic history they are Earth's largest animals), the larger their range and further they are likely to travel. Thus, in accordance with the study's results, the farther they spread P and other nutrients from the areas where these would otherwise concentrate, such as rivers and floodplains. In this way, large animals – or megafauna – act as nutrient distributors of the environment. As the authors of the paper eloquently put it "This service is analogous to that played by arteries in the human body, with large animals acting as arteries of ecosystems transporting nutrients further and smaller animals acting as capillaries distributing nutrients to smaller subsections of the ecosystem."

Nowadays, there aren't actually very many big Amazonian animals. Not like there were up until about 10,000 years ago. The Pleistocene extinctions saw the demise of 70% of animals over 10 kg, implying average home range decreased by at least 50 km2! This would have had serious implications for nutrient diffusion. Doughty and colleagues estimate that even now, thousands of years after the megafaunal extinctions that closed the Pleistocene, the Amazon's P levels have not recovered entirely.

South American giants: extinct species are in grayscale; extant are in colour.
*Disclaimer: I do NOT own this image. It is by my favourite palaeoartist, Mauricio Antón.*

Bringing to light the role of large animals in lateral nutrient diffusion across the landscape is very relevant in today's world, where wildlife is under immense pressure from human activities. It highlights the crucial need to protect the world's remaining megafauna. Fortunately for mammals, it is easier to raise awareness for the big, furry things than, say, equally or more important creepy crawlies; however, studies like this one show that large mammals aren't only useful as "poster species" (which was pretty much what I thought about most of them, up until very recently).

Doughty's et al.'s formula can be applied to other environments and for other nutrients. The paper briefly discusses the differences in loss of megafauna and reduction in nutrient diffusion across different continents at the end of the Pleistocene. South America was the most severely affected of all continents, whereas Africa – whose native wildlife had evolved alongside humans – suffered least. Whether or not this has anything to do with human populations spreading at that time is something that needs more looking into. Without implying one thing or another, it may be an interesting point to take into consideration in the controversial argument of whether humans or climate change caused the Pleistocene extinctions.

Main source:

Doughty, C. E., Wolf, A., and Malhi, Y. (2013). The legacy of the Pleistocene megafauna extinctions on nutrient availability in Amazonia. Nature Geoscience. doi:10.1038/ngeo1895

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