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How much did the Pleistocene extinctions hurt nutrient flow?

If you live in North America, or South America, or Europe, mainland Asia, or Australia, you might notice that there's not a whole lot of 100kg or larger animals that you can see in the wild parts of your continent. Which is somewhat weird because, in sediments just thousands of years old, fossils of large animals exist. This massive die-off of large-bodied mammals on four continents is called the Pleistocene megafaunal extinctions, and its legacy can be seen in many places.

One place where its legacy has been recently explored is in how the loss of large herbivores affected nutrient flow. If you're an organism that can't move on its own, your ability to grow is going to be directly affected by the geographical availability of nutrients, particularly limiting nutrients like phosphorus. While being eaten by a mammoth is not so great for you, being eaten by a mammoth who then (to be technical) defecates you out far from where you live is good for your species; megafaunal defecation spreads nutrients around.

So how much does the loss of megafauna decrease the homogeneity (spread-out-ness) of nutrients in an ecosystem? Apparently, a lot.


This figure shows the concentration of phosphorous changing over time in the Amazon Basin. A: how phosphorous was spread 15,000 years ago, just before South American megafauna suffered an extinction event. B: current levels. C: an estimate of levels in 28,000 years from now. D: The amount of change between 15,000 years ago and today.

Areas that were red (high in concentration) 15,000 years ago mostly continue to be high today; a highly active river system helps move nutrients in the terrestrial biosphere. But the amount of nutrient spread has rapidly dropped, such that areas that used to be relatively moderate in their levels of phosphorous now have very minimal values, and this trend is only expected to continue into the future.

Such research has practical applications in the sense of informing decisions on whether or not notions of "Pleistocene rewilding" are worth the cost: knowing how much nutrient availability is dependent on large herbivores might change the expected benefits of rewilding.

Science Daily write-up.

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