Why the past matters: megafaunal extinctions, excretions and global nutrient depletions

The paper, “Global nutrient transport in a world of giants”, published just this August, has exciting implications for ecology and conservation. I would argue that the findings of this research and related work provide 1) historical support for a trait and ecosystem process based approach to conservation, 2) support for the potential benefits of rewilding, and 3) insight into possible solutions for dealing with phosphorus depletion.

Before I go on to explain these points, I will give a brief overview of what the paper is about. The research highlights the relationship between the late-Quaternary megafauna extinctions on land, as well as the mass reduction of whale populations in the oceans, and the global nutrient cycling problem we are seeing today. These large animals of the Pleistocene – including the mammoth, mastodon, and woolly rhinoceros – were giants in comparison to what we see on our modern planet.

Size, it turns out, is an excellent predictor of how much they would have eaten, moved around, and, well, pooped. So, to use that classic (read: cliché) joke that every science teacher loves, “size does matter”. Larger animals move important nutrients (e.g. phosphorus, nitrogen, and potassium) around a lot more than smaller animals. Such nutrients are necessary for plant growth, and therefore, are extremely important to humans as well.

It was considered that perhaps humans and their domesticated animals took over the role of the Pleistocene giants as nutrient cyclers. After all, livestock farming is a massive industry and every year we dump large quantities of fertilizers onto the ground. The difference is, we concentrate these nutrients in areas of agricultural cropland or farmland. Similarly, farmers tend to keep a small variety of domesticated animal species behind enclosures to limit their ability to roam. Animals of the same species will tend to have similar behavioural patterns and therefore nutrient dispersal patterns, and the fences cause restrictions to nutrient dispersal. So, compared to the Pleistocene, the Anthropocene is seeing a lot more heterogeneity across the landscape in terms of nutrient concentration; nutrients aren’t as spread out and homogenous in distribution as they used to be.

tractor-spraying-fertilizer

Image: phosphate fertilizer

By 2015, not only have our Pleistocene land giants gone extinct, but whale populations have been reduced by 66%-99%.  Whales play a large role in this process of nutrient cycling. Nutrients flow off the land, into the oceans, and sink down to the ocean floor. Whales then come along and feed at the bottom of the water, and when they finally rise to the surface, they release faecal plumes, bringing all the nutrients ingested at the ocean depths back to the surface. These nutrients are then cycled back to the land by seabirds feeding at sea and anadromous fish swimming up rivers and inland. This process and its change over time is summarized and quantified in the image below.

megafauna-poop-cycling

Image: system of nutrient recycling Doughty et al., 2015

Now that the research has been explained, let me return back to why it is important. Conservation efforts have historically often sought to restore nature back to an arbitrary pristine state. The nutrient cycling dilemma illustrates that it is not always a particular “state” that has been lost, but rather a process broken. This process can potentially be restored if the right functional components were to be re-established. So, for example, we should not necessarily be pumping resources into conservation efforts such as mammoth “de-extinction”, when the functional traits of the mammoth can be assumed by other proxy species for various processes.

mammoth-cover-660x267

Image: mammoth de-extinction

Rewilding sits within this ecosystem process paradigm, wherein process restoration and hands-off conservation is the end goal and rewilding is a method for achieving it. For the marine realm, rewilding could simply involve better protection of whales, so that population numbers could increase and percentage of nutrient recycling within the oceans could also increase. A core part of rewilding is the concept of trophic cascades. Whales and other large animals or top predators could play a part in shaping landscapes as well as homogenizing nutrient concentrations across them.

Global supplies of phosphate rock are estimated to be depleted within 50 years. This is a major problem if left untreated because phosphorus is essential to plant growth, crop growth, and therefore human prosperity. Now that scientists have managed to get a better understanding about the reasons behind the shortfalls of the current nutrient recycling process, it could drive a process-based solution to the problem. The need to restore large-scale nutrient recycling  might therefore provide impetus for the protection of biodiversity, rewilding or simply changes to management of farmlands.

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Image: mining for phosphate rock, photo by Lorrie Graham 

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