The collective impact of animals in fertilising ecosystems across the world can offer much to management systems for climate resilience and nature conservation.
Animals influence Earth’s biogeochemistry
Animals are not simply passive recipients of a world organised by powerful non-living forces such as climate, soil and fire. Instead they are key actors, exerting their own biological control on the composition and function of ecosystems across the world .
Most animals are motile; that is to say that they can move independently using metabolic energy. The evolution of locomotory structures allows animals to find food, sexual partners, and places of security. However, it also confers to them an ability to connect the landscapes through which they pass. Like an erratic public transport service, animals pick up passengers of seeds, nutrients and microorganisms in one location before dropping them off sometime later in another. Because of their high food consumption rates, long gut residence times and sizeable movement ranges, large animals – or megafauna as they are known – are thought to be disproportionately important to this network of mass transportation .
A biologically linked system: The phosphorus cycle
Phosphorus (P) is vital to all of life on Earth and constitutes one of the main ingredients in agricultural fertilisers. Ultimately, P is derived from the weathering or mining of rocks, before spending time in various pools of soil, vegetation and animals. Eventually though, P succumbs to the pull of gravity and is washed out of the biosphere and into the deep ocean, where it is buried in sediments. Accordingly, the geography and abundance of this element is essential for the structure and functioning of life across the planet.
Long before human civilisations began plying their fields with excessive loads of P, however, wild systems had their own internal mechanism of replenishing landscapes with this key nutrient. Animals provide mobile linkages between nutrient-rich and nutrient-poor areas. By eating, moving and defecating, animals disperse P and other minerals more evenly throughout the landscape and in doing so, stimulate more fertile, healthier ecosystems. Or in other words, life makes the planet more habitable for life.
There are four key ways in which animals play an important role in the P cycle:
Retention: Some species, such as beavers, engineer ecosystems in ways that can reduce P runoff, retaining vital nutrients in upstream aquatic and riparian environments.
Filtering: Animals such as oysters and mussels filter nutrients from the water column. The bio-deposits that bivalve suspension feeders deposit play an important role in reducing the deleterious effects of eutrophication.
Upstream movement: Many wild animals, including whales, fish and seabirds transport P from the deep ocean back to terrestrial systems. This biological pump importantly bioaccumulates a distributed source of P and converts it into a concentrated form in the bodies of marine animals .
Distribution: Herbivores, scavengers and carnivores then distribute P across the land. Animal digestion accelerates the cycling of nutrients to more labile forms in excreta.
In this biologically linked system, the megafauna can be considered as nutrient arteries, carrying P great distances as they move along their migration routes, find water or defend territories. Whilst smaller animals, such as dung beetles, play an equally important role as nutrient capillaries, diffusing P more evenly in their local environment.
The loss of megafauna
Since the late-Pleistocene, megafauna have been selectively removed from ecosystems across the world. Due to their low population densities, slow reproductive rate and threat to early human civilisations, these animals were particularly prone to extinction . Whilst some megafauna species escaped complete elimination, their population sizes and ranges have been profoundly altered as they survived into the Anthropocene. A recent global assessment of the world’s remaining 362 megafauna species found that >70% had decreasing populations with 59% facing extinction . The loss of these animals has drastically reduced their contribution to nutrient dispersal. Figure 1 highlights the global decrease in animal-mediated P transport over the last 10,000 years .
Applications to the UK
Like many places across the world, the UK is living in the shadows of its lost giants. Colossal animals including the woolly mammoth (Mammuthus primigenius) and Irish elk (Megaloceros giganteus) once roamed freely here. But today domesticated livestock command the shores, having forged a powerful (albeit one-sided) partnership with people. Our countryside is a patchwork of fenced fields, no longer home to a diversity of free ranging wild animals. We artificially pump our fields with industrial fertiliser, whilst our wilder ecosystems become depleted and undernourished.
So, today we must instead look to the ocean to find Britain’s last remaining large wild animal species. Here, whales, dolphins and seals are still masters of the waves. Whilst low in number due to whaling, pollution and fishing-net entanglement, these marine mammals play a crucial role by transporting lost nutrients from ocean depths back to the coastal environment. In doing so they create life for many others. Puffins, gannets and kittiwakes flock in great numbers, further distributing these same nutrients from the ocean onto land as they return home to their rocky cliffs. Our remaining migrating fish, too, on their journey up tumbling mountain rivers also deliver nutrients to the land. Britain’s marine animals enrich not only their own aquatic homes, but also the earth beneath our feet.
If current extinction trends continue unabated, in two hundred years time, the largest animal to roam our Earth will be the domesticated cow (Bos taurus) . Humanity is currently grappling with the breakdown of both our climate and biodiversity systems. The collective impact of animals in altering global biogeochemical cycles links these crises, and thus offers limited hope. Under the growing conservation movement of rewilding, the practice of restoring biodiversity can remove carbon dioxide from the atmosphere, fertilise wild ecosystems and reduce the deleterious consequences of eutrophication. Where they are allowed to flourish, feedbacks within natural systems will help degraded landscapes bounce back over the coming century, increasing their contribution to ecological and climate stability.
In medieval England, peasants could graze their sheep on the land of nobility, but faced severe punishment if caught removing their droppings . The importance of animal manure has been understood on a local scale for hundreds of years. Research now shows that it also plays a role at a much larger scale. In light of the growing body of evidence for the role of animals within global nutrient cycles, it is imperative that we design land management systems that facilitate the natural fertilisation of landscapes throughout all of Planet Earth.
- Malhi, Yadvinder et al. (2016) Megafauna and ecosystem function from the Pleistocene to the Anthropocene. PNAS, 113 (4), 838-846.
- Wolf, Adam et al. (2013) Lateral Diffusion of Nutrients by Mammalian Herbivores in Terrestrial Ecosystems. PLOS One, 8(8): e71352.
- Roman, Joe and McCarthy, JJ. (2010) The Whale Pump: Marine Mammals Enhance Primary Productivity in a Coastal Basin. PLOS One, 5(10): e13255.
- Sandom, Christopher et al. (2014) Global late Quaternary megafauna extinctions linked to humans, not climate change. Proceedings of the Royal Society B, 281: 20133254.
- Ripple, William et al. (2019) Are we eating the world’s megafauna? Conservation Letters, e12627.
- Doughty, Christopher et al. (2016) Global nutrient transport in a world of giants. PNAS, 113 (4), 868-873.
- Smith, Felissa et al. (2018) Body size downgrading of mammals over the Quaternary. Science, 360, 310-313.
- Ashley, Ken et al. (2011) A brief history of phosphorus: From the philosopher’s stone to nutrient recovery and reuse. Chemosphere, 84, 737-746.