Doughty, Christopher et al. (2016) Global nutrient transport in a world of giants. Proceedings of the National Academy of Sciences, 113 (4), 868-873. doi.org/10.1073/pnas.1502549112
Animals Influence Earth’s Biogeochemistry
It is increasingly recognised that 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 – they can move independently using metabolic energy. The evolution of locomotory structures has allowed 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
However, since the late-Pleistocene, 10,000 years ago, 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 human-dominated world today. 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 2 highlights the global decrease in animal-mediated P transport over the last 10,000 years.
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. Plants, microbes, animals and humans all benefit from the legacy of bygone behemoths and their capacity to disperse nutrients. But will we let today’s contemporary giants continue this ecosystem service?