Large Mammals Help Maintain Ecosystems by Fertilizing Plants

Humans love animals of all kinds and sizes. Our enjoyment and care for these creatures fuels our passion to preserve them. To buttress our efforts, new research has revealed insight about animal-plant ecosystems that may motivate us even more.

A synergy exists between plants and animals. Plants supply food to animals. Animals, in turn, supply nutrients to plants¹ in the form of waste products and contribute to an ecosystem’s biogeochemistry.² Nutrients in animal waste affect the primary production of plants in two ways: (1) the amounts of nutrients deposited, and (2) the ratios of different nutrients.

Mammal Body Size and Nutrient Distribution
A new field study conducted in Hluhluwe-iMfolozi Park, a large game reserve in the Republic of South Africa, by an international team of four conservation ecologists led by Elizabeth le Roux sought to determine the impact of mammal body size on nutrient distribution.³ Le Roux’s team affirmed two conclusions from previous field studies and ecological models and made a new discovery.

One affirmation of previous studies⁴ indicated that for any given habitat region, the overall herbivore biomass remains roughly constant. Evidently, plant productivity establishes an upper limit on the total herbivore biomass. A second affirmation of a previous study by le Roux’s team⁵ revealed that the total amounts of urine and feces deposited by herbivores in a given region is roughly independent of the average herbivore body size.

The new discovery made by le Roux’s team shows that as the average herbivore body size increases, the ratio of nitrogen to phosphorus in the nutrients deposited by the herbivores also increases. Specifically, her team measured the difference between habitat regions where the largest herbivores present were impalas compared to regions where elephants were the largest herbivores. They also noted outcomes for varying ratios of impala populations relative to elephant populations. They established a direct correlation between the average body size of a herbivore community and the nitrogen-to-phosphorus ratio in the herbivore deposits.

Le Roux’s team determined that the fecal nitrogen content increased logarithmically with adult mammal body size. On the other hand, fecal phosphorus content decreased logarithmically with adult mammal body size. Nevertheless, average adult mammal body size was a poor predictor of the nitrogen-to-phosphorus ratio in the ecosystem grasses.

Predation Risk, Rainfall, Nitrogen Fixation, and Nutrient Distribution
The reason why the nitrogen-to-phosphorus ratio was roughly constant across ecosystem grasses is because the largest mammalian herbivores (such as elephants) preferentially feed in regions where there is a low abundance of nitrogen-fixing plants. Conversely, the smaller mammalian herbivores preferentially feed in regions that possess a high abundance of nitrogen-fixing plants. Predation risk and rainfall explain these preferences.

Herbivore mammals with body masses greater than 1,000 kilograms (2,200 pounds), such as elephants and rhinoceroses, are virtually invulnerable to predation but they need to consume enormous quantities of food. Hence, these megaherbivores preferentially feed in higher rainfall regions that support dense vegetation, typically a mix of grasses, brush, and trees. Smaller herbivore mammals avoid such regions owing to the heightened predation risk. They seek high visibility regions, ecosystems with few trees and bushes and shorter grasses, where it is much more challenging for predators to ambush them.

Crucial Roles of Mammalian Megaherbivores
The team’s field studies reveal the unique and important roles that megaherbivores play in maintaining ecosystem balances. Without them, ecosystem soils would receive uneven amounts of nutrients and high-predator-risk regions would become overgrown with low-quality vegetation.

Le Roux and her colleagues did not mention that megafauna (large animals) roam over territories much larger in size than those of smaller animals. Hence, they fulfill vitally important roles in redistributing nutrients from hotspots to nutrient-poor regions.

Le Roux and her team end their paper by pointing out that the rise of human civilization has dramatically reduced the population levels of mammalian megaherbivores. These megaherbivores, while invulnerable to nonhuman predation, are prime and easy targets for human predation. The team cites research that establishes that between 50,000 years ago and now, about 50% of mammal species with adult body masses greater than 44 kilograms (97 pounds) have become extinct.⁶ They also cite research showing that at the start of the late Pleistocene, about 20,000 years ago, the average body mass of the world’s mammals was greater than 100 kilograms (220 pounds) but since then has collapsed to less than 10 kilograms (22 pounds).⁷

One consequence of the severe reduction in mammalian megaherbivore population levels has been an equally severe reduction in long-distance cycling of key nutrients (fertilization). In some cases, the long-distance cycling of key nutrients has plummeted to just 4–8% of what it was several centuries ago.⁸

All these scientific studies provide ecological and economic reasons for doing whatever we can to restore mammalian megaherbivore population levels. However, there are also aesthetic and theological reasons. Wild megaherbivores bring delight to humans of all ages. Wouldn’t it be wonderful if humans did not need to travel thousands of miles to a few protected preserves to enjoy seeing just a few megaherbivores in their natural environment?

We have also been given a mandate from God. According to the creation texts in Genesis and Job, we humans have been appointed as stewards to manage Earth’s resources for our benefit and the benefit of all other life. That stewardship includes caring for Earth’s megafauna.

Endnotes

1. S. J. McNaughton, F. F. Banyikwa, and M. M. McNaughton, “Promotion of the Cycling of Diet-Enhancing Nutrients by African Grazers,” Science 278, no. 5344 (December 5, 1997): 1798–1800, doi:10.1126/science.278.5344.1798; Richard D. Bardgett and David A. Wardle, “Herbivore-Mediated Linkages between Aboveground and Belowground Communities,” Ecology 84, no. 9 (September 2003): 2258–68, doi:10.1890/02-0274.
2. Oswald J. Schmitz et al., “Animals and the Zoogeochemistry of the Carbon Cycle,” Science 362, no. 6419 (December 7, 2018): id. eaar3213, doi:10.1126/science.aar3213.
3. Elizabeth le Roux et al., “Animal Body Size Distribution Influences the Ratios of Nutrients Supplied to Plants,” Proceedings of the National Academy of Sciences USA 117, no. 36 (September 8, 2020): 22256–63, doi:10.1073/pnas.2003269117.
4. John Damuth, “Population Density and Body Size in Mammals,”Nature 290 (April 23, 1981): 699–700, doi:10.1038/290699a0; M. J. Coe, D. H. Cumming, and J. Phillipson, “Biomass and Production of Large African Herbivores in Relation to Rainfall and Primary Production,” Oecologia 22 (December 1976): 341–54, doi:10.1007/BF00345312.
5. Elizabeth le Roux, Graham I. H. Kerley, and Joris P. G. M. Cromsigt, “Megaherbivores Modify Trophic Cascades Triggered by Fear of Predation in and African Savanna Ecosystem,” Current Biology 28, no. 15 (August 6, 2018): 2493–99, doi: 10.1016/j.cub.2018.05.088.
6. Anthony D. Barnosky, “Megafauna Biomass Tradeoff as a Driver of Quaternary and Future Extinctions,” Proceedings of the National Academy of Sciences USA 105, supplement 1 (August 12, 2008): 11543–48, doi:10.1073/pnas.0801918105.
7. Felisa A. Smith et al., “Body Size Downgrading of Mammals over the Late Quaternary,” Science 360, no. 6386 (April 20, 2018): 310–13, doi:10.1126/science.aao5987.
8. Christopher E. Doughty et al., “Global Nutrient Transport in a World of Giants,” Proceedings of the National Academy of Sciences USA 113, no. 4 (January 26, 2016): 868–73, doi:10.1073/pnas.1502549112.