The eaten outpace the eaters in biomass pyramid

You may have learned in biology that as the number of prey animals increases in an ecosystem, so does the number of predators, since they have more food to eat, which helps to support a larger population. That’s a good general understanding about populations within an ecosystem, but it may be overly simplistic, according to a new study.

(Brevard Zoo, MELBOURNE, Fla., 2009) — A kookaburra feeds on a mouse (Will Montague / Flickr Creative Commons)

Yes, the population of predators increases as the population of prey increases, but the two populations don’t increase at the same rate. The biomass in the lower trophic level, with prey species, increases faster than the biomass in the upper trophic level, with the predators. Furthermore, the ratio of proportionality is about the same in many diverse ecosystems on Earth. That’s what Ian A Hatton, professor of biology at McGill University in Montréal, et al reported in a study published in the journal Science.

The limit on the number of organisms an ecosystem can support is known as the “carrying capacity” of that ecosystem. The capacity is limited by the availability of resources within the ecosystem, including living and nonliving resources like food, water, space, and other environmental conditions.

Graph shows alternation between reproducing organisms and replenishing environment. (Mainland High School)

The document for the Next Generation Science Standards lists predation and disease among the factors that affect carrying capacity, and I suppose they do in some regards; they just aren’t the most influential factors. This is yet another gap in the disciplinary core ideas in the standards, and it should be closed before we test these new standards.

Learning about carrying capacity helps us understand many facts about our planet’s ecology, including how we might protect endangered species. “If we went to India, for example, it could help figure out how many tigers there should be,” Dr Hatton was quoted as saying. The NGSS should do better in prescribing how we teach important concepts like this, or they should just stay out of making any attempt to tell biology teachers what information is important.

This research, which looked for common patterns in hundreds of diverse ecosystems around the world, shows that resources are finite. Predators typically go after young and old individuals in their prey populations, so more prey biomass needs to be created in order to compensate for individuals taken out of the populations through predation.

Plotting the log of the predator population against the log of the prey population in terms of biomass, Dr Hatton and his colleagues found, results in a slope of about ¾. This rule seems to be consistent whether we’re looking at ecosystems on the African plain, in a pond, or even in the body of other organisms.

Major factors affecting carrying capacity

  • Biotic factors

Food availability is the biggest biotic factor affecting carrying capacity. Carnivores need prey to be available and won’t suffer from food stress as long as prey is abundant. Herbivores, on the other hand, can suffer from either a shortage of food or a shortage of nutritious food. They have “preferred” foods, which satisfy their nutritional needs, known as staple foods, but they can resort to “emergency foods” that satiate their appetites but don’t increase their body weight.

  • Abiotic factors

Water is needed for food digestion, regulation of body temperature, and elimination of waste products from animals’ bodies. The amount of water needed for the normal functioning of their organs is proportional to their size. Plants just die if water is scarce, which means animals emigrate or die. Animals that remain then have to compete for any water, and they become weak, can’t fight off disease, and can’t fend off predators.

Conditions in the area also affect its carrying capacity. The present study showed that in a lush savanna ecosystem, there are three times more prey per predator than in a dry desert ecosystem. Also, if an environment is close to humans, this may bring pollution and other negative impacts on an ecosystem’s carrying capacity. Natural disasters—especially hurricanes and floods—also change how well an environment can support animal or plant populations. Erosion can also affect the carrying capacity, such as when it’s difficult to support crops in bad soil to feed humans and farm animals.

Finally, the amount and quality of space affects the carrying capacity of an ecosystem. The present study suggests “a systematic form of density-dependent growth and is intriguing given the parallels it exhibits to growth scaling at the individual level, both of which independently follow near ¾ exponents.” What happens, of course, in deserts for example, is that animals don’t have a place to hide and keep their young safe from predators. Those prey animals then have a higher chance of being eaten and won’t reach reproductive age. Deserts therefore have much lower carrying capacities for animals than forests, not only because of the limited water but because of the lower quality of space provided.

About the Author

Paul Katula
Paul Katula is the executive editor of the Voxitatis Research Foundation, which publishes this blog. For more biographical information, see the About page.