Newsroom: Animals poop – a critical ecological function!

The distribution of nutrients across a landscape is critical in determining primary productivity (i.e. plant growth) and organism survival. Historically, ecologists have focused on larger-scale environmental forces that affect this distribution, like wind, current, gravity, and erosion. However, in 2019, researchers at UC Davis developed a theoretical framework outlining that animal behavior is another critical force influencing nutrient transport that should be considered as we try to understand how different ecosystems function.

Animals can be a source of nutrients in a variety of ways; for instance, they serve as prey for other organisms, their bodies decay into the ground, and they poop. Several studies have already demonstrated that nutrient input by organisms can be necessary to the organization of certain ecosystems. In this study, a team of UC Davis behavioral ecologists including Alexandra McInturf, Lea Pollack, Dr. Louie Yang and Dr. Orr Spiegel, build on these findings to suggest that animals may also serve a unique role in how nutrients are distributed. Specifically, through their behavior, animals provide different pathways through which nutrients can move. This study highlights three major features that allow them to do so.

First, animals can move in ways unachievable by passive forces. For instance, different organisms exhibit behaviors that take them up hills, opposite winds and currents, and against gravity. Spawning salmon (Oncorhynchus spp.) swim upstream from the ocean hundreds of kilometers to spawn, which can double or triple the amount of nitrogen and phosphorous (key nutrients) in upstream habitats [1, 2, 3]. Similarly, deep-diving whales in areas like the Gulf of Maine have to surface (move against gravity) in order to breathe. Once at the top of the water column, these animals will often poop. In doing so, they bring nutrients acquired from prey they find at depth to the upper layers of the ocean [4].

The “Whale Pump” (Roman & McCarthy, 2010). Great whales recycle nitrogen from deeper waters into the photic zone through a mechanism known as the “whale pump”. These organisms feed at depth (>100 meters) on fish and zooplankton in the waters through which they migrate, but must rise to the surface to breathe, often releasing waste in the process. Nutrients from the waste (ie. nitrogen) are utilized in the surface waters for growth and photosynthesis by phytoplankton, which are then consumed by zooplankton and fish. This whale pump counter-acts the pull of gravity on poop and other biological material from the surface (here, described as the “Biological Pump”).

Second, animals actively respond to the patterns of nutrients that they generate. By continuing to occupy a certain area where they defecate and feed, they can create ecological “hotspots” and actively improve their own habitat. The nutrient input into the ground from their food remnants and feces allows plants to grow. These plants can either feed the organism itself (if it’s an herbivore), or attract other organisms to the area that may serve as prey. The cycle then continues through a positive feedback loop. Similarly, negative feedback loops can occur when organisms actively avoid certain areas and limit the amount of nutrients those areas are getting. Because those habitats do not have sufficient nutrient input, other organisms and species then also avoid those areas, and the negative feedback loop continues. In sum, organisms are able to cultivate or avoid certain areas or habitats. In doing so, they affect where nutrients will be transported.

Third, animals interact with each other. Those interactions also affect nutrient distribution. Animals that are social or tend to gather, like wildebeest, deer, and seabirds, deposit vast amounts of nutrients in the same location at the same time. However, the presence of predators may determine the areas that potential prey species can safely occupy, which in turn would also affect where those species can deposit nutrients.

Example of behavioral features of animals that affect nutrient transport (McInturf et al. 2019). Many marine birds bring nutrients obtained from fish prey out at sea to otherwise barren islands (orange arrows). In these cases, 1) the birds transport nutrients counter-directionally to gravity and over greater distances than other environmental forces could achieve. 2) This influx of nutrients increases primary productivity (plant growth) on the islands which transforms the island into habitat more suitable for the seabirds themselves. 3) Different seabirds may be attracted to the presence of others on the island, and roost here themselves (bringing in more nutrients to continue to the positive feedback loop).

Generally, this study concludes that the role of animal behavior in nutrient distribution remains underappreciated. The authors suggest that ecologists continue to integrate behavior into future empirical studies. This may be particularly important if human-induced environmental change begins to affect animal movement patterns, as such alterations could lead to dramatic and unexpected consequences for how ecosystems continue to function in the future.  

For more information:

McInturf, A. G.*, Pollack, L.*, Yang, L. H.*, & Spiegel, O.* (2019). Vectors with autonomy: what distinguishes animal‐mediated nutrient transport from abiotic vectors?. Biological Reviews94(5), 1761-1773.

*Stars denote authors currently or previously affiliated with UC Davis.

References:

  1. Helfield, J.M. & Naiman, R.J. (2001) Effects of salmon-derived nitrogen on riparian forest growth and implications for stream productivity. Ecology 82, 2403–2409
  2. Holtgrieve, G.W. & Schindler, D.E. (2011) Marine-derived nutrients, bioturbation, and ecosystem metabolism: reconsidering the role of salmon in streams. Ecology 92, 373–385.
  3. Deacy, W., Leacock, W., Armstrong, J.B. & Stanford, J.A. (2016) Kodiak brown bears surf the salmon red wave: direct evidence from GPS collared individuals. Ecology 97, 1091–1098.
  4. Roman, J. & McCarthy, J.J. (2010) The whale pump: marine mammals enhance primary productivity in a coastal basin. PLoS ONE 5, e13255.

Cover Photo Citation: Wikimedia Commons

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