The value of a species

The earth is currently undergoing the sixth mass extinction crisis in the last half-billion years.  Some have described our current crisis as the largest loss of plants and animals since the dinosaurs 65 million years ago.  With so much loss it is hard to understand how these extinctions are impacting our ecosystems.  After all, how important is a single species to its environment? A number of scientists have delved into this very question and below I discuss some of their findings.

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In 2014, the Living Planet index, which measured more than 10,000 representative populations of mammals, birds, reptiles, amphibians and fish estimated that more than half of those groups combined have been lost in the last 50 years. Source: http://www.usatoday.com/story/news/nation-now/2015/06/26/wildlife-statistics-extinction/29245459/

Each species in an ecosystem plays a role that impacts other species and the environment around them.  These roles can sometimes be performed by more than one species and when this happens it is called functional redundancy.  As an oversimplified example, if two different predators prey on a fish population, and one of those predators is lost, the other could theoretically pick-up the slack and continue to prey on the fish population, keeping the ecosystem in balance.

foodweb

An example of the complexities of an ecosystem and how each species can impact other species and the environment around them, By Thompsma – Own work, CCo, Source: http://commons.wikimedia.org/w/index.php?curid=9406352

One of the most well-known, obvious, and dramatic examples of a species with no functional redundancy is the sea otter in kelp forests.  Sea otters prey on sea urchins, which in-turn prey on kelp.  If the sea otters are removed, the sea urchin population explodes, mowing-down the kelp and killing off the forest; a bad situation for all of those species that rely on that kelp forest to survive (to learn more about these types of trophic cascades see here).

640x360-no-otters-no-kelp1

Images of a kelp forest with a healthy population of sea otters (left) and without (right). The picture on the right is also known as an urchin barren. Source: http://seaotters.com/2013/05/why-are-sea-otters-important-no-sea-otters-no-kelp-forest/

However, there are other less obvious examples of species that can play important roles with little functional redundancy, and go unnoticed until lost.  One study in particular demonstrated that the removal of a single lower trophic level species in a tropical environment impacted an entire river.  This species was a migratory detritus-eating fish called, flannelmouth characin (Prochilodus mariae).

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Flannelmouth characin (Prochilodus mariae). Source: http://www.fishbase.org/summary/51243

Detritus is dead organic matter that typically includes the bodies or fragments of dead animals and plants, as well as fecal material.  Because flannelmouth can dig-up, eat, and egest large volumes of detritus all while migrating during a time when water and river transport is low, they transport and cycle the nutrients from detritus back into the base of the upstream and downstream foodwebs.

fisher

A fisher uses a cast net to catch flannelmouth characin. Flannelmouth characin are a major protein source for two South American watersheds (Orinoco and Amazon). Source: http://www.dartmouth.edu/~btaylor/VZ_btaylor.html

To test the effect of losing this species from the river, researchers split the river with a barrier running down the center and selectively removed flannelmouth from one side of the barrier, leaving the remaining fish assemblage intact.  Results demonstrated that flannelmouth regulated the fundamental processing, synthesis and degradation of organic carbon for the river with very little functional redundancy.  Thus, unsustainable harvest of these fish from these ecosystems could negatively impact the carbon flow for the whole river.

split-river

Visual differences in benthic particulate matter after removing flannelmouth characin (right) compared with the intact fish assemblage (left). Source: Taylor et al. 2006

Because tropical ecosystems, like coral reefs, are often very diverse (lots of different species), they are assumed to have a lot of functional redundancy (or a high number of species performing similar functions).  However, as seen in the example above, studies are beginning to find that that is not always the case.  In another study examining 6316 species from various places across the world, one group of researchers found 39% to 54% of ecosystem functions had no redundancy, relying on a single species to fill those specific roles.  This means that even with high-diversity many ecosystem functions are vulnerable to species loss.

fig-func-vuln

The number of species per ecosystem function plotted against those ecosystems functions from six tropical-reef fish faunas. The number under the “Red.” to the right side of each figure represents the average functional redundancy. The functional vulnerability (percentage of ecosystem functions having only one species) is illustrated by the horizontal colored lines with arrows. As can be seen, there are a large percentage of functions that only have one species to perform them and only a few functions that have an overwhelming number of species to perform them. Source: Mouillota et al. 2014

A single species can shape its environment, impacting numerous other species within that environment and beyond.  With our current high rate of species loss, scientists are concerned about these losses altering key ecosystem processes, such as nutrient cycling.  Thus, a better understanding and appreciation of each species functional role within our ecosystems may help to prevent the loss of vital ecosystem functions by identifying those species that perform those functions and are in need of conservation efforts.

 

By: Dana Sackett

 

References and other reading material:

 

Bellwood DR, Hoey AS, Choat JH. 2003. Limited functional redundancy in high diversity

systems: resilience and ecosystem function on coral reefs. Ecology Letters, 6:281–285

 

Ceballos G, Ehrlich PR, Barnosky AD, García A, Pringle RM, Palmer TM. 2015. Accelerated modern human–induced species losses: Entering the sixth mass extinctionScience Advances,  1, e1400253. DOI: 10.1126/sciadv.1400253

 

Micheli1 F, Halpern BS. 2005. Low functional redundancy in coastal marine assemblages. Ecology Letters, 8:391–400

 

Mouillota D, Villégera S, Parravicinic V, Kulbickic M, Arias-Gonzáleze JE, Bendera M, Chabanetg P, Floeterf SR, Friedlanderh A, Vigliolai L, Bellwoodb DR. 2014. Functional over-redundancy and high functional vulnerability in global fish faunas on tropical reefs. PNAS, 111:13757–13762. http://www.pnas.org/cgi/doi/10.1073/pnas.1317625111

 

Rosenfeld JS. 2002. Functional redundancy in ecology and conservation. OIKOS, 98:1.

 

Taylor BW, Flecker AS, Hall Jr RO. 2006. Loss of a Harvested Fish Species Disrupts Carbon Flow in a Diverse Tropical River. Science, 313:833-836.

 

http://www.usatoday.com/story/news/nation-now/2015/06/26/wildlife-statistics-extinction/29245459/

 

http://www.biologicaldiversity.org/programs/biodiversity/elements_of_biodiversity/extinction_crisis/

 

http://science.sciencemag.org/content/355/6325/558.full

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