Trophic downgrading

While visiting the Monterey Bay Aquarium for the first time, I was reminded of an article in Science discussing the consequences of globally removing large apex predators from our ecosystems.  Below I share some information from my visit and from this article.

I walked up to the renowned three-story display of the kelp forest in the Monterey Bay Aquarium and stared. Despite the crowds and clatter of people around me, this beautiful floating forest with a diverse array of fish darting through the swaying stalks and leaves was mesmerizing. At the front of the crowd, an employee of the aquarium stood describing how kelp forests provide nursery grounds and habitat to a large number of fish species.

 

Pictures of a kelp forest. Source

Being a native to the east coast, I had never before had the opportunity to see a kelp forest. As I stood watching the display, I could hear my undergraduate ecology professor in my head describing how kelp forests held one of the most common examples of a key stone species, the sea otter, and top-down control of an ecosystem. I also remembered that kelp are the fastest growing plant in the world (20in/day); nearly double the growth rate of bamboo.



Source

Despite this amazing growth rate, kelp can be decimated into “sea urchin barrens” (former kelp forests where sea urchins cover the substrate) in the absence of sea otters. Sea otters eat invertebrates and small fish, but most importantly to kelp, sea urchins. Sea urchins are herbivorous and can clear-cut a kelp forest quickly and with relative ease by scraping the base of the kelp off the sea floor. Without the pressure of sea otters to control the population of sea urchins, kelp forests and the habitat they provide to fishes can be destroyed.

Landscape level affects of a trophic cascade. Shallow seafloor community at Amchitka Island, Aleutian archipelago, before (right; 1971; photo credit: P. K. Dayton) and after (left; 2009) the collapse of sea otter populations. Sea otters enhance kelp abundance by limiting herbivorous sea urchins (Estes and Duggins 1995). Source: Estes et al. 2011.

A recent article in Science describes how the widespread removal of large apex predators from natural ecosystems is one of the most pervasive influences humans have had on nature, particularly because of unexpected impacts from trophic cascades such as the one described above. The problem lies not just in the loss of a species but in the loss of the ecological interactions associated with that species.



Landscape level affects of a trophic cascade. Coral reef ecosystems of uninhabited Jarvis Island (right, unfished) and neighboring Kiritimati Island (left, with an active fishery that removes large fish). Fishing alters the patterns of predation and herbivory, leading to diminished populations of reef-building corals and coralline algae along with large fish (Sandin et al. 2008). Source: Estes et al. 2011. 

Ecological interactions connect an array of species at a range of spatial scales and include biological processes such as predation, competition, mutualism, and physicochemical processes such as influencing water temperatures, nutrients levels, and pollution affects.

  
Landscape level affects of a trophic cascade. A plot in the rocky intertidal zone of central California before (right; September 2001) and after (left; August 2003) seastar exclusion. Seastars increase species diversity by preventing competitive dominance of mussels. [Photo credits: D. Hart]. Source: Estes et al. 2011.

For example, the number of people impacted by malaria near freshwater systems has been linked with the population of predatory fishes because these predators prey on smaller fish that eat mosquito larvae. On an even broader scale, the absence of top predatory fishes can alter phytoplankton density, affecting the rate of primary production and uptake of carbon dioxide, changing the carbon flux between lakes and the atmosphere.



Landscape level affects of a trophic cascade. Long Lake (Michigan) with largemouth bass present (right) and experimentally removed (left). Bass indirectly reduce phytoplankton (thereby increasing water clarity) by limiting smaller zooplanktivorous fishes, thus causing zooplankton to increase and phytoplankton to decline (Carpenter et al. 2001). Source: Estes et al. 2011. 


The authors call this phenomemon of removing large apex predators and the resultant cascading affects on our ecosystems trophic downgrading. The difficulty with identifying a system with top-down control (and the potential for trophic cascades) is that the affects are often not seen until after the apex predator has been removed, at which point our ability to restore top-down control is very limited if not lost entirely.


Landscape level affects of a trophic cascade. Pools in Brier Creek, a prairie margin stream in south-central Oklahoma with (right) and lacking (left) largemouth and spotted bass. The predatory bass extirpate herbivorous minnows, promoting the growth of benthic algae (Power et al. 1985). Source: Estes et al. 2011.

The authors of this article also provide numerous examples of how worldwide trophic downgrading has altered and is still changing human and wildlife diseases, carbon sequestration, invasive species, and biogeochemical cycles. A serious problem considering scientists suggest we are currently in the early to mid-stages of the sixth mass extinction of our planet; a mass extinction largely caused by humans.

Leave a comment about your own experience with trophic downgrading below.

Also be sure to read about how we are giving away a free fishing rod and reel!



Dana

References:

http://libraries.ucsd.edu/historyofucsd/newsreleases/1960/19600729.html

Estes JA, et al. 2011. Trophic downgrading of planet earth. Science. 333:301-306.

Raup DM, Sepkoski Jr. JJ.  1982.  Mass extinctions in the marine fossil record.  Science 215:1501.

Ray J, Redford KH, Steneck R, Berger J, Eds.  2005.  Large Carnivores and the Conservation of Biodiversity, Island, Washington, DC.

Wake DB, Vredenburg VT. 2008. Are we in the midst of the sixth mass extinction? A view from the world of amphibians.  PNAS. 105:11466.

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