Trophic Downgrading and its Effects on Biodiversity
In our world that is constantly changing, wildlife ecosystems are no stranger to change either. As time goes on and humans use more space on land and sea, the removal of high trophic level predators (also known as apex consumers) can lead to drastic changes in community structures within different ecosystems. This relates to the idea of trophic downgrading and the effect it can have on biodiversity.
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Trophic downgrading defined
Trophic downgrading occurs when apex predators are removed from an ecosystem. This leads to a cascading effect down the food web that the consumers are in and can have ecological consequences on lower trophic level groups. These consequences can shift the dynamic of animal communities and make them more vulnerable to new disease or invasion from a new species. Some examples of these effects can be seen with the removal of sea otters from shallow seawater, the removal of sea stars from intertidal zones, and more across different ecosystems on the planet.
The following pictures from James Estes’s “Trophic Downgrading of Planet Earth” depict the outcome various environments face depending on the presence or absence of apex consumers in those ecosystems¹.
(A) Sea otters play a major role in enhancing kelp growth by eating sea urchins to prevent them from consuming kelp.
(B) Similarly, sea stars are important in promoting species diversity by preventing mussels from being too dominant over other groups.
(A) The Arctic fox plays a role in shifting the ecosystem they are present in from grassland to tundra by consuming seabirds, in turn limiting nutrients from the ocean.
(B) Both jaguars and cougars promote plant survival by preying on different herbivores that normally cause the forest to not be as green.
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Effect on biodiversity and different food webs
The effect trophic downgrading can have on ecosystems is not just limited to an apex consumer’s own food chain. Their removal can affect other food webs and overall biodiversity as well. By analyzing different apex consumer’s impact in more detail, it can be seen that the absence of sea otters results in the collapse of kelp forests after these primary producers are consumed by sea urchin populations that are left uneaten. Resultantly, kelp forest fish abundance decreases with the lack of kelp and forces a shift in the diet of animals that consume these fishes in a separate food web. Typically, bald eagles and gulls eat these fishes in ecosystems where sea otters are present but they have to switch to another food source as a result of trophic downgrading.
Recognizing the overarching effect trophic downgrading can have on our planet is a driver for more research to be conducted in order to better understand how to mitigate and potentially prevent trophic downgrading from occurring.
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Potential Research
In terms of marine ecosystems, anthropogenic activity is one outlet to be explored as the extent of influence it can have on trophic downgrading is still unclear. Being more prevalent now than ever with industrialization, some factors that can affect marine ecosystems include overfishing, oil drilling, coastal development, and foot traffic.
As the ocean is large as it is diverse, these factors can prove to hasten the occurrence of trophic downgrading and can have detrimental effects on producer abundance such as phytoplankton that provide for so many different groups. One apex consumer that can indirectly affect phytoplankton are sea bass as they consume zooplanktivorous fishes, leading to increased zooplankton and decreased phytoplankton.
With deeper research and understanding, conversation strategies can be developed that will prevent the removal of important species that manage these critical food webs.
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References:
[1] Estes, J. et al. “Trophic Downgrading of Planet Earth.” Science 333(6040), 301–306 (2011).