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Top-Down Forcing:  The Complexity of Nature

Earths many ecosystems contain millions of animal species.  From the smallest mutlticellular organism to the massive blue whales, the our planet serves as host to the most diverse group of guests.  Among these organisms, none are more prolific and acknowledged than the large apex consumers that dominate Earth's food chains.  The animals however are subject to some of the most pervasive of human influences, a reality that has lead to the decline of numerous populations in both aquatic and terrestrial biomes.  Species that reside on the top trophic levels of food webs have a large influence over the structure, function and biodiversity of the their ecosystems.  Because these animals are so influential as apex consumers, their decline has had direct and indirect effects on species much farther down the food chain as well as the disruption of ecosystem functioning.

Top predators have widespread influence upon species further down the trophic levels in a cascading effect known as top-down forcing, sometimes called tropic downgrading.  Added pressures or release from prior pressures is placed upon a species from species above it ("top" to "bottom") rather than from species below it, referred to as bottom-up forcing.   Countless studies have tested the effects of top-down forcing all over the world.  In 2011, an international team of 24 scientist synthesized information from over 30 studies regarding these cascade effects and effectively combined the results to say that "The loss of these animals [apex consumers] may be humankind's most pervasive influence on nature"(Estes 2011).

I will discuss case studies in which the decline of top predators, as well as large herbivores, has resulted in adverse effects of  trophic levels below.  Top-down forcing has been associated with the theory of mesopradoter release- the increase in populations of species that the apex consumer once controlled via a predatory relationship.  Better understanding of top-down forcing is imperative.  It should  be included in biologist's conceptual overviews of nature and integrated into wildlife management and conservation plans in  the future.  In the final page of this website, I proved commentary on a study of large sharks species of the U.S. Atlantic Coast.   It is meant to provide different explanations for the decline of the Bay Scallop in the region.  Although top-down forcing is highly influential on the ecology of many species, it alone is not the sole determinant of the number and abundance of animal species.  Ecosystems are the integration of thousands of different ecological occurrences and species interactions.