What is a Trophic Cascade?

What is a Trophic Cascade?


According to the Encyclopedia Britannica, a trophic cascade is 'an ecological phenomenon triggered by the addition or removal of top predators and involving reciprocal changes in the relative populations of predator and prey through a food chain, which often results in dramatic changes in ecosystem structure and nutrient cycling.  In a three-level food chain, an increase (or decrease) in carnivores causes a decrease (or increase) in herbivores and an increase (or decrease) in primary producers such as plants and phytoplankton.'

In the language of non-biology majors, a trophic cascade is simply a chain of events, and their subsequent results, that take place when a predator at or near the top of the food chain of a particular ecological environment is either added to or removed from that environment.

The Pacific Northwest, the area I grew up and continue to live in today, is home to several well known examples of trophic cascades: the sea otter - sea urchin - sea kelp chain along the Pacific Coast, and the grey wolf - deer - plant community chain in Yellowstone National Park.


How do Trophic Cascades Occur?


There are many factors that have a hand in causing cascades.  The biggest players in the game are climate change and human impact.

Climate change has always been a naturally occurring phenomenon, but the rate at which it occurs has been dramatically increased in recent decades due to human factors, particularly the burning of fossil fuels.  As the Earth warms, habitat ranges of many animals can either extend, contract, or shift altogether.  Predators at the top of the food chain could be hard put to find food, and thus their population numbers will decrease over time.  A decrease in predators leads to a blooming increase in mid-level prey.  In many cases, this level consists of herbivores.  Without carnivorous predators, the herbivores see huge a population increase.  This seems to be good for the population at first, but over time the population becomes so big that the environment becomes stripped of resources to support it.  The plant community steadily decreases and thus, the mid-level prey population crashes.  The decrease in availability of food for the herbivores is likely to have adverse affects on the plant diversity of the environment.


Humans can easily cause the same results as climate change in two general ways.  First, we decrease habitat by both amplifying climate change and through human population growth.  Second, we directly remove predators from their environment by either over exploiting their populations or their particular choice of food and other resources.

Trophic cascades are not unique to the Pacific Northwest.  In fact, they occur in every environment, from tropical rain forests to the world's oceans.  Some biological chains are more difficult to study than others, but all have similar affects on animal and plant populations and species diversity.