What are they?
Trophic Cascade Experiments
The trophic level of an organism is the position it occupies in a food chain.
Trophic levels can
be represented by numbers, starting at level 1 with plants. Further
trophic levels are numbered subsequently according to how far the
organism is along the food chain.
Trophic Levels (above)
The path along the chain can form either a one-way flow or a food "web".
Ecological communities with higher biodiversity form more complex trophic paths.
level. The plants in this image,
Second trophic level: Rabbits eat plants at the first trophic level, so they are primary consumers.
Third trophic level: Foxes eat rabbits at the second trophic level, so they are secondary consumers.
Fourth trophic level: Golden eagles eat foxes at the third trophic level, so they are tertiary consumers.
Research in a wide variety of terrestrial and aquatic environments has shown that trophic cascades control:
Ecological Cascade Effect
· A series of secondary extinctions that is triggered by the primary extinction of a key species in an ecosystem.
When do trophic cascades occur?
Top Down Cascade
· Is a trophic cascade where the food chain or food web is disrupted by the removal of a top predator, or a third or fourth level consumer.
Bottom Up Cascade
· Occurs when a primary producer, or primary consumer is removed, and there is a reduction of population size through the community.
Ecosystems Without Top Predators
In many instances, trophic cascades have been initiated by human persecution and harvesting of top carnivores such as:
· wolves and big cats in terrestrial ecosystems
· sharks, tunas, and game fish in aquatic ecosystems.
Biomanipulation in Lakes
In lakes, trophic cascades are used to improve water quality through biomanipulation.
Terrestrial Trophic Cascades
Earliest documented trophic cascades all occurred in lakes and streams.
Subsequent research has documented trophic cascades in terrestrial ecosystems, including:
Reintroduction of Gray Wolves
The gray wolf was locally extinct by 1920.
Was reintroduced to Yellowstone NP in 1995 and 1996.
Since then a three-tiered trophic cascade has been reestablished involving:
Examples of this phenomenon include:
· A 2-3 fold increase in deciduous woody vegetation cover, mostly of willow, in the Soda Butte Creek area between 1995 and 1999.
· Heights of the tallest willows in the Gallatin River valley increasing from 75 cm to 200 cm between 1998 and 2002.
· Heights of the tallest willows in the Blacktail Creek area increased from less than 50 cm to more than 250 cm between 1997 and 2003. Additionally, canopy cover over streams increased significantly, from only 5% to a range of 14-73%.
· In the northern range, tall deciduous woody vegetation cover increased by 170% between 1991 and 2006.In the Lamar and Soda Butte Valleys the number of young cottonwood trees that had
The diagram (above) illustrates trophic cascade caused by removal of the top predator. When the top predator is removed the population of deer is able to grow unchecked and this causes over-consumption of the primary producers.
Trophic Cascade Impact on Biodiversity
Trophic cascades also impact the biodiversity of ecosystems, and when examined from that perspective wolves appear to be having multiple, positive cascading impacts on the biodiversity of Yellowstone National Park. These impacts include:
· Scavengers, such as ravens (Corvus corax), bald eagles (Haliaeetus leucocephalus), and even grizzly bears (Ursus arctos horribilis), are likely subsidized by the carcasses of wolf kills.
· In the northern range, the relative abundance of six out of seven native songbirds which utilize willow was found to be greater in areas of willow recovery as opposed to those where willows remained suppressed.
· Bison (Bison bison) numbers in the northern range have been steadily increasing as elk numbers have declined, presumably due to a decrease in interspecific competition between the two species.
· Importantly, the number of beaver (Castor canadensis) colonies in the park has increased from one in 1996 to twelve in 2009. The recovery is likely due to the increase in willow availability, as they have been feeding almost exclusively on it. As keystone species, the resurgence of beaver is a critical event for the region. The presence of beavers has been shown to positively impact the following:
o streambank erosion,
o sediment retention,
o water tables,
o nutrient cycling, and
o both the diversity and abundance of plant and animal life among riparian communities.