Trophic Cascades

 

 

 

 

 

 

 

 

 

What are they?

Trophic Cascade Experiments

  • Freshwater lakes
  • Those experiments showed that trophic cascades controlled:
    • biomass and production of phytoplankton
    • recycling rates of nutrients
    • the ratio of nitrogen to phosphorus available to phytoplankton
    • activity of bacteria
    • sedimentation rates

 

Trophic Levels

The trophic level of an organism is the position it occupies in a food chain.

  • The number of steps an organism is from the start of the chain is a measure of its trophic level.

 

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.
 

  • Level 1: Plants and algae make their own food and are called primary producers.
  • Level 2: Herbivores eat plants and are called primary consumers.
  • Level 3: Carnivores that eat herbivores are called secondary consumers.
  • Level 4: Carnivores that eat other carnivores are called tertiary consumers.
  • Level 5: Apex predators that have no predators are at the top of 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.

 

First trophic level. The plants in this image,
and the algae and phytoplankton in the lake,
are primary producers. They take nutrients
from the soil or the water, and manufacture
their own food by photosynthesis, using energy
from the sun.

 

 

 

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:

  • species composition

  • biomass

  • production of herbivores and plants

 

 

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.

 

 

Examples

  • Overfishing of cod

 

  • The commercial harvesting of sea otters

Biomanipulation in Lakes

In lakes, trophic cascades are used to improve water quality through biomanipulation.

  • stocking of game fish

  • Lake Nokomis Biomanipulation Project aims to reduce algae in Lake Nokomis by adjusting fish populations and reducing bottom-feeding fish that stir up the lake bottom.

 

 

Terrestrial Trophic Cascades

Earliest documented trophic cascades all occurred in lakes and streams.

Subsequent research has documented trophic cascades in terrestrial ecosystems, including:

  • Coastal prairie of Northern California

Yellow lupines

 

 

  • Costa Rican rain forest

Clerid beatle

 

 

Pheidole bicornis

 

 

Piper plant

 

 

 

 

 

Reintroduction of Gray Wolves
to Yellowstone National Park

 

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:

  • wolves

  • elk (Cervus elaphus)

  • woody browse species such as:

    • aspen (Populus tremuloides)

    • cottonwoods (Populus spp.)

    • willows (Salix spp.)

 

 

Aspen

 

Cottonwoods

 

Willows

 

 

 

 

 

 

Link to Yellowstone NP pdf map

 

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.

 

 

 

 

Reintroduction of Wolves
to
Yellowstone National Park
(video)