Download PowerPoint - Susan Schwinning

Community Ecology
Definitions:
A community is a group of species populations that
occur together in the same place and the same time.
Community ecology studies the patterns of species
distributions and abundance, rules of species
assembly, dynamics and stability.
The Topics of Community Ecology
Succession: how does open habitat become
populated by first few than many species?
Assemblage: Are there any rules that determine
which species live together?
Coexistence: how do many species requiring
essentially the same habitats and resources coexist?
Food webs: what are the feeding relationships
between many members of a community?
Food chains and webs
- the pathway of food and energy transfer through the organisms of a community -
Simplified food web for the
Northwest Atlantic
Southern California kelp forest
Definition: Trophic Level
Organisms can be categorized by how many levels they are removed from
eating primary producers- plants. An organism’s trophic level is its “distance”
from the primary producers at level 1.
1st :
2nd :
3rd :
4th :
primary producers
primary consumers (vegetarians)
secondary consumers (carnivores I)
tertiary consumers (carnivores II)
gulls
oystercatchers
mussels
barnacles
Lottia digitalis
Wootton 1992.
(Tatoosh Island, Washington)
Lottia strigatella
algae
Lottia pelta
birds present
(Townsend, Begon and Harper 2003)
birds excluded
(Townsend, Begon and Harper 2003)
Effect of bird removal:
+
+
release from
predation
release from
predation
increased
competition
mussels
Lottia digitalis
barnacles
Lottia strigatella
Lottia pelta
+
increase in
safer habitat
Wootton 1992.
(Tatoosh Island, Washington)
increased
+
increased
competition
competition
-
decrease in
safer habitat
Complex food webs contain complex indirect interactions:
• Enemies of your friends are your enemies (reduced predation of a
mutualist can lead to increases in abundance).
• Enemies of your enemies are your friends (reduced predation of a
competitor can lead to decreases in abundance).
Sometimes, indirect effects are stronger than direct effects.
Sometimes, indirect and direct effects cancel one another.
The presence or absence of top predators often fundamentally
changes community structure.
What controls the herbivores in food webs?
Bottom-up control:
Primary production sets a limit on herbivore density.
(in low-productivity environments)
To-down control:
Carnivores control herbivore density which in turn
control the density of primary producers.
(in high-productivity environments)
Keystone species:
a species whose removal would have significant effects on
community structure and/or diversity.
In the construction of arches, the
keystone holds all other stones in place.
Some recognized keystone species:
Black-tailed prairie dogs to the prairie community
Salmon to Canada’s coastal rainforests
Sea otters to Pacific coastal waters
Sea otters maintain a healthy kelp forest:
Salmon maintain Canada’s coastal rainforests:
Prairie dogs maintain prairies.
Food chains and trophic structure
Most food chains have 4 trophic levels or less.
Schoener 1989
Schoenly 1989
Why are so many food chains short?
• Food chains run out of energy to support viable population sizes at
higher trophic levels.
• Long food chains are unstable (Pimm and Lawton 1977).
• There could be longer food chains, but area in each of earth’s
ecosystems is too small to support another trophic level.
Productivity pyramid:
Tertiary consumers
Secondary consumers
Primary consumers
Primary producers (plants)
On average, a consecutive trophic level has only 10-20% of the
biomass growth (often quantified as biomass-C) of the previous level.
Productivity
Biomass
Species
(excluding plants and parasites)
(Terrestrial Ecosystems)
(Aquatic Ecosystems)
More productive ecosystems do not have longer food chains!
Mean net primary productivity:
5-100 g m-2 yr-1
Mean net primary productivity:
1200 g m-2 yr-1
But, some low productivity biomes do have shorter food chains.
Mean net primary productivity:
10 - 400 g m-2 yr-1
Why are so many food chains short?
 Some low productivity food chains run out of energy to support higher trophic
levels, but high productivity food chains that could support more trophic levels do
not.
Pimm and Lawton compared the stability of a large
number of hypothetical food chains, all including four
species, but differently arranged:
Increasing tendency to return to
equilibrium after perturbation
With omnivory and competition:
Increasing tendency to return to
equilibrium after perturbation
However, each omnivorous trophic web was much less
stable than its non-omnivorous counterpart
Why are so many food chains short?
 Some low productivity food chains run out of energy to support higher trophic
levels, but high productivity food chains that could support more trophic levels do
not.
 Long food chains are more unstable (Pimm and Lawton 1977).
- perhaps, but the model is really too simple.
The limited area hypothesis:
Tertiary consumers
Secondary consumers
Primary consumers
Primary producers (plants)
If we have more area at the bottom, we get more total primary
production over a whole region and perhaps enough fuel to sustain one
more trophic level at the top.
Larger areas contain more species.
(Unpublished data from Jake Vander Zanden for lakes in Canada)
Species diversity usually declines with increasing trophic levels.
Schoenly et al. 1991
Why are so many food chains short?
 Some low productivity food chains run out of energy to support higher trophic
levels, but high productivity food chains that could support more trophic levels do
not.
 Long food chains are more unstable (Pimm and Lawton 1977).
- perhaps, but the model is really too simple.
 Long food chains (> 4 trophic levels) do seem to require a larger ecosystem
(larger area for terrestrial, greater volume for aquatic).
- whether larger areas would allow longer trophic chains is
untestable on this earth.
Summary

Food chains and webs are generally very complex, such that many
species are weakly interacting either directly (through a feeding
relationship) or indirectly (through feeding relationships once or twice
removed).

This implies that the removal of one species can have many trickledown effects in the food web.

Some species are more important than others: top predators and other
key stone species. Their removal can lead to the loss of an entire
community.

There appear to be strong limits on trophic chain length but we don’t
know why.

In theory food chains could be longer but in practice they seldom are.
Do higher trophic levels run out of energy or do they become too
unstable?