Biology 2108
Lecture
Ecology: Communites
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Community
- all populations in a specific area
Examples of questions in
community
ecology include: to what degree do populations 'behave' differently
together
then when alone; how do interactions maintain diversity; how important
are these interactions in
determining species composition of a
community;
and how stable are species assemblages over time?
Let's examine the first question: do
populations 'behave'
differently together then when alone?
Types
of interactions between two populations (species)
- Mutualism
- both
species benefit
- Commensalism
-
one species benefits, the other neither benefits or is harmed
- Competition
- both
species are harmed
Species that share similar
niches (
the niche is the range of abiotic and biotic
under which a species can live) are likely to be competing for a
similar limiting resource. What
can happen if two species living in the same community occupy very
similar niches?
- Predation
- one
species benefits, the other is harmed
Are
parasites predators? Are herbivores predators?
The
importance of community interactions:
Persistence
of interactions (how
interactions maintain diversity)
If population
are
not always stable and many interactions between populations are
harmful,
how do species persist over time (how is diversity maintained)?
Examples
of
competition
and predation coexistence:
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 The importance of these
interactions in maintaining
diversity is demonstrated by success of exotic (invasive)
species at
the
expense of native species.
What happens when
humans introduce species
into communities in which they have not evolved?
Why do some species do better in
environments
in which they did not evolve than in they do in the environment
in
which they did evolve?
For example, Rabbits
in Australia
  
Non-native species placed in
a new environment
no longer have to interact with the species that have adapted to take
advantage
of them.
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'The
intermediate disturbance hypothesies' - diversity of a community
tends to be highest where disturbances are of intermediate intensity:
Why is diversity higher where there
is
some disturbance than where there is no disturbance?
Disturbances create
opportunities for fugitive species (those with adaptations to tolerate
harsh environments and to disperse widely)
How does human activity affect the rate and intensity of
disturbance?
So, both species
interaction and phyiscal disturbances are important in maintaining
diversity!
Ecological
communities are dynamic (how species
composition changes over time)
The
number of species may remain fairly constant over time.
For example, the number of species on an island is directly related to
island size (as a result of an
equilibrium between species extinction
and immigration).
How might this be useful in
predicting loss of biodiversity as a result of habitat destruction and
fragmentation?
Changes
in species composition of community over time are well documented:
Ecological
Succession - a fairly
predictable
change in species compostion over time.
Typically,
the series of community
changes begins with a disturbance.
Examples
of succession:
- U.S. Southeast Piedmont Region (e.g.
the KSU
Aboretum)
Succession usually emphasizes plant
community
changes, but these in turn cause succession in animal species.
What
drives ecological succession? How is the
presence of later-succession species dependent upon early
ones modifying the environment so that it is favorable to the
later?
Lakes
are at the bottom of
their watersheds
so that lakes accumulate the nutrients that runoff the watershed,
causing
drastic changes in lake ecosystems over time. Lakes change
over time in a process know as
eutrophication,
typically going from clear lakes (few nutrients, so few nutrients) to
turbid lakes (many nutrients resulting in thick blooms of phytoplankton
and little or no oxygen in deeper waters).
Bottom
line: Species interact in numerous ways within
communities and many of these interactions important in maintaining
diversity. But non-equilibrium processes can also be important in
positively influencing diversity, demonstrating that natural systems
are dynamic.
