Transcript Community Ecology

Community Ecology
Definition of community:
A community is a group of populations that coexist in space
and time and interact with one another directly or indirectly.
Note that this definition does not exclusively consider plants;
a community includes all the decomposer, plant, herbivore,
and carnivore populations that coexist, even if we will
concentrate on plants here.
Are communities like ‘super-organisms’, the species bound to
each other by their interactions,
or
Are communities associations of species assembled by
coincidence, and with species independent of each others’
presence and absence?
The first view is sometimes called a closed community. The
idea was developed in the 1920s by a plant ecologist named
F.E. Clements. He even coined the term ‘superorganism’ to
describe it.
The second view is called an open community. This was
proposed by H.A. Gleason.
The implications of these two views of community structure
are distinct, and allow us to assess whether one view works
better than the other, or whether we need some compromise
intermediate between them.
The organismal community suggests (due to interdependence)
that only particular groups of species should be found
together, and that the boundaries of communities should
sharply divide one group of species from another.
This is almost a common sense view. Foresters talk about
particular kinds of forests, defining them by the most common
species: oak-hickory forests or oak-maple forests that
dominate around here, pinyon pine or ponderosa pine forests
in the Rockies and the west. Tall grass prairies are dominated
by Andropogon gerardii (big bluestem), the characteristic
species of the community.
Oak-hickory forest
Tall grass prairie
Andropogon
gerardii
Gleason’s view, of an open community comprised of
‘individualistic’ species, suggests that community boundaries
should be indistinct, that there may well be species
substitutions in what we might describe as an oak-hickory
forest just because those species are the most common ones
in it. As well, when we look at the boundaries of distributions
of species, those boundaries should not be linked.
The boundaries are transitions between one habitat/
community and another, called ecotones. They are usually
quite sharply evident. Here are some examples:
There is a transition evident from the photograph. Consider
the picture on the right. It isn’t evident from the picture that
the soil beneath the grassland is a serpentine soil, with much
different nutrients than beneath the forest.
Clements’ organismal community concept suggests clear and
relatively sharp community boundaries. Are ecotones always
clear and sharp? No!
On a continental scale Eastern deciduous forest has relatively
sharp transitions to boreal forest to the north (due to cold), to
open grassland to the west (due to limits on rainfall), to firetolerant pine forests to the south, and to ocean on the east.
However, eastern deciduous forest is not ‘monolithic’. There
are different species dominant, and different component
species in different parts of the eastern deciduous forest. This
sounds more like Gleason’s view, not only described as open,
but as the basis of the continuum concept for communities.
We can draw logical predictions from the two views:
These two views produce different theoretical distributions of
component species of communities:
Here is what Whittaker found along mountainsides in Oregon
(the Siskyou Mts.), in the Santa Catalina Mts. of
Arizona/California and, in the figure, in the Great Smoky
Mts. Of Tennessee. Are there any signs of organismal
interdependence and correlation in distribution limits among
species?
Species’ responses to environmental gradients similarly
suggest their individuality.
What set the distributional limits that Whittaker observed
along the various mountain ranges? Even without detailed
environmental measurements, we can surmise that there were
differing abiotic conditions at work.
The evidence we have points generally to the open,
individualistic view of living communities.
However, there is another way to learn about communities:
study of the fossil record they leave behind. Trees (and other
plants) that are wind pollinated leave behind evidence of
their presence in the form of pollen grains.
By aging segments of sediment cores (usually taken from the
bottom of lakes) and identifying the types of pollen present
in these segments, the component species in communities at
different times in the past can be learned.
In northern temperate North America, we can see what
happened to communities as the glaciers receded northward
over the 15,000 years since the last (Wisconsin) ice age
(glaciation).
Pollen diagrams tell us that the communities (or at least
species coexisting) as the glaciers began to recede are not
combinations we would observe today. During recession
various other combinations occurred. The diagram below
comes from a marsh in Minnesota…
In this area of Minnesota spruce (Picea) and ash (Fraxinus)
were dominant 20,000 years ago.
After that there were waves of dominance by birches (Betula),
pines (Pinus), elms (Ulmus) and oaks (Quercus), then they
declined and grasses (Poaceae) became dominant.
In recent times (the last 2000 years or so) pines and oaks have
returned, now sharing the space with sedges (Cyperaceae),
whose pollen is common in part because the samples come
from a marsh.
The dominant plants are different in southern Ontario, but the
process is conceptually similar…
In southwestern Ontario 10,000 years ago, the forest was
dominated by white pine…
Pinus strobus
eastern white pine
There was no hemlock or hickory in those forests. But, by
5,000 years ago, with the gradual recession of the glacial
edge northwards, hemlock and hickory had migrated
northward from their glacial refuge from the southeastern
U.S. White pine remained a component of the community.
Tsuga canadensis
eastern hemlock
Carya glabra
pignut hickory
These post-glacial migrations tell us two things:
1. The boundaries of communities shift continuously in
response to environmental conditions.
2. There was no necessary community integrity involving
all, or even most of the species that now comprise a
recognized community during post-glacial migration.
There is much more information about the northward
migration. For example, some species basically moved
straight northward. Others moved back into Canada by way
of the Atlantic provinces and eastern Quebec. Still others
migrated all the way around the Great Lakes, re-entering
Canada a little west of Lake Superior.
What had been a community before the Wisconsin glaciation
fragmented for the duration of the glaciation and migrated
separately northward as the glaciers receded.
All this does not suggest Clements’ organismal community.