Transcript Community Structure
Understanding that we cannot consider interactions as if they
occur individually and independently, we can move to
consider the ways in which ecologists study communities,
and what we have learned about them…
Community Ecology – Multi-species communities
What is a community?
A community is “an association of interacting populations,
usually defined by the nature of their interaction or the place
in which they live.”
Are communities like ‘super-organisms’, the species bound to
each other by their interactions,
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
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.
The organismal community suggests 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, the characteristic species of the
Tall grass prairie
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:
farm field to forest
forest to grassland
There is a transition in each case, though it may not be
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.
Here’s the distribution of species and soil minerals from
forest to serpentine barren in northern California
There is good reason why
some plant species can’t
grow on serpentine soil.
Both nickel and chromium
are at much higher
concentration in serpentine
soil, and both tend to be
toxic to plants that lack
Clements’ organismal community concept suggests clear and
relatively sharp community boundaries. Are ecotones always
clear and sharp? No!
Eastern deciduous forest as a whole 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:
Here is the way these two views compare in terms of the
theoretical distributions of component species of communities:
Here is what Whittaker found along mountainsides in Oregon
(the Siskyou Mts.) and in the Santa Catalina Mts. of
Arizona/California. Each line represents the distribution of a
single species. Are there any signs of organismal
interdependence and correlation in distribution limits among
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
In southwestern Ontario 10,000 years ago, the forest was
dominated by white pine…
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.
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.
The ‘opposite side of the coin’ provides the same sort of
evidence against Clement’s organismal community. Here,
from your text (Fig.21.7) is the very messy set of distributions
of tree species co-occurring in forests of eastern Kentucky.
There are 12 different outlines overlapping in the diagram, but,
with the exception of some similarity in range limits along the
eastern slope of the Appalachian Mts. And in southern
Louisiana, there is no apparent pattern.
The change in community structure and composition
following glaciation occurred quite slowly.
Communities also change over much shorter times.
Communities may be altered by disturbances (fire,
hurricane, etc.). Following the disturbance, they recover.
The process of change that occurs in recovery is called
Communities also develop where new, never-before
occupied space becomes available (e.g. by volcanic
formation of a new island, like Karakatau. This process is
called primary succession, since this is the development
and succession of a first community on the site.
For Krakatau, a previous volcanic island exploded on
August 27, 1883. What remained was, in effect, a new
island, devoid of life, with a surface made up of volcanic
ash. The new island, now called Rakata, wasn’t static. It,
too, was volcanic and grew to a much larger size. It still is
not very stable, and there are frequent rockfalls down one
side of the island.
Ecologists rapidly recognized the value of studying how
Within 3 years there were 24 plant species recorded on the
new island. 10 of them were dispersed to the island by the sea
(e.g. coconuts), almost all the remainder were wind dispersed
– blown from nearby sources Sumatra and Java. One was
By the 1920s, there was a closed forest on the island, and
initial colonists had been marginalized. With the forests came
forest-dwelling animals (birds, bats), and with them came
more animal dispersed seeds.
This is the most remarkable, well-studied primary
succession. There are many more studies of secondary
For example, Bob M’Closkey followed the secondary
succession at Ojibway Prairie after a managed fire.
In grassland, fire is an important and valuable management
tool. Before humans began breaking prairie for agriculture,
fires were a ‘regular’ phenomenon. Spring thunder storms
started fires in the litter (the dry, dead, grasses from previous
years). That released nutrients, raised the temperature of
surface soil, and increased germination and growth of the
Now, to prevent forest from taking over at Ojibway, there is
an established program that burns different parts of the
prairie each year.
Ojibway Prairie before a burn, and what the fire looks like
Since nutrients, seeds, and some species with protected
structures below ground are already there, secondary
succession proceeds much faster than primary succession.
What is the succession like after an intense (forest) fire that
clears the space down to the soil surface…
First – a grassland made up (mostly) of annuals, then, within
a year or two, some flowering plants like annual
sunflowers. This phase lasts 3-4 years.
Second – an old field of perennial grasses and flowering
plants like goldenrods, asters, coneflowers. This phase
lasts from around 4 years to about 10 years after the fire.
Third – a shrub phase that lasts from 10-20 years. Typical
shrubs are sumacs, dogwoods, locust trees.
Fourth – the entry and development of the trees of a mature
forest, e.g. oaks and maples.
Here are photos from your text showing the succession
following a forest fire in an oak-hornbeam forest in southern
Poland…time in years
Succession (whether primary or secondary) leads eventually
to what is called a climax community. The climax is the end
point of a successional sequence like the one indicated to
follow a forest fire.
Clements recognized 14 climax communities in North
• 2 types of grassland: prairie & tundra
• 3 types of shrub: sagebrush, desert shrub, chapparal
• 9 types of forest: beech-oak etc.
A short summary of succession:
Species comprising the early stages of succession generally
have a high capacity for dispersal (only they can get to the
new area rapidly).
As the community matures, the ratio of biomass to
productivity (B/P) increases. Body size and the increasing
presence of woody tissue increase biomass, but, while the
photosynthetic area (area of leaves) increases, it doesn’t keep
Climax species are typically more shade tolerant, grow more
slowly (see above), and disperse less widely (K-strategists,
larger seed size) than the species they replace.
Diversity generally increases through succession, but the
highest level is frequently achieved only partway through
the sequence. Why?
Because part way through, the species of early successional
stages are still ‘hanging on’, while species of later
succession have already appeared. As succession approaches
closer to climax, those early successional species are lost.
Do we know much about the mechanism of succession?
There are two ways that species modify their environments
leading to succession:
Species change environmental conditions in ways that
support the colonization and establishment of later
One example – trees of the locust family have root
nodules that support nitrogen-fixing bacteria
(Rhizobium). The increased nitrogen supports the
growth of species that require higher soil nitrogen.
Here one species prevents others from becoming
established. This is not frequently by means of
allelopathy (suppression by chemicals leached into the
soil from leaves or roots, e.g. the California sage
you’ve seen before). Rather, a dominant species may
block light from reaching smaller, younger plants, so
that they die. Their roots may compete very strongly
for nutrients, and deny them to other species. Oaks are
dominant in many forest communities for both these
To introduce ideas about diversity (anticipating the next
lecture), it is important to recognize the way in which
interactions among species can influence/determine the
number of species in a community. Typically, successionally
advanced communities are at least moderately diverse. There
are very few (if any) communities built from one or more
Instead, each trophic level, and each species in a level,
usually has multiple food sources (species below it). The
result is termed a food web. You’ve already seen the effect
of a keystone predator. However, it is virtually the only
predator in its community (along the Pacific coast of
Washington). In a seemingly similar community from
further south (the Gulf of California), there is a diversity of
intermediate predators and a large diversity of herbivores.
The more complex the food web (indicated by the number
of links) the more diverse the community and, by
controversial argument, the more stable the community.
You can think of the basis of the argument in terms of
available alternatives for any species. A species can persist,
even when one food source goes locally extinct, by using
However, the number of species and the number of links do
not necessarily increase in parallel.
Here’s the example from your text…