Geography of Communities

Geography of Communities
Often, groups of unrelated species will show similar distribution patterns.
Spruces, kinglets, and red-backed voles are all found in coniferous forests
that spread across the northern part of North America and extend to the
south at high elevations.
Species occur together in
assocations known as
ecological communities.
The nature of these
associations was a subject
debate for much of the
20th Century. Many of the
earliest community
ecologists studied plant
F.E. Clements compared
the community to a
“superorganism”, with its
own life and structure.
Community – an
assemblage of different
species living together in
the same place, typically
interacting with one
Ecosystem – an
assemblage of organisms
considered together with
the physical factors acting
upon them.
Ecologists study two primary aspects
about communities:
Community structure – includng
composition, biomass, diversity, etc.
Community function – movement of
energy through the system, changes
over time, etc.
Two characteristics of species that impact their effect on community
organization are their:
Body mass
Trophic status
The larger an organism, the more
energy it takes to maintain it.
Basal metabolic rate (m) varies
with body mass (M) according to
the relationship:
m  cM
0 . 75
with the constant c varying somewhat
among taxonomic groups.
Since the exponent is less
than 1, this means that the
mass-specific metabolic
needs (per unit of mass)
are greater for small
organisms than for larger
A mouse needs about 25
times more energy per
gram of body mass than
an elephant.
Body size is significant in that it
influences the scale at which
organisms interact with the
Since small organisms use fewer
resources, they can utilize
smaller areas and specialize on
more specific resources, yet still
maintain population densities
high enough to avoid extinction.
They can subdivide the
environment more finely.
An examination of frequency distribution of the number of
species relative to body size for different animal groups
reveals the following:
There is a general pattern of small species being more
numerous than larger ones (note the logarithmic scale).
There are many more species of smaller organisms (like
insects) than larger organisms (like birds and mammals).
Large animals must have broad
geographic ranges. They require more
space, and a larger area of the
environment is required to provide the
resources they need.
Mountain lion range in
A depiction of the geographic range
and body mass among North
American terrestrial mammals.
In general, larger mammals require
a larger home range.
The areas of the smallest rangest
ranges are smaller for herbivores.
Trophic status refers to the
manner by which they acquire
energy. This also influences
the role they play in community
For virtually all organisms, the
ultimate source of energy is the
Food chains are made up of trophic levels.
Quarternary consumer
(third-level carnivore)
Tertiary consumer
(second-level carnivore)
Secondary consumer (first-level
Primary consumer (herbivore)
Primary producer
Most communities
have more complex
trophic relationships
that are illustrated as
a “food web”.
The laws of thermodynamics come into play in the consideration of food chains.
First law of thermodynamics – energy is neither created nor destroyed, but may
be converted from one form into another.
Second law of thermodynamics – as energy is converted from one form to
another, its capacity to do work is diminished and entropy increases (in other
words, the process is imperfect – there is a loss of energy at each step).
Most animals are able to incorporate a very small percentage of the energy
they ingest. The efficiency of transfer is often less than 10%.
This means that less energy is available at successively higher trophic levels.
This can be illustrated in the form of ecological pyramids (next slide).
Since the carrying capacity of an
area is lower for successively
higher trophic levels, predictions
can be made about their ecological
roles and geographic distributions.
There are fewer species of
carnivores than of herbivores and
Carnivores tend to be larger and
more generalized than herbivores.
Carnivores typically have to be
large enough to overpower their
Carnivores tend to be generalists
in their prey selection.
Carnivores tend to have broad
geographic ranges.
Distribution of Communities in Space and Time
The distribution of species along environmental gradients can differ
greatly depending on the nature of the species and the nature of the
The study of such patterns, known as coenoclines, has resulted in a
number of hypotheses about how communities transition from one to
A. Groups of species exhibit similar
ranges along the gradient and are
distributed as discrete communities.
B. Individual species abruptly exclude
one another along sharp
boundaries, but most species are
not associated in discrete
C. Species form discrete communities,
but replacement of these
communities along a gradient is
D. Individual species gradually appear
and disappear, independent of other
species. Species replacement
along a gradient is random. There
are no discrete communities.
E. Ranges of most species are nested
within the ranges of a few dominant
Five Hypothetical Coenoclines
Actual coenoclines showing
the distributions of tree species
along two moisture gradients.
In these cases, species
replacement is gradual and
seemingly independent of one
This corresponds to pattern D
in the previous figure.
However, there are some who
believe this may be an artifact
resulting from the manner in
which the data was collected.
Elevational distribution of
three-needled (A) and fiveneedled (B) pines on the
western slopes of the
Sierra Nevada.
Similar species show little
overlap in elevation.
There also seems to be
some degree of community
Ecological succession is the
progressive change in community
structure and function over
ecological time. In short, one
assemblage is replaced over time
by another assemblage.
Succession is a normal process in
any system in which disturbance
eliminates entire communities.
If the substrate is removed and the
community must start “from
scratch”, we call it primary
If the soil remains, it is known as
secondary succession.
We can also examine paleoecological
records. Pollen analysis lets us look at
the historical distribution of tree species.
We can see that distribution of beech
and hemlock were considerably further
south after the Pleistocene glaciation
(~10,000 years ago).
World Distribution of Major Terrestrial Biomes
A climograph is a way of examining the relationship of terrestrial biomes to
two critical environmental factors, mean annual temperature and mean
annual precipitation.
Climate diagrams provide
similar information. These
diagrams were developed by
Heinrich Walter to help
examine the relationship
between climate and
terrestrial vegetation. They
summarize much climatic
information. With some
practice, a glance at a
climate diagram can give an
overview of a location’s
Climate diagram for a
rain forest
Climate diagram for a hot desert
Climate diagram for a cold desert
Global Distribution of Tropical and Temperate Rain Forests
Tropical Rain Forests
Tropical rain forests are found near the equator
in three primary regions: Southeast Asia, West
Africa, and South and Central America. Most
rain forest occurs within 10º of latitude north or
south of the equator.
The distribution of rain forest corresponds to
areas where conditions are warm and wet yearround. Temperatures vary little from month to
month. They are not extremely hot. Average
temperatures are typically from 25 º to 27 º C.
Annual rainfall ranges from 2000 to 4000 mm
Tropical rain forest distribution and climate
Tropical rain forest soils are
typically poor, as heavy rains leach
nutrients and rapid decomposition
keeps soil organics low. Rain
forest plants are adapted to
conserving nutrients. They often
make use of mutualistic
relationships with fungi associated
with their roots. These
relationships are known as
Epiphyte mat in rain forest canopy
Life in tropical rain forests is
very much three-dimensional
Tropical Dry Forest
These forests are typically found between 10º
and 25º latitude. The climate here is more
seasonal than in rain forests. The dry season
lasts 6-7 months, followed by a wet season of
heavy rains. There is also more seasonal
variation in temperature than is seen in the
tropical rain forest.Soils in tropical dry forests
are usually less acidic and more nutrient-rich
than those in rain forests. Highly vulnerable to
erosion during the wet season.
Tropical dry forest distribution and climate
Plant and animal life in the dry forest is tightly
linked to cycle of dry and wet seasons. Trees in
wet regions tend to be taller than those in dry
regions. In addition, trees in wet regions may be
evergreen, while those in dry regions typically
lose their leaves during the dry season.
Many animal species are shared with the rain
forest. These include monkeys, parrots, and
jaguars in South America. Many animals migrate
to wetter habitats during the dry season.
Tropical Savanna
Found between 10º and 20º latitude.
Found in Africa, South America,
Australia, and parts of Asia.
Alternating wet and dry seasons, but
drier than tropical dry forest. Seasonal
droughts combine with another
important ecological feature….fire.
Fires kill young trees while grasses
survive and resprout.
Soil layers have low permeability.
Retain water near surface. Maintains
enough water in surface soils to
support grasses in dry areas. In wetter
areas, trees cannot move onto
savannas because the surface soil is
waterlogged in the wet season.
Tropical savanna distribution and climate
landscapes are
somewhat 2dimensional.
Trees are present,
but much of the
biological activity
takes place at
ground level.
Occupy about 20% of the earth’s
surface. Most are found are 30º
latitude. Correspond to regions
of descending, dry, subtropical
air. Other deserts are found in
the interior of continents or in the
rain shadow of mountains.
Environmental conditions vary.
Some deserts actually receive
significant rainfall. However,
water loss through evaporation
and transpiration exceeds
precipitation during most of the
year. Soils are low in organic
material, and are classified as
lithisols. May have high salt
Desert distribution and climate
Plant cover is absent from many
places. Where present, it is sparse.
Often gray-green in color, because
of protective covering.
Desert plants show adaptations to
prevent water loss. Animal
abundance tends to be low, but
diversity can be high. Many
animals are adapted to deal with
environmental extremes.
Temperate Shrubland and Woodland
Occur on all continents except
Antarctica. Most extensive around
the Mediterranean and in western
North America. Most are found
between 30º and 40º degrees
latitude. Cool and moist during the
fall, winter, and spring. Summers
hot and dry.
Frost danger varies, but frosts are
not often severe. Fire is frequent
and intense. Soils are of relatively
low fertility and are fragile. Erosion
threat can be increased by fire.
Temperate woodland/shrubland distribution and climate
Organisms are adapted
for drought. Trees and
shrubs typically
evergreen with small,
waxy leaves. Fireselected plants
dominate. Animals are
highly diverse. Includes
many migratory birds
and insects. Native
browsers include deer,
wild sheep, antelope,
and kangaroos.
Temperate Grassland
Largest biome in North
America (Great Plains).
Even more extensive in
Eurasia. In southern
hemisphere, found in
South American and New
Receive between 300 and
1000 mm of precipitation
annually. Most in the
summer. Shorter growing
season than more tropical
climates. Best grassland
soils are deep, nonacidic,
and rich in organics.
Temperate grassland distribution and climate
Grassland vegetation is dominated
by….grasses. Drought and high
summer temps encourage fires,
which help exclude woody
vegetation. Can be a very high
diversity of other vegetation as well.
Temperate grasslands once
supported large populations of large
grazing herbivores, which often
formed huge herds. Smaller
grassland animals are often either
burrowers, or fast-moving predators.
Temperate Forest
Typically found between 30º and
55º degrees latitude. May be
either coniferous or deciduous.
Found where temperatures are not
extreme and where annual
precipitation averages between
650 and 3000 mm. Generally
receive more winter precipitation
than temperate grasslands.
Winters relatively mild. Soils
usually moist and fertile, usually
neutral or slightly acidic.
Temperate forest geography and climate
Diversity is typically lower than
in tropical forests, but biomass
may be higher.
Vertically stratified. Canopy
height may vary from 40 to
100 m. Birds, mammals and
insects make use of all layers
from forest floor to the canopy.
Most important consumers are
fungi and bacteria, which
recycle organic material.
Much temperate forest habitat has
been lost. About 1 to 2% of North
America’s old-growth forests remain.
Boreal Forest (Taiga)
Extends around the world in the
northern hemisphere. Pattern of woods
and water. Found between 50ºN and
65ºN latitude. Bounded by temperate
forests and grasslands to the south and
tundra to the north.
Long winters and short summers.
Include some of the most variable
climates on earth. Verkhoyank, Russia,
has an annual temperature range of
over 100º C.
Precipitation is moderate (200 mm to
600 mm). Low evaporation rates make
drought rare. Soils of thin and acidic,
with low fertility. Nutrients largely tied
up in plant litter.
Boreal forest geography and climate
Generally dominated by evergreen
conifers. Low diversity. Some
deciduous trees like aspen and
birch, and willows are common
around bodies of water. Little
herbaceous vegetation. Small
shrubs like blueberry common.
Winter home of caribou. Moose
and woodland bison found
yearround. Major predator is the
wolf. Also contains bears,
wolverines, snowshoe hare,
porcupine, and many smaller
mammals. Nesting habitat for
crossbills and spruce grouse.
Although humans
have lived in boreal
forests since the ice
ages, impact has
been minor until
recent times. Now,
hunting, trapping,
and forestry are
beginning to
severely impact
taiga animals and
Open landscape of
mosses, lichens, and
low shrubs. Covers
most of the lands
north of the Arctic
Circle. Cold and dry.
Temperatures not
usually as extreme as
in the boreal forest.
Precipitation ranges
from a little less than
200 mm to a little
over 600 mm.
Average precipitation
exceeds evaporation.
Organic matter accumulates in peat and
humus deposits. Surface soils thaw in
the summer; underlain by permafrost.
Tundra geography and climate
Many grasses,
sedges, mosses, and
lichens. Woody
vegetation includes
dwarf willows and
Large numbers of
large mammals,
including caribou,
musk ox, bear, and
wolves. Many
smaller mammals
also, including
lemmings, weasels,
and foxes.
Many migratory bird
species in the
summer. Insects not
diverse, but very
As with taiga, the tundra
was long free from
serious human impact.
Now, much attention as
the result of oil
Also, airborne pesticides
and radionuclides have
been deposited there.
Radioactive Cs137 from
Chernobyl has become
so concentrated in
Norway tundra that the
reindeer were
considered unfit for
Marine and freshwater ecologists and biogeographers do
not classify aquatic communities into categories analagous
to those used for terrestrial biomes.
Oceanographers recognize
biogeographic and climatic
1 – Arctic
2 – Subarctic
3 – Northern temperate
4 – Northern subtropical
5 – Tropical
6 – Southern subtropical
7 – Southern temperate
8 – Subantarctic
9 - Antarctic
Climatic regions;
A – Arctic
B – northern boreal
SB – Southern boreal
T – tropical
E – equatorial
N – northern notal
SN – southern notal
ANT - antarctic
Marine communities are also
classified based on bathymetry (the
depth and configuration of the
location at which they are found).
A similar classification
system has been
developed for
freshwater systems.
Another factor
controlling the
distribution of
ecosystems is net
primary productivity.
Productivity is larger
a factor of the
available solar
energy, although
available moisture
and nutrients also
play a role.
Ecosystem geography is the
branch of science that examines
how ecological processes have
influenced the distribution of
Practitioners have developed a
hierarchical scheme of
This ecoregion approach allows
us to look at processes on a
scale that may range from local
sites to landscape mosaics to
A hypothetical map
of the ecoregion
scheme at three
spatial scales
Ecoregions of the terrestrial realms based on Bailey’s scheme. Check
out this web site.
…and for aquatic realms.
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