Geography of Communities

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Geography of Communities
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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.
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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 associations. F.E.
Clements compared the community to a
superorganism, with its own life and structure.
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Community an assemblage of different species
living together in the same place, typically
interacting with one another. Ecosystem an
assemblage of organisms considered together with
the physical factors acting upon them.
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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.
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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
with the constant c varying somewhat among
taxonomic groups.
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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 ones. A mouse needs about 25 times more
energy per gram of body mass than an elephant.
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Body size is significant in that it influences
the scale at which organisms interact with the
environment. 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.
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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).
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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 California
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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.
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Trophic status refers to the manner by which they
acquire energy. This also influences the role
they play in community structure. For virtually
all organisms, the ultimate source of energy is
the sun.
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Food chains are made up of trophic levels.
Quarternary consumer (third-level
carnivore) Tertiary consumer (second-level
carnivore) Secondary consumer (first-level
carnivore) Primary consumer (herbivore) Primar
y producer
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Most communities have more complex trophic
relationships that are illustrated as a food
web.
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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).
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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 plants. Carnivores tend to be larger and more
generalized than herbivores. Carnivores typically
have to be large enough to overpower their
prey. Carnivores tend to be generalists in their
prey selection. Carnivores tend to have broad
geographic ranges.
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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
gradient. The study of such patterns, known as
coenoclines, has resulted in a number of
hypotheses about how communities transition from
one to another.
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1. Groups of species exhibit similar ranges along
   the gradient and are distributed as discrete
   communities.
2. Individual species abruptly exclude one another
   along sharp boundaries, but most species are not
   associated in discrete communities.
3. Species form discrete communities, but
   replacement of these communities along a gradient
   is gradual.
4. Individual species gradually appear and
   disappear, independent of other species. Species
   replacement along a gradient is random. There
   are no discrete communities.
5. Ranges of most species are nested within the
   ranges of a few dominant species.

Five Hypothetical Coenoclines
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Actual coenoclines showing the distributions of
tree species along two moisture gradients. In
these cases, species replacement is gradual and
seemingly independent of one another. 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.
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Elevational distribution of three-needled (A) and
five-needled (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 organization.
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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. Successi
on 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 succession. If
the soil remains, it is known as secondary
succession.
Primary Succession
Secondary Succession
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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).
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World Distribution of Major Terrestrial Biomes
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A climograph is a way of examining the
relationship of terrestrial biomes to two
critical environmental factors, mean annual
temperature and mean annual precipitation.
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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 locations climate.
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Climate diagram for a rain forest
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Climate diagram for a hot desert
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Climate diagram for a cold desert
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Global Distribution of Tropical and Temperate
Rain Forests
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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 year-round. 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
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Tropical rain forest distribution and climate
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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
mycorrhizae.
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Epiphyte mat in rain forest canopy
Life in tropical rain forests is very much
three-dimensional
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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.
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Tropical dry forest distribution and climate
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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.
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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.
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Tropical savanna distribution and climate
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Savannas landscapes are somewhat 2-dimensional.
Trees are present, but much of the biological
activity takes place at ground level.
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Desert Occupy about 20 of the earths 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
concentrations.
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Desert distribution and climate
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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.
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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.
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Temperate woodland/shrubland distribution and
climate
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Organisms are adapted for drought. Trees and
shrubs typically evergreen with small, waxy
leaves. Fire-selected plants dominate. Animals
are highly diverse. Includes many migratory
birds and insects. Native browsers include deer,
wild sheep, antelope, and kangaroos.
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Temperate Grassland Largest biome in North
America (Great Plains). Even more extensive in
Eurasia. In southern hemisphere, found in South
American and New Zealand. 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.
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Temperate grassland distribution and climate
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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.
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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.
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Temperate forest geography and climate
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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.
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Much temperate forest habitat has been lost.
About 1 to 2 of North Americas old-growth
forests remain.
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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.
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Boreal forest geography and climate
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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.
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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 plants.
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Tundra 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.
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Tundra geography and climate
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Many grasses, sedges, mosses, and lichens. Woody
vegetation includes dwarf willows and birches.
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 abundant.
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As with taiga, the tundra was long free from
serious human impact. Now, much attention as the
result of oil exploration. 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 consumption.
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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 regions 1 Arctic 2 Subarctic 3
Northern temperate 4 Northern subtropical 5
Tropical 6 Southern subtropical 7 Southern
temperate 8 Subantarctic 9 - Antarctic
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Climatic regions A Arctic B northern
boreal SB Southern boreal T tropical E
equatorial N northern notal SN southern
notal ANT - antarctic
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Marine communities are also classified based on
bathymetry (the depth and configuration of the
location at which they are found).
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A similar classification system has been
developed for freshwater systems.
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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.
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Ecosystem geography is the branch of science that
examines how ecological processes have influenced
the distribution of ecosystems. Practitioners
have developed a hierarchical scheme of
ecoregions. This ecoregion approach allows us to
look at processes on a scale that may range from
local sites to landscape mosaics to ecoregions.
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A hypothetical map of the ecoregion scheme at
three spatial scales
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Ecoregions of the terrestrial realms based on
Baileys scheme. Check out this web site.
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and for aquatic realms.