Download Community Ecology: Structure, Species Interactions, Succession

Community Ecology: Structure,
Species Interactions, Succession,
and Sustainability
Miller Chapter 8
Section 8-1
Community Structure
The structure of an ecosystem described in terms of:
Physical appearance – Things like size, stratifications, &
distribution of its population & species
Lots of differences in aquatic life zones
Size differences of vegetation patches
Differences in abiotic factors at edges of boundaries &
ecotones  edge effects
Species diversity - # of different species
Species abundance - # of individuals of each species
Niche structure - # of ecological niches, how they
compare to each other, & species interactions
Biodiversity
The World’s Places with the Most
Biodiversity places are:
Rain forests
Coral Reefs
Deep Sea
Large tropical lakes
In general, high biodiversity = low
species abundance
Species Diversity
Species Diversity is affected by:
Latitude – species diversity decrease as you move
away from the equator

Latitudinal species diversity gradient leads to the highest
species diversity in tropical areas and lowest in the polar
areas
Depth in aquatic systems – diversity increases
from the surface to 2,000 meters then begins to
decline until sea bottom is reached, where species
diversity is high.
Pollution in aquatic systems – the more
pollution, the less species diversity & abundance
Species Diversity con’t
In terrestrial communities, species
diversity increases with increasing
solar radiation, increased
precipitation, decreasing elevation, &
seasonal variations.
Species Equilibrium Model
(Theory of Island Biogeography)
States that the # of species on
an island depends on the rate at
which new species immigrate to
the island & the rate at which
species become extinct on the
island.
Size & degree of isolation
determine the number of
species found on an island.
This suggests that a small island
will have a lower species
diversity because there is
generally lower immigration
because it is a smaller target & a
higher extinction rate because
of resource limitation.
Colonization
Extinction
Gaia Hypothesis
Hypothesis that earth is alive and
can be considered a system that
operates and changes by feedback
of information between its living
and nonliving components.
Section 8-2
Types Of Species
Native
Non-native
Indicator
Keystone
Native Species
Species that normally live in a
particular ecosystem
Non-native Species
Species that
migrate or are
deliberately or
accidentally
introduced into
an ecosystem
AKA: exotic
species, alien
species
Ex. African Bees
Indicator Species
Species that are
indicators of
problems within
an ecosystem or
community
Ex. birds, fish,
snails
Keystone Species
Species whose interactions with other
species affect the health & survival of
these species.
In short, these are organisms that
without which the ecosystem
could collapse.
Ex. dung beetles
Keystone Species con’t
Roles:
Pollination of flowers by bees, hummingbirds, etc.
Seed dispersal by bat feces
Habitat modification (elephants, bats, beavers)
Predation of species by top carnivores
Top predator species – help regulate
populations by feeding on certain species,
creating a stabilization effect.
Loss of a keystone species can cause
population crashes & extinctions of species
that depend on the keystone species.
Section 8-3
Community Interactions
Intraspecific competition
Competition between members of the
same species for the same resource
Ex. dandelions competing for space &
nutrients
Community Interactions
Territoriality
Organisms of the same species patrol or mark the
area around their home, nest, or feeding ground
and defend it
Ex. robins, hummingbirds
Good territory = abundant food supply, good
nesting site, few predators, etc.
Disadvantages: exclusion of male members from
breeding & large energy expenditure during
defending the territory
Community Interactions con’t
Interspecific competition
Competition between members of different species
for food, space, & other limited resources
Interference competition – limiting
access to a resource
Exploitation competition – one species
can exploit/obtain the common resource
faster than another
Competitive Exclusion Principle
No 2 species can occupy the same niche at the
same time
Avoiding Competition
Resource
partitioning
Dividing scarce
resources so that
competing species
use them at different
times, in different
ways, or in different
places
Figure 8-9 Miller p. 175
Predator-Prey Relationships
Predation – member of one species feeding
directly on all or part of a living organism of
another species
Predator – the killer/eater  benefits
Pursuit & ambush (carnivores)
Prey – the killed/eaten  harmed (usually)
Prey can also benefit because weak members of a
species will be killed off thereby freeing up resources
for the more fit members
Defense mechanisms: mobility, keen sight & smell,
protective armor, bark or spines, camouflage,
chemical warfare.
Symbiosis
Symbiosis
species living together in close
association
Types:
Parasitism
Mutualism
Commensalism
Parasitism
One species lives in
or on another species
& harms it (+,-)
Host – lived on/in 
harmed
Parasite – lives
within the host 
benefits


Ascaris
Smaller than the
host
Weakens host
over time, but
rarely kills it
Ex. heartworms,
ticks, tapeworms,
mosquitoes
Guinea Worm (Dracunculus medinensis)
Mutualism
Both species
benefit from the
interaction (+,+)
3-way mutualism
Ants eat from nectar ducts on
the caterpillar & the caterpillar
eats from the flower. The ants
provide protection for both the
flower & the caterpillar.
Protection
Reproductive
assistance
Being supplied
with food
Commensalism
One species
benefits & the
other is neither
helped nor harmed
(+,ø)
Ex. whales &
barnacles
Ecological Succession
The gradual change in species
composition of a given area; some
populations grow & some decline
2 Types:
Primary
Secondary
Primary Succession
Characteristics:
Occurs in an area where no life
has ever been; occurs gradually
EX. Island formation as a result of
volcanic eruption, rock exposed from
melting glaciers
Characteristics con’t
Begins in an area with no soil (terrestrial)
or no bottom sediment (aquatic);
producers, consumers & decomposers
need soil
Soil formation begins when pioneer
species attach themselves to bare rock.
Ex. Lichens, moss
They trap wind-blown soil & detritus,
produce small amounts of organic matter,
and secrete acids that break down rock
(weathering).
Characteristics con’t
Pioneer species are eventually replaced by
small grasses or herbs which grow close
to the ground & can flourish in harsh
conditions.
Midsuccessional plant species  take
over following hundreds of yrs of soil
formation.

These are herbs, grasses, & small shrubs
Late successional plant species  occur
following the growth of trees that provide the
necessary shade.
Primary Succession cont’d
Figure 8-14 Miller p. 180
Secondary Succession
Reestablishment occurring in an area
where life once had been; more
common
EX. Land destroyed by wildfires
New plants usually begin to grow in a
few weeks
Secondary Succession con’t
Figure 8-15 Miller p. 181
Succession and Wildlife
Figure 8-16 Miller 182
Succession involves lots of changes in the community
structure, so different species are present at various stages of
succession.
Succession
3 Factors affect how and how quickly
succession occurs:
1. Facilitation – one species makes an area
suitable for another species; EX. lichens &
mosses role in soil formation
2. Inhibition – early species hindering the
growth/establishment of other species; EX.
interference competition (chem. warfare)
3. Tolerance – late successional plant
species are unaffected by earlier plant species
Succession
Disturbances
Changes in the environmental
conditions that disrupt an ecosystem
Can be gradual or catastrophic
Not all are harmful

Ex. wildfires renew resources in soil, can
also discourage or eliminate species
thereby freeing up niches
Section 8-6
Stability
Balance in an ecosystem maintained by constant
dynamic changes in response to the changing
environment.
3 Aspects of Stability
Inertia, or persistence is the ability of a living
system to resist being disturbed or altered
Constancy is the ability of a living system to keep its
numbers and live within its resources.
Resilience is the ability of a living system to
rebound after a mild external disturbance.
Stability con’t
Recent research indicates that ecosystems
with higher species diversity have higher net
productivities and are more resilient. In these
same ecosystems, however, species
populations fluctuate more widely.
Populations, communities and
ecosystems are rarely balanced.
Disturbance, fluctuation & change is
more the norm.
Precautionary Principle
When evidence indicates that an
activity can harm human health or the
environment, we should take
precautionary measures to prevent
harm even if some of the cause & effect
relationships have not been
scientifically established.