Transcript Community Dynamics

Community
Dynamics
What is a Community?
 Ecological
communities are composed
of populations which share a defined
area and interact.
 Community ecology examines how
species interactions influence community
structure and function.
Communities are Dynamic
 Community
structure is not static, but
changes over time as a result of things like
the arrival and loss of species, and the
effects of outside forces such as fires and
floods.

Dynamic – constantly changing
Disturbance & Succession
 Disturbance
and succession are two
common causes of change in community
structure.
Disturbance
 Disturbance
is "any relatively discrete
event in time that disrupts ecosystem,
community, or population structure and
changes resources, substrate availability,
or the physical environment.”
 This broad definition covers nearly
anything that creates open space,
including hurricanes, volcanoes, floods,
footprints, and last but not least, fire.
Disturbances result from forces
originating outside the community.
Succession
 Succession
is the repeatable change in
community composition through time
following a disturbance.
Fire in Yellowstone
 Fire
is a natural occurrence in the
Yellowstone ecosystem.
 Many, if not all, of the species in these
communities are fire-adapted and slowly
die out if fire is excluded from the
landscape.
Succession in Yellowstone
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
After 1988 fires,
studies of succession
past & present.
In 1988, most trees
were the same age –
regrowth after a big
fire in the 1700s.
Regrowth follows a
pattern - succession
1988
Succession in Yellowstone
1991
Succession in Yellowstone
1992
Succession in Yellowstone
1997
Succession in
Yellowstone

Lodgepole pines require
fire to reproduce.

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
Cones don’t open to
release seeds until
exposed to a fire.
Burned areas soon have
lodgepole seedlings
sprouting.
Annuals common early
on, shrubs & trees take
longer to grow, but
eventually the pines are
dominant once more.
Processes Driving Succession
 Self-thinning
– lots of seedling lodgepole
pines begin growing, naturally thin out as
the trees grow.
Processes Driving Succession
 Life
history trade-offs
 Early succession traits


Roots or seeds that suvive or even require fire.
Light seeds float on the wind into burned
area.
 Late


succession traits
Slow growers
May require conditions created by early
successional species.
Life History Traits
 The
differences in physiology and
behavior that make each species well
suited to grow in particular environments
are called the life history traits of the
species.
Life History Traits
 There
are often trade-offs involved with
life history traits. For instance, it may not
be possible to have a physiology that
allows rapid growth in both low and highlight environments.
Life History
Traits
 Species
that do
well in early
succession often
have different life
history traits than
those that appear
in late succession.
Types of Succession
 Primary
rock.


succession – Starting from bare
After volcano erupts
After a glacier recedes
 Secondary
succession – Growth has been
removed, but soil & seed bank is intact.


Abandoned agricultural field
Fires
Three Models for Succession
 Facilitation
 Inhibition
 Tolerance
Facilitation



Barren ground is uninhabitable by all
but the most stress-tolerant of
colonists (small, yellow flower).
Over time, early stress-tolerant
colonists make the environment more
suitable for successive species (such
as the purple flower) by increasing
nutrient availability, developing soils,
reducing pH, or providing shade from
the sun and shelter from the wind.
This sequence continues until the
most competitively dominant species
(here the tall orange flower) no
longer facilitate the invasion and
growth of any other species.
Inhibition

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All species arriving on an unoccupied
site can survive. Thus, the initial
community composition is simply a
function of who gets there first.
Once a colonist becomes established,
it inhibits growth of subsequent arrivals
by monopolizing space and/or other
resources.
Only when space and/or resources are
released through the death or decay of
dominant residents can new colonists
invade and grow.
Because short-lived early species die
more frequently, succession slowly
progresses from short-lived to long-lived
species.
Tolerance

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All species arriving on an unoccupied
site can survive. Thus, the initial
community composition is simply a
function of who gets there first.
Species that appear later simply
arrived later or arrived early but grew
more slowly.
Late arriving species tolerate the
presence of early species and grow
despite the presence of earlysuccessional species because they
are better competitors for light and
nutrients. Over time, late-successional
species exclude other species.
Early-successional species
have no effect on late-successional
species.
Climax Community
 If
there are no more disturbances, the
community will eventually reach a stable
state – the climax community.

An equilibrium state
 Something
almost always provides a
disturbance – keeps the community out of
equilibrium.
Fire Creates Habitat Diversity
 Smaller
burned patches were colonized
more quickly and tended to have greater
forb, grass, and total cover then more
severely burned sites.
 These results indicate that rare years
characterized by widespread fires of
different sizes and intensities create much
of the habitat diversity in the Park.
 The
intermediate
disturbance
hypothesis
predicts that the
greatest diversity
in the community
will be found with
intermediate
levels of
disturbance.
Food Chains &
Indirect Effects
Food Chains
 Species
can be grouped according to
where in the community they obtain their
food, and groups can be linked together
to form a food chain, where each link
comprises a different trophic level.
Food Chains
 Primary
Producers – Plants & algae that
make food through photosynthesis.
 Herbivores – Feed on primary producers
 Carnivores (predators) – Feed on
herbivores.
 Omnivores – Feed on plants & animals.
Yellowstone Food Chain
 Aspen
Elk
Wolves
 Wolves removed
 Elk hunted, but then allowed to rebound
 Reintroduce wolves to keep elk
populations in balance
 Controversy
Aspens
 Quaking
aspens are the primary
hardwood in forests around Yellowstone.
 They have been declining. Why?


Fire suppression – fires may provide ideal
conditions for aspens to sprout.
Elk herbivory – too many elk may feed on
saplings before they have a chance to get
established.
Top-Down Control
 Elk
controlling density of aspen is an
example of top-down control.

Population is controlled by the trophic level
above.
Top-Down Control
 Marine

example:
Sea urchins feed on kelp
 Adding
urchins reduces kelp density
 Removing urchins increases kelp density
Trophic Cascade
 Adding

a third link in the food chain:
Otters feed on urchins
 Urchin
population declines
 Kelp increases
Trophic Cascade
 Adding

a fourth link in the food chain:
Orcas feeding on sea otters
 Otters
decline
 Urchins increase
 Kelp decline
Behavioral Cascade
 In
a traditional trophic cascade,
predators actually reduce the population
of herbivores.
 Wolves reduce the elk population
somewhat, but mostly they change elk
behavior.

Hide & feed in pine forest rather than being
in open feeding on aspen.
Predation Risk
 Prey
species,
like elk or
gerbils, will
forage more
in areas
where they
feel safe from
predation.
Ecosystem Engineers
 Animals
at different trophic levels effect
community structure through feeding.
 Ecosystem engineers affect the
ecosystem itself – indirect effects on
community structure.
Ecosystem Engineers
 In
order for an organism to be considered
an ecosystem engineer, it must alter the
availability of environmental resources.
Corals are considered autogenic
ecosystem engineers because their
physical structure alters ocean currents
and siltation rates, creating conditions
that many reef communities require.
Ecosystem Engineers
 Ecosystem
engineers
can also alter the
environment by
directly redistributing
or transforming living
and non-living
materials.
Beavers & Willow - Yellowstone
 In
Yellowstone, elk
herbivory also
reduced willow
density.


Beavers
disappeared.
Would returning
beavers be
beneficial?
Top-Down vs Bottom-Up
Control
 Some
ecologists have suggested that
communities are not so much structured
by consumers as by resource availability.
 Support for this suggestion comes from
farms where additions of nutrients like
nitrogen and phosphorus have
successfully increased crop yields.
Ecologists taking a bottom-up perspective
hypothesize that organisms on each
trophic level are resource limited.
Bottom-Up Control
 When
leaf litter in
the stream was
limited, primary
consumers and
predators declined.
Food Chain Length

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
What limits the size of
food chains?
Productivity- more
producers = more links
Ecosystem size – bigger
lake = more links
Productive-space – both
aspects important.
Community
Stability
Resistance
 How
resistant is the community to
disturbance?

How big of a change did the disturbance
cause?
 The
term resistance is used to describe
how much the community changes due
to a particular disturbance. More resistant
communities will change less.

Disturbance specific
Return Time
 How
quickly did the community recover
from the change?
 Return time is the amount of time it takes
for the community to stop changing
(reach an equilibrium) after the
disturbance.
Resilience
 How
closely did the post-recovery
community resemble the pre-disturbance
community?
Response to Disturbance
 Communities
differ in their response to
disturbance in each of the three ways you
saw in the simulation. The response can
also change depending on what the
disturbance is. For instance, the size of a
disturbance affects recovery time. The
larger the disturbance, the longer (in
general) you expect recovery to take.
Persistence
 The
overall degree to which a community
stays the same over time, especially (but
not exclusively) after disturbances, is
called the community's persistence.
Alternative Stable States
 Different
potential communities in the
same place are called alternative stable
states and occur when more than one
type of community can exist in a
particular environment.
Why are some communities
more stable than others?
 The
diversity-stability hypothesis argues
that species-rich communities are more
stable.
Why are some communities
more stable than others?
 Food
web
theory suggests that
communities with
higher connectance(i.
e., with many species
interacting trophically)
should be more stable.
Keystone Species
 Keystone
species,
which have a larger
impact on
community stability
than would be
expected based on
relative
abundance.
Dominant Species
 Dominant
species, which have a large
impact due to their high abundance.