Ch. 14 Flashcards
oscillating dynamics.
What is beneficial at one time point, is not at the next due to alterations in
environmental conditions over time.
coexistence can be
stable or unstable.
Stable coexistence
means that species tend to recover from low densities and that
species densities may fluctuate over time, but don’t disappear.
Equalizing Mechanisms
Reduce fitness difference between species
All species in the community respond in the same way to the environment
The more similar species are in fitness the longer it would take for competition to favor one species at the expense of another
Cannot facilitate the recovery of species from low densities
Cannot lead to stable coexistence
Unstable coexistence
means that even if species coexist for long periods of time
there are no ecological mechanisms that will promote species recovery when at
low abundance leading to species extinction and decline in species richness.
Stabilizing Mechanisms
Stable coexistence requires stabilizing mechanisms
The species similar in fitness requires less stabilizing mechanisms to maintain coexistence
Include: resource partitioning and frequency-dependent selection – because these operate whether the environment fluctuates or not
and thus are called fluctuation-independent mechanisms.
Mechanisms that depend on population densities and environmental factors are called fluctuation-dependent mechanisms.
fluctuation-dependent mechanisms.
Mechanisms that depend on population densities and environmental factors
Origins of the Idea of the Storage Effect
Discovered by Peter Chesson in the 1980’s
He was exploring a hypothesis proposed by another scientist: Peter Sale
Sale’s hypothesis stated that temporal variation in recruitment could allow the
coexistence of many species of fish competing for territories on a coral reef.
fluctuation-independent mechanisms.
resource partitioning and frequency-dependent selection – because these operate whether the environment fluctuates or not
Fluctuation-Dependent Mechanisms
Can be classified into two categories:
Nonlinearity of competition
Storage Effect
The storage effect
temporal variation in environmental conditions leads to
species coexistence
competition for territories on the reef is intense; once a territory is
obtained
it is held until the death of the individual
Sale predicted that even if fish have generalized diets and habitat usage….
as long as there is
temporal variation in the availability of territories (due to death of individuals) and variation in the
settlement of fry of different fish species on those territories, a wide diversity of species can
coexist within the coral reef habitat.
‘lottery model’
territory establishment is on a
first-come-first serve basis.
Temporal variation in recruitment, coupled with relatively long-lived adult stage
that can “store” the population contributions allows these temporal fluctuations to be
a stabilizing mechanism.
Key features of the Lottery Model (fix)
Adult fish hold territories on the reef until death. Adults are iteroparous (reproduce
more than once), and each species has overlapping generations.
- To survive and reproduce territories are necessary. Space is limiting. There are
always more fry fish attempting to establish territories than there are territories.
Allocation of territories is a first-come-first-serve basis.
Fry are produced in abundance and dispersed.
Fry survival depends on environmental conditions, which fluctuate over time, species
differ in their response to those conditions. So this means that all species have relatively
“good” and “bad” years for fry survival and territory recruitment.
Adler (2006)
Empirical testing of the storage effect: Prairie Grasses
Adler (2006) tested this idea in prairie plants in plots in Kansas. what did he find
All three species have long life spans, differential species-specific responses to
environmental variation (i.e. precipitation and temperature).
Found that the effects of competition were more severe in more favorable years,
but neighboring species had weak or even facilitative effects during bad years.
Environmental variation may promote
species coexistence
Environmental variation can have important consequences
on a global scale over
geologic time in shaping:
Glacial cycles
Shift in species ranges
Species extinction
Alterations to biodiversity
An equilibrium, or stable state, is a state in which
the population isn’t changing.
Alternative stable state
describes a situation where communities may exist in
different configurations (of species richness, species composition, food web
structure, size structure) and these different community configurations represent
different equilibrium states.
who determines that alternative states may exist?
Mathematical modeling by Robert May first elucidated that in fact, alternative
states may exist.
The concept of alternative stable states is represented by a ball on a landscape of
hills and valleys.
Here the picture shows that communities remain stable if in the valley until an
environmental disturbance or changes causes it to shift to a new state.
A change from one community state to another is termed
Regime Shift or
Phase Shift
The critical threshold at which a system undergoes a regime shift is referred to as the
tipping point.
Resistance is the term used to describe
the strength of perturbation needed to shift a community from one
state to another.
the strength of perturbation needed to shift a community from one
state to another is called
resistance
Resilience is the term used to describe
the speed at which a community recovers after a perturbation.
the term used to describe the speed at which a community recovers after a perturbation.
Resilience
Ecological Resilience
the magnitude of a perturbation that a system can withstand before
undergoing a regime shift.
Engineering Resilience –
the time it takes for a system to recover from a perturbation.
Two definitions of resilience
Engineering Resilience and Ecological Resilience
Ecological shifts can be at
the macro or microscales:
Coral reef systems → macroalgae
Savannas → Deserts
Microbial communities in the gut of Humans after antibiotics
Anthropogenic changes have resulted in
an increase in regime shifts in recent years over shorter
periods of time.
Rate of change to environments can happen
smoothly, gradually over time
OR
In large steps, abruptly over time
Mechanisms that lead to alternative stable states
Positive Feedbacks
Nonlinear Species Interactions (remember the species that are better competitors
at low resource levels vs species that are better competitors at high resource
levels)
Size-based refuges from predation
Priority Effects (who arrives in an ecosystem first)
( see slide) Zooplankton community composition changes
smoothly with changes in planktivore density
Environments are always
changing
Environments are always changing, and as a result, communities of organisms
also change
