Stability
• One of the most important features of any
ecological system is its inherent stability
• Understanding the stability properties of any
system, and the characteristics of structure
and dynamics which enhance or jeopardize
that stability, is fundamental to our
comprehension of natural systems
Stability
• Stability is a complex phenomenon influenced
simultaneously by many population or
community processes (which may interact)
• Many studies relate stability to a specific
characteristic of a population or community
(and frequently in isolation)
Stability
• Consequently, we will
review some of the
different ‘unitary’
relationships which have
been established in an
attempt to give an
overview of the larger
picture
Stability: Population Interaction
Stability: Population Interaction
Stability: verbiage
• MacArthur (1955) defined
stability as ‘the ability of
both populations and
communities to withstand
environmental
perturbation, to
accommodate change,’
Stability: verbiage
• Orians (1975) identified a number of different
elements which may be recognized within the
overall concept of stability, which may be
grouped into 3 types: constancy, resilience,
and inertia (see Table 9.1)
Stability: verbiage
Stability: verbiage
• Attributes of such stability functions are
Orian’s persistence, elasticity and amplitude
(Table 9.1)
Stability: verbiage
• Elasticity is a measure of the speed with
which a system returns to its former state
following some perturbation (aka resilience in
web theory)
• The amplitude of a system defines that
domain over which it is stable
• E.g. a system with a high amplitude can be
considerably displaced and still return
Stability: verbiage
• When a system can return to its previous state
following any perturbation, however large, it
is considered ‘globally stable’
• If it can only return after a minor
perturbation, then it is considered ‘locally
stable’
Stability: verbiage
• Persistence is the survival time of the system
or of some defined element within it
• E.g. if one population has a greater time to
extinction than another, it may be considered
more stable
Stability: verbiage
• There are also the concepts of cyclical or
trajectory stability
• Cyclical: the property of a system to oscillate
around some central point (limit cycles)
• Trajectory: the property of a system to move
towards some final end point or zone despite
different starting points
Stability
• While populations may have inherent stability,
it is unclear if there are emergent properties
of the community which may, or may not, add
more stability to the system
Stability
• Several authors have reviewed the stability of
a species (or even a pair of species), which we
will not review (too extensive)
Stability
• Evidence to date suggests the relationship
between individuals and the community is
two-way each contributing to the other (but
may be greater than sum of its parts)
• The strength of an interaction between
species will depend upon the type (e.g.
predatory, competitive…)
Stability
• Consequently, it has been shown that
observed communities of a given size are
consistently more stable randomly
constructed systems with the same number of
species
Stability
• One of the most frequently cited examples of
this is the stability of trophic structure
reported by Heatwole and Levins (1972) in
their analysis of the organization and structure
of arthropod communities established on
mangrove islets in FL following defaunation
(Simberloff and Wilson 1969)
Stability
• If you recall the results appeared to show
remarkable consistency in trophic structure
(in terms of number of species per trophic
level) of the subsequent colonizing
communities relative to the previous ones
(although there was a great deal of taxonomic
turnover)
Stability
• Of course, many other studies have not
provided such ‘clean’ support for the idea of
such stability (recall Simberloff’s own
subsequent analysis)
Stability
• It is not clear that Heatwole and Levins study
really even shows stability
• Communities may simply go back to the
similar structure as a result of the physical
parameters of the habitat itself
• Furthermore, is it really resilience if the
community has been completely destroyed
rather than just perturbed?
Stability
• The fact that secondary
succession in communities
generated after some
perturbation eventually returns
to its previous climax state (albeit
using different pathways at times)
may be adduced as evidence for
resilience and the constancy of
climax communities
Stability
• Another good example comes from analyzing
the faunal assemblages of West Malaysia
before and after selective logging
• Logging was relatively modest (18 trees/ha)
• Immediate effects were clear (dramatic
declines of birds and mammals) and a large
shift in species composition
Stability
• There was considerable turnover with
complete loss of some mammalian species (30
of 45 lost) and colonization by several new
species (5) that had not been recorded
• Even of the 15 sp that remained their relative
dominance changed
• Bird communities showed similar dramatic
changes
Stability
• After 5-6 years the logged areas began to
show evidence of recovery
• Trophic structure of the mammal fauna had
returned towards its original state, although
the species composition and relative
abundances had changed
Stability
Stability
• This study suggested that
the communities showed
little inertia (low stability in
the presence of a small
perturbation), low
constancy (some trophic,
little species), but
reasonable resilience
Stability: Population Interaction
• Since communities are influenced by
population stability, let’s review the general
consequences of different types of interaction
for population stability
Stability: Population Interaction
• We earlier established that predation and
interspecific competition generally have a
destabilizing effect
• There are aspects of these relationships which
may tend to stabilize the prey population
(through density dependent processes), other
aspects of the relationship clearly contribute
to the destabilization of the relationship
Stability: Population Interaction
• Few interactions are symmetrical in their
effects
• E.g. top-down relationships where those at
the bottom are being more strongly
influenced than those at the top; whereas in
‘donor-controlled’ interactions the prey are
relatively independent of predator pressures
Stability: Population Interaction
• As a general rule, top-down interactions are
far more likely to be destabilizing for both
populations
• Wider interactions, such as those of diffuse
competition and competitive mutualism, may
help improve the dynamical stability of
interaction populations (predator-prey or
competitors)
Stability: Population Interaction
• We also know that complexity or
heterogeneity of the environment may further
facilitate stable interactions
Stability & Diversity
• The fact that the destabilizing effects of
primary interactions such as predation or
interspecific competition may be lessened
within the context or a more complex matrix
of interaction has sometimes led to the idea
that ‘dynamic stability’ is a result of their own
complexity
Stability & Diversity
• Basic observations have led to the idea that
habitats comprised of greater complexity (e.g.
late succesion forests, tropical forests) also
have greater diversity
in comparison
Stability & Diversity
• Consider the destabilizing effects of
competition may be reduced by indirect
interaction within a multi-species array – the
effects of a third competitor or predator
• The injurious effects of predation upon the
dynamics of the prey may be ameliorated if
the predator has alternative prey species to
switch to if the initial prey declines too much
Stability & Diversity
• So, is stability the result of the observed
diversity or is the diversity a consequence of
the inherent stability?
• Early works (May) noted a tendency for model
communities to become less stable with
increasing diversity
• Hence, these communities can only exist in
predictable environments
Stability & Diversity
• By contrast, in an unpredictable environment,
there is need for the region over which any
one population remains stable to be
extensive, with the implication that the
system must be
relatively simple to
persist
Stability: Population Interaction
• May concluded that the frequently observed
correlation of high diversity with high
apparent stability was not in itself a causal
relationship, but rather a reflection of two
independent consequences of a common
cause: that environmental stability permitted
the development of high diversity while at the
same time ensuring the continued stability of
a high-diversity system
Stability: Population Interaction
• Random assembly is perhaps an unreasonable
assumption; however, additional research has
supported the idea that adding diversity to a
system (community) generally decreases
stability
• However, experimental evidence (Tilman) of
grassland assemblages suggest otherwise
Stability: Population Interaction
• Tilman’s
research on
grassland
communities
Stability: Population Interaction
• ‘May’s Paradox’ addresses simple constancy:
the more complex a system the greater the
potential to accommodate change through
minor adjustments elsewhere in the system
• This is really ‘redirecting’ the community
relationships to absorb the perturbation (is
really resilience)
Stability: Population Interaction
• Thus May’s conclusions are intuitive: the more
complex a system becomes the less likely it
may be to maintain constancy of species
composition
• Conversely, the more complex it is flowing
perturbation, because of its capacity to absorb
such perturbation
Stability: Population Interaction