ch. 13 Flashcards
Andrewartha and Birch (1954) were the first to think about populations in
a spatial context
Studied insects and found great variance in the abundance across habitats.
They concluded that populations occur in patches of suitable habitat,
surrounded by unsuitable habitat. These patches are connected by the
migration of individuals – so that if a patch goes extinct can reestablish by
migration from other nearby suitable patches.
Richard Levins (1969, 1970) formally introduces the concept of
metapopulations and develops mathematical models.
The classic Levins metapopulations model
Serves as a foundation for all other metapopulation models
Examines population growth in patchy environments
classic Levins metapopulations model assumptions
The environment is composed of a large number of discrete patches, all identical and all connected to each other via migration
.
2. Patches are either occupied or not (actual sizes of populations within patches are ignored, and we assume that each colonized patch quickly reaches its carrying capacity).
Populations within patches have a constant (per patch) rate of extinction (m).
The rate of patch colonization is proportional to the per patch colonization rate (c) times the fraction of currently occupied patches (P) times the fraction of currently empty patches (1 – P), which are the targets of colonization.
know levin’s model equations
Rate of population growth can be described as:
dP/dt = cP (1 – P) – mP
We can rearrange it to yield:
dP/dt = (c – m) P (1- P/1-m/c)
Dispersal limitation
Occurs when a species is unable to occupy all suitable patches in the
environment.
The degree to which species are dispersal-limited has important outcomes
for population dynamics, species coexistence, and community structure.
Metapopulation theory
is the conceptual underpinning now in conservation
biology.
Levins’ model helps us to understand how habitat loss may affect species
persistence.
Important insight gained from Levins’ model:
We should expect to find unoccupied patches of suitable
habitat in all metapopulations.
A significant fraction of unoccupied patches should be
found in metapopulations of species that have low
colonization rates (c) relative to their rate of extinction within patches
(m).
Levins also pointed metapopulation extinction is
time delayed
Metapopulation extinction is not immediate after critical loss of habitat.
Tilman et al. (1994) referred to this time delay in extinction after critical
habitat loss as the extinction debt.
Hanski (1997) described metapopulations facing slow extinction as the
‘living dead’.
Preserving existing habitat may
not be enough to save a species whose
dynamics are those of a metapopulation.
The only advantage is that because extinction does happen slow,
conservationists may have time to intervene, many times by increasing
dispersal of the existing population…
Also, establishing movement corridors can assist
distribution of populations.
Found corridors facilitated the movement of tropical bird species between
isolated forest patches.
see slide 13
Gillies and St. Clair (2008)
Captured territorial individuals of barred antshrikes and rufous-naped
wrens in a highly fragmented tropical dry forest of Costa Rica.
Removed birds and translocated them to several different habitats.
Birds that were habitat specialists (antshrikes) in treatments with corridors
back to their “home” territories returned faster than those without
corridors. Birds that were habitat generalists (wrens) had no significant
difference in return rate.
Movement corridors may
also serve to increase
species richness
Global climate change is occurring
at a rapid rate.
Clear evidence of species shifting ranges in response to changing
environments.
The ranges of a variety of organisms have shifted poleward at average rates
of 6-17km per decade over the past 50 years.
Species that are vulnerable to rapid climate change.
that are less mobile and with small ranges are particularly
Some conservationists and ecologists have proposed__________ in response to climate change
assisted dispersal –
dispersing species to where future climates are projected to be more
This assisted migration/colonization carries many risks and uncertainties.
METAPOPULATIONS AND EPIDEMIOLOGY
In epidemiology there is something called the threshold, which refers to the…
To make parallels between these two fields we can easily substitute infections for
colonization, and host recovery, death, or immunization as patch extinction (Lawton et al.
,1994).
In epidemiology there is something called the threshold, which refers to the required number
of susceptibles for a disease outbreak to occur.
This is directly analogous to the minimum number of patches required to maintain a
metapopulation.
The work from metapopulation theory demonstrates that not everyone needs to be
immunized to eradicate a disease, but that there should be a threshold with which once the
number of susceptibles decreases to the disease is unable to persist (although this threshold is
many times in the 90% range).
Thus, whether working with endangered species an number of suitable patch habitats, or a
microbe that requires a certain number of susceptible hosts.
TYPES OF METAPOPULATIONS
Levins’ model was fairly simple and since he published it ecologists have
determined other types of spatial structures exist.
Harrison (1991) categorized these spatial structures into four main types:
- Classic Metapopulations
- Mainland-Island Metapopulations
- Patchy Populations
- Nonequilibrium populations
CLASSIC METAPOPULATIONS
Characterized by Levins’ model
All local populations are similar in size/type
All populations may go extinct, but have the potential to be
reestablished by colonization.
MAINLAND-ISLAND METAPOPULATIONS
A large population or patch exists without significant risk of extinction
Smaller surrounding populations are supported by immigrants from the
mainland population
Source-Sink model
Assumes habitat patches differ in quality
High quality habitats (source habitats), which have high birth
rates and low death rates experience migration of individuals to low
quality habitats (sink habitats) where birth rates are lower than death
rates.
Source habitats have a very low probability of going extinct, like the
mainland habitats.
Sink habitats are only maintained by low of individuals from source
populations.
Source habitats
Source habitats have a very low probability of going extinct, like the
mainland habitats.
Sink habitats
Sink habitats are only maintained by low of individuals from source
populations.
PATCHY POPULATIONS
Individuals within a single interbreeding population are clumped in space
Clumps do not exist as separate populations (gene flow between clumps is
high)
NONEQUILIBRIUM POPULATIONS
Extinction is not balanced by recolonization
Local populations are effectively isolated from one another, the extinction
of local populations eventually leads to regional species extinction.
slides 34-35
look at
COMPETITION AND COEXISTENCE IN A
PATCHY ENVIRONMENT
Hutchinson’s realizations:
Niche partitioning could ameliorate the effects of competition and allow for species
coexistence
Dispersal of species between habitat patches could provide another mechanism for the
maintenance of species diversity
Hutchinson also coined the term fugitive species.
fugitive species
an inferior competitor to coexist with a superior competitor
if the inferior competitor has the ability to consistently move to open un-utilized
patches, thus why it is called a fugitive species.
TILMAN (1994)
Observed competition/colonization in five grass species in abandoned
agricultural fields as a result of
differential allocation of biomass to roots
and reproductive structures.
Apparent trade-off between the species ability to compete for soil
nitrogen and their ability to colonize old fields (measured as the number
of years required to colonize fields).
A large number of species can theoretically coexist via the competition/colonization
trade-off if:
- species exhibit a competitive hierarchy
- the better competitor always wins within a patch
- there is a strict trade-off
CONSEQUENCES OF PATCH
HETEROGENEITY
Heterogeneous environments may allow species coexistence via combinations of different colonization and competitive abilities.
