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Lecture

BIOC50H3 Lecture Notes - Northern Spotted Owl, Patch Dynamics, Switch


Department
Biological Sciences
Course Code
BIOC50H3
Professor
Marc Cadotte

Page:
of 4
BIOB50 LECTURE 19: Metapopulations and Metacommunities
Metapopulations
Isolated populations do affect one another’s pop dynamics because individuals/gametes
occasionally disperse from one pop to another such a group of interacting populations=
metapopulation=population of populations
Many species have a metapopulation structure in which sets of spatially isolated pops are linked
by dispersal for many species areas of suitable habitat exist as a series of favorable sites that
are spatially isolated from one another
Meta= Greek for about itself in science it it translated as something of something i.e. meta-
analyses= analysis of analyses
Hufaker: can see through time where and when prey/predators are over time and how they
occupy different locations at different times
Metapopulations: spatially isolated pops that are linked by the dispersal of individuals or
gametes
Metapopulations are characterized by repeated extinctions and colonizations
Patches are prone to extinction for two reasons
o 1) if small, they have small pop sizes= more likely to go extinct= demographic
stochasticity
o 2) environmental conditions often change in a rapid and unpredictable manner =
environmental stochasticity
Metapopulations aid persistence of populations although the individual pops may be prone to
extinction, the collection of pops, the metapopulation persists since it includes pops that are
going extinct and new pops established by colonization
Proportion (p) of patches occupied can be predicted in terms of extinction and colonization of
habitat patches
o dp/dt= immigration rate-extinction rate
o extinction and colonization of habitat patches can be described by this equation
dp/dt=cp(1-p)-ep
o p=proportion of habitat patches that are occupied at time t
o c=patch colonization rate
o e=patch extinction rate
o for a metapopulation to persist for a long time, the ratio e/c must be less than 1
o this equation has several assumptions
1)there is an infinite number of identical habitat patches=unrealistic
2) all patches have an equal chance of receiving colonists (i.e.
distance)=unrealistic
3) All patches have an equal chance of extinction (i.e. size)
4) once a patch is colonized, its pop increases to its carrying capacity more
rapidly than the rates of colonization and extinction (allows pop dynamics
within patches to be ignored)
o Levins: dp/dt=cp(1-p)-ep cp(1-p)= immigration=qudratically related (parabola with
maximum seen on graph) and ep=extinction =linearly related (straight diagonal line)
o From the intersection of the two plots of extinction and immigration, we can find the
equilibrium point
Proportion of patches occupied= 1-e/c (at equilibrium) p=1-e/c
Metapopulations have led to research on key issues
o How to estimate factors that influence patch colonization and extinction
o Importance of the spatial arrangement of suitable patches
o Extent to which the landscape between habitat patches affect dispersal
o How to determine whether empty patches are suitable habitat or not
o It also helps to study how fragmentation of habitat affects patch occupation (p)
parches may become ever smaller and more isolated, reducing colonization rate and
increasing extinction rate e/c ratio increases via habitat fragmentation
o Metapopulation can go extinct even when suitable habitat remains i.e. the northern
spotted owl the population decreased due to logging since they cant use the trees for
habitat
in stuldies of the northern spotted owl in old growth forests in the Pacific NW, it
was estimated that the entire metapopulation would collapse is logging were to
reduce the fraction of suitable patches to less than 20%
real metapopulations often violate the assumptions of the Levins model i.e. dp/dt=cp(1-p)-ep
extinction and colonization rates often vary among patches
patches may vary population size and ease of colonization; extinction and colonization rates can
vary greatly among patches these rates can also be influenced by nonrandom environmental
factors
research on the skipper butterfly in the UK highlighted 2 important features of many
metapopulations
o 1) Isolation by distance (colonization)
Can affect chance of extinction a patch that is near an occupied patch may
receive immigrants repeatedly making extinction less likely
High rates of immigration to protect a population form extinction is known as
the rescue effect
Isolation by distance occurs when patches located far away from occupied
patches are less likely to be colonized than are nearby patches
o 2) The effect of patch area (or pop size) i.e. small patches tend to have small pop sizes=
extinction
Patches with the largest area and were closest to occupied patches were most
likely to be colonized
Rescue effect: the tendency for high immigration rates to protect a pop from
extinction (by reducing the problems associated with small pop size)
o Hanski’s modification: dp/dt-cp(1-p) -ep(1-p)= includes a rescue effect into extinction
rate= makes it parabolic not linear anymore
with this modification, plotting extinction and immigration ate there is no
equilibrium only exists @0 or 1 since if c>e then p goes to 1 and if c<e then p
goes to 0
Metacommunities
Metacommunities: patches that are occupied by multiple species
Multiple species within a patch are likely to compete, this the ability to move can be critical for
coexistence
There are 4 metacommunity patterns/paradigms/perspectives that try to explain why dispersal
among patches is important for coexistence
o 1) Neutral perspective: species are competitively equal, patches are identical, and there
is limited dispersal= species mingle in same patch
o 2)Patch dynamics perspective: species compete in identical patches with limited
dispersal= species co-exist by not coexisting locally a tradeoff is required usually
between competition and colonization abilities
o 3) Species sorting perspective: species compete in different patches with low dispersal:
competitive exclusion is fast relative to dispersal and differences in patches is due to
abiotic factors i.e. soil, water exposure, etc each species in its own patch =isolated in
diff patches
o 4) Species compete, patches differ, and there is high dispersal= most realistic
dispersal is higher than the rate of competitive exclusion= different species can exist in
the same patch
Differences between species sorting and mass effects
o LV equation:
o Adding births to patches (i)
o Species sorting assumes i is too small to affect the outcome
o Mass effects, i can be large enough to move dN/dt<1 to dN/dt>1
All paradigms but neutral predict coexistence at larger scales, even though a single species may
exclude all other species within a patch
o for how this happens, patch heterogeneity is critical
o heterogeneity: mass effects
o homogeneity: patch dynamics (tradeoff)
o NOTE: *Remember the 3 conditions of how each paradigm differs for the exam*
Scale dependent diversity: alpha, beta, and gamma