Transcript Temporal and spatial dynamics of populations

Temporal and spatial
dynamics of populations
Reading; Smith and Smith
Chapters 10-12 (read)
Every Species Has a Range

Range can be determined by physical tolerances,
and by the availability of critical resources
– can also be determined by history and geographic
barriers preventing dispersal.
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In the United states, most species have a range of
4-24 states.
– Cosmopolitan species are an exception, they are
worldwide in distribution.
– Endemic species are found in only a small, restricted
area.
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Example; trees are very sensitive to moisture and
temperature
– The northern and southern limits of the range of North
American maple species are determined by winter
temperatures and summer temperatures
• differences in their ranges reflect different tolerances
– The western limit is imposed by summer drought
– Within the range, each species tends to grow within areas
where certain conditions predominate
• Sugar maple-forest species, prefers well developed soil and
over 50cm (north) and 100 (south) cm of rain
• Black maple-like silver maple but drier soil, higher
Calcium
• Silver maple-moist, well drained soil-river valleys
• Red maple-either wet, swampy soils, or dry, poorly
developed soil-where other species will not grow
Distribution
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Populations of a species occur within areas of
suitable habitat-but the absence of a species does
not necessarily mean that the habitat is unsuitable..
– For example-Islands separated from a
continental area will harbor some species from
the mainland, but not others. Certain birds,
arthropods, and reptiles disperse well to islands,
but many other species do not.
– The range of a species may be in the process of
change, but dispersal has lagged behind climate
change. This is the case for certain North
American trees, notably the Beech.
– An important geographical barrier may keep a
species restricted to a given continent or area, even
though suitable habitat exists elsewhere
– Recent human activities have broken down
many of these historical barriers to dispersal,
resulting in the widespread introduction of
exotics to new areas. I.e., the Nile Perch.
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Within its range, a species is distributed into one
or several populations.
– Populations are all the members of a single
species, within an area, that can potentially
interbreed.
– Populations are frequently defined by areas of
suitable habitat-but, for some species,
population boundaries can be rather arbitrary.
– Within these areas, interaction and gene flow is
frequently localized-Populations are often
subdivided into subpopulations or demes.
– An array of heirarchially structured clusters of
individuals is sometimes referred to as a
metapopulation
Example: An endemic perennial shrubWithin the geographic range of
Clematis fremontii,
individuals are restricted to limestone
glades.Within Glades, individuals
are clustered
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These are Idealized Population Structures
– A) Standard (Levins) metapopulation-Isolated patches
harbor populations that persist for a while, but eventually
may go extinct (arrival of a predator or competitor, or
simply outstripping its own resources).
– Once empty, patches may be colonized.
– Surviving patches send out colonists that colonize empty
patches.
– Landscape is a dynamic matrix of empty and occupied
patches-whole system is stable even though isolated
populations are not
– B) Source-Sink-As above, but some areas are more or
less permanent “sources” and some are “sinks”-areas that
cannot sustain permanent populations
– C) ”Patchy Environment or Habitat Network”-more or
less permanent corridors link habitat patches
Example-Checkerspot Butterfly
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Paul Ehrilch studied the Checkerspot butterfly
Euphydryas editha bayensis for over 20 years.
– The species occupied serpentine grasslands
(containing high metal content), a patchily
distributed plant community at Jasper Ridge, near
Stanford, CA.
– Slight changes in solar intensity on the serpentine grasslands
regulate moisture and temperature variation that dictate the
reproductive success and ultimately determine whether a
patch is a “source” or a “sink”.
– Certain patches of
habitat are “sources”,
which can sustain
permanent populations.
– Other patches are
“sinks”, unsuitable
habitat which cannot
sustain populations.
– Still other areas can
sustain suitable
populations for a while,
but ultimately too small
to persist, and
occasionally go extinct,
only to be recolonized.
– For Checkerspots, Weather Affects Suitability of
Patches;
• Sunny weather supports increased larval growth
• Rainy weather inhibits larval growth
• Weather also affects host plants-cold weather inhibits the
rate of their development.
• During droughts, only larvae on cool slopes survive,
because plants senesce before larvae complete development
• During moderately wet years, the population expands to
colonize patches on sunny slopes.
• Area C-Large, flat area.
Supported a single,widely
fluctuating population till 1991.
• Area H. Smaller, but a
topographically heterogeneous
metapopulation. It was more
stable, but ultimately went
extinct in 1998.
• The butterfly is currently
endangered, but it still may
recolonize the Jasper ridge area
from the outside.
Example: Mud Daubers
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The organ-pipe mud dauber, Trypoxylon politum,
builds mud nests-the female stocks her nest with
paralyzed spiders of many types, but prefers
Neoscona sp.
The species was probably originally restricted to
cliff overhangs and caves near water in the
Southern United States.
– Now it nests wherever a bridge crosses a slow
moving stream, or where a garage is near a
source of mud.
– Females are excellent fliers, but once they start
building a nest, they tend to stick with it till
something kills them.
– Mud daubers in any given area form a local
metapopulation, composed of a local
subpopulation wherever there is a suitable nesting
site.
– Individuals usually disperse from their local
subpopulation at birth, Molumby (1995) showed
about half of them tend to return.
– These local subpopulations may go extinct at any
time floodwaters get high enough to wash away the
nests, or whenever the State finds the money to
clean the bridges.
– Old populations accumulate parasites, and
ultimately become “sinks”. Newer, smaller, and
isolated populations are “sources”.
– Because of this extinctionrecolonization dynamic,
the mud-dauber
metapopulation is quite
stable, even though the
population at any bridge
site, shed, or cave, is
temporary and might go
extinct at any time.
– The parasites of mud
daubers, and other
species that use their old
nests, have similar
metapopulations.
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Some populations move
For example;
– Pacific salmon migrate from the ocean upstream to
spawning sites in freshwater pools
– Atlantic eels migrate to breed in the Sargasso Sea, an
area of immense seaweed growth in the Central
Atlantic. Larvae swim to Europe and colonize
freshwater rivers.
– Monarch butterflies migrate to overwintering grounds
In Mexico and California, returning to the Midwest the
next year. No single individual ever makes the trip, it
takes two generations each way.
– Arctic terns migrate from feeding grounds in the Arctic
to feeding grounds in the Antarctic every year, breeding
on shorelines in the North Atlantic.
Some migratory
vertebrates of
North America
Population Growth
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It is a fundamental characteristic of all living
things that under some conditions, they reproduce
and increase in number
The growth of biological populations is of
tremendous interest to ecologists– Many populations of organisms undergo
conspicuous cycles and changes in abundance
– The individual propensity for reproduction is
the basis of an organism’s fitness
– The ability of a population to grow under a
given set of circumstances reflects its
competitive ability.
– Exponential growth is a simple model that
describes population growth in an essentially
unlimited environment.
– It assumes that the rate of births and deaths are
essentially constant, thus;
• b=instantaneous per capita birth rate
• d=instantaneous per capita death rate
• N=population size
• dN/dt=bN-dN, since b-d=r;
• dN/dt=rN, so calculus fans, you can integrate to
get:
–N(t)=N0
rt
e
• where r is the exponential growth parameter
• N0 is the starting population
• t is the time elapsed
– r=0 if the population is constant, r>0 if
population is increasing, r<0 if the
population is decreasing.
 Exponential population growth is only appropriate
for describing the initial colonization of empty
habitats.
 Ultimately, the growth of every population is limited
by some resource.

Actual organisms compete with each other for
limiting resources.
– As a population becomes more dense, reproduction of
individuals can be inhibited.
– Mortality can increase with increasing population
density as well.
– These patterns are called density dependence.
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Ultimately, all organisms have a carrying
capacity based on limiting resources in the
environment-thus they cannot grow forever.
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Example-fecundity in Drosophila melanogaster
is inversely proportional to population density
– primary mechanism is mortality of larvae
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Example-larger populations of mud daubers
experience higher rates of parasitism
Example-many
plants go through a
process called
“self thinning”
Mortality rates for
seedlings increase
with density,
because only so
many adults will
survive-regardless
of initial seedling
number
The logistic growth model accounts for
density dependence by designating a
fixed carrying capacity.
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– dN/dT=rmaxN(K-N)/K where
K= Carrying Capacity, this is the maximum
number of individuals that the population can
sustain.
N=The Number of individuals in the population at
a given time.
rmaxis the maximum population growth rate
 The population growth rate is positive when
populations are less than K:
dN/dt = rN(1 - N/K):
 If N < K, then N/K < 1, so the term inside the
parentheses is positive thereby making the growth rate
positive, so the population will increase.
 The population growth rate when populations
are greater than K:
 If N = K (when the population has reached carrying
capacity), N/K = 1, and the term in the parentheses = (1 1) = 0.
 The growth rate is zero when the population is at
K, and the population size will not change.
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Simple, laboratory populations frequently exhibit logistic
population growth.
– For example, if light and nutrients are kept constant,
populations of algae, such as Anabaena, will reach
carrying capacity
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Most natural populations, however, exhibit patterns not
predicted by the simple logistic model.
– Many undergo periodic cycles.
This figure describes population fluctuations in Australian moths.
Pp. 190 of Smith and Smith lists four other examples of cyclic
populations.
Lag time-oscillations
– For all but the shortest-lived organisms, there is a delay
between the growth of a population, and the effects of the
increase in population density on the survivorship and/or
reproduction of its members.
– This delay is called lag time.
• It is given the symbol t
– Lag time can have major consequences on the dynamics of a
population.
• 1. Populations often show overshoots of the carrying
capacity or oscillations around K
• 2. The logistic equation will produce oscillations if time lags
are introduced.
• 3. The population grows according to a population density
prior to the current density of the population
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When lag time is incorporated into the
logistic equation, the behavior is dymamic-N
at any given time depends upon the history of
the system.
Essentially, there are three patterns;
– rt<p/2-damped oscillations-ultimately the
population reaches a stable size at carrying
capacity.
– rt<e-1-no oscillations at all, population goes to K
– rt> p/2-oscillations form limit cycles, with
periods from 4t to 5t
– very large rt produces chaotic behavior-very
complex patterns that skyrocket and crash
This is actually a discrete time model, continuous time
models look pretty similar, except the parameters are
different.
Nicholson Blowfly Experiment
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A. J Nicholson, an Australian entomologist,
studied the population dynamics of an
artificial population of blowflies.
– Experiment 1-Provided blowfly larvae with
50grams of ground liver per day (as an
oviposition substrate). Adults had essentially
unlimited food.
– Monitored numbers of eggs, larvae, and adults
for over a year.
– Result-populations of adults fluctuated from
over 4000 to 0. Eggs and larvae underwent
similar fluctuations.

Nicholson noted that when the numbers of
adults were large, they would lay so many
eggs that nearly all the larvae died. He
postulated this to be the mechanism for the lag
time
– Experiment 2-Provided blowfly larvae with
50grams of ground liver per day (as an oviposition
substrate). Adults had 1 gram/day of food.
– Monitored numbers of eggs, larvae, and adults for
over a year.
– Result-populations of eggs, larvae, and adults
reached a stable, high population.
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Actual populations of organisms vary in their stabilitysome may persist for thousands of years, others last
only a short time.
– For actual species, K varies from year to year as
well, because weather and other factors influence
available resources-bad years can lead to the
extinction of a population (catastrophe).
– For populations with a high rt, chaotic behavior can
cause a population to crash and go extinct.
– Small populations can also go extinct due to random
factors (stochasticity) or inbreeding.
– As we shall see, competition or predation may also
drive a population extinct.
Metapopulations and Persistence
– 1. Almost all populations are fragmented to some
degree and thus have patch structure
• 2. Most ecological processes are strongly affected by the
degree to which populations are subdivided
• 3. Occupation of presumably habitable sites are affected by
both ecological processes such as local mortality rate as well
as the size of the site and its distance from other sites
• 4. The metapopulation persists as a balance between extinction
and colonization of the subpopulation sites.
• 5. The overall population may be stable but the subpopulations
are not.
• 6. Local populations that are part of a metapopulation may be
unstable in their dynamics
A Simple Model
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A series of populations linked by dispersal can be
much more stable than any single population.
– Simple model p=1-e/c where;
• p=proportion of patches occupied
• e=population extinction rate
• c=colonization rate

– for e=0, p=1 all patches occupied
– for e<p metapopulation is unstable, eventually
it goes extinct (takes a while)
– The metapopulation is stable for c>e, if both are
between 0 and 1.
In actual species, these assumptions are not likely
to be met-neither e nor c is likely to be constant
Dispersal
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Nearly every species has evolved some means of
dispersal, but dispersal ability varies from one to
the next
– Dispersal is dangerous, and for a few,
exceptional species, there is selection against
dispersal;
• Insects on oceanic islands frequently evolve
the loss of wings.
– For species that exist in a matrix of unstable
habitats, dispersal is essential to persistence.
• Individuals without dispersal ability will produce
progeny that are ultimately doomed to extinction.
Modes of Dispersal
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These are a few examples, actual modes of dispersal
are as various as organisms themselves
Animals-walking, crawling, directed flight, swarms
pushed by prevailing weather (locusts), attached to
animals (parasites), planktonic larvae
Plants-seeds with wings or parachutes, floating seeds
(impatiens, coconuts), seeds with spines or prickles
that attach to animals, seeds that pass through the
digestive tract of animals
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Protists, bacteria-resistant stages that can be carried
on, or in, other organisms, passive dispersal
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Example-roadside weeds in urban areas.
– Most people don’t think about them much, but
the urban landscape is a matrix of patches, each
of which supports populations of weeds.
– Landscapers, city workers, drought, and
development destroy patches on a regular basis,
but there is no city that is free of weeds.
– Urban weeds all have one thing in commonexcellent powers of dispersal.
• This offsets the high extinction rate by ensuring a
high rate of colonization for each species.

The interaction between a population’s
capacity for growth, its dispersal abilities, its
ecological tolerances, and its competitive
ability relative to similar species determines
whether a species can live in a particular
environment.
– Some species are good dispersers, adept at
colonizing disturbed habitats.
– Other species do not disperse well, but once
established, are excellent competitors and cannot
be displaced by invading species-they have a low
extinction rate.

Every species has a life history which has
evolved to maximize fitness. Only life
histories that are also compatible with the
existence of the species have survived.