Population Growth. Factors Affecting Population Growth Populations grow and shrink in response to...
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Transcript of Population Growth. Factors Affecting Population Growth Populations grow and shrink in response to...
Population Growth
Factors Affecting Population Growth Populations grow and shrink in response
to abiotic and biotic factors. Abiotic – physical & chemical factors
such as water & light availability, soil structure, salinity, pH, etc.
Biotic – factors having to do with living organisms, like competition & predation.
Population Growth Models We can study the way populations grow
by using models. What happens when resources are
unlimited? What limits population growth?
Example – Waterhemp Waterhemp
populations can grow quickly. When it occurs in soybean fields, it can reduce the crop of soybeans available to harvest.
Geometric Growth When there are plenty
of resources, the population can grow very rapidly. As the population
grows, there are more individuals available to reproduce, so it grows faster.
Geometric Growth If the estimated
growth rate is 2.0, each individual will produce 2 offspring each year. 2,4,8,16,32,64 etc
Geometric Growth A simple measure of
population growth is the ratio of the population size at one time (Nt+1) to the population size in the previous time step (Nt
). This is known as the finite rate of increase, denoted by lambda (λ).
Geometric Growth The geometric
population growth model: Nt = N0λt
Population that reproduce all at once sometimes follow this growth model. Sockeye salmon
Exponential Growth Some organisms can reproduce multiple
times throughout the year. We need to adjust our growth equation.
r=lnλ
Exponential Growth dN/dt = rN N= population size t = time r = intrinsic rate of increase So, change in population size over time is
the population size times r. Populations grow increasing fast due to
increase in reproductive individuals. Positive feedback
Exponential Growth When conditions
are optimal, with unlimited resources, the population can grow at its maximum rate. rmax
Exponential Growth The human
population is growing exponentially.
Logistic Growth In reality,
populations usually can not sustain exponential growth for long. Resources
become limiting.
Logistic Growth Carrying Capacity (K) – the number of
individuals the environment can support. The population grows exponentially, then
levels off as K is reached and resources start to run low.
Logistic Growth Example: sheep in
Tasmania (southern Australia) Introduced in 1810 Reached carrying
capacity around 1860 Fluctuates around
carrying capacity
Logistic Growth Equation Adds a carrying
capacity component to the exponential growth equation: dN/dt = rN (1-
N/K)
Factors that Affect Population Growth Density dependent factors will affect
population growth more when there are more individuals in a given area. Disease / parasites Food Light Space Predation
Factors that Affect Population Growth Density independent factors affect
population size in the same way regardless of population density. Storms Falling trees Natural disasters
Example - Aphids Aphid populations will increase until
they reach the carrying capacity. This is determined by limited resources –
soybean leaves for example. When the population nears the carrying
capacity, some individuals will get enough food to survive & reproduce, some get enough to just survive, and some will starve.
Aphids Is food limitation an example of a
density-dependent factor or a density-independent factor?
Density-dependent Density-independent
Dispersal and Metapopulations
Growth Rate Growth rate = births – deaths
We must also account for immigration and emigration.
Growth rate = births – deaths + immigration – emigration r = b-d + i-e
Immigration If a habitat patch is
small, it may not be able to permanently support a population. Immigrants from other
patches can come in and rescue the population periodically.
Source-Sink Populations Immigrants usually come
from a neighboring population. If there is a large population surrounded by smaller populations, the small ones may be constantly re-populated by immigrants from the large one.
Source-Sink Populations Population size and distance
between ponds affects the probability of California tiger salamanders colonizing one pond from another in a system of ponds in Monterey County, CA. Trenham and colleagues estimated how far rescue effects from source ponds extend to other ponds that might be sinks, based on dispersal patterns and pond characteristics. Dispersal estimates are shown by arrows. Ponds with many emigrants, such as LC, are sources, while ponds that primarily have immigrants but not emigrants, such as CRP, are sinks.
Dispersal Dispersal is important to help small
populations avoid extinction. Allows organisms to escape competition,
find mates, find new resources, etc. Take advantage of new space opened up
after a landslide etc.
Metapopulations A metapopulation is a
set of subpopulations connected via dispersal. A metapopulation inhabits a shifting set of occupied and empty patches that together sustain the metapopulation for much longer than any one patch sustains a subpopulation.
Variability in Populations Environmental stochasticity - refers
to seemingly random variability in resource availability, ecological community composition, predation pressure, weather events, etc., which often causes fluctuations in a population's growth rate.
Variability in Populations Demographic Stochasticity -
Population growth rate is determined by birth and death rates. By chance, there could be many births in a row, leading to a higher population growth than you would expect. Or by chance, there could be a number of deaths in a row, leading to unexpected extinction.
Variability in Populations The Allee effect
occurs when populations reproduce at a slower rate than expected at low population densities. Meerkats form social
groups where only one pair reproduces.
Variability in Populations All of these sources of variability can
make modeling and predicting population dynamics challenging.