Lecture 14: Population spatial structure Flashcards
Ecologists try to understand what factors determine
the distribution and abudance of species
Population
group of individuals of the same species that live within a particular area and interact with one another
When species interact, it means
compete, share pathogens, reproduce
Distribution
Geographic area where individuals of a species occur
Abundance
Number of individuals in a given area
Measure of population abundance
- population size (individuals)
- population density (individuals/area or individuals/volume)
Genetic population
group of individuals that mate with one another and produce offspring; group of individuals that exchange genes
Ecological population
group of individuals of the same species that compete for same resources, share pathogens, parasites, predators, and mutualists
they are linked together by ecological processes
What is the spatial extent of genetic population vs. ecological population?
Think about conifers are all wind pollinated
In this case, Genetic population is much larger because of the genetic connectivity (gene flow) provided by long distance pollen dispersal
Dispersion of individuals within a population =
spacing with one respect to one another
Dispersion of individuals can be described as 3 distributions:
- Regular distribution
- Random distribution
- Clumped distribution
From regular –> random –> clumped distribution is
overdispersed –> regularly dispersed –> underdispersed
Causes of overdispersed (even spacing) distribution
Animals defending territories (overdispersion at individual level and group level)
In plants: resource limitation (ex. water limitation)
Causes of underdispersed (clumped) distribution
Animals: animal flocks, cooperative breeding
Plants: heterogenous (=patchy) physical environment
Spatial structure may vary by
scale
Ex of how herbacceous has spatial structure varrying by scale
patchy at large scales (limestone meadows)
more random/uniform at smallest scales
Dispersal limitation
absence from suitable habitat due to limited dispersal
-particularly annoying to theoreticians
can be easy to test for
Ex. Hawaiian islands only have one native terrestrial mammal, hoary bat
Measuring a population
Measuring subsamples for sessile organisms
Mark-Recapture for motile organisms
Area based counts of a subsample
individuals in a given area of volume are counted
Used to estimate abundance of sessile organisms like plants
Individuals are counted in several quadrats; counts are averaged to estimate population size
Quadrats
sampling areas of specific size
Distance methods
distance of individuals from a line or point are converted into estimates of abundance
Line transects
observer travels along a randomly placed line, counts individuals and determine distance from the line
used for sessile organisms (trees)
Mark Recapture studies
Measure population abundance for mobile organisms
A subset of individuals are captured and marked or tagged, then released
Those released individuals mix evenly with rest of population
At later date, individuals are captured again and proportion of previously marked individuals is noted
For mark-recapture studies, the proportion of marked individuals in recaptured population should equal
the original proportion of the population caught during the first sampling
Variables for measuring population size (N) with mark recapture
N is ____
A=
B=
R=
A/N=
R/B=
N is unknown
A= # in 1st sample = # marked
B= # of 2nd sample
R=# of recaptured in 2nd sample
A/N= true proportion marked
R/B= observed proportion marked in 2nd sample
Assuming no bias, equation R/B __ A/N
equal
N=
N= (A * B) / R
Geographic ranges vary in
size
Ex. many tropical plants live in small ranges vs. coyotes live all over North American continent
Endemic species
occurs in one location only, and no where else on Earth
Geographic range includes
all the areas a species occupies during all life stages
Really important to think about geographic range and distribution pattern for species that
mirgate and whose biology is poorly understood
Within a species’ range, not all habitats are ___ so distributions are ____
suitable, patchy
Some species have very specific _____ others tolerate ______
habitat requirements, a broader range
Ex. Creosote bush tolerates both dry and cold vs. saguaro cactus can tolerate only dry but not cold
Habitat suitability determines
distribution and abundance
Habitat suitability/ species distribution influenced by
abiotic factors: moisture, temperature, pH, light, nutrients
biotic factors: herbivores, predators, competitors, parasites, and pathogens
disturbance
Disturbance
events that kill or damage some individuals, creating opportunities for other individuals to grow and reproduce
Ex. Fires
Influence species distribution
Species distribution models
Attempt to determine climatic rules determining a species’ abundance
Apply those rules to predict distribution as climate changes
Why are species distribution models useful
You don’t know the whole range of a species, but have samples of its presence
Predicting spread of pest species
Predicting shifts with climate change
Chameleons in Madagascar Species Distribution Model Example
Models predicted distributions from known climatic associations
Able to see where species was found not known before
Use known samples with climate change
Not perfect!
Large scale spatial structure
Effects of climatic change on geographic ranges
Map Climate velocity
The distance needed to move to maintain the same climate
Can be done for different details of climate
Population dynamics
the study of population size over time
simple assumptions about individuals
assume all individuals are the same
Use of population dynamics
Predator-prey cycles
Disease dynamics
Population viability analyses
If △N > 0
population is increasing
If △N < 0
population is decreasing
△N =
births - # deaths + # immigrants -# emigrants
Population dynamics models used to answer questions:
How does the population size change over time?
What determines whether a population is increasing or decreasing?
Assumptions in Population Dynamic model 1
Births per unit time of the population are constant (B)
Deaths per unit time of the population are constant (D)
Immigration = Emigration
Population Dynamics model 1
Linear increase/decrease in population size
Constant population growth
Assumptions in Population Dynamic model 2 = Discrete model
All of the individuals are the same
Each individual gives birth at the same rate (b)
Each individual has same chance of dying (d)
Immigration and emigration are equal
per capita
rate of change in population size
per capita = per individual
λ
Geometric rate of increase
Conditions of geometric growth
Unlimited Resources
Often: small population size and rapidly growing organisms (time lags)
Good “null” model for comparison: If there are no changes to per capita births, deaths, and immigration = equal emigration
Ex of geometric growth
Bacteria in petri dish with abundant food, invasive species, global human populations
Discrete geometric growth model
Time is discrete
The per-capita rate of change in population size is constant.
▪ Each individual contributes the same amount to the change in
population size over time.
▪ This means the population will rapidly increase or die out
▪ Or stay the same if lambda = 1.
Good null model to figure out if the per-capita rate of change
in population size is changing.
