Population Ecology. Populations Population ecology is the study of populations in relation to the...
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Transcript of Population Ecology. Populations Population ecology is the study of populations in relation to the...
Population Ecology
Populations
Population ecology is the study of populations in relation to the environment
A population is a group of individuals of the same species that live in the same area
Population Ecology: Vocabulary
To describe populations, we have to talk about how they are situated
Density: The number of individuals per unit
area/volume Example: 47 elephants/km2
Dispersion: The pattern of spacing among individuals in a
population Clumped Uniform Random
Uniform
Environmental conditions are uniform
Causes COMPETITION or antagonism between organisms (territoriality)
Clumping
Most common Reproductive
patterns favor clumping
Social behaviors lead to clumping
Optimal density is usually intermediate (medium)
Random
No competition No tendency to
group/clump Conditions are
uniform Rarely happens!
Estimating Population Size
Rarely, it is possible to count the number of species in a population
Usually, a statistical method is needed to determine population size
The mark-recapture method can be used to estimate the size of a population
Capture, mark, release Recapture and count Equation: N = Number marked x Total catch 2nd time
Number of marked recaptures
Factors That Influence Population Size
There are 3 major factors that influence population size:1. the number of births
2. the number of deaths3. the number of individuals that enter or leave a population
- immigration: individuals entering an existing population
- emigration: individuals leaving an existing population
DEMOGRAPHICS
In looking at populations, biologists must also look at demographics (the vital statistics of a population and how they change over time) Birth rates Death rates Life tables Survivorship curves Reproduction rates Growth rates Age structures
DEMOGRAPHICS
One tool in demographics is to estimate the life span of organisms
To do this they often create a life table, age summary of a populationA cohort (group of individuals of the
same age) are studied to see what percentage of the population dies
This can determine life expectancy, survivorship at each age category, or male vs. female survivorship
Survivorship Curves
A way to represent a life table is a survivorship curve, a plot of the proportion of individuals that survive at each age group
When discussing survivorship curves, there are 3 general categories
Survivorship Curves
Type I- live to old age & die (most large mammals)
Type II- constant mortality rate (rodents, lizards, hydra)
Type III- high mortality at young age, but if they survive they live a long life (sea turtles).
DEMOGRAPHICS
Another important aspect of demographics is reproductive rates For obvious reasons, biologists who study
reproductive rates only focus on females Age of fertility Number of offspring for each age
group Time of year Spawning cycles
Reproductive tables summarize this data
Life Histories
Life history are traits that affect an organism’s schedule of reproduction and survival
Clutch size: Number of offspring produced at each
reproductive episode Semelparity
A life history in which an organism spends most of its energy in growth and development, expend their energy in one large reproductive effort, and then die
Many insects, annual plants, salmon, etc.
Life Histories
IteroparityA life history pattern in which
organisms produce fewer offspring at a time over a span of many seasons
Example: humans, panda bears, etc.
Life Histories (Semelparity vs. Iteroparity)
Many factors contribute to the life history of an organism Finite resources Reproduction vs. survival Number and size of offspring Paternal investment in offspring
POPULATION GROWTH MODELS
Different models of how populations grow
Formulas off your Cheat Sheet
Rate: dY/dt Generic expression for change of some
variable (Y) over time (t) Population Growth: dN/dt = (B – D)
The change in population size (N) over time (t) is the same as the birth rate (B) minus the death rate (D)
Exponential Growth: dN/dt =rmax •N The change in population size over time is
equal to the growth rate (rmax) multiplied with the current population size
Formulas off your Cheat Sheet
Logistic Growth: dN/dt =rmax •N • [(K-N)/K]The growth rate over time is the
same as exponential growth adjusted for carrying capacity (K)
Types of Population Growth (start here)
One of the biological imperatives is to reproduce and pass on genetic material to succeeding generations.
Yet population growth is controlled by the environment and limited resources
This causes different patterns of population growth
Patterns of Population Growth Exponential Growth:
Occurs in ideal conditions with unlimited resources
J shaped curve Example:
1 bacterium (reproducing every 20 minutes) could produce enough bacteria to form a 1-foot layer over the entire surface of the Earth in 36 hours
Exponential Growth
Exponential growth is a useful model when studying populations that are introduced into a new, unfilled, environmentRecovery after a catastropheExponential Growth:
dN/dt =rmax •Nrmax is the maximum rate of
population growth for the species
Patterns of Population Growth
Exponential growth cannot continue indefinitely
It is characteristic of populations who are entering a new environment OR those whose numbers are rebounding from a catastrophic events
Patterns of Population Growth
Logistic Growth: Pattern of population
growth which takes into account the effect of population density on population growth
Occurs when resources become more scarce
Characterized by an S-shaped curve
Patterns of Population Growth
dN/dt =rmax •N • [(K-N)/K] Carrying capacity (K):The maximum number of individuals that a particular environment can support over a long period of timeDetermined by such limiting factors as crowding and food resourcesGraph levels off at carrying capacityK-selected populations (equilibrial populations) live near or at the carrying capacity
Carrying Capacity
K-strategists (Life history)
Density stays near carrying capacity.
Large, slow growing organisms Small population sizes Long life span; slow maturation Few young/small clutch size Reproduce late in life Parental care Most large mammals; endangered
species
r- strategists (Life history)
Grow exponentially when environmental conditions allow; when conditions worsen, population size plummets.
Short life span Reproduce early in life Many offspring/large clutch size Usually small in size Little or no parental care Bacteria, some plants, insects
Environmental Factors
Abiotic and biotic influences on population size
Limiting Factors
There are a number of factors that limit the size of populations:Density-dependent limiting factors
(depends on the size of the population)
Density-independent limiting factors (does not matter the size of the population)
Density-Dependent Limiting Factors
The effect of density-dependent limiting factors intensifies as the population increases Intraspecific competition
Food, space, etc.TerritorialityPredationWaste build upDisease (if caused by
pathogen/contagious)
Density-Independent Limiting Factors
The occurrence and severity of
density-independent limiting factors are unrelated to population size Climate Disease (if not caused by pathogen/not
contagious) Pollution
The Interaction of Limiting Factors
Density-dependent and density-independent limiting factors often work together to regulate the size of a populationDeer in snowy winter
Starve from lack of food (density-dependent)
Severity of winter/depth of snow determines access to food (density-independent)
Population Dynamics
Population dynamics is the study of the environmental factors that cause variations in the population size Looks at fluctuations in population over
time to examine stability Immigration and emigration also affect
population Metapopulations are when you have
several interconnected populations
Boom-and-bust cycle Another phenomena that affects population are
predator-prey relationships. Each population is interdependent and causes a
boom-and-bust cycle The prey population increases which causes
the predator population to increase The prey are over hunted and their population
crashes This causes the predator population to crash Now, with fewer predators, the prey
population can again increase (recovery gives a geometric increase)
Boom-and-Bust Cycles
Human Population Growth
Increase in Human Population Agricultural Revolution -
Major period of population growth began when humans started to cultivate crops and domesticate animals
Industrial Revolution – Improved food production and distribution
Health Care – germ theory lead to improved hygiene, better waste removal and water treatment
Decrease in Human Population Plague – disease that
greatly reduces the size of population (Black Plague in 1300’s reduced the population in England by 50%)
Famine –a severe food shortage causing starvation and death (Potato Famine of 1840’s/China 1870-1890)
War – death by combat, disease, cut off from food supply (Germany 1618-1648/WWI/WWII)
Human Population Growth
The human population is unlike any other organism
Since about 1650, we have remained in an exponential population increase
Population increases by about 201,000 people/day worldwide
Human Population Growth
Human Population Growth
Even though there is a tremendous increase in human population, it is not evenly distributed around the globe Regional areas have different
population trendsSome regions have stable regional
human populations (birth rate is the same as death rate)
Other regions show incredible growth rates
Industrialized Nations
An emerging nation usually has a very high birth rate, but also a high death rate (disease, lack of modern medical treatment, famine)
An industrialized nation usually has a low death rate, but also a low death rate Moving from an emerging nation to an
industrialized nation is known as a demographic transition
Human Population Growth
In the 1950s, mortality rates began to rapidly drop (advances in medicine and sanitation)
Yet, the birth rates have not always dropped Has caused a huge increase in
population in some nations About 80% of the world’s population
lives in emerging nations
Age Structure
One way to determine human population growth is to look at the nations’ age structure, relative number of individuals at each age
By looking at the age structure of a population, you can determine the population growth
Age-Structure Diagrams
Human Population Growth
Implications of exponential human population growth: Lack of food supplies Lack of space Lack of natural resources (metals, fossil fuels, etc) Lack of sites for waste disposal
Ecologists cannot agree on a carrying capacity for Earth (2 – 40 billion) Are we going to reach carrying capacity
through individual choices and/or government programs?
OR Is Earth’s population going to “level off” as a
result of mass deaths?