Population Ecology Chapter 53

Population Ecology The study of populations in relation to their environment Members of a population rely on the same resources, are influenced by similar environmental factors, and are likely to interact and breed with one another 3 fundamental characteristics Density Dispersion Demographics

Density The number of individuals per unit or volume The number of oak trees per square km Counting all individuals is impractical so there are a variety of surveying methods to make a viable estimate Sample plots Use of an indicator (nests, burrows, tracks) Mark-recapture method This is dynamic and is under constant change via birth, emigration, immigration, death, etc.

Dispersion The pattern of spacing among individuals within the boundaries of the population Clumped – most common pattern; organisms group together based on needs (i.e. food, mate, etc.) Uniform – evenly spaced pattern Random – the position of each individual is independent of other individuals

Demographics The study of the vital statistics of populations and how they change over time Birth and death rates are particularly important Life tables look at the survival patterns of a given population based on cohort groups (age groups) Data in a life table can then be represented graphically using a survivorship curve Classified into 3 types: Type I, II and III

Life History Traits that affect an organism’s schedule of reproduction and survival (from birth to death) When reproduction begins How often the organism reproduces How many offspring are produced during each reproductive episode Several kinds of life histories Big bang reproduction (semelparity) Repeated reproduction (iteroparity) There is a trade off between reproduction and survival Reproduction can lower the chance of survival for the parent

Exponential Model Change in population size (ΔN/Δt) = rN ΔN = the change in population size Δt = the time interval r = per capita rate of increase (annual per capita birth rates – annual per capita death rates) Under ideal conditions, population will increase exponentially The resulting curve is J shaped

Logistic Model Actual populations have limitations to population growth known as the carrying capacity (K) Limited by a variety of resources including energy, shelter, predators, nutrient availability, water, etc. The resulting curve is an S Not uncommon for populations to overshoot K and then fall back

Population Density Density independent – a birth rate or death rate does not change with population density Dune fescue grass mortality is due to physical factors such as drought, rather than density Density dependent - death rate will rise as population density rises; birth rate will fall as density increases

Density Dependent Density dependent is a type of negative feedback loop that works to halt population growth as density increases Competition for resources Territoriality Disease Predation Toxic Wastes Intrinsic factors Natural drop in reproduction via aggression, hormonal changes, delay in maturation, immune system depression

Population Dynamics Focuses on the complex interactions between biotic and abiotic factors that cause variation in the size of populations. Populations are not generally stable, they fluctuate for a variety of reasons Harsh weather can weaken the organism and diminish food supply causing a higher death rate. The fewer number of animals allows the food source to rebound and become plentiful causing a higher birth rate.

Population Cycles Some organisms undergo boom-and-bust cycles with average time lengths Voles and lemmings 3-4 yr cycles Ruffed grouse and ptarmigans 9-11 yr cycles Ex: Lynx and snowshoe hare populations are connected because of their predator prey relationship; they rise and fall together This cycle is thought to be related to predator-prey interactions and sunspot activity, which effects the quality of the hare’s food

Human Population In 1650, human population was 500 million, 1850 reached 1 billion, today almost 7 billion. The human population was experiencing exponential growth, but in the 1960s the growth rate began to decline We are still growing, but at a slower rate (currently ~1.15%) This is largely due to new diseases and population controls

Population Dynamics Populations vary widely from region to region Many areas are in the process of moving from high birth rate, high death rate to low birth rate, low death rate This is demographic transition This is related to healthcare and education, especially for women

Population Dynamics Age structure is very important in determining growth trends Populations with many more children than adults will find population increasing Understanding age structure and its potential effects can help countries plan accordingly Infant mortality and life expectancy play a role in planning at the family level

Global Carrying Capacity Estimates for carrying capacity vary widely, but average in the 10-15 billion range Estimate can be based on a variety of information such as extrapolation of current data or limiting factors such as the potential of available food based on diet Must be realistic about our needs/wants Scientists have also come up with the ecological footprint which accounts for the amount of land and water area required by each person to produce the resources consumed