Chapter 52 Population Ecology

Population ecology - The study of population’s and their environment. Population – a group of individuals of a single species living in the same general area. The members of a population rely on the same resources, are influenced by similar environmental factors, and are likely to interact and breed with each other.

Characteristics Density - number of individuals per unit area or volume. Dispersion - the pattern of spacing between individuals within the boundaries of the population.

Dispersion Clumped - when individuals are in patches, this is the most common. Uniform - this is when individuals are evenly spaced. Random - this is unpredictable spacing in a population.

Demography - study of vital statistics of populations and how they change over time Age Structure – the coexistence of generations gives populations age structure. This is the relative number of individuals of each age. Sex ratio – the proportion of individuals of each sex that affects population growth. For example the number of females is usually directly related to the number of births. Fecundity - (birthrate) the number of offspring produced during a certain amount of time.

Survivorship Curve - a plot of numbers in a cohort (group of individuals of same age, from birth until all are dead) still alive at each age.

Type 1 The curve is relatively flat in the beginning which shows a low death rate in early and middle life, and then drops steadily as the death rates increase among older age groups. Humans are an example of this as well as other large mammals.

Type 2 This curve is intermediate with the mortality constant over the lifespan. Annual plants are an example of this.

Type 3 This curve drops sharply at the left of the graph, which shows a high death rate for the young, but then the curve flattens out as the death rates decline for those few individuals who have survived to a certain critical age.

Age Structure Pyramids Population growth is important to determine what the rate of growth will be.

Which group will likely have a rapid increase in growth?

Life History Traits Has three basic variables – When reproduction begins – How many offspring are produced during each reproductive episode – How often organism reproduces.

Tradeoffs between reproduction and survival Natural selection cannot maximize all reproductive variables simultaneously. – Time – energy – Nutrients limit the reproductive capabilities of all organisms.

Semelparity (semel –once) Is a “one shot” pattern of reproduction. Unpredictable environments in these environments adults are less likely to survive, so the production of a large number of offspring should increase the probability that some offspring will survive.

Iteroparity (iterare – to repeat) Is repeated reproduction. Dependable environments where adults are more likely to survive to breed again. Competition for resources may be intense, and large offspring should have a better chance of survival.

Number of offspring Large numbers of offspring – Usually for plants and animals whose young are subject to high mortality rates. Fewer offspring – extra investment on the part of the parent in some organisms greatly increases the offspring’s chances of survival.

Age of Reproduction Breeding late in life is usually a K selected population. When Semelparity organisms reproduce early they have a shorter life span. When Iteroparity organisms reproduce later in life they have to invest more energy in maintenance and growth.

Population Growth models Exponential vs. Logistic growth

Exponential Growth Unlimited growth with unlimited resources. For example human population growth.

Logistic Growth Most populations are limited and do not have exponential growth. They have a carrying capacity which is the limit of their population. If there is a K in the equation it is logistic growth because of the carrying capacity.

K vs. R selected Populations K population density is usually near the carrying capacity. Long life span, few young, slow maturing process, reproduction later in life, and extensive care of young. (Slow growing populations) R Grow exponentially when environmental conditions allow them to reproduce. Short life span, reproduce early in life, have many offspring, mature rapidly with little or no parental care. (rapidly growing populations )

Density Dependent Factors Effect a population more as the population increases. The rate at which they become depleted depends upon the population density of the population uses them.

Density Independent Factors Environmental conditions that can affect a population independent of population density. For example weather, climate, and severe disturbances.

Boom and Bust Cycles These are populations that fluctuate at regular intervals. The cycles may be caused by food shortages or predator- prey interactions.

Human Growth Human growth for now seems to be exponential and the only thing that may limit this growth is the lack of space. One of the most important ecological questions is how many humans the earth can support.