Introduction to Ecology  Ecology is the study of organisms and their interactions with their environment.  The environment includes 2 types of factors: Biotic factors – the living parts of the environment  Plants, animals, bacteria, fungi, protists Abiotic factors – the nonliving parts of the environment  Water, oxygen, light, temperature, etc.

Levels of Organization  Ecologists can look at the world in levels of increasing complexity: Population – a group of organisms of the same species that live together in a certain area  Example: Community – a group of organisms of different species that live together in a certain area  Example: Ecosystem – all the living and nonliving things in a certain area  Example: Biome – a group of ecosystems that have similar climates and communities  Example: Biosphere – the entire portion of the planet that supports life

Population Ecology  A population is a group of individuals of the same species that live in the same area

Population Ecology: Vocabulary  Density: The number of individuals per unit area/volume Example: 47 elephants/km 2  Dispersion: The pattern of spacing among individuals in a population  Clumped  Uniform  Random

results from intense competition or antagonism between individuals where environmental conditions are fairly uniform. uniform distribution

occurs when there is no competition, antagonism, or tendency to aggregate where environmental conditions are fairly uniform It is rare for all these conditions to be met. Random distribution

is the most common distribution environmental conditions are seldom uniform reproductive patterns favor clumping animal behavior patterns often lead to congregation. Clumping

Factors That Influence Population Size  There are 3 major factors that influence population size: 1. the number of births 2. the number of deaths 3. the number of individuals that enter or leave a population - immigration: individuals entering an existing population - emigration: individuals leaving an existing population

Limiting Factors  factors that limit the size of a populations Density-dependent limiting factors  effect intensifies as the population increases Ex’s. Intraspecific competition (food & space), Predation Density-independent limiting factors  The occurrence and severity are unrelated to population size Ex’s. Climate, Disease, Pollution

Estimating 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 2 nd time Number of marked recaptures

Patterns of Population Growth  Exponential Growth: Occurs in ideal conditions with unlimited resources Characterized by a 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

 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

Pattern of population growth which takes into account the effect of population density on population growth Takes carrying capacity into account Occurs when resources become more scarce Characterized by an S-shaped curve Logistic Growth

Patterns of Population Growth  Carrying capacity: The maximum number of individuals that a particular environment can support over a long period of time Symbolized as K Determined by such limiting factors as crowding and food resources The point at which a graph “levels off” is the environment’s carrying capacity for that particular species

Where the curve levels off, births and deaths are in balance and the population has zero population growth. This occurs because environmental limitations become increasingly effective in slowing population growth as the population density rises. When the density approaches the carrying capacity, the limitation becomes severe.

Most populations oscillate around the carrying capacity. When a population goes above the carrying capacity, the environment can not sustain it the population will fall. If it falls below the carrying capacity, then resources are plentiful and population will increase.

The populations of many small short-lived animals, or those living in variable environments, go through a period of exponential growth, followed by a sudden crash (boom and bust curve). The crash occurs before the populations reach the carrying capacity; it is due to a density-independent limitation such as weather or other physical environmental factors.

K-selected populations (equilibrial populations) live near or at the carrying capacity r-selected populations (opportunistic populations) live in environments where the population densities fluctuate or where they face little competition Populations can be K-selected or r-selected

Characteristic r-selected K-selected populations populations -Maturation timeshortlong -Lifespanshortlong -growth curve boom-bust logistics curve -Death rate in younghighlow -Investment of nonesome time in offspring -Number of offspring manyfew per reductive episode -# reproductionsoneseveral per lifetime -Timing of first early in lifelate in life reproduction -Habitatunstablestable

Some populations show regular fluctuations in density in a cyclic way. Mammals -Small herbivores (lemmings) 3-5 year -Large herbivores (snowshoe hares)9-11 year cycle Hypotheses for fluctuations -Stress from high densities may alter hormone balance and reduce fertility or may cause physiological changes -High density may cause changes in the immune system,

Oscillation in the population of the hare preceded changes in the lynx density. This was once considered to be interrelated but the hare shows such oscillations on islands where the lynx is absent. The lynx population oscillation may be the result of hare population but not the other way around.

the study of factors that affect birth and death rate. Age structure -Every age group has characteristic birth and death rate a. Birth rate (fecundity) is greatest for those of intermediate age b. Death rate is often greatest for the very young and very old c. A population with more older, non-reproductive individuals,grows slower than a population with a larger percentage of young, reproducing individuals. Demography

Sex ratio is the # males vs. # females. It is not always 50:50. If the male mates with more than one female, then he may have a harem of females e.g. elk. If the species form monogamous pairs, the ratio is more 50:50. In addition to the birth rate and death rate, the potential life-span, the average life expectancy, and the average age of reproduction are important determinants of the makeup of a population.

A major factor in the variation of the growth rates among countries is the variation in their age structure. The relatively uniform age distribution in Italy, for example, is due to the fact that the growth is rate is stable. For every birth there is a death on the average. It has been this way for some time. This keeps the population at large from growing. The size of each age class from pre-reproductive through early post-reproductive is roughly equal. In the U.S. the death rate is closely equal to the birth rate, however this is recent. Because of that, the U.S. has a disproportionate percentage of pre-reproductive and younger reproductive individuals. The U.S. would reach equilibrium like Italy in about 25 years, however, the high rate of immigration will keep it like the population distribution as it is shown. In Kenya the age distribution is heavily weighted toward the bottom, with larger percentages of younger age classes in the bottom. Diagrams

Determining the mortality rates for the various age groups in the population gives a survivorship curve. type I curve - all the organisms live to old age and die quickly type II curve - constant mortality rate at all ages type III curve - mortality among the young is very high, but those who survive the early stages tend to live for a long time. In nature, high mortality among the young is the rule. survivorship curves

Human Population Growth So far, the history of human population growth has shown exponential growth. Humans have “changed” many of their limiting factors (agriculture, medicine, eliminate interspecific competition). But that cannot continue forever!

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 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?