It is usually measured as the large number of individuals found per sample. How species abundances are distributed within an ecosystem is referred to as relative species abundances.

Relative species abundance is calculated by dividing the number of species from one group by the total number of species from all groups.

• Based on the taxonomic similarities of the organisms living there
• Boundaries are set where species composition changes dramatically over short distances

• Climate determine the distribution of biomes
• Terrestrial biomes are based on the structure of their dominant vegetation, Aquatic biomes on their Physical / Chemical differences
• In each biome vegetation has similar traits, but may not be evolutionary closely related

• Climate is the average atmospheric conditions found over time (Temperature, Precipitation, Wind Velocity)
• Climate varies because of differences in the amount of solar energy (Drives global atmospheric and oceanic circulation)

• Edward Forbes 1887, H.C. Cowles 1899, - community regulation and succession

Any ecologically integrated group of species of microogansisms, plants, animals inhabiting a given area

Negative interaction where organisms are harmed by the association.

• occurs intraspecifically or interspecifically

• occurs over resources (food, space, light, nutrients, etc.)

N1 = αN2

• expressing species #2 in units of species #1

N2 = β N1

• expressing species #1 in units of species #2

α< 1

• species #2 has less competitive effect on species #1 than species #1 has on itself

α > 1

• species #2 has greater competitive effect on species #1 than species #1 has on itself

Apply same line of reasoning to species #2 (β)

• Farr 1843 - Farr’s rule (relationship between the density of the population and the death rate)

Density = the number of individuals per unit area or volume

Factors that alter per capita birth or death rates in a population are dependent on population density (can stabilize population density)

Ex: Parasitism, predation, competition can be density- dependent.

Factors that alter per capita birth or death rates in a population are independent of population density (do not stabilize population density)

Ex: Physical/chemical factors: weather, drought, freezes, flood and fire

The movement of organisms from one geographical location to another.

• Most organisms are mobile

• Geographical barriers are common in nature

An abiotic factor; Catastrophic disturbance such as fires, hurricanes, tornadoes and volcanoes can devastate organisms and limit distribution

• Frequent disturbance may be predictable and organisms may be evolutionarily adapted to the disturbance

• Infrequent disturbance may be unpredictable and organisms will not have acquired the evolutionary adaptation to them

Growth, especially in the number of organisms in a population, which is a geometric function of the size of the growing entity: the larger the entity, the faster it grows.

dN =(b-d)N dt
dN = r N dt

Graph is in increasing parabola

• Organisms can move but choose not to live in certain habitats

• Distribution may be limited by the behavior of individuals in selecting their habitat

r = per capita growth rate

IF: r < 0 (population decreases), r = 0 (population size is constant,    r > 0 (population size increases)

• No natural populations can maintain exponential growth indefinitely
• Population density typically fluctuates around a constant number of individuals
• As population size increases resources become limiting for growth and reproduction
• Has a Carrying Capacity

1) in the absence of predators, prey populations grow exponentially

2) in the absence of food, predator populations decline exponentially

Oscillations are neutrally stable and are due to time delays in the response of predator density to that of prey density and vice versa

Metapopulation dynamics model: How does the fraction
of occupied sites ( f ) change with time?

1. If the immigration rate is high and the extinction rate is low the occupied sites will increase with time.
2. If the immigration rate is low and the extinction rate is high the fraction of occupied sites will decrease with time.

Predation: (+ , -) interaction
Predation can limit the distribution of organisms via:

1. direct consumption of prey

2. behavioral modifications of prey in the presence of predators

Niche - The biological role of an organism within the environment

Realized Niche - where an organism actually exists due to ecological constraints

Stable Equilibrium (birth = death)

• If perturbed population will return to initial density
• Stabilizing forces dampen population fluctuations
• Density dependent control

• If perturbed, population may not return to initial density.
• Destabilizing forces enhance population fluctuations
• Inverse density dependence, D-D with long time lags

• The major conceptual techniques used to determine the factors limiting distribution
• Move organism and see if it can survive and reproduce in new environment
• Follow through at least one complete generation

Transplant successful: potential range of a species is larger than its actual range

Transplant unsuccessful: potential range of a species is the same as its actual range