To better understand interactions between organisms in nature, ecologists need to define a number of terms

Terms

Species

group of organisms which have ability to interbreed, under natural conditions, to produce fertile offspring

Population

group of organisms of same species that are living in same area at same time

Population Density

number of individuals of species per unit area or volume

Crude Density

number of individuals per total area

Ecological Density

number of individuals per unit area that is actually used

Habitat

place where species normally lives

Niche

role or function of species within an ecological community

Community

all populations in a given ecosystem at a given time

Ecosystem

living (biotic) and non-living (abiotic) components that function as a unit in a given area or volume

Population Dispersion

Since resources are not always distributed throughout the environment in the same way, all populations do not show the same pattern of dispersion

Clumped

most populations show this pattern

Examples: birds “flock”, fish “school”, zebra “herd”, mushrooms on rotted logs

Random

when resources are evenly distributed and there is little interaction between individuals

Examples: trees in tropical rain forest (very rare)

Uniform

individuals are equally spaced apart (almost never seen in nature, but this is how crops are planted)

Examples: due to strong competition for nest sites, King penguins show this pattern

Counting Populations

For organisms that are stationary (plants) use of quadrats is standard method of sampling

Quadrat (stationary organisms)

1. frame of a given size that is placed at random within a habitat

2. Count is taken of population size within quadrat

3. Used to extrapolate size of entire population

For organisms that are stationary (plants) use of quadrats is standard method of sampling

quadrat sampled may fall in a clump

(overestimation)

quadrat sampled may fall between clumps

(underestimation)

Mark-Recapture Sampling (mobile organisms)

For organisms that are mobile most practical sampling method is mark-recapture sampling

1. Group of organisms is captured, marked in some way and released

2. After allowing enough time for marked organisms to mix randomly into population, researchers capture a second sample of organisms

3. Proportion of marked to unmarked organisms in second sample is used to estimate overall population size

Rationale is: If you can measure proportion of marked individuals in second sample (i.e. 10 out of 100 = 10%)

Equal to proportion of all marked organism in population

(i.e. if 50 were marked, then that’s 10% of all individuals, so population size is 500)

Following equation is used to estimate population size in mark-recapture sampling:

Accuracy of mark-recapture sampling depends on following assumptions:

• all individuals in population have an equal chance of being captured

• during time between samples, ratio of marked to unmarked individuals is constant (i.e. marking does not affect chance of survival)

• during study period population size is constant