8: Population Distribution and Abundance

What is a population?

A group of individuals of the same species living in the same geographic area at the same time

What factors do ecologists consider when studying populations?

Conservation of species, controlling pests/diseases, managing natural resources, and understanding human population growth

What is distribution?

The size, shape, and location of the area a population occupies

What is density?

The number of individuals per unit area (e.g., per km²)

What is abundance?

The total number of individuals or total biomass in a given area

What is a niche?

The full set of environmental conditions and resources required for a species to survive, grow, and reproduce

What is a fundamental niche?

The full range of environmental conditions where a species could potentially live without biotic limitations

What is a realized niche?

The actual conditions where a species lives after limits like competition, predation, and environmental stress

What limits a realized niche?

Biotic interactions (competition, predation) and abiotic factors (temperature, moisture)

What did Hutchinson define a niche as?

An n-dimensional hypervolume representing all environmental conditions and resources needed for survival

Key difference between fundamental vs realized niche?

Fundamental = potential range

Realized = restricted

actual range due to limiting factors

What environmental factors limit species (barnacle example)?

Moisture availability and temperature extremes

Do Chthalamus and Balanus have larger fundamental or realized niches?

Both species have larger fundamental niches than their realized niches

Why is Balanus’ realized niche restricted to lower zones?

It cannot tolerate desiccation and extreme temperature changes in the upper intertidal

Why does Balanus experience high mortality in the upper intertidal?

Exposure during low tide leads to drying out (desiccation)

Where are Chthalamus and Balanus typically found?

Chthalamus = upper intertidal

Balanus = middle to lower intertidal

Why is Chthalamus absent from the lower intertidal?

It is outcompeted by Balanus in those regions

Compare tolerance of Chthalamus vs Balanus

Chthalamus tolerates drying better; Balanus is less tolerant but better competitor in moist areas

Why is scale important in ecology?

Ecological patterns and processes change depending on spatial and temporal scale

What happens at small vs large scales?

Small scale = relatively uniform conditions

Large scale = strong environmental gradients (e.g., mountains, continents)

Distribution: What are the three main population distribution patterns?

Random, uniform (regular), and clumped

What is random distribution?

Individuals are spaced unpredictably, often due to weak interactions or random processes

What is uniform (regular) distribution?

Individuals are evenly spaced, often due to competition or territorial behavior

What is clumped distribution?

Individuals are grouped together, often due to resource availability or social behavior

What determines distribution patterns?

Physical environment, interactions among individuals, or a combination of both

How do interactions influence patterns?

Ignore each other → random; repel each other → uniform; attract each other → clumped

Examples of distribution patterns?

Aggressive stingless bees = uniform

Non-aggressive bees = random

Desert shrubs = influenced by competition

How do patterns differ across scales?

Local scale = influenced by interactions and random colonization

Large scale = populations tend to be clumped

What is population density?

The number of individuals in a given area or volume

How does body size relate to population density?

As body size increases, population density generally decreases

Why is the size–density relationship important?

It helps predict rarity, resource use, and extinction risk across species

What three factors determine species rarity or commonness?

Geographic range (wide vs restricted), habitat tolerance (broad vs narrow), and local population size (large vs small)

Why are rare species more vulnerable to extinction?

They often have small populations, limited ranges, and narrow habitat requirements

Example of species with wide range but small populations?

Peregrine falcon

Example of species with large populations but narrow habitat?

Passenger pigeon (now extinct)

Example of species with restricted range and small populations?

Mountain gorilla and California condor