11

Biology 205Ecology and Adaptation

Lecture 8: Population distribution and abundance

Dr. Erik D. Davenport

22

Population

How do ecologist define population?

What are some characteristics of a population?

33

Population

Ecologists usually define a population as a group of individuals of a single species inhabiting a specific area.– Characterized by the number of individuals and

their density.– Additional characteristics of a population include

age distributions, growth rates, distribution, and abundance.

44

Outline

What determines or limits the population distribution?

What are the distribution patterns of a population?

What is the relationship between Organism Size and Population Density?

Commonness and Rarity --- determines whether a population (species) is a common species or a rare species.

55

Distribution Limits

Physical environment limits geographic distribution of a species.

– Organisms can only compensate so much for environmental variation.

66

Distributions and Climate:Kangaroo study

77

Distributions and Climate:Kangaroo Study

Caughley found a close relationship between climate and distribution of the three largest kangaroos in Australia.– Macropus giganteus - Eastern Grey

Eastern 1/3 of continent.– Macropus fuliginosus - Western Grey

Southern and western regions.– Macropus rufus - Red

Arid / semiarid interior.

88

Distributions and Climate:Kangaroo study

Limited distributions may not be directly determined by climate.– Climate often influences species distributions via:

Food production Water supply Habitat Incidence of parasites, pathogens and competitors.

99

Tiger Beetle of Cold Climates

1010

Tiger Beetle of Cold Climates

Tiger Beetle (Cicindela longilabris) lives at higher latitudes and elevations than most other species in NA.– Schultz et. al. found metabolic rates of C.

longilabris are higher and preferred temperatures lower than most other species.

– Supports generalization that the physical environment limits species distributions.

1111

Distributions of Plants along a Moisture-Temperature Gradient

Encelia species distributions correspond to variations in temperature and precipitation.

1212

1313

Distribution of Individuals on Small Scales

1414

Distribution of Individuals on Small Scales

Random: Equal chance of being anywhere.– Uniform distribution of resources.

Regular: Uniformly spaced.– Exclusive use of areas.– Individuals avoid one another.

Clumped: Unequal chance of being anywhere.– Mutual attraction between individuals.– Patchy resource distribution.

1515

09_11.jpg

1616

Distribution of Tropical Bee Colonies

Hubbell and Johnson predicted aggressive bee colonies would show regular distributions while non-aggressive species would show random or clumped distributions.– As predicted, four species with regular distributions

were highly aggressive.– Fifth was non-aggressive and randomly distributed.– Prospective nest sites marked with pheromones.

1717

09_12.jpg

1818

09_13.jpg

1919

Distributions of Desert Shrubs

Phillips and MacMahon proposed as plants grow, some individuals in clumps die, reducing clumping.– Competition among remaining plants produces

higher mortality.– Eventually creates regular distributions.

2020

Distributions of Desert Shrubs

Traditional theory suggests desert shrubs are regularly spaced due to competition of what?– Phillips and MacMahon found distribution of

desert shrubs changes from clumped to regular patterns as they grow.

– Young shrubs clumped for (3) reasons: Seeds germinate at safe sites Seeds not dispersed from parent areas Asexual reproduction

2121

Distributions of Individuals on Large Scales

Bird Populations Across North America– Root found at continental scale, bird populations

showed clumped distributions in Christmas Bird Counts.

– Clumped patterns occur in species with widespread distributions.

– Brown found a relatively small proportion of study sites yielded most of records for each bird species in Breeding Bird Survey.

2222

09_15.jpg

2323

Plant Abundance Along Moisture Gradients

2424

Plant Abundance along Moisture Gradients

Whittaker examined distributions of woody plants along moisture gradients in several North American mountain ranges.– Documented moisture gradient from moist canyon

bottoms up to the dry southwest-facing slopes.– Tree species showed a highly clumped distribution along

moisture gradients, with densities decreasing substantially toward the edges of their distribution.

2525

Metapopulations

A metapopulation is made up of a group of subpopulations living on patches of habitat connected by an exchange of individuals.– Alpine Butterfly - Roland et.al.– Lesser Kestrels - Serrano and Tella.

2626

09_22.jpg

2727

Organism Size and Population Density

2828

Organism Size and Population Density

In general, population density declines with increasing organism size.– Damuth found the population density of herbivorous

mammals decreased with increased body size.– Peters and Wassenberg found aquatic invertebrates

tend to have higher population densities than terrestrial invertebrates of similar size.

– Mammals tend to have higher population densities than birds of similar size.

2929

Plant Size and Population Density

Plant population density decreases with increasing plant size.– Underlying details are very different.– Tree seedlings can live at very high densities, but as

the trees grow, density declines progressively until mature trees are at low densities.

3030

Commonness and Rarity

Rabinowitz devised commonness and rarity classification based on (3) factors:

Geographic Range of Species Habitat Tolerance Local Population Size

Populations that are least threatened by extinction, have extensive geographic ranges, broad habitat tolerances, and some large local populations.

All seven other combinations create some kind of rarity.

3131

09_25.jpg

3232

Rarity

Rarity I– Extensive Range, Broad Habitat Tolerance, Small

Local Populations Peregrine Falcon

Rarity II– Extensive Rage, Large Populations, Narrow

Habitat Tolerance Passenger Pigeon

3333

Rarity

Rarity III– Restricted Range, Narrow Habitat Tolerance,

Small Populations California Condor

3434

Main concepts

The physical environment limits the geographic distribution of species

On small scales, individuals within populations are distributed in patterns that maybe random, regular, or clumped; on larger scales, individuals within a population are clumped.

Population density declines with increasing organisms size.

Rarity is influenced by geographic range, habitat tolerance, and population size; rare species are vulnerable to extinction.