SUMMARY

CONCEPT 9.1 Populations are groups of individuals of the same species that vary in size over space and time.

9.1.1 Define the terms population, population size, and population density.

A population is a group of individuals of the same species that live in the same area and at the same time and interact with one another.

Population abundance can be defined as population size (the number of individuals in the population) or as population density (the number of individuals per unit of area).

9.1.2 Compare the different ways in which individuals are defined, including the terms clones, genets, and ramets.

In sexually reproducing species, the offspring is a single genetic individual.

In asexually reproducing species, multiple genetically identical individuals, or clones, can be produced. The single genetic unit is known as the genet and the genetically identical individuals are known as ramets.

9.1.3 Compare the different methods used to measure the abundance of individuals within populations or species.

The most direct way to determine the number of individuals in a population is to count all of them, obtaining their absolute population size. When this is not possible, the relative population size, or the number of individuals standardized for area or time, can be measured.

Relative population size measures can include area-based counts, distance methods, or mark-recapture studies.

CONCEPT 9.2 Species vary in their distribution and abundance across their geographic range.

9.2.1 Describe the relationship between populations, metapopulations, and geographic ranges for species.

Groups of individuals of the same species can be divided into populations, and groups of populations (metapopulations) are linked together by dispersal.

At larger spatial scales, the entire geographic range, or distribution, of a species might consist of one or multiple metapopulations.

9.2.2 Compare the different dispersion patterns of populations.

The dispersion of individuals within a population may be regular, random, or clumped. In the field, clumped dispersions are more common.

9.2.3 Describe how the size and patchiness of geographic ranges vary among species and can be predicted using models.

Geographic ranges can be as small as a few meters to as large

as multiple continents.

Many species have a patchy distribution within their geographic range depending on the amount and variability of suitable habitat for that species.

Species distribution models can be used to estimate the geographic range of a species when there are insufficient data on its distribution or when we want to predict the future locations of its populations.

CONCEPT 9.3 Species are limited in their distribution and abundance by habitat suitability, historical factors, and dispersal.

9.3.1 Describe the factors important to the suitability of habitat for populations and species.

The suitability of habitat depends on both abiotic and biotic features of the environment, including factors that affect physiological tolerances, resources, and species interactions.

Some species are thwarted or dependent on regular forms of disturbance, or abiotic events that kill or damage some individuals.

9.3.2 Explain how the distribution and abundance of species can reflect their evolutionary and geologic history.

The distribution of species may be the result of their past evolutionary history, which has shaped their adaptations to the abiotic and biotic environment.

The distribution of species may be the result of past geologic changes on Earth such as continental drift.

9.3.3 Describe the role of dispersal and migration in distributing organisms across the landscape.

Dispersal, or the movement of individuals into (immigration) or out of (emigration) an existing

population, can link or limit individuals among populations over small or large spatial scales.

Migration involves round-trip movement, includes the entire population, and is often in response to seasonal variation in resources.

CONCEPT 9.4 In metapopulations, sets of spatially isolated populations are linked by dispersal.

9.4.1 Describe how the rates of colonization and extinction of populations affect metapopulations.

The populations within metapopulations experience repeated colonization and extinction events.

If the rate of population extinction is greater than the rate of population colonization, the metapopulation will eventually collapse.

9.4.2 Describe how the amount of suitable habitat and population isolation can affect metapopulation persistence or extinction.

A metapopulation can be doomed to extinction, even when some suitable habitat remains, if the remaining habitat is not large enough to sustain individual populations or if the habitat is isolated by distance and is thus unable to receive immigrants

from other populations (rescue effect).

REVIEW QUESTIONS

1. Describe some of the complicating factors that can be encountered in studying a population.

2. No species is found everywhere on Earth. Why? Your answer should include an explanation of why organisms are not found in all places where you might expect them to thrive.

3. What is a species distribution model? Describe how such a model could be used to predict the future distribution of an organism that is spreading into a new geographic region.

4. Sea otters can eat 20% to 23% of their body weight in food each day. An average sea otter weighs 23 kg (roughly 50 pounds), and there are 20 to 30 otters per square kilometer where they are present. An average sea urchin weighs 0.55 kg. Assuming that the otters eat only sea urchins, use these data to calculate a conservative estimate of the number of sea urchins per square kilometer that an otter population would be expected to eat each year.

HONE YOUR PROBLEM-SOLVING SKILLS

Climate change has enabled the long-spined sea urchin (Centrostephanus rodgersii) to expand its range poleward along the east coast of Tasmania, Australia (see Figure 9.9).