40 Population Ecology and the Distribution of Organisms

Discovering Ecology

• Ecology is the scientific study of the interactions between organisms and the environment.

• These interactions determine the distribution of organisms and their abundance.

• Modern ecology includes observation and experimentation.

Global Ecology

• Global ecology is concerned with the biosphere, or global ecosystem, which is the sum of all the planet’s ecosystems.

• Global ecology examines the influence of energy and materials on organisms across the biosphere.

Ecosystem Ecology

• Ecosystem ecology emphasizes energy flow and chemical cycling among the various biotic and abiotic components of the environment.

• An ecosystem is the community of organisms in an area and the physical factors with which they interact.

Community Ecology

• Community ecology considers the whole array of interacting species in a community.

• A community is a group of populations of different species in an area.

Population Ecology

• Population ecology considers factors affecting population size over time.

• A population is a group of individuals of the same species living in an area.

Organismal Ecology

• Organismal ecology focuses on the adaptations and interactions of individual organisms with their environment.

Concept 40.1: Earth’s climate influences the distribution of terrestrial biomes.

• The long-term prevailing weather conditions in an area constitute its climate.

• Four major physical components of climate are temperature, precipitation, sunlight, and wind.

• Abiotic factors are the nonliving chemical and physical attributes of the environment.

• Biotic factors are the other organisms that make up the living component of the environment.

Global Climate Patterns

• Global climate patterns are determined largely by solar energy and the planet’s movement in space.

• The warming effect of the sun causes temperature variations, which drive evaporation and the circulation of air and water.

• This causes latitudinal variations in climate.

• Latitudinal variation in sunlight intensity is caused by the curved shape of Earth.

• Sunlight strikes the tropics, regions between 23.5° north and 23.5° south latitude, most directly.

Seasonality

• Seasonal variations of light and temperature increase steadily toward the poles.

• Seasonality at high latitudes is caused by the tilt of Earth’s axis of rotation and its annual passage around the sun.

• Belts of wet and dry air straddling the equator shift throughout the year with the changing angle of the sun.

• Changing wind patterns affect ocean currents.

Bodies of Water

• Oceans, their currents, and large lakes moderate the climate of nearby terrestrial environments.

• The California Current carries cold water southward along western North America.

• The Gulf Stream carries warm water from the equator to the North Atlantic.

Mountains

• Rising air releases moisture on the windward side of a peak and creates a “rain shadow” as it absorbs moisture on the leeward side.

• Many deserts are found in the “rain shadow” of mountains.

Climate and Terrestrial Biomes

• Biomes are major life zones characterized by vegetation type (terrestrial biomes) or physical environment (aquatic biomes).

• Climate determines vegetation type and limits the distribution of terrestrial biomes.

• Latitudinal patterns in terrestrial biomes reflect the latitudinal patterns of climate.

Concept 40.2: Aquatic biomes are diverse and dynamic systems that cover most of Earth.

• Aquatic biomes are characterized primarily by their physical and chemical environment.

• For example, marine biomes have saltwater concentrations that average 3%, whereas freshwater biomes have salt concentrations of less than 0.1%.

Aquatic Biome Stratification

• Aquatic biomes are stratified into vertical and horizontal zones.

• Light is absorbed by water and photosynthetic organisms, so its intensity decreases rapidly with depth.

• The upper photic zone has sufficient light for photosynthesis, while the lower aphotic zone receives little light.

• The photic and aphotic zones together make up the pelagic zone.

• The organic and inorganic sediment at the bottom of the pelagic zone is called the benthic zone.

• The communities of organisms in the benthic zone are collectively called the benthos.

Lakes

• In lakes, the aquatic biomes can be divided horizontally into the littoral zone and the limnetic zone.

• The littoral zone includes waters close to shore that are shallow enough for rooted plants.

• The limnetic zone includes waters farther from shore that are too deep to support rooted plants.

Concept 40.3: Interactions between organisms and the environment limit the distribution of species.

• Species distributions are a consequence of both ecological factors and evolutionary history.

• Geographic isolation can result in the evolution of unique lineages restricted to specific areas.

Dispersal and Distribution

• Dispersal is the movement of individuals away from centers of high population density or from their area of origin.

• Dispersal contributes to the global distribution of organisms.

Biotic Factors

• Biotic factors that affect the distribution of organisms may include predation, herbivory, mutualism, parasitism, and competition.

Abiotic Factors

• Abiotic factors affecting the distribution of organisms include temperature, water and oxygen, salinity, sunlight, and rocks and soil.

Concept 40.4: Biotic and abiotic factors affect population density, dispersion, and demographics.

• Population ecology explores how biotic and abiotic factors influence density, distribution, and size of populations.

• A population is a group of individuals of a single species living in the same general area.

• Populations are described by their boundaries and size.

Density

• Density is the number of individuals per unit area or volume.

• Dispersion is the pattern of spacing among individuals within the boundaries of the population.

Density: A Dynamic Perspective

• In most cases, it is impractical or impossible to count all individuals in a population.

• Sampling techniques can be used to estimate densities and total population sizes.

• Population density can be estimated by extrapolation from small samples or an indicator of population size (for example, number of nests).

• Density is the result of an interplay between processes that add individuals to a population and those that remove individuals.

• Additions occur through birth and immigration, the influx of new individuals from other areas.

• Removal of individuals occurs through death and emigration, the movement of individuals out of a population.

Patterns of Dispersion

• Environmental and social factors influence the spacing of individuals in a population.

• The most common pattern of dispersion is clumped, in which individuals aggregate in patches.

• A clumped dispersion may be influenced by resource availability and behavior.

Life Tables

• A life table is an age-specific summary of the survival pattern of a population.

• It is best made by following the fate of a cohort, a group of individuals of the same age, from birth to death.

• Life tables constructed for sexually reproducing species often ignore males because only females produce offspring.

Survivorship Curves

• A survivorship curve is a graphic way of representing the data in a life table.

• Survivorship curves plot the proportion or numbers of a cohort still alive at each age.

• Survivorship curves can be classified into three general types:

  • Type I: high survivorship during early and middle life followed by a steep drop due to increase in death rates among older age groups.

  • Type II: survivorship declines linearly due to a constant death rate over the organism’s life span.

  • Type III: low survivorship due to high death rates for young age-groups and stable survivorship later in life due to a lower death rate for survivors.

Concept 40.5: The exponential and logistic models describe the growth of populations.

• Unlimited growth occurs under ideal conditions; in nature, growth is limited by various factors.

• Ecologists study growth in both idealized and realistic conditions.

Changes in Population Size

• Change in population size can be defined by the equation:

  • If immigration and emigration are ignored, the change in population size equals births minus deaths.

• The population growth rate can be expressed mathematically as:

  • Where ΔN is the change in population size, Δt is the time interval, B is the number of births, and D is the number of deaths in the population during the time interval.

  • Where R represents the difference between the number of births (B) and the number of deaths (D).

Exponential Growth

• Exponential population growth is population increase under idealized conditions (food is abundant and all individuals reproduce at physiological capacity).

• Under these conditions, the population increases in size by a constant proportion at each instant in time.

• The equation of exponential population growth is:

  • Where r is the intrinsic rate of increase, the per capita rate at which a population increases in size at each instant in time.

• The J-shaped curve of exponential growth characterizes populations in new environments or rebounding populations.

Carrying Capacity

• Exponential growth cannot be sustained for long in any population.

• A more realistic population model limits growth by incorporating carrying capacity.

• Carrying capacity (K) is the maximum population size the environment can support.

• Carrying capacity varies with the abundance of limiting resources.

• As a population approaches carrying capacity, the per capita birth rate will decrease or the per capita death rate will increase.

• Such changes in either (or both) of these rates cause the per capita growth rate (r) to drop.

The Logistic Growth Model

• In the logistic population growth model, the per capita rate of increase approaches zero as carrying capacity is reached.

• The logistic model starts with the exponential model and adds an expression that reduces per capita rate of increase as N approaches K.

The Logistic Model and Real Populations

• The growth of many laboratory populations, including paramecia, fits an S-shaped curve when resources are limited.

• These organisms are grown in a constant environment lacking predators and competitors.

• The logistic model fits few real populations.

• Some populations overshoot K before settling down to a relatively stable density.

• Some populations fluctuate greatly and make it difficult to define K.

Mechanisms of Density-Dependent Population Regulation

• Density-dependent birth and death rates are an example of negative feedback that regulates population growth.

• Population growth declines at high density due to factors such as competition for resources, predation, disease, toxic wastes, territoriality, and intrinsic factors.