43. Populations. Chapter 43 Populations. Key Concepts 43.1 Populations Are Patchy in Space and Dynamic over Time 43.2 Births Increase and Deaths Decrease Population Size 43.3 Life Histories Determine Population Growth Rates 43.4 Populations Grow Multiplicatively, but Not for Long.
43.5 Extinction and Recolonization Affect Population Dynamics
43.6 Ecology Provides Tools for Managing Populations
How does understanding the population ecology of disease vectors help us combat infectious diseases?
Population—all the individuals of a species that interact with one another within a given area at a particular time.
Humans have long been interested in understanding species abundance:
to increase populations of species that provide resources and food and to conserve species for ethical and aesthetic reasons
to decrease abundance of crop pests, pathogens, etc.
Population density—number of individuals per unit of area or volume
Population size—total number of individuals in a population
Counting all individuals is usually not feasible; ecologists often measure density, then multiply by the area occupied by the population to get population size.
Abundance varies on several spatial scales.
Geographic range—region in which a species is found
Within the range, species may be restricted to specific environments or habitats.
Habitat patches are “islands” of suitable habitat separated by areas of unsuitable habitat.
Population densities are dynamic—they change over time.
Density of one species population may be related to density of other species populations.
Change in population size depends on the number of births and deaths over a given time.
“Birth–death” or BD model of population change:
Population growth rate (how fast it is changing):
Per capita birth rate (b)—number of offspring an average individual produces
Per capita death rate (d)—average individual’s chance of dying
Per capita growth rate (r) = (b – d) = average individual’s contribution to total population growth rate
If b > d, then r > 0, and the population grows.
If b < d, then r < 0, and the population shrinks.
If b = d, then r = 0, and population size does not change.
Life history—time course of growth and development, reproduction, and death during an average individual’s life
Life histories are quantitative descriptions of life cycles.
Example: the life cycle of the black-legged tick.
A life table shows ages at which individuals make life cycle transitions and how many individuals do so successfully.
Life tables have two types of information:
survivorship—fraction of individuals that survive from birth to different life stages or ages
fecundity—average number of offspring each individual produces at those life stages or ages
Life histories vary among species: how many and what types of developmental stages, age of first reproduction, frequency of reproduction, how many offspring they produce, and how long they live.
Life histories can vary within a species. For example, different human populations have different life expectancies and age of sexual maturity.
Individual organisms require resources (materials and energy) and physical conditions they can tolerate.
Rate at which an organism can acquire resources increases with the availability of the resources.
Examples: photosynthetic rate increases with sunlight intensity, or an animal’s rate of food intake increases with the density of food.
Principle of allocation—once an organism has acquired a unit of some resource, it can be used for only one function at a time: maintenance, foraging, growth, defense, or reproduction.
In stressful conditions, more resources go to maintaining homeostasis.
Once an organism has more resources than it needs for maintenance, it can allocate the excess to other functions.
In general, as average individuals in a population acquire more resources, the average fecundity, survivorship, and per capita growth rate increase.
Life-history tradeoffs—negative relationships among growth, reproduction, and survival
Example: investments in reproduction may be at the expense of adult survivorship or growth.
Environment is also a factor: if high mortality rates are likely, it makes sense to invest in early reproduction.
Species’ distributions reflect the effects of environment on per capita growth rates.
A study of temperature change in a lizard’s environment, combined with knowledge of its physiology and behavior, led to conclusions about how climate change may affect survivorship, fecundity, and distribution of these lizards.
Laboratory experiments have also shown the links between environmental conditions, life histories, and species distributions.
Population growth is multiplicative—an ever-larger number of individuals is added in each successive time period.
In additive growth, a constant number (rather than a constant multiple) is added in each time period.
Charles Darwin was aware of the power of multiplicative growth:
“As more individuals are produced than can possibly survive, there must in every case be a struggle for existence.”
This ecological struggle for existence, fueled by multiplicative growth, drives natural selection and adaptation.
Multiplicative growth has a constant doubling time.
The time it takes a population to double in size can be calculated if r is known.
Populations do not grow multiplicatively for very long. Growth slows and reaches a more or less steady size:
r decreases as the population becomes more crowded; r is density dependent.
As the population grows and becomes more crowded, birth rates tend to decrease and death rates tend to increase.
When r = 0, the population size stops changing—it reaches an equilibrium size called carrying capacity, or K.
Spatial variation in environmental factors can result in variation of carrying capacity.
Temporal variation in environmental conditions may cause the population to fluctuate above and below the current carrying capacity.
Example: the rodents and ticks in Millbrook, New York.
The human population is unique. It has grown at an ever-faster per capita rate, as indicated by steadily decreasing doubling times.
Technological advances have raised carrying capacity by increasing food production and improving health.
In 1798 Thomas Malthus pointed out that the human population was growing multiplicatively, but its food supply was growing additively, and predicted that food shortages would limit human population growth.
His essay provided Charles Darwin with a mechanism for natural selection.
Malthus could not predict the effects of technology such as medical advances and the Green Revolution.
Many believe that the human population has now overshot its carrying capacity for two reasons:
Technological advances and agriculture have depended on fossil fuels, a finite resource.
Climate change and ecosystem degradation have been a consequence of 20th century population expansion.
If the human population has indeed exceeded carrying capacity, ultimately it will decrease.
We can bring this about voluntarily if we continue to reduce per capita birth rate.
Regional populations (metapopulations) are made up of subpopulations in habitat patches.
Individuals may move in or out of subpopulations.
The BIDE model of popultion growth adds the number of immigrants (I) and emigrants (E) to the BD growth model.
Small subpopulations in habitat patches are vulnerable to environmental disturbances and chance events and may go extinct.
Individuals from other subpopulations can recolonize the patch if dispersal is possible.
Understanding life history strategies can be useful in managing other species.
Black rockfish grow throughout their life. The number of eggs a female produces is proportional to her size. Older, larger females also produce eggs with oil droplets that give the larvae a head start on growth.
Fishermen prefer to catch big fish. Intense fishing reduced the average age of female rockfish from 9.5 to 6.5 years.
These younger females were smaller, produced fewer eggs, and larvae didn’t survive as well.
Management may require no-fishing zones where some females can mature and reproduce.
Reducing disease risk:
The black-legged tick’s life history indicates that success of larvae in obtaining a blood meal has greatest impact on the abundance of nymphs.
Thus, controlling the abundance of rodents that are hosts for the larvae is more effective in reducing tick populations than controlling the abundance of deer, the hosts for adults.
Conserving endangered species:
Larvae of the endangered Edith’s checkerspot butterfly feed on two plant species found only on serpentine soils.
The two plant species are being suppressed by invasive non-native grasses.
Grazing by cattle can control the invasive grasses.
Conservation plans begin with inventories of habitat and potential risks to the habitat.
Largest patches can potentially have the largest populations and are given priorty.
Quality (carrying capacity) of the patches is evaluated; ways to restore or maintain quality are developed.
Ability of the organism to disperse between patches is evaluated.
For some species, a continuous corridor of habitat is needed to connect subpopulations and allow dispersal.
Dispersal corridors can be created by maintaining vegetation along roadsides, fence lines, or streams, or building bridges or underpasses that allow individuals to avoid roads or other barriers.
By understanding the factors that control abundance and distribution of pathogens and their vectors, we can devise ways to control their abundance or avoid contact.
Black-legged ticks are vectors for the bacterium that causes Lyme disease.
For these ticks, abundance of hosts for larvae (rodents) determines tick abundance.
Rodent abundance depends on acorn availability.
Acorn production can be used to predict areas that are likely to become infested with ticks, and measures can be taken to minimize human contact.