Chapter 6: Population Dynamics

Dynamics of Population Growth

Exponential Growth and Doubling Times

• Exponential growth: growth at a constant rate of increase per unit time.

- Growth curve called a J-curve because of its shape. - Also called geometric growth

• Arithmetic growth: growth that increases at constant amount per unit time

Table 6.1. Doubling times at various compound interest rates

Annual % Increase Doubling Time (years) 0.1 700 0.5 140 1.0 70 2.0 35 5.0 14 7.5 9 10.0 7

100.0 0.7

• At 1% per year, a population or a bank account doubles in about 70 years

• Therefore:

70/annual % growth = approximate doubling time in years

Biotic Potential

• Biotic Potential: maximum reproductive rate of an organism

• Given unlimited resources and no limiting factors, many species can produce enormous populations very quickly (e.g. houseflies).

Table 6.2. Biotic potential of houseflies (Musca domestica) in one year Assuming that: - a female lays 120 eggs per generation - half of these eggs develop into females - there are seven generations per year

Total Population: Generation If all females in each If all generations survive one year

generation lay 120 eggs and all females reproduce maximallyand then die in each generation

1 120 120 2 7,200 7,320 3 432,000 446,520 4 25,920,000 27,237,720 5 1,555,200,000 1,661,500,920 6 93,312,000,000 101,351,520,120 7 5,598,720,000,000 6,182,442,727,320 Source: Data from E.J. Kormondy, Concepts of Ecology, 3rd ed., 1984 Harper and Row Publishers, Inc.

Population Oscillations and Irruptive Growth

• In the real world, there are limits to growth.

• Dieback: when the population decreases as fast as, or faster , than it grows

- Some limiting factor comes into effect.

• Overshoot: extent to which a population exceeds the carrying capacity of its environment.

• Irruptive or Malthusian growth: pattern of population explosion followed by a population crash.

• Populations may go through repeated oscillating cycles of exponentialgrowth and catastrophic crashes.

Growth to a Stable Population

• Not all populations go through cycles of irruptive population growth and catastrophic decline. • Growth rates of many species are regulated by internal and external factors so that they can come into equilibrium with their environmental resources.

• Logistic growth: exponential growth when resources are unlimited and slowed growth as species approach carrying capacity of environment.

- Growth curve called an S-curve because of its shape.

• Environmental resistance: factors that tend to reduce population growth rates.

Chaotic and Catastrophic Population Dynamics

• Populations under natural conditions rarely follow linear dynamics and smooth growth curves.

• Chaotic systems: systems that exhibit variability that can be of a complexity whose pattern is not observable over a normal human timescale. -The "Butterfly effect" described by meteorologist Edward Lorenz

is a metaphor for describing chaotic dynamics. “If a butterfly flapping its wings in Rio de Janeiro could, through

a long series of unpredictable multiplying effects in weather systems, eventually result in a tornado in Texas.”

• A catastrophic system may jump abruptly from one seemly steady state without any intermediate changes.

Strategies of Population Growth

• Malthusian Strategies (externally controlled growth) - Insects, rodents, marine invertebrates, parasites, and annual

plants

• Logistic Strategies (intrinsically controlled growth) Wolves, elephants, whales, and primates

Table 6.3. Characteristics of contrasting reproductive strategies Externally Controlled Growth Instrinsically Controlled Growth

1.Short life 1.Long life2.Rapid growth 2.Slower growth3.Early maturity 3.Late maturity4.Many small offspring 4.Fewer large offspring5.Little parental care or protection 5.High parental care and protection6.Little investment in individual offspring 6.High investment in individual offspring7.Adapted to unstable environment 7.Adapted to stable environment8.Pioneers, colonizers 8.Later stages of succession9.Niche generalists 9.Niche specialists10.Prey 10.Predators11.Regulated mainly by extrinsic factors 11.Regulated mainly by intrinsic factors12.Low trophic level 12.High trophic level

Factors that Increase or Decrease Populations

Natality, Fecundity, and Fertility

• Natality: production of new individuals by birth, hatching, germination, or cloning. - Sensitive to environmental conditions.

* Nutritional levels * Climate

* Soil * Water conditions• Fecundity: physical ability to reproduce. • Fertility: measure of actual number of offspring produced.

Immigration

• Organisms are introduced into new ecosystems by a variety of methods. - Seeds, spores, and small animals may float on winds or water

currents. - Sometimes organisms are carried on the fur, feathers, or

intestines of animals. - As adults, some animals travel by flying, swimming, or walking.

Mortality and Survivorship

Mortality: death rate determined by dividing the number of organisms that die in a certain time period by the number alive atthe beginning of the period.

Survivorship: percentage of a cohort that survives to a certain age.

Life expectancy: probable number of years of survival for an individual of a given age.

• Life span: longest period of life reached by a given type of organism.

• Maximum age is dictated primarily by physiological aspects of the organism itself.

• Enormous difference exists in the life span between different species . - Microorganisms live whole life cycles in a matter of hours or minutes. - Bristle cone pine trees have life spans up to 4600 years.

• Most individuals in a population do not live anywhere near themaximum life span for their species. - Many factors are involved in early mortality. * Predation * Parasitism * Disease * Accidents * Fighting * Environmental influences (e.g. climate and nutrition)

Age Structure

• Growing or declining populations will have very different proportions of individuals in various age classes.

• Populations will often be distinguished by differences in distributionamong prereproductive, reproductive, and postreproductive age classes.

• Population momentum: potential for rapid increase in natality onceyoungsters reach reproductive age in populations with a large number ofprereproductive individuals.

Emigration

• Emigration: movement of members out of a population

• Many organisms have specific mechanisms to facility migration of one or more of each generation of their offspring.

- Genes will still be carried by descendants in other places that may be better suited for their survival.

Factors that Regulate Population Growth

Various factors regulate population growth, primarily by affecting natality or mortality and can be classified in different ways.

• Intrinsic: operating within individual organisms or between organisms in the same species. • Extrinsic: imposed from outside the population. • Biotic: caused by living organisms. • Abiotic: caused by non-living components of the environment.

These factors can act in two ways.

• Density-dependent: effects are stronger or a higher proportion of the population is affected as population density increases. • Density-independent: effect is same or a constant proportion of the population is affected regardless of population density.

Biotic regulation of factors tend to be density-dependent, while abiotic factors tend to be density-independent.

Density-Independent Factors

Factors that affect natality or mortality independently of population density tend to be abiotic components of ecosystem

• Extreme cold or even moderate cold during the wrong time of year. • High heat • Drought • Excess rain • Severe storms

Abiotic factors can have beneficial effects.

• Desert blooms can occur after rainfall. • Fire can dominate or create many biomes.

Density-Dependent Factors

Density-dependent mechanisms tend to reduce population size by decreasing natality or increasing mortality as population sizeincreases.

Reduction in population size can result from interactions between populations or interactions within a population.

Interspecific interactions

Prey species may actually benefit from predation by helping to prevent population overshoot.

Predator and prey populations often oscillate in synchrony with each other (e.g. lynxand hare)

Parasites and the host often develop some sort of balance with each other • If the parasite kills the host, food will be unavailable for offspring. • If parasite and host have lived together for a long time, the host is likely to have developed some resistance. • If parasite is introduced into a new ecosystem, the effects can be disastrous.

In interspecific competition, two species can compete for the same environmental resources in an ecosystem.

Not all interspecific interactions are harmful to one of the species involved (e.g. mutualism and commensalism).

Intraspecific interactions

Individuals within a population can also compete for resources.

Territoriality is one principal way many animal species control access to environmental resources.

• Individual, pairs, or group that holds the territories will drive off rivals if possible either by threats, superior features (colors, size, dancing ability), or fighting equipment.

Stress and Crowding

Stress shock or stress-related diseases • Loose set of physical, physiological, and/or behavioral changes that are thought to result from the stress of too much competition and too close proximity to other members of the same species. • Best evidence for existence of these diseases comes from experimentation in the lab.

- Lab animals are grown in very high densities with plenty of food and water, but little living space.

* Symptoms reported include 1) reduced fertility; 2) low resistance to infectious diseases, 3) hypoactivity;4) hyperactivity; 5) aggression; 6) lack of parental instincts; 7) sexual deviance; and 8) cannibalism.