Population dynamics

Chapter 6                      POPULATION DYNAMICS

DYNAMICS OF POPULATION GROWTH

POPULATIONS CAN GROW EXPONENTIALLY

The unrestricted increase in populations is called exponential growth.

Exponential growth (or geometric growth) is the increase that takes place at a constant rate per unit of time, e. g. 2, 4, 8, 16, 32, 64…

dN  = rN       dN = change in the number of individuals; 
dt                     r = the average individual contribution to population growth: biotic potential.        
dt =  change in time

If r is positive, the population is growing; if r is negative, the population is decreasing in numbers; if r is 0, there is not change in numbers.

Dividing 70 by the annual percentage growth, you will get the approximate doubling time of the population.

• Countries growing at a 4% per year will double their population in 17.5 years.

 

Arithmetic growth occurs when a constant amount is added to the population, e.g. 1, 2, 3, 4 or 1, 3, 5, 7, etc.

Biotic potential refers to the maximum reproductive rate of an organism given unlimited resources and ideal environmental conditions.

• Some species can produce an enormous number of offspring given an unlimited amount of resources, e.g. rabbits, housefly.

 

POPULATIONS CAN GO THROUGH BOOM AND BUST CYCLES

Exponential growth produces a J curve.

In the real world there are limitations to growth. Exponential growth for an unlimited period of time does not happen.

Carrying capacity refers to the maximum number of individuals of any species that can be supported by a particular ecosystem on a long-term basis.

• Overshoot: when the population exceeds the carrying capacity.
• Dieback or crash: a fast decrease of the population.

 

Irruptive or Malthusian growth takes place when a population crashes after a population explosion.

 

POPULATIONS CAN GROW TO A STABLE SIZE

Species may grow exponentially at the beginning but their growth slows down as they approach carrying capacity. This slow down in growth produces what is called an S curve.

Logistic growth occurs when a population grows exponentially at first when the resources are unlimited but the growth slows down as the population approaches the carrying capacity of the environment.

dN = rN (1- /K)        N = number of individuals; 
dt                           r = biotic potential           
t = time
K= carrying capacity

1-N/K represents the relationship of number in the population and K, the carrying capacity, the number of individuals the environment can support.

If 1-N/K is positive, the population is below what the environment can support, and is growing, though slowly; if 1-K/N is negative, the population growth is negative.

 

MANY FACTORS LIMIT POPULATION GROWTH

The growth rate of many species is regulated by internal and external factors so that they come into equilibrium with the environment.

Various factors affect natality and mortality and regulate population growth. These factors may be acting simultaneously on a population.

Some of these factors are dependent on the number of individuals in the population; how crowded is the population: density dependent factors.

Others are independent of the density of the population, e. g. floods, droughts: density independent factors.

More on population growth:
http://ic.ucsc.edu/~wxcheng/envs23/lecture19/human_popu.htm
http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/P/Populations.html

 

STRATEGIES OF POPULATION GROWTH: SPECIES CAN BE EITHER K-ADAPTED OR r-ADAPTED

• r-adapted species tend to grow exponentially, overshoot the carrying capacity and dieback.
• K-adapted species reproduce slowly as they approach the carrying capacity of their habitat.

 

r-ADAPTED

• Species that exhibit a r-adapted type of growth usually occupy low trophic levels in the ecosystem or are pioneer species.

 

• Generalists and opportunists: invade disturbed habitats quickly, grow fast, mature early and produce many offspring.

• Short life.

 

• Do not care for the young.

• They are controlled by predators and other outside factors (extrinsic).

 

• Depend on large numbers for the perpetuation of the species.

• Examples: mice and many rodent species, housefly and many insects, weeds and pests, parasites.

 

K-ADAPTED

• Species that show a logistic growth pattern usually grow more slowly and occupy higher trophic levels in the ecosystem.

 

• They are normally adapted to stable environments.

• They are usually large, live longer, grow slowly, mature late and produce very few offspring.

 

• They provide care and protection for their young.

• They have few predators and are usually controlled by intrinsic factors.

 

• Examples: elephants, wolves, whales, monkeys and apes.

Where do we fit? What strategy do we use as a species? Are we going to surpass our carrying capacity or are we going to come into balance with the environment?

More of reproductive strategies: http://www.bio.miami.edu/tom/bil160/bil160goods/16_rKselection.html

 

FACTORS THAT AFFECT POPULATION GROWTH

Natality: number of individuals added to the population by birth, hatching, cloning or germination.

Fecundity: the physical ability to reproduce. Fecund individuals may not contribute to the growth of the population due lack of opportunities to mate or other external factor.

Fertility: a measure of the number of individuals produced. It is usually describe in terms of the number of individuals per female.

Immigration: introduction of organisms from other communities.

 

MORTALITY AND SURVIVORSHIP MEASURE LONGEVITY

Mortality or death rate: the number of organism that die in a certain time period divided by the number of organisms that were alive at the beginning of the time period.

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

• There are four basic patterns of survivorship.

 

Life expectancy: the probable number of year of survival for an individual of a certain age.

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

• Parasitism, disease, accidents fighting, climate and others influence early mortality.
• The maximum life span for humans appears to be 120 years.

 

Emigration: the movement of organisms out of a population.

 

http://euclid.dne.wvfibernet.net/~jvg/ENV101/lectures/population_dynamics.html

 

FACTORS THAT REGULATE POPULATION GROWTH

Regulatory factors can be biotic or abiotic.

Intrinsic factors operate within individual organisms or between organisms in the same species.

Extrinsic factors are imposed from outside the population.

Density-dependent factors affect the population according to the population density.

• Biotic factors tend to be density-dependent.
• Interspecific interactions: predation, competition, parasitism, commensalism and mutualism.
• Intraspecific interactions: territoriality, competition within the species.
• Stress and crowding: stress shock and stress-related diseases. Symptoms: low fertility, low resistance to diseases, cannibalism, pathological behavior, hyper or hypoactivity, lack of parental instinct, and others.
• Subordinate animals are usually most affected by stress and crowding.

 

Density-independent factors: their effect on the population is the same regardless of the population, e.g. weather and climate.

• Abiotic factors tend to be density-independent factors.
• They do not regulate the population in the same way as the biotic factors do.
• They can have strong impact in the population.

 

Factors that tend to reduce population growth are called environmental resistance.

 

CONSERVATION BIOLOGY

Small populations are susceptible to catastrophic declines due to genetic problems, environmental change or random events (stochastic events).

Island Biogeography

The number of species on an island is the result of a balance between immigration and extinction.

An island will contain less species than an area the same size on the nearest mainland.

• Dispersal was thought to be the problem.
• Recent studies have shown than many species arrive at islands but few become established.

 

Some reasons for the lower number of species are...

• Islands do not have an overflow from adjacent areas as the mainland does.
• Limited habitat can support less species.
• The energy base is too small to provide for top carnivores.

 

More on Island Biogeography:  http://darwin.bio.uci.edu/~sustain/bio65/lec12/b65lec12.htm#Island_Biogeography:_the_Species-area_Cu
http://www.geog.ox.ac.uk/research/bie/islandbio/book.html

 

Conservation Genetics

In large populations, the gene frequency in the offspring population is the same as in the parent population (Hardy-Weinberg Law of Equilibrium). Genetic diversity is maintained.

Small isolated populations are subject to decline and extinction due to genetic changes in the population.

The addition or loss of a few individuals can make great difference in a small population by altering the gene make up of the group

A gradual change in the gene frequencies of a population due to random events is called genetic drift.

Loss of genetic diversity causes a number of harmful effects that limit adaptability, reproduction, and species survival.

When a few individuals survive a catastrophic event or colonize a new habitat geographically isolated, the gene pool is very limited. A genetic bottleneck or founder effect occurs.

Any deleterious genes present in the founder group will be overrepresented in future generations.

Inbreeding also makes the expression of rare or recessive genes more likely.

Population Viability Analysis

A minimum viable population (MVP) size or number of individuals is needed for long-term survival of rare and endangered species for long time, 100 years or more.

The probability of surviving more than 100 years should be 90-95%.

Conservation biologists use the concepts of island biogeography, genetic drift and founder effects to determine the minimum viable population size. 

Computer modeling help conservation biologists estimate the MVP.

Metapopulations

A metapopulation is collections of small populations of a species that exchange genes with some frequency.

Separated population can exchange genes if the there are suitable corridors between the populations.

If the small populations are well interconnected through immigration, they actually function as a single large population.

Habitat fragmentation can prevent gene flow between subgroups of the species. This isolation of subpopulation can bring about genetic bottlenecks.

When a population becomes extinct in a habitat, re-colonization of area can take place with relative ease.

Source habitats produce more individuals that it can support and these excess individuals migrate to other population.

In sink habitats natality is not high enough and these populations must recruit members from source populations.

If immigration is prevented for some reason, the species will become extinct in sink habitats.

Interesting site:
http://www.env.duke.edu/lel/env214/le_popn.html
http://crssa.rutgers.edu/courses/lse/Web_Patch/final/Tanya/Rohrbach_Final.htm