Populations, Communities, and Species Interaction

Who Lives Where, and Why?

  • Why does a particular species live where it does?

  • How is it able to live where it does?

  • How does it deal with the physical resources of its environment?

  • How does it interact with the other species present?

  • What gives one species an edge over another species in a particular habitat?

Critical Factors and Tolerance Limits

Leibig's Law of Minimum: In 1840, Justus von Liebig proposed that the single factor in shortest supply relative to demand is the critical determinant in the distribution of that species.

Sheldford's Law of Tolerance: Victor Shelford later expanded Liebig's principal by stating that each environmental factor has both minimum and maximum levels called tolerance limits beyond which a particular species cannot survive.

A specific critical factor that, more than any other, may determine the abundance and distribution of a species in a given area.

Sometimes the requirements and tolerances of species are useful indicators of specific environmental characteristics

The presence or absence of these environmental indicators can tell us something about the community and ecosystem as a whole.

Natural Selection, Adaptation, and Evolution

Species acquire traits that allow them to be adapted to their environment.

The term adapt can be used in two ways:

  1. Acclimation: limited range of physiological modifications available to individual organisms
  2. Evolution: Operates at the population level, brought about by inheritance of specific genetic traits that allow species to live in a particular environment.

Species change gradually through two mechanisms:

  1. Competition for scarce resources.

  2. Natural selection: members of a population that are best suited for a particular environment will survive and produce offspring more successfully than their ill-suited competitors

What environmental factors cause selective pressure and influence fertility or survivorship in nature?

Given enough geographical isolation or selective pressure, members of a population can become so different from their ancestors that they can be considered a new species that has replaced the original one.

Alternatively, isolation of population subsets by geographical or behavior factors that prevent exchange of genetic material can result in branching off of new species that coexist with their parental line

Convergent evolution: unrelated organisms coming to look and act very much alike due to natural selection and adaptation.

The Ecological Niche

Habitat: place or set of environmental conditions in which a particular organism lives.

Ecological niche: description of either the role played by a species in a biological community or the total set of environmental factors that determine species distribution.

Species Interactions and Community Dynamics

Competition for scarce resources and Predation are major factors in evolution and adaptation.


Law of competitive exclusion: no two species will occupy the same niche and compete for exactly the same resources in the same habitat for very long.
  • One species will have a competitive edge, and will gain a larger share of resources.

  • Other species will migrate to a new area, become extinct, or change its behavior in a way to minimize competition.

  • Process of niche evolution is called resource partitioning.

  • Niche specialization can create behavior separation that allows subpopulations of a single species to diverge into separate species.

For what do organisms compete?

Intraspecific competition: competition among members of the same species

Interspecific competition: competition among members of different species


Predator: an organisms that feeds directly upon another living organism, whether or not it kills the prey to do so.

All forms of organisms which feed on living things can be considered predators:

Exceptions include scavengers, detritivores, and decomposers (which feed on dead things)

Predation is a potent and complex influence on the population balance of communities involving:

Predators play a role in evolution by

Coevolution: process in which species exert selective pressure on each other

Prey species evolve many protective or defensive adaptations to avoid predation.

Predators evolve mechanisms to overcome the defenses of their prey.

Keystone Species

Keystone species: species or set of species whose impact on its community or ecosystem is much larger and more influential than would be expected from mere abundance.

Both top predators (e.g. wolves) as well as less conspicuous species (e.g. tropical figs and some microorganisms) play essential community roles.

Often a number of species are intricately interconnected in biological communities so that it is difficult to tell which is the essential key.

The filter-feeding Krill is a keystone species in the complex food web of the Antarctic.


Symbiosis: the intimate living together of members of two or more species.
  • In contrast to predation and competition, symbiotic interactions between organisms can be non antagonistic.

  • Symbiotic relationships often entail some degree of coadaptation or coevolution of the partners, shaping, or at least in part, their structural or behavior characteristics (mutualistic coadaptation).

Commensalism: type of symbiosis in which one member clearly benefits and the other is neither benefited nor harmed.

  • Cattle and cattle egrets
  • Many mosses, bromeliads, and other plants growing on trees.

Mutualism: association in which both members of the partnership benefit.

  • Lichens (combination of fungi and a photosynthetic partner)

Parasitism: type of symbiosis in which one member benefits and the other is harmed.

Defensive Mechanisms

Many plants and animals have toxic chemicals, body armor, and other defensive adaptations to protect themselves from competitors or predators.

Batesian mimicry: harmless species will evolve colors, patterns, or body shapes that mimic species that are unpalatable or poisonous.

Muellerian mimicry: two species, both of which are unpalatable or dangerous have evolved to look alike so that when predators learn to avoid either species, both benefit.

Species also evolve amazing abilities to avoid being discovered.

Predators use camouflage to hide as they lay in wait for their prey.

Population Growth Dynamics

Population growth (r) is controlled by variety of factors depending on scale examined:

Global Scale -- no. of births v. no. of deaths.

r = b - d

"r" is expressed as a proportion

(e.g., 0.1 , -0.05 ; equiv. to 10% increase and 5% decrease).

Example --

Given:   Population of 10,000

400 births/yr (40 per 1,000 people)
200 deaths/yr (20 per 1,000 people)

Calcs:   r = b - d

b = 400/10000 = 0.04
d = 200/10000 = 0.02

Therefore:   r = 0.04 - 0.02 = 0.02

(The population is growing at an annual percentage rate of 2%.)

Local Scale -- (b-d) + migrations in and out

Migrate in = immigration (i)
Migrate out = emigration (E)

So, on local scale, population growth rate (r) is calculated taking into account births, deaths, immigrants and emigrants:

r = (b - d) + (i - E)


Given:   Population of 10,000

400 births/yr (40 per 1,000 people)
200 deaths/yr (20 per 1,000 people)
20 immigrants/yr
50 emigrants/yr

Calcs:   r = (b - d) + (i - E)

b = 400/10000 = 0.04
d = 20010000 = 0.02
i = 20/10000 = 0.002
E = 50/10000 = 0.005

Therefore:   r = (0.04 - 0.02) + (.002 - .005) = 0.017

(The population is growing at an annual percentage rate of 1.7%.)

The Exponential Function

Population growth follows a pattern that is described by a particular mathematical formula called the exponential function.

A common function, that can also describe phenomena such as radioactive decay, uptake and elimination of drugs, and others...

In terms of population growth, the exponential function is expressed as:


N(t) is number of people at any time "t"
N(o) is number of people at time zero
e is the base of the natural system of logarithms (2.7182...)
r is the population growth rate
andt is time in years

Example 1:

What will the population size be in 10 years for the first population above?

N(o) = 10,000
r = 0.02 (1/years)
t = 10 (years)

N(t) = 12,214

Example 2:

How long will it take for the population to double in size?

Implies that N(t) = 2N(o), so can replace in formula:

Now, only unknown in the equation is the parameter for time. If solve for time in this equation, will solve for time for population to double in size (doubling time = t(d)).

Use rules of logarithms to simplify equation:

ln(2N(o)) = ln(N(o)) + r*t(d)

Rearrange, and then solve for t(d):

ln(2) + ln(N(o)) = ln(N(o)) + r*t(d)

ln(2) + ln(N(o)) - ln(N(o)) = r*t(d)

ln(2) = r*t(d)

t(d) = ln(2)/r

t(d) = 0.693/r = 0.693/0.02 = 34.6 years

Doubling times at various growth rates

Annual % Increase

Doubling Time (years)

















Characteristics of Exponential Growth:

Under ideal conditions a population can achieve a maximum growth rate: BIOTIC POTENTIAL

In the absence of limiting factors, the population will grow following the exponential function and this is termed EXPONENTIAL GROWTH.

Because of shape of curve, has been termed a "J"-shaped, exponential growth curve.

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 exponential growth 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.

Strategies of Population Growth

Characteristics of contrasting reproductive strategies

r-adapted species

K-adapted species

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

Community Properties

This section focuses on how fundamental properties of biological communities and ecosystems are affected by factors such as tolerance limits, species interactions, resource partitioning, evolution, and adaptation.


Primary productivity: rate of biomass production is an indication of the rate of solar energy conversion to chemical energy.

Tropical forests, coral reefs, and estuaries have high levels of productivity because they have abundant supplies of all of the above resources.

Other systems do not have sufficient levels of the necessary resources.

Even in the most photosynthetically active ecosystems, only a small percentage of the available sunlight is captured and used to make energy-rich compounds.

Abundance and Diversity

Abundance: expression of the total number of organisms in a biological community

Diversity: measure of the number of different species, ecological niches, or genetic variation present.


Complexity: number of species at each trophic level and the number of trophic levels in a community.

Resilience and Stability

Three types of stability or resiliency in ecosystems

The more complex and interconnected a community is, the more stable and resilient it will be in the face of disturbance.

In highly specialized ecosystems, removal of a few keystone species can eliminate many other associated species.

Community Structure

Structure: patterns of spatial distribution of individuals and population within the community and the relation of a particular community to its surroundings.

Edges and Boundaries

Edge effects: relationship of the boundary between one habitat and its neighbors.

Ecotones: boundaries between adjacent communities.

Closed community: community that is sharply divided from its neighbors.

Open community: community with gradual or indistinct boundaries over which many species cross.

Many game animals such as white-tailed deer are often most plentiful in boundary zones between different types of habitat.

Game managers were once urged to develop as much edge as possible to promote large game populations.

Today, however, most ecologists recognize that edge effects associated with habitat fragmentation are detrimental to biodiversity.

Communities in Transition

Ecological Succession

Ecological succession: process by which organisms occupy a site and gradually change environmental conditions by creating soil, shelter, and increasing humidity. .

Both types of succession usually follow an orderly sequence of stages.

Communities of organisms often become more diverse and increasingly competitive as development continues

Climax community: either a primary or secondary community that seemingly resists further change.

  • Climax communities may still be changing because rate of succession is slow.

Equilibrium communities: landscapes that never reach a stable climax because they are characterized by and adapted to periodic disruption.

  • Fire-climax: communities that are shaped and maintained by periodic fires.

Introduced Species and Community Change