Population Ecology and Community Interactions

Population Ecology and Community Interactions

Population and Community

• A population is a group of the same species living in the same area.
• A community is a group of different species living in the same area.
• An ecosystem is a community of living organisms (plants, animals, microorganisms) interacting with each other and their physical environment (air, water, soil) in a specific area.
• A biome is made up of many ecosystems that share similar environmental characteristics and organisms adapted to those conditions.

Population Characteristics

• A population refers to a group of organisms of the same species located in the same place at the same time.
  • Example: Population of human beings in a city, all Pine trees in a given land.
• Populations are identified by a number of characteristics:
  • Gene Flow and Gene Frequency
  • Population size and density
  • Population and Age distribution
  • Natality (birth)
  • Mortality (death)

Gene or Allele Frequency

• Allele frequency or Gene frequency is the number of copies of a particular allele divided by the number of copies of all alleles in a population.
• Allele frequencies are used to depict the amount of genetic diversity at the population and species level.
• The more genetic or species variation a population has, the better its ability to adapt to changes in its environment through natural selection.
• Example: If the frequency of an allele is 20% in a given population, then among population members, one in five chromosomes will carry that allele.

Calculation of Allele Frequency

• Example: Consider two alleles W (dominant) and w (recessive) in a population.
• If W=13 and w=5, the total number of gene copies in the whole population is 13 + 5 = 18.
• Frequency of W = 13/18 = 0.72 = 72\%. Frequency of w = 5/18 = 0.28 = 28\%.

Population Size and Growth

• The growth and size of a population is dependent on:
  • The rate at which organisms enter the population
    • Natality = the number of new individuals added by birth/1000 individuals
    • Immigration (entering into the population)
  • The rate at which organisms leave the population
    • Mortality = the number of individuals leaving by death/1000 individuals
    • Migration from the population

Population Growth Curve

• Population growth curves graph the change in population size over time.
• Two types of population growth curve:
  • The Exponential curve (also known as a J-curve)
    • Occurs when there is no limit to population size.
    • Constant growth without environmental constraints
  • The Logistic curve (also known as an S-curve)
    • This curve predicts that the population starts off rapidly, but unlike exponential growth, it slows down as the population reaches a maximum capacity, known as the carrying capacity (K).
    • This curve shows the effect of a limiting factor

J-Curve

• No limit to population size.

S-Curve

• Population grows until it reaches carrying capacity (k).
• Carrying capacity is the maximum number of organisms of a particular species that can be supported in a given environment.
• Logistic curve” is shaped like an "S“.
  • Initially, it increases slowly (the lower part of the "S").
  • Then, it accelerates sharply in the middle (the steep middle part of the "S").
  • Finally, it levels off as it approaches the carrying capacity (the top part of the "S").
• Carrying capacity (K): is the maximum population size of a biological species that can be sustained by that specific environment, given the food, habitat, water, and other resources available.

r and K-strategist

• r-strategist (follow J-Curve, short life span).
  • Example: May Fly: lives 24 hours.
  • Oyster produces 500 million offspring a year and provides no parental care.
  • Population growth is characterized by a rapid growth followed by rapid death.
• K-strategist (follow S Curve, long life span).
  • Example: Greenland Shark lives 400 years.
  • Apes (Chimpanzee) produces 1 offspring every 5 years and provide extensive parental care.
  • Usually for large organisms who live a long time and reach a population size that can be sustained (reach carrying capacity).

Exponential Growth Details

• Biotic potential: is the unrestricted growth of populations resulting in the maximum growth of that population.
• Overshoot: often in this type of curve population exceeds the carrying capacity before it collapses. This is called overshoot.
• Dieback: A sudden sharp reduction in the size of a population that can be caused by disease, environmental stress (such as pollution), or when its numbers exceed the carrying capacity of its habitat.
• Exponential Growth (J-Curve): The growth is exponential meaning the population increases rapidly because there’s an abundance of resources, and the individuals are reproducing at a high rate.

Limiting factors for Population Size

• Limiting factors are environmental factors that limit population size.
• Two Types of limiting factors:
  • Density-Dependent Limiting Factors- Control K- strategists
    • Example-Predators, and Competition for food
  • Density-Independent Limiting Factors-Control r- strategists
    • Example-Weather pattern; Drought/excessive rainfall
    • Natural disasters

Survivorship Curve

• A survivorship curve is a graphical representation that illustrates how different species or populations survive, reproduce, and age.
• There are three main types of survivorship curves, often classified as Type I, Type II, and Type III. Each represents different patterns of survival throughout the lifespan of a species.
  • Type I Curve (Convex Curve): Low mortality rate (mortality increases with age i.e. old age).
  • Type II Curve (Diagonal/Linear Curve): Constant mortality rate (Mortality is fairly consistent at all ages).
  • Type III Curve (Concave Curve): High mortality rates in early life.
• x-axis: represents age (from birth to death).
• y-axis: represents the number or proportion of individuals still alive at each age.
• After drawing the curve, interpret its shape to understand the survival strategy of the population.

Community Ecology (Community Interaction)

• A community consists of all the populations of different species that interact in a particular location.
• Before an ecologist can study a community, the community boundaries must be defined.
• The boundaries of an aquatic community are naturally defined by the water’s edge.

Habitat and Niche Concept

• Scientist learn how a community works by focusing on the activities of different population. Two concepts are mainly used-Habitat and Niche Concept
• A habitat is the place where the organism lives, such as a forest or river.
  • Tuna habitat is the open ocean, Cactus habitat is desert
• A Niche is specific role that the organism plays in the ecosystem or it’s habitat.
  • Niche involves the organism’s interaction with biotic (living) and abiotic (non-living) factors

Community Interactions

• An important aspect of an organism’s niche that is how it interacts with other species in the community.
• Some interactions are positive, some are negative, and some are neutral.
• There are three major category of interaction
  • Competition (-/-)
  • Predation (+/-)
  • Symbiosis (+/+ or 0/+ or +/-)

Competition

• It is an interaction between organisms in which both organisms might be harmed (-/-)
  • Intraspecific Competition: Occur between organisms of same species (for food and nesting sites)
  • Interspecific Competition: Occur between organisms of different species (Lion and Vulture fighting for food)

Predation

• It is an interaction where one organism kills the other for food
• A predator (an organism that is hunting) feeds on its prey (the organism that is attacked).
  • In this interaction, one species benefits, and the other is harmed (+/- )
  • Predators exhibit traits such as sharp teeth, claws, and venom that enhance their ability to catch food
  • Prey species exhibit traits that help them avoid detection or capture. Many, such as leaf insects, moths, a variety of frogs and small lizards, and herbivorous mammals, are cryptically colored (Camouflage) to make them more difficult to see

Herbivory

• Herbivory is the consumption of plant material by animals, and herbivores are animals adapted to eat plants.
• As in predator-prey interactions, this interaction drives adaptations in both the herbivore and the plant species it eats.
  • For example, to reduce the damage done by herbivores, plants have evolved defenses, including thorns and chemicals.
  • Scientists have identified thousands of plant chemical defense compounds, including familiar compounds such as nicotine and cocaine

Symbiosis

• A symbiosis is a long-term interaction between members of different species.
• These relationships can take several forms, depending on the benefits (or harms) experienced by the organisms involved.
  • Symbiosis ( 3 types)
    • Parasitism (+/-)
    • Commensalism (+/o)
    • Mutualism (+/+)

Parasitism

• Involves one organism living in or on another living organism in order to obtain nutrients
• Example-Parasite and Host relationship
  • In this relationship parasite benefits and the host harmed (+/-)
    • Parasites usually do not kill their host immediately because they need nutrients from it.
    • However, as the parasite drains nutrients from the host, the host is weakened and may die.
  • Bacteria, viruses, protists, insects, worms, mites have some species that are parasites.

Commensalism

• Commensalism is a relationship in which one organism benefits and the other is not affected (+/0)
  • Example-Barnacles & turtle; Cattle & Egrets
    • Barnacles attaching to the shell of a turtle. The barnacles get access to nutrient-rich waters as the turtle swims, while the turtle is unaffected by their presence.
    • The egrets always remains cattle are grazing because the cattle, as they move, stir up and flush out insects from the vegetation that otherwise might be difficult for the egrets to find and catch.

Mutualism

• Mutualism is a relationship in which both species benefit (+/+)
  • Usually involves a close physical association
    • Lichens contain both fungus and algae cells living together. The algae carry on photosynthesis and the fungus provides moist surroundings for algae.
    • Ruminants (cow and deer) have bacteria and protozoa in their guts that digest cellulose. The ruminant provides a moist environment for the bacteria and protozoa to survive.

Symbiosis: Major Types