Population Interactions, Limiting Factors & Carrying Capacity (SL IB Environmental Systems & Societies (ESS))

Population Interactions

• A population is a group of organisms of the same species living in the same area at the same time
  • Populations can be characterised by their size, density, distribution, age structure, and growth rate
• The carrying capacity of a habitat or ecosystem is the maximum number of individuals of a particular species (i.e. the maximum population size) that can be sustained in a given area over a given time period
• Ecosystems consist of numerous species with different interactions occurring between populations, including:

   Competition

• • Competition can be divided into intraspecific competition (competition between members of the same species) and interspecific competition (competition between members of different species)
  • Intraspecific competition can lower the carrying capacity of a population due to a decrease in food availability caused by high population density
  • Interspecific competition occurs between species with similar niches, causing a decrease in the carrying capacity of one or both species

Intraspecific competition between two grey squirrels (same species) for a limited resource

Interspecific competition between a grey squirrel and a red squirrel (different species) for a limited resource

   Predation

• • When one animal eats another
  • This lowers the carrying capacity of the prey species
  • This can have negative feedback effects, lowering the carrying capacity of the predator species due to a decrease in prey numbers

   Herbivory

• • When an organism (known as a herbivore) feeds on a plant
  • The carrying capacity of herbivore species is affected by the quantity of plants they feed on
  • An area with more plant resources will have a higher carrying capacity for herbivore species
  • This can also have negative feedback effects - the carrying capacity of the herbivore species may decrease if herbivory rates are too high and the plant population decreases too much

   Parasitism

• • Parasites are organisms that are adapted to live very closely with another species, known as the host species
  • A parasitic relationship is one in which the parasitic organism benefits from the host organism
  • The parasite lives either in or on the body of the host species and gains resources (i.e. what it needs in order to survive) from the host, including food, shelter and a suitable location to reproduce (where offspring can feed and grow)
  • However, the host does not benefit from this relationship and parasites often harm the host in some way 
  • This can lower the host's carrying capacity
  • An example of a parasitic relationship is fleas being a parasite to mammals (e.g. dogs) - the fleas feed on the host's blood but don't provide anything to the host in return and may transmit diseases to the host
  • Another example is the parasite that causes malaria
    • This parasite infects red blood cells in humans and causes recurrent episodes of fever and can be fatal in certain instances
    • The malarial parasite has a life cycle that includes the mosquito as a vector

Part of the malaria parasite's life cycle is in humans and the other part is in mosquitos

Mutualism

• • A mutualistic relationship between species is one in which both species benefit
  • This increases the carrying capacity of both species in the relationship
  • An example of a mutualistic relationship is the one that exists between bees and many species of flowering plants
  • Bees gain nectar (i.e. food to provide them with energy) from flowers
  • When bees visit flowers, pollen is transferred to their bodies
  • As bees visit multiple different flowers, they spread the pollen to these flowers, pollinating them
  • In this way, the flowers gain help in reproducing

Disease

• • Pathogens (bacteria, viruses, fungi, and protozoa) are organisms that cause diseases
  • These diseases lower the carrying capacity of the species that the pathogens infect
  • Changes in the incidence of diseases can cause populations to fluctuate around their carrying capacity

Limiting Factors and Carrying Capacity

• Limiting factors restrict the growth of a population (prevent it from increasing further)
  • Plant populations have several limiting factors, including light, nutrients, water, carbon dioxide, and temperature
  • Animal populations have several limiting factors, including space, food, mates, nesting sites, and water
• Limiting factors will slow the growth of a population as it approaches the carrying capacity of the system