Populations and Communities

Populations

• A population is an interacting group of organisms of the same species that inhabit the same geographical area and are capable of breeding with one another (no reproductive isolation).
• Members of the same species that are reproductively isolated are considered different populations.
• Populations are interacting groups of organisms of the same species living in an area.
• Community: Group of populations.
• Habitat: Abiotic environment.
• Ecosystem: Habitat + Community.

Population Growth Curve

• Stable populations demonstrate a sigmoidal (S-shaped) population growth curve comprising three key stages:
  1. Exponential Growth Phase (J-curve):
     • Rapid growth due to an abundance of resources
  2. Transitional Phase:
     • Growth slows (environmental resistance sets in)
  3. Plateau Phase:
     • Population stays stable at the carrying capacity
• The population growth curve illustrates the relationship between time and population size.

Carrying Capacity

• The carrying capacity represents the maximum number of individuals of any species that an environment can support.
• In a population growth curve, the carrying capacity \kappa represents the point where the plateau phase is reached.
• The carrying capacity for a given species is not a static value and may be influenced by either abiotic or biotic factors.
• Limited resource availability creates competition, leading to death, restricting more population growth.
• Populations will tend to oscillate around the carrying capacity \kappa.

Human Population Growth

• Population clocks can be used as a tool for identifying changing demographics.
• The human population is currently growing at a rate unprecedented in world history.
• Declining birth rate doesn't mean the population is decreasing; it means the population is still increasing, but at a slower rate than before.
• Historical Trends:
  • The world population grew slowly for most of human history because the birth rate and death rate were roughly balanced.
  • Around the 19th century, births started to outnumber deaths, leading to a significant population increase.
• Current Trends:
  • While the rate of population growth has been declining in recent decades, it still remains positive, meaning the number of births is higher than the number of deaths.
  • The United Nations projects that the world population will continue to grow until the 2080s, eventually peaking above 10 billion, but with no significant decline expected this century.

Density-Dependent Factors

• Population size can be impacted by density-dependent factors, which push populations back to the carrying capacity (negative feedback).
• Examples of density-dependent factors include:
  • Predation: Larger populations will experience more predation.
  • Access to habitats: There will be more competition for territory.
  • Nutrient supply: There is less food availability in large populations.
  • Disease / pathogens: Quicker spread in denser populations.
  • Accumulation of wastes: Metabolic by-products can be toxic.

Intraspecific Interactions

• Intraspecific interactions occur between members of the same species
• Cooperation (both members benefit from the specific interaction)
  • Pack animals (wolves) working in unity to capture and feed on prey
  • Insects (ants) using chemotactic signals to search for food sources
• Competition (one member derives a bigger benefit from an interaction)
  • Territorial animals defending a defined space against other animals
  • Animals (gorilla) fighting for the opportunity to mate with females
  • Woodland trees (oak) vying for access to light, water and minerals

Predator-Prey Dynamics

• The predator-prey relationship functions as an example of density-dependent control in several animal populations.
• A predator is an organism that hunts another (the prey).
• Predator and prey populations experience linked changes:
  • If a prey population drops, then predator numbers will dwindle as intra-specific competition for prey increases.
  • If the prey population rises, predator numbers increase as a result of the over-abundance of food source (prey).
• Predator-Prey Example: Arctic fox (predator) feeds on the snowshoe hare (the prey).

Communities

• A community is a group of populations living together and interacting with each other within a given area (i.e. it is a combination of all of plant, animal, fungal, and bacterial populations).
• As communities consist of organisms, they represent the biotic component of ecosystems.

Interspecific Relationships

• Different species within a community form a variety of interspecific interactions, including:
  • Feeding relationships (herbivory, predation)
  • Interspecific competition (‘inter’ = between)
  • Symbiosis (mutualism, parasitism)
  • Pathogenicity (host invasion)
• The type of interaction is typically defined according to the effect on the organisms involved.
• Herbivory.
• Predation.
• Mutualism.
• Parasitism.
• Competition.
• Pathogenicity.

Feeding Patterns

• Species within a community may interact via their feeding patterns.
• These feeding requirements also form the basis for competition.
• Herbivory: The act of eating only plant matter (primary consumers).
  • Some herbivores only eat certain plant parts (honeybees = nectar).
  • Other herbivores feed to the detriment of the plant (cows grazing).
• Predation: A predator hunts and feeds on another organism (prey).
  • All predators are carnivores or omnivores (plants are not hunted).
  • E.g., Owls hunting mice, lions hunting zebras, wolves hunting deer.

Symbiosis

• Symbiosis describes a close and persistent (i.e., long-term) interaction between two species.
• Mutualism: Both species benefit from the interaction.
  • E.g., Clownfish attracts food and is protected by sea anemone.
• Commensalism: One species benefits, and the other is unaffected.
  • E.g., Barnacles transported to food source on whales.
• Parasitism: One species benefits, and the other is harmed.
  • E.g., Ticks and fleas feed on the blood of a host animal.

Mutualism

• Mutualism is an interspecific relationship between two species that benefits both organisms.
• Legume root nodules (Fabaceae) trade resources with nitrogen-fixing bacteria (Rhizobium).
• Fungi and orchids can form a filamentous network called a mycorrhizae (↑ surface area).
• Photosynthetic algae (zooxanthellae) provide nutrition inside a coral polyp’s endodermis.

Pathogenicity

• Pathogenicity involves infectious microorganisms living either inside or on the surface of a host organism and causing disease.
• The infectious agent (pathogen) causes the disease by disrupting the homeostatic processes occurring within the host organism.
• Pathogens can include bacteria, protozoa, fungi, and multicellular parasites – viruses are also pathogenic but are not alive (abiotic).
• Mycobacterium (a bacteria) causes tuberculosis in animals.
• Powdery mildew (a fungus) can cause foliar disease in plants.

Antibiotics

• Antibiotics are biological compounds that are released by certain organisms to kill or impede the growth of bacterial pathogens.
• Antibiotics specifically target components of prokaryotic cells.
• A specific example of an antibiotic is penicillin, which is released by the fungus Penicillium and targets the cell wall of a bacterium.
• Humans have since extracted and purified this compound to now use penicillin as a medication to treat bacterial diseases.
• Note: Antibiotics are not effective against eukaryotic organisms.
• The efficacy of penicillin is demonstrated by a zone of inhibition around the fungus.

Interspecific Competition

• Interspecific competition occurs when different species compete for access to a limited resource (‘inter’ = between, ‘specific’ = species).
• The competition can either be direct (vying for common territory) or indirect (consuming a resource and thereby depleting its availability).
• Examples of interspecific competition may include:
  • Lions and hyenas competing for the same source of food (prey).
  • Pine trees and beech trees competing for access to sunlight.
  • Crabs and tube worms compete for the space within a rockpool.

Invasive Species

• The species within an ecosystem can be broadly described as being either endemic or alien.
  • Endemic: Native to a region (indigenous).
  • Alien: Transferred from a different area.
• If an alien species has a detrimental effect upon existing food chains, it is classed as invasive.
  • Invasive species typically possess a larger fundamental niche (can occupy a wider region).
  • They often have faster reproduction rates (form larger populations than endemic species).
  • They commonly lack a predator capable of limiting their survival (due to being non-native).
  • They may possess certain features (adaptations) that make them better suited to an area.
• Consequently, they often pose a threat to the viability of the endemic species within a region.

Red Fox versus Quoll

• Red foxes were introduced to Australia following British colonisation for the sport of hunting.
• The red fox shares a common diet with the native quoll and occupies a similar niche.
• Quoll populations have since declined dramatically, and certain species are now endangered.
• Foxes are now being actively targeted for eradication to protect quoll populations.

Population Control

• When an organism exerts control over another organism, it can have one of two effects:
  • Top-down control – Pressure exerted by a higher trophic level (induces oscillating effects).
  • Bottom-up control – Pressure exerted by lower trophic level (reduces all higher levels).
• A keystone species may exert top-down control; low nutrients may exert bottom-up control.

Habitat

• A habitat is the physical environment in which a community, species, population or organism normally lives (therefore, a habitat will represent the abiotic components of an ecosystem).
• Habitat descriptions can include geographical and physical locations, or type of ecosystem.

Examples

• Examples of Species and their habitats:
  • Penguin: Antarctica, Polar.
  • Great White Shark: Ocean, Indian Ocean.
  • Rattlesnake: Desert, South-West U.S.
  • Tapir: Rainforest, Brazil.
  • Snow Leopard: Mountain, Himalayas.
  • Salmon: River, Central Canada.
  • Giant Squid: Ocean, Pacific Ocean.
  • Bactrian Camel: Desert, Mongolia.
  • Blue Wildebeest: Grassland, South Africa.
  • Gorilla: Rainforest, Central Africa.
  • The Andean Condor: Mountain, S. America.
  • Leopard: Grassland, Southern Africa.