Brent Cornell

An organism’s capacity to survive will be determined by the characteristics it possesses that enable it to interact with the environment

• Environmental pressures (both abiotic and biotic factors) will limit the survival or organisms that lack suitable characteristics
• Hence, genetic diversity influences the survival prospects of a population or species by determining the frequency of specific traits

Ecological Organisation

Specific ecology terms are used to describe the interactions between organisms within a given environment, including:

• Species:  A group of organisms that can potentially interbreed to produce fertile, viable offspring
• Population:  A group of organisms of the same species, living in the same area at the same time
• Community:  A group of populations living together and interacting with each other within a given area
• Habitat:  The environment in which a species normally lives, or the location of a living organism
• Ecosystem:  A community and its abiotic environment (i.e. habitat)
• Ecology:  The study of the relationship between living organisms, or between living organisms and their environment

Survival Conditions

In ecology, a limiting factor is a component of an ecosystem which limits the distribution or numbers of a population

• A limiting factor defines optimal survival conditions according to its effect on a species when in deficiency or excess
• Limiting factors can be categorised as either being biotic (living) or abiotic (non-living)

The distribution of a species in response to a limiting factor can be represented as a bell-shaped curve with 3 distinct regions:

• Optimal zone – Central portion of curve which has conditions that favour maximal reproductive success and survivability
• Zones of stress – Regions flanking the optimal zone, where organisms can survive but with reduced reproductive success
• Zones of intolerance – Outermost regions in which organisms cannot survive (represents extremes of the limiting factor)

Abiotic Factors

Abiotic factors are the non-living conditions of an environment that influence the survival of organisms within an ecosystem

Abiotic factors may include: 

• The resources an organism needs to survive (such as light, water, oxygen or carbon dioxide concentrations)
• Conditions that need to be maintained within a certain range (such as temperature, pH, radiation levels, salinity)
• Specific environmental events (such as weather patterns, climate conditions, natural disasters like fires or floods)

Abiotic factors will impact survival prospects and affect the density, distribution and size of a population within a given ecosystem

Biotic Factors

Biotic factors are the living components of an ecosystem that influence the survival of individual organisms and collective populations

• Certain species may need to interact (co-exist) within a shared environment in order to survive (they share a positive association)
• Examples of species interactions include predator-prey dynamics and symbiotic relationships (e.g. commensals, parasites, etc.)

Predator–Prey Relationship

Predation is a biological interaction whereby one organism (predator) hunts and feeds on another organism (prey)

• Because the predator relies on the prey as a food source, their population levels are inextricably intertwined
• If the prey population drops (e.g. due to over-feeding), predator numbers will dwindle as intra-specific competition increases
• If the prey population rises, predator numbers will increase as a result of the over-abundance of a food source

Symbiotic Relationships

Symbiosis describes the close and persistent (long-term) interaction between two species

• Symbiotic relationships can be obligate (required for survival) or facultative (advantageous without being strictly necessary)

Symbiotic relationships can be beneficial to either one or both organisms in the partnership:

• Mutualism – Both species benefit from the interaction (anemone protects clownfish, clownfish provides fecal matter for food)
• Commensalism – One species benefits, the other is unaffected (barnacles are transported to plankton-rich waters by whales)
• Parasitism – One species benefits to the detriment of the other species (ticks and fleas feed on the blood of their canine host)  

Keystone Species

A keystone species is a species that has a disproportionately large impact on the environment relative to its abundance

• It is analogous to a keystone in an arch – it fundamentally supports the whole structure and prevents it from collapsing

Keystone species may influence communities in a number of ways:

• Predators – they can exert pressure on lower trophic levels to prevent them from monopolising certain resources
• Mutualism – they can support the life cycle of a variety of species within a community (e.g. pollinators / seed dispersal)
• Engineers – they can refashion the environment in a manner that promotes the survival of other species

There are numerous examples of keystone species within different communities:

• Sea stars (predator) prey on urchins and mussels, preventing mussel overpopulation and coral reef destruction by urchins
• Honey bees (mutualist) pollinate a wide variety of plant species, ensuring the continuation of the plant life cycle 
• Beavers (engineer) build dams that transform the environment in a manner that allows certain other species to survive

Keystone species are not the dominant species (most numerous) within a community, nor do they have to be apex predators

Population Control

Limiting factors can exert their influence over populations via either top down or bottom up control mechanisms

• The difference between the two types of control relates to the position of the trophic level that is influencing the community

Top Down Control

• Top down factors are pressures applied by a higher trophic level to control the population dynamics of the ecosystem
• The top predator either suppresses the abundance of its prey or alters its behaviour to limit its rate of population growth
• Top down control results in an oscillating trophic cascade (suppression at one level increases numbers at the next level)
• Keystone species commonly exert top down control by preventing lower trophic levels from monopolising essential resources

Bottom Up Control

• Bottom up factors are pressures that limit the availability of resources to lower trophic levels (e.g. producers)
• A lack of resources at lower trophic levels suppresses the abundance of organisms at higher trophic levels
• Population growth will be reduced for all higher levels as the suppression of the 'bottom’ restricts energy supply to the ‘top'
• Human activity can often limit resource availability and hence inadvertantly exert bottom up pressure on an ecosystem