C.1 Species and communities

Question 1

What is a limiting factor (in ecology)?

Answer 1

A component of an ecosystem which limits the distribution of a species
These limiting factors can be either abiotic or biotic.

Question 2

Examples of abiotic limiting factors

Answer 2

Light, water, temperature, pH, salinity, wind velocity

Question 3

Examples of biotic limiting factors

Answer 3

Relationships between organisms: predatory-pray, competition, symbiosis
- Includes interactions between organisms – either intraspecific (within species) or interspecific (between species)

Question 4

Law of Tolerance

Distribution of species depends on their tolerance of limiting factors

Answer 4

The law of tolerance was proposed by an American zoologist, Victor Ernest Shelford, in 1911.
- According to the law of tolerance, populations have optimal survival conditions within critical minimal and maximal thresholds
- As a population is exposed to the extremes of a particular limiting factor, the rates of survival begin to drop

Question 5

How would a law of tolerance graph look like?

Answer 5

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)

Question 6

Distribution of one plant species to illustrate limits of tolerance and zones of stress: Black mangrove

Answer 6

Black mangrove (Avicennia germinans) is a very widespread mangrove tree.
- It can survive and grow in a wide range of
salinity levels from 0 to 96 part per thousand (ppt).
- Greatest growth rates occur at salinity levels of 24 and 48 ppt, the optimal zone, outside of this range the Black Mangrove
trees experience the zones of stress.

Question 7

Distribution of one animal species to illustrate limits of tolerance and zones of stress: coral reefs

Answer 7

• Coral species form connected reefs that are greatly impacted by changes in oceanic temperature
• Coral polyps receive nutrition from photosynthetic zooxanthellae (an algae) that lives within the polyp’s endodermis
• The zooxanthellae cannot survive in lower ocean temperatures (i.e. < 18ºC)
• Increases in ocean temperature cause zooxanthellae to leave the coral tissue, leading to coral bleaching (i.e. > 35ºC)
• Reef-building coral species therefore have a typical optimal growth range in temperate waters between 20 – 30ºC
• This correlates to the tropical and sub-tropical regions of the world (i.e. near the equator)

Question 8

Explain the symbiotic relationship between Zooxanthellae and reef-building coral reef species

Answer 8

The coral provides the algae with:
- carbon dioxide
- a protected environment - coral polyps secrete calcium carbonate to build the stony skeletons which house the coral polyps (and zooxanthellae)

The zooxanthellae provide the coral with:
- oxygen, glucose and other organic molecules (produced via photosynthesis)
- helps the coral to remove wastes

Question 9

State one limiting factor on Zooxanthellae which affects coral reef formation.

Answer 9

light / temperature / salinity / carbon dioxide / pH

Question 10

What is an ecological niche?

Answer 10

An ecological niche describes the functional position and role of an organism within its environment.
- It consists of all physical and biological conditions which determine the organism’s survival and reproductive prospects

An ecological niche will be comprised of various components, including:
- The habitat in which an organism lives
- The activity patterns of the organism (e.g. periods of time during which it is active)
- The resources it obtains from the environment (e.g. food sources, territorial boundaries, etc.)
- The interactions that occur with other species in the community (e.g. predator prey relationships, competition, etc.)

Question 11

What happens if two distinct species share an identical niche?

Answer 11

Two species cannot survive indefinitely in the same habitat if their niches are identical. There will be interspecific competition for available space and resources.
- This competition between the two species will result in the fitness of one being lowered by the presence of the other
- Inevitably, the less well-adapted species will struggle to survive and reproduce – it will eventually be eliminated from the niche

Interspecific competition within a shared niche will typically prompt one of two responses:
- Competitive exclusion – One species uses the resources more efficiently, driving the other species to local extinction
- Resource partitioning – Both species alter their use of the habitat to divide resources between them (i.e. niche separation)

Question 12

Fundamental niche

definition + explanation

Answer 12

The full range of environmental and social conditions under which a species could potentially survive and reproduce.
- It is the theoretical habitat and may not be fully occupied due to the presence of competing species

Question 13

Realised niche

Answer 13

The specific set of conditions under which a species would survive in a given habitat or ecosystem, with the limitations of other species being present.
- The actual conditions under which a species lives

Question 14

Analysis of a data set that illustrates the distinction between fundamental and realized niche.

Answer 14

• The fundamental niche can be deduced from the populations of the isolates species
• The realised niche can be deduced by the populations of different species after they are allowed to interact

Question 15

Herbivory

Answer 15

Herbivory is the act of eating only plant matter (e.g. primary consumers are considered herbivores)
Herbivory can be either harmful or beneficial to the plant species as a whole:
- Certain types of beetle may feed voraciously on the leaves / foliage of crop plants (folivores), causing crop failure
- Fruit-eating animals (frugivores) spread the seeds from a fruit in their faeces, promoting overall seed dispersal

Examples: rabbits

Question 16

Predation

Answer 16

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

Example: arctic fox and snowshoe hare

Question 17

Symbiosis

Answer 17

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, commensalism, parasitism

Question 18

Mutualism

Symbiotic relationship

Answer 18

A mutually beneficial symbiotic relationship.

Example: zooanthellae and coral polyp

Question 19

Commensalism

Symbiotic relationship

Answer 19

When one species benefits and the other is unaffected.

Example: nurse shark and remora

Question 20

Parasitism

Answer 20

When one species benefits at the other’s expense

Example: ticks and human hosts

Question 21

Explain coral bleaching - symbiotic relationship between Zooxanthellae and reef-building coral reef species

Answer 21

It is the zooxanthellae within the polyp endodermis which gives the coral its vibrant pigmentation
- When there is a large scale loss of zooxanthellae from the coral (due to environmental stress), bleaching occurs
- When bleaching occurs, coral begins to starve and will die unless the zooxanthellae are restored

Conditions which can cause coral bleaching include:
- Changes in light availability (e.g. sedimentation may increase the opacity of the oceanic waters)
- Temperature increases (water temperatures in excess of 30ºC can irrevocably stress the zooxanthellae)
- Ocean acidification (the build up of carbon dioxide concentrations in the ocean can lower pH and stress the algae)

Question 22

What is a keystone species?

Answer 22

A keystone species is a species that has a disproportionately large impact on the environment relative to its abundance.
- This species is so important to the ecosystem that if something were to happen to it, the ecosystem could fall apart.

Question 23

How may keystone species influence communities in a number of ways?

predators, mutualism, engineers

Answer 23

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) (honey bees)
- Engineers – they can refashion the environment in a manner that promotes the survival of other species (beavers: build dams))

Question 24

Example of keystone species

Sea otter

Answer 24

• Sea otters regulate sea urchin populations, which feed upon kelp and other macroalgae.
• The otters keep the sea urchin populations in check, thus allowing enough kelp forests to remain as a habitat for a variety of other species.
• As a result, the entire ecosystem is kept in balance.