c.1- species and communities

Question 1

what is the distribution of species affected by?

Answer 1

limiting factors

Question 2

define limiting factors

Answer 2

the factor that is most scarce in relation to an organism’s needs

Question 3

Limiting factors can be either

Answer 3

biotic (living) or abiotic (non-living):
- Biotic factors include interactions between organisms – either intraspecific (within species) or interspecific (between species)
- Abiotic factors include environmental conditions – such as light, temperature, salinity, rainfall, wind velocity, soil pH, etc.

Question 4

describe the law of tolerance

Answer 4

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

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

Answer 5

• 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

draw a normal distribution curve (labelled) for the distribution of a species

Question 7

what can we use to correlate the distribution of a plant or animal species with an abiotic variable?

Answer 7

• line transects: a tape is laid along the ground between two poles. sampling can be confined to describing all of the organisms that touch the line or distance of samples from the line
• belt transects: where sampling is carried out between two lines separated by a fixed distance
• point transects, used mostly in studies of bird populations: researcher stands at randomly selected points and makes observations within a certain radius

Question 8

describe the niche concept

Answer 8

each species plays a unique role within a community because of the unique combination of its spatial habitat, how it obtains food, and its interactions with other species

Question 9

define the competitive exclusion principle

Answer 9

two species cannot survive indefinitely in the same habitat if their niches are identical.

Question 10

describe what would happen in the case of completely overlapping niches

Answer 10

• the two species cannot coexist in the same habitat
• either one species will lead to the decline and extirpation of the other, or one or both of the competitors will narrow their niches to avoid competition

Question 11

describe the difference between fundamental and realised niches

Answer 11

the fundamental niche of a species is the potential mode of existence, given the adaptations of the species. it refers to the broadest range of habitats it can occupy and roles it can fulfil. The realised niche is the actual mode of existence, which results from the combination of its adaptations and competition with other species

Question 12

give 5 types of interspecific interactions

Answer 12

herbivory: primary consumers feeding on producers

predation: a consumer feeding on another consumer

parasitism: when one organism feeds off another but does not normally kill it

mutualism: two species live in a close association where both organisms benefit from the association

commensalism: one organism benefits and the other is neither harmed nor helped.

Question 13

describe the symbiotic relationship between zooxanthellae (photosynthetic algae) and reef-building coral reef species

Answer 13

• the coral provides the alga with a protected environment and a substrate that can hold it in place for photosynthesis to occur
• the zooxanthellae provide the coral with molecules such as glucose and amino acids

Question 14

define a keystone species and its effects

Answer 14

a keystone species is one which has a disproportionate effect on the structure of an ecological community

fundamentally supports the whole structure and prevents it from collapsing

Question 15

give 3 ways keystone species may influence a community

Answer 15

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