Identify and describe abiotic and biotic factors that influence the distribution and abundance of organisms in an ecosystem, including tolerance ranges and ecological niche

What this dot point is asking

QCAA wants you to name and describe the abiotic and biotic factors that determine where organisms live and how many of them are there, to apply the concept of a tolerance range, and to use ecological niche correctly (including the distinction between fundamental and realised niche). Stimulus questions usually present a gradient (intertidal zonation, an altitudinal transect, salinity in an estuary) and ask you to interpret distribution data.

The answer

The distribution (where) and abundance (how many) of organisms in an ecosystem are the joint outcome of abiotic and biotic factors acting through each species' tolerance range. The niche concept ties these together.

Abiotic factors

Abiotic factors are the non-living, physical and chemical conditions of the environment.

• Light. Intensity, duration (photoperiod) and quality. Limits photosynthesis in plants and algae; cues flowering, migration and breeding.
• Temperature. Sets metabolic rates and protein stability. Ectotherms are particularly constrained; endotherms expend energy buffering temperature.
• Water availability. Includes rainfall, soil moisture and humidity. The single biggest determinant of vegetation distribution across Australia.
• Soil (edaphic factors). Texture, depth, drainage, organic content. Sandy soils retain little water; clay soils retain water but drain poorly.
• Soil and water pH. Influences nutrient availability and enzyme function. Most plants prefer pH 5.5 to 7.5; the highly weathered Australian soils tend acidic.
• Salinity. Concentration of dissolved salts. Critical in estuaries, dryland salinity zones and coastal soils.
• Dissolved oxygen. Aquatic environments only. Cold, fast-flowing water carries more oxygen than warm, stagnant water.
• Wind and exposure. Increases evapotranspiration, shapes plant form, transports pollen and seeds.
• Fire regime. In Australian ecosystems, fire is a recurrent abiotic factor that shapes plant traits (lignotubers, serotiny) and prevents succession to rainforest in many areas.

Biotic factors

Biotic factors are the influences other organisms have on the species in question.

• Competition. Intraspecific (between members of the same species, e.g. for territory) and interspecific (between species for shared limiting resources). Often shrinks the realised niche.
• Predation. Predators reduce prey abundance and can restrict prey to habitats where they are less vulnerable.
• Herbivory. Plants are consumed by animals; heavy herbivory limits plant distribution and selects for defensive traits.
• Symbiosis. Long-term close interactions. Three main types appear on QCAA exams.
  • Mutualism. Both species benefit (mycorrhizal fungi and eucalypts; flying foxes and rainforest trees).
  • Commensalism. One benefits, the other is unaffected (epiphytic orchids on rainforest trees).
  • Parasitism. One benefits, the other is harmed (cane toad and its native nematode parasites; mistletoe on eucalypts).
• Disease. Pathogens regulate populations and can drive local extinction (Phytophthora dieback in jarrah forests; chytrid fungus in frogs).
• Pollination and seed dispersal. Animals enable plant reproduction; loss of a pollinator narrows the plant's realised distribution.

Tolerance ranges

Every species has a range of values of each abiotic factor within which it can survive and reproduce. Plotted as a tolerance curve (performance against the factor), the curve is bell-shaped with the following zones:

1. Optimum range. Performance is at its maximum. Growth, reproduction and survival are highest.
2. Zones of physiological stress. Performance declines but the organism survives. Reproduction may fail.
3. Limits of tolerance. The minimum and maximum values beyond which the organism cannot survive.
4. Zones of intolerance. The species is absent.

The width of the tolerance curve matters.

• Eurytopic species have wide tolerance ranges and tend to be widespread.
• Stenotopic species have narrow tolerance ranges and tend to be restricted (alpine species, deep-cave species).

Multiple factors interact. A species may tolerate higher temperatures when water is abundant but be restricted to cooler sites when it is dry. Liebig's law of the minimum states that the factor in shortest supply relative to need limits performance.

Ecological niche

A species' ecological niche is its multidimensional role in an ecosystem: where it lives, when it is active, what it eats, what eats it, what it requires and what it provides. The niche is summarised along several axes.

• Spatial. Physical position (canopy, understorey, soil).
• Temporal. Time of activity (diurnal, nocturnal, crepuscular, seasonal).
• Trophic. Position in food webs (primary producer, herbivore, predator).
• Reproductive. Breeding season, mate selection, parental care.

Fundamental niche. The full set of conditions under which the species can survive and reproduce, with no competitors or predators present.

Realised niche. The actual subset occupied once species interactions are factored in. Competition usually shrinks the niche; facilitation can expand it.

Competitive exclusion principle. Two species with identical niches cannot coexist indefinitely; the better competitor displaces the other. Coexistence requires niche differentiation (different resources, different times, different microhabitats).

Worked example: intertidal zonation

The rocky shore between high and low tide shows clear horizontal bands of species. Distribution is driven by tolerance to two main factors interacting with competition.

• High shore (upper littoral). Long exposure between tides. High desiccation, large temperature swings. Limpets and periwinkles dominate because they tolerate desiccation.
• Mid shore. Intermediate. Barnacles and mussels reach high densities.
• Low shore. Short exposure between tides. Lower desiccation but higher predation by sea stars and crabs. Algae, anemones and small fish dominate.

Joseph Connell's experiments with Chthamalus and Balanus barnacles showed that Chthamalus could survive across the whole shore (broad fundamental niche), but was outcompeted by Balanus in the lower zone, restricting its realised niche to the upper shore. Distribution is the joint outcome of an abiotic gradient and a biotic interaction.

Common traps

Confusing habitat and niche. Habitat is the where; niche is the role and the conditions across all axes.

Treating tolerance curves as identical for all factors. A species can have a wide tolerance to temperature but a narrow tolerance to salinity. Plot each factor separately.

Calling everything mutualism. Make sure both species genuinely benefit; one-sided benefit is commensalism, harm is parasitism.

Ignoring interactions. Distribution rarely reflects abiotic factors alone. Competition, predation, disease and disturbance routinely shrink the realised niche.

In one sentence

The distribution and abundance of a species is set by its tolerance range for abiotic factors such as light, temperature, water, soil, pH, salinity and dissolved oxygen, modified by biotic interactions (competition, predation, symbiosis, disease) so that its realised niche is usually a subset of the fundamental niche it could occupy in isolation.