What Shapes a Community
Why are there thousands of species packed into a rainforest, but only a handful scattered across a salt lake? Why does one barnacle live high on a rock and another low down, never overlapping? The answer is a constant tug-of-war between the non-living conditions and the living organisms, and together these forces decide how many of each species there are, where they live, and how varied the community becomes.
Orient yourself
Make a prediction, scan the goals and the key terms, and see which misconception this lesson is built to fix.
Before You Read
warm-upThink of two places you know: a lush, shady forest and an exposed, dry hilltop nearby.
Before reading: the forest has far more species than the hilltop. List one abiotic (non-living) and one biotic (living) reason this might be so, and define what "abundance", "distribution" and "diversity" each mean to you.
Learning Intentions
goalsKnow
- What abundance, distribution and diversity mean
- Examples of abiotic and biotic factors that influence each
- The idea of a limiting factor and tolerance range
Understand
- How abiotic conditions set where and how many organisms live
- How biotic interactions (competition, predation) reshape the community
- Why both kinds of factor act together
Can Do
- Explain how a given factor influences abundance, distribution or diversity
- Classify a factor as abiotic or biotic
- Use examples to explain a pattern in a community
Scan these before reading
vocabMatch each term to its meaning: use the Key Terms card above.
- Abundance
- Distribution
- Diversity
- Limiting factor
- Tolerance range
- The range of a factor within which a species can survive
- Where in an ecosystem a species is found, and its spatial pattern
- The factor in shortest supply, which restricts a population's size or range
- How many individuals of a species are present in an area
- The variety of different species present in an ecosystem
Misconceptions To Fix
watch outAbundance, Distribution & Diversity
The three things we are trying to explain
Core Content
Abundance, Distribution & Diversity
+5 XPThe three things we are trying to explain
When ecologists describe a community, they focus on three measures, how many, where, and how varied, and all three are shaped by abiotic and biotic factors.
Abundance = how many individuals of a species are present. Distribution = where the species is found (its spatial pattern). Diversity = the variety of different species in the ecosystem. A factor can influence one or all three: for example, low water availability can reduce a plant's abundance, restrict its distribution to wetter patches, and lower the overall diversity of the ecosystem.
Factors fall into two groups, both important: abiotic (non-living: temperature, light, water, pH, salinity, nutrients, oxygen) and biotic (living: competition, predation, disease, food supply, symbiosis).
Pause, copy the highlighted definitions of abundance, distribution and diversity into your book before moving on.
The variety of different species present in an ecosystem is called its _____.
How Abiotic Factors Shape Communities
Tolerance ranges and limiting factors
How Abiotic Factors Shape Communities
+5 XPTolerance ranges and limiting factors
We just defined the three measures. That raises a question: how does a non-living factor actually change them? This card answers it → through tolerance ranges and limiting factors.
Every species can survive only within a tolerance range of each abiotic factor, and the factor in shortest supply limits where and how many can live.
Each species has a tolerance range for factors like temperature, salinity or pH; inside it the species thrives, near the edges it is stressed and rarer, beyond it the species is absent. This sets distribution, for example, on a rocky shore, species are zoned up the beach by how much drying-out (an abiotic factor) they can tolerate. The factor in shortest supply or at an extreme is the limiting factor that caps abundance: a desert's lack of water limits how many plants grow. And harsh or extreme abiotic conditions usually lower diversity, because few species can tolerate them, which is why a salt lake or a hot spring has so few species while a stable rainforest has many.
Text description
- Abundance (up) against an abiotic factor such as temperature (across).
- Optimum (centre): the species is most abundant.
- Too low or too high (either edge): the species is absent.
Abiotic factors set distribution (each species lives within its tolerance range, e.g. rocky-shore zonation by drying-out), cap abundance (the limiting factor, e.g. water in a desert), and influence diversity (harsh/extreme conditions = few tolerant species = low diversity; stable, favourable conditions = high diversity).
Pause, copy the tolerance-range/limiting-factor idea and one example into your book.
The rocky shore is the standard worked example, and it was settled by an experiment. On a rocky shore, species occupy distinct horizontal bands, and it is tempting to assume each band is simply the stretch of shore whose drying-out that species can tolerate. Joseph Connell tested this on Scottish shores by removing one barnacle species and watching what the other did (Connell, Ecology, 1961). The result splits the two kinds of factor cleanly: the upper limit of the lower-shore species was set by how much drying-out it could survive, an abiotic tolerance limit, while the lower limit of the upper-shore species was set by being crowded out by its competitor, a biotic limit that vanished as soon as the competitor was removed. Zonation is therefore produced by abiotic and biotic factors acting on different edges of the same band, which is exactly the point this dot point is making.
A desert supports few plants mainly because water is scarce. In this case, water is acting as a:
Apply it: Classify and Explain the Factors
Work the activity in your own book before you read on. This is the first point where you are on your own.
Activity 1
Classify and Explain the Factors
Pattern A, Classify, then justify
For each factor, state whether it is abiotic or biotic and which measure it most affects (abundance, distribution or diversity): (a) a sudden frost; (b) a new predator arriving; (c) a region with rich, deep soil; (d) two bird species competing for the same nest holes. Give one line each explaining the effect.
How Biotic Factors Shape Communities
Competition, predation and disease reshape the picture
How Biotic Factors Shape Communities
+5 XPCompetition, predation and disease reshape the picture
We just saw the abiotic forces. That raises a question: what role do the living organisms themselves play? This card answers it → biotic interactions can override the physical conditions.
Living organisms compete, eat one another and spread disease, and these interactions strongly shape who lives where, in what numbers, and how varied the community is.
Competition for the same resource (food, space, light) can reduce the abundance of the weaker competitor or exclude it from part of its range, on rocky shores, one barnacle species is out-competed for the lower zone, restricting its distribution. Predation controls prey abundance, and food supply limits predator abundance. Surprisingly, predators can raise diversity: by keeping a dominant prey species in check, a predator stops it monopolising the space and lets other species persist (you will meet this as the keystone idea in Year 12). Disease and parasites can reduce a host species' abundance and distribution.
So the full explanation always combines both: abiotic factors set the physical possibilities, and biotic factors fine-tune (and sometimes override) which species actually fill them. A species can be physically capable of living somewhere yet be absent because a competitor, predator or disease keeps it out.
Biotic factors: competition can reduce abundance or restrict distribution of the weaker species; predation controls prey numbers and can even raise diversity by stopping one prey dominating; disease/parasites lower host abundance. Abiotic factors set the physical possibilities; biotic factors decide which species fill them, both act together.
Add one biotic factor and its effect, plus the "both act together" point, to your notes before the check below.
A species may be physically able to live in an area yet be excluded by a competitor or predator.
Only abiotic factors influence the abundance and distribution of species; biotic factors do not.
A predator can increase species diversity by stopping one prey species dominating all the space.
See where it lands in the real world
Why this matters: managing land and pests
Why this matters: managing land and pests
Knowing how abiotic and biotic factors shape a community is the basis of land and pest management. Restoring an abiotic factor, re-watering a wetland, reducing soil salinity, can bring species back, while managing a biotic factor, removing an invasive predator or competitor, can let native species recover. In Australia, programs that control foxes and cats (biotic factors) have allowed threatened native mammals to increase in both abundance and distribution, showing that the two kinds of factor must be considered together to protect biodiversity.
Odd one out: three of these follow from the card above about land and pest management. Which one does not?
Explain what is meant by a species' tolerance range, and explain how it determines where that species is found within an ecosystem.
Explain means giving cause and effect. Here the cause is a property of the organism and the effect is a pattern on the ground, so the answer must travel from one to the other rather than defining both separately.
5 marks. Define the tolerance range and its zones, connect it to the idea of a limiting factor, then derive the distribution pattern, then note what complicates the picture.
For any abiotic factor, a species can survive only between an upper and a lower limit, and that span is its tolerance range for that factor. Within the range there is a narrower optimum zone where the species grows and reproduces best. Towards either end lie zones of physiological stress, where individuals survive but grow poorly and may not reproduce, and beyond the limits the species cannot survive at all.
A species has a separate tolerance range for every factor that affects it, including temperature, water availability, light, pH, salinity and dissolved oxygen. It can only live where conditions fall within all of those ranges at once, so its presence is governed by whichever factor is furthest from its optimum at that place. That factor is the limiting factor, and it can differ from place to place for the same species.
Because conditions vary across an ecosystem, the region where every factor lies inside the species' tolerance ranges is only part of it, and that region is where the species is found. Where conditions are near the optimum the species is abundant; towards the edges of its range it becomes progressively scarcer as more individuals fall into the zones of stress; and beyond the limits it is absent. This is why distribution and abundance are related rather than separate: the same gradient that sets the boundary also shapes the numbers inside it.
Different species have different tolerance ranges, so a place with conditions inside many species' ranges supports more species than one whose conditions are extreme or highly variable. This is why diversity is generally lower in harsh or fluctuating environments: few species have tolerance ranges wide enough to cover the whole swing, so only specialists persist.
Tolerance ranges set where a species could live, not always where it does. Biotic factors narrow the outcome further: a competitor may exclude a species from part of its tolerable range, a predator or disease may prevent it establishing, and the absence of a required mutualist or food species may exclude it from conditions that otherwise suit it perfectly. The observed distribution is therefore usually smaller than the tolerance ranges alone would predict, which is why abiotic and biotic factors have to be considered together.
For any abiotic factor a species can survive only between an upper and a lower limit, and that span is its tolerance range for that factor; within it lies a narrower optimum zone where the species grows and reproduces best, towards either end lie zones of physiological stress where individuals survive but grow poorly and may not reproduce, and beyond the limits the species cannot survive. A species has a separate tolerance range for every factor affecting it, including temperature, water, light, pH, salinity and dissolved oxygen, and can live only where conditions fall inside all of them at once, so its presence is governed by whichever factor lies furthest from its optimum at that place, which is the limiting factor and may differ from place to place for the same species. Because conditions vary across an ecosystem, the region where every factor lies within the species' ranges is only part of it, and that region is where the species occurs: near the optimum it is abundant, towards the edges of its range it becomes progressively scarcer as more individuals fall into the zones of stress, and beyond the limits it is absent, which is why distribution and abundance are related rather than separate, the same gradient setting both the boundary and the numbers within it. Different species have different tolerance ranges, so a place whose conditions lie inside many species' ranges supports more species than one that is extreme or highly variable, which is why diversity is generally lower in harsh or fluctuating environments, few species having ranges wide enough to span the whole swing. Tolerance ranges nevertheless set where a species could live rather than always where it does: a competitor may exclude it from part of its tolerable range, a predator or disease may prevent it establishing, and the absence of a required mutualist or food species may exclude it from conditions that otherwise suit it, so the observed distribution is usually smaller than the tolerance ranges alone would predict, and abiotic and biotic factors must be considered together.
Distinguish between abundance, distribution and diversity, and explain how a single change in an ecosystem could alter one of them without altering the others.
Distinguish asks for the differences between the three. The second clause then tests whether you really have separated them, because you can only produce such an example if the three are genuinely independent measures.
4 marks. Define each with what it measures and what it is measured in, then give one worked example of a change that moves exactly one.
Abundance is how many individuals of a species are present, counted as a number or as a density such as individuals per square metre. Distribution is where those individuals occur within the ecosystem, described as the area occupied and the pattern within it, such as clumped, uniform or random. Diversity is how many different species are present, and in its fuller sense also how evenly individuals are spread among those species.
The three can move independently because they measure different things. A species can be abundant in a small area or sparse across a large one, so abundance and distribution are not the same. A community can contain many individuals of very few species, or few individuals of many, so abundance and diversity are not the same either.
Suppose a disease affects one abundant grass species in a paddock and reduces its numbers by half, without eliminating it anywhere it currently grows. Abundance of that species falls substantially. Distribution is unchanged, because the grass still occupies the same area, just less densely within it. Diversity is unchanged, because no species has been lost or gained, only the numbers of one of them.
Contrast that with a change that moves all three. If the same disease eliminated the grass entirely, abundance would fall to zero, distribution would disappear, and diversity would fall by one species. And if a new patch of deep, nutrient-rich soil allowed a species to spread into ground it had not occupied, its distribution would expand and its abundance would rise while diversity stayed unchanged, since the species was already present in the ecosystem. The three therefore describe different properties of a community and should be reported separately, since a statement that a species "declined" does not say which of them has changed.
Abundance is how many individuals of a species are present, counted as a number or a density such as individuals per square metre; distribution is where those individuals occur within the ecosystem, described as the area occupied and the pattern within it, whether clumped, uniform or random; and diversity is how many different species are present, and in its fuller sense how evenly individuals are spread among them. The three can move independently because they measure different things: a species may be abundant in a small area or sparse across a large one, so abundance and distribution differ, and a community may contain many individuals of very few species or few individuals of many, so abundance and diversity differ. An example that moves only one is a disease affecting an abundant grass in a paddock and halving its numbers without eliminating it anywhere it currently grows: abundance of that species falls substantially, distribution is unchanged because the grass still occupies the same area but less densely, and diversity is unchanged because no species has been lost or gained. By contrast, if the disease eliminated the grass entirely, abundance would fall to zero, distribution would disappear and diversity would fall by one species; and if a patch of deep, nutrient-rich soil allowed a species to spread into ground it had not occupied, distribution would expand and abundance rise while diversity stayed the same, the species already being present. The three therefore describe different properties of a community and should be reported separately, since an unqualified statement that a species has declined does not say which of them has changed.
Practise independently
Answer every question in your own words first. The model answers are for checking against, not for copying.
Multiple Choice
+5 XPA fresh set drawn from this lesson's question bank, feedback shown immediately. +5 XP per correct · +25 XP all correct
Pick your answer, then rate your confidence, that tells the system what to drill next.
Short Answer, 13 marks
+5 XPUnderstandBand 5(6 marks) 1. Explain, using examples, how both abiotic and biotic factors influence the abundance, distribution and diversity of species in an ecosystem.
1 mark: abundance/distribution/diversity correctly used · 2 marks: an abiotic factor linked to a measure with an example · 2 marks: a biotic factor linked to a measure with an example · 1 mark: states that both kinds of factor act together
ApplyBand 4(3 marks) 2. Classify each of the following as abiotic or biotic, and state which of abundance, distribution or diversity it most directly affects: a sudden frost; the arrival of a new predator; a band of deep, nutrient-rich soil across part of a paddock.
1 mark: all three correctly classified as abiotic, biotic and abiotic · 1 mark: a defensible measure named for each · 1 mark: at least two of the three linked with a one-line mechanism rather than an assertion
EvaluateBand 5-6(5 marks) 3. On a rocky shore, removing one barnacle species allowed a second species to extend further down the shore than it had ever been found, while the upper limit of the first species did not change. Evaluate what this experiment shows about how abiotic and biotic factors set the distribution of a species.
1 mark: identifies the extension downward after removal as evidence that a biotic factor, competition, was setting that lower limit · 1 mark: identifies the unchanged upper limit as evidence that an abiotic factor, drying-out, sets that edge · 1 mark: recognises that a species can be physically capable of living somewhere and still be excluded · 1 mark: notes what the removal design contributes, that it distinguishes cause from correlation in a way observation alone cannot · 1 mark: reaches a judgement that both kinds of factor act, on different edges of the same distribution
Show all answers
Multiple choice
MC answers and full explanations are shown inline as you complete each question. Use the retry button to attempt a fresh set from the lesson bank.
Short Answer Model Answer
Q1 (6 marks): Both abiotic and biotic factors shape a community's abundance (how many), distribution (where) and diversity (variety of species). Abiotic example: water availability is often a limiting factor, in a desert, the scarcity of water caps the abundance of plants, restricts their distribution to wetter patches such as creek lines, and lowers overall diversity because only drought-tolerant species can survive; more broadly, each species lives only within its tolerance range of factors like temperature or salinity, which is why species are zoned up a rocky shore by how much drying-out they can withstand. Biotic example: living interactions are equally important, competition for a shared resource can reduce the abundance of the weaker species or exclude it from part of its range (e.g. one barnacle out-competed from the lower shore), while predation controls prey abundance and can actually raise diversity by preventing a single dominant prey species from monopolising the space and crowding out others. Crucially, the two act together: abiotic conditions set the physical possibilities for an ecosystem, and biotic interactions then determine which species actually fill them, a species may be physically capable of living somewhere yet be absent because of competition, predation or disease.
Q2 (3 marks): A sudden frost is abiotic and most directly affects abundance, because individuals outside their tolerance range for low temperature die and the population falls [1]. The arrival of a new predator is biotic and most directly affects abundance as well, since predation removes individuals, though it can also restrict distribution if the prey survives only where cover is available. A band of deep, nutrient-rich soil is abiotic and most directly affects distribution, because plants requiring those nutrients are found in the band and not outside it [1]. The mechanisms matter as much as the labels: the frost acts by exceeding a tolerance limit, the predator by increasing the death rate, and the soil by supplying a resource that is limiting elsewhere in the paddock [1].
Q3 (5 marks): The downward extension is the key result. If the second species could not previously be found low on the shore but spreads there once the first is removed, then nothing physical was stopping it: the limit was being set by competition, a biotic factor, and removing the competitor removed the limit [1]. The unchanged upper limit points the other way. Its upper edge did not move when the competitor was taken away, so it was not being set by anything the competitor did; it is set by an abiotic factor, how much drying-out and heating the species can survive when the tide is out, which is a tolerance limit no removal can change [1]. Together these show that a species can be physically capable of living in a place and still be absent from it, so the range where an organism is actually found is narrower than the range where it could survive [1]. The design is what makes this conclusive. Observing zonation alone shows only that the bands correlate with height on the shore, and correlation is consistent with either explanation; removing one species and watching the other respond manipulates the suspected cause and lets the two be told apart, which observation cannot do [1]. My judgement is that the experiment shows abiotic and biotic factors are not competing explanations for distribution but act on different edges of the same distribution, with the physical tolerance limit setting one boundary and the biotic interaction setting the other. Any explanation of where a species is found needs both [1].
Retrieve and reflect
Do the spaced-review set from memory first, then look back at the prediction you made before you read anything.
Check what actually stuck
A short spaced-review set drawn from this lesson and the lessons before it. Answer from memory first, that is what makes it stick.
Lock it in
bossYou can now explain how abiotic and biotic factors together influence abundance, distribution and diversity. Next lesson gives you the tools to measure abundance and distribution properly, comparing sampling techniques in a field practical.
↩ Back to focus-area hubHow did your thinking change?
You gave reasons the forest has more species than the hilltop. Abiotically, the forest has more reliable water, richer soil and a milder, more stable temperature, so more species fall within their tolerance ranges and diversity is high. Biotically, the forest's many niches allow many species to coexist, while interactions like predation can keep any one species from dominating. Abundance, distribution and diversity are all set by these abiotic and biotic forces acting together.
The surprising hook, losing a predator reducing diversity, fits this: predation can hold a dominant prey species in check, so removing the predator lets it take over and crowd others out, lowering diversity.