14.1 Essential ideas

14.1.1 Species and communities

  • Community structure is based on the principle that each species occupies a unique position in an ecosystem, called a niche.
  • Individual species are adapted to living within a certain range of tolerance for different abiotic conditions. Outside of this range, species distribution is limited.
  • A zone of stress and limits of tolerance graph is a model showing how an environmental factor affects the distribution of species.

Zones of tolerance

Figure 14.1.1a – Zones of tolerance
A general pattern showing how limiting factors affect the distribution of species

Limiting factors

The factors that affect the distribution of species are called limiting factors. Some limiting factors are:

Related to water quality:
  • pH
  • oxygen concentration
  • salinity
  • temperature
Related to air quality:
  • temperature
  • humidity
Related to soil quality:
  • mineral and nutrient content
  • salinity
  • pH
  • depth
Related to habitat:
  • number of nesting or breeding sights
  • availability of territories
  • food/prey availability



Think of other examples of how each of the limiting factors listed in the table affects the distribution of a number of different species. Consider adaptations. Share your thoughts with the class.

Interspecific interactions are classified by their effect

  • Each species plays a unique role within a community because of the combination of spatial habitat and interactions with other species.
  • Interactions between species are classified by their effects, as shown in the table below:
Interaction Overall effect Effect on species 1 Effect on species 2 Example
Commensalism One organism benefits while the other is unharmed + Neutral A spider builds a web on a tree
Mutualism Both organisms benefit + + Bacteria living in the gut of cows digest cellulose and provide nutrients
Herbivory A producer is ingested by a primary consumer +

Goats graze on grasses in a field
Predation One consumer hunts and consumes another +

Lions hunt and kill gazelle on the savannah
Parasitism A parasite takes nutrients and resources from a host +

Fleas survive by sucking blood from the skin of host mammals

Niche concepts and the competitive exclusion principle

Each species uses a range of resources, occupies a specific habitat and has a unique life cycle that is not identical with any other species. This unique combination of requirements is called a niche.

  • Many species are capable of living in very similar niches, but no two species can occupy an identical niche indefinitely.
  • Species generally have a broad fundamental niche. The fundamental niche is the full range of conditions that a species would occupy if there was no competition from other species.
  • In reality, most species occupy a narrow realised niche. This is because if two species occupy the same niche in an ecosystem, they will compete for resources, and exclude each other from certain areas of their own range. This is known as the competitive exclusion principle.

competitive exclusion

Figure 14.1.1b – Competitive exclusion
On the x axis of the diagram above is a gradient for an environmental variable. Each colour represents a species that has been limited from its fundamental niche (dotted lines) to its realised niche (solid lines) by competitive exclusion.

Keystone species

Every species plays a role in maintaining the structure of an ecosystem. However, some species play a more critical role than others. These are called keystone species. The removal of a keystone species may have a disproportionate effect on the functioning of the ecosystem, especially in terms of stability or productivity or biodiversity.

For each community, there is only one keystone species, but the keystone species does not always occupy the same trophic level. The keystone species could be:

  • A small predator that prevents a herbivorous animal from eliminating a dominant plant species. For example, sea kelp, a producer, provides shelter and food for many species and would be devoured if populations of sea urchins were not controlled by predation by sea otters.
  • A larger predator that controls populations at lower trophic levels from the top down. An example is the gray wolf, which occupies the highest trophic level.
  • An ecosystem engineer – these animals manipulate the habitat to a degree that makes only certain types of ecosystem possible. Beavers are an example. By building dams, beavers change fast-moving river ecosystems into pools of still water, where different species thrive. Beavers are primary consumers.

Key concept

Community structure is an emergent property of an ecosystem.

Nature of Science

Zones of stress and limits of tolerance graphs are models of the real world that have predictive power and explain community structure. This model takes into account only one limiting factor. How could you represent a multi-dimensional niche that takes into account many limiting factors?

buttress roots

Figure 14.1.1c – Species adaptation
Buttress roots are shallow enough to gain nutrients from rainforest soils and provide stability for tall trees. They can spread up to 30m away from the tree trunk.

giant panda

Figure 14.1.1d – Availability of food is a limiting factor
The endangered giant panda, Ailuropoda melanoleuca, has a diet consisting of 99% bamboo leaves. The distribution of both giant and red pandas is decreasing as humans encroach on bamboo forests.

International mindedness

Billions of disposable bamboo chopsticks end up in trash bins every year. Isn’t there a more sustainable way to enjoy your take-aways? Do your bit by using your own chopsticks in restaurants.

cave snail

Figure 14.1.1e – Zones of tolerance
This cave snail, Zospeum tholussum, has a water-permeable translucent shell.  Within the cave, humidity is 95–100%. The snail could not survive outside the cave, where humidity is much lower.

Language tool

Symbiosis is the general term used to describe relationships between species. Most people think symbiosis means living together in a mutually beneficial relationship, but it actually means ‘living together’. Some types of symbiotic relationship are described below.

Language tool


Figure 14.1.1f – Keystone
A keystone is the stone at the top of an arch that holds the structure of the arch together.

sea otter

Figure 14.1.1g – Small predator
Sea otters are a keystone species, although they are not at the top of the food chain.

grey wolf

Figure 14.1.1h – Large predator
The gray wolf is a keystone species at the top of the food chain.


Figure 14.1.1i – Ecosystem engineer
Beavers and other ecosystem engineers such as elephants are examples of keystone species.