Species in a community do not obtain resources in isolation. Their interactions determine who gains access to food, nutrients, light, and space, shaping survival, reproduction, and how energy and matter move through the community.

Core idea: interactions regulate access to resources

Energy and matter are acquired through shared pathways

Organisms require energy (to fuel cellular work) and matter (atoms for biomass). In communities, those requirements are met through overlapping resource pools (prey, plant biomass, detritus, dissolved nutrients, light, nesting sites). Because multiple populations depend on the same pools, interactions among them strongly affect resource access.

Interactions shape the “used” portion of a niche

A species’ potential resource use is often narrowed by other species’ presence, because access depends on whether resources are defended, depleted, or made easier to obtain.

Simplified niche diagram showing how biotic interactions can contract each species’ fundamental niche into a smaller realized niche. The overlap region highlights where competition occurs, illustrating how interactions restrict which resources are actually accessible in nature. Source

How interactions change access to energy and matter

Resource depletion and resource defence

When one population consumes or sequesters a shared resource, less remains for others.

Zero-growth isoclines from a Lotka–Volterra competition model, showing how each species’ population growth depends on both its own density and the density of a competitor. The diagram illustrates that competition shifts the conditions under which growth is positive vs. negative, linking reduced resource access to reduced population growth. Source

Alternatively, organisms may defend resources (territories, host tissues, shelter sites), reducing access even when the resource is not fully depleted. These interactions can:

• Reduce growth rates by lowering energy intake

• Lower reproductive output by limiting limiting nutrients (often nitrogen or phosphorus)

• Shift activity to less profitable resources, decreasing net energy gain

Consumer–resource links determine who gets energy first

Communities are built from feeding relationships: the ability of one population to capture another (or its parts) controls energy acquisition and transfers matter between bodies and wastes. Outcomes depend on traits affecting access:

• Detection (sensory abilities, timing of activity)

• Capture/handling (speed, morphology, toxins)

• Avoidance/defence (armor, camouflage, deterrent chemicals)

Because consumer pressure can reduce a resource population, other populations that depend on the same resource may experience reduced access even without directly interacting.

Facilitation can increase access to limiting materials

Not all interactions restrict resources. Some species increase resource availability to others by altering conditions or providing access routes, for example:

• Making nutrients more available through waste products that others can assimilate

• Increasing access to food by concentrating prey/food items in predictable places

• Creating physical conditions (shade, moisture retention) that reduce stress and improve resource uptake efficiency

These effects can increase community-level productivity by allowing organisms to acquire energy or matter that would otherwise be inaccessible.

Indirect effects and feedbacks in resource access

Interactions often propagate beyond two species. A change in one population can alter the abundance or behaviour of another, which then changes a third population’s access to energy and matter.

Three-level trophic cascade schematic showing how predators can indirectly increase primary producers by suppressing herbivores. This diagram is a clear example of an indirect interaction pathway that reshapes who gains access to energy-rich biomass across trophic levels. Source

Indirect pathways commonly arise through:

• Shared resources (one population reduces a resource, indirectly limiting another)

• Shared consumers (one population increases a consumer, indirectly reducing another)

• Behavioural shifts (avoiding a consumer can force organisms into lower-quality habitats or diets)

What to look for in data and explanations

Signals that interactions are controlling access

When asked to interpret community data, evidence that interactions are determining resource access often includes:

• Resource levels declining as one population increases

• Reduced body condition, growth, or fecundity when another population is present

• Spatial shifts in habitat use coinciding with another species’ distribution

• Diet shifts indicating altered access to preferred energy sources

These patterns connect directly to the syllabus focus: interactions among populations determine how they access energy and matter within a community.