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IB DP Biology Study Notes

4.3.1 Interactions Between Species

Interactions between species form the foundation of ecological communities, influencing their structure, function, and diversity. Key interactions include competition, predation, and various forms of symbiosis. This comprehensive overview will detail these interactions and their multifaceted impacts on community structure. Additionally, understanding the roles of autotrophic and heterotrophic nutrition in species is crucial in exploring how these interactions are facilitated.


Competition is a fundamental ecological interaction, influencing community structure by affecting species distribution, abundance, and diversity.

Intraspecific Competition

  • Definition: Competition among members of the same species.
  • Effect on Population Dynamics: Can cause density-dependent regulation, influencing population size.
  • Effect on Behaviour: May lead to territoriality and increased aggression within a species.

Interspecific Competition

  • Definition: Competition between members of different species.
  • Outcomes: Can lead to competitive exclusion, niche differentiation, or coexistence. Understanding population dynamics can provide further insight into how these outcomes manifest.
  • Examples: The famous Gause's experiment with Paramecium species.

Competitive Exclusion Principle

  • Statement: No two species can occupy the same ecological niche in the same environment for a prolonged time.
  • Implications: Leads to differentiation in resource use to reduce direct competition.
  • Challenges: In some cases, species do coexist despite sharing resources, indicating other factors at play.


Predation shapes community structure by influencing population sizes and the evolution of various adaptations.

Types of Predation


  • Definition: Predators that consume animal flesh.
  • Impact on Community: Can control prey populations and indirectly affect other species, similar to the roles played by consumers, detritivores, and saprotrophs.


  • Definition: Animals consuming plant materials.
  • Impact on Plant Communities: Affects plant growth, reproduction, and diversity.


  • Definition: Organisms that live on or within a host, causing harm.
  • Impact on Host Populations: Can influence host health, reproduction, and survival.

Predator-Prey Dynamics

  • Lotka-Volterra Equations: Mathematical models describing the cyclic nature of predator-prey relationships.
  • Adaptations: Include predator strategies to locate, capture, and consume prey, and prey mechanisms to avoid detection or capture.


Symbiosis encompasses various close, long-term interactions between different species, ranging from mutually beneficial to harmful.


  • Definition: Both species benefit.
  • Types: Obligatory, where species cannot survive without the other; and Facultative, where mutualism is beneficial but not essential.
  • Examples: Mycorrhizal fungi and plants.


  • Definition: One species benefits; the other is neither harmed nor helped.
  • Challenges: Sometimes difficult to categorize, as unnoticed harm or benefit may exist.
  • Examples: Barnacles on whales.


  • Definition: One species benefits at the host's expense.
  • Types: Ectoparasites live outside the host, while endoparasites live inside.
  • Impact: May lead to complex host-parasite dynamics and coevolution.

Impact on Community Structure

Species interactions significantly influence community structure and dynamics, including the energy flow in ecosystems, which is a critical component of these dynamics.

Stability and Resilience

  • Role: Interactions contribute to community stability, influencing how communities respond to disturbances.
  • Examples: Recovery after natural disasters.

Trophic Structure

  • Definition: Interactions define the feeding relationships within an ecosystem.
  • Effect: Influence energy flow, nutrient cycling, and the overall functioning of ecosystems.


  • Role: Interactions may increase or decrease species diversity.
  • Mechanisms: Include predation reducing competitive exclusion or competition driving species out.

Keystone Species

  • Definition: Species with disproportionately large effects on their environment.
  • Impact: Maintain community structure by controlling other species' populations.
  • Examples: Sea otters in kelp forest ecosystems. The concept of species and reproductive isolation further explains the importance of keystone species in maintaining biodiversity.


Commensalism is a type of interaction where one species benefits and the other is neither helped nor harmed. An example is the relationship between barnacles and whales, where barnacles attach to the whale's skin and gain access to different feeding areas as the whale moves. Mutualism, on the other hand, is a relationship where both species benefit, such as the relationship between bees and flowering plants. The key difference between commensalism and mutualism is that in mutualism, both species benefit, while in commensalism, one species benefits without affecting the other.

Keystone species are species that have a disproportionate impact on the community structure relative to their abundance or biomass. They often play a crucial role in maintaining biodiversity and ecosystem function. Removal of a keystone species can lead to a dramatic shift in community composition. For example, sea otters in kelp forest ecosystems feed on sea urchins, controlling their population. Without otters, sea urchins can overgraze kelp, leading to a shift in the ecosystem.

Predators play a vital role in shaping community structure by controlling the population sizes of prey species. Through predation, they can prevent a single species from dominating an ecosystem, allowing for greater biodiversity. Predators can also indirectly affect the populations of other species through trophic cascades, where changes in one population cascade down through the ecosystem, influencing other levels.

No, two species cannot have exactly the same ecological niche indefinitely. According to the Competitive Exclusion Principle, if two species are competing for exactly the same resources in the same way, one species will eventually outcompete the other, leading to the latter's decline or extinction in that specific environment. The competing species will either adapt to exploit different resources (niche differentiation) or will be excluded from the ecosystem.

Intraspecific competition is the competition between individuals of the same species for resources like food and space. It can lead to increased variability within the species, as individuals compete for niche space. Interspecific competition, on the other hand, occurs between members of different species and often results in one species dominating the other in a particular ecosystem. Intraspecific competition drives adaptations within a species, while interspecific competition may influence the distribution and abundance of species.

Practice Questions

Define interspecific competition and explain its potential outcomes in the context of the Competitive Exclusion Principle. Use an example to illustrate your answer.

Interspecific competition refers to the competition between members of different species for the same resources in an ecosystem. This can lead to three potential outcomes: competitive exclusion, where one species outcompetes the other, leading to the latter's extinction in that environment; niche differentiation, where species evolve to utilise different resources, reducing direct competition; or coexistence, where both species continue to live together with some competition. The Competitive Exclusion Principle asserts that no two species can occupy the exact same ecological niche indefinitely; one will always outcompete the other. An example is Gause's experiment with Paramecium, where one species outcompeted the other in a controlled environment.

Describe the concept of mutualism in symbiosis, and differentiate between obligatory and facultative mutualism. Provide examples for each.

Mutualism is a form of symbiosis where both species involved benefit from the interaction. In obligatory mutualism, the relationship is essential for the survival of at least one of the species involved. An example is the relationship between certain termites and the gut bacteria that help them digest cellulose; neither can survive without the other. In facultative mutualism, the relationship is beneficial to both parties but not essential for survival. An example would be the interaction between bees and flowering plants; bees gain nourishment from the nectar, and the plants benefit from pollination, but neither species is entirely dependent on this relationship for survival.

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Written by: Dr Shubhi Khandelwal
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