Competition Between Two Species Occurs When

The struggle for survival intensifies when two species find themselves vying for the same limited resources, highlighting the core of competition between two species. This interaction, a cornerstone of ecological dynamics, shapes the distribution, abundance, and even the evolution of the involved species. Understanding the nuances of this competition is vital for comprehending the intricate web of life that sustains our planet.

Defining Interspecific Competition

Interspecific competition occurs when individuals of different species compete for the same resources in an ecosystem. These resources can include:

• Food: Both plants and animals require specific nutrients and energy sources to survive.
• Water: Essential for all life processes, water scarcity can trigger intense competition.
• Light: Crucial for plants in performing photosynthesis, and indirectly impacting species that rely on plant life.
• Space: Territory for nesting, breeding, or simply growing can be a limiting factor.
• Nutrients: Particularly important in plant communities where soil nutrients are essential for growth.
• Nesting sites: Cavities, burrows, or specific locations for laying eggs or raising young.

Competition isn't always a straightforward, head-to-head battle. It manifests in various forms, each with distinct characteristics and impacts on the competing species.

Types of Interspecific Competition

Competition manifests in two primary forms: interference competition and exploitative competition. Recognizing these different mechanisms helps understand the nuanced ways species interact and influence each other's survival.

Interference Competition

This involves direct interaction between species, where one species actively prevents another from accessing resources. This can include:

• Aggression: Physical confrontations, like one bird species chasing another away from a feeding site.
• Territoriality: Defending a specific area to exclude other species.
• Chemical Inhibition: Some plants release chemicals into the soil that inhibit the growth of other plants, a phenomenon known as allelopathy.
• Predation of Competitors: One species might prey on a competing species, directly reducing its population size.

Exploitative Competition

This is more indirect, occurring when species compete for the same resources without directly interacting. The species that is more efficient at utilizing the resource depletes it, leaving less for the other species. Examples include:

• Two plant species competing for limited nitrogen in the soil. The species that can absorb nitrogen more quickly will thrive, while the other may struggle.
• Different insect species feeding on the same plant. The species that consumes the most foliage will reduce the food available for the others.

The Competitive Exclusion Principle

A foundational concept in ecology, the Competitive Exclusion Principle states that two species competing for the exact same limited resource cannot coexist indefinitely. The species that is even slightly more efficient at utilizing the resource will eventually outcompete and eliminate the other.

Gause's Experiments: This principle is famously illustrated by the experiments of Georgy Gause in the 1930s. He studied competition between different species of Paramecium in laboratory cultures.

• When grown separately, each species thrived.
• However, when grown together and competing for the same food source, one species consistently drove the other to extinction.

Real-World Complexity: While the Competitive Exclusion Principle is a powerful theoretical tool, its application in natural ecosystems is often more complex. Factors like environmental variability, resource partitioning, and predation can allow species to coexist even when they share similar resource needs.

Factors Influencing the Intensity of Competition

Several factors can amplify or diminish the intensity of interspecific competition:

• Resource Availability: Competition intensifies when resources are scarce. In times of abundance, the effects of competition may be less pronounced.
• Environmental Conditions: Climate, temperature, and other environmental factors can favor one species over another, influencing the outcome of competition.
• Population Density: Higher population densities increase the demand for resources, leading to greater competition.
• Niche Overlap: The more similar the ecological niches of two species, the greater the competition between them.
• Evolutionary History: Species that have co-evolved in the same environment may have developed mechanisms to reduce competition, such as resource partitioning.

Resource Partitioning: A Strategy for Coexistence

Resource partitioning is an evolutionary process where species adapt to utilize resources in different ways, reducing direct competition and allowing them to coexist. This can involve:

• Dietary Specialization: Different species consuming different types or sizes of food within the same habitat. For example, different species of warblers in a forest may feed on insects in different parts of the trees.
• Spatial Partitioning: Species utilizing different areas within the same habitat. For example, different species of fish in a lake may occupy different depths or areas with different vegetation.
• Temporal Partitioning: Species being active at different times of the day or year. For example, some plant species may flower earlier in the spring than others, reducing competition for pollinators.

The Ecological Niche: Defining a Species' Role

The ecological niche encompasses all the factors that influence a species' survival and reproduction, including its resource requirements, interactions with other species, and tolerance to environmental conditions. It essentially defines the species' "role" in the ecosystem.

• Fundamental Niche: The full range of environmental conditions and resources a species could potentially occupy and use, if there were no competition from other species.
• Realized Niche: The actual range of conditions and resources a species occupies and uses, taking into account the limitations imposed by competition, predation, and other factors.

Competition often forces species to occupy a realized niche that is smaller than their fundamental niche. This is because the presence of competitors restricts their access to resources and habitats.

The Impact of Competition on Species Distribution and Abundance

Competition is a major driver of species distribution and abundance patterns. It can:

• Limit Distribution: Competition can prevent a species from occupying certain areas, even if those areas are otherwise suitable.
• Reduce Abundance: Competition can reduce the population size of a species, particularly if it is a weaker competitor.
• Alter Community Structure: The outcome of competition can influence the composition and diversity of ecological communities.

Competition and Evolution

Competition can act as a selective force, driving evolutionary changes in the competing species. This can lead to:

• Character Displacement: The evolution of differences in traits between competing species, reducing niche overlap and competition. For example, Darwin's finches on the Galapagos Islands evolved different beak sizes and shapes to exploit different food sources.
• Evolution of Competitive Abilities: Species may evolve traits that enhance their ability to compete for resources, such as increased foraging efficiency or greater aggression.

The Role of Competition in Invasive Species Ecology

Invasive species often have a significant impact on native ecosystems because they are highly effective competitors. They may:

• Outcompete Native Species: Invasive species can drive native species to decline or extinction by outcompeting them for resources.
• Alter Ecosystem Structure and Function: Invasive species can change the physical environment, nutrient cycles, and other ecosystem processes, further disadvantaging native species.
• Lack Natural Enemies: Invasive species often lack the natural predators, parasites, or diseases that would control their populations in their native range, allowing them to proliferate rapidly and outcompete native species.

Examples of Competition in Nature

• Barnacles: Classic studies by Joseph Connell on intertidal barnacles demonstrated how competition limits the distribution of different species. Balanus barnacles were found to outcompete Chthamalus barnacles in the lower intertidal zone, excluding Chthamalus from this area.
• Plants: Many plant communities exhibit intense competition for light, water, and nutrients. Taller plants often outcompete shorter plants for light, while plants with extensive root systems may be more effective at absorbing water and nutrients.
• African Wild Dogs and Lions: These predators compete for the same prey in the African savanna. Lions, being larger and stronger, can often steal kills from wild dogs, influencing the hunting strategies and distribution of the wild dog populations.
• Red and Grey Squirrels: In the UK, the introduced grey squirrel has largely replaced the native red squirrel. Grey squirrels are more efficient at foraging for food and are also more resistant to a squirrel poxvirus, giving them a competitive advantage.

Distinguishing Competition from Other Interactions

It's important to differentiate competition from other types of species interactions:

• Predation: One species (the predator) kills and consumes another species (the prey).
• Parasitism: One species (the parasite) benefits at the expense of another species (the host).
• Mutualism: Both species benefit from the interaction.
• Commensalism: One species benefits, and the other is neither harmed nor helped.

While these interactions are distinct, they can also interact with competition in complex ways. For example, predation can reduce the population size of a strong competitor, allowing a weaker competitor to coexist.

Studying Competition

Ecologists use a variety of methods to study competition in the field and in the laboratory:

• Removal Experiments: Removing one of the competing species and observing the response of the remaining species. This can reveal the extent to which the removed species was limiting the other.
• Addition Experiments: Adding individuals of one species to a community and observing the effects on other species. This can reveal whether the added species is able to outcompete native species.
• Observational Studies: Monitoring the distribution, abundance, and behavior of competing species over time. This can provide insights into how competition is influencing their populations.
• Mathematical Models: Developing mathematical models to simulate the dynamics of competing populations. These models can help to predict the outcome of competition under different conditions.

The Importance of Understanding Competition

Understanding interspecific competition is crucial for:

• Conservation Biology: Managing endangered species and mitigating the impacts of invasive species.
• Ecosystem Management: Predicting the effects of environmental changes on ecological communities.
• Agriculture: Optimizing crop yields by minimizing competition between crops and weeds.
• Understanding Evolution: Gaining insights into the evolutionary processes that shape the diversity of life on Earth.

Current Research and Future Directions

Research on interspecific competition continues to evolve, with ongoing studies focusing on:

• The Role of Environmental Change: How climate change, habitat loss, and other environmental changes are altering the dynamics of competition.
• The Impact of Invasive Species: Understanding the mechanisms by which invasive species outcompete native species and developing strategies for controlling their spread.
• The Evolution of Competition: Investigating the evolutionary responses of species to competition and how these responses can lead to character displacement and diversification.
• The Complexity of Multi-Species Interactions: Examining how competition interacts with other types of species interactions, such as predation and mutualism, to shape ecological communities.

Conclusion

Competition between two species is a fundamental ecological interaction that shapes the distribution, abundance, and evolution of life. By understanding the mechanisms, factors influencing, and consequences of competition, we gain critical insights into the intricate workings of ecosystems and the challenges of conserving biodiversity in a changing world. From the microscopic battles of bacteria to the epic struggles of large predators, competition drives the dynamics of life and continues to be a central focus of ecological research.