Realised Niche in Action: How Barnacle Competition Shapes Intertidal Zonation

The rocky intertidal zone is a dynamic and harsh environment, home to a variety of organisms that have evolved unique strategies to survive the constant battering of waves, changing tides, and intense competition. Among these hardy creatures, barnacles stand out as a classic example of how competition shapes species distribution. When two barnacle species, species 1 and species 2, are observed to occupy the upper and lower strata of intertidal rocks respectively, and only when species 2 is removed does species 1 expand into both zones, we witness a powerful demonstration of the realised niche concept in ecology. This article explores the ecological principles at play, the significance of realised versus fundamental niches, and the broader implications for understanding species interactions and community structure.

What Is a Niche in Ecology?

In ecology, a niche refers to the role and position a species has in its environment—how it meets its needs for food, shelter, and reproduction, and how it interacts with other species. The niche concept is central to understanding why species are found where they are and how they coexist with others.

There are two main types of niches:

• Fundamental Niche: The full range of environmental conditions and resources a species can theoretically use in the absence of competition, predation, or other biotic interactions.

• Realised Niche: The actual range of environmental conditions and resources a species occupies in the presence of competitors, predators, and other biotic factors.

The Barnacle Example: A Classic Case Study

The observations described—species 1 limited to the upper strata and species 2 to the lower strata, with species 1 expanding when species 2 is removed—are not just a laboratory curiosity but a real-world phenomenon first documented in classic ecological studies. This pattern is a textbook illustration of how competition determines species distribution and the concept of realised niche.

Initial Observations

• Both Species Present: When both species 1 and species 2 are present on intertidal rocks, species 1 is found only in the upper strata, while species 2 occupies the lower strata.

• Species 2 Removed: When species 2 is experimentally removed from the lower strata, species 1 is able to occupy both the upper and lower strata.

Ecological Interpretation

This pattern can be explained as follows:

• Competitive Exclusion: Species 2 is a better competitor in the lower strata, preventing species 1 from establishing there.

• Realised Niche: The upper strata is the realised niche of species 1 when both species are present—the part of the habitat it actually occupies due to competition.

• Fundamental Niche: The full range (upper and lower strata) is the fundamental niche of species 1—the area it could occupy if no competitors were present.

The Role of Competition in Niche Partitioning

Competition is a fundamental force shaping the distribution and abundance of species in ecosystems. In the intertidal zone, space is a critical and limited resource. Different species of barnacles have evolved to exploit different parts of the rock surface, reducing direct competition.

• Upper Strata: Often more exposed to air, sunlight, and desiccation, the upper strata may be less favorable for some species but more tolerable for others.

• Lower Strata: More frequently submerged, the lower strata offers more stable conditions but is dominated by species better adapted to competition in this zone.

When two species compete for the same resource (in this case, space on the rock), the principle of competitive exclusion comes into play. This principle states that two species with identical resource requirements cannot coexist indefinitely—one will outcompete the other134. In the barnacle example, species 2 outcompetes species 1 in the lower strata, restricting species 1 to the upper strata.

Realised Niche vs. Fundamental Niche

Understanding the difference between realised and fundamental niches is crucial for interpreting species distributions.

• Fundamental Niche: The potential range a species could occupy if there were no competitors, predators, or other biotic constraints. For species 1, this includes both the upper and lower strata.

• Realised Niche: The actual range a species occupies when biotic interactions are taken into account. For species 1, this is only the upper strata when species 2 is present.

When species 2 is removed, species 1 can expand into the lower strata, revealing its fundamental niche. This experiment demonstrates how competition limits the distribution of species in nature.

Why Is the Upper Strata the Realised Niche of Species 1?

When both species are present, the upper strata is the only area where species 1 can persist. This is because:

• Competitive Exclusion: Species 2 is a superior competitor in the lower strata, preventing species 1 from establishing there.

• Environmental Tolerance: Species 1 may be better adapted to the harsher conditions of the upper strata, where species 2 cannot thrive as well.

Thus, the upper strata represents the realised niche of species 1 under competitive pressure.

Broader Ecological Implications

The barnacle example has far-reaching implications for understanding how species interact and coexist in nature.

Niche Partitioning and Community Structure

Niche partitioning—the division of resources among species to reduce competition—is a key mechanism promoting biodiversity. By occupying different parts of the habitat (upper vs. lower strata), barnacle species reduce direct competition and allow for coexistence.

The Role of Disturbance and Predation

In addition to competition, other factors such as predation and physical disturbance (e.g., wave action) can influence species distribution. However, in the classic barnacle experiment, competition is the primary driver of the observed patterns.

Conservation and Management

Understanding realised and fundamental niches is important for conservation and habitat management. Protecting a variety of habitats and ensuring that key competitors are present (or absent) can influence the distribution and survival of species.

Common Misconceptions

It is easy to confuse the concepts of realised and fundamental niches, especially when interpreting experimental results.

• Realised Niche Is Not Fundamental Niche: The upper strata is the realised niche of species 1, not its fundamental niche. The fundamental niche is broader and includes both strata.

• Competition Is Not Mutualism: The interaction between species 1 and species 2 is competitive, not mutualistic. There is no evidence that they benefit each other.

• Competitive Exclusion Is Not Absolute: While species 2 excludes species 1 from the lower strata, this does not mean species 1 is always the weaker competitor in all environments or under all conditions.

Summary Table: Key Concepts in the Barnacle Example

Practical Applications

The principles illustrated by the barnacle example are widely applicable in ecology, conservation, and environmental management.

• Restoration Ecology: Understanding realised and fundamental niches helps guide the reintroduction of species and the restoration of habitats.

• Invasive Species Management: Invasive species often outcompete natives, restricting them to suboptimal habitats (their realised niche). Removing invasives can allow natives to reclaim their fundamental niche.

• Climate Change Adaptation: As environmental conditions change, species may shift their distributions. Understanding niche concepts helps predict how species will respond.

Conclusion

The observation that species 1 is restricted to the upper strata of intertidal rocks when species 2 is present, but can expand into the lower strata when species 2 is removed, is a classic demonstration of the realised niche concept. The upper strata is the realised niche of species 1 under competitive pressure, while the full range (upper and lower strata) represents its fundamental niche. This example highlights the importance of competition in shaping species distributions and the structure of ecological communities. Understanding these concepts is essential for ecologists, conservationists, and anyone interested in the complex interactions that sustain biodiversity.