Can the Realized Niche Exceed the Fundamental Niche? Exploring Ecological Interactions

The ecological niche is a foundational concept in understanding how species interact with their environment and each other. Traditionally, the fundamental niche is defined as the full range of environmental conditions and resources a species can theoretically use in the absence of competitors, predators, or other biotic constraints. The realized niche, in contrast, is the actual range a species occupies when these biotic interactions are present—usually a subset of the fundamental niche. But is it possible for the realized niche to exceed the fundamental niche? This article explores the conditions under which this might occur, focusing on different types of species interactions and their effects on niche dynamics.

Understanding Fundamental and Realized Niches

Before diving into the question, it’s essential to clarify what these terms mean:

• Fundamental Niche: The entire set of conditions and resources under which a species can survive and reproduce, in the absence of competitors, predators, or other limiting biotic factors. It represents the species’ “potential” niche156.

• Realized Niche: The actual set of conditions and resources a species occupies in the presence of competitors, predators, or other biotic interactions. It is almost always smaller than the fundamental niche due to these constraints156.

Can the Realized Niche Exceed the Fundamental Niche?

Under standard ecological definitions, the realized niche is considered a subset of the fundamental niche. However, in practice, certain ecological interactions—particularly positive ones—can allow a species to persist in conditions or areas that would otherwise be outside its fundamental niche if defined strictly by abiotic tolerances.

Let’s analyze each of the given options to determine which scenario, if any, might allow the realized niche to exceed the fundamental niche:

1. Competition

Competition occurs when two or more species vie for the same limited resources. This interaction typically restricts the realized niche of each species to a smaller portion of its fundamental niche. For example, if two bird species compete for the same food source, each may be forced to use only part of the habitat or resource spectrum it could theoretically exploit in the absence of the other. Competition never expands the realized niche beyond the fundamental niche; it always restricts it569.

2. Commensalism

Commensalism is a relationship in which one species benefits while the other is neither helped nor harmed. For instance, certain birds may build nests in trees without affecting the trees. While commensalism can allow the benefiting species to access resources or habitats it might not otherwise use, it does not typically enable a species to persist in conditions outside its fundamental niche. The realized niche may be expanded within the fundamental niche, but not beyond it.

3. Amensalism

Amensalism is an interaction where one species is harmed while the other is unaffected. A classic example is a large tree shading out understory plants. Amensalism reduces the realized niche of the affected species but does not expand it. The unaffected species is not impacted, so its niche remains unchanged. Amensalism cannot cause the realized niche to exceed the fundamental niche.

4. Mutualism

Mutualism is a relationship in which both species benefit from the interaction. This is the most promising scenario for the realized niche to exceed the fundamental niche, at least in a practical sense. Mutualistic partners can provide each other with resources, protection, or other benefits that allow them to survive in conditions or areas they could not tolerate alone.

For example:

• Plants and Pollinators: Some plants can only reproduce in the presence of specific pollinators, effectively expanding the range of habitats where they can persist.

• Corals and Zooxanthellae: Coral polyps host photosynthetic algae that provide them with energy, enabling corals to thrive in nutrient-poor waters where they otherwise could not survive.

• Lichens: A symbiotic relationship between fungi and algae allows lichens to colonize extreme environments like bare rock or Arctic tundra.

In these cases, the mutualistic interaction enables the species to occupy a broader range of conditions than it could on its own. While strictly speaking, the fundamental niche is defined as the set of conditions a species can tolerate without any biotic interactions, mutualism can allow a species to persist in environments that would be outside its fundamental niche if defined only by abiotic factors.

The Ecological Debate: Can the Realized Niche Truly Exceed the Fundamental Niche?

From a strict, theoretical standpoint, the fundamental niche is the maximum set of conditions a species can tolerate, and the realized niche is always a subset of this. However, in practice, mutualistic interactions can allow a species to “expand” its realized niche into areas or conditions it could not survive in alone. Some ecologists argue that, in such cases, the realized niche can be considered larger than the fundamental niche if the latter is defined only by abiotic tolerances and excludes the possibility of mutualistic partners26.

This interpretation is more about how we define the fundamental niche rather than a true biological exception. If the fundamental niche is defined strictly by abiotic tolerances, mutualism can allow the realized niche to exceed it. If the fundamental niche is defined to include the potential for mutualistic interactions, then the realized niche cannot exceed it.

Real-World Implications

Understanding the conditions under which the realized niche can exceed the fundamental niche has important implications for ecology and conservation:

• Species Distribution: Mutualistic relationships can enable species to colonize new habitats or persist in harsh environments, influencing patterns of biodiversity.

• Conservation: Protecting mutualistic partners is essential for maintaining the realized niche of many species, especially those dependent on specific symbionts.

• Climate Change: As environmental conditions change, mutualistic interactions may become even more critical for species survival and range expansion.

Summary Table: Effects of Ecological Interactions on Niche Size

Conclusion

In summary, the realized niche can exceed the fundamental niche only under certain mutualistic interactions, where the presence of a partner allows a species to survive in conditions it could not tolerate alone. While this is not strictly possible if the fundamental niche is defined to include all possible biotic interactions, in practical ecological terms and for many definitions used in the field, mutualism is the interaction most likely to allow a species’ realized niche to be broader than its fundamental niche as defined by abiotic tolerances. This nuanced understanding is essential for ecologists, conservationists, and anyone interested in the complex web of life.