Understanding patterns of species abundance and diversity is central to ecological research. One of the most informative visual tools for this purpose is the dominance-diversity curve, also known as the rank-abundance plot. This curve helps ecologists interpret the structure of biological communities by plotting the sequence of species (from most to least abundant) on the X-axis and their relative importance (often log-transformed abundance) on the Y-axis.

What Is a Dominance-Diversity Curve?

A dominance-diversity curve visually summarizes the abundance and evenness of species within a community. By ranking species from most to least abundant and plotting their relative abundances, researchers can quickly assess community structure, dominance patterns, and evenness. The slope and shape of the curve provide insights into how resources are partitioned among species and how diverse or dominated a community is.

The Geometric Model Explained

When the dominance-diversity curve results in a straight line (especially when the Y-axis is on a logarithmic scale), it is indicative of the geometric model of species abundance. This model is also known as the niche pre-emption hypothesis.

Key Features of the Geometric Model:

• Resource Partitioning: The model assumes that the first species to colonize a community preempts a fixed proportion of the available resources. Each subsequent species then preempts the same proportion of what remains.

• Mathematical Expression: The relative abundance of the i-th species is given by a geometric series, where each species’ abundance is a fixed fraction of the remaining resources.

• Graphical Signature: On a log scale, plotting relative abundance against species rank produces a straight line, reflecting the geometric decrease in abundance from the most to least common species.

Ecological Interpretation:

• Early Successional or Stressed Communities: The geometric model is most often observed in communities with low diversity, early successional stages, or environments under stress, where a few species dominate and niche overlap is minimal.

• Low Evenness: The straight line indicates high dominance by the most abundant species and low evenness across the community.

Why Not Other Models?

Other models, such as the log-normal or broken stick models, produce different shapes on a rank-abundance plot:

• Log-normal model: Results in a bell-shaped curve when plotted on a log scale, reflecting a more even distribution of abundances.

• Broken stick model: Produces a more gradual decline, indicating a more equitable distribution of resources among species.

• Random-niche boundary hypothesis: Is associated with the log-normal or broken stick models, not with the straight-line geometric model.

Correct Answer and Its Significance

A straight line in a dominance-diversity curve (species sequence on X, log relative importance on Y) represents:

(1) geometric model and niche pre-emption hypothesis.

This relationship is well-established in ecological literature and is supported by mathematical and empirical studies. The geometric model provides a useful baseline for understanding how resource partitioning and competitive dominance shape community structure, especially in less diverse or disturbed ecosystems.

Conclusion

Dominance-diversity curves are powerful tools for visualizing and interpreting the structure of ecological communities. When these curves form a straight line on a log scale, they reveal a geometric model of species abundance, rooted in the niche pre-emption hypothesis. Recognizing this pattern helps ecologists identify communities where a few species dominate, resources are unevenly partitioned, and overall diversity is low—insights that are crucial for biodiversity management and conservation planning.