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Richness and evenness: two Alpine ponds

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3.1.7 Species diversity in communities is a product of richness and evenness.

Richness is the number of species in a community, and evenness is how similar the population sizes of each species are. Consider the significance of these two variables for biodiversity.

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Imagine two freshwater ponds in the French Alps. Both have been sampled for aquatic invertebrates using kick-sampling. Pond A sits in undisturbed alpine meadow, fed by snowmelt. Pond B sits at the edge of agricultural land, receiving nutrient-rich runoff. Both ponds contain exactly six species.

Pond A: Alpine meadow pond

Pond B: Agricultural runoff pond

Both ponds have the same species richness: six species each. Yet Pond A has individuals spread fairly evenly across all species, while Pond B is overwhelmingly dominated by water beetles. Pond A has high evenness; Pond B has low evenness.

This matters because species diversity is a product of both richness and evenness. A community with high evenness implies a complex ecosystem with many available niches that can support a wide range of species. A community dominated by one species suggests fewer niches or conditions that favour one species disproportionately.

Richness alone is therefore insufficient to describe biodiversity. A community could contain many species, yet if 90% of all individuals belong to a single species, diversity is low. This is why ecologists developed mathematical indices that combine both richness and evenness into a single comparable number.

1. Both ponds contain six species. Explain why Pond A would be described as having higher species diversity than Pond B.

2. Suggest why nutrient-rich agricultural runoff might lead to the dominance of one species in Pond B.
   • Hint: Think about what happens when nutrient levels increase. Which species might be most tolerant?

3. A third pond, Pond C, contains 12 species, but 90% of all individuals belong to a single chironomid midge species. Would you expect Pond C to have high or low species diversity? Justify your answer.

4. Explain why measuring species richness alone is not sufficient to assess the health of an ecosystem.

Simpson's reciprocal index: quantifying diversity

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3.1.8 Simpson's reciprocal index is used to provide a quantitative measure of species diversity, allowing different ecosystems to be compared and for change in a specific ecosystem over time to be monitored.

Consider appropriate sampling procedures for comparing diversity in areas containing the same type of organism in the same ecosystem. Calculate diversity (D) if provided with data and the formula in which N is the total number of individuals in the population and n is the number of individuals of a single species. D = N(N-1) / Σn(n-1). The value of D will be higher where there is greater richness (number of species) and evenness (similar abundance), with 1 being the lowest possible value.

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The scenario

A research team is comparing ground beetle (Carabidae) communities at two sites on a rewilded estate in the Scottish Highlands. Site 1 has been rewilded for 15 years: sheep removed, natural regeneration of birch and Scots pine. Site 2 is still under traditional sheep grazing. Pitfall traps (small containers sunk into the ground so beetles fall in) were used at both sites.

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This is a valid comparison because they are comparing the same type of organism (ground beetles) in the same type of ecosystem (upland heath), using the same sampling method (pitfall traps) with the same effort.

The formula

D = N(N - 1) / Σn(n - 1)

Where:

D = Simpson's reciprocal index (the diversity value)

N = the total number of all individuals of all species in the sample

n = the number of individuals of each individual species

Σ = "the sum of" (add up all the n(n - 1) values)

You do not need to memorise this formula. It will be provided in an exam. You must understand what each symbol means and be able to use it.

1. What type of sampling method is a pitfall trap, and why is it appropriate for ground beetles?

2. Explain why it would not be valid to compare a Simpson's reciprocal index calculated for beetles in a grassland with one calculated for birds in a forest.

Worked example: Site 1 (rewilded area)