🦉Intro to Ecology Unit 6 Review

Species richness is simply the total number of distinct species present in a community. A pond with 12 fish species has higher richness than one with 5, regardless of how many individuals of each species are swimming around.

Species evenness describes how equally individuals are distributed among those species. Picture two forests, each with 4 tree species and 100 total trees:

Forest A: 25 oaks, 25 maples, 25 birches, 25 pines (high evenness)
Forest B: 91 oaks, 3 maples, 3 birches, 3 pines (low evenness)

Both have the same richness, but Forest A has much higher evenness.

Diversity combines richness and evenness into a single measure. This matters because a community can have high richness but low evenness (many species, but a few dominate), or high evenness but low richness (few species, all similarly abundant). Neither situation alone tells the full story.

Types of ecological diversity

Ecologists distinguish three spatial scales of diversity:

Alpha diversity is diversity within a single area or habitat. Think of the number of tree species in one forest plot, or fish species in one lake.
Beta diversity captures the change in species composition between communities or along environmental gradients. If a forest and a nearby grassland share very few bird species, beta diversity between them is high.
Gamma diversity is the total species diversity across an entire landscape or region, encompassing multiple communities. For example, the total mammal diversity across an entire mountain range.

These three levels are connected: gamma diversity reflects the combination of how diverse individual sites are (alpha) and how different sites are from each other (beta).

Diversity indices for community structure

Common diversity indices

Raw species counts don't capture the full picture, so ecologists use mathematical indices that incorporate both richness and abundance patterns.

Shannon-Wiener Index (H')

$H' = -\sum (p_i \times \ln p_i)$

Here, $p_i$ is the proportion of total individuals belonging to species $i$ . You calculate $p_i \times \ln p_i$ for each species, sum them up, and flip the sign (since the natural log of a proportion is always negative). Higher values mean greater diversity. A forest with many evenly distributed tree species might have $H' = 2.5$ , while a heavily dominated community might sit below 1.0.

Simpson's Diversity Index (D)

$D = 1 - \sum (p_i^2)$

This measures the probability that two randomly chosen individuals belong to different species. It ranges from 0 (one species completely dominates) to nearly 1 (many species at similar abundances). A diverse coral reef community might score $D = 0.8$ .

Pielou's Evenness Index (J')

$J' = H' / \ln(S)$

where $S$ is the total number of species. This isolates the evenness component by dividing the observed Shannon index by the maximum possible Shannon index (which occurs when all species are equally abundant). It ranges from 0 to 1, with 1 representing perfect evenness.

Interpreting diversity indices

A higher index value doesn't automatically mean a "better" or healthier ecosystem. Some naturally low-diversity systems (like salt marshes or boreal forests) function perfectly well. Interpretation always depends on context: what's normal for that habitat type, and whether the community has been disturbed.

These indices are also sensitive to sampling effort. A quick survey will miss rare species, deflating richness and skewing abundance proportions. Two practical tools help address this:

Rarefaction curves standardize comparisons by estimating how many species you'd find in each community if you had taken the same number of samples. This is essential when comparing, say, bird diversity between a small and a large forest fragment where survey effort differed.
Margalef's Richness Index ( $D_{Mg}$ ) accounts for sample size directly:

$D_{Mg} = (S - 1) / \ln(N)$

where $N$ is the total number of individuals sampled. This helps compare richness between communities with very different total abundances, like sparse desert vegetation versus a dense tropical understory.

Community composition and ecosystem function

Biodiversity-ecosystem functioning relationships

Ecologists have found consistent links between community composition and how ecosystems work. Two mechanisms help explain why more diverse communities often function better:

The complementarity effect: species with different niches use resources more completely. In mixed tree plantations, for instance, species with different rooting depths or light requirements often achieve higher total productivity than monocultures.
The selection effect: more diverse communities are statistically more likely to include a highly productive or influential species. A grassland with many plant species is more likely to contain a nitrogen-fixer that enriches the soil for everyone.

Functional diversity, the range of functional traits (like leaf size, root depth, or feeding strategy) in a community, is often a stronger predictor of ecosystem functioning than species counts alone. The diversity of leaf traits in a forest, for example, can better predict carbon storage than simply counting tree species.

Community composition and ecosystem stability

Several concepts connect diversity to an ecosystem's ability to withstand and recover from disturbance:

Functional redundancy means multiple species perform similar ecological roles. If one pollinator species declines, others can compensate, maintaining pollination services. This buffering is central to the insurance hypothesis, which predicts that higher diversity stabilizes ecosystem functioning across variable environmental conditions.
Keystone species have effects on the community far out of proportion to their abundance. Sea otters, for example, control sea urchin populations; without them, urchins overgraze kelp, collapsing the entire kelp forest ecosystem.
Ecosystem engineers physically modify habitats in ways that shape the whole community. Beavers dam streams to create wetlands, generating habitat for dozens of other species. Corals build reef structures that support some of the most diverse communities on Earth.

Factors influencing community composition and diversity

Local-scale influences on community structure

At the local scale, three main forces shape which species are present and how abundant they are:

Environmental filtering selects for species whose traits match local abiotic conditions. Soil pH strongly influences which plant species can establish, and temperature gradients determine which insects can survive in a given area.
Biotic interactions further sort species through competition, predation, mutualism, and parasitism. Two plant species competing for the same light and soil nutrients may not coexist indefinitely (competitive exclusion), while predator-prey dynamics create population cycles in small mammal communities.
Disturbance regimes introduce variation in both space and time. Periodic fires in grasslands prevent any single species from dominating, and treefall gaps in forests create patches of light where shade-intolerant species can establish. Both processes can actually increase diversity by preventing competitive exclusion.

Broader-scale factors shaping communities

Zooming out, additional forces come into play:

Neutral theory proposes that random processes like dispersal, speciation, and extinction can explain many patterns in community composition, especially at larger spatial scales. It challenges the assumption that every species' presence must be explained by niche differences.
Metacommunity dynamics link local communities through dispersal. Source populations can sustain sink populations in less favorable habitats, and the spatial arrangement of habitat patches (like islands in an archipelago or pools along a stream network) shapes which species end up where.
Historical and biogeographic factors set the stage for everything else. The regional species pool available for local assembly reflects millions of years of evolutionary history. Wallace's Line in Southeast Asia, for example, marks a sharp boundary in species distributions shaped by ancient tectonic events. Post-glacial recolonization patterns still influence temperate communities today.
Anthropogenic impacts increasingly reshape communities at every scale. Habitat fragmentation isolates populations, climate change shifts species ranges, and invasive species introductions can restructure native communities. Urbanization, for instance, tends to homogenize bird communities across cities worldwide, favoring the same handful of generalist species.

🦉Intro to Ecology Unit 6 Review