#2 Classifying communities and diversity

Why is it important to define and classify ecological communities? (first 3)

allows ecologists to:

• Manage different community types appropriately

• Identify interactions that are characteristic of specific communities

• Understand how ecosystem functions differ among communities

Why is it important to define and classify ecological communities? (last 3)

• Track changes in a community over time or space

• Use knowledge of one community type to inform understanding of others

• Assess rarity and prioritize communities for conservation or protection

Ecological community

A community is a group of populations that coexist in space and time and interact directly or indirectly

Population

a group of individuals of the same species living in the same area

Plant community

vegetative subset of the community

• i.e., all plants occupying a particular area

What did Clements propose about community structure?

viewed communities as highly organized “superorganisms” composed of interdependent species

What did Clements believe about community structure?

• Species occur together because they depend on each other

• Removing one species could destabilize the entire community

• Community boundaries should be abrupt due to strong species associations

• Biotic interactions are the primary determinants of structure

What pattern would a Clementsian “superorganism” predict across a landscape?

Abrupt transitions between community types, with species grouping tightly and limited overlap between communities

What did Gleason propose about community structure?

Communities result from interactions between individual species and the environment

What did Gleason believe about community structure?

• Not tightly integrated

• Defined arbitrarily by humans

• Shaped primarily by abiotic factors and individual species' tolerances

• Often lacking sharp boundaries

What pattern would a Gleasonian view predict across a landscape?

Gradual species turnover along environmental gradients, with overlapping distributions and no sharp boundaries

What is the “middle ground” between Clements and Gleason?

• Although abiotic and biotic boundaries do not always align, they are connected.

• Community composition changes along environmental gradients, and abrupt transitions may occur due to sudden environmental shifts (e.g., fires, plowing) or strong interactions

Why is biodiversity an important property of communities to measure?

diversity gives insights into:

• Number of species present

• Resilience and stability

• Genetic variability

• Complexity of species interactions

• Potential ecosystem functioning

What are the major diversity metrics used in community ecology?

• Species Richness (S)

• Dominance (d or 1/d; Berger–Parker Index)

• Shannon Diversity Index (H′)

• Evenness (E)

Species richness (S)

number of species present in an area

Why must richness surveys be conducted over multiple seasons?

Because species vary in detectability

• some appear only in certain seasons

How is richness often measured in plant ecology?

Using biomass or percent cover rather than counting individuals

• since many plants are clonal or morphologically ambiguous

How is the Berger–Parker Dominance Index (d) calculated?

d = N_max / N
Where:

• N_max = mean cover (or abundance) of the most common species

• N = total mean cover (or abundance) of all species

What does a dominance value of 0 vs. 1 mean?

• 0: Equal abundance → high diversity

• 1: Single species dominates → low diversity or richness of 1

What does the Shannon Diversity Index (H′) measure?

quantifies diversity by incorporating both richness and evenness using species' relative abundances

How is evenness (E) calculated?

E = H′ / ln(S)

• This measures how evenly individuals are distributed among species

Historically, what were the major disturbances shaping tallgrass prairie ecosystems?

Fire and grazing

What happened to diversity when these disturbances were removed?

Diversity decreased

How do burning and grazing influence tallgrass prairie diversity?

• Burning: Favors C4 grasses → decreases diversity

• Grazing: Bison prefer C4 grasses → increases forb abundance → increases diversity

• Burning + grazing: Highest diversity because processes counterbalance each other

What is the purpose of ordination in ecology?

simplify multivariate data by arranging sites or species in low-dimensional space (1–3 axes) so that distances reflect ecological similarity

What is ordination typically used for?

Pattern detection and hypothesis generation about species–environment relationships

What does ordination allow ecologists to detect?

Major gradients, shifts in community composition, and grouping patterns among species or sites

What does the theory predict about species richness and ecosystem function when niches are complementary? (snowballs on the barn)

Productivity should increase linearly with added species because species use different resources or occupy different niches

Why does the relationship between richness and function saturate at high richness?

Because niche overlap increases, leading to competition and limiting additional productivity gains

What did Tilman’s biodiversity experiment show?

• Productivity increases with species richness

• Productivity increases even more strongly with the number of functional groups

• Ecosystem processes depend more on functional diversity than taxonomic diversity

How was plant biomass measured in Tilman’s experiment?

Plants were cut, dried, and weighed until biomass stabilized

Why are functional groups considered more informative than taxonomy in ecosystem studies?

Functional groups reflect ecological roles and processes, which determine ecosystem functioning

• (e.g., water redistribution, decomposition, light capture)

What are the main challenges associated with biodiversity–function experiments?

• Difficulties measuring belowground productivity

• Short spatial or temporal scales

• Sampling effect/portfolio effect:

  • Communities with more species likely include a highly productive species by chance

    • inflating richness effects

• Artificial communities may violate natural assembly rules