🧮Section 4: Grass lands, Graphs and Algal experiments 🌿

🧮Species richness

The number of species in a community.

🧮Five possible productivity–richness relationships

U-shaped, negative, none, positive and hump-shaped.

🧮Mittelbach et al. 2001

Reviewed many studies and found the hump-shaped productivity–richness relationship was the most common in nearly every group.

🧮Why richness is hump-shaped with productivity

At low productivity few species can survive; at intermediate levels many coexist; at high productivity a few strong competitors take over.

🧮Park Grass Experiment

Started in 1856 at Rothamsted, England. Adding more types of resources (fertilizers) to grassland plots lowered the number of plant species.

🧮Park Grass regression

Species = 34.1 − 3.1 × (number of resources), R² = 0.49, p = 0.02. Each added resource meant about 3 fewer species, and resources explain about half the variation.

🧮MacArthur & MacArthur 1961

Foliage height diversity (number of vertical vegetation layers) predicted bird species diversity better than plant species diversity did.

🧮Why habitat structure increases diversity

More structural layers create more niches, so more species can coexist.

🧮Intermediate Disturbance Hypothesis

Species richness is highest at intermediate (occasional, moderate) disturbance, compared with rare or very frequent disturbance.

🧮IDH: low disturbance

Dominant competitors exclude other species, so richness is low.

🧮IDH: high disturbance

Only a few tough or fast-colonizing species survive, so richness is low.

🧮Lubchenco 1978

🦠Algal species richness in tide pools peaked at intermediate snail (grazer) densities.