Species Diversity

What do we mean by diversity?

1. Species RichnessCountSpecies/areaSpecies/number

2. Heterogeneity = Richness + evenness3. Scales of diversity

AlphaBetaGamma

Measures of diversity sensitive to both richness and

evenness

Simpson’s Index D = 1/Σ pi

2

Shannon IndexH’ = - Σ( pi log pi) or exp(H’)

What do we mean by diversity?

1. Species RichnessCountSpecies/areaSpecies/number

2. Heterogeneity = Richness + evenness3. Scales of diversity

AlphaBetaGamma

β=γ/αγ = α + β

Additive Partitioning of Diversity:

(Wagner et al. 2003)

α1

α2

α3

α4

β 3

β 2

β 1

γ

Global Patterns of diversity

1. Islands2. Climate 3. Latitude

Dependence of these patterns on grain size?

Number of species of reptiles on Caribbean islands

Species – Area RelationshipS = c A z z ~ 0.3

•Galapagos – Land Plants.325•West Indies – Reptiles. & Amph..301•Bahamas – Orchids .31•West Indies – Carabids .34•East Indies – Ants .30•East Indies – Birds .28

MacArthur and Wilson 1967

Assumptions:

1. The immigration rate decreases as the number of species on the island increases. This is expected because competition increases and the number of available niches decreases.

2. The extinction rate increases with increasing species number. This is expected because more species implies greater competition.

Equilibrium theory

Assumptions:

1. The immigration rate decreases as the number of species on the island increases. This is expected because competition increases and the number of available niches decreases.

2. The extinction rate increases with increasing species number. This is expected because more species implies greater competition.

3. For a given number of species, immigration decreases with increasing distance from the mainland. That is, the farther the island is from the mainland, the less frequent Long-distance dispersal events will be.

Assumptions:

1. The immigration rate decreases as the number of species on the island increases. This is expected because competition increases and the number of available niches decreases.

2. The extinction rate increases with increasing species number. This is expected because more species implies greater competition.

3. For a given number of species, immigration decreases with increasing distance from the mainland. That is, the farther the island is from the mainland, the less frequent Long-distance dispersal events will be.

4. For a given number of species, the extinction rate increases with decreasing island size. That is, populations on smaller islands have a greater risk of extinction because their population sizes are lower.

Equilibrium theory has led to a large body of theory and observation to which we will return in the next lecture .

Climate as a determinant of diversity

Latitudinal gradient

Breeding bird diversityGreenland 56 (840,000 mi2)New York 105N Am. North of Mexico 650Guatemala 469 (42,000 mi2)Columbia 1395+ (440,000 =1/16 N. Am area)

Ant species Arctic Alaska 3 (66-72) Alaska 7 (55-72) Iowa 73 (41-43) Cuba 101 (20-

23 N) Trinidad >134 San Paulo, Brazil >222 (20-25 S) Tucuman, Argentina 139 (26-

28) Buenos Aires, Argentina 103 (33-39) Humid western Patagonia 19 (40-52) Tierra del Fuego 2 (43-55)

Latitude and grain size

Snakes per political unit (large grain)Canada 22US 126Mexico 293

Trees per 1000 m2 (small grain)Canadian boreal forest ~2N. US Deciduous forest ~8North Carolina Piedmont ~15

(to 30)Panama & Columbia ~100 Pluvial Columbia >260

Willig et al. 2003. Annual Reviews E&S

Willig et al. 2003. Annual Reviews E&S

Exceptions?• Often narrow specialist taxonomic groups

– Ichneumonid wasps– Saxifrages

• Buffered environments– endo and ecotoparasites of vertebrates– aquatic plants– secondary marine vertebrates

Willig et al. 2003. Annual Reviews E&S

Willig et al. 2003. Annual Reviews E&S

Global Mechanisms1. Area, Heterogeneity & Geometry2. Time (Age, Time , History, Stability?)3. Climate/Environment

- Favorableness of climate or environment- Constancy, stability or predictability of

climate or environment - Energy-diversity or Species-energy Theory - Productivity

4. Biotic interactions or Coevolution- Competition - Predation

Area & Heterogeneity • More connected (contiguous) area permits

greater population size, lower extinction.• Premise: More contiguous area of uniform

environment in tropics then in temperate to arctic latitudes; tropical area in one block

• Question: Corrected for area, does diversity reach similar levels across latitudes?

Mid-domain effect• Random placement of species ranges within

a bounded domain• Null models all produce latitudinal gradients,

but with different specific attributes.– Unconstrained– Constrained by range midpoints– Constrained by the distribution of range

sizes• Species wholly contained in any geographic

domain should exhibit a mid-domain peak.

Colwell & Hurtt 1994

Time (Age, Time , History,

stability?)• Evolutionary time:

– More time for evolution to produce species;

– fewer extinctions in stable environments.• Ecological time:

– More time for species to colonize appropriate habitat.

• Premise: the tropics have sustained less drastic change in environmental conditions over time

• Question: Does species richness increase without limit?

Favorableness of climate or environment

• Fewer species can tolerate climatic extremes.

• Premise: ideal conditions for life are found in the tropics

• Questions: What is the limit to evolutionary rate as a function of latitude?

Constancy, stability or predictability of climate or

environment • Fewer species can tolerate varying

environments; those that do tolerate great ranges of environment have broad niches

• Premise: seasons less pronounced in tropical latitudes

• Problem: some species-rich environments do occur in seasonal environments; some stable environments are poor in species.

• Questions: Do fluctuating environments select for broad tolerance, broad niches, and low specialization?

Variant – speed of speciation

Rapoport-Rescue Hypothesis

• Range size varies inversely with latitude• Because seasonality increases with

latitude, species with broad tolerance are found at higher latitudes

• Northern hemisphere fits better than soutehrn hemisphere

Energy & Productivity• Without production, no diversity• More primary production allows more

energy and thus more species• The Paradox of Enrichment (diversity

increases and then decreases with productivity)

• Problem: many species poor habitats are highly productive, and some unproductive habitats are highly diverse

• Question: Why do competitive dominants evolve in some ecosystems

Biotic interactions or Coevolution

• Species diversity begets possible interactions, leading to more species

• Premise: tropics, being more diverse, have more specialized coevolutionary relationships

• Questions: – Does this argument require that there already be

a diversity gradient for this effect to be more pronounced in the tropics?

– Does the latitudinal gradient reflect a gradient from selection by biotic interaction to selection by physical factors

Competition • Competitive exclusion limits richness.• Competition promotes specialization,

divergence, and niche partitioning.• Premise: tropics have higher competition,

more niche divergence.• Question: competitive pressure to

specialize would not occur without diversity—which came first?

Predation • Predation prevents competitive exclusion.• Janzen-Connell hypothesis on tree

regeneration vs. density• Premise: tropics, being more diverse, have

more predators, pests, and diseases, so competitive exclusion less likely.

• Question: Does a latitudinal gradient in predators, pests, and diseases exist and how did this come about?

Global Mechanisms1. Area, Heterogeneity & Geometry2. Time (Age, Time , History, Stability?)3. Climate/Environment

- Favorableness of climate or environment- Constancy, stability or predictability of

climate or environment - Energy-diversity or Species-energy Theory - Productivity

4. Biotic interactions or Coevolution- Competition - Predation

Species pools

Zobel 1997

Regional patterns & mechanisms

- Moisture & Elevation- Substrate- Production- Succession

Succession in a neotropical rain forest (0.5ha)

• Years 3-5 30-50 100-150 >300

• Birds 21 49 127 236• Primates 0 2-6 6-8 8-12• Trees 20 33 64 112

River FloodplainRiver Floodplain

River Bedrock Scour BarRiver Bedrock Scour Bar

Scoured IslandScoured Island

Native

Exotic

Upland(1090 plots)

Riparian (121 plots)

31.12

0.20 268 plots with exotics

55.66

7.98 110 plots with exotics

Mean Species Richness

After Brown & Peet 2003

Not discussed in lecture

Community patterns and mechanisms

- Environmental tolerance- Competition – the paradox of

enrichment- Slow dynamics- Suppression of dominance- Immediate disturbance hypothesis- Spatial mass effects – Propagule pressure- Temporal mass effects- Asymmetry of competition- Ecological equivalency

Willig et al. 2003. Annual Reviews E&S

Does diversity matter?- Stability ?- Productivity ?- Invasibility ?