Diversity and Gradients

Question 1

Species Richness

Answer 1

the number of species within a prescribed area, without regard to numerical abundance or ecological importance in a given community

Question 2

Three levels of diversity

Answer 2

alpha: #species within a locality or a habitat
beta: change in the species composition between localities across space or an environmental gradient
Gamma: number of species within a larger region. Gamma is a fxn of both alpha and beta diversity

Question 3

Explanation of alpha, beta and gamma

Answer 3

If alpha diversity is = gamma diversity then there is low beta diversity (i.e less difference between regional pools)

If alpha diversity < gamma diversity then high beta diversity (i.e. more difference between regional pools)

Question 4

Species turnover

Answer 4

is the beta diversity examined along an axis of variation

Question 5

Example of species turnover

Answer 5

From the cloud forest to the dry seasonal forest in Costa rica (change in elevation) - the bird community changes composition between two areas.
The difference in 500m elevation results in 100% turnover in species

Question 6

Increase in latitude

Answer 6

decrease in alpha, beta and gamma diversity. Increased richness in tropics: observed in mammals, birds, and brachiopod fossil fauna
exceptions: penguins, pelagic seabirds, seals, pine trees, aphids, zooplankton

Question 7

Causes of the Latitudinal Diversity Gradient

Answer 7

Two hypotheses:

1. Null versus deterministic hypothesis
2. equilibrial versus non-equilibrial

Question 8

LDG null hypotheses

Answer 8

Patterns observed are artifacts of the way data are collected or presented (eg mid domain effect: random distribution of geographic range boundaries are towards the poles so the greatest range overlaps mid-way between the poles).
Ex. if you place a bunch of blocks in a box they will most likely overlap in the middle of the box than the edges

Question 9

LDG deterministic hypotheses

Non equilibrial hypotheses

Answer 9

A) Non equilibrial hypotheses:
-communities have no yet reached a steady state (eg historical perturbations: species diversity is still in the process of increasing or decreasing after some historical disturbance)

Question 10

LDG deterministic hypotheses

Equilibrial hypothesis Productivity

Answer 10

B) equilibrial hypothesis Productivity
A steady state has been reached. The forces that increase diversity are exactly balanced by those that reduce diversity such that species diversity remains fixed through time.
*Productivity: evapotranspiration and solar radiation are equilibrating - exceptions are in salt marshes and shallow eutrophic lakes (which are cyclical)

Question 11

LDG deterministic hypotheses

Equilibrial hypothesis Harshness

Answer 11

Harshness: harsh environments have higher extinction rates, lower colonization potential, and less opportunity for resource specialization than more benign environments

Question 12

LDG deterministic hypotheses

Equilibrial Temporal stability

Answer 12

Temporal stability: variable climates prevent resource specialization and hence are able to support fewer species. Temporal variability tends to favour generalist
exceptions (stable, low diversity): deep ocean

Question 13

LDG deterministic hypotheses

Equilibrial hypothesis Habitat heterogeneity

Answer 13

Habitat heterogeneity: diverse physical environments promote isolation, resource specialization, speciation and co-existence. Example: BC very diverse topography, and 1140 native vertebrates
exceptions (low heterogeneity, high diversity): marine plankton

Question 14

LDG deterministic hypotheses

Equilibrial hypothesis Interspecific interactions

Answer 14

Interspecific interactions: More species create a positive feedback of diversification through increased competition, predation, parasitism

Cause or consequence?

Question 15

LDG deterministic hypotheses

Equilibrial hypothesis Area

Answer 15

Area: Tropics have a greater area than either polar region due to curvature of the earth. Species-area relationship is well established.

Question 16

LDG deterministic hypotheses

Equilibrial hypothesis Age

Answer 16

Age: Latitudinal gradients for clades originating in warm climates are steeper with a strong tropical affinity. For a variety of plants and animals of both marine and terrestrial realms, most clades radiated in tropical climates. Extant tropical diversity peak is created from lineages that adapted to a planet with tropical climate.

Question 17

Synthesis of LDG hypotheses

Answer 17

Multiple factors together are responsible for the diversity gradient: Area, stability, productivity, and age account from increased richness at equator.

Harshness suggests increased diversity at temperate regions

Question 18

Diversity cradle or museum?

Answer 18

Cradle means that the sister species are relatively young.
Museum means that the sister species are old.
According to data sister species are older at equator and younger at poles

Question 19

Rapoport’s Rule

Answer 19

Species tend to have larger range sizes at higher latitudes
Ex. across bird species in north american species with smaller geographical ranges tend to have range centers at lower latitudes.
Also, very few species at high latitude with small geographic ranges

Same is true with marine mollusks and trees

Question 20

Janzen’s Hypothesis consistent with Rapoport’s rule

Answer 20

Mountain passes are greater barriers to dispersal in the tropics than in temperate regions because there is less overlap in thermal regimes experienced at low and high altitudes in the tropics.
i.e. the range size of animals are smaller at the equator and therefore do not overlap or cross over the mountain passes

Question 21

Relationship between dispersal barriers and species richness

Answer 21

Since there are stronger physiological dispersal barriers in the tropics it is expected that species elevational ranges along mountainsides would be narrower in the tropics

Therefore higher beta diversity in tropical mountains, or high turnover rate moving across elevations compared to temperate regions.

Allopatric speciation is also higher in the tropics.

Question 22

Species Dominance

Answer 22

Tropical areas tend to have more species but those species are numerically rare. Any individual tropical species amounts for a smaller proportion of the total abundance of species of individuals summed across all species than the average species in the temperate areas.
Example: tropical tree communities: they have higher species richness overall but most species are rare
Temperate communities have fewer species but those species are higher abundance.

Question 23

Peninsula Effect

Answer 23

Tendency for species richness to decrease towards the tip of a peninsula.
Example: florida ground beetles. Temperate and endemic species decrease towards the tip. But tropical species tend to increase

Question 24

Spatial diversity gradient

Answer 24

Altitude and depth
Increase in elevation = decrease in diversity (ex. vegetation in santa catalina mountains)
increase in depth = decrease in diversity (ex. marine fauna)