Diversity and Gradients Flashcards
Species Richness
the number of species within a prescribed area, without regard to numerical abundance or ecological importance in a given community
Three levels of diversity
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
Explanation of alpha, beta and gamma
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)
Species turnover
is the beta diversity examined along an axis of variation
Example of species turnover
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
Increase in latitude
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
Causes of the Latitudinal Diversity Gradient
Two hypotheses:
- Null versus deterministic hypothesis
- equilibrial versus non-equilibrial
LDG null hypotheses
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
LDG deterministic hypotheses
Non equilibrial hypotheses
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)
LDG deterministic hypotheses
Equilibrial hypothesis Productivity
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)
LDG deterministic hypotheses
Equilibrial hypothesis Harshness
Harshness: harsh environments have higher extinction rates, lower colonization potential, and less opportunity for resource specialization than more benign environments
LDG deterministic hypotheses
Equilibrial Temporal stability
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
LDG deterministic hypotheses
Equilibrial hypothesis Habitat heterogeneity
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
LDG deterministic hypotheses
Equilibrial hypothesis Interspecific interactions
Interspecific interactions: More species create a positive feedback of diversification through increased competition, predation, parasitism
Cause or consequence?
LDG deterministic hypotheses
Equilibrial hypothesis Area
Area: Tropics have a greater area than either polar region due to curvature of the earth. Species-area relationship is well established.
LDG deterministic hypotheses
Equilibrial hypothesis Age
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.
Synthesis of LDG hypotheses
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
Diversity cradle or museum?
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
Rapoport’s Rule
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
Janzen’s Hypothesis consistent with Rapoport’s rule
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
Relationship between dispersal barriers and species richness
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.
Species Dominance
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.
Peninsula Effect
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
Spatial diversity gradient
Altitude and depth
Increase in elevation = decrease in diversity (ex. vegetation in santa catalina mountains)
increase in depth = decrease in diversity (ex. marine fauna)
