Dominance/diversity relations

Graphics

"Dominance" is used several ways:

physiognomic dominance--sp. w/ greatest cover or size

aspect dominance--most apparent or noticeable

functional (sociological) dominance--sp. whose removal causes greatest change in community

e.g., Pearson (1942, Ecol. Monogr. 12:315-338), investigated effects of grass spp., root depth, and season of growth on ponderosa pine establishment

Grime linked dominance and diversity w/ competition:

Competition between plants reaches a maximum in circumstances that allow for rapid development of large biomass material, i.e., in conditions of high productivity and minimal disturbance. Under these conditions, the species that prevail are those that are best equipped to capture resources and maximize production: high competitive ability is represented by a combination of genetic characteristics which, by maximizing production, facilitate the exclusive occupation of fertile, relatively undisturbed environments.

Good competitors, acc. to Grime, have the following characteristics:

1. tall stature

2. morphological adaptations

3. opportunistic phenological growth pattern, able to quickly exploit ppt. or sunlight when it becomes available

4. produce thick litter layer

In his 1973 paper (Nature 242:344-347), Grime asked "Is there a correlation between the appearance of these characteristics and a decrease in species richness because subordinates are eliminated by dominants?"

Competition, based on Grime's characteristics:

1. largely ignores below-ground portion of plant

2. indicates a species' ability to exert competition, not withstand it

3. is site-specific

Nonetheless, Grime concludes that there is strong evidence that competitive exclusion is a causal factor in low species diversity

In summary, acc. to Grime:

1. Competition between plants is greatest in relatively productive, stable habitats.

2. The species that prevail in these habitats tend to exhibit a suite of characteristics (e.g., tall stature, rapid growth rates) which maximize the capture of resources;

3. this results in dominance over competitors.

4. And these dominance relations are related to species diversity (number of species) of the site.

Caveat: competitive dominance as a phenomenon depends on environment.

We have already defined one measure of diversity (richness = number of species = s)

This is diversity in the strictest sense, and is a measure of (within-community) diversity

Problems w/ s:

1. Depends on area sampled (i.e., comparisons between communities require same sampling area)

   transformations can be used to minimize this problem:

   d = s/[log(area)]^*, or

   d' = (s-1)/[log(area)]^*, where

   d and d' represent rate at which spp. are added w/ increasing area

   ^*assumes areal sampling--w/ measures not based on area, equivalent formulae are:

   d = s/[log(N)], or

   d' = (s-1)/[log(N)], where

   N = sample size

   Use of log(area) and log(N) assumes known logarithmic relationship between s and area or sample size

   
   
   

2. May be vulnerable to high sampling variability (spp. are not randomly distributed, esp. rare spp.)

3. Can be affected by dominance relations

   e.g., the expected value of s, when sampling, is dependent on total population size, number of species, and sample size

   • Given N=990, n=5, s=3 in 1:1:1 ratio (N_i=330 for all i):

     E(s) = 2.6 different species

   • Given N=990, n=5, s=10, w/ N₁=900 and N₂ = N₃ = N₄ = ... = N₁₀=10:

     E(s) = 1.4 different species