Dominance/diversity models

Lecture graphics

The number of species in a community and the relative abundance of each species are integral and intrinsic properties of communities. These properties lead to theories about aspects of community organization.

Now, the question that emerges is: where do these diversity relationships come from?

Niche pre-emption model

Suppose the percent of total available resources used by a species is determined by the species' success in pre-empting for its own use some portion of available resources

Less successful spp. utilize resources that are left. And so on, for all spp.

Graphically, w/ k=0.5:

In model: let	I1 = importance value of most successful sp.,
	I2 = I.V. of 2nd-most important sp., and so on ...
	Is = I.V. of least successful sp.
	Ii = N

e.g., given k = 0.4:

Sp. i	Resources available	Resources pre-empted by sp. i	Ii/Ii-1 = c	k = 1-c = Ii/res. avail.
1	100	40	--	--
2	60	24	24/40 = 0.6	24/60 = 0.4
3	36	14.4	14.4/24 = 0.6	14.4/36 = 0.4
4	21.6	8.64	8.64/14.4 = 0.6	8.64/21.6 = 0.4
5	12.96	5.184	0.6	0.4
.
.
.
s

= 100

Ii = Nk(1-k)i-1; e.g.,	I1 = 100(.4)(1-.4)1-1 = 40
	I2 = 100(.4)(1-.4)2-1 = 24

Note Ii = Nk(1-k)i-1 = I1ci-1; e.g.,	I1 = 40(.6)1-1 = 40
	I2 = 40(.6)2-1 = 24

This model predicts that spp. importance values look like this:

Question: do they really?

Conclusion: this model appears to represent reality in:

1. coniferous spp. of low species richness; e.g.,:
   • Frasier fir forest (GSMNP)
   • pine-heath forest
   • pygmy conifer-oak scrub type (AZ)

2. early-successional stages of old-field succession

3. may be appropriate for some strata not very rich in spp. even though community as a whole is not geometric (i.e., strata w/in a community)

4. in "severe" environments

In these situations, dominance tends to be strongly developed, and spp. may be related in resource use thru strong competitive interactions