Chapter 7. Commonness & Rarity

We have seen how a number of ecological principles can be combined to describe how communities of multiple populations (of different species) can exist on a single site. You may not have noticed, or at least not seen the implications, but we also predicted that the populations of the various species in the community will not be equal, even when each species has the same potential at its optimum environment. This has been called “resource partitioning.” This also predicts that communities should exhibit a “distribution of commonness and rarity” among its member species. In this chapter, we shall examine the distribution of commonness and rarity, as it relates to our theory of plant community ecology.

Diversity

the number of species (often of a single higher taxon, such as Order) which can potentially be found on any site in a given geographic area. Although for particular studies, diversity has been used as the number of species in the sample data, the more common usage is to include all species which could be on the study site [the field Ecologists who prefer the restricted definition, use the term ‘Species Richness’ to refer to the broader definition. I prefer the broader definition, because I have observed year to year differences in those species in the sample data. This variation can be explained, at least in part, by the statistical theory of sampling. In any sample data, some of the rarer species will not appear although they would be found in a complete census. In Chapter 4 “Limiting Factors,” I hinted that ‘absence’ in the sample data is not the equivalent of concluding that the species is not present, but is merely “missed in the sampling”]. I would suggest that we refer to the restricted definition as ‘sample diversity’ to distinquish it from the broader definition. Whittaker [Whittaker, R.H. (1972). “Evolution and measurement of species diversity.” Taxon, 21, 213-251.] defined ‘alpha diversity’ to be diversity within a community or ecosystem [equivalent to my definition of sample diversity], and ‘beta diversity’ to be the a description of changes in diversity between communities or ecosystems. In my view, the broader definition is needed because the theory of plant community ecology must account for all species which could be present, not just those which are known to be present.

Diversity, or species richness, is the number of species available to be in a community within the study area. For example, for a study of a site in the historic Grand Kankakee Marsh (including a transect from wetland to wet prairie to dry prairie) designed to test a hypothesis of mine concerning year to year variation in plant species in sample data, and to provide a field Ecology study for undergraduate students at a small private two-year Liberal Arts College in Northwest Indiana, the species richness was approximately 2,500 spp of vascular Plants. Over the three years of the study (terminated early due to a curriculum change which caused enrollment to drop below the minimum necessary to offer the course), the sample diversity was 54 species, although the students were upset because several other species were observed outside of the 1 meter square quadrats used to collect the sample data. The sample diversity by year was 27 in 2002, 32 in 2003, and 20 in 2004. I was able to show that there was a correlation (correlation coefficient = -0.8170, compared to the critical value = 0.9969 at 95% confidence; not quite strong enough to be statiatically significant [but significant at 80% confidence) between the species composition (list of all 54 species in the data set with abundances) and annual precipitation for the 12 months (June 1 to May 31) prior to each year's growing season).

Dominance

The concept of dominance was introduced as an estimate of the influence [of the species] on community function. [“Ecological dominance refers to the exertion of a major controlling influence of one or more species upon all other species by virtue of their number, size, productivity or related activities.” (“Glossary of Environment Statistics,” Studies in Methods, Series F, No. 67, United Nations, New York, 1997.) downloaded 9 Mar 2011 from Organisation for Economic Co-operation and Development] Various measures have been suggested for dominance. The simplest is abundance (or population density). Others have included cover (estimate of the size of the shadow of the plant as illuminated from directly overhead), frequency (the number of sample units in which the species occurs divided by the total number of sample units; this has also been called ‘evenness’ or the probability that the next individual found is the same as the last one) ), biomass (the dry weight of the harvested above ground parts of the plant).

Distribution of commonness & rarity

When we look at actual field data, for example using the data from the study in the Grand Kankakee Marsh cited earlier, it becomes obvious that a few species account for most of the individuals in the community.

Almost 43% of the individuals are in the first species, about 30.5% of the individuals are in the next two species, 20.8% of the individuals are in the next four species, and the remaining 5.6% of the individuals are in the last forty seven species. This is fairly typical of the distributions of commonness & rarity from published papers. Another view of this would be to see all 54 species in the data.

Here we see that the first species probably comes close to its carrying capacity. The next 6 species appear to realize their carrying capacity, assuming that they occupy the site by niche overlap with the previous, dominant species as predicted in the previous chapter (Chap 6. “Competition & Niche”). The remaining species appear to ‘fill in’ the ‘unused resource’ from the dominants (those occupying the site by niche overlap), and may be at, or may be below, their carrying capacity as suggested in chapter 6.
It is this graph (or ones like it) that lead me to think that this graph should continue to include all 2,500 vascular plant species whose ranges include the Grand Kankakee Marsh. Since the relative abundance estimates the probability that the next sample will include each species, and the 54^th species has a probability of 0.0011%, the 2,500^th species probably has a probability of less than 0.00005%, but greater than 0.000000…%. As I hope to demonstrate by the end of this text, even these extremely rare species do fulfill a function in natural Communities.

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