PARENT SESSION
Oral Session #104: Plant Communities: Vegetation Analysis.
Presiding: S. Will-Wolf
Friday, August 9. 9:00 AM to 12:00 PM. Grand Ballroom West, Radisson.

Do uncommon habitats host fewer plant species? A test of the species pool hypothesis.

Schamp, Brandon^*,1, Laird, Robert¹, Aarssen, Lonnie¹, ¹ Queen's University, Kingston, Ontario, Canada

ABSTRACT- Numerous studies have reported a significant pattern of decreasing plant species richness across regional habitat gradients of increasing productivity or community biomass. This decrease is typically characterized by either a unimodal or decreasing monotonic productivity-richness relationship. Several hypotheses have been developed to explain this pattern, but none have emerged with strong supporting evidence. The species pool hypothesis postulates that the commonly reported decline in species richness with increasing habitat productivity is due to a relatively low rate of origination of adapted species in such habitats. This reduced rate of speciation in highly productive habitats is attributed to the predicted geographic rarity of such habitats through time. In this study, the prediction of relative rarity of highly productive habitats is tested for the first time with a meta-analysis of 11 published studies. A new statistical technique using Monte Carlo randomization procedures to identify significantly unimodal and/or decreasing monotonic frequency distributions of habitat productivity types was developed and applied to data from all 11 studies. In all but one study the most productive habitat types were not only lower in species richness, but were also less common in the data set than habitat types of lower and/or intermediate productivity, regardless of whether the data were interpreted as showing decreasing monotonic or unimodal species richness patterns. This relative spatial rarity of extremely fertile habitats follows practically inevitably, we suggest, from the 'left wall' effect in generating a right-skewed unimodal distribution of habitat productivity types. The relative scarcity of resident species in these habitats is perhaps, therefore, no less inevitable and hence, explainable largely in terms of the restricted historical opportunity predicted for the origination of species possessing the adaptations, including competitive ability, necessary to leave descendents under these rare habitat conditions.

KEY WORDS: species richness, productivity, species pool hypothesis, Monte Carlo methods