Elizabeth P. Binney

Ph.D. (1997)

Thesis Title:

Comparative Analysis of Community and Population Levels of Organization in the Rare Grass, Achnatherum Hendersonii

Abstract:  Studies of rarity tend to focus on the population level of the species in question.  A complete understanding of rarity however, requires understanding both population and community patterns and processes.  To that end, population and community levels of organization were compared in the rare grass, Achnatherum hendersonii. Achnatherum hendersonii occurs in small, local populations in central Washington and north-central Oregon.  I compared sympatric and disjunct populations of A. hendersonii from its northern and southern distribution limits.  I also compared A. hendersonii with the closely related common grass, A. lemmonii.  Plant species coverage in vegetation plots was used to estimate community level organization, and morphology of inflorescence and vegetative leaves was used to estimate population level organization.  Principal components analysis (PCA) was used to make comparisons.  Organization was assessed through the calculation of 95% confidence level bivariate ellipses about the first two PCA axis scores.  The ellipse statistics theta, eccentricity, and area were used to quantify three components of organization for each group: theta reflects the covariance or organization sensu stricto; eccentricity reflects the strength of correlation or organization as order; area relates to the amount of variation.  Community level organization differed throughout the distribution range of A. hendersonii.  At the northern distribution limit, variation in the vegetation was less for the rare grass than the common grass, however, in the southern limit A. hendersonii vegetation had the greatest overall variation.  In the southern end of the distribution range, variation in the vegetation was influenced by the coverage of cryptogamic crust.  Population level organization was similar throughout the distribution range of A hendersonii.  Achnatherum hendersonii morphology was congruent within and among populations.  There was less morphological variation in Achnatherum hendersonii than A. lemmonii.  Overall, community and population levels of organization were dissimilar.  Comparisons of community and population levels of organization of rare species increases our theoretical understanding of biological organization, by increasing our power of examination.  In this case, they reveal different responses by the plant as compared with its vegetation.  This different response argues for different causal agents affecting morphology and vegetation.