Ecological and evolutionary importance of neighbors in the grass Anthoxanthum odoratum.

A field experiment was performed to estimate the relative importance of neighbors and the rest of the environment for the growth, mortality and reproductive output of cloned individuals of the perennial bunchgrass Anthoxanthum odoratum. Single cloned Anthoxanthum tillers (targets) were reciprocally transplanted between a xeric and a mesic grassland site with one of four neighbor treatments: (1) no neighbors, (2) Anthoxanthum neighbors transplanted from the xeric site, (3) Anthoxanthum neighbors from the mesic site, and (4) Holcus lanatus neighbors. Targets without neighbors had a twofold higher two year reproductive output (RO) than those with neighbors, but there was no difference among neighbor treatments. No overall site effect on two year RO was found, because the site with the highest mortality among targets produced larger plants, with more inflorescences. Neighborhood competition was more intense at the xeric site than at the mesic site. The effects of environmental and neighborhood variation on Anthoxanthum were additive, rather than interactive. Population origin did not affect target performance significantly. Anthoxanthum neighbors of different origin did respond differentially to transplant site. There was a strong target genotype x site interaction, but no genotype x neighborhood interaction.

The relationship of environment size and population parameters Oxytrema proxima (say) (Gastropoda: Pleuroceridae).

Environmental size, as measured by cross-sectional area of the inhabited streams, and population density of Oxytrema proxima are related by the power function Y=8.074X -0.584, where Y=population density and X=cross-sectional area of the stream. Relationships were also found between maximum size and biomass and cross-sectional area, but maximum size and biomass were more closely related to population density by the equations Y=8.832X -0.064 (Y=maximum individual size, X=density) and Y=6.730X 0.459 (Y=biomass (total live weight), X=density). This is interpreted as evidence that environment size acts directly on density, which in turn produces effects on maximum size, total live weight, and to a lesser extent mean size. The inverse relationship between population size and environment has not been previously reported. It is clear that in this case this distribution and abundance of O. proxima are closely related, and that this can be so when populations disperse extremely poorly as well as extremely well. This must mean that dispersal is secondary in importance to population stability as an influence on distribution and abundance.