Ecological Sorting Research Articles

The origins and detection of plant community structure: Reproductive versus vegetative processes

The exchange of ideas and information between vegetation ecology and pollination ecology is relatively restricted, yet both fields have devised methods to detect the structure of species assemblages and communities. To promote the exchange of ideas between fields I compare approaches, concepts, and problems faced by researchers working in each area. Both vegetative and reproductive interactions may generate assemblage structure through ecological sorting or through character displacement. Vegetative interactions may lead to assemblage organization more often by ecological sorting and reproductive interactions more often by character displacement. Vegetative interactions generally operate over shorter temporal and smaller spatial scales than reproductive interactions and may be affected more strongly by temporal and spatial heterogeneity in abiotic and biotic environments. These differences affect how the concept of ecological niche should be applied to plants. The Hutchinsonian concept of niche needs to be significantly modified before it can be usefully applied to plants.

Floral Character Displacement Generates Assemblage Structure of Western Australian Triggerplants (Stylidium)

We studied the pollination ecology and assemblage structure of 31 species of Stylidium (Stylidiaceae) at 25 sites in Western Australia. The number of species per study site varied between two and size. Stylidium species are pollinated by a variety of nectar—seeking solitary bees and bombyliid flies. Within and among species there is significant variation in nectar—tube length (and therefore in the insects that visit the flowers) and in pollen placement on pollinators. Pollen is placed “explosively” on the insect by a motile column of fused staminate and pistillate tissues; the position and reach of the column varies within and among species, thereby causing variation in site of pollen deposition. When discrete pollination niches were defined for all species, only one niche overlap was observed across the 86 interacting pairs of Stylidium species at the 25 sites. To determine whether this was a nonrandom assemblage structure we compared our observation with the outcome of null models. We developed three null models to cover the most likely structuring processes: that communities are organized by (1) ecological sorting, (2) evolution of plant phenotypes, or (3) both processes. We concluded that it was unlikely (P = .055—.002) that so few overlaps in pollination niches would occur by chance. We developed another null model to test whether chance could have created the apparent pattern of character displacement in pollination niches exhibited by the nine species showing intraspecific variation. The analysis indicated that character displacement has probably occurred (P = .014). This study is one of the clearest demonstrations to date of reproductive interactions generating assemblage structure and character displacement in plants.

Habitat selection by chemically differentiated races of lichens.

The maritime European lichens of the aggregate species Ramalina siliquosa represent six chemical races. Where the races are sympatric they populate different habitats. Such intensive local ecological sorting of morphologically similar individuals accumulating different, highly specialized metabolic end products appears to be unknown in other plants.