Taxonomic and Ecological Differences Between Rare and Common Plants in Southwestern Ontario
Taxonomic and Ecological Differences Between Rare and Common Plants
Cadotte, MW, Lovett-Doust, J. 2002. Ecological and taxonomic differences between rare and common plants of southwestern Ontario. Écoscience 9 (3):397-406.
This study, conducted in the Carolinian ecoregion of Ontario, Canada, compared the frequency of selected characteristics between rare and common plant taxa. The researchers tested the null hypothesis that phylogeny, habitat association, and life history characteristics did not differ between common and rare species. They compiled lists of rare plants with national and regional abundance ranges, drawing information from life history literature.
Results
Taxonomic Patterns
Rare species were found in families with a lower average number of species compared to common species. Generally, the number of rare species in a family correlated with the number of common species per family. Additionally, the number of rare species per family correlated with the overall number of species per family and the overall number of threatened species per family. Many families were significantly over-represented by rare species.
Habitat Patterns
Rare species were over-represented in the following life history features: absence of clonal growth and insect pollination. They were significantly under-represented in terms of bushy growth habit, clonal growth capacity, dioecy, wind pollination, and fruits with multiple pits.
Woody Taxa
Woody species were significantly over-represented by flowering periods limited to a single month, relatively larger fruits (> 50mm diameter), and animal dispersal. They were significantly under-represented among species inhabiting wet and inundated soils and those whose propagules were dispersed by wind.
Discussion
Taxonomic and Phylogenetic Patterns
The results indicate that some plant orders (e.g., Fabales) and families (e.g., Boraginaceae and Fabaceae) have significantly more rare species than expected, while others have significantly fewer. Similar results have been found in other studies. Five of the seven families containing rare species in Ontario are primarily distributed in warm tropical and subtropical regions, suggesting that these taxa may be at the edge of their biogeographical ranges.
Habitat Patterns
The findings generally align with results from a study in England, where aquatic and wetland habitats had a greater representation of rare taxa. Natural communities (forest, aquatic, and open habitats) had a higher number of rare species, with open habitats significantly over-represented. Regarding habitat differences, rare species were significantly over-represented in open and dry habitats and under-represented in aquatic habitats. The association between rare species and dry, open habitats may be weakly related to dispersal and local historical habitat.
Life History Patterns
Pollination
Biotic pollination is disproportionately associated with species with smaller geographic ranges. The finding that rare species are less likely to be wind-pollinated than more abundant species aligns with these results. Biotic interactions are strongly associated with rarity, as biotically pollinated species require suitable habitats for both the plant and pollinator. Habitat fragmentation, particularly in grassland systems, isolates plant populations and their pollinators. Isolation and reduced population sizes increase the risk of localized extinction events due to reduced immigration (population rescue events) and increased susceptibility to stochastic extinctions. Notably, rare species were significantly under-represented among dioecious species.
Fruits and Dispersal
The results indicate that regionally rare species tend to have smaller fruits and potentially fewer seeds, suggesting reduced fertility potential. No significant effects of dispersal were found, possibly because the categories of biotic versus abiotic dispersal were too broad. Kelly and Woodward’s research suggests that the association between small range size and animal pollination does not necessarily imply a strong relationship with animal dispersal.
Vegetative Reproduction
The results show that rare species across many monophyletic groups are under-represented among clonal species.
Conclusions
This study suggests that plant species ill-suited to thrive in degraded and fragmented landscapes are more likely to be listed as rare by conservation authorities. These rare species generally possess characteristics that make them poor colonizers and appear restricted to less degraded natural habitats.