What is it that makes some plants rare?

There are some plants that you see everywhere and some that you have to go somewhere special to see. Why is it that some plants seem to be inherently rare? Jennifer Nagel Boyd and colleagues set out to see if there are some common factors that rare plants share. They found that rare plants tend to have small reproductive organs, depressed fitness and reduced genetic diversity. 

The findings are due to a series of comprehensive phylogenetically controlled meta-analyses of comparative congeneric studies of plants. The team examined and organised over four thousand articles from the past forty years. To determine how a plant could be rare, they classified the plants studied using Rabonowitz’s three primary axes, looking at geographic distribution, habitat specialisation and local abundance.

Thirty-one studies had genetic data, allowing the authors to determine how genetically diverse the rare plants were compared to their more common neighbours. They also took data from over seventy studies on components of fitness, such as recruitment or damage from natural enemies.

One conclusion from the paper is that some plants are rare because there are so few of them. This isn’t as tautological as it sounds. With a small population comes reduced genetic diversity within a species. When conditions change, the species has a much smaller gene pool to dip into to find genes that can help cope with adversity.

Less genetic diversity could also mean that when a plant does find a partner, it’s not suitable. Being so genetically similar to a partner could lead to inbreeding depression.

The authors found few differences in functional traits between rare and common plants, though rare plants tended to have smaller reproductive organs. This is a drawback as insects tend to prefer bigger flowers, though it’s not a simple preference.

“Discerning the ecological and evolutionary causes and consequences of rarity remains challenging, and our meta-analyses revealed persistent gaps in our understanding of species rarity,” write Nagel Boyd and colleagues in their conclusion. “To fill those gaps, we advocate for ecologically-relevant studies that examine additional elements of rarity, estimate quantitative genetic variation in fitness and functional traits, quantify the extent of adaptive phenotypic plasticity, evaluate the contributions of life history and mating system variation to rarity, and explicitly assess the potential for rare species to persist through global change via adaptation, acclimation, and migration.”

ORIGINAL ARTICLE

Nagel Boyd, J., Anderson, J.T., Brzyski, J., Baskauf, C. and Cruse-Sanders, J. (2022) “Eco-evolutionary causes and consequences of rarity in plants: a meta-analysis,” New Phytologist https://doi.org/10.1111/nph.18172