Paper 29 by Rebecca Kiat

Paper 29 by Rebecca Kiat

Rabinowitz, D. 1981. Seven forms of rarity. Pages 205-215 in H. Synge, ed. The biological aspects of rare plant conservation. Wiley, New York.

Paper Author: Deborah Rabinowitz

(Commentary by Kevin J. Gaston)

Deborah Rabinowitz

-       Ph.D. from the University of Chicago

-       First female faculty member in the Department of Ecology and Evolutionary Biology at the University of Michigan

-       Tenured at Cornell in e Section of Ecology and Systematics within the Division of Biological Sciences until August, 1987

-       Passed away at the age if 39 in August 1987 from cancer complications

“Although Deborah had only twelve years between her Ph.D. degree and her death at the age of thirty-nine, she made substantial contributions to the general field of plant population biology. By far her most significant contribution is to our understanding of why some kinds of plants are so much less common than others. The question of differences in species abundances has had a long history of interest in ecology, but Deborah brought to it a fresh and highly original approach. In 1981 she published a landmark paper in which she described seven different meanings of the concept of “rarity”.” – From University of Cornell commons website

Kevin J. Gaston

-       Ph.D. from the University of York

-       Founding Director of Environment & Sustainability Institute, University of Exeter

Professor of Biodiversity & Conservation, University of Exeter

-       Website: http://kevingaston.com/

“I lead basic, strategic and applied research in ecology and conservation biology, with particular emphases at present including common ecology, ecosystem goods and services, land use strategies, and urban ecology.” – From University of Exeter website

What makes a species rare? What makes some species more scare compared to other species e.g. is it due to inferior competitive abilities?

There are different ways a species can be rare. As Rabinowitz mentions, a species may have large local abundances but they may only be found in specific, limited habitat types, while other species may be very common across different habitats, even though that species is never dominant in any one of those habitats (low population size).

In her chapter, “Seven forms of rarity”, Rabinowitz discusses a theoretical framework of a way to classify rare species with different forms of rarity using examples from North American flora. Rabinowitz also discusses experimental studies she conducted (Harper 1977: de Wit plots) with prairie grasses from Missouri to assess competitive effect on abundance in terms of population size as well as the size of the individuals.

-       Focus on exploring the biological (evolutionary and ecological) consequences of rarity.

-       Note: Difference between causes of rarity and consequences of rarity.

-       Rabinowitz proposed a simple scheme to focus thoughts instead of focusing on specifics with monolithic rarity – allow for further discussion when looking at species comparatively instead, perhaps allowing for a shift in perspective.

A classification of rare species

Three categories for rarity:

i)              Geographic range – large vs. small

ii)            Habitat specificity – wide vs. narrow

iii)          Local populations size – large, dominant vs. small, non-dominant

-       Used these categories to form a 2 x 2 x 2 block to compare between and look at different species.

-       Talk about results vs. processes that result in observed rarity

-       Extinction – In light of these categories, how might this affect the risk of a species to extinction?

Examples:

o   Demographic stochasticity with small samples in causing local extinctions

o   Habitat destruction – species endemic to mangrove swamps even if locally common may be subject to high risk with anthropogenic intervention

-       Importance of comparisons: “Monitoring rare species (for instance, Bradshaw's long term assessments of the Teesdale rarities) tells us a lot about the characteristics of these taxa. However, in the absence of comparative data for related common taxa, essentially control species, we cannot judge whether the traits of rare plants are unique to them or are some random sample of plant traits in general and unrelated to the rare state.”

Competitive abilities of sparse species

-       Prior claims – a species is rare because it is an inferior competitor?

-       Setup of de Wit plots to compare competitive effect of different species on abundance and size of plants – different proportions of each species in pairwise experiments with combinations of rare and/or common species

-       Compare monocultures with yield when grown in pairwise setup

-       General results:

o   Sparse grasses yields fall ABOVE expectation with monocultures.

o   Common grasses yields fall BELOW expectation with monocultures.

o   Conclusion – Rarer species are superior competitors? Other factors that may result in rarity?

-       Note: Is abundance the only way to gauge the success of a species?

Natural selection and sparse species

 – What are the other factors that may explain why a species is rare?

-       Advantage to a rare species only when it is rare?

o   E.g. density-dependent fungal pathogen being a limiting factor with chestnut species