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Why are some plants rare?

The Proteaceae contains a fair number of plant species which are listed in the Red Data Book for plants: 124 from the Cape Floral Kingdom to be exact. Thus, 38 per cent of the 325 species in the Cape Floral Kingdom are rare in some way. But what makes a plant species rare?

Man is the cause of the majority of rare plant species. There are many ways in which we threaten plants: agriculture, dams, quarrying and urbanization make land unsuitable for indigenous plant species; grazing and burning may disadvantage some plant species and favour others; introduced alien plants may outcompete some indigenous plants while favouring others; herbicides, fertilizers, pesticides and pollution from neighbouring fields may poison some plants, fertilize others and play havoc with insects which feed on plants or pollinate them.

The Red Data Books categorize species in terms of their threat. Most heavily impacted by man are those species which are eXtinct in the wild - these may still be kept alive in botanical gardens, or even be a popular garden plant, or simply be irrevocably lost to mankind. Then there are those species in imminent danger of extinction unless something is done to reduce the threats: these Endangered species form the focus of conservation strategies. Also threatened, but not in imminent danger of extinction, are Vulnerable species: here the threats are operative, but the species is currently not in danger of extinction.

Number of Proteaceae species in Red Data Book categories in the Cape Floristic Region. (T = no longer threatened or rare; N = Not in Red Data Book)

Of course, man is not the only cause of rarity: many species are naturally Rare. In fact, many threatened species were already naturally rare before we started making things unsuitable for their continued existence. A species can be rare for a variety of reasons:

- It may occupy only a few locations (or even only one) in a geographically restricted area, and be known from nowhere else (Localized). At these locations it may occur in huge numbers and perhaps even dominate the landscape.

- It may occur at very low densities (Sparse). Even though it may occur over a wide area in many habitats, the low densities mean that there are not very many plants in existence. Some of our resprouters (Pr speciosa, and in many areas, Pr cynaroides) are usually sparse.

- It may exist in a rare habitat (Habitat specific). It may occur in huge numbers, perhaps even dominating the habitat, but it is strictly confined to a specific habitat. It may occur over a wide geographical range if the habitat is widespread; such species will be rare if the habitat is rare.

Of course, there is nothing to prevent a species from being Localized, Sparse and Habitat-specific at the same time, or any combination of these three factors. A little arithmetic gives us seven forms of rarity and only one of commonness (i.e. species that are geographically widespread, have dense populations and are not habitat-limited).

Rabinowitz categories of Rarity.

But what makes a species rare? One may think of the life of a species as: starting off; spreading; in full glory; fragmenting; dying out; perhaps spreading and achieving glory several times; and finally dying out and going extinct. The vast majority of species start off their existence as rare. These species are most likely to be Localized. Depending on competition with other plant species, predation on the plant and its seeds by fungi, insects and vertebrates, and on associations the species may have with animals (for pollination, seed dispersal, or browsing) or fungi (for nutrient uptake or diseases) the species may be limited to specific habitats (Habitat specific), or not, or may form dense stands, or be Sparse. On its way to extinction the species may again be Localized, but extinction may be warded off for millennia. Extinction may not imply that the species leaves no descendants: by hybridization or speciation the "extinct" species may live on in a new guise. We can thus identify several causes for the rarity of species: predation, competition, symbiotic associations, "recent birth" or "impending death".

Whatever the cause of rarity, when species are rare they are far more vulnerable to everyday events than are common species. A single flood, drought, hurricane or farmer may wipe out a localized species that would have survived if its geographical range was merely 50ÿm larger. By chance, a small population could contain only males and thus not be able to breed, whereas twice as many plants would have made that less likely. Rare genes will be lost in small populations, simply because there are too few individuals in which to store them: these lost genes may, for example, be the ones allowing the species to survive periods of rare fungal attack. When genes are lost the surviving individuals become more alike: they will thus react more similarly to changes in their environment - if this is unfavourable the species may go extinct, whereas in a larger population some of the most different individuals would survive and breed and allow the species to recolonize its habitat. Del Wiens, University of Utah, contends that declining species tend to become increasingly specialized to local habitat conditions, so much so that the plant may "seem out of place" in its habitat.

Note the importance of numbers! The more individuals that comprise the population the better the future survival of the species. Work done on animals suggests a magical number of between 50 and 500 to ensure survival over 200 years. This number is "magical", because like all magic, it does not really exist and can trick one into a false sense of security. Each species must be treated separately, and there are no estimates of minimum population sizes for plants. All we know is that the more individuals the better; and the fewer, the costlier it will be to prevent loss of genes, skewed sex ratios and other stochastic processes from eliminating the species.

It is thus crucial that we know just how threatened different species are. It is crucial that we know what type of rarity (or commonness) a species displays. Protea Atlassers will soon discover for themselves which species are Habitat-specific, Localized or Sparse. These data will allow conservationists to plan strategies for conserving species with different types of rarity. For instance, localized species can be preserved in a small reserve protecting their locality. Sparse species will require much larger reserves to ensure that a large enough population is preserved. Habitat specific species (e.g. wetlands) will only be preserved if their habitat is preserved. Species with combinations of these characters will require more complicated strategies for their preservation.

Your data on population size and dispersion (Protea Present Box), the data in the Habitat Box, and the localities of the SRS will allow us to re-evaluate which species are Sparse, Habitat specific and Localized, respectively. The various fields will also allow us to determine threats to species (picking, agriculture, alien invasion, etc.). Your data will be invaluable in planning the conservation of our flora.

Rabinowitz table for the Proteaceae in the Cape Floristic Region.

It is clear from the Rabinowitz table that by far the majority of Proteaceae species are either common, only habitat specialists, or both habitat specialists and localized. Sparse species tend to be equally localized or not localized. However, sparse-localized species are never habitat specific: this combination also appears to be impossible in the United Kingdom, but the reasons for this are unknown. Any ideas?

What is interesting is that the proportion of localized-habitat specialists (including those which are also sparse) is far higher in the Cape than in other regions of the world for which data are available. By providing more accurate data, the Protea Atlas will shed considerable light on the causes of rarity in southern Africa. These data will thus form the backbone of our understanding of why southern Africa is the botanical treasure house of the world - richer per unit area than even the tropical rain forests.

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