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Taxonomy is the tool that measures plant diversity – and our level of knowledge

Volume 4 Number 1 - January 2007

Charlotte M. Taylor

A species without a name does not exist in terms of science and conservation. Each plant species has a unique scientific name that is the tag that allows it to be found, counted, researched, and monitored, and the index key that retrieves everything we know about it from the Internet, books, studies, databases and specimens. The specimens in turn document the physical attributes of the plant, and its geographic distribution.

Taxonomy is the work that gives the plant its scientific name and classifies it among all the other plants in the world. The classification encodes information about which other species are related to our plant, and thus indexes yet more information about our plant - the diseases that attack its relatives and so might also attack our plant, poisonous chemicals or drugs those other species contain that might be found in our plant too, or other species that might be crossed with our plant to introduce resistance traits.

Taxonomy is the same as other areas of biology: it is only as good as the available information. More information is always useful, and sometimes makes us change our minds; in taxonomy a change based on new information takes the form of a change in scientific name. Thus as new information flows in, names sometimes change in response; and new information is flowing in rapidly for tropical plants. Tropical regions are areas of high priority for conservation work because of their rich biodiversity and the rapid rates of habitat destruction; and ironically, tropical organisms are very much less known scientifically than temperate organisms. As a result, taxonomy and conservation run up against each other in these regions, where the plants are being catalogued and identified scientifically at the same time as these areas are being urgently assessed for conservation priority and management.

South America is estimated to have one of the richest floras in the world, if not the richest of all. The estimates of its plant richness vary widely depending on who does the estimate, as do counts of plant species for individual tropical countries, parks and other sites. Generally the estimate of species number for a tropical site starts with a count of the species already known from there, and extrapolates to add some proportion of species expected to be discovered there in the future. This discovery rate is relatively high for tropical areas, with discoveries coming from both bootson- the-ground exploration and detailed taxonomic study of the plants the explorers collect there. Exploration takes different forms; one widely used approach in South America is the Rapid Biological Inventory (RAP), which documents the plants found, with dried scientific specimens being made for later taxonomic study. RAP inventories make specimens because good taxonomic knowledge of an area’s plants is absolutely basic to a good conservation assessment. Knowing the level of taxonomic knowledge of the region is just as essential for conservation as understanding its geology and history of human occupation.

One measure of an area’s level of taxonomic knowledge is its rate of change. The rate of change in our taxonomic knowledge of South American plants has recently been measured for two of its most botanically rich countries, Ecuador and Peru. A comprehensive catalogue of all the plant species known from each country was published first for Peru (Brako & Zarucchi, 1993), then shortly afterwards for Ecuador (Jørgensen & León-Yánez, 1999). Aferwards all the subsequent taxonomic changes for these countries’ plants were tracked and compiled (Peru: Ulloa et al., 2004; Ecuador: Ulloa & Neill, 2005). The original checklist numbers and subsequent taxonomic changes are summarized below:

The number of changes show specifically the improvement in knowledge of the flora of each country: about what plant species grow there, and about what these plants “are” - widespread common species, local endemic species, rare species of conservation concern, invasive plants, living fossils, species of economic importance, etc.

Where do these taxonomic changes come from? There are three categories of changes, which are shown below as the number of species in each category followed by the percentage this category comprises of the total number of changes for that country:

1. “New Records”, or range extensions, are species that are already known to science from other regions, which have now been newly discovered in this particular country. These all constitute additions to the country’s known plant diversity. These additions depend on good taxonomic knowledge of the whole regional flora, so the same species can be identified throughout the region. Knowing these regional patterns of species distributions is essential for evaluation of the biological uniqueness of an area, and these range extensions often extend or refine the limits of areas of conservation concern.

2. “Taxonomic Changes” result when our improved knowledge of a plant species shows that it needs a different name. These are therefore changes in the scientific name but not usually in the identities or number of the species in the area, and thus do not change the diversity estimates for the flora. These changes do indicate our level of knowledge of the plants: more changes indicate that our knowledge is increasing.

Some taxonomic changes arise from the unfortunate discovery that two plants in different regions actually belong to the same species, so although they previously had two different names, now they have only one name and the other name is subsumed, or “synonymized”, under the first name. Other taxonomic changes arise when new information shows that the previous classification was not correct, and the plant now needs to be “moved” to a new genus or a new family. Such name changes can be frustrating to a field biologist – but can assist in finding close relatives of that plant, which may possess for example, resistance genes for a devastating fungus.

3. “New Species” are plants that are presented to science for the very first time, by receiving their first scientific description and their first scientific name. Some of these are surprising organisms that have never been seen before; while others have been “hiding” inside known species, with the same name being unknowingly applied to two different plants until a taxonomist found the confusion. The addition of new species directly increases the known diversity of the region.

When our knowledge of a region’s plants is deep, the names are generally stable, few new species are discovered, and few significant range extensions turn up. Thus a high rate of taxonomic change, as in the examples of Ecuador and Peru, shows that our knowledge was limited; while relatively stable names, as for many temperate European and North American plants, shows that our knowledge is rather good. The rate of taxonomic change thus provides a quick estimate of the level of knowledge on which conservation assessments are being based.

Do the numbers above indicate that Peru is richer in plant species than Ecuador? Not necessarily, because Peru covers a much larger area than Ecuador so more plant species would be expected to live there. Here in fact the generally similar numbers of species reported for these two countries together with the large difference in their areas actually suggest that the plants of Peru are less well known than Ecuador’s flora. Looking at this in more detail, RAP inventories of some generally comparable areas in these two countries support this taxonomic impression that Peru may be more diverse botanically. For example, the inventory of Serranías Cofá, Bermejo, and Sinangoe in Andean Ecuador (Pitman et al., 2002) estimated 2,000- 3,000 plant species for the region, while the inventory of the Río Biabo-Cordillera Azul region in Peru (Alverson et al., 2001) was done by the same team with the same methodology around the same time and estimated 4,000-6,000 plant species for that region.

Does the higher number of taxonomic changes indicate that Peru has more active taxonomic work underway, and thus our knowledge of its flora is better? Not necessarily, because the larger number of taxonomic changes for Peru’s plants suggests that actually the flora was less well known to begin with, so more work was needed to bring its scientific understanding to the same level as that of Ecuador’s flora. Thus probably the plant diversity assessments from Ecuador are more taxonomically reliable because of the apparently better knowledge of the flora.

If Peru may have more plant species than Ecuador does, why are the numbers of known species similar for both countries? This indicates in part that Ecuador is probably better explored and known, and also that we are probably much closer to the frontiers of taxonomy in Peru than in Ecuador. The scientific frontier here is the difference between the high number of species that we expect to find in Peru vs. the lower number of species we have actually found. This taxonomic gap, or “taxonomic impediment”, is a gap in our knowledge: the plant species we know about now and the species yet to be discovered and named, which do not yet “exist” scientifically.

This taxonomic gap will never completely close, but it will diminish in direct proportion to the amount of time and effort applied to the study of the plants of both these regions. In the meantime this taxonomic gap must be kept in mind, for both local work and regional comparisons, as a confidence limit for our knowledge of species richness and biodiversity. This taxonomic gap exists for all tropical floras, though its size varies widely between regions. Some on-line indexes to the publication of new plant names allow simultaneous searches by year of publication and country, which allow comparable estimates to be made for the rate of taxonomic change in different regions. An example is the World Checklist of Selected Plant Families of the Royal Botanic Gardens, Kew, England: http://www.rbgkew.org.uk/wcsp/home.do Only selected families are indexed here, but this information provides a preliminary estimate of the recent rate of taxonomic change for a given region. However the information in this type of index does not track range extensions, nor indicate the starting level of knowledge of the regional flora. That sort of information depends on detailed taxonomic tracking of a particular flora, as done by Ulloa et al. (2004) and Ulloa & Neill (2005).

Looking again now from our taxonomic viewpoint at current estimates of species richness, is South America really the richest continent for plant diversity? We can now see that this answer depends in large part on the size of the taxonomic gap in South America vs. that of other regions. The number of species known from various continents is not greatly controversial, but the total estimated richness depends also on the expected rates of discovery for each region, and those rates are sometimes difficult to estimate accurately. The African flora is well enough known to confirm that South America is more diverse. However the level of knowledge of much of the flora of Asia and Oceania is probably lower than that of South America, so the taxonomic gap cannot even be accurately estimated until taxonomy advances in all of these regions.

References

  • Alverson, W. S., L. O. Rodríguez & D. K. Moskovits (eds.), 2001. Rapid Biological Inventories: 02: Peru: Biabo Cordillera Azul. The Field Museum, Chicago.
  • Brako, L. & J. L. Zarucchi, 1993. Catalogue of the Flowering Plants and Gymnosperms of Peru. Monogr. Syst. Bot. Missouri Bot. Gard. 45: 1- 1286.
  • Jørgensen, P. M. & S. León-Yánez (eds.), 1999. Catalogue of the Vascular Plants of Ecuador. Monogr. Syst. Bot. Missouri Bot. Garden 75: 1-1181.
  • Pitman, N., D. K. Moskovits, W. S. Alverson & R. Borman A. (eds.), 2002. Rapid Biological Inventories: 03. Ecuador: Serranías Cofán, Bermejo, Sinangoe. The Field Museum, Chicago.
  • Ulloa Ulloa, C., J. L. Zarucchi & B. León, 2004. Diez años de adiciones a la flora del Perú: 1993-2003.Arnaldoa, Ed. Espo.
  • Ulloa Ulloa, C. & D. A. Neill, 2005. Cinco Años de adiciones a la flora del Ecuador, 1999-2004. Missouri Botanical Garden, Universidad Técnica Particular de Loja, & Funbotánica, Ecuador.
Charlotte M. Taylor
Postal address: Missouri Botanical
Garden, P.O. Box 299, St Louis,
MO 63166-0299
E-mail: Charlotte.Taylor@mobot.org
Tel: (314) 577 5100
Fax: (314) 577 0820