Grouping and prioritization of vascular plant species for conservation: combining natural rarity and management need

Abstract

National and International Red Lists and Legal Acts specify species with conservation needs, mainly on the basis of personal experiences. For effective conservation we need scientifically justified prioritization and grouping of these species. We propose a new combined approach where species are grouped according to the similar activities needed for their conservation. We used the national list of vascular plant species with conservation need for Estonia (301 species), and linked these species to eight qualitative conservation characteristics, four reflecting natural causes of rarity (restricted global distribution; restricted local distribution within a country; always small populations; very rare habitat type), and four connected with nature management (species needing the management of semi-natural grasslands; species needing local disturbances like forest fires; species needing traditional extensive agriculture; species which may be threatened by collecting). Only one positive association occurred among the characteristics – between restricted local distribution and small size of populations. Thus, natural causes of rarity and management aspects are not overlapping, and both should be used in conservation activities. Species grouping by different conservation characteristics allows one to focus on species groups with similar conservation needs instead of individual species. Prioritization of species with conservation needs can be based on the number of conservation characteristics that are associated with a particular species. Our prioritization did not correlate with the categories of the national Red Data Book, but a positive association was found with legal protection categories. The legislation, however, covers only the natural causes of rarity. We propose a new combined approach for plant species’ conservation planning that starts by considering human induced rarity and progresses through to natural rarity causes.

Introduction

There is now a growing worldwide concern about the status of biodiversity. The IUCN Red List is being developed as one of the tools that helps to organize biodiversity conservation (Lamoreux et al., 2003). In recent years, Red Lists have enjoyed an increasingly prominent role in guiding national conservation activities (Gärdenfors, 2001). In Europe, there are also lists of species that deserve attention across all the European countries and are listed in the annexes of the European Commission Habitats Directive (1992; 92/43).

Species listed in Red Data Books, lists of species protected by national laws, and species in EC Directives are all accorded certain ‘conservation status’, i.e. they are considered to be endangered by some kind of factor on a certain scale, and active or passive means have to be taken to stabilize or improve their status. Red Lists describe how threatened a species is (low number of localities found or low number of individuals), and what the major reasons of its decline may be (Hilton-Taylor, 2000). Habitat loss, direct exploitation, indirect human influence through changing local ecological interactions, natural disasters, pollution and intrinsic factors (unfavorable species traits) have been listed among the main threats. Such information is frequently based only on the personal experience of a few conservation biologists; knowledge is often limited. Conservation legislation, however, also includes other types of arguments such as commercial importance and aesthetics. The conservation assessment of different taxonomic groups is unequal. For example, conservation priorities for European birds have already been assessed scientifically 10 years ago (Tucker et al., 1994). Works on insects, plants and fungi are still very preliminary, probably due to larger numbers of species and smaller numbers of people working with them.

Biodiversity conservation needs careful planning (e.g. Younge and Fowkes, 2003). An example is a scientifically reasoned action plan for a particular species (Palmer, 1996). The number of species with conservation needs has, however, increased to a level where we no longer have the time and resources to elaborate action plans for each individual taxon. When focusing on some subjectively selected ‘hot’ species, the status of overall diversity may worsen. Instead, one might collect a kind of general information about the possible factors and mechanisms behind the real or potential decrease of the local or global abundance and distribution area of these species. On the basis of such information, one could prioritize species conservation needs and establish groups of species that could be subjected to similar action plans.

Besides being important for practical nature conservation, the study of the causes of plant species rarity has been of fundamental importance for understanding the distribution and dynamics of plant species (Kunin and Gaston, 1997, Rosenzweig, 1997). There have been two predominating approaches to the study of possible causes of plant rarity. The first ‘inductive approach’ has focused on the comparative study of the traits of rare and common plant species. Despite high expectations, the results of such studies have not always made much conceptual advance, since no clear differences between the traits of rare and common plant species have emerged (Bevill and Louda, 1999, Murray et al., 2002) and connections between traits and rarity are evidently context-dependent (e.g. Pilgrim et al., 2004). Until the ecological mechanisms behind plant rarity are unveiled, one cannot expect too much for practical species conservation from the study of species traits.

The second ‘deductive approach’ is based on classifying plant species into so-called rarity categories according to the nature of their distribution and habitat requirements. Rabinowitz (1981) offered a general scheme where species were classified into categories according to their geographic range, habitat specificity and local population size. These categories offer a general hypothetical explanation as to why a species is rare. Species with narrow geographic range may be rare for historical reasons. In the case of habitat specificity, the basic reasons of rarity are of evolutionary origin, but habitat destruction may play a role as well. If local populations are small, local factors such as biotic interactions and human impact are probable causes of rarity. There are rather few studies that have attempted to use Rabinowitz’s scheme in regard to the flora of certain regions, e.g. British Isles (Rabinowitz et al., 1986), France (Médail and Verlaque, 1997), Spain (Blanca et al., 1998) and Amazonia (Pitman et al., 1999). Until now, such an approach has been mostly theoretical and the link to practical conservation has been weak. In particular, one must note that human impact is not specifically considered in Rabinowitz’s system.

There is, however, also a third ‘synthetic approach’, which tries to combine the two previous ones and study plant traits in a wider geographic context or to screen longer lists of plant species (Lahti et al., 1991, Saetersdal, 1994, Gustafsson, 1994, Kull et al., 2002, Rogers and Walker, 2002). Though most of these trials have evidently not yet reached the stage where results could be useful for practical conservation, this approach clearly has higher application potential than the two previous ones, since it indicates the main threats for species and the main habitats for rare species. Successful examples, where the link to practical conservation already exists, include priority analysis for Central European plants, where threat status was combined with information about world-wide distribution (Schnittler and Günther, 1999), and a statistical study on rarity and threat factor relations in Iberian flora (Lozano et al., 2003).

Our intention is to develop the synthetic approach to the stage where it can both help to understand possible causes of plant rarity in a particular region, as well as to meet the needs of practical nature conservation, like the elaboration of conservation priorities within the list of species of conservation interest and the grouping of plant species according to the similar activities needed for their conservation. Many earlier decisions about conservation status and possible threats rely on tradition, not on analysis of scientific information. In the ideal case, determination of the status of a management plan for species has to rely on scientific information about the distribution and ecology of the target species. Because of that, there is an evident need to screen the lists of species with conservation need in relation to factors responsible for rarity.

In particular, we aimed to analyze the national list of vascular plant species with conservation need in order to understand the main possible causes of plant rarity, to suggest new priority categorization within the list and to group plant species according to different conservational aspects. Grouping could make it possible to compile management plans, not for single species, but for the whole set of species. In this way, the national plant diversity would be protected more efficiently. We used the full list of species with conservation status in Estonia – a small country with a well-known vascular plant flora and a long tradition of nature conservation – as a model for the survey (Sepp et al., 1999). We tested whether our conservation characteristics are related to each other, and how our conservation characteristics are related to the present conservation legislation and national Red Data Book criteria. Finally we propose a new synthetic approach for plant species conservation that starts by considering human induced rarity and progresses through to natural rarity causes.