No abstract

Plants provide fundamental support systems for life on Earth and are the basis for all terrestrial ecosystems; a decline in plant diversity will be detrimental to all other groups of organisms including humans. Decline in plant diversity has been hard to quantify, due to the huge numbers of known and yet to be discovered species and the lack of an adequate baseline assessment of extinction risk against which to track changes. The biodiversity of many remote parts of the world remains poorly known, and the rate of new assessments of extinction risk for individual plant species approximates the rate at which new plant species are described. Thus the question ‘How threatened are plants?’ is still very difficult to answer accurately. While completing assessments for each species of plant remains a distant prospect, by assessing a randomly selected sample of species the Sampled Red List Index for Plants gives, for the first time, an accurate view of how threatened plants are across the world. It represents the first key phase of ongoing efforts to monitor the status of the world’s plants. More than 20% of plant species assessed are threatened with extinction, and the habitat with the most threatened species is overwhelmingly tropical rain forest, where the greatest threat to plants is anthropogenic habitat conversion, for arable and livestock agriculture, and harvesting of natural resources. Gymnosperms (e.g. conifers and cycads) are the most threatened group, while a third of plant species included in this study have yet to receive an assessment or are so poorly known that we cannot yet ascertain whether they are threatened or not. This study provides a baseline assessment from which trends in the status of plant biodiversity can be measured and periodically reassessed.

Plants of a self-incompatible species, which occur in small populations, may have reduced fitness due to the limited availability of compatible mates. Self-incompatibility decreases inbreeding by allowing successful mating to occur only with individuals which differ by at least one-allele at the S-locus. A computer simulation model was developed to test the effect of small population size upon the diversity and the relative frequency of the S-alleles which determine the number of available mates. In a large population at equilibrium, the greater the number of S-alleles the greater the frequency of available mates for all individuals in the population. In small populations (less than 50 individuals), they are unable to maintain a high diversity of S-alleles and therefore there is a decrease in the frequency of available mates. In addition, in small populations there is an increase in the variance of available mates. The number of mates in these populations depends on the genotype of a particular individual. Two patterns would be expected in a small population of incompatible species: (1) a lower seed set per individual due to limited mates, and (2) an increase in variation of seed set among individuals due to the variance in available mates. Lower seed set would lead to a decrease in fitness of particular genotypes and could increase the possibility of local extinction of the species.

The world's governments have identified reducing the rate of biodiversity loss as a global priority. However, we lack robust measures of progress toward this target. Developing indicators that are generally representative of trends in global biodiversity has presented the scientific community with a significant challenge. Here we discuss the development and implementation of the IUCN Red List Index with a new sampled approach, permitting the assessment of the conservation status and trends of large, speciose taxonomic groups. This approach is based on the IUCN Red List and measures trends in extinction risk through time. The challenges in developing this new approach are addressed, including determining the species groups to be included in the index, identifying the minimum adequate samples size, and aggregating and weighting the index. Implementing this approach will greatly increase understanding of the status of the world's biodiversity by 2010, enabling the first assessment of a number of key groups.

It is widely recognized that there are basic conflicts between the resource needs of a plant for paternal versus maternal functions. In dioecious species, these divergent demands, and the selection pressures they impose, can lead to the evolution of sexual dimorphism. The present study was conducted to assess the potential for the evolution of sexual dimorphism in Silene latifolia by evaluating the genetic variation and genetic correlation between characters and between the sexes for a range ofgrowth and reproductive characters. Sexual dimorphism is largely restricted to reproductive characters, particularly flower number and flower size. A canonical correlation analysis revealed considerable intercorrelation between growth characters, such as germination date, height, and leaf size, and reproductive characters; plants that grow fast early on also flower earlier, and plants that produce big leaves also produce big flowers. There was genetic variation for several sexually dimorphic characters; much of the focus in this analysis was on flower size, particularly calyx diameter. Finally, genetic correlations within and between the sexes were found that limit the rate of evolutionary divergence between the sexes. The genetic results suggest that S. latifolia has been subject to divergent selection on the two sexes for a long period of time, bringing about a gradual fixation of sex-limited gene effects, so that the remaining genetic effects are expressed in both sexes. Genetic correlations between the sexes that arise from this residual variation impose limits on further evolutionary change.