J. M. Iriondo

Spatial analysis of genetic diversity as a tool for plant conservation

Development of suitable approaches to the analysis of genetic diversity in a spatial context, where factors such as pollination, seed dispersal, breeding system, habitat heterogeneity and human influence are appropriately integrated, can provide new insights in the understanding of the mechanisms of maintenance and dynamics of populations. In this sense, it is important to recognise that patterns and processes may take place at different scales at the same time, and that the scales of a study must be chosen in accordance with the objectives pursued. Apart from conventional approaches to genetic structure, spatial autocorrelation and related techniques, such as Mantel test, correlograms, Mantel correlograms, join-counts, variograms and point pattern analysis, can detect and characterise the existence of spatial genetic structures and lead the way to discussing the environmental and biological factors responsible for them. An alternative way of including spatial variability in modelling approaches that deal with genetic patterns or processes is through the use of constrained ordinations. Although scarcely used at present, these methodologies have great applicability in conservation biology and can lead a way to an effective integration of genetic, demographic and ecological perspectives.

Plant conservation: old problems, new perspectives

A review is given of the major conceptual changes that have taken place during the last 50 years in our understanding of the nature of plant conservation and of the principal methodological advances in undertaking conservation assessments and actions, largely through the incorporation of tools and techniques from other disciplines. The interrelationships between conservation and sustainable use are considered as well as the impact of the development of the discipline of conservation biology, the effects of the general acceptance of the concept of biodiversity and the practical implications of the implementation of the Convention on Biological diversity. The effect on conservation policy and management of the accelerating loss or conversion of habitats throughout the world and approaches for combating this are discussed.

Structural equation modelling: an alternative for assessing causal relationships in threatened plant populations

Structural equation modelling (SEM) is a powerful tool to explore and contrast hypotheses on causal relationships among variables using observational data. It constitutes an alternative to experimental approaches that is especially useful in the conservation of small populations where the implementation of treatments may have a negative effect on population viability. We are presently applying SEM to study the factors that condition reproductive success, seed emergence and plantlet survival in several plant species. We are also using model comparisons through multi-sample analysis to assess the implications of different microhabitats on the viability of a population. The most outstanding advantages of this tool are the global perspective used in the study of complex problems, the ability to discern the essential from the accessory, and the possibility of evaluating ones own hypotheses. The basic procedure, the limitations of this method and further applications in conservation and management are also discussed.

Factors affecting establishment of a gypsophyte: the case of Lepidium subulatum (Brassicaceae)

The restriction of vascular plants to gypsum-rich soils under arid or semiarid climates has been reported by many authors in different parts of the world. However, factors controlling the presence of gypsophytes on these soils are far from understood. We investigated the establishment of Lepidium subulatum, a gypsophyte, in a nondisturbed semiarid gypsum-soil landscape in central Spain, both from spatial and temporal perspectives. Over 1400 seedlings were tagged, and their growth and survival were monitored for a 2-yr period. Several biotic and abiotic variables were measured to determine the factors controlling the emergence and early survival. These variables included the cover of annual plants, bryophytes, lichens, litter, gypsum crystals, bare fraction and cover of each perennial plant, and several soil properties (gravel, fine gravel, and fine-earth fraction, conductivity, pH, gypsum content, organic matter and penetrometer soil resistance). Our results support the linkage of gypsophily with some physical properties of the surface crust. Seedlings tended to establish on the gypsum surface crust, and their survival was size dependent, probably as a consequence of the necessity of rooting below the surface crust before summer drought arrives. However, once seedlings emerged, a higher survival rate occurred on the alluvial soils of the piedmont-slope boundary where soil crusts are absent or thinner. We conclude that Lepidium subulatum may be considered a refuge model endemic with a distribution range that occupies a reduced fraction of a wider habitat from which it is probably excluded by competition.

Genetic structure of an endangered plant, Antirrhinum microphyllum (Scrophulariaceae): allozyme and RAPD analysis.

Thirteen allozyme loci and 68 random amplified polymorphic DNA (RAPD) markers were analyzed to assess the genetic diversity and population structure of threatened Antirrhinum microphyllum (Scrophulariaceae), a narrow endemic of central Spain known from only four populations. According to allozyme data, species genetic diversity (p = 46.15%, A = 2.61, and H(e) = 0.218), as well as within-population genetic diversity (p = 44.23%, A = 2.10, and H(e) = 0.204), were high when compared to average estimates for other narrowly distributed plant species. Ninety-four percent of species genetic diversity corresponded to within-population genetic diversity. Nevertheless, significant differences were found among populations in allele frequencies of four of the six polymorphic loci, and three private alleles were detected. Inbreeding coefficients (F(IS)) suggest that populations are structured in genetic neighborhoods. The RAPDs also showed high levels of genetic diversity (p = 89.71% and H(e) = 0.188 at the species level, and p = 67.65% and H(e) = 0.171 at the population level). Neis genetic distances estimated both from allozymes and RAPDs indicated low differentiation among populations. In spite of this, the low frequencies of certain alleles and the presence of private alleles indicate that efforts should be made to conserve all four remaining populations.

Short communication: Weighted-Interaction Nestedness Estimator (WINE): A new estimator to calculate over frequency matrices

We propose a new nestedness estimator that takes into account the weight of the interactions, that is, it runs over frequency matrices. A nestedness measurement is calculated through the average distance from each matrix cell containing a link to the cell with the lowest marginal totals, in the packed matrix, using a weighted Manhattan distance. The significance of this nestedness measure is tested against a null model that constraints matrix fill to observed values and retains the distribution of number of events. This is the first methodological approach that allows for the characterization of weighted nestedness. We have developed a graphical user interface (GUI) running in Matlab to compute all these parameters. The software is also available as a script for R-package and in C++ version.

FEMALE REPRODUCTIVE SUCCESS OF NARROW ENDEMIC ERODIUM PAULARENSE IN CONTRASTING MICROHABITATS

In 1996, we investigated the female reproductive success of the narrow endemic Erodium paularense in a population with a mixture of rock and lithosol micro- habitats. The main purposes of this study were to determine the main factors that affected reproductive success and whether these factors were influenced by differences in the two contrasting microhabitats. In both microhabitats, fruit set was low, and seed abortion rates were very high. Lithosol plants were larger and had higher flower, fruit, and seed production than rock plants. We used structural equation modeling (SEM) to analyze the effects of plant size, phenological variables, competition, and flowering synchrony on fecundity and to compare path coefficients among microhabitats. In both microhabitats reproductive suc- cess was strongly predicted by plant size and to a lesser extent by flowering synchrony. Flowering moment and intensity did not have a significant effect on fruit set, suggesting that pollinator availability is not a limiting factor in total seed production. This was further corroborated by hand-pollination experiments. Resource limitations, inbreeding depression, and environmental restrictions may be responsible for the high rates of seed abortion. Multisample comparison of path coefficients for the two microhabitats rejected the pos- sibility that reproductive patterns could be described by one single model. The main di- vergences in both microhabitat models were related to competition. Reproductive data from 1995, a year with below-average rainfall, showed remarkably lower seed production at the lithosol microhabitat and similar values at the rock microhabitat as compared to 1996. The variation of the reproductive parameters between the two years was evaluated by multi- sample analyses in which 1995 data were fed into the 1996 models. At the lithosol micro- habitat, two constrained paths involving seed production produced a significant decrease in fit, whereas at the rock microhabitat, significant differences were found in three paths involving only phenological variables. Thus, the lithosol microhabitat may contribute to greater seed production in favorable years while the rock microhabitat may provide more stable conditions over time.

Pollination patterns limit hybridization between two sympatric species of Narcissus (Amaryllidaceae)

Natural hybrids between rare and common sympatric species are commonly eradicated to avoid the potential extinction of the rare species, although there is currently no clear predictive framework to quantify this risk. As hybrids can have intrinsic value as new evolutionary pathways, further knowledge on the factors controlling hybridization is needed. In this study we evaluated the role of pollination patterns in hybridization events in two sympatric populations of Narcissus cavanillesii and N. serotinus in Portugal. Narcissus cavanillesii is a rare species, while N. serotinus is widely distributed across the Mediterranean. The hybrid, N. ×perezlarae, is quite frequent in southeastern Spain but is scarce in Portugal. Reciprocal manual crossings confirmed compatibility between the two species, although hybridization was more successful when N. cavanillesii participated as female. Narcissus cavanillesii and N. serotinus only shared one pollinator, Megachile sp. (Hymenoptera), which had low visitation rates and high flower constancy. No single isolation mechanism was fully effective in preventing hybridization. Temporal displacement of flowering peaks, strong pollinator specificity, and high flower constancy in the shared pollinator all contributed to limiting hybridization in this site. In other sympatric occurrences, different phenological windows and pollination assemblages may allow greater frequency of the hybrid.

Growing with siblings: a common ground for cooperation or for fiercer competition among plants?

Recent work has shown that certain plants can identify their kin in competitive settings through root recognition, and react by decreasing root growth when competing with relatives. Although this may be a necessary step in kin selection, no clear associated improvement in individual or group fitness has been reported to qualify as such. We designed an experiment to address whether genetic relatedness between neighbouring plants affects individual or group fitness in artificial populations. Seeds of Lupinus angustifolius were sown in groups of siblings, groups of different genotypes from the same population and groups of genotypes from different populations. Both plants surrounded by siblings and by genotypes from the same population had lower individual fitness and produced fewer flowers and less vegetative biomass as a group. We conclude that genetic relatedness entails decreased individual and group fitness in L. angustifolius. This, together with earlier work, precludes the generalization that kin recognition may act as a widespread, major microevolutionary mechanism in plants.

Patch Dynamics and Islands of Fertility in a High Mountain Mediterranean Community

Abstract Vegetation in high Mediterranean mountains usually consists of patchy communities. Patch structures have been interpreted as a result of the prevalence of facilitation phenomena in highly stressful environments. Several mechanisms have been proposed in order to explain the factors that control the existence of these clumped structures. However, they have not been evaluated in these mountains. Our hypothesis is that patchy structure in high mountain Mediterranean vegetation is a consequence of facilitative and competitive interactions in a very harsh environment which ultimately involve strong localized effects on soil properties. Our results show that levels of soil nutrients were higher under vegetation patches than in bare ground areas, confirming the hypothesis of an amelioration of soil resources under canopies. Pairwise associations and repulsions suggest the existence of two contrasting composition stages. Contrasting models relating patch species composition (cover and biomass) and soil resources indicated a weak relationship between species features and soil nutrient levels. Finally, structural modeling showed that patch size has a relevant but indirect effect on soil resource levels through grass and total biomass. We conclude that patch structure and dynamics in high Mediterranean mountain communities may be partly controlled by an endogenous process involving facilitation and competition for soil key resources. These interactions may operate through some community traits related to patch size but not to composition.