Dominique Strasberg

Why does the biota of the Madagascar region have such a strong Asiatic flavour

A corollary of island biogeographical theory is that islands are largely colonized from their nearest mainland source. Despite Madagascar’s extreme isolation from India and proximity to Africa, a high proportion of the biota of the Madagascar region has Asian affinities. This pattern has rarely been viewed as surprising, as it is consistent with Gondwanan vicariance. Molecular phylogenetic data provide strong support for such Asian affinities, but often not for their vicariant origin; most divergences between lineages in Asia and the Madagascar region post‐date the separation of India and Madagascar considerably (up to 87 Myr), implying a high frequency of dispersal that mirrors colonization of the Hawaiian archipelago in distance. Indian Ocean bathymetry and the magnitude of recent sea‐level lowstands support the repeated existence of sizeable islands across the western Indian Ocean, greatly reducing the isolation of Madagascar from Asia. We put forward predictions to test the role of this historical factor in the assembly of the regional biota.

An Assessment of Habitat Diversity and Transformation on La Réunion Island (Mascarene Islands, Indian Ocean) as a Basis for Identifying Broad-scale Conservation Priorities

Most natural habitat in the Mascarene Islands (Mauritius, La Réunion and Rodrigues) has been transformed. Although urbanisation, agriculture and alien plant invasions have transformed large areas in La Réunion, the island has by far the greatest area of intact habitats in the Mascarenes, but remaining natural areas are under threat. We propose a protocol for defining a system of habitat types and for using these to provide a preliminary assessment of conservation priorities for La Réunion. The protocol draws on existing data and expert knowledge to map habitat types, assesses the extent of habitat transformation, and quantifies heterogeneity between habitat types based on climate, topography and geology. The pattern of habitat transformation was uneven among the nineteen habitat types identified. While three habitats have lost > 95% of their original area, four still retain> 80% of their original extent. Habitat types could be grouped into the following categories: (i) transformed habitats with low levels of plant endemism, (ii) habitats confined to homogenous geology with high levels of plantendemism, and (iii) species-rich heterogeneous habitats on diverse geological types. Priority habitats were also identified using municipalities as a basis for implementation. Urgent action is required for several habitat types where a large part of the original extent has been transformed. Three municipalities which contain more than 10 habitat types should receive conservation priority. The analysis provides the basis for setting conservation priorities in La Réunion at the regional and local scale. Implications of the results are discussed with reference to requirements for finer-scale conservation planning.

Effects of alien plant invasions on native vegetation remnants on La Reunion (Mascarene Islands, Indian Ocean.

Although most of the lowland endemic forests of the mountainous Indian Ocean island of La Reunion have been converted by Man to other land-uses, relatively intact ecosystems still survive at higher elevations. Given the particular conservational value of these remnants of primary forest, detailed knowledge of the threats faced by these native ecosystems was urgently needed. Accordingly we conducted field surveys through the various plant communities on the island in order to answer particular questions regarding the vulnerability of native vegetation remnants to invasive, introduced plant species. Our results indicated that most invaders colonize human-disturbed sites most successfully. However, such alien plants are also frequent in the Reunion primary forest remnants, with at least 62 species penetrating into areas which have experienced no or little human disturbance. Nevertheless, the forest remnants were still mainly dominated by indigenous or endemic plant species and maintain what is, presumably, their original vegetational structure. The most threatening plant invaders are the trees Psidium cattleianum and Ligustrum robustwn , the bramble Rubus alceifolius , the shrubs Fuchsia magellanica and Lantana camara , and the herbaceous Hedychium gardnerianum, Boehmeria macrophylla , and Boehmeria penduliflora . We found the early stages of primary succession on young volcanic sites to be dominated by aggressively expanding populations of alien species. It appears very unlikely that native species will replace the invaders as the succession proceeds, and furthermore the survival of many indigenous species probably hinges on the active control of alien species. In order for alien plant management efforts to be successful, we propose a priority ranking based on the current and potential future extent of specific infestation, the rate of spread, the difficulty with which a species can be controlled, and its ecological impact on the areas invaded. Finally, the long-term survival of the Reunion ecosystems depends largely upon the drawing up and implementation of an adequate overall conservation policy regarding human-caused invasions by alien plants in the native vegetation.

Diversity, size composition and spatial aggregation among trees on a 1-ha rain forest plot at La Réunion

Because lowland rain forests of oceanic islands have almost always been destroyed soon after human colonization, there exist few quantitative descriptions of tree species composition and diversity in such forests. For this reason, the diversity and structure of a lowland tropical rain forest were studied on a 1-ha permanent plot on the oceanic island of La Réunion. A total of 1270 individuals of gbh ≥25 cm (girth at breast height), including at least 43 species, were mapped, measured and identified. Several species were represented only by large individuals. Ten species among the 23 common enough to test are randomly distributed over the hectare, while 13 show aggregated spatial distributions. The tree species diversity, size structure and spatial distribution observed in this island plot are discussed in light of patterns occurring in continental lowland tropical rain forests. In the study area, the high density of trees and the tendency of conspecific individuals to be clumped may be linked to several factors: hurricane disturbance, lack of seed dispersers and patchy seed rain.

Altitudinal variation in fertility and vegetative growth in the invasive plant Rubus alceifolius Poiret (Rosaceae), on Réunion island.

Rubus alceifolius Poiret (Rosaceae) was introduced to the island of Réunion in the southeastern Indian Ocean about 1850 and is now highly invasive. This bramble, native from southeastern Asia and Malaysia, has invaded a wide variety of habitats (lowland rainforest, mountain and submountain rainforest, Acacia heterophylla rainforest) from sea level to 1700 m. It is suspected to be monoclonal so, its remarkable success may be due in part to great phenotypic plasticity. On Réunion, bud, flower, fruit and seed production, the duration of the flowering period and the importance of the seed bank were found to be negatively correlated with elevation (50-1500 m a.s.l.). At a lowland site, fruit production in mature stands averaged between 30 and 80 fruits/m2 during 1999 and 2000. No fruit set occurred above 1100 m. This fruit production pattern was similar over two years. Although the number of leaves per unit area is similar along the whole gradient studied, decrease of fruit set in upland areas might be compensated for by an increase in vegetative growth. Temperature variation is very sharp along the elevation gradient and may control the fruit and the seed production. Fruit production allows establishment of new populations all around the island via bird dissemination. Once established, R. alceifolius maintains dense patches that can grow vegetatively. Our results may be relevant for eradication programs that should take into account variation in reproductive strategy in lowland vs. highland habitats.

In and out of Madagascar: Dispersal to Peripheral Islands, Insular Speciation and Diversification of Indian Ocean Daisy Trees (Psiadia, Asteraceae)

Madagascar is surrounded by archipelagos varying widely in origin, age and structure. Although small and geologically young, these archipelagos have accumulated disproportionate numbers of unique lineages in comparison to Madagascar, highlighting the role of waif-dispersal and rapid in situ diversification processes in generating endemic biodiversity. We reconstruct the evolutionary and biogeographical history of the genus Psiadia (Asteraceae), a plant genus with near equal numbers of species in Madagascar and surrounding islands. Analyzing patterns and processes of diversification, we explain species accumulation on peripheral islands and aim to offer new insights on the origin and potential causes for diversification in the Madagascar and Indian Ocean Islands biodiversity hotspot. Our results provide support for an African origin of the group, with strong support for non-monophyly. Colonization of the Mascarenes took place by two evolutionary distinct lineages from Madagascar, via two independent dispersal events, each unique for their spatial and temporal properties. Significant shifts in diversification rate followed regional expansion, resulting in co-occurring and phenotypically convergent species on high-elevation volcanic slopes. Like other endemic island lineages, Psiadia have been highly successful in dispersing to and radiating on isolated oceanic islands, typified by high habitat diversity and dynamic ecosystems fuelled by continued geological activity. Results stress the important biogeographical role for Rodrigues in serving as an outlying stepping stone from which regional colonization took place. We discuss how isolated volcanic islands contribute to regional diversity by generating substantial numbers of endemic species on short temporal scales. Factors pertaining to the mode and tempo of archipelago formation and its geographical isolation strongly govern evolutionary pathways available for species diversification, and the potential for successful diversification of dispersed lineages, therefore, appears highly dependent on the timing of arrival, as habitat and resource properties change dramatically over the course of oceanic island evolution.

HYBRIDIZATION AND BARRIERS TO GENE FLOW IN AN ISLAND BIRD RADIATION

While reinforcement may play a role in all major modes of speciation, relatively little is known about the timescale over which species hybridize without evolving complete reproductive isolation. Birds have high potential for hybridization, and islands provide simple settings for uncovering speciation and hybridization patterns. Here we develop a phylogenetic hypothesis for a phenotypically diverse radiation of finch‐like weaver‐birds (Foudia) endemic to the western Indian Ocean islands. We find that unlike Darwins finches, each island‐endemic Foudia population is a monophyletic entity for which speciation can be considered complete. In explaining the only exceptions—mismatches between taxonomy, mitochondrial, and nuclear data—phylogenetic and coalescent methods support introgressive hybridization rather than incomplete lineage sorting. Human introductions of known timing of one island‐endemic species, to all surrounding archipelagos provide two fortuitous experiments; (1) population sampling at known times in recent evolutionary history, (2) bringing allopatric lineages of an island radiation into secondary contact. Our results put a minimum time bound on introgression (235 years), and support hybridization between species in natural close contact (parapatry), but not between those in natural allopatry brought into contact by human introduction. Time in allopatry, rather than in sympatry, appears key in the reproductive isolation of Foudia species.

Molecular characterization of trophic ecology within an island radiation of insect herbivores (Curculionidae: Entiminae: Cratopus)

The phytophagous beetle family Curculionidae is the most species‐rich insect family known, with much of this diversity having been attributed to both co‐evolution with food plants and host shifts at key points within the early evolutionary history of the group. Less well understood is the extent to which patterns of host use vary within or among related species, largely because of the technical difficulties associated with quantifying this. Here we develop a recently characterized molecular approach to quantify diet within and between two closely related species of weevil occurring primarily within dry forests on the island of Mauritius. Our aim is to quantify dietary variation across populations and assess adaptive and nonadaptive explanations for this and to characterize the nature of a trophic shift within an ecologically distinct population within one of the species. We find that our study species are polyphagous, consuming a much wider range of plants than would be suggested by the literature. Our data suggest that local diet variation is largely explained by food availability, and locally specialist populations consume food plants that are not phylogenetically novel, but do appear to represent a novel preference. Our results demonstrate the power of molecular methods to unambiguously quantify dietary variation across populations of insect herbivores, providing a valuable approach to understanding trophic interactions within and among local plant and insect herbivore communities.

Effects of canopy gap size on recruitment and invasion of the non-indigenous Rubus alceifolius in lowland tropical rain forest on Réunion.

An important challenge of invasion biology is to understand how interactions between species traits and ecosystem properties enable alien species to become invasive at particular locations. We investigated how gap dynamics in a tropical rain forest on the island of Reunion affected the invasiveness of alien plants. In the 12 000-m 2 study area, alien plants occupied 24.9% of the area of gaps, which represented 5.62% of the forest area, but only 0.8% of the understorey area. The most abundant invasive species was Rubus alceifolius , which formed dense, monospecific stands in the largest gaps (> 25 m 2 ). Although plants could persist in the shade, a germination experiment revealed that canopy openings were essential for seedling establishment. A cyclone that struck the study area in 2002 caused a temporary thinning of the canopy, increasing light levels to above the threshold needed for germination of R. alceifolius and also stimulating the growth of established plants. We conclude that the ability of this and other alien species to colonize intact lowland tropical rain forest is strongly influenced by the prevailing gap dynamics. Because gaps are also essential for the regeneration of many native trees in our study area, there is a real danger of the forest being progressively degraded by alien plants. There are no simple solutions to controlling species such as Rubus alceifolius , but efforts should be focused mainly upon the larger gaps where the species are most invasive.

Checklist of the Bryophytes of Madagascar

Abstract This updated checklist of the bryophytes of Madagascar was compiled from the literature, herbarium and recent collections. A total of 1144 species and infraspecific taxa are recorded for Madagascar composed of 751 mosses, 390 liverworts and 3 hornworts. 28.67% of the species reported for the Island are endemics. Twenty-four taxa are newly recorded for Madagascar and two of them, Fissidens cyathaeicola Brugg.Nann. and Taxithelium nepalense (Schwägr.) Broth. are reported for the first time for the Sub-Saharan Africa region.