Can species richness be maintained in logged endemic Acacia Heterophylla forests (Reunion Island, Indian Ocean) ?

It is assumed that forests can serve multiple uses, including wood production and maintenance of high biodiversity level. We tested this hypothesis by studying eradication methods of invasive plants currently implemented in exploited endemic Acacia heterophylla forests located in a tropical highland forest region on the island of Réunion. We also compared species richness in logged (over time) and natural forests. Our results show that all individuals of the widespread alien invasive plant Rubus alceifolius were generated from cuttings. We quantifi ed the high growth capability of this species by comparing with those of Acacia heterophylla along with Rubus apetalus var. apetalus, a close non-invasive congener. The substantial multiplicative and vegetative growth ability of R. alceifolius enabled it to form monospecific patches in only 2.5 years. The species richness of the exploited Acacia heterophylla forest was thus very affected. Although the species richness increased over time after logging, the extent of the resulting richness was not as great as that in natural forests which, moreover, included numerous exotic plants. The authors assess the impact of highly disturbing logging operations and, based on the results of the comparisons, are very pessimistic about the possibility of maintaining species richness in logged forests. Nevertheless, the results indicate that the situation is not beyond hope and an eradication program specifi cally tailored to this kind of forest is proposed if exploitation will stop

Les effets de l’ouverture de sentiers sur l’invasion des zones protégées de l’île de la Réunion (archipel des Mascareignes, océan Indien) par des espèces végétales exotiques.

Les effets de l'ouverture de sentiers sur l'invasion des zones protégées de l'île de la Réunion (archipel des Mascareignes, océan Indien) par des espèces végétales exotiques. - L'envahissement par les espèces exotiques est devenu un problème majeur au niveau des aires protégées. Une meilleure connaissance de leur impact sur le fonctionnement des écosystèmes est nécessaire pour améliorer les pratiques de gestion et la conservation des habitats. Nous suggérons que les effets des perturbations comme l'ouverture de chemins forestiers et l'invasion par les plantes exotiques sont sous-estimés. Nous avons donc quantitativement mesuré le rôle de l'ouverture de chemins forestiers dans la colonisation par les plantes envahissantes, un phénomène en progression constante mais jamais étudié auparavant. Notre étude montre que Rubus alceifolius, la plante exotique envahissante la plus répandue à La Réunion, est capable de germer aussi bien au niveau d'un chemin étroit que d'un chemin large mais ne persiste qu'au niveau du chemin le plus large. Nos résultats montrent aussi qu'un chemin large favorise l'établissement des plantes exotiques et détermine une réduction significative de la richesse spécifique. Cette étude conduit à suggérer aux forestiers de construire plutôt des chemins étroits, ce qui permettrait d'empêcher l'installation et la propagation de R. alceifolius et des autres plantes non indigènes.Exotic species have become a major problem in protected areas. A better knowledge of ecosystem functioning is necessary to improve management practice that facilitates habitat conservation. We suggest that small-scale disturbances such as trail opening and the effects on exotic plants invasions are underestimated. We have, therefore, quantitatively measured the effect of forest trail building on invasive plant colonization, a phenomenon in constant progression but never studied before. Our study shows that Rubus alceifolius, the widespread exotic invasive plant on La Réunion, is able to germinate on both narrow and wide trails, but persists only on the wider ones. Our results show moreover that wide forest trails favored establishment of exotics and significantly decreases species richness. We suggest to forest managers that building narrow forest trails is preferable as it will stop the persistence and propagation of R. alceifolius and other non-indigenous plants

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 Réunion affected the invasiveness of alien plants. In the 12 000-m2 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 m2). 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 invasiv

In and out of Madagascar : dispersal to peripheral islands, insular speciation and diversification of Indian Ocean daisy trees (Psiadia, Asteraceae)

This study was supported by the European Union’s HOTSPOTS Training Network (MEST-2005-020561)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.Publisher PDFPeer reviewe

Statut démographique et biologie de la reproduction d'une espèce endémique menacée de l'île de la Réunion: Lomatophyllum macrum (Haw.) Salm-Dyck (Siliacées)

Knowledge of the demography and of the reproductive biology of endangered plant species is crucial for the establishment of an efficient conservation strate gy. The authors show that Lomatophyllum macrum, an endangered lily endemic to La Réunion island, is represented by less than 15 populations of extremely reduced size. The analysis of fecundity in natural conditions shows that the fructification rate is no more than 20 %. The breeding system study, performed on 9 cultivated plants in the greenhouse, reveals that the species is partially self-compatible but poorly self-fertile. In the light of these results, the survival of these populations is questioned. Future studies are proposed to improve the basic knowledge of the biology of this species for a long term conservation strategy of its populations in the wildLa connaissance du statut démographique et de la biologie reproductive des plantes en voie d'extinction est une étape cruciale, incontournable pour la mise en place de stratégies de conservation des espèces. Les auteurs montrent que Lomatophyllum macrum n'est aujourd'hui plus représentée que par quelques populations de taille extrêmement réduite. L'analyse de la fécondité en conditions naturelles montre que le taux de fructification ne dépasse pas 20 %. L'étude du système de reproduction à partir de 9 plantes multipliées en serre met en évidence que l'espèce est partiellement autocompatible mais très faiblement autofertile. Des perspectives d'études sont proposées afin d'enrichir les connaissances de la biologie de cette espèce dans le but d'élaborer un plan de conservation efficace de ses populations dans la nature