Overview of the Distribution, Habitat Association and Impact of Exotic Ants on Native Ant Communities in New Caledonia

Ants are among the most ubiquitous and harmful invaders worldwide, but there are few regional studies of their relationships with habitat and native ant communities. New Caledonia has a unique and diverse ant fauna that is threatened by exotic ants, but broad-scale patterns of exotic and native ant community composition in relation to habitat remain poorly documented. We conducted a systematic baiting survey of 56 sites representing the main New Caledonian habitat types: rainforest on ultramafic soils (15 sites), rainforest on volcano-sedimentary soils (13), maquis shrubland (15), Melaleuca-dominated savannas (11) and Acacia spirorbis thickets (2). We collected a total of 49 species, 13 of which were exotic. Only five sites were free of exotic species, and these were all rainforest. The five most abundant exotic species differed in their habitat association, with Pheidole megacephala associated with rainforests, Brachymyrmex cf. obscurior with savanna, and Wasmannia auropunctata and Nylanderia vaga present in most habitats. Anoplolepis gracilipes occurred primarily in maquis-shrubland, which contrasts with its rainforest affinity elsewhere. Multivariate analysis of overall ant species composition showed strong differentiation of sites according to the distribution of exotic species, and these patterns were maintained at the genus and functional group levels. Native ant composition differed at invaded versus uninvaded rainforest sites, in the absence of differences in habitat variables. Generalised Myrmicinae and Forest Opportunists were particularly affected by invasion. There was a strong negative relationship between the abundance of W. auropunctata and native ant abundance and richness. This emphasizes that, in addition to dominating many ant communities numerically, some exotic species, and in particular W. auropunctata, have a marked impact on native ant communities

Estimation of herbaceous fuel moisture content using vegetation indices and land surface temperature from MODIS data

The monitoring of herbaceous fuel moisture content is a crucial activity in order to assess savanna fire risks. Faced with the difficulty of managing wide areas of vegetated surfaces, remote sensing appears an attractive alternative for terrestrial measurements because of its advantages related to temporal resolution and spatial coverage. Earth observation (EO)-based vegetation indices (VIs) and the ratio between Normalized Difference Vegetation Index (NDVI) and surface temperature (ST) were used for assessment of herbaceous fuel moisture content estimates and validated against herbaceous data collected in 2010 at three open savanna sites located in Senegal, West Africa. EO-based estimates of water content were more consistent with the use of VI as compared to the ratio NDVI/ST. Different VIs based on near-infrared (NIR) and shortwave infrared (SWIR) reflectance were tested and a consistent relationship was found between field measurements of leaf equivalent water thickness (EWT) from all test sites and Normalized Difference Infrared Index (NDII), Global Vegetation Moisture Index (GVMI) and Moisture Stress Index (MSI). Also, strong relationships were found between fuel moisture content (FMC) and VIs for the sites separately; however, they were weaker for the pooled data. The correlations between EWT/FMC and VIs were found to decrease progressively as the woody cover increased. Although these results suggest that NIR and SWIR reflectance can be used for the estimation of herbaceous water content, additional validation from an increased number of study sites is necessary to study the robustness of such indices for a larger variety of savanna vegetation types

Sharp transitions in microclimatic conditions between savanna and forest in New Caledonia: Insights into the vulnerability of forest edges to fire

Fires are one of the main causes of forest loss in the tropics. Understanding the dynamic edge effects is critical for managing fires and protecting forests. We measured and analysed trends in microclimatic conditions (air temperature, relative humidity and vapour pressure deficit) over 7 months along three transects extending from core savanna areas to core forest areas. We tested two hypotheses: (i) that the forest edge is subject to microclimatic edge effects, and (ii) that the depth of these edge effects increases during dry periods. Sharp changes in each microclimatic variable were consistently observed between savanna and forest throughout the study period. Microclimatic transitions took place within 5m outside the forest boundary. Drought levels increased homogenously throughout the forest and were not disproportionately severe in the vicinity of the forest edge. We suggest that these results were related to the fact that the studied period was abnormally humid due to a La Nina episode, and that under such conditions the vulnerability of the forest edge to savanna fires is relatively low. Relatively wet conditions in the savanna close to the forest edge may promote forest expansion by limiting fire spread. Prescribed fires during humid years could reduce fuel loads in savanna without affecting the forest edge, which would prevent fires during the dry years associated with El Nino episodes from having severe impacts

Dynamiques des forêts denses humides et des savanes en réponse aux incendies en Nouvelle-Calédonie.

La Nouvelle-Calédonie qui présente une biodiversité à la fois exceptionnelle et très menacée, fait partie des points chauds de biodiversités définis à l échelle globale comme zones prioritaires pour la conservation. Les incendies, d origines anthropiques et constituant l une des principales menaces pesant sur les écosystèmes naturels néo-calédoniens, conduisent à un recul des forêts denses humides (forêts par la suite) principalement au profit des savanes sur substrats volcano-sedimentaires. Au cours de cette thèse, les processus écologiques mis en jeu dans les dynamiques des forêts et des savanes, liés aux variations du régime d incendie, ont été étudiés à différentes échelles spatiales et temporelles. L analyse de la distribution spatio-temporelle des forêts et des savanes à l échelle du paysage a mis en évidence différents facteurs dirigeant la dynamique paysagères et identifié des zones de recolonisation forestière. Différents modèles de succession secondaire et cortèges d espèces pionnières ont été identifiés à partir de l analyse de ces zones de recolonisation. La mesure de traits fonctionnels et l utilisation de modèles semi-physiques de dommages causés par les incendies ont révélé une faible tolérance aux incendies de ces espèces. Enfin, une analyse multivariée de la structure, de la composition floristique et des conditions micro-climatiques des zones clés de transitions entre la savane et la forêt a permis de mieux comprendre les processus d expansion et de contraction forestière. Ces travaux de thèse ouvrent un nouveau champ de recherche en Nouvelle-Calédonie avec d importantes implications en terme de restauration des écosystèmes forestiers et de gestion durable.New Caledonia, which presents both an exceptional and highly endangered biodiversity, is one of the worldwide biodiversity hotspots for conservation priority. Human-induced fires, which are one of the main threats to natural ecosystems in New Caledonia, lead to the expansion of savannas on volcano-sedimentary substrates at the expense of rainforests. In this thesis, the ecological processes, which are involved in the dynamics of rainforests and savannas and related to changes in fire regime, have been studied at different spatial and temporal scales. The analysis of the spatio-temporal distribution of rainforests and savannas across the landscape allowed us to both point-out the different drivers of their dynamics and to identify areas of rainforest recolonization. Different secondary succession patterns and pioneer species assemblages were identified from the analysis of these areas of recolonization. The analysis of measured functional traits and the use of semi-physical models of fire-caused damages highlighted the low tolerance of these pioneer species to fire. Finally, a multivariate analysis of the structure, the floristic composition and the micro-climatic conditions of transitional zones between savanna and rainforest, has allowed us to better understand the processes of rainforest's expansion and contraction. This thesis opens a new field of research in New Caledonia with important implications in rainforest s restoration and sustainable management.AIX-MARSEILLE3-Bib. élec. (130559903) / SudocSudocFranceF

Approche bayésienne de la reconstruction des paléoclimats à partir du pollen : vers la modélisation des mécanismes écologiques

Le pollen conserve dans les sédiments lacustres constitue un indicateur essentiel pour reconstruire l' evolution de la végétation et du climat passés sur les continents. Actuellement, les reconstructions climatiques se basent sur des modèles statistiques décrivant le lien climat-pollen. Ces modèles posent des problèmes méthodologiques car ils sont tous basés sur l'hypothèse que la relation pollen-climat est constante au cours du temps, impliquant que les paramètres non climatiques déterminant cette relation aient une influence faible. Cela est contredit par les développements récents en écologie et en écophysiologie. C'est pourquoi, dans ce travail, nous développons une approche intégrant un modèle dynamique de végétation et les processus majeurs liant la végétation au pollen capté par les lacs. Le cadre bayésien fournit une base théorique ainsi que les outils pour inférer les paramètres des modèles et le climat passé. Nous utilisons ces nouveaux modèles pour reconstruire le climat de l'Holocène en différents sites européens. Cette approche qui permettra des reconstructions spatio-temporelles requiert encore des développements autour de l'inférence de modèles semi-mécanistes.AIX-MARSEILLE3-BU Sc.St Jérô (130552102) / SudocSudocFranceF

Interactions between vegetation and climate variability: what are the lessons of models and paleovegetation data

International audienceThe climate reconstruction by using pollen data is classically done by statistical methods. The use of a conceptual model of niches show a few weaknesses of this approach which is not enough supported by causal relationships. A solution is to take into account ecophysiological processes through a mechanistic model. On the one hand, these models help to test the biases that can bring some changes in the distribution of the extremes, and, on the other hand, they help to understand the effect of some external constraints such as the concentration of the atmospheric CO, At a shorter time scale, dendrochronological series are useful to test the response of the Aleppo pine to a warming combined to a stronger water stress. This approach is facilitated by the use of vegetation models used in inverse mode or with climatic scenarios. This paper illustrates that purpose through several examples. To cite this article: J. Guiot et al., C R. Geoscience 340 (2008). (C) 2007 Academie des sciences. Published by Elsevier Masson SAS. All rights reserved

Analysis of African climate variability at pre-industrial, 4kyr and 6kyr BP simulations by Self-Organizing Maps

International audienceResults from three simulations of the IPSL-CM5A (96 x 96) coupled ocean-atmosphere model from the modern pre-industrial control (1000-year long) and mid-Holocene at 4kyr (400-year long) and 6kyr (910-year long) BP have been used to investigate changes in the seasonal sub-Saharan rainfall patterns and their links with sea surface temperatures (SST) of the main oceanic basins. Self-organizing maps and hierarchical ascendant classifications have been performed over the whole data set to highlight the main rainfall anomaly patterns ("monsoon regimes") for each season of the year. Then, composite maps of SST, mean sea level pressure and low-levels wind anomalies, have been computed for every class of rainfall anomaly patterns. In northern summer, eight monsoon regimes are defined. Three of them are mostly present in the pre-industrial period, other three in the 6kyr period, and the last two are evenly distributed in the three periods. On the other hand, the 4kyr period is scattered within all the eight monsoon regimes. A discriminant analysis shows that these eight regimes can be well differentiated by the main SST modes of the oceanic basins, in particular the Guinea gulf and the Mediterranean basin. Similar analyses have been conducted for the other seasons and will be presented too. This work has been funded by the French Agency for National Research through the research project C3A

Risque d'incendie pour la végétation d'un hotspot de biodiversité : une approche par modélisation en Nouvelle-Calédonie, sud-Pacifique

[Departement_IRSTEA]Territoires [TR1_IRSTEA]SEDYVIN [Departement_IRSTEA]Territoires [TR1_IRSTEA]SEDYVINInternational audienceWildfire has been recognized as one of the most ubiquitous disturbance agents to impact on natural environments. In this study, our main objective was to propose a modelling approach to investigate the potential impact of wildfire on biodiversity. The method is illustrated with an application example in New Caledonia where conservation and sustainable biodiversity management represent an important challenge. Firstly, a biodiversity loss index, including the diversity and the vulnerability indexes, was calculated for every vegetation unit in New Caledonia and mapped according to its distribution over the New Caledonian mainland. Then, based on spatially explicit fire behaviour simulations (using the FLAMMAP software) and fire ignition probabilities, two original fire risk assessment approaches were proposed: a one-off event model and a multi-event burn probability model. The spatial distribution of fire risk across New Caledonia was similar for both indices with very small localized spots having high risk. The patterns relating to highest risk are all located around the remaining sclerophyll forest fragments and are representing 0.012% of the mainland surface. A small part of maquis and areas adjacent to dense humid forest on ultramaficsubstrates should also be monitored. Vegetation interfaces between secondary and primary units displayed high risk and should represent priority zones for fire effects mitigation. Low fire ignition probability in anthropogenic-free areas decreases drastically the risk. A one-off event associated risk allowed localizing of the most likely ignition areas with potential for extensive damage. Emergency actions could aim limiting specific fire spread known to have high impact or consist of on targeting high risk areas to limit one-off fire ignitions. Spatially explicit information on burning probability is necessary for setting strategic fire and fuel management planning. Both risk indices provide clues to preserve New Caledonia hot spot of biodiversity facing wildfire

Plant migration and plant communities at the time of the "Green Sahara"

Around 8500 cal years BP, at the time of the maximum of the African Humid Period, lakes and wetlands expanded in the present-day Sahara while large paleodrainages were formed or re-actived, in response to an orbitally-induced increase in monsoon rainfall. It has been suggested that the direct consequence of this increase in rainfall was the northward displacement of the Sahara/Sahel boundary, thought to have reached 23 degrees N in central and eastern Africa. Here, we show a more complex situation characterized by an increase in biodiversity as the desert accommodated more humid-adapted species from tropical forests and wooded grasslands: tropical plant species now found some 400 to 500 km to the south probably entered the desert as gallery-forest formations along rivers and lakes where they benefited from permanent fresh water. At the same time, Saharan trees and shrubs persisted, giving rise to a vegetation that has no analogue today. In this article, we present distribution maps of selected plant species to show both the amplitude of the vegetation change compared to the present and the composition of the past plant communities. We also estimate the migration rate of tropical plant taxa. to their northernmost position in the Sahara. This study is based on the use of several data sets: a data set of the modern plant distribution in northern Africa and a data set of modem and fossil pollen sites (from the African Pollen Database, http://fpd.mediasfrance.org/ and http://medias.obs-mip.fr/apd/). To cite this article: J. Watrin et al., C R. Geoscience 341 (2009). (C) 2009 Published by Elsevier Masson SAS on behalf of Academie des sciences