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Mangroves meet their limits: where and how climate sets the latitudinal limits of mangroves in the context of global warming
Quisthoudt, K. (2013). Mangroves meet their limits: where and how climate sets the latitudinal limits of mangroves in the context of global warming. PhD Thesis. VUB Brussels University Press: Brussel. ISBN 978 90 5718 273 0. 190 pp.

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Documenttype: Doctoraat/Thesis/Eindwerk

    Global warming

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  • Quisthoudt, K., meer

    Mangrove ecosystems are an important host of a large part of the global marine biodiversity and offer important services such as carbon sequestration and protection of the coasts against storms. However, mangrove systems are threatened directly by human land use and indirectly by climate change as well as by distal causes such as human-induced changes in the river catchments. Habitat destruction and climate change are therefore two major components of the ongoing global change that the mangrove ecosystem undergoes. Accurately predicting the impacts of global change on the mangrove ecosystem requires a better understanding of the drivers of the distribution and of the dynamics at the latitudinal limits of the structuring mangrove species. As mangroves have well-defined geographic upper latitudinal limits in both hemispheres, the mangrove distribution can be expected as being primarily controlled by climate. The intertidal dynamics can be considered as a local regulator acting on top of climate. All mangrove species occur around the equator and species richness declines with increasing latitude. Therefore, the upper latitudinal limits of mangrove species in the northern and the southern hemisphere correspond to their respective leading edges (i.e. the forward edges of the range of a species during range expansion). One expected consequence of global warming on the mangrove distribution is a poleward shift of the upper latitudinal limits of its species. This will happen (i) if the current latitudinal limits of the mangrove species are controlled by climate factors and in particular by temperature, (ii) if species have reached geographically the equilibrium with their climatic niche (or in other words if the climatic niche is filled) and (iii) if dispersal to the new climatically suitable sites will be possible. In this research, the main objective was to clarify whether the first two conditions are fulfilled. In part 1, we searched for common climate attributes (air and sea surface temperature as well as aridity) that set the latitudinal limits of Avicennia and Rhizophora, the two mangrove genera occurring globally throughout the range of mangroves. We started with the most parsimonious hypothesis which was that there is only one critical temperature-based factor that sets all latitudinal limits of a mangrove genus (Chapter 1). This was supported by the fact that all upper latitudinal limits of mangroves are leading edges. This hypothesis was not retained because the variability of the temperature-based variables among the latitudinal limits was high for both mangrove genera. In addition to that, we found that mean air temperature was warmer at arid limits than at non-arid limits of Avicennia and Rhizophora. Therefore we further investigated the role of aridity on the temperature requirements of these two mangrove genera at their latitudinal limits. Our second hypothesis was that the minimum temperature requirements at the upper latitudinal limits of Avicennia and Rhizophora increase with increasing drought stress (Chapter 2). It was suggested that higher aridity is compensated by a higher minimum temperature requirement. Thus, we first investigated the relationship between temperature and water balance at the upper latitudinal limits of the two genera. Second, we classified the limits into four classes of aridity and extracted limiting temperatures per aridity class. We accepted the second hypothesis because the minimum temperatures that these two mangrove genera could withstand increased with increasing water balance and aridity class. Climate (i.e. interactions between temperature and aridity) could explain the position of half of the limits. It was well-known that Avicennia is better adapted to high salinity levels and variation in salinity levels than Rhizophora and that Avicennia is more tolerant to freezing events than Rhizophora. As Avicennia has a larger latitudinal extent than Rhizophora, it was expected that along frost free coastlines Rhizophora reached its latitudinal limits at warmer temperatures than Avicennia. Indeed, we found that (i) yearly mean AT and SST at the Rhizophora limit were warmer than at the nearby Avicennia limit (Chapter 1) and (ii) the limiting temperatures of the non-arid/dry and extremely dry class were warmer at the Rhizophora limits than at the Avicennia limits of the corresponding class (Chapter 2). The advantage of Avicennia compared to Rhizophora on the basis of its specific adaptations to salinity stress (i.e. a ‘safer’ water transport system and successive cambia) shows up in its minimum temperature tolerance in extremely dry climates where Avicennia could withstand much colder temperatures than Rhizophora. Results from Part 1 suggested that mangroves do not spatially fill their climatic niche. In part 2, we investigated whether and where the current geographic range limits of the 39 dominant mangrove species and the mangrove forest coincide with the limits of their climatic niche by using an empirical modelling approach (Chapter 3). This is a key question that needs to be answered before making projections of range shift under climate change. We tested the most parsimonious hypothesis that the geographic distributions of the mangrove species are at equilibrium with climate. This hypothesis was accepted at the ecosystem level because the species distribution models predicted the distribution of the mangrove forest worldwide fairly well. However, we rejected the hypothesis on the species level, because for many mangrove species not all potential latitudinal limits set by the climatic niche were reached. In particular, we showed that species filled less their niche along non-continuous coastlines distributed from the northern to the southern hemisphere. Residuals of predicted and observed species richness were high at low- and mid-latitudes and increased within each biogeographic region with distance from the place with the highest species richness towards the west and the east. This pattern was strongest in the Indo-West Pacific (IWP) biogeographic region. Besides the Indian Ocean that acts as a dispersal barrier between East-Africa and the rest of the IWP, a dispersal time lag and the changing compositions of the islands by sea level oscillations might be the causes. In the areas where mangrove species reach the potential latitudinal limits set by their climatic niche, global warming may provide new suitable sites beyond the current limits. Climatic equilibrium was reached at the latitudinal limits of the mangrove species in South Africa. Finally, in part 3, the objective was to quantify the predicted shift for mangrove latitudinal limits that are driven by climate. The hypothesis was that in South Africa global warming will make move the latitudinal limit of the mangrove forest and its species southwards beyond the current limits (Chapter 4). The hypothesis was accepted for two of the three species because species distribution models predicted an expansion of the latitudinal limit polewards before the end of the 21st century for these two species.

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