VULNERABILITY OF ISLAND ECOSYSTEMS
Composition of the Theme - Projects - Phd theses in progress
Head: Jean-Claude GAERTNER (IRD)
Scientific context and general objectives
The vulnerability of an ecosystem is a complex concept, the definition of which varies according to the authors and the disciplinary field of investigation (ecological, economic or social). It may nevertheless be considered that the vulnerability of an ecosystem designates "the exposure to unforeseen phenomena and to stress, and the difficulty of facing up to them". This perception of vulnerability is based on definitions that are widely accepted in ecology and elsewhere. The level of vulnerability of an ecosystem thus results essentially from the combination of two main elements: (1) the risk of exposure to forcing (i.e. the nature, frequency and intensity of external structuring factors), and (2) the response of organisms and ecosystems to these forcings, which depends in particular on their sensitivity and their capacity to absorb the alterations. For several decades, the combined impact of the increase in anthropic pressures and in environmental variability have aggravated the vulnerability of many tropical ecosystems worldwide. This situation is manifested in a loss of biodiversity and a reduction in the number of ecosystem services (ES), which are particularly marked for the island ecosystems of Oceania, such as those of French Polynesia. These are highly sensitive to disturbances, while their societies are strongly dependent on ecosystem services produced at local scale. The vulnerability of the ecosystems is thus today at the core of a range of issues for island societies in general and for those of the South Pacific in particular. The vulnerability of the ecosystems is however difficult to assess and even more to anticipate, in particular since it is manifested in a variety of ways, at various scales and through different, often interdependent 'processes'.
From the scientific point of view, determining the vulnerability of ecosystems is thus a complex process which requires the characterisation of the intrinsic properties of the systems studied (complexity, diversity, productivity, specific interactions, distribution of functional traits, etc.), and the integration of the impact of multiple factors on a wide range of taxons, levels of organisation (e.g. communities, ecosystems) and scales of observation. The work of Theme 3 comes within this framework, and is focused on several groups of plants and animals which play a key role in the functioning of island ecosystems (e.g. phytoplankton, algae, fishes) and/or in the production of major ES for the island societies, in particular in French Polynesia (e.g. pearl oysters, giant clams). French Polynesia represents a study zone that is particularly suited to the illustration of the diversity of the forms of vulnerability of island ecosystems. The observation and research questions are applied to islands and atolls, broadening the field of investigation to the high seas, in particular for taking into account the impact of oceanic forcing on the coastal environment (e.g. island effects), and ocean-lagoon exchanges. In addition, the impact of the interactions between 'living resources - environment - uses' on human societies is dealt with on the basis of multidisciplinary approaches.
From the organisational point of view, Theme 3 includes 3 research axes. The 1st axis, centred on the study of processes, is focused on the analysis of the structure and functioning of island ecosystems. The 2nd axis is focused on the response of various key components in the face of various types of natural and anthropic disturbance. Finally, the aim of the 3rd axis, of a more methodological nature, is to develop and/or test tools contributing to the definition of new systems for monitoring the state of the OIEs. These 3 research axes are closely complementary. Thus the ecological knowledge derived from Axes 1 and 2 is integrated in the actions of Axis 3 (search for indicators, definition of protocols). In return, the methodological developments derived from Axis 3 may enrich the methods of observations and analysis to respond to the ecological questions of Axes 1 and 2.
Axe 1 : Structure and functioning of ecosystems
Knowledge of the processes that aggravate the vulnerability of tropical island ecosystems (e.g. dystrophic crises, toxic bloom, loss of resistance / resilience) is still too patchy. To make good this lack requires better understanding of the structure and functioning of ecosystems in a wide range of environmental and anthropic situations. In this perspective, the aim of the work of Axis 1 is to characterise the interactions between the key taxons (e.g. algae, invertebrates, fishes). A further aim is to quantify the role of these taxons and of their interactions with regard to the major ecosystem functions (e.g. productivity, recycling of matter, resistance) and to the properties involved in the production of ecosystem services (e.g. growth of pearl oysters, risk of ciguatera outbreaks).
For example, it is accepted that the planktonic microorganisms play a major role in the productivity of filtering organisms (e.g. oysters, giant clams). In return, the filtering activity of filtering organisms such as bivalves in culture (e.g. pearl oysters and associated epibiont species) also have an impact on the planktonic compartment. However, the effects of these complex interactions on the vulnerability of systems are still poorly known. They are part of the research focus of Axis 1 on the relation 'Pearl culture - Environment', in relation with Theme 1. Similarly, the interactions between the algal assemblages (benthic cyanobacteria and macroalgae) and the animal coral communities (fishes, invertebrates) take a variety of forms (e.g. primary production, diazotrophy, reef construction). The macroalgal communities are in particular formidable competitors for space. In the commercially exploited systems, this situation often results in a tendency towards dominance by the macroalgae, to the detriment of the coral communities ('phase shift' phenomenon). This phenomenon is particularly marked in the zones where the fishing pressure on herbivorous fishes is such that they are prevented from regulating the balance between algae and corals. Although the phase shift has been the focus of numerous studies, the impact of this phenomenon on the diversity of functions, and more broadly on the functioning of ecosystems, are areas of research that remain wide open, and which are explored as part of the activities of Theme 3.
The state of knowledge on the pelagic ecosystem of the high seas is still more limited, in particular as concerns the central South Pacific. This ecosystem plays an important role for the island societies, which derive a significant part of their resources from it (fishing). In addition, it plays a determining role in several aspects of the ocean dynamic of the Pacific (El Niño) and in the lagoon ecosystems (transfer of scales, ocean-lagoon exchanges, inter-lagoon connectivity, etc.). Nevertheless, mainly because of the difficulty of collecting data representative of its components, the structure and the functioning of this ecosystem remain very poorly known. The high seas environment of the Marquesas islands is the site of a remarkable biological enrichment (island effect). This island effect, the origin of which has yet to be determined, has an impact on the higher trophic levels and makes the Marquesas a region rich in biodiversity and in marine resources, in particular the large pelagic fish species. Part of our work is thus dedicated to characterising the physical and biogeochemical processes at the origin of this island effect. More broadly, given the primordial role of the commercial exploitation of the tuna species (especially the albacore tuna) in the economic development of the island states and territories of the South Pacific, our work should serve to complete the recent studies carried out in the South-West Pacific on environment-prey relations. The aim is to better understand the interactions between the primary trophic levels (phytoplankton communities) and intermediate levels (nekton) with regard to the availability of food for the tuna species in function of current and future high climatic variability.
Axe 2 : Reponse to forcing
Over the past decades, the structure (biomass, composition, diversity) and functioning of tropical island communities have been impacted by multiple factors. Many of these forcings have had a major impact on the biodiversity of the Polynesian islands and on the ecosystem services which it engenders. In this context, the aims of the work of Axis 2 are to assess the response, in terms of structure and function, of key communities (phytoplankton, algae, invertebrates, fishes) in the face of climatic forcing (i.e. temperature, acidification) and anthropic forcing (e.g. eutrophication, chemical contamination, aquaculture, fishing).
The impact of climate warming can have a range of consequences for the island ecosystems and the activities which are dependent on them. For example, if they prove to be valid, the current environmental forecasts (e.g. temperature increase and acidification of the oceans) could endanger the sustainability of the aquaculture sectors, such as pearl culture (decline in the effectiveness of collection of oyster spat, environmental conditions less favourable for the growth of oysters, greater vulnerability to disease). The same is true for the giant clam, a resource that is both commercially exploited, emblematic and protected, and which has already shown in French Polynesia signs of vulnerability to climate warming. The particular situation of French Polynesia with regard to this resource from the cultural, ecological (e.g. certain areas present densities among the highest worldwide) and economic (traditional fisheries and current emergence of aquaculture) points of view make the giant clam a biological model of particular interest. In this context, we pursue our research on the response of the giant clam to climate warming on the basis of a range of approaches, in particular by means of schlerochronology. The analysis of the impact of climate change on the ocean environment and its consequences with regard to the resources exploited by the island societies will complete the work focused on the lagoon environment. It will involve, for example, studying the impact of past and future climate change on the oceanic and plankton dynamic at the scale of the South Pacific and the Polynesian Exclusive Economic Zone, and the repercussions for the richness of the waters in terms of fisheries (the impact for the local populations could prove dramatic).
In addition, several actions of Axis 2 are focused on the impact of the degradation of the coral habitat as the result of local enthropic activities (e.g. urbanisation, fishing). The increasing anthropisation of certain coastal areas of French Polynesia is the cause of heavy domestic, industrial and agricultural waste discharges likely to upset the balance of the phytoplankton communities, and more broadly the whole ecosystem. The degradation of the habitat may also induce alterations in the balance between the algal communities and the animal communities (fishes and corals), in particular through the phase shift phenomenon and its interactions with fishing (cf. Axis 1, above). The current trend towards the degradation of the coral habitats also has an indirect impact on human health, in particular through the development of microalgal blooms. The spread of ciguatera is a good illustration of this problem. Knowledge in this field shows that the occurrence of outbreaks of ciguatera depends on the convergence of a range of factors and on the level of disturbance of the environment. Nevertheless, the mechanisms and the interactions between the factors involved are still very poorly understood. They are studied in the framework of Axis 2 in relation with the research undertaken in Theme 2.
Axe 3 : Development of indicators for monitoring the state of ecosystems
In the context of the management of the environment, there is increasing demand for the development of indicators as the result of the current demographic increase, which is accompanied by the diversification of pressures and the increase of potential conflicts between uses. The European Union has produced directives (Water Framework Directive, Marine Strategy Framework), which have led to the development of indicators for the qualification of the marine environment, including in the French overseas départements (Départements d'outre-mer). On the other hand, there are no equivalent regulations concerning the South, in particular for the Polynesian atolls, the vulnerability of which is increasingly the focus of discussion, without there being any proper objective basis. In this context, the methodologies developed in Axis 3 are applied with the aim of enriching and improving the techniques of assessment and monitoring (observation protocols, indices, simulation techniques based on scenarios adapted to the constraints of the OIEs, knowledge transfer tools, etc.) of the state of the island ecosystems as an aid to their management. In fine, this research will provide a basis for providing specifications for the monitoring systems required by managers or for the purposes of observatories, which could contribute to the development of products such as methodology guidelines.
Thus, on the basis of the surveys carried out and/or the knowledge acquired in the first two Axes of Theme 1, we will characterise the descriptors and identify the indicators of the current state and the patterns of change in the OIEs, while determining how they should be deployed taking into account the cost-effectiveness (feasibility and statistical power in relation with the sampling strategies). For example, although the diversity is recognised as one of the key elements which conditions the vulnerability of an ecosystem, it has hitherto been rarely integrated in management plans. Furthermore, the current monitoring processes are generally limited to the study of a very limited number of components of the diversity (starting with specific richness). Yet whatever the taxon, it would appear that other facets of diversity (e.g. beta-diversity, functional diversity) are capable of providing information that is better suited to understanding the rules of structuration of assemblages of species and for monitoring and predicting the response of assemblages in the face of anthropic pressure and the impact of climate change. Integrating these components in future ecosystem monitoring and management processes will require the development of new methodological approaches. In this context, part of the work of Axis 3 will be focused on the development and application of multi-component approaches for studying diversity, and also undertaking research on the properties of diversity indices for functional diversity in order, in both cases, to seek series of indicators specifically adapted to the context and to the main issues concerning island ecosystems. Ultimately, the actions developed in Axis 3 should contribute to the facilities already in existence in French Polynesia, whether for scientific research (GOPS), conservation (e.g. recognition as UNESCO World Heritage site), classification of aquaculture facilities (ICPE), or finally the undertaking of an initial state survey of the environment (EIE).