Identification of ecological indicators for monitoring ecosystem health in the trans-boundary W Regional park: A pilot study

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Abstract

The sustainable management of the W Regional park and its peripheral areas is based on a trade-off between conservation and the generation of economic income for local populations. This work is a pilot study for the identification of ecological indicators to monitor ecosystem health in Sudanian Savannah ecosystems. Ecological indicators are needed to warrant the efficiency of the protection measures, particularly in the mosaic landscapes of the peripheral areas. Two insect families (Coleoptera: Scarabaeidae (Cetoniinae) and Lepidoptera: Nymphalidae) were trapped along transects crossing landuse units submitted to various human pressures (none, hunting, traditional and intensive crops, grazing) in two countries (Burkina Faso and Bénin). Plant species richness was found to be correlated with the abundance of four fruit-feeding insect species and with the fruit-feeding butterflies species richness, but not with the Cetoniinae species richness. The abundance of Nymphalidae species generally dropped with human activities, but that of Cetoniinae species followed the intermediate disturbance theory. The likely impact of the various management practises on the general ecosystem health is discussed, as is the potential value of fruit-feeding insects as bioindicators and the points that still need to be clarified.

Introduction

The W Regional park (Bénin, Burkina Faso and Niger) was recently classified as the first trans-boundary biosphere reserve in Africa by the UNESCO-MAB (November 2002). Thanks to the involvement of the neighbouring populations in the management and the exploitation of the peripheral areas surrounding the protected one, it is becoming a model of sustainable conservation for West-African Savannah ecosystems. To be sustainable, the conservation policies must reconcile the conservation of the diversity with the generation of income for local populations of the peripheral areas (buffer areas, transition areas). Monitoring of the health of ecosystems appeared to be of critical importance while evaluating the various technical strategies involved in the project. To achieve this aim, the identification of ecological indicators, sensitive to slight ecosystem changes in a predictive manner, thus allowing the detection and measurement of the effect of various human pressures and activities, was of high priority. These indicators should fulfil two main attributes:

  • be a tool to select the best management strategies to maintain the maximal biodiversity given the space available,

  • be demonstrative for the local populations and environmental services, so that they can use it themselves and be encouraged to apply the conservation measures.

Our selection scheme (presented in Section 2) led to the choice of two fruit-feeding insect groups and to test the relation between their apparent densities at a species level, and the various landuse units encountered in the park periphery. Insects represent half of the global diversity and are influenced by many ecologic factors, like micro-climate, geology or vegetation structure (Ramos, 2000). Their relation to the ecosystem attributes are not still fully understood. Both bait-attracted insect families are taxonomically well-known and ecologically highly diversified (Allard, 1991, Sakay and Nagai, 1998, Larsen, 2006). Bait-attracted butterflies are well studied, as attested by the many references available and their recent classification in well-defined categories of habitat and distribution by Larsen (2006). Bait-attracted Cetoniinae are not as well studied, but we are preparing a similar classification than Larsen for this group (only the species of West-African Savannah areas), thanks to other trials in various localities in Burkina Faso, Niger and Bénin (Legrand et al., 2006).

The relations between ecosystem disturbance and insects in general (Holloway et al., 1992, Eggleton et al., 1995, Brown, 1997, Hamer et al., 1997), fruit-feeding butterflies in particular, have been investigated in tropical forests (Kremen, 1992, Beck and Schulze, 2000, Fermon et al., 2000, Ramos, 2000, Bobo et al., 2006). Fruit-feeding butterflies revealed themselves sensitive to disturbance and allowed an evaluation of the ecological impact of various conservation strategies (thinning, agroforestry, crops). However, the impact of disturbance on species richness is ambiguous, a slight disturbance or a border effect being able to increase local biodiversity (Lovejoy et al., 1986, Brown, 1997, Wood and Gillman, 1998), thus jeopardising the use of this parameter as a bioindicator. Authors generally agree that the use of species level parameters in well known environments is much more precise, provided that the sampling method can be standardised. This is particularly easy in the case of fruit-feeding insects that can be attracted to banana traps. All these studies were achieved in forest ecosystems and no data concerning the relation between disturbance and fruit-feeding butterflies or beetles densities is available in Savannah ecosystems. We chose to study both families together because their ecological roles and trophic levels of their larval stages (phytopageous and saprophytic) are completely different, thus ensuring a better indication of the global ecosystem health.

Section snippets

Selection of indicator taxa

The selection of our indicator taxa is based on three reviews presenting the general properties of bio-indicator taxa and step-wise decision-making frameworks for the selection of indicator taxa (Noss, 1999, Hilty and Merenlender, 2000, Dale and Beyeler, 2001).

Authors generally agree that the first step is “to decide what ecosystem attribute indicator taxa should reflect” (Hilty and Merenlender, 2000). The activities considered as creating an income for local populations in the peripheral areas

Vegetational transition

In the southern transect, the vegetation has been regularly burned for a very long time. The characteristic and dominant tree is Isoberlinia doka Craib & Stapf, which gives the aspect of natural woodland to this vegetation, corresponding to a fire-climax, especially from trapping sites 1–6 (Table 1). The herbaceous stratum is dominated by grasses, principally Schizachyrium sanguineum (Retz.) Alston and Diheteropogon amplectens (Nees.) Clayton. Under anthropogenic pressure, the vegetational

Evaluation of the indicators

The four species selected as indicators are widely distributed and do not display high ecological fidelity. H. daedalus and C. epijasius are butterflies of open formations, centered on the Guinean Savannahs, but even found up to the disturbed places of the rainforest area (Joly, 2003, Larsen, 2006). The P. marginata- and P. cordata-complexes are ubiquitous and found in most African countries (Sakay and Nagai, 1998). However, in England, it was demonstrated that widespread species may have

Conclusion

The present study is a step toward the use of fruit-feeding insects in the management of Savannah ecosystems, but a better understanding of communities that will be obtained only through long-term studies is still needed to design an appropriate management of the W regional park. The question whether these insect taxa are good predictors for other groups also need further study.

In the South-western part of Burkina-Faso, these ecological indicators will be used together with vegetation follow-up

Acknowledgements

This study is part of the ECOPAS Program funded by the European Union, following a regional collaboration between Burkina-Faso, Bénin and Niger. We are grateful to the CIRDES and CIRAD authorities who allowed the work to be carried out, and particularly to Prof. A.S. Gouro and Dr. F. Monicat. We also want to thank J.P. Legrand, P. Juhel and A. Monfort for their help in identifying the species and reading the manuscript, and Torben Larsen and an anonymous referee for their help in improving the

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