Variabilité spatio-temporelle des insectes mangeurs de fruits utilisés comme bioindicateurs en Afrique de l'ouest

Abstract

Les insectes mangeurs de fruits se sont récemment avérés de très bons bioindicateurs de santé des écosystèmes savanicoles d'Afrique de l'Ouest. Au Burkina Faso, la fragmentation grandissante des paysages dans diverses régions rend le développement de ce genre d'outils de suivi particulièrement utile. Cette étude a été réalisée pour mesurer la variabilité spatio-temporelle des densités apparentes d'insectes, de la famille des Nymphalidae et des Cetoniinae, attirés par des pièges appâtés de bananes fermentées, afin de fournir aux gestionnaires des zones protégées des recommandations quant à leur utilisation comme bioindicateurs. Deux paysages (une zone de pâture et une zone de culture) ont ainsi été échantillonnés plusieurs fois pendant différentes périodes de la saison des pluies et pendant deux années, et ce sur une durée variable (5 à 10 jours). L'étude s'est déroulée à Koro, au Sud-Ouest du Burkina Faso. L'impact de ces facteurs de variation sur la comparaison des deux types de pression anthropique a ainsi été analysé. L'étude a confirmé l'utilité de ces bioindicateurs et un impact plus important des cultures sur l'écosystème. Les résultats sont discutés et des recommandations présentées pour l'utilisation de ces insectes comme bioindicateurs, dans le cadre de la gestion des terroirs. (Résumé d'auteur)

Environmental impact of tsetse eradication in Senegal

“…Their characteristics include taxonomical and ecological diversity, ecological fidelity, sedentary (species with short dispersal range), endemicity (non-invasive species), taxonomical knowledge and ease of identification, abundance and ease of monitoring, indication of other species and specific resources, temporal and spatial fluctuations and predictable, rapid, sensitive, and a linear response to disturbance that is easy to analyze 21,22 . The seasonal variation of the apparent densities of these ecological indicators is the only limitation and therefore, the monitoring must be done during the same period of the year 23 . For this study, the traps were deployed for eight consecutive years in October-November, a period that coincides with the end of the rainy season – beginning of the cold dry season and is the period of their highest diversity and density.…”

“…The scores considered the following characteristics of these groups:both insect families are taxonomically well-known and ecologically highly diversified 38–41 ;the species selected as indicators are widely distributed in open savannah from the Guinean to the Sahelian regions in Africa and do not display high ecological fidelity;the mobility of both insect groups do not affect their indicator value since their densities drop or increase very quickly with distance depending on human activity;the selected species are generally caught in high densities and capture scores allow strong statistical analyses, which lead us to switch three of Brown’s criteria (1991) to the maximum level for bait-attracted Nymphalinae in past studies (abundant, non-furtive, easy to find in the field, damped fluctuations, easy to obtain large random samples of species and variation). Seasonal variations, previously described in Burkina Faso 23 , were taken into consideration (see below) and vertical variations are not likely in savannah ecosystems, where the canopy is not continuous and the trees generally small (<10 m).the importance of both groups in the ecosystem is probably modest, although the Cetoniinae larvae are saprophytic and play a role in the recycling of organic matter 42 ;we previously demonstrated that the response of the selected species to ecosystem disturbance is sensitive and analysable 21 . However, the response to disturbance is not linear in the Cetoniinae subfamily, actually favored by a slight disturbance;finally, a linear relationship was found between plant species richness and the apparent densities of the most abundant species, which were also correlated together, although belonging to different families.…”

“…This study focused on fruit-feeding Cetoniinae and Nymphalidae species only because this group was validated as indicating ecosystem health in similar ecosystems in previous studies 21,23 . Both groups were previously evaluated thanks to Brown’s evaluation criteria (1991) and scored 19 each (on a maximum score of 24), and even 23 when evaluated together, like in the present study.…”

“…the selected species are generally caught in high densities and capture scores allow strong statistical analyses, which lead us to switch three of Brown’s criteria (1991) to the maximum level for bait-attracted Nymphalinae in past studies (abundant, non-furtive, easy to find in the field, damped fluctuations, easy to obtain large random samples of species and variation). Seasonal variations, previously described in Burkina Faso 23 , were taken into consideration (see below) and vertical variations are not likely in savannah ecosystems, where the canopy is not continuous and the trees generally small (<10 m).…”