Influence des activités anthropiques sur la myrmécofaune de deux zones agroécologiques (Hauts plateaux de l’Ouest et zone humide à pluviométrie bimodale) du Cameroun

Abstract

Ants are the main animal taxa present in natural and anthropogenic ecosystems, and play a key role in ecosystem functioning. In Cameroon, little is known about ant ecology, due to the diversification of land use systems and the many changes that have occurred because of human disturbance. The intensification of human activities in Cameroon, following demographic growth and strong self-subsistence needs characterized by, among other things: the agricultural exploitation of vast areas of land to increase the economic sector through commercial activities (1) and, to meet primordial needs such as food, health (2). Thus, uncontrolled human activity is one of the main threats to the integrity of natural biodiversity in humid tropical environments. Their real impact in certain ecoregions, such as tropical rainforests and dry and humid savannahs, remains little studied. The present study was carried out in two agroecological zones: Koutaba, in the Western Highlands of Cameroon, and Obout, in the Ecological Region of Cameroon, dominated by evergreen forest and humid savannah vegetation respectively. Data collection was carried out in four land-use systems, surveyed to assess the influence of anthropogenic activities on the diversity, composition and spatial distribution of ants (Formicidae). Ants were sampled along a 100-m linear transect per plant formation. Three capture techniques were used: pitfall-traping, food-bait trapping, and sight captures in quadrats. Identifications were made on the basis of morphological characters, using dichotomous keys proposed by various authors. Data were analyzed using various indices: species richness, Shannon and Piélou indices for specific diversity, and Sorensen's index for species composition. The Venn Diagram was used to analyze species distribution between land-use systems. A total of 90 ant species belonging to 30 genera and six subfamilies were collected at Koutaba. Moreover, the Myrmicinae taxon is the richest subfamily in terms of genera and species. Moreover, at Koutaba, palm groves, with 63 species, appear to be richer and more diverse than other land-use systems. The specific composition of ant assemblages seems similar between palm groves and savannahs, while that of orchards differs from all others in the different land-use systems. The ant community in coffee plantations was numerically dominated by Myrmicaria opaciventris (53.31%) and Camponotus acvapimensis (11.08%). The spatial distribution of these two species is linked to land use patterns and hence human disturbance. However, at Obout, the study of the structure, variation in specific diversity and distribution of ants was carried out in the forest, cocoa grove, palm grove and forest orchard respectively. In total, 88 ant species belonging to 31 genera and six subfamilies were collected. Myrmicinae and Formicinae are the subfamilies with the highest number of genera and species. Palm (S= 59; H'= 1.68, J= 0.41) xv and cocoa plantations (S= 50; H'= 0.87, J= 0.22) appear richer but less diverse than forests (S= 45; H'= 3.81, J= 0.65). Ant community composition was similar between the different land-use systems. Eight species were numerically dominant: Dorylus (Anomma) nigricans (64.64%) followed by Myrmicaria opaciventris (11.06%). Paltothyreus tarstus and Pheidole megacephala were the indicator species for cocoa plantations; in the forest, Camponotus (Myrmopelta) vividus, C. (Myrmopelta) barbarossa and Crematogaster (Crematogaster) sp.1 were the indicator species. Two species are indicators of the palm grove: Cataulacus guineensis affects ant diversity and distribution. Nevertheless, the cocoa plantation has played a positive role in maintaining a high level of diversity in a context of natural habitat loss. At Koutaba, the first network of interactions involves species such as M. opaciventris, O. longinoda, T. aculeatum, A. tenella and Cr. striatula associated with species such as C. vividus, C. acvapimensis, Od. troglodytes and L. guineensis. The second interaction network involves species such as Cr. clariventris, Ct. Guineensis, An. Africanus, Ct. Guinensis and C. barbarossa, and the third network involves genera such as: Camponotus, Mesoponera, Tapinoma, Polyrachis and also,ajustement of distributions obtained with diagram rank-frequence and the model Zipf in the cocoa grove (AIC=1630.9 ; BIC=1634.8) and the orchard (AIC=415.20 ; BIC=417.79) and the model log-normal in the palm grove (AIC= 790.86 ; BIC=795.14) and the savannah (AIC= 473.24 ; BIC= 795.14). As far as Obout is concerned, the interaction networks and functional structures of ant species present three levels. The first network of interactions characterizes species such as Dorylus (Anoma) nigricans, Myrmicaria opaciventris, Camponotus (Tanaemyrmex) equatorialis, Anoplolepis tenella. These ant species are also associated with Crematogaster clariventris. The second level of interactions includes the following species: Camponotus brutus, Cataulacus guineense, Odontomachus troglodytes and Palthotyreus tarsatus. The third interaction network characterizes genera such as Hypoponera, Mesoponera, Monomorium, Polyrachis, Tapinoma and Tetramorium.