Arboreal ants are a key component of the canopy fauna in tropical forests. Indeed, they include a wide variety of species with diverse nesting sites [1]. In the canopy, ants exert significant predation pressure and thus significantly influence the diversity, structure, and dynamics of other arboreal arthropods [2], hence their use as biological control agents [3][4]. Furthermore, these ants are considered pollinators for certain plants [5]. Despite the key role of arboreal ants, studies conducted on ant diversity and distribution in Côte d’Ivoire have primarily focused on ground-dwelling ants [6]-[8]. Consequently, there is a significant lack of data on the diversity of arboreal ants in Côte d’Ivoire. Indeed, deforestation caused by intensive agriculture, unregulated logging, and population growth and industrialization has led to the loss of natural habitats [9]. Thus, one of the major consequences of this phenomenon is habitat fragmentation. In the long term, this leads to changes in the structure and functioning of the resulting habitat fragments [10].

Central Côte d’Ivoire is home to a unique type of vegetation known as a forest-savanna mosaic, which lies at the intersection of two major biomes (forest and savanna) [11]. Indeed, these habitats are of particular importance for biodiversity conservation [12], and agricultural activity is significant there, as they offer the dual opportunity to grow food crops typical of savanna areas such as yams and export crops like cocoa and coffee, which originally require forest plots [13]. Recently, rubber cultivation has been added to these export crops in this region of the country [14], exacerbating the phenomenon of habitat fragmentation. Evergreen forests or semi-deciduous forests embedded within savannas are often interpreted as remnants of what were originally continuous forests, while the savannas are viewed as forests degraded by human activities [15].

However, studies examining the effects of habitat fragmentation on animal diversity focus on vertebrates. Few studies have examined the impact of habitat fragmentation on the species and functional diversity of invertebrates, particularly insects like ants [16], even though they play a key role in ecological processes such as nutrient cycling, pollination, seed dispersal, and predation [17]. This work aims to understand the impact of habitat changes on arboreal ant populations in the Lamto Scientific Reserve area. Specifically, it consists of 1) assessing the arboreal ant assemblage and their distribution in their natural habitats within the various habitats of the Lamto Reserve and surrounding areas, and 2) assessing those of the habitats in the rural area adjacent to the reserve, namely, the cocoa and rubber plantations, as well as the remnant forests found there.

Observed species richness was calculated by adding up the number of species and morphospecies observed across all samples within each habitat type. The Chao2 index species richness estimator was calculated to extrapolate the species richness from our data. Sample coverages were determined to estimate sampling efficiency based on different sampling methods using EstimateS v.9.1.0. Ant species richness across habitats was compared using a one-way ANOVA and Tukey’s pairwise comparison test. The ANOSIM test allowed us to compare the ant species composition. The similarity of ant assemblages between habitats was estimated using the Bray-Curtis index and displayed using Non-metric Multidimensional Scaling with PAST 5.03 Software [30]. The rank-frequency plot was used to study community structure by evaluating the relationship between species richness and the number of individuals [31]. Indicator species (IndVal) analysis was also conducted to identify species associated with different habitats, and significance was tested using Monte Carlo permutation in order to verify whether a species’ preference for a particular habitat type is significantly higher than would be expected from a random distribution [32][33]. This analysis was performed using the vegan, permute, and Indic species packages in R [34]. Additionally, we measured morphological traits of two dominant arboreal species to demonstrate the potential effects that a reduction in the size of forest patches or their conversion to plantations might have on the species’ functions within their habitat [35]. Thus, six traits were selected [36]: clypeus width, femur length, head width, mandible length, scape length, and alitrunk length (Table 1). These various morphological traits are associated with different functions performed by ants in their environment [37]. Past Software v3.09, R Software v4.4.3 and Microsoft Excel were used for statistical analyses, considering the value of p ≤ 0.05 as an indicator of statistical significance.

Table 1. Characteristics taken into account in the measurement of morphological traits according to Santoandre [36].

A greater number of ant species was associated with the forest island in the Lamto Reserve, while few species were associated with forest remnants and monoculture plantations in the rural area. A possible explanation may be that the forest islands in the reserve, being in a protected habitat, are certainly subject to much less human pressure than the forests in the rural area. This likely gives them greater complexity than rural forests, which are constantly subjected to human activities (harvesting of firewood and timber, collection of medicinal plants, hunting, and land clearing for the expansion of neighboring plantations) that further reduce their size. These observations are consistent with [38], who demonstrated that ant species richness is positively correlated with habitat complexity. This fact has also been mentioned by Wilson and MacArthur [39], who supported the habitat heterogeneity hypothesis that greater habitat complexity should provide more nesting and foraging sites, as well as a greater abundance of food. This would therefore explain the high species richness observed in the forest patches within the reserve.

Furthermore, changes in vegetation structure could act as a selective filter for species movements, thereby influencing the distribution of ants [40]. Indeed, another reason could be related to the size of forest fragments, as it has been shown that in other forest types (dry tropical and montane), a reduction in forest size decreases ant species richness [41].

The rank-frequency diagram revealed that in the Lamto Forest, four groups of species can be distinguished based on their abundance. In contrast, three groups were identified in rural forests and in cocoa and rubber plantations. Furthermore, certain species may lose their status as dominant species when moving from one habitat to another. For example, Crematogaster solenopsides is more abundant only in the reserve’s forest island, whereas Oecophylla longinoda is abundant in all habitats. Indeed, for some arboreal ant species, dominance depends on habitat quality [42]. Some species of the genus Crematogaster are dominant in one habitat type but not in another. These species are sometimes involved in mutualistic interactions with plants, and their abundance may also depend on the abundance of these plants within the habitat [43].

In contrast, species belonging to the genus Oecophylla are highly aggressive and competitive dominant species in both natural habitats and plantations [44][45]. Furthermore, the absence of very rare species collected in the reserve’s forest patches, such as Tetramorium rhetidum, Cataulacus vorticus, and Tapinoma sp. 4, in rural forests and plantations may suggest that these species are the first victims of the ecological disturbances affecting these habitats. This could be related to the dispersal capacity of these species in question, as not all species are capable of dispersing across forest fragments, even if they are close to one another. For example, in the Atlantic forests of Brazil, the species Dinoponera lucida Emery cannot disperse from one forest fragment to another, even when the fragments are close to one another [46].

Here, in this study, the species identified as indicator species in each habitat are rare. The number of these indicator species decreases drastically from the forest island within the reserve to rural forests and is obviously missing in monoculture plantations. Indeed, although rural forests share the same origins as the forest islands within the reserve [18], the indicator species of the Forest patches within the reserve do not retain the same status in rural forests. This may be due to the stress caused by human activities or to interactions between species. The study by Alohou [47] also identified different indicator species between native forests and remnant forests that had been converted into sacred forests in Benin. Thus, the plant species found in these sacred forests as indicators became indicators of anthropogenic disturbances in this type of habitat. Consequently, we could classify the ant species Monomorium sp. 4 and Polyrhachis decemdentata as indicators of disturbed forests. Meanwhile, the changes observed in arboreal ant communities in rural forest fragments do not appear to affect only rare species. Certain changes may also occur in abundant species, particularly in terms of their functional traits. This study shows that individuals of the species Oecophylla longinoda in rural forests are smaller than those in the forest islands within the reserve. This result does not corroborate the observations of David & Capy [48], who stated that in an environment with low diversity, reduced competition should lead to an increase in body size. However, this might be entirely accurate when the contested resources are limited to food. Also, in reduced habitats, additional nutritional resources, nesting sites and foraging areas become highly sought-after resources. Thus, Nooten [38] showed that ants in simplified habitats comprised fewer species and exhibited a different set of functional traits compared to those in more complex habitats, which corroborates our results. Similarly, Gibb et al. [49] showed that the body size of ants in disturbed habitats was smaller. However, numerous studies have shown that reduced body size in certain ant species is correlated with colony size [50]-[52]. This finding could represent an adaptive strategy for the species Oecophylla longinoda in rural forest fragments. However, not all species, despite their dominant nature, respond in the same way to stress caused by environmental conditions. This may explain why, for the species Crematogaster solenopsides, no differences were observed between the forest islands within the reserve and rural forests. This species appears to be resistant to changes in its environment.

An analysis of tree-dwelling ant communities, taking vertical stratification into account, showed that between the reserve’s forest island and those in the rural area, differences in species richness and abundance occurred within 2-meter strata. This may suggest that most of the species collected are generally found at or above this height. However, a comparison of the strata across the different habitats revealed nearly identical species compositions, suggesting that the differences observed in these various habitat types depend primarily on the higher canopy layers. Indeed, the two vegetation formations differed in terms of canopy structure [53].

This work has shown that the large forest island in the Lamto Reserve is home to a greater number of ant species, in contrast to the smaller rural forests, which are more heavily impacted by human activities. Other differences were also observed in community structure, such as the number of rare species and the number of indicator species. For all these parameters, species richness decreases from the forest patches in the Lamto Reserve to the forests in rural areas. This study has demonstrated the importance of protected areas for the conservation of the smallest species, such as ants. Indeed, plans for managing or creating biodiversity conservation areas must necessarily take into account the size of the habitat to be preserved.

The authors would like to express their sincere thanks to the World Bank Group and the African Excellence Center on Climate Change, Biodiversity and Sustainable Agriculture (CEA-CCBAD) of Felix Houphouët-Boigny University of Côte d’Ivoire for funding our research. We also thank Kouassi Dorgeles and Koffi Kouame François for their useful help during this work.

This paper is a result of several ant-course training projects with financial support from the Belgian Directorate-General for Development Cooperation (DGD), Global Taxonomy Initiative, within the framework of the CEBioS programme.

Conceptualization: Christine V. Dakele Yode and Souleymane Konate; Methodology: Christine V. Dakele Yode; Software: Christine V. Dakele Yode; Validation: Philippe K. Kouassi, Kanvaly Dosso, and Lombart M. Maurice Kouakou; Formal Analysis: Christine V. Dakele Yode; Investigation: Christine V. Dakele Yode; Resources: Christine V. Dakele Yode; Data Curation: Christine V. Dakele Yode; Writing—Original Draft Preparation: Christine V. Dakele Yode; Writing—Review and Editing: Christine V. Dakele Yode; Visualization: Boris Dodji Kasseney; Supervision: Lombart M. Maurice Kouakou; Project Administration: Christine V. Dakele Yode; Funding Acquisition: Souleymane Konate. All authors have read and agreed to the published version of the manuscript.