Small-scale drivers on plant and ant diversity in a grassland habitat through a multifaceted approach

Semi-natural grasslands are characterized by high biodiversity and require multifaceted approaches to monitor their biodiversity. Moreover, grasslands comprise a multitude of microhabitats, making the scale of investigation of fundamental importance. Despite their wide distribution, grasslands are highly threatened and are considered of high conservation priority by Directive no. 92/43/EEC. Here, we investigate the effects of small-scale ecological differences between two ecosites present within the EU habitat of Community Interest of semi-natural dry grasslands on calcareous substrates (6210 according to Dir. 92/43/EEC) occurring on a Mediterranean mountain. We measured taxonomic and functional diversity of plant and ant communities, evaluating the differences among the two ecosites, how these differences are influenced by the environment and whether vegetation affects composition of the ant community. Our results show that taxonomic and functional diversity of plant and ant communities are influenced by the environment. While vegetation has no effect on ant communities, we found plant and ant community composition differed across the two ecosites, filtering ant and plant species according to their functional traits, even at a small spatial scale. Our findings imply that small-scale monitoring is needed to effectively conserve priority habitats, especially for those that comprise multiple microhabitats.

Genetic Factors Associated with Variation in Abundance of the Invasive Yellow Crazy Ant (Anoplolepis gracilipes)

<p>A key component of successful invasion is the ability of an introduced population to reach sufficient abundance to persist, spread, and alter or dominate the recipient biological community. Genetic diversity is one of many factors that may contribute to population dynamics, but has important ramifications for biological fitness, and thus invasion success in the long term. I explored genetic factors associated with variation in abundance (i.e., differential invasion success) of the yellow crazy ant Anoplolepis gracilipes in the Indo-Pacific region, primarily focussing on Arnhem Land in Australia's Northern Territory. I explored five aspects that I hypothesised could contribute to variation in the abundance of this ant: 1) I investigated the unusual reproductive mode of A. gracilipes, and tested whether it involved dependent-lineage genetic caste determination (DL GCD) in Arnhem Land. In DL GCD systems populations require hybridisation between genetically distinct groups to produce both reproductive and worker castes. Asymmetry in the ratio of different lineages may result in low abundance and population collapse. I found no evidence for a DL GCD system in A. gracilipes, and thus its abundance in Arnhem Land does not appear to be constrained by any lineage ratio asymmetry. Worker reproduction (either of males or asexual production of other workers) also appeared unlikely. The reproductive mode of the species remains fascinating but enigmatic; 2) I explored whether multiple source populations were responsible for the observed variation in abundance in Arnhem Land (i.e., is abundance associated with propagule pressure, or populations from different sources), and if the population has diverged since introduction. The A. gracilipes population in Arnhem Land stemmed from a single source, and thus propagule pressure was apparently not responsible for variation in abundance. In contrast to many invasive ants, population divergence has occurred since introduction; 3) I tested the hypotheses that genetic variation was associated with variation in abundance in Arnhem Land, and that ecological success was density-dependent. While the population divergence found in Chapter 3 was not related to variation in abundance, genotypic diversity was higher in more abundant nest clusters. These more abundant nest clusters were in turn associated with lower native ant species diversity, and a difference in composition of the invaded ant community (i.e., greater ecological success); 4) I revisited the invasion of the yellow crazy ant in Tokelau to determine whether a haplotype that was linked to greater abundance and dominance of the ant community has increased in distribution. Although ants of the inferred dominant haplotype were implicated in most new invasions, their abundance was substantially lower than previously observed in Tokelau; 5) I conducted a preliminary analysis of the metagenomic diversity of A. gracilipes endogenous parasites and symbionts among populations from Christmas Island, Okinawa, Samoa and Arnhem Land. Bacterial community composition and diversity differed between the study populations. Variation in abundance among A. gracilipes populations in Arnhem Land was not due to parasite load on populations with low abundance. However, low abundance of A. gracilipes was correlated with lower microbial diversity overall, and higher prevalence of some groups, notably two that confer antibiotic properties. Together, my findings suggest that propagule pressure, reproductive mode and haplotype-specific effects do not appear to be associated with variation in A. gracilipes abundance. Other genetic factors I investigated do appear to be associated with the variation in A. gracilipes abundance and effects on the invaded ant communities. Genotypic diversity was positively related to the abundance of A. gracilipes in Arnhem Land, and this relationship may be affected by population divergence through population bottlenecks. In addition, differences in bacterial diversity among populations highlighted several candidate bacteria that could be associated with variation in abundance, which would be a topic of future work. Although genetic factors are often implicated in the successful establishment of invasive species, my thesis demonstrates that genetic factors may also be associated with post-establishment population dynamics.</p>

Genetic Factors Associated with Variation in Abundance of the Invasive Yellow Crazy Ant (Anoplolepis gracilipes)

<p>A key component of successful invasion is the ability of an introduced population to reach sufficient abundance to persist, spread, and alter or dominate the recipient biological community. Genetic diversity is one of many factors that may contribute to population dynamics, but has important ramifications for biological fitness, and thus invasion success in the long term. I explored genetic factors associated with variation in abundance (i.e., differential invasion success) of the yellow crazy ant Anoplolepis gracilipes in the Indo-Pacific region, primarily focussing on Arnhem Land in Australia's Northern Territory. I explored five aspects that I hypothesised could contribute to variation in the abundance of this ant: 1) I investigated the unusual reproductive mode of A. gracilipes, and tested whether it involved dependent-lineage genetic caste determination (DL GCD) in Arnhem Land. In DL GCD systems populations require hybridisation between genetically distinct groups to produce both reproductive and worker castes. Asymmetry in the ratio of different lineages may result in low abundance and population collapse. I found no evidence for a DL GCD system in A. gracilipes, and thus its abundance in Arnhem Land does not appear to be constrained by any lineage ratio asymmetry. Worker reproduction (either of males or asexual production of other workers) also appeared unlikely. The reproductive mode of the species remains fascinating but enigmatic; 2) I explored whether multiple source populations were responsible for the observed variation in abundance in Arnhem Land (i.e., is abundance associated with propagule pressure, or populations from different sources), and if the population has diverged since introduction. The A. gracilipes population in Arnhem Land stemmed from a single source, and thus propagule pressure was apparently not responsible for variation in abundance. In contrast to many invasive ants, population divergence has occurred since introduction; 3) I tested the hypotheses that genetic variation was associated with variation in abundance in Arnhem Land, and that ecological success was density-dependent. While the population divergence found in Chapter 3 was not related to variation in abundance, genotypic diversity was higher in more abundant nest clusters. These more abundant nest clusters were in turn associated with lower native ant species diversity, and a difference in composition of the invaded ant community (i.e., greater ecological success); 4) I revisited the invasion of the yellow crazy ant in Tokelau to determine whether a haplotype that was linked to greater abundance and dominance of the ant community has increased in distribution. Although ants of the inferred dominant haplotype were implicated in most new invasions, their abundance was substantially lower than previously observed in Tokelau; 5) I conducted a preliminary analysis of the metagenomic diversity of A. gracilipes endogenous parasites and symbionts among populations from Christmas Island, Okinawa, Samoa and Arnhem Land. Bacterial community composition and diversity differed between the study populations. Variation in abundance among A. gracilipes populations in Arnhem Land was not due to parasite load on populations with low abundance. However, low abundance of A. gracilipes was correlated with lower microbial diversity overall, and higher prevalence of some groups, notably two that confer antibiotic properties. Together, my findings suggest that propagule pressure, reproductive mode and haplotype-specific effects do not appear to be associated with variation in A. gracilipes abundance. Other genetic factors I investigated do appear to be associated with the variation in A. gracilipes abundance and effects on the invaded ant communities. Genotypic diversity was positively related to the abundance of A. gracilipes in Arnhem Land, and this relationship may be affected by population divergence through population bottlenecks. In addition, differences in bacterial diversity among populations highlighted several candidate bacteria that could be associated with variation in abundance, which would be a topic of future work. Although genetic factors are often implicated in the successful establishment of invasive species, my thesis demonstrates that genetic factors may also be associated with post-establishment population dynamics.</p>

Strong pairwise ant-plant mutualism has limited spillover effects on an ant community

Pairwise mutualisms are embedded within rich communities of co-occurring species. Mutualism by definition benefits partner species and theory predicts these effects can spill over into broader communities, with consequences for abundance, diversity, and composition. Specifically, positive feedbacks between partner species may shift competitive hierarchies and drive reductions in non-partner species. We used long-term monitoring data spanning 16 years to determine the ant partner species of tree cholla cacti (Cylindriopuntia imbricata), which reward ants with extrafloral nectar in exchange for anti-herbivore defense. We coupled these long-term data with short-term characterization of the ant community via pitfall trapping, which sampled partner and non-partner species across ten plots that varied naturally in cactus density. The long-term data revealed one dominant ant partner (Liometopum apiculatum) and two other less common partners (Crematogaster opuntiae and Forelius pruinosus). In the short-term census, the dominant ant partner occupied more cacti in plots of higher cactus density, and was also found at higher occurrence within the pitfall traps in the high cactus density plots, suggesting strong positive feedbacks that promote ant occurrence where plant partners are available. Despite the consistency of L. apiculatum partnership with cacti through time and the dominance of this partnership in areas of high cactus density, spillover effects from this mutualism appear limited. Of the common ant species, a single non-partner ant species showed a modest reduction in occurrence at high cactus density, possibly in response to increased presence of L. apiculatum. Additionally, the composition and diversity of the ant community in our plots were insensitive to cactus density variation, indicating that positive effects on the dominant ant partner did not have cascading impacts on the ant community. This study provides novel evidence that pairwise mutualisms, even those with strong positive feedbacks, may be limited in the scope of their community-level effects.

Scattered Trees as Crucial Elements in Maintaining Urban Diversity: A Case Study with Canopy ants in a Biodiversity Hotspot

Urban environments are under a high degree of human disturbance, which profoundly changes their abiotic characteristics, affecting the biotic communities. A conspicuous element in cities are scattered trees, often included in urban planning around the world. These trees are key elements for urban ecological processes and services and act as islands of resources (i.e., food and shelter) for a considerable arthropod’s diversity, such as ants. Studies involving ants in urban environments often focus on ground-dwelling ants, and few studies seek to glimpse the ants that occupy scattered trees and, even less, how these ants and trees interact within cities. This study aimed to investigate the ant communities foraging day and night on trees in an urban area inside a biodiversity hotspot, the Brazilian Atlantic Forest. Specifically, we investigated how tree characteristics (tree size, tree isolation, presence of extrafloral nectaries, and the tree origin, i.e., native and exotic) modulate ant diversity and ant-plant interactions. We found that the species composition is different between day and night, as well as the central core of generalist ant species of day and night interaction networks. The tree size increases beta-diversity among trees only of the nocturnal ant community, while none of the tree characteristics shaped the ant community. This study is the first to address the role of scattered trees in maintaining the diversity of arboreal ants in urban landscapes, focusing on species diversity and their interaction networks. Overall, we provide insights supporting the conservation value of scattered trees in maintaining urban biodiversity.

Tree Preference and Temporal Activity Patterns for a Native Ant Community in an Urbanized California Woodland

Anthropogenic disturbances, including land use change and exotic species, can alter the diversity and dynamics of ant communities. To examine foraging behavior in an urbanized habitat in northern California, we surveyed the presence of 9 ant species on 876 trees across 4 seasons during both day and night in a 9.5-hectare urbanized oak-exotic woodland. Ants were more likely to be observed on native, evergreen trees, suggesting that native evergreen species may help maintain ant diversity. Species showed clear patterns of temporal partitioning of foraging activity. Ant species varied in their use of native evergreen Quercus agrifolia trees across season and day/night axes. Of the 3 ant species most frequently observed, Camponotus semitestaceus was most active during spring and summer nights, Formica moki was most active during spring and summer days, and Prenolepis imparis was most active during both day and night during fall and winter. Liometopum occidentale was the second most active species during summer day and night, and winter day. Our findings demonstrate that an oak-exotic urban woodland in Northern California was able to maintain a native ant community, and strong temporal partitioning within that community.

A comparative assessment on ant communities in three vegetation types located at Mfou Suburban locality of Yaoundé (Cameroon)

Urban expansion in Cameroon and many other Congo basin countries, involves the destruction of natural habitat previously colonized by a rich and diversified invertebrate and vertebrate fauna. In order to understand the dynamic of transformation of natural landscape into urban area on ant communities, a study was conducted at Mfou, a suburban agglomeration of Yaoundé. We aimed to evaluate the variation in ant diversity and ant community structure in relation to the type of habitat. Ants were collected on the ground and trees in cocoa farms, secondary forests, and palm groves using visual catch, pitfall traps, quadrat, and baits. We recorded 144 species belonging to 39 genera and 6 subfamilies. Cocoa farms (S= 102; H’= 3.83; E=0.83) were richer and more diversified than secondary forests (S= 100; H’= 3.83; E=0.83) and palm groves (S= 70; H’= 3.61; E=0.85). Myrmicinae, Formicinae and Ponerinae were the richest subfamilies both at genus and species levels. Based on their frequency of occurrence, Myrmicaria opaciventris (18.6%), Crematogaster striatula (17.1%), Crematogaster gabonensis (14.9%) and Camponotus crawleyi Emery, 1920 (14.2%) species were numerically dominant. Strumigenys sp.1, Strumigenys sp.2, Strumigenys sp.3, and Strumigenys sp.4 species were found only in secondary forests, suggesting the relatively stability of this habitat despite anthropogenic disturbance. Camponotus brevicollis, Technomyrmex sp.2 and Tetramorium guineensis were the indicator species in cocoa farms. In the secondary forests, Camponotus wellmani, Hypoponera punctatissima and Pheidole pulchella were found as indicator species while in palm groves H. punctatissima was the only indicator species.

Ants community structure in the urban and the city suburbs areas of Douala (Littoral-Cameroon)

The Douala harbour represents the main gateway through which human activities introduce invasive ants, so that Solenopsis geminata (Fabricius, 1804) originating from Neotropics has been reported in several areas of the Littoral region of Cameroon. But nothing is known about the ant community structure and composition of the coastal zone. We hypothesized that environmental perturbations around Douala have repercussions on the native litter-dwelling ants. Collections conducted in 33 houses, six gardens of 225 m² each, 41 plantations of one hectare each and 34 two-year old fallows of one hectare each suggested lowly even communities, low species richness, low diversity and low dominance by a few species. Among 28 species recorded four species were mostly represented: two foreign origin species [S. geminata (Fabricius, 1804) and Trichomyrmex destructor (Jerdon, 1851), from tropical America and India respectively], one native species [Pheidole megacephala (Fabricius, 1793)] and two ambiguous native range species [Paratrechina longicornis (Latreille, 1802) and Tapinoma melanocephalum (Fabricius, 1793)] since they have long been recorded as having a widespread distribution. Alien species were highly represented than native ones (52.9% and 47.1% respectively, p<0.001). Inside houses, gardens and plantations the ant species were distributed according to the geometrical progression (Motomura’s model) while in the old fallows abundance distribution suggested an evolved ecosystem (Mandelbrot’s model). Between the three dominant species [Pa. longicornis (Latreille, 1802), Ph. megacephala (Fabricius) and S. geminata (Fabricius)], the first species was positively correlated with the third one while other associations were not significant. These dominant species appeared influencing the abundance of the rare species. Dominance and high abundance of a few species indicated that areas were influenced mostly by interspecies competition and/or disturbance by human activities.

An insight into the quality of sacred groves – an island habitat – using leaf-litter ants as an indicator in a context of urbanization

Sacred groves (SGs) of India are islets of forests providing ecosystem and spiritual services to man. Studies suggest that SGs are deteriorating on their quality due to urbanization, invasive species, land-use change, and religious modernization. We explored diversity, community, and abundance of overall and different functional groups of litter ants, including Anoplolepis gracilipes – an invasive ant – on paired SG-neighbouring home garden (HG) sites in rural and urban landscape to (a) assess the quality of SGs and (b) examine whether the variation in ant community of the two habitats was predicted by urbanization and abundance of A. gracilipes. We considered species and local contribution to β-diversity to identify species and sites crucial for conservation of sites. Abundance and richness of overall ants, proportional trap incidence of species, and abundance of A. gracilipes were similar on SG and HG, but species diversity and abundance of certain ant functional groups were higher on SG. Ant community of SG was different from HG, but was not affected by urbanization. A. gracilipes and rural SGs contributed the most to β diversity. A. gracilipes gave little pressure on native ant community. The study concludes that SGs, despite invaded by A. gracilipes, have potential for conserving biodiversity.