Woody Encroachment Affects Multiple Dimensions of Ant Diversity in a Neotropical Savanna

Although savanna woody encroachment has become a global phenomenon, few studies have simultaneously evaluated its effects on multiple dimensions and levels of savanna biodiversity. We evaluated how the progressive increase in tree cover in a fire-suppressed savanna landscape affects the taxonomic, functional, and phylogenetic diversity of neotropical ant communities. We sampled ants along an extensive tree cover gradient, ranging from open savannas to forests established in former savanna areas due to fire suppression, and found that Leaf Area Index explained much of the observed variation in ant diversity at both the alpha and beta levels. However, ant responses to variation in tree cover were largely non-linear as differences in alpha diversity and in the dissimilarities of the sampled communities were often much more marked at the savanna/forest transition than at any other part of the gradient. The patterns of functional and phylogenetic diversity mirrored those of taxonomic diversity, notably at the beta level. At the alpha level, functional diversity tended to increase, whereas taxonomic and phylogenetic diversity decreased or was unrelated to tree cover. Our results indicate that savanna ant communities switch rapidly to an alternative state once savanna turns into forest. Ant communities in the newly formed forest areas lacked many of the species typical of the open habitats, suggesting that the maintenance of a fire suppression policy, is likely to result in a decrease in ant diversity and in the homogenization of the ant fauna at the landscape level.

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.

Phylogenetic α- and β-Diversities Jointly Reveal Leaf-Litter Ant Community Assembly Mechanisms Along a Tropical Elevational Gradient

Despite the long-standing interest in the organization of ant communities across elevational gradients, few studies have incorporated the evolutionary information to understand the historical processes that underlay such patterns. Through the evaluation of phylogenetic α and β-diversity, we analyzed the structure of leaf-litter ant communities along the Cofre de Perote mountain in Mexico and inferred its putative driving forces. Lowland and some highland sites showed phylogenetic clustering, whereas intermediate elevations and the highest site presented phylogenetic overdispersion. We infer that strong environmental constrains found at the bottom and the top elevations are favoring closely-related species to prevail at those elevations. Conversely, more benign conditions at intermediate elevations suggest interspecific interactions being more important in these environments. Total phylogenetic dissimilarity was driven by the turnover component, indicating that the turnover of ant species along the mountain is actually shifts of lineages adapted to particular locations resembling their ancestral niche. The greater phylogenetic dissimilarity between communities was related to greater temperature distances probably due to narrow thermal tolerances inherit to several ant lineages that evolved in more stable conditions. Our results suggest that the interplay between environmental filtering, interspecific competition and habitat specialization plays an important role in the assembly of leaf-litter ant communities along elevational gradients.

Disentangling factors that assemble New Zealand's ant communities

<p>Several biotic and abiotic stressors can influence community assembly. The negative co-occurrence patterns observed within many communities, for example, may derive either from behavioural similarities (e.g. species displaying high aggression levels towards each other) or habitat preference. I evaluated the role of several stressors that may shape New Zealand’s ant communities. First, I investigated (in chapter 2) the co-occurrence patterns of two native ant communities located within transitional grassland-forest habitats. I also monitored the temperature variation in these habitats over a one-year period. I found that grasslands are exposed to higher temperature variation than forest habitats. I also found that some ants are mostly associated with forest habitats and others with grasslands. Using null models to examine these communities, I found evidence that two ant species (Monomorium antarcticum and Prolasius advenus) exhibit negative co-occurrence patterns. In the reminder of my thesis I developed a series of laboratory-based experiments to examine the processes that could explain the co-occurrence patterns that I observed in these ant communities. In chapter 3, I subjected heterospecific groups of ants to interactions in controlled conditions. I asked if interspecific aggression predict the survival probability and co-occurrence patterns described in chapter 2. My results demonstrated that aggression predicted the survival probability of interacting ant species and their co-occurrence patterns. I argued that aggressive behaviour might reflect the risks imposed by competitors. Differences in aggression may thus be a key factor influencing sympatric and allopatric co-occurrence patterns of these ant communities. In chapter 4, I tested the hypotheses that arrival sequence and diet influence the strength of interactions between colonies of two species that exhibited negative co-occurrence patterns (P. advenus and M. antarcticum). When arriving first, P. advenus displayed increased aggression and M. antarcticum a defensive reaction. The adoption of a defensive reaction by M. antarcticum increased their colony survival probability. Changes in carbohydrate and protein availability modulated colony activity rates of both species. These results indicate that arrival sequence can modulate the territorial behaviour displayed by interacting species in situations of conflict. Also, I showed that these ant species adjust their foraging activity rates in according to their diet, but different species do so differently. In chapter 5, I expanded the scope of chapter 4 and asked if aggression and foraging behaviour of P. advenus and M. antarcticum change in different conditions of temperature, diet and group size. For both ant species, changes in temperature had stronger effects on small than large colonies. Small groups of M. antarcticum displayed higher foraging activity at lower temperatures. Conversely, small groups of P. advenus displayed higher foraging activity at high temperatures. Also, small M. antarcticum colonies displayed increased aggression and significantly reduced the size of large P. advenus colonies, regardless of temperature and diet. These results suggest that P. advenus and M. antarcticum perform differently at different temperatures. Furthermore, I demonstrated that the persistence of these small colonies might be related to their ability to modulate foraging activities and interspecific aggression according to the environment. I also investigated (in chapter 6) the effects of a neurotoxic pesticide (neonicotinoid) on a native (M. antarcticum) and an invasive ant (Linepithema humile). I tested whether sublethal contamination with a neonicotinoid affects foraging, fitness and the outcome of interspecific interactions between these ants. Overall, pesticide exposure increased aggression of the invasive ant and reduced the aggression of the native species. Importantly, non-exposed individuals of the invasive species subjected to interactions against exposed natives were less aggressive, but more likely to survive. These results suggest that the modification of the physicochemical environment by pesticide contamination could change the dynamics of communities and influence invasion success. Overall, this thesis highlights that synergistic effects between several biotic and abiotic factors influence community assembly. My results suggest that non-random allopatric patterns of niche occupancy observed in these ant communities are better explained by high levels of aggression displayed between pairs of species that seldom co-occur, though I was unable to falsify the hypothesis that habitat preference also plays a role in determining their distribution and co-occurrence patterns. The modification of behaviour by external factors – either natural (e.g. temperature) or human mediated (e.g. pesticide exposure) – likely has broad effects on population and community dynamics and on patterns of species co-existence.</p>

Disentangling factors that assemble New Zealand's ant communities

<p>Several biotic and abiotic stressors can influence community assembly. The negative co-occurrence patterns observed within many communities, for example, may derive either from behavioural similarities (e.g. species displaying high aggression levels towards each other) or habitat preference. I evaluated the role of several stressors that may shape New Zealand’s ant communities. First, I investigated (in chapter 2) the co-occurrence patterns of two native ant communities located within transitional grassland-forest habitats. I also monitored the temperature variation in these habitats over a one-year period. I found that grasslands are exposed to higher temperature variation than forest habitats. I also found that some ants are mostly associated with forest habitats and others with grasslands. Using null models to examine these communities, I found evidence that two ant species (Monomorium antarcticum and Prolasius advenus) exhibit negative co-occurrence patterns. In the reminder of my thesis I developed a series of laboratory-based experiments to examine the processes that could explain the co-occurrence patterns that I observed in these ant communities. In chapter 3, I subjected heterospecific groups of ants to interactions in controlled conditions. I asked if interspecific aggression predict the survival probability and co-occurrence patterns described in chapter 2. My results demonstrated that aggression predicted the survival probability of interacting ant species and their co-occurrence patterns. I argued that aggressive behaviour might reflect the risks imposed by competitors. Differences in aggression may thus be a key factor influencing sympatric and allopatric co-occurrence patterns of these ant communities. In chapter 4, I tested the hypotheses that arrival sequence and diet influence the strength of interactions between colonies of two species that exhibited negative co-occurrence patterns (P. advenus and M. antarcticum). When arriving first, P. advenus displayed increased aggression and M. antarcticum a defensive reaction. The adoption of a defensive reaction by M. antarcticum increased their colony survival probability. Changes in carbohydrate and protein availability modulated colony activity rates of both species. These results indicate that arrival sequence can modulate the territorial behaviour displayed by interacting species in situations of conflict. Also, I showed that these ant species adjust their foraging activity rates in according to their diet, but different species do so differently. In chapter 5, I expanded the scope of chapter 4 and asked if aggression and foraging behaviour of P. advenus and M. antarcticum change in different conditions of temperature, diet and group size. For both ant species, changes in temperature had stronger effects on small than large colonies. Small groups of M. antarcticum displayed higher foraging activity at lower temperatures. Conversely, small groups of P. advenus displayed higher foraging activity at high temperatures. Also, small M. antarcticum colonies displayed increased aggression and significantly reduced the size of large P. advenus colonies, regardless of temperature and diet. These results suggest that P. advenus and M. antarcticum perform differently at different temperatures. Furthermore, I demonstrated that the persistence of these small colonies might be related to their ability to modulate foraging activities and interspecific aggression according to the environment. I also investigated (in chapter 6) the effects of a neurotoxic pesticide (neonicotinoid) on a native (M. antarcticum) and an invasive ant (Linepithema humile). I tested whether sublethal contamination with a neonicotinoid affects foraging, fitness and the outcome of interspecific interactions between these ants. Overall, pesticide exposure increased aggression of the invasive ant and reduced the aggression of the native species. Importantly, non-exposed individuals of the invasive species subjected to interactions against exposed natives were less aggressive, but more likely to survive. These results suggest that the modification of the physicochemical environment by pesticide contamination could change the dynamics of communities and influence invasion success. Overall, this thesis highlights that synergistic effects between several biotic and abiotic factors influence community assembly. My results suggest that non-random allopatric patterns of niche occupancy observed in these ant communities are better explained by high levels of aggression displayed between pairs of species that seldom co-occur, though I was unable to falsify the hypothesis that habitat preference also plays a role in determining their distribution and co-occurrence patterns. The modification of behaviour by external factors – either natural (e.g. temperature) or human mediated (e.g. pesticide exposure) – likely has broad effects on population and community dynamics and on patterns of species co-existence.</p>

Variation in the Persistence and Effects of Argentine Ants throughout Their Invaded Range in New Zealand

<p>Invasive ants are a serious ecological problem around the world. The Argentine ant has had devastating effects on resident ant communities and may negatively impact other invertebrates in its introduced range. First detected in Auckland in 1990, this invader has since spread widely around the country. The effect of Argentine ants on invertebrates in New Zealand was investigated by comparing ground-dwelling arthropod species richness and abundance between and among paired uninvaded and invaded sites in seven cities across this invader's New Zealand range. In order to study density-dependent effects, invaded sites were chosen so as to differ in Argentine ant population density. The effects of rainfall and mean maximum temperature on Argentine ant abundance and the species richness and abundance were also examined. Argentine ant population persistence in New Zealand was examined by re-surveying sites of past infestation across this species range. The influence of climate on population persistence was investigated, and how this effect may vary after climate change. Additionally, the potential of community recovery after invasion was also examined. Epigaeic (above ground foraging) ant species richness and abundance was negatively associated with Argentine ant abundance; however, no discernable impact was found on hypogaeic (below ground foraging) ant species. The effect of Argentine ant abundance on non-ant arthropod species richness and abundance was mixed, with most arthropod orders being unaffected. Diplopoda was negatively influenced by Argentine ant abundance while Hemiptera was positively influenced. Annual rainfall and mean maximum temperature were found to have no effect on Argentine ant abundance or resident ant species richness and abundance, though these variables did help explain the distribution of several non-ant arthropod orders. Argentine ant populations appear to be collapsing in New Zealand. Populations had a mean survival time of 14.1 years (95% CI= 12.9- 15.3 years). Climate change may prolong population survival, as survival time increased with increasing temperature and decreasing rainfall, but only by a few years. Formerly invaded Auckland ant communities were indistinguishable from those that had never been invaded, suggesting ant communities will recover after Argentine ant collapse.</p>

Variation in the Persistence and Effects of Argentine Ants throughout Their Invaded Range in New Zealand

<p>Invasive ants are a serious ecological problem around the world. The Argentine ant has had devastating effects on resident ant communities and may negatively impact other invertebrates in its introduced range. First detected in Auckland in 1990, this invader has since spread widely around the country. The effect of Argentine ants on invertebrates in New Zealand was investigated by comparing ground-dwelling arthropod species richness and abundance between and among paired uninvaded and invaded sites in seven cities across this invader's New Zealand range. In order to study density-dependent effects, invaded sites were chosen so as to differ in Argentine ant population density. The effects of rainfall and mean maximum temperature on Argentine ant abundance and the species richness and abundance were also examined. Argentine ant population persistence in New Zealand was examined by re-surveying sites of past infestation across this species range. The influence of climate on population persistence was investigated, and how this effect may vary after climate change. Additionally, the potential of community recovery after invasion was also examined. Epigaeic (above ground foraging) ant species richness and abundance was negatively associated with Argentine ant abundance; however, no discernable impact was found on hypogaeic (below ground foraging) ant species. The effect of Argentine ant abundance on non-ant arthropod species richness and abundance was mixed, with most arthropod orders being unaffected. Diplopoda was negatively influenced by Argentine ant abundance while Hemiptera was positively influenced. Annual rainfall and mean maximum temperature were found to have no effect on Argentine ant abundance or resident ant species richness and abundance, though these variables did help explain the distribution of several non-ant arthropod orders. Argentine ant populations appear to be collapsing in New Zealand. Populations had a mean survival time of 14.1 years (95% CI= 12.9- 15.3 years). Climate change may prolong population survival, as survival time increased with increasing temperature and decreasing rainfall, but only by a few years. Formerly invaded Auckland ant communities were indistinguishable from those that had never been invaded, suggesting ant communities will recover after Argentine ant collapse.</p>