Alan N. Andersen

Leaf-cutting ant populations profit from human disturbances in tropical dry forest in Brazil

Anthropogenic disturbance often results in the proliferation of native species of particular groups that leads to biotic homogenization. Leaf-cutting ants are an example of such winner organisms in tropical rain forests, but their response to disturbance in dry forests is poorly known. We investigated Atta colony density in areas of tropical dry forest in Brazil with different distance to roads and vegetation cover. Atta colonies were surveyed in 59 belt transects of 300 × 20 m, covering a total area of 35.4 ha. We found 224 Atta colonies, 131 of which were active and belonged to Atta opaciceps (87 colonies, 2.45 ha −1 ), A. sexdens (35 colonies, 0.98 ha −1 ) and A. laevigata (9 colonies, 0.25 ha −1 ). The density of active colonies sharply decreased from 15 ± 2.92 ha −1 in the 50-m zone along roads to only 2.55 ± 1.65 ha −1 at distances up to 300 m. The reverse pattern was observed for inactive colonies. Active Atta colonies preferentially occur in areas with low vegetation cover, while inactive colonies prefer areas with high vegetation cover. We demonstrate for the first time that anthropogenic disturbances promote the proliferation of leaf-cutting ants in dry forest in Brazil, which may affect plant regeneration via herbivory and ecosystem engineering as demonstrated for rain forests.

Bioclimatic transect networks: powerful observatories of ecological change

Abstract Transects that traverse substantial climate gradients are important tools for climate change research and allow questions on the extent to which phenotypic variation associates with climate, the link between climate and species distributions, and variation in sensitivity to climate change among biomes to be addressed. However, the potential limitations of individual transect studies have recently been highlighted. Here, we argue that replicating and networking transects, along with the introduction of experimental treatments, addresses these concerns. Transect networks provide cost‐effective and robust insights into ecological and evolutionary adaptation and improve forecasting of ecosystem change. We draw on the experience and research facilitated by the Australian Transect Network to demonstrate our case, with examples, to clarify how population‐ and community‐level studies can be integrated with observations from multiple transects, manipulative experiments, genomics, and ecological modeling to gain novel insights into how species and systems respond to climate change. This integration can provide a spatiotemporal understanding of past and future climate‐induced changes, which will inform effective management actions for promoting biodiversity resilience.

Dominance-diversity relationships in ant communities differ with invasion

The relationship between levels of dominance and species richness is highly contentious, especially in ant communities. The dominance-impoverishment rule states that high levels of dominance only occur in species-poor communities, but there appear to be many cases of high levels of dominance in highly diverse communities. The extent to which dominant species limit local richness through competitive exclusion remains unclear, but such exclusion appears more apparent for non-native rather than native dominant species. Here we perform the first global analysis of the relationship between behavioral dominance and species richness. We used data from 1,293 local assemblages of ground-dwelling ants distributed across five continents to document the generality of the dominance-impoverishment rule, and to identify the biotic and abiotic conditions under which it does and does not apply. We found that the behavioral dominance-diversity relationship varies greatly, and depends on whether dominant species are native or non-native, whether dominance is considered as occurrence or relative abundance, and on variation in mean annual temperature. There were declines in diversity with increasing dominance in invaded communities, but diversity increased with increasing dominance in native communities. These patterns occur along the global temperature gradient. However, positive and negative relationships are strongest in the hottest sites. We also found that climate regulates the degree of behavioral dominance, but differently from how it shapes species richness. Our findings imply that, despite strong competitive interactions among ants, competitive exclusion is not a major driver of local richness in native ant communities. Although the dominance-impoverishment rule applies to invaded communities, we propose an alternative dominance-diversification rule for native communities.

When macroecological transitions are a fiction of sampling: comparing herbarium records to plot-based species inventory data

Natural history collections are alternative data sources to plot-based species inventories for analysing macroecological species turnover. Herbarium records sample diversity well at regional level and are taxonomically validated. However, they are ad hoc from a sampling perspective, generating spatial and taxonomic biases. The implications of biased sampling on beta diversity (β) estimation, and use of herbarium data to identify macroecological transitions, remain unexplored. We tested sampling influences by comparing herbarium data with systematically collected inventory data from the Mount Lofty–Flinders Ranges region of Australia. We calculated β within moving windows across bioclimatic gradients using metrics varying in sensitivity to richness differences (pairwise/multi-site Sørensen β; Simpson β; Harrison et al. β-2), and correlated β to species sampling and between herbarium and plot data. We tested whether generalised dissimilarity modelling (GDM) revealed the same compositional transitions in herbarium and plot data along environmental gradients. Sørensen, Simpson and multi-site Sørensen β had strong negative correlations with richness (indicating sampling bias) for herbarium data (Pearson’s r = –0.85, –0.80, –0.81, respectively) but not plots (r = –0.27, –0.28, –0.11). Harrison et al. β-2 correlated poorly with richness (herbarium: r = –0.16; plots: r = –0.14) but herbarium and plot data were only weakly correlated (r = 0.18). All other metrics correlated poorly (–0.03  r  0.16) between datasets, suggesting biases. GDMs differed in variable importance but revealed similar transition zones for key gradients. We conclude that untransformed herbarium data are unsuitable for detecting macroecological transitions because turnover is linearly related to sampling intensity and correlates poorly with systematic surveys. Herbarium data should be used cautiously for β, even with methods insensitive to richness differences. However, herbarium data can robustly reproduce transition zones when modelled along environment gradients. We recommend this approach for detecting macroecological transitions using natural history data in the absence of plot data.

Habitat disturbance selects against both small and large species across varying climates

Global extinction drivers, including habitat disturbance and climate change, are thought to affect larger species more than smaller species. However, it is unclear if such drivers interact to affect assemblage body size distributions. We asked how these two key global change drivers differentially affect the interspecific size distributions of ants, one of the most abundant and ubiquitous animal groups on earth. We also asked whether there is evidence of synergistic interactions and whether effects are related to species’ trophic roles. We generated a global dataset on ant body size from 333 local ant assemblages collected by the authors across a broad range of climates and in disturbed and undisturbed habitats. We used head length (range: 0.22–4.55 mm) as a surrogate of body size and classified species to trophic groups. We used generalized linear models to test whether body size distributions changed with climate and disturbance, independent of species richness. Our analysis yielded three key results: 1) climate and disturbance showed independent associations with body size; 2) assemblages included more small species in warmer climates and fewer large species in wet climates; and 3) both the largest and smallest species were absent from disturbed ecosystems, with predators most affected in both cases. Our results indicate that temperature, precipitation and disturbance have differing effects on the body size distributions of local communities, with no evidence of synergistic interactions. Further, both large and small predators may be vulnerable to global change, particularly through habitat disturbance.

An experimental test of whether pyrodiversity promotes mammal diversity in a northern Australian savanna

26 1. The increasing awareness that a fire regime that promotes biodiversity in one system can 27 threaten biodiversity in another has resulted in a shift away from fire management based on 28 vague notions of maximising pyrodiversity, towards determining the optimal fire regime based 29 on the demonstrated requirements of target species. 30 2. Here, we utilised a long-running, replicated fire experiment on Melville Island, the largest 31 island off the northern Australian coast, to test the importance of pyrodiversity for native 32 mammals in a northern Australian savanna landscape. We first developed statistical models to 33 determine how native mammal abundance has responded to nine years of experimentally34 manipulated fire frequency. Next, given each species’ modelled response to fire frequency, we 35 identified the level of pyrodiversity and optimal mix of fire frequencies that would be expected 36 to maximise mammal diversity and abundance, and minimise extinction risk, This was done for 37 both the entire mammal assemblage and for the mammal species currently declining on 38 Melville Island. 39 3. Fire frequency was a significant predictor of abundance of the northern brown bandicoot 40 (Isoodon macrourus), black-footed tree-rat (Mesembriomys gouldii), brush-tailed rabbit-rat 41 (Conilurus penicillatus), grassland melomys (Melomys burtoni), pale field-rat (Rattus tunneyi), 42 and mice/dunnarts but not of the common brushtail possum (Trichosurus vulpecula). 43 4. The geometric mean abundance (GMA) of the entire mammal assemblage was positively 44 associated with pyrodiversity, but peaked at an intermediate value. Hence, maximising 45 pyrodiversity would reduce native mammal assemblage GMA below its potential maximum. 46 The fire history for an area that maximised the entire native mammal assemblage GMA 47 consisted of 57% long-unburnt, 43% triennially burnt and <1% annually burnt. Pyrodiversity 48 did not reduce the extinction risk, nor increase the GMA of declining mammals above that 49 predicted in areas entirely annually or triennially burnt. 50 5. Synthesis and applications: We demonstrate a useful approach with which to develop fire 51 management strategies based on the demonstrated requirements of target species. By comparing 52 the optimal fire regime identified for the conservation of threatened species and that identified 53 for the entire mammal assemblage, we demonstrate the flexibility of this approach to tailor fire 54 management to address specific management priorities in other fire-prone environments. 55

Rapid response of habitat structure and aboveground carbon storage to altered fire regimes in tropical savanna

Rapid response of habitat structure and aboveground carbon storage to altered fire regimes in tropical savanna Shaun R. Levick1,2,3, Anna E. Richards2, Garry D. Cook2, Jon Schatz2, Marcus Guderle1, Richard J. Williams2, Parash Subedi3, Susan E. Trumbore1, and Alan N. Andersen3 1Max Planck Institute for Biogeochemistry, Hans-Knoell-Str. 10, 07745 Jena, Germany 2CSIRO Land and Water, PMB 44, Winnellie, 0822 NT, Australia 3Research Institute for the Environment and Livelihoods, Charles Darwin University, NT 0909, Australia Correspondence: Shaun R. Levick (shaun.levick@csiro.au)

Ants of the Caatinga: Diversity, biogeography, and functional responses to anthropogenic disturbance and climate change

Despite the outstanding diversity and ecological relevance of ants in most terrestrial ecosystems, current knowledge of the ants of the Caatinga is still incipient. This chapter offers an overview covering the diversity, taxonomy, biogeography, and functional composition of the Caatinga ant fauna, and a synthesis on ant response to chronic anthropogenic disturbance and increased aridity. We compiled a database consisting of 572 presence–absence ant records and 276 ant species from 37 localities in the Caatinga. As expected, most of the Caatinga has not been intensively sampled for ants, with the intensive sampling that has been conducted revealing high rates of species turnover across localities. Most ant species recorded in the Caatinga are widely distributed in other biomes, especially in Cerrado, and few species can be considered endemic to the Caatinga. Thus, the Caatinga ant fauna appears to represent an impoverished subset of the Cerrado’s fauna. Such a reduced endemism and the occurrence of a highly depauperate ant fauna at a regional level contrast to the diversity patterns exhibited by the Caatinga flora and other faunal groups. Significant changes in ant taxonomic and functional composition in response to human disturbance are observed, with a predictable winner–loser replacement. Disturbance winners consist of generalist species exhibiting wide environmental tolerances and those inhabiting open habitats (Opportunists and Dominant Dolichoderinae). Highly specialized species are disturbance losers (Specialist predators). Aridity also affects both species occurrence and functional-group composition of local assemblages. Since several ant species and functional groups are sensitive to increasing disturbance and aridity, ant-mediated ecological services are already threatened in the Caatinga biota.