María Natalia Lescano

Exotic thistles increase native ant abundance through the maintenance of enhanced aphid populations

Exotic species change the structure and composition of invaded communities in multiple ways, but the sign of their impact on native species is still controversial. We evaluated the effects of the thistles Carduus thoermeri and Onopordum acanthium—two of the most abundant exotic plant species in disturbed areas of the Patagonian steppe—on the native tending ant assemblage. Exotic thistles showed an increased number of plants with aphids and had greater aphid density than native plants. Since native tending ants were present only in plants with aphids, their abundance was higher in infested thistles than in native plants. Path analyses confirmed that ant activity depended more on aphid density than on thistle traits. Our results suggest that the presence of exotic thistles in disturbed areas of NW Patagonia indirectly benefit the native ant assemblage through the maintenance of an increased aphid population. This illustrates how the impact of exotic on native species can depend on the ecological context.

Bottom-up effects may not reach the top: the influence of ant–aphid interactions on the spread of soil disturbances through trophic chains

Soil disturbances that increase nutrient availability may trigger bottom-up cascading effects along trophic chains. However, the strength and sign of these effects may depend on attributes of the interacting species. Here, we studied the effects of nutrient-rich refuse dumps of the leaf-cutting ant, Acromyrmex lobicornis, on the food chain composed of thistles, aphids, tending ants and aphid natural enemies. Using stable isotopes tracers, we show that the nitrogen accumulated in refuse dumps propagates upward through the studied food chain. Thistles growing on refuse dumps had greater biomass and higher aphid density than those growing in adjacent soil. These modifications did not affect the structure of the tending ant assemblage, but were associated with increased ant activity. In contrast to the expectations under the typical bottom-up cascade effect, the increase in aphid abundance did not positively impact on aphid natural enemies. This pattern may be explained by both an increased activity of tending ants, which defend aphids against their natural enemies, and the low capacity of aphid natural enemies to show numerical or functional responses to increased aphid density. Our results illustrate how biotic interactions and the response capacity of top predators could disrupt bottom-up cascades triggered by disturbances that increase resource availability.

Effects of fire severity on early recovery of Patagonian steppes

Fire severity influences the recovery and biodiversity of plant communities. We compared the early post-fire recovery of plant functional groups in two sites (one burnt with high severity and one with low severity) in the north-western Patagonian steppe. We sampled the post-fire soil seed banks, as well as the composition and cover of aboveground plant species, and calculated richness, diversity and the percentage of exotic species. Low fire severity increased the diversity of aboveground vegetation and seed banks, whereas high fire severity reduced the cover of all functional groups, with the exception of fugitive species. Tiller production of perennial grasses decreased due to bud bank mortality. In the seed bank, high severity fire reduced fugitive species and increased exotic species. Fugitive species need fire to recruit, and the combination of fire cues and post-fire environmental conditions can explain the high abundance of these species after fire. In Patagonia, the use of fire is not considered a management option because fires are believed to be dangerous to humans, goods and services. However, north-western Patagonia is sparsely populated and the frequent wildfires pose no danger. It is necessary to increase the knowledge of the ecological processes of Patagonian ecosystems related to fire regime and land use, to achieve a reasonable balance between sustainable use and conservation in this region.

Nocturnal resource defence in aphid-tending ants of northern Patagonia

Interspecific competition plays a key role in the organisation of ant communities. In ant–plant interactions, individual host plants are usually occupied by a single ant colony, and co‐occurring ant species compete for hosts. Here indirect evidence of competition between three dominant ant species that tend aphids on two biennial thistles in northern Patagonia is described, and a novel defensive behaviour in temperate ant assemblages is reported. This study has found that: (i) dominant ant species were not spatially segregated, thus enhancing the probability of fights and invasions of host plants; (ii) ant species did not show preferences for a thistle species or for any plant characteristic, and thus all plants have similar chances of being colonised by all dominant ant species; (iii) the resident ant species remained on the same plant during the whole plant life cycle, monopolising plant resources (aphids); and (iv) all dominant species, whose nests are on the ground, assigned some ants to stay on the host plant during the night, when the low temperatures typical of this temperate environment greatly reduce foraging activities. When these ‘nocturnal guards’ were experimentally removed from the host plant, other ants from the same colony rapidly appeared showing aggressive behaviours. Taking all these findings together, it is suggested that interspecific competition influences the distribution of ants on their host plants and involves nocturnal defensive behaviours despite unfavourable thermal conditions. This illustrates how habitat features, such as the short life cycle of thistles and the low night‐time temperatures that reduce ant foraging and thus make plants more vulnerable to invasion, might affect the distribution and behaviour of ants.

Synergy between roads and disturbance favour Bromus tectorum L. invasion

Background Global change produces pervasive negative impacts on biodiversity worldwide. Land use change and biological invasions are two of the major drivers of global change that often coexist; however, the effects of their interaction on natural habitats have been little investigated. In particular, we aimed to analyse whether the invasion of an introduced grass (Bromus tectorum; cheatgrass) along roads verges and the disturbance level in the natural surrounding habitat interact to influence the degree of B. tectorum invasion in the latter habitats in north-western Patagonia. Methods Along six different roads, totalling approximately 370 km, we set two 50 m × 2 m sampling plots every 5 km (73 plots in total). One plot was placed parallel to the road (on the roadside) and the other one perpendicular to it, towards the interior of the natural surrounding habitat. In each plot, we estimated the B. tectorum plant density in 1 m2 subplots placed every 5 m. In the natural habitat, we registered the vegetation type (grassy steppe, shrub-steppe, shrubland, and wet-meadow) and the disturbance level (low, intermediate, and high). Disturbance level was visually categorized according to different signs of habitat degradation by anthropogenic use. Results B. tectorum density showed an exponential decay from roadsides towards the interior of natural habitats. The degree of B. tectorum invasion inside natural habitats was positively related to B. tectorum density on roadsides only when the disturbance level was low. Shrub-steppes, grassy steppes and shrublands showed similar mean density of B. tectorum. Wet-meadows had the lowest densities of B. tectorum. Intermediate and highly disturbed environments presented higher B. tectorum density than those areas with low disturbance. Discussion Our study highlights the importance of the interaction between road verges and disturbance levels on B. tectorum invasion in natural habitats surrounding roads of north-western Patagonia, particularly evidencing its significance in the invasion onset. The importance of invasion in road verges depends on disturbance level, with better conserved environments being more resistant to invasion at low levels of B. tectorum density along road verges, but more susceptible to road verges invasion at higher levels of disturbance. All the habitats except wet-meadows were invaded at a similar degree by B. tectorum, which reflects its adaptability to multiple habitat conditions. Overall, our work showed that synergies among global change drivers impact native environments favouring the invasion of B. tectorum.

Pay it forward: refuse dump from leaf-cutting ants promotes caterpillar digestive performance by increasing plant nitrogen content

1. The activity of soil‐disturbing animals that increase soil nutrients can affect the carbon : nitrogen (C : N) ratio of plants, which, in turn, may determine the transfer of energy and nutrients through higher trophic levels. However, the strength and sign of this indirect effect depend on whether enhanced nutrient substrates increase plant foliar nutrients and/or plant defensive traits.

La leña como vía para la introducción de insectos. ¿Cuáles son los riesgos de las invasiones de hormigas en la Patagonia?

Firewood can act as a long-distance dispersal vector for wood-infesting insects both within and among countries. Recently, we detected the alien carpenter ant Camponotus mus in firewood transported from central Argentina to Patagonia. This species generates significant economic losses in its native range and has invader potential. Moreover, global warming and the increasing anthropogenic disturbance in Patagonian ecosystems make them highly susceptible to insect invasions. This is especially alarming considering the current lack of sanitary controls of incoming goods into the region. To prevent insect introductions via firewood, it is crucial to implement a joint effort among the scientific community, control organisms, government and end user.https://doi.org/10.25260/EA.17.27.2.0.478

Outcomes of competitive interactions after a natural increment of resources: the assemblage of aphid-tending ants in northern Patagonia

Ant-aphid relationships provide excellent opportunities to study how changes in resource availability may affect the outcome of competitive interactions. Variations in soil fertility may affect host plant quality, with concomitant effects on aphid abundance and the amount/quality of aphid honeydew. This may determine the intensity at which tending ants defend aphids against natural enemies and competing ants. In a shrub-steppe of northern Patagonia, aphid-infested thistles naturally grow on contrasting fertility substrates: organic waste piles of leaf-cutting ants (refuse dumps) and nutrient-poor steppe soils. Thistles growing on refuse dumps have much larger aphid colonies than thistles growing on steppe soils. We took advantage of the co-occurrence in the field of plants with contrasting aphid density to study the effect of natural variation in food availability (aphid density) on aphid-tending ant species richness and agonistic interactions among them. Enhanced aphid density did not promote the coexistence of aphid-tending ant species. Although all ant species are potential colonizers of the study plants, thistles were often monopolized by a single ant species, regardless of aphid density. Field experiments showed that increased aphid density did not modify aggressiveness toward an intruder ant, nor the probability of coexistence between two rival ant species after the invasion of a host plant. We discuss several hypotheses to explain why increased resource availability does not necessarily reduce competitive interactions in ant-aphid relationships.