Leaf‐cutting ants as ecosystem engineers: topsoil and litter perturbations around Atta cephalotes nests reduce nutrient availability

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Performance and fate of tree seedlings on and around nests of the leaf‐cutting ant Atta cephalotes: Ecological filters in a fragmented forest

Habitat fragmentation is currently the most pervasive anthropogenic disturbance in tropical forests and some species of leaf‐cutting ants of the genus Atta (dominant herbivores in the neotropics) have become hyper‐abundant in forest edges where their nests directly impact up to 6% of the forest area. Yet, their impacts on the regeneration dynamics of fragmented forests remain poorly investigated. Here we examine the potential of Atta cephalotes nests to function as ecological filters impacting tree recruitment. Growth, survival and biomass partitioning of experimentally planted seedlings (six tree species) were examined at eight spatially independent A. cephalotes colonies in a large Atlantic Forest fragment. Seedling performance and fate (leaf numbers and damage) were monitored up to 27 months across three habitats (nest centre, nest edge and forest understorey). Plants at illuminated nest centres showed twice the gross leaf gain as understorey individuals. Simultaneously, seedlings of all species lost many more leaves at nests than in the forest understorey, causing a negative net leaf gain. Net leaf gain in the shaded understorey ranged from zero (Licania and Thyrsodium species) to substantial growth for Copaifera and Virola, and intermediate levels little above zero for Protium and Pouteria. Also seedling survival differed across habitats and species, being typically low in the centre and at the edge of nests where seedlings were often completely defoliated by the ants. Lastly, seedling survival increased strongly with seed size at nest edges while there was no such correlation in the forest. Our results suggest that Atta nests operate as ecological filters by creating a specific disturbance regime that differs from other disturbances in tropical forests. Apparently, Atta nests favour large‐seeded tree species with resprouting abilities and the potential to profit from a moderate, nest‐mediated increase in light availability.     

Population structure of Leptothorax ambiguus,a facultatively polygynous and polydomous ant species

We examined the genetic and spatial structure of Leptothorax ambiguus in a Vermont site. Nests of this tiny ant species have variable queen number and comprise larger polydomous colonies, as do their closest relatives in North America. Nests are patchily distributed in the forest, and sometimes occur in local abundance. We collected 121 nests in four years from plots in which all nests were mapped; furthermore, we subjected nests collected in two separate years to starch gel electrophoresis and estimated relatedness according to the Queller—Goodnight (1989) algorithm. Queens that share a nest site also share 33% of their alleles on average, and relatedness among worker nestmates is about 0.5. The existence of diploid males and nonzero F-values demonstrate inbreeding in this species, an unusual phenomenon for social insects in general. Mapping data showed that nests with like genotypes tended to cluster in space, forming polydomous colonies. Colonies consisted of 1–6 nest subunits, and about half of all colonies were polygynous. We compare these features of L. ambiguus to its close relative L. longispinosus and a European congener L. acervorum. These comparisons allow us to conclude that an interplay between ecological and genetic factors produces the observed pattern of multiple queening and nest spatial distribution in this species.     

How leaf-cutting ants impact forests: drastic nest effects on light environment and plant assemblages

Leaf-cutting ants (Atta spp.) have become a topical issue in Neotropical ecology, particularly because they are reaching hyper-abundance due to escalating levels of fragmentation in recent years. Yet, despite intensive research on their role as dominant herbivores, there is still insufficient documentation on the impacts of their large, long-lived nests on plant assemblage structure and ecosystem functioning. Our study aimed at investigating the magnitude, nature, and spatial extent of nest influence by assessing 11 attributes of ant nest, canopy structure, light environment and sapling assemblage for 20 colonies in four plots along nest-understorey gradients in a large remnant of Atlantic forest. We also monitored the performance of seeds and seedlings of Chrysophyllum viride, an abundant shade-tolerant species. Previously unrecognized canopy gaps above ant nests (0.04–87.9 m²) occurred in 95% of all colonies surveyed. Overall, canopy openness and light availability at least doubled in ant nest plots compared with distant understorey plots. These drastic changes in the light environment paralleled those in plant assemblage: sapling density almost tripled (mean ± SE: 0.42 ± 0.1 saplings m^?2) and sapling species richness doubled (0.16 ± 0.02 species m^?2) in distant plots, as did shade-tolerant species. After a 1-year period, only 33 ± 15.6% of the seeds germinated and all seedlings died on nests, whereas seed germination reached 68 ± 5.1% in distant plots and 66.4 ± 7.6% of their seedlings survived after 12 months. Therefore, plot location was the most significant explanatory variable for predictable and conspicuous changes in the light environment and structure of sapling assemblages. Our findings greatly extend knowledge on the role played by leaf-cutting ants as ecosystem engineers by demonstrating that ant nest-mediated disturbance promotes environmental modifications in tens of meters around nests and is thus, strong enough to drive plant recruitment and consequently alter both the floristic and functional signature of plant assemblages.     

Effects of pasture and forest microclimatic conditions on the foraging activity of leaf-cutting ants

The fragmentation and transformation of land cover modify the microclimate of ecosystems. These changes have the potential to modify the foraging activity of animals, but few studies have examined this topic. In this study, we investigated whether and how the foraging activity of the leaf-cutter ant Atta cephalotes is modified by microclimatic variations due to land cover change from forest to pasture. We characterized the microclimate of each habitat and identified alterations in foraging behavior in response to relative humidity (RH), air temperature, and surface temperature along ant foraging trails by synchronously assessing foraging activity (number of ants per 5 min including incoming laden and unladen and outgoing ants) and microclimatic variables (air temperature, RH, and maximum and minimum surface temperature along the foraging trail). There were climatic differences between habitats during the day but not throughout the night, and A. cephalotes was found to have a high tolerance for foraging under severe microclimatic changes. This species can forage at surface temperatures between 17 and 45°C, air temperatures between 20 and 36°C, and an RH between 40% and 100%. We found a positive effect of temperature on the foraging activity of A. cephalotes in the pasture, where the species displayed thermophilic behavior and the ability to forage across a wide range of temperatures and RH. These results provide a mechanism to partially explain why A. cephalotes becomes highly prolific as anthropogenic disturbances increase and why it has turned into a key player of human-modified neotropical landscapes.     

Life Inside an Acorn: How Microclimate and Microbes Influence Nest Organization in Temnothorax Ants

Nests provide a buffer against environmental variation, but conditions may also vary at different locations within a nest. Conditions can vary based on abiotic factors, such as moisture and temperature, as well as biotic factors, such as the presence of microbes and potential pathogens. Therefore, characterizing how animals adjust their position inside their nests to track microclimate preferences while at the same time preventing pathogen exposure is necessary to understand the benefits nests provide. Here we studied how colonies of the acorn‐nesting ant Temnothorax curvispinosus responded to experimental manipulation of moisture, temperature, and microbial growth inside their nests. Colonies showed no response to differences in moisture and moved to the bottom of the acorn regardless of moisture treatment. When nests were heated from the top to simulate warming by the sun, workers preferentially moved brood to the warm, upper half of the acorn, which would stimulate brood development. Finally, the strongest factor that influenced colony position was the presence of microbes inside the nest—colonies avoided the bottom of the nest when it was inoculated with microbes, and colonies in new acorns shifted to the top of the acorn over time as mold and other microbes had time to grow. The relatively strong response of T. curvispinosus to microbial growth inside their nests suggests that pathogen pressures—in addition to microclimate—have a significant impact on how colonies use nest spaces. Social insects are known to invest heavily in antimicrobial compounds that kill or slow the growth of microbes, but avoidance may represent an additional line of defense to prevent pathogen exposure.     

Overwintering colonies of German (Vespula germanica) and common wasps (Vespula vulgaris) (Hymenoptera: Vespidae) in New Zealand

Abstract

Nests of both common (Vespula vulgaris) and German wasps (V. germanica) sometimes overwinter in New Zealand. Three overwintering common wasp colonies were found in low-altitude honeydew beech forest; about 2% of the colonies initiated there in 1988 survived the winter. Wasp traffic rates from nests in Nelson city and a nation wide survey of wasp abundance, showed that more German wasp than common wasp workers were on the wing in winter and spring. German wasp colonies in Nelson city were more likely to overwinter than were common wasp colonies. Of the active German wasp colonies recorded in Nelson in January and February 1989, only two (4%) had previously overwintered, but these two nests accounted for 38% of all German wasp workers estimated to leave nests in the area. Had other overwintering colonies not been poisoned, overwintering colonies might have accounted for up to 11 % of nests and produced up to 64% of German wasp workers on the wing in January and February 1989.

Overwintering common wasp colonies did not produce queens or drones in their first year or second spring, but all 10 overwintering German wasp colonies examined produced sexuals at both times. German wasp queens produced in winter and spring may influence the number of colonies successfully initiated and affect the population dynamics of German wasps in New Zealand.     

Effects of Formica podzolica ant colonies on soil moisture,nitrogen, and plant communities near nests

1. Ants are widely regarded as ‘ecosystem engineers’ because their nest construction and contributions to nutrient cycling change the biological, chemical, and physical properties of the soil around their nests. Despite increasing attention to ant manipulation of soil ecosystems, the extent to which many common species influence soil properties, as well as nutrient uptake and community composition of plants near nests, is still unknown. 2. This study tested hypotheses that activities of a common subalpine ant, Formica podzolica, alter soil moisture and pH, redistribute nitrogen around nests, and affect plant species abundance and ground cover. 3. A combination of field sampling techniques showed that distance from a nest had a positive relationship with soil moisture and a negative relationship with plant abundance next to and downhill from nests. Slope aspect also affected plant communities, with downhill transects having higher plant cover and above‐ground biomass than uphill transects. A stable isotope analysis did not reveal that plants near nests had enriched ¹⁵N, but there were substantial differences in ¹⁵N among sites. 4. Overall, this study uncovers significant impacts of F. podzolica on the subalpine microhabitats directly surrounding their nests.     

Tanzanian Forest Edge Microclimatic Gradients: Dynamic Patterns1

Air temperature, vapor pressure deficit, and light intensity microclimatic gradients were examined along four forest edge and four paired forest interior transects in the East and West Usambara Mountains, Tanzania. Between 14 August 1995 and 11 August 1998, 287, 282, and 196 air temperature, vapor pressure deficit, and light intensity gradients, respectively, were measured along the four forest edge and four interior transects. The relationship between microclimate and distance from the forest edge was examined using piecewise linear regression. All microclimatic gradients were classified into one of nine shapes based on the sign and the size of the two estimated slopes. The relative frequency in the shapes of 65 percent of air temperature gradients, 52 percent of vapor pressure deficit gradients, and 62 percent of light intensity gradients along forest edge transects exceeded the relative frequency of these same shapes along forest interior transects, indicating that a majority of the forest edge microclimatic gradients measured were influenced by edge effects. Yet this result also indicated that approximately one‐third of all air temperature and light intensity gradients and nearly one‐half of all vapor pressure deficit gradients recorded during this study were affected by factors independent of edge effects per se, and that forest edge microclimatic gradients were temporally nonconstant. For air temperature and vapor pressure deficit gradients, low spatial but high temporal variation existed in estimated edge width and the relative change in microclimate between the forest edge and interior. For light intensity gradients, both high spatial and temporal variability characterized estimated edge width and relative change in microclimate between the forest edge and interior. The pooled mean edge width and relative change in microclimate between die forest edge and interior across the four forest edge transects for air temperature, vapor pressure deficit, and light intensity gradients were 94.1 m and 2.00°C, 82.6 m and 0.29 kPa, and 60.5 m and 10.6 joules/sec/m², respectively. These results suggest that forest edge microclimatic gradients in general may be inherently dynamic and nonconstant.     

The Role of Wood Ants (Formica rufa group) in Carbon and Nutrient Dynamics of a Boreal Norway Spruce Forest Ecosystem

Wood ants (Formica rufa group) are regarded as keystone species in boreal and mountain forests of Europe and Asia by their effect on ecosystem carbon (C) and nutrient pools and fluxes. To quantify the impact of their activity on boreal forest ecosystems, C, nitrogen (N), phosphorus (P), potassium (K) and calcium (Ca) pools and fluxes in wood ant nests (WAN), and soil were assessed along a 5-, 30-, 60-, and 100-year-old Norway spruce (Picea abies L. Karsten) dominated successional gradient in eastern Finland. Amounts of C and nutrients in WAN increased with stand age, but contained less than 1% of total C and nutrient pools in these stands. The CO₂-efflux from nests was also insignificant, as compared to CO₂-efflux from the forest floor. Annually, the amount of C brought by wood ants into their nests as honeydew, prey and nest-building materials ranged from 2.7 to 49.3 kg ha^?1 C, but this is only 0.1–0.7% of the combined net primary production of trees and understorey in boreal forests. The difference between wood ant nest C inputs and outputs was very small in the younger-aged stands, and increased in the older stands. Carbon accumulation rates in nests over a 100 year period are estimated to be less than 10 kg ha^?1 a^?1. In contrast to C, annual inputs of N, P, and K are larger compared to wood ant nest nutrient pool size, ranging from 3 to 6% of the annual tree stand and understorey uptake. This indicates a more rapid turnover and transport of N, P, and K out of WAN, and suggests that wood ants increase the cycling rate of these nutrients in boreal forests.     

Population Density of Chimpanzees and Gorillas in the Petit Loango Reserve,Gabon: Employing a New Method to Distinguish Between Nests of the Two Species

We studied the vegetation and population density of chimpanzees (Pan troglodytes troglodytes) and gorillas (Gorilla gorilla gorilla) in the Petit Loango Reserve, Gabon. The study site consists mainly of primary and secondary forest, and it also includes patches of savanna, inundated forest, and coastal scrub. The density of herbaceous plants of Marantaceae and Zingiberaceae, which serve as important foods for great apes at other study sites, is extremely low. It was very difficult to distinguish between the nests of chimpanzees and gorillas because most gorilla nests were made in trees. We developed a new method to identify the builder of nests via a combination of field observation of nests and fecal samples, and examination of hair samples under a scanning electron microscope. Nests of chimpanzees were mainly in primary forest, and the estimated density (0.78 individual per km ² ) is higher than the average density in Gabon and comparable to the nest density in the Lopé Reserve, Gabon. Nests of gorillas were mainly in secondary forest, and their estimated density (0.21) is similar to the average next density in Gabon but lower than the gorilla nest density in the Lopé Reserve. The low density of herbaceous foods seemed to influence the density of gorillas but not the density of chimpanzees.     

Nest orientation of the Spotted Barbtail, <Emphasis Type="Italic">Premnoplex brunnescens</Emphasis>, is strongly correlated with stream flow

The placement and orientation of bird nests may strongly influence reproductive success. For many species, nest orientation is related to the cardinal directions and has to do with prevailing winds or solar exposure. Nest orientation differs among species in different environments, variously cooling or warming nests to maintain a nest microclimate optimal for successful development of eggs and young. The Spotted Barbtail (Premnoplex brunnescens) builds mossy, enclosed nests along shaded streams in Neotropical cloud forests. It shows a unique pattern of nest orientation, whereby nests are oriented in relation to the direction of water flowing below the nest entrance, rather than compass bearing. Nests face in one of three directions in relation to the stream: downstream, upstream, or perpendicular to the flow of the water. I discuss the ability of various hypotheses to explain this pattern in the context of factors affecting nest orientation in other species. While the reasons for this pattern of orientation remain unclear, orientation is likely driven by the need to create and maintain an optimal internal nest microclimate.     

Nest Predation Risk on Ground and Shrub Nests in Forest Margin Areas of Sulawesi, Indonesia

Forest loss and fragmentation in Indonesia may seriously affect the survivorship of forest birds and lead to local extinction of bird populations. We used 786 artificial nests baited with quail eggs to examine the effect of habitat alteration on nest predation in Lore Lindu National Park, Sulawesi. Natural forest and four habitats of forest margin areas: forest edge, forest gardens, coffee plantations, and secondary forest, were studied. Two types of artificial nests, ground and shrub nests were placed in these habitats at two different locations for a period of 8 days. In addition, we used automatic cameras and cage-traps to identify the predators. Nests in shrubs experienced significantly higher predation rates in forest margin areas than in natural forest. Predation on ground nests did not differ significantly between these habitat types, but was significantly higher than that on shrub nests in each habitat except forest edge. Rodents were the most common predators of both nests, but shrub nests were also susceptible to Dwarf cuscus (Strigocuscus celebensis), squirrels, and tree snakes. The nest predation rates we found were among the highest found in tropical rainforests, probably a consequence of the unique predator assemblages of Sulawesi. These results suggest that egg survival is negatively affected by human intervention and that human-induced habitats might have only limited importance for the conservation of Sulawesi's largely endemic understorey avifauna. These considerations might be important since forest margins comprise significant proportions of protected areas on Sulawesi and play an important role in future Park zoning concepts as well as in conservation-oriented land use management.     

Foraging activity of leaf‐cutting ants changes light availability and plant assemblage in Atlantic forest

1. Leaf‐cutting ants (LCAs) have often been denoted as ecosystem engineers because of their multifarious effects on the vegetation, particularly via nest‐driven environmental changes. However, the non‐trophic impacts of LCAs on forest dynamics via foliage harvesting across sizeable foraging zones (so‐called associated ecosystem engineering) are still poorly investigated. 2. Here, light availability and sapling assemblages were assessed within foraging areas and ant‐free control zones of 16 Atta cephalotes colonies located in a large remnant of Atlantic forest in northeastern Brazil. 3. Canopy openness and total light transmission were 1.4 and 1.6 times higher in foraging zones than in control areas. In parallel, sapling density and species richness decreased constantly from control to foraging zone plots. Additionally, shade‐tolerant species exhibited reduced abundance across foraging zones. A non‐metric multidimensional scaling ordination based on taxonomic similarity primarily segregated foraging zone and control plots; foraging zone plots converged to be more similar to each other as well. Finally, some plant species emerged as indicators of LCA‐free zones. 4. These results suggest that LCA foraging activity in the forest canopy directly increases the light availability and indirectly affects the recruitment and the structure of local plant assemblages. 5. Such a biologically significant effect on the light environment and its cascades confirms LCAs as potent ecosystem engineers, particularly as a plant assembly force, which operates beyond the spatial reach of their well‐described nest effects.     

Species turnover regulates leaf‐cutter ant densities in environmental gradients across the Brazilian Cerrado

Environmental changes resulting from Atta leaf‐cutter ant activities characterize them as ecosystem engineers of ecological and economic importance in multiple habitats. Although Atta effects depend strongly on colony density, there has been limited and inconsistent information available on their abundance and interspecific dominance in natural areas and across environmental gradients in the Brazilian Cerrado. Our study bridges this gap by testing the hypothesis that Atta nest density at genus level is high and relatively constant across locations regardless of vegetation type due to strong effects of dominant species turnover. To do so, we conducted a natural experiment surveying Atta nest abundance in five different vegetation types representing the environmental gradient from forest to savannah habitats found in the Cerrado. Our linear transect surveys covered 48.9 km in four well‐preserved areas across two Brazilian states in which we recorded 124 colonies. Of these, 84 nests belonged to A. laevigata and 40 to A. sexdens, the two dominant Atta species in the region. We also found no nests in 30% of the sampled area, which when combined with strong variation in density measured per transect indicates colonies tend to be aggregated in the landscape. Furthermore, we observed a strong dominant species turnover from forest habitats where 90% of nests belonged to A. sexdens to savannah habitats where 92% of nests were A. laevigata. Turnover effect was reflected in the absence of a significant difference in density between locations and vegetation types, indicating that density at genus level remains high and relatively constant regardless of nest aggregation. Our findings allow larger scale inferences about Atta effects and reveal new insights of their dynamics into natural areas that could affect plant species distribution and contribute to spatial heterogeneity of vegetation, having important implications for Cerrado conservation.     

Potential positive effect of the ant species Lasius niger on linyphiid spiders

Ants are highly abundant generalist predators and important ecosystem engineers which can strongly affect the composition of animal communities. We manipulated the density of the ant species Lasius niger with baits in a small‐scale field experiment to study the role of intraguild predation, top‐down control and bottom‐up effects of ants in a dry grassland surrounded by agricultural fields. Two different kinds of baits (honey and tuna) were presented near to the nests and at a distance of 2 m from six L. niger colonies in a dry grassland habitat, where L. niger was a highly abundant, omnipresent species. The experiments were performed for 1 month in spring. Additionally, the natural abundance of L. niger varying with the distance to their nests was used to study the effects on spiders and potential prey groups. The activity of L. niger was significantly higher at tuna baits compared with that at honey baits and empty control dishes. We found no effects of higher activity of L. niger on the arthropod community. However, there is evidence for a facilitation effect of ants on Collembola near to their colonies, probably due to habitat modification, which also influenced the density of Linyphiidae. Both groups had up to four times higher denisities next to L. niger colonies than at a distance of 2 m. Furthermore, δ¹³C values demonstrated that linyphiid spiders and L. niger predominantly feed on Collembola. We conclude that there is no evidence of top‐down effects of L. niger in a grassland in spring, but we found a facilitation of linyphiid spiders and their prey by the ants, which acted as ecosystem engineers.     

Seasonal spatial dynamics and causes of nest movement in colonies of the invasive Argentine ant (Linepithema humile)

Abstract.  1. Colony organisation and movement behaviour of the Argentine ant ( Linepithema humile ) was studied over 3 years in field populations in California and in captive colonies in the laboratory. This invasive species is highly polydomous and unicolonial; colonies consist of expansive and fluid networks of nests and trails. The spatial and temporal organisation of colonies may contribute to ecological dominance.
2. Argentine ant nests and inter-nest trails shift in size, abundance, and location, so that colony networks are spatially contracted in the winter and expanded spring to autumn. Colonies occupy permanent sites; ants migrated to and from the same winter nest locations year after year, and occupied 30% of the same nests repeatedly during seasonal migrations.
3. Nests were moved on average 2–3 m. Forty-two per cent were occupied less than 1 month, 4% the entire study, and the other 54% lasted 3.9 ± 2.3 months (mean ± SD).
4. Nests were located within 2–4 m of woody plants, in warm sites in the winter and cool sites in the summer. Both humidity and food availability influenced nest-site choice in laboratory colonies. However, when faced with a trade-off between factors, the ants chose humid nest boxes over nest boxes near food, and ants moved nests only in response to changes in humidity and not distance to food.
5. The results indicate that L. humile colonies are seasonally polydomous, and that nest movements are driven by changes in microclimate. Colony organisation maintains high local density and increases food supply, which may improve the competitive ability of L. humile colonies and reduce opportunities for species coexistence.     

Atta sexdens (L.) (Hymenoptera: Formicidae) demography in the Colombian Amazon: an evaluation of the palatable forage hypothesis

This study tested expectations of the palatable forage hypothesis for Atta sexdens (L.). Literature records on Atta and Acromyrmex nest density in the Neotropics were analyzed and A. sexdens demographical aspects were studied in six transects located along the Colombian Amazon River. Literature research results did not show that Atta and Acromyrmex nest densities are consistently higher in disturbed habitats. In transects, 56 active and 182 inactive nests were found in 2008. Higher active nest numbers and densities were determined for 1992, 1999, and 2006. Nests were present in old forest, in agricultural plot and grassland secondary forest, and absent from open spaces around houses, grasslands, agricultural plots, and a particular sector of old forest. Nest densities were higher in secondary forest. Between 1992 and 2008, the size of disturbed areas increased, whereas nest numbers and densities diminished. Average nest age was higher in secondary forest, but maximum nest age was comparable in the three habitats. On average, nests were superficially larger in old forest. Nests were mostly established in old forest, at sites without fallen trees. Almost all of those currently present in secondary forest were established in old forest before any disturbance occurred. In disturbed habitats, nest inactivity was slightly higher and possibly affected by human control efforts. This study suggested that the palatable forage hypothesis alone did not explain observed demographic patterns. Soil physiochemical variables or microclimatic conditions also are thought to play important roles in determining A. sexdens demography.     

Ants accelerate succession from mountain grassland towards spruce forest

Question: What is the role of mound‐building ants (Lasius flavus) in successional changes of a grassland ecosystem towards a spruce forest? Location: Slovenské Rudohorie Mountains, Slovakia; ca. 950 m a.s.l. near the Obrubovanec point (1020 m a.s.l.; 48°41′N, 19°39′E). Methods: Both chronosequence data along a successional gradient and temporal data from long‐term permanent plots were collected on ants, spruce establishment, and vegetation structure, together with additional data on spruce growth. Results: There are more spruce seedlings on ant mounds (4.72 m^?2) than in the surrounding vegetation (0.81 m^?2). Spruce seedlings grow faster on these mounds compared to surrounding areas. The first colonization wave of seedlings was rapid and probably occurred when grazing prevailed over mowing. Ant colony presence, mound volume, and plant species composition change along the successional gradient. Mounds become bigger when partly shaded but shrink in closed forest, when ant colonies disappear. Shade‐tolerant acidophylic species replace grassland plants both on the mounds and in surrounding areas. Conclusions: The massive occurrence of Lasius flavus anthills contributes to a runaway feedback process that accelerates succession towards forest. The effect of ants as ecosystem engineers is scale‐dependent: although they stabilize the system at the scale of an individual mound, they may destabilize the whole grassland system over a longer time scale if combined with changes in mowing regime.     

Local and landscape‐scale effects of an ant nest construction in an open dry forest of Uruguay

1. Ants modify soil properties via nest construction and by doing this modulate soil resources for other organisms. In this sense ants are recognised as ecosystem engineers. 2. In this framework, two less well‐studied issues are focused on: (i) the permanence of the effects of ant nests on plant communities after colonies have died, and (ii) the scaling up from patch to landscape‐scale effects. 3. The aim of the present study was to address these issues in the open dry forests of Uruguay, inhabited by the ant Atta vollenweideri Forel. The active and abandoned nests of this ant represent different and conspicuous patches (30–60 m²) in the landscape. 4. The soil concentration of sodium, a key element in the system, was substantially higher among active nests, and remains high during the early stages of abandoned nests. Woody species abundance, richness, and composition were affected at the patch scale, and simulation models suggested an increase in species richness at the landscape scale. 5. The present study highlights the importance of abandoned nests for plant‐species richness in the ecosystem engineer framework and the need to advance in an integrative approach to study both local and landscape effects of ant's nests.