Paula Sackmann

Effect of fire on ground beetles and ant assemblages along an environmental gradient in NW Patagonia: Does habitat type matter?

ABSTRACT The response of beetle and ant assemblages to fire (2–5 y old) and the dependence of that response on habitat type were analyzed. Using pitfall traps, beetles (Coleoptera) and ants (Formicidae) were sampled in replicated forest, scrub, and steppe areas including paired unburnt and burnt plots. A total of 176 species of beetles (8245 individuals) and 22 species of ants (115,056 individuals) were captured. Most beetle families (65%) were captured in the forest, while most ant species were most frequently captured in the steppe (45%). Beetle abundance was the same in unburnt and burnt plots in the three habitats, but ant abundance was higher in burnt than in unburnt plots. Fire usually decreased species richness of both taxa, but the strength of this effect depended on the habitat type. Coleoptera richness was lower in burnt than in unburnt plots in the forest and steppe but was the same in unburnt and burnt scrub plots. Ant richness was lower in burnt forest and scrub plots and was similar between burnt and unburnt steppe plots. For both taxa, species composition of the forest assemblages was different between unburnt and burnt forest plots (difference almost significant for ants) but not between unburnt and burnt steppe plots. Beetle species composition in the scrub was different between unburnt and burnt plots, but ant species composition was the same. Our results support the idea that the consequences of fire on native beetle and ant assemblages depend on the habitat type but also on the taxonomic group under analysis. Beetles and ants showed differences in their habitat preference and in the way that they use the habitats (e.g., preference for vegetated or bare soil patches) along the environmental gradient. These differences explain why beetles were in general more severely affected than ants, and why fire differentially affected both taxa in the scrub (habitat of intermediate complexity). A good knowledge of the habitat utilization by different groups at the local scale and a regional perspective (e.g., habitat preference along an environmental gradient) are necessary to fully understand the effect of disturbances on native arthropod assemblages.

Beetle abundance–environment relationships at the Subantarctic–Patagonian transition zone

Abstract. 1 The spatial variation in the abundance of individuals may be associated with the variation in environmental variables. The productivity hypothesis proposes that climate affects plant productivity, which may limit the abundance of beetles. The thermal‐limitation hypothesis proposes the direct effect of ambient temperature may limit beetle abundance. We analysed the abundance of epigaeic beetles at the Subantarctic–Patagonian transition to test for these abundance–environment relationships. 2 We collected beetles using 450 pitfall traps within a ~150 × 150 km area representative of the Subantarctic–Patagonian transition. We used path analysis to evaluate the relationships between beetle abundance and plant cover, litter biomass, averaged minimum and daily temperature range, and mean annual precipitation. We used principal coordinates of neighbour matrices to model the spatial autocorrelation of the data. 3 The abundance of beetles increased strongly with tree canopy cover and less strongly with herb cover. The increase in shrub cover had a positive effect on beetle abundance in areas to the east of the transition, within the scrubland‐steppe, but it has a negative effect on beetle abundance in areas to the west of the transition, within the forests. The association between beetle abundance and minimum daily temperature was negative or weak throughout. Increased temperature variation had a negative effect on beetle abundance. 4 We suggest that indirect positive climatic effects mediated through plant cover are important to account for the variation in beetle abundance, which favours the productivity hypothesis. Thermal limitation may operate locally through variation in daily temperature range.

Competition for food between the exotic wasp Vespula germanica and the native ant assemblage of NW Patagonia: evidence of biotic resistance?

The success of a biological invasion may depend on the interactions between the invader and the native biota. However, little experimental evidence demonstrates whether local species can successfully compete with exotics. We experimentally determined the existence of competition for food between the exotic wasp Vespula germanica, one of the most recent Patagonian invaders, and the native ant assemblage. Both wasps and ants are generalist predators and scavengers, sharing habitat and food resources. We selected 30 sites within scrubland habitats where both ants and wasps were present. At each site, we placed containers with protein baits under three treatments: wasp exclusion, ant exclusion, and control (i.e., free access for wasps and ants). Ant exclusion increased the number of wasps (with regard to a control), but wasp exclusion did not affect ant abundance. This result suggests that native ants affect the foraging activity of exotic wasps but not vice versa. Aggressive behaviors and worker aggregation may explain the competitive advantage of ants. Ants bite wasp legs and massively aggregate on food sources, physically limiting the landing of wasps on baits. If the outcome of interactions at baits reported here influence wasp population-level parameters, this competitive interaction could be one of the factors explaining the low abundance of this exotic wasp in NW Patagonia in comparison with other invaded regions.

Environmental determinants of the distribution and abundance of the ants, Lasiophanes picinus and L. valdiviensis, in Argentina

Abstract The distribution and abundance variation of the terrestrial ants, Lasiophanes picinus and Lasiophanes valdiviensis Emery (Formicinae: Lasiini), which are endemic in Patagonia (Argentina and Chile), are described and a set of environmental factors are examined to explain the observed patterns. Ants were collected using 450 pitfall traps arranged in 50, 100 m2 grid plots each with nine traps within a roughly 150 × 150 km area representative of the subantartic-patagonian transition of Argentina. Five sampling periods each 8-days long were carried out between November 2004 and March 2006. To understand the distributional patterns and their link to environmental variables discriminant analysis was used. Path analysis was performed to test for direct and indirect effects of a set of environmental variables on species abundance variation. L. picinus was more frequently captured and attained higher abundance in the forests, while L. valdiviensis was more frequently captured and more abundant in the scrubs. The maximum daily temperature and mean annual precipitation explained L. picinus distribution (i.e. presence or absence) with an accuracy of 90%. L. valdiviensis distribution was predicted with almost 70% accuracy, taking into account herbal richness. The maximum daily temperature was the only climatic variable that affected ant abundance directly; an increase in temperature led to an increase of L. picinus abundance and a decrease of L. valdiviensis abundance. The amount of resources, as indicated by the percent plant cover, explained the variation of the abundance of both species better than the variety of resources as indicated by plant richness (i.e. models including plant richness had low fit or no fit at all). A direct effect of habitat use by cattle was found, as indicated by the amount of feces in the plots, only when variables related to the amount of resources were replaced by variables with less explanatory power related to the variety of resources. This study provides new data on the ecology of Lasiophanes species in relation to existing hypotheses proposed to explain patterns of abundance variation. Evidence is provided that changes in temperature (i.e. global climate change) may have important consequences on populations of these species.

The impact of an exotic social wasp (Vespula germanica) on the native arthropod community of north-west Patagonia, Argentina: an experimental study

Abstract 1. Biological invasions are usually thought to have a negative impact on native communities. However, data supporting this idea are often based on comparative studies between invaded and non‐invaded areas, and are spatially and temporally limited.

Richness-environment relationships in epigaeic ants across the Subantarctic-Patagonian transition zone

Abstract.  1. We analysed ant species richness‐environment relationships across the Subantartic‐Patagonian transition, in southern South America. We tested the predictions of the (i) thermal limitation hypothesis: temperature limits ant species richness, (ii) the productivity hypothesis: ant richness is driven by the indirect effect of climate (temperature and precipitation) mediated by changes in plant environment (plant cover and litter accumulation). We also evaluated the effects of (iii) plant species richness, and (iv) habitat use by cattle on richness.

Effects of the bittering agent denatonium benzoate on the success of toxic baiting of pestiferous German wasps ( Vespula germanica )

Our aim was to study the effects of the addition of varying concentrations of the bittering agent, denatonium benzoate (DB), to toxic baits used for the control of German wasps (Vespula germanica). We tested the level of detection of different concentrations of DB by wasps, and measured the overall effectiveness of freeze-dried baits containing 0.1% fipronil and DB to control local wasp activity. The addition of DB concentrations higher than 10 ppm to fresh beef baits made them less attractive to wasps compared with controls. Lyophilized toxic baits containing DB 10 ppm and fipronil 0.1% were as attractive as control baits and reduced local wasp abundance by, on average, 74%. The results for lyophilized baits containing 0.1% fipronil but lacking DB were similar (average reduction: 80%). The addition of DB to toxic baits against Vespula spp. can bring about undesirable effects on toxic baiting efficacy. In the case of V. germanica, we suggest using the lowest concentration possible (i.e. 10 ppm or lower) that may still elicit an aversive response in humans and/or non-target species.