We investigated the effects of recent fires on the native ant communities in two habitats of north-west Patagonia that differ in vegetation structural complexity. Using bait traps, we sampled ants in replicated scrub and steppe areas including paired burned and unburned sites. Fires significantly reduced plant cover and ant diversity only in scrub sites. The drop in diversity was due to (a) a reduction in the abundance of rare species associated with woody vegetation, and (b) an increase in the abundance of the dominant species, which thrive in more xeric microclimatic conditions. Consequently, ant assemblage structure of burned scrub approaches that of steppe sites. Our findings suggest that the effects of disturbances on ant assemblages depends both on habitat characteristics, which in turn determine the extent of the changes induced by the disturbance, and on the regional context of the ant fauna, which in turn determines the ability of the ants to deal with the post-disturbance conditions.

No abstract

1. In environments in which resources are distributed heterogeneously, patch choice and the length of time spent on a patch by foragers are subject to strong selective pressures. This is particularly true for parasitoids because their host foraging success translates directly into individual fitness. 2. The aim of this study was to test whether: (i) females of the parasitoid Ibalia leucospoides (Hymenoptera: Ibaliidae) can discriminate among patches according to host numbers; (ii) the surrounding context affects the initial choice of patch, as well as time spent on patch; and (iii) the perceived quality of a given patch is affected by the quality of the surrounding patches. 3. Each female was randomly exposed to one of three different three‐patch environments which differed in host number per patch, mean environment host number and host distribution among patches. For each treatment level, the first patch chosen and the time allocated to each patch visited by the female were recorded. 4. Females of I. leucospoides were able to discriminate different levels of host numbers among patches from a distance. The patch bearing the highest number of hosts was, predominantly, the first choice. Patch host number in association with mean habitat profitability influenced the length of time spent on the first patch visited. By contrast, variance in habitat profitability did not influence time allocation decisions. Contrary to the study prediction, there were no significant habitat‐dependent time allocation differences among patches holding the same number of hosts. 5. The results indicate that, for I. leucospoides, patch exploitation decisions are partially influenced by information obtained from the habitat as a whole, a behaviour that may prove to indicate adaptive ability in highly patchy environments, as well as suggesting the presence of good cognitive abilities in this parasitoid species.

Drylands worldwide are experiencing ecosystem state transitions: the expansion of some ecosystem types at the expense of others. Bees in drylands are particularly abundant and diverse, with potential for large compositional differences and seasonal turnover across ecotones. To better understand how future ecosystem state transitions may influence bees, we compared bee assemblages and their seasonality among sites at the Sevilleta National Wildlife Refuge (NM, USA) that represent three dryland ecosystem types (and two ecotones) of the southwestern U.S. (Plains grassland, Chihuahuan Desert grassland, and Chihuahuan Desert shrubland). Using passive traps, we caught bees during twoweek intervals from March-October, 2002-2014. The resulting dataset included 302 bee species and 56 genera. Bee abundance, composition, and diversity differed among ecosystems, indicating that future state transitions could alter bee assemblage composition in our system. We found strong seasonal bee species turnover, suggesting that bee phenological shifts may accompany state transitions. Common species drove the observed trends, and both specialist and generalist bee species were indicators of ecosystem types or months; these species could be sentinels of community-wide responses to future shifts. Our work suggests that predicting the consequences of global change for bee assemblages requires accounting for both within-year and among-ecosystem variation. Drylands worldwide are experiencing ecosystem state transitions: the expansion of some ecosystem types at the expense of others 1,2. These transitions include encroachment of C 3 shrubland into C 4 grassland 3 and conversion of woodland to savanna 4. It is through these transitions that the largest changes in dryland ecosystem processes are occurring 5-7. State transitions can produce dramatic changes in carbon fluxes 8,9 , nutrient dynamics 10,11 , spatial heterogeneity in vegetation 12 , and consumer community composition 13,14. Because drylands cover ~45% of land area on Earth 15 and support over 2 billion people 16 , understanding how much dryland ecosystems currently differ in community composition can help to predict changes in future communities-and the ecosystem services they provide-under state transitions. Bees may serve as important bio-indicators of state transitions and sentinels of altered ecosystem services 17,18. In drylands, bees are important pollinators of both wild plants and agricultural crops 19,20 , and are particularly abundant and diverse. North America's highest bee diversity occurs in the southwestern U.S. and northwest Mexico, and 75% of the continent's bee species are found in the western U.S. 21,22. Relative to mesic ecosystems, drylands can also host higher proportions of specialist bee species, which pollinate one or a few closely related plant species 23. For example, creosote bush (Larrea tridentata (DC.) Coville), a widespread and abundant shrub in North American warm deserts 24 , is visited by 22 documented specialist bee species 25. Cacti also host many...

17Drylands worldwide are experiencing ecosystem state transitions: the expansion of some 18 ecosystem types at the expense of others. Bees in drylands are particularly abundant and diverse, with 19 potential for large compositional differences and seasonal turnover across ecotones. To better 20 understand how future ecosystem state transitions may influence bees, we compared bee assemblages 21 and their seasonality among sites at the Sevilleta National Wildlife Refuge (NM, USA) that represent three 22 dryland ecosystem types (and two ecotones) of the southwestern U.S. (Plains grassland, Chihuahuan 23Desert grassland, and Chihuahuan Desert shrubland). Using passive traps, we caught bees during two- 24week intervals from March -October, 2002 -2014. The resulting dataset included 302 bee species and 25 56 genera. Bee abundance, composition, and diversity differed among ecosystems, indicating that future 26 state transitions could alter bee assemblage composition in our system. We found strong seasonal bee 27 species turnover, suggesting that bee phenological shifts may accompany state transitions. Common 28 species drove the observed trends, and both specialist and generalist bee species were indicators of 29 ecosystem types or months; these species could be sentinels of community-wide responses to future 30 shifts. Our work suggests that predicting the consequences of global change for bee assemblages 31 requires accounting for both within-year and among-ecosystem variation. 33Introduction 34 Drylands worldwide are experiencing ecosystem state transitions: the expansion of some 35 ecosystem types at the expense of others 1,2 . These transitions include encroachment of C3 shrubland into 36 C4 grassland 3 and conversion of woodland to savanna 4 . It is through these transitions that the largest 37 changes in dryland ecosystem processes are occurring [5][6][7] . State transitions can produce dramatic 38 changes in carbon fluxes 8,9 , nutrient dynamics 10,11 , spatial heterogeneity in vegetation 12 , and consumer 39 community composition 13,14 . Because drylands cover ~45% of land area on Earth 15 and support over 2 40 billion people 16 , understanding how much dryland ecosystems currently differ in community composition 41 can help to predict changes in future communities ¾ and the ecosystem services they provide ¾ under 42 state transitions. 43 3 Bees may serve as important bio-indicators of state transitions and sentinels of altered 44 ecosystem services 17,18 . In drylands, bees are important pollinators of both wild plants and agricultural 45 crops 19,20 , and are particularly abundant and diverse. North America's highest bee diversity occurs in the 46 southwestern U.S. and northwest Mexico, and 75% of the continent's bee species are found in the 47 western U.S. 21,22 . Relative to mesic ecosystems, drylands can also host higher proportions of specialist 48 bee species, which pollinate one or a few closely related plant species 23 . For example, creosote bush 49 (Larrea tridentata (DC.) Coville), a widespread and ...