Aridity and chronic anthropogenic disturbance as organizing forces of fruit-feeding butterfly assemblages in a Caatinga dry forest

Anthropogenic disturbances and climate change are expected to reorganize biodiversity on multiple ecological levels from populations to ecosystems, especially in arid and semiarid regions due to environmental filtering imposed by water stress. This paper examines the individual and combined effects of chronic anthropogenic disturbance and increased aridity on the structure of fruit-feeding butterfly assemblages in a human-modified landscape of Caatinga dry forest, in the northeast of Brazil. Butterflies were recorded monthly across old-growth forest stands and their assemblages were described in terms of taxonomic and functional community-level attributes confronted to different levels of chronic disturbance and aridity. Butterfly assemblages were species-poor but had high species replacement (turnover) along both the chronic disturbance and aridity gradients. We observed a negative effect of aridity on alpha and beta diversity of butterfly assemblages. Butterfly assemblages across forest stands exposed to high levels of chronic disturbance and aridity had a nested structure. Functional diversity (Rao's Q) and the community-weighted means (CWM) of ocellus-bearing species and monocot-feeding larvae were negatively and positively affected by increased aridity and chronic disturbance, respectively. Our findings suggest that aridity and its combination with chronic disturbance have a drastic effect on the structure of butterfly assemblages in the Caatinga dry forest. These findings highlight that rainfall and chronic disturbances as major drivers of biological reorganization in human-modified landscapes. As aridity increases, Caatinga tends to support taxonomically and functionally impoverished and highly distorted assemblages. Abstract in Portuguese is available with online material     

Foraging in highly dynamic environments: leaf‐cutting ants adjust foraging trail networks to pioneer plant availability

Major shifts in the availability of palatable plant resources are of key relevance to the ecology of leaf‐cutting ants in human‐modified landscapes. However, our knowledge is still limited regarding the ability of these ants to adjust their foraging strategy to dynamic environments. Here, we examine a set of forest stand attributes acting as modulating forces for the spatiotemporal architecture of foraging trail networks developed by Atta cephalotes L. (Hymenoptera: Formicidae: Attini). During a 12‐month period, we mapped the foraging systems of 12 colonies located in Atlantic forest patches with differing size, regeneration age, and abundance of pioneer plants, and examined the variation in five trail system attributes (number of trails, branching points, leaf sources, linear foraging distance, and trail complexity) in response to these patch‐related variables. Both the month‐to‐month differences (depicted in annual trail maps) and the steadily accumulating number of trails, trail‐branching points, leaf sources, and linear foraging distance illustrated the dynamic nature of spatial foraging and trail complexity. Most measures of trail architecture correlated positively with the number of pioneer trees across the secondary forest patches, but no effects from patch age and size were observed (except for number of leaf sources). Trail system complexity (measured as fractal dimension; Df index) varied from 1.114 to 1.277 along the 12 months through which ant foraging was monitored, with a marginal trend to increase with the abundance of pioneer stems. Our results suggest that some leaf‐cutting ant species are able to generate highly flexible trail networks (via fine‐tuned adjustment of foraging patterns), allowing them to profit from the continuous emergence/recruitment of palatable resources.     

Leaf-cutting ants negatively impact the regeneration of the Caatinga dry forest across abandoned pastures

The mechanisms affecting forest regeneration in human-modified landscapes are attracting increasing attention as tropical forests have been recognized as key habitats for biodiversity conservation, provision of ecosystem services, and human well-being. Here we investigate the effect of the leaf-cutting ants (LCA) Atta opaciceps on regenerating plant assemblages in Caatinga dry forest. Our study encompassed 15 Atta opaciceps colonies located in landscape patches with a gradient of forest cover from 8.7% to 87.8%, where we monitored regenerating individuals (seedlings and saplings of woody and herbaceous plants) in different habitats (nests, foraging areas, and control areas) over one year. We recorded 2,977 regenerating plant individuals, distributed among 55 species from 23 families. Herbaceous plants represented 82.1% and 58.2% of the total number of individuals and species, respectively. Species richness of both the whole and herbaceous plant assemblages increased along the forest cover gradient, but without difference between the habitats. Total plant abundance was highest in control areas followed by foraging areas and nests and this pattern held for both woody and herbaceous plants. Although forest cover did not influence the abundance of herbaceous plants and the whole plant assemblage, it positively affects woody plant abundance across control areas. Forest cover and habitat changed species composition of both the entire regenerating and the herbaceous assemblages. These results together indicate that LCA negatively impact regenerating plant assemblages, particularly in those sites with increased forest cover. As LCA proliferate in human-modified landscapes, they may prevent plant regeneration of disturbed areas.     

In vivo protein cyclization promoted by a circularly permuted Synechocystis sp. PCC6803 DnaB mini-intein.

A synthetic Synechocystis sp. PCC6803 DnaB split mini-intein gene was constructed for the in vivo cyclization of recombinant proteins expressed in Escherichia coli. The system was used to cyclize the NH(2)-terminal domain of E. coli DnaB, the structure of which had been determined previously by NMR spectroscopy. Cyclization was found to proceed efficiently, with little accumulation of precursor, and the product was purified in high yield. The solution structure of cyclic DnaB-N is not significantly different from that of linear DnaB-N and it unfolds reversibly at temperatures approximately 14 degrees C higher. Improved hydrogen bonding was observed in the first and last helices, and the length of the last helix was increased, while the 9-amino acid linker used to join the NH(2) and COOH termini was found to be highly mobile. The measured thermodynamic stabilization of the structure (Delta Delta G approximately 2 kcal/mol) agrees well with the value estimated from the reduced conformational entropy in the unfolded form. Simple polymer theory can be used to predict likely free energy changes resulting from protein cyclization and how the stabilization depends on the size of the protein and the length of the linker used to connect the termini.     

Drought stress drives intraspecific choice of food plants by Atta leaf‐cutting ants

Leaf‐cutting ants (LCA) are polyphagous and dominant herbivores throughout the Neotropics that carefully select plant individuals or plant parts to feed their symbiotic fungus. Although many species‐specific leaf traits have been identified as criteria for the choice of food plants, the factors driving intraspecific herbivory patterns in LCA are less well studied. Herein, we evaluate whether or not drought‐stressed native plants are a preferred food source using free‐living colonies of two leaf‐cutting ants, Atta sexdens L. (Hymenoptera: Formicidae: Attini), in combination with five plant species, Ocotea glomerata Nees (Lauraceae), Lecythis lurida S. A. Mori (Lecythidaceae), Miconia prasina DC (Melastomataceae), Tovomita brevistaminea Engl. (Clusiaceae), and Tapirira guianensis Aubl. (Anacardiaceae), and Atta cephalotes L., in combination with two plant species, O. glomerata and Licania tomentosa Benth. (Chrysobalanaceae). In dual‐choice bioassays, ants removed about three times more leaf area from drought‐stressed plants compared to control plants. Both leaf‐cutting ant species consistently preferred drought‐stressed plants for all species tested, except T. guianensis. The mean acceptability index – expressing the preference for one of two options on a scale of 0 to 1 – of drought‐stressed plants ranged from 0.65 to 0.86 across plant species, and the preference did not differ significantly among the tested plant species. Our results suggest that selection of drought‐stressed individuals is a general feature of food plant choice by leaf‐cutting ants irrespective of ant or plant species. As human‐modified forest assemblages across the Neotropics are increasingly prone to drought stress, the documented preference of Atta for drought‐stressed plants may have tangible ecological implications.     

Disturbance Winners or Losers? Plants Bearing Extrafloral Nectaries in Brazilian Caatinga

Plants bearing extrafloral nectaries (EFNs) often have traits typical of pioneer species, and may be expected to proliferate in disturbed habitats. However, a negative effect of disturbance on visitation by attendant ants could prevent EFN‐bearing plants from acting as disturbance winners. Here, we address the effects of chronic anthropogenic disturbance on the abundance of EFN‐bearing plants and their interactions with attendant ants in Caatinga vegetation of northeastern Brazil. We recorded the abundance of EFN‐bearing plants, proportion of plants visited by ants and composition of ant attendant species at 24 sites varying in levels of disturbance. EFN‐bearing plants as a whole did not behave as a disturbance winner group. The responses of the 13 species to increasing disturbance were highly variable, with three species declining in abundance (loser species). The richness of ant species attending EFNs did not vary with disturbance, but species composition did. The overall proportion of EFN‐bearing plants attended by ants per 5‐min period was not affected by disturbance. However, for the three loser species, attendance decreased from about 50 percent with low and moderate disturbance to half that with very high disturbance. We hypothesize that disturbed sites are more stressful for loser species compared with other EFN‐bearing plants, with physiological stress resulting in lower production of EFN secretions and reduced attraction of ants. This would make such species double losers, with physiological stress at disturbed sites not only directly influencing their performance but also indirectly affecting it through the disruption of a key mutualism.     

Out-and-back 13C–13C scalar transfers in protein resonance assignment by proton-detected solid-state NMR under ultra-fast MAS

We present here ¹H-detected triple-resonance H/N/C experiments that incorporate CO–CA and CA–CB out-and-back scalar-transfer blocks optimized for robust resonance assignment in biosolids under ultra-fast magic-angle spinning (MAS). The first experiment, (H)(CO)CA(CO)NH, yields ¹H-detected inter-residue correlations, in which we record the chemical shifts of the CA spins in the first indirect dimension while during the scalar-transfer delays the coherences are present only on the longer-lived CO spins. The second experiment, (H)(CA)CB(CA)NH, correlates the side-chain CB chemical shifts with the NH of the same residue. These high sensitivity experiments are demonstrated on both fully-protonated and 100 %-H^N back-protonated perdeuterated microcrystalline samples of Acinetobacter phage 205 (AP205) capsids at 60 kHz MAS.     

Euphorbiaceae responses to chronic anthropogenic disturbances in Caatinga vegetation: from species proliferation to biotic homogenization

Chronic anthropogenic disturbances (CAD) have posed tangible threats to biodiversity-relevant tropical biotas, but community- and ecosystem-level impacts still remain poorly understood. Here we address a 152 km² Caatinga landscape in northeast Brazil in order to investigate how Euphorbiaceae species and these seasonally dry tropical plant assemblages respond to a CAD gradient. Woody plant species were recorded across 26 0.06 ha spatially independent plots exposed to CAD. Euphorbiaceae species accounted for 78.9% of all plants and 21.5% of all species, with some species reaching up to 283 individuals per 0.06 ha or 92% of all recorded plants. Despite such contributions, Euphorbiaceae total and relative abundance, as well as total and relative richness, did not correlate with disturbance intensity at plot scale. At species level, some Euphorbiaceae species responded positively to disturbance, while others declined or did not exhibit a consistence response (i.e., positive, negative, and neutral responses). CAD intensity affected patterns of community similarity considering the whole plant assemblage as follow: first, plot-level disturbance correlated positively with NMDS scores; second, C. sonderianus experienced a 100% increment in terms of relative abundance along the disturbance gradient, and its plot-level abundance correlated positivity with NMDS scores; and finally, cross-plot similarity correlated positively with cross-plot geographical distance. Among others, our results suggest that CAD may favor some particular species (i.e., proliferating taxa), leading to community-level changes, including biotic homogenization as disturbance increases.