Do abandoned nests of leaf-cutting ants enhance plantrecruitment in the Atlantic Forest?aec_2141 220..232

ANA GABRIELA D. BIEBER,1,2 MARCONDES A. OLIVEIRA,1 RAINER WIRTH,3

MARCELO TABARELLI4 AND INARA R. LEAL4*1Programa de Pós-Graduação em BiologiaVegetal, Departamento de Botânica, Universidade Federal dePernambuco, Av. Prof Moraes Rego, s/n, Cidade Universitária, Recife, PE, Brazil; 2Present address:Programa de Pós-Graduação em Ecologia, Instituto de Biologia, Universidade Estadual de Campinas,Campinas, Brazil; 3Plant Ecology and Systematics, University of Kaiserslautern, Kaiserslautern,Germany; and 4Departamento de Botânica, Universidade Federal de Pernambuco, Av. Prof MoraesRego, s/n, Cidade Universitária, 50670-901, Recife, PE, Brazil (Email: irleal@ufpe.br)

Abstract The role played by abandoned nests of leaf-cutting ants (Atta spp.) as a small-scale disturbance regimethat affects plant recruitment, species coexistence and forest regeneration remains poorly investigated. Here weexamine whether abandoned nests of Atta cephalotes serve as regeneration niches and operate as particular plantrecruitment habitats, favouring forest regeneration after ant activities cease and leading to the establishment oftaxonomically/ecologically distinct plant assemblages. Soil properties, canopy openness, light availability andregenerating plant assemblages were evaluated across 18 nests and adjacent control plots in a large remnant ofAtlantic Forest in north-east Brazil from December 2004 to December 2005. Surprisingly, nests and control plotsexhibited very similar light environments irrespective of nest age, but nest soils exhibited substantial reductions incarbon content (1.45 � 0.24 vs. 1.79 � 0.13%) and organic matter (2.50 � 0.41 vs. 3.08 � 0.23%), and provedto be much more resistant to penetration (30.57 � 6.08 vs. 39.48 � 7.53 mm). Functional signature of regener-ating plant assemblages exhibited little variation across both habitat types, as they were dominated by pioneer,small-seeded and vertebrate-dispersed species. However, abandoned nests exhibited less dense, impoverished andmore homogeneous regenerating plant assemblages at local and landscape scale; they clearly lacked nest-dependentplant species and represented floristic subsets of the flora inhabiting the undisturbed forest. This recruitmentbottleneck was transient in the long term because nest-related effects ameliorated in older nests. Our results suggestthat, unlike treefall gaps, abandoned nests represent temporary (relatively long-lasting) islands of unsuitablesubstrate that reduce plant recruitment, retard forest regeneration, and fail in providing a special regeneration nicheable to promote species coexistence and plant diversity.

Key words: Atta cephalotes, bottom–up gaps, disturbance, forest regeneration, regeneration niche.

INTRODUCTION

Natural disturbances have been proposed to shape thestructure of plant assemblages across tropical forests,including species diversity (Petraitis et al. 1989;Huston 1994). A prominent theoretical explanationfor the positive effects of disturbance on species diver-sity is the intermediate disturbance hypothesis, whichstates that moderate levels of disturbance mightincrease species diversity by facilitating the coexistencevia niche partitioning of both colonizing and competi-tively dominant species (Connell 1978; Molino &Sabatier 2001). In tropical forests, treefall gaps com-prise the predominant (and most studied) natural dis-turbance regime, covering up to 15% of the entire

forest landscape (Richards 1996). Briefly, canopy gapspermit increased levels of sun light to reach the forestunderstorey and thus create suitable habitat forrecruitment of light-demanding plant species(Denslow 1980; Brokaw 1982). Such gap-inducedrecruitment of light-demanding species, particularly ofshort-lived pioneers, has been advocated to represent astrong force permitting species coexistence at the land-scape scale, shaping tree assemblage structure anddriving forest regeneration dynamics (Denslow 1980;Hartshorn 1980; Brokaw 1982). In addition to treefallgaps, tropical forests also experience moderate levels ofunderstory or bottom–up gaps formed by the activityof animals such as agoutis, armadillos, wild pigs, ter-mites and ants (Clark 1990; Farji-Brener & Illes 2000;Wirth et al. 2003). Although very common acrosstropical forests, the opportunities created by animal-mediated gaps in terms of plant niche differentiation

*Corresponding author.Accepted for publication March 2010.

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and partitioning remain poorly explored (but see Farji-Brener & Illes 2000; Wirth et al. 2003; Farji-Brener2005).

Leaf-cutting ants (Atta and Acromyrmex spp., For-micidae, Hymenoptera) are among the most polypha-gous and voracious herbivorous insects in neotropicalforests, harvesting up to 15% of the standing leaf crop(Wirth et al. 2003; Urbas et al. 2007) and up to 50% ofthe species of a given forest flora (Vasconcelos 1990;Wirth et al. 2003) in their colony’s territory each year.Moreover, they act as dominant ecosystem engineers(Meyer 2008) owing to the construction and mainte-nance of their nests, which can reach 250 m2 or morein surface area (Cherrett 1989). These prodigiousstructures: (i) enhance nutrient availability through theaccumulation of nutrient-rich leaf material used tofeed fungus gardens in underground chambers (Farji-Brener & Silva 1995; Moutinho et al. 2003; Sternberget al. 2007) or above-ground waste piles (Hudson et al.2009); (ii) substantially improve soil penetrability rela-tive to non-nest soils (Moutinho et al. 2003); (iii)promote active removal of leaf litter, resulting in bareground on and in the vicinity of nests (Weber 1972;Meyer 2008); and (iv) create ‘nest clearings’ or veg-etation gaps above and immediately around Atta nestsites, where understorey vegetation growing on oroverhanging the immediate nest surface is constantlycleared leading to an increased light availability (Farji-Brener & Illes 2000; Hull-Sanders & Howard 2003;Corrêa et al. 2010).

Abandoned leaf-cutting ant nests are therefore likelyto provide enhanced soil and light microsites for theestablishment and recruitment of seedlings for thosespecies with a narrow or particular regeneration niche,such as light demanding species (Farji-Brener & Illes2000). Hence the spatial dynamics of ant colony aban-donment may impact not only patterns of plant regen-eration and forest structure, but also the provision ofdistinct regeneration niches, therefore resembling therole played by treefall gaps (Fowler et al. 1989; Per-fecto & Vandermeer 1993; Farji-Brener & Illes 2000;Wirth et al. 2003; Corrêa et al. 2010). Precisely, nestsabandoned by Atta have been proposed to favour therecruitment of pioneer, nutrient-limited, small-seededlitter-gap and/or abiotically dispersed plant speciesleading to the establishment of particular plant assem-blages in terms of both taxonomic and ecological/functional composition (Farji-Brener & Illes 2000;Farji-Brener 2005). In some non-forested ecosystemssuch as savannas, cerrados and steppes, previous inves-tigations have documented increased abundance andspecies richness on and around Atta and Acromyrmexnests, either while the colony is still active or followingnest abandonment (Farji-Brener & Silva 1995, 1996;Farji-Brener & Ghermandi 2000). Despite a consider-able literature on the role played by leaf-cutting ants interms of environmental shifts and plant recruitment

(see Hull-Sanders & Howard 2003; Corrêa et al.2010), studies addressing the long-term impacts ofabandoned Atta nests on plant regeneration andspecies diversity of tropical forests are still scarce.

Here we examine whether abandoned nests ofAtta cephalotes operate as particular plant recruitmenthabitats or distinct regenerating niches, favouringspecies coexistence and forest regeneration after antactivities cease and leading to the establishment oftaxonomically/ecologically distinct plant assemblages.We first test the idea that soil properties (nutrients andphysical factors) and light environment differ on aban-doned leaf-cutting ant nests compared with adjacentundisturbed forests as suggested by Farji-Brener andIlles (2000). Furthermore, regenerating plant assem-blages on abandoned nests and undisturbed forest arecompared in terms of plant abundance, species rich-ness and ecological composition (i.e. seed size, regen-eration and seed dispersal strategy). We also examinepatterns of plant species similarity and distribution,and identify indicator species across both habitats.Finally, we address some of the potential forcesshaping the physical attributes of abandoned nests andtheir potential as an exclusive or particular regenerat-ing niche that promotes species coexistence and plantdiversity.

METHODS

The study was carried out at Usina Serra Grande, a 667-km2

private sugar cane landholding in the state of Alagoas, north-eastern Brazil (8°30′S, 35°52′W). Serra Grande is situatedon a low-altitude plateau (300–400 m a.s.l.) covered by twosimilar classes of dystrophic and clay-laden soils: yellow-redlatosol and yellow-red podzol according to the Braziliansystem of soil classification (IBGE 1985). Annual rainfall isapproximately 2000 mm, with a 3-month dry season(<60 mm per month) from November to January; and thenatural vegetation is a lowland rain forest (sensu Whitmore1998). Regionally, Leguminosae, Lauraceae, Sapotaceae,Chrysobalanaceae, Euphorbiaceae and Lecythidaceae arethe most speciose families (�10 cm d.b.h.; Grillo et al.2006). This site still retains approximately 9000 ha of forestcover assigned to a unique biogeographic zone of the AtlanticForest biota – the Pernambuco Center of Endemism (Santoset al. 2007). Sugar cane cultivation in this landscape, whichdates back to the 19th century or earlier, provided the stron-gest incentive for clearing large tracts of pristine old-growthforests (Pôrto et al. 2006). Currently, this hyper-fragmentedlandscape retains 9.2% of forest cover, including theCoimbra Forest – the largest (3500 ha) and best preservedpatch of Atlantic Forest in north-eastern Brazil (Grillo et al.2006). The Coimbra Forest is embedded into a stable, rela-tively old (at least 60 years) and homogeneous matrix ofsugar cane fields, and its pioneer-dominated edge zone witha 20–25 m tall upper canopy layer (Oliveira et al. 2004) hasbeen shown to support impoverished plant assemblages, par-ticularly in terms of shade-tolerant and large-seeded trees(Melo et al. 2006).

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Atta cephalotes and abandoned nests

Atta cephalotes is a leaf-cutting ant of neotropical forests, witha continuous distribution from Mexico to Bolivia (Kempf1972); and the recently published record of the species in theCoimbra Forest marks its most southern occurrence in theAtlantic Forest region (Corrêa et al. 2005). Atta cephaloteshas been found to achieve increased densities (up to sixcolonies per ha) across the edges of Coimbra (Wirth et al.2007; Meyer et al. 2009), and in this forest patch it buildsconspicuous and compact nests (covering up to 260 m2),with nest clearings that frequently extend to the uppercanopy layers (i.e. almost vegetation-free zones), thus per-mitting increased light to reach the forest ground (Meyer2008; Corrêa et al. 2010). When colonies die, their largesingle nest mounds remain visible for several years, therebyfacilitating their localization as spots of forest regeneration;along the forest edge zone of Coimbra we have recordedapproximately 1.7 abandoned nests per hectare (Bieber2006), but the density is even higher in patches of advancedforest regeneration (Silva et al. 2009).

Sample design of abandoned nests

We tested our predictions by randomly selecting 18 aban-doned A. cephalotes nests along the edge zone of CoimbraForest, all of them consisting of a large single mound (24.14 � 12.49 m2 in size; mean � SD; 10.3–43.8 m2) charac-teristic for A. cephalotes nests (Urbas et al. 2007). Nests werefurther classified into two mutually excluding and arbitrarycategories: recently abandoned nests (less than 5 years ofinactivity, n = 8) or decayed/long-abandoned nests (10–15 years of inactivity, n = 10). Nest age was determined bypersonal observation (especially for recently abandoned nestssince we have monitored A. cephalotes colonies at CoimbraForest since 2001) and comprehensive interviews with long-term residents and company employees who had long been incharge of forest protection at this site. As control sites weadopted equal-sized forest plots in the nest vicinity – eachlocated 5–10 m away from the border of the nest mounds,avoiding areas with apparent disturbances, such as canopygaps. This paired approach (i.e. nest and control plot withequal size) allowed us to set each nest and its respectivecontrol in the same macroenvironment, therefore reducinginfluences not related to the nest itself.

Physical environment of abandoned nests

Forest structure and light transmission in abandoned nests(n = 18) and respective control plots (n = 18) were charac-terized via hemispherical photographs taken in the centre ofeach plot at 0.7 m above-ground level using a Nikon Coolpix4500 camera with a LC-ER1 fisheye converter, as alsoadopted by Corrêa et al. (2010) in the same study site. Foravoiding direct solar radiation, pictures were taken only atdusk after sunset. Before each photograph, north/south ori-entation was identified using a pointer that protruded 5 mmabove the front edge of the camera lens. Camera configura-tion, based on pertinent literature information, was as

follows: FISHEYE1 black and white mode, FINE imagequality, aperture (f) = 2.5, automatic shutter speed, underex-posure (�) = -0.7 (Frazer et al. 1999).The analyses of hemi-spherical photographs were performed with the software GapLight Analyzer (version 2.0, Frazer et al. 1999). Photographswere analysed twice to minimize subjectivity in thresholdselection for the distinction between visible sky and foliage.The descriptors of canopy structure and light regime derivedfrom image analysis included canopy openness (the percent-age of open sky seen from beneath a forest canopy) and totallight transmission (the fraction of radiation reaching theground relative to that outside the canopy) (Frazer et al.1999). Considering that the spatial heterogeneity in lighttransmission in and around canopy gaps is mostly due tovariation in the amount of direct-beam radiation (Wirth et al.2003), we also estimated direct light transmission (theamount of direct solar radiation transmitted by the canopy)and diffuse light transmission (the amount of diffuse solarradiation transmitted by the canopy).

To characterize soil nutrient content in both nest andcontrol plots, soil samples were taken at five locations perplot, at each of the four plot corners and one at the plotcentre.The litter layer was excluded and samples were takenat the superficial soil layers (<10 cm), where seedling radicleswere frequently located. The five samples were storedtogether in a single plastic bag, air-dried and homogenizedfor the analysis, resulting in one soil sample per plot. Thefollowing soil attributes were analysed: macronutrients (C,H, P, K, Ca and Mg) and micronutrients (Zn, Cu, Fe, Mn,Na and Al), pH and organic matter; nitrogen was not exam-ined because of operational limitations and the fact thatoxisols (such as those found in our study site) are likely Prather than N limited (Vitousek & Sanford 1986). Soil analy-ses were conducted by a commercial lab (LAGRI – Labo-ratório de Análise Agrícolas Ltda, Recife, Pernambuco)following the standard measurement techniques definedby the Empresa Brazileira de Pesquisas Agropecuárias(EMBRAPA 1997). In addition to nutrients, soil resistanceto penetrability was estimated at the same five spots per plotused to collect soil samples. A sharpened steel rod (1 m longand 3 mm diameter) was released from a height of 1.6 m andthe penetration depth of the rod into the soil was recorded(modified from Passos & Oliveira 2004).

Regenerating plant assemblages inabandoned nests

All non-adult woody plant species inhabiting nests andcontrol plots were recorded and assigned into mutuallyexclusive stages of plant development: (i) seedlings – indi-viduals �50 cm tall; (ii) saplings, >50 cm tall and <1.5 cmd.b.h.; and (iii) poles, individuals between 1.5–10 cm d.b.h.(modified from Chapman & Chapman 1995). Plants werealso assigned into morphospecies with the help of a localparataxonomist and via comparisons with specimens depos-ited at the UFP Herbarium (vouchers n° 34 445–51 604 forthe study site) and at the Plant Ecology Laboratory (SerraGrande seedling and sapling collections) of the UniversidadeFederal de Pernambuco, Brazil. A checklist of the local florais available in Pôrto et al. (2006). Poles were further identi-fied to species level and assigned into mutually exclusive

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categories of: (i) regeneration strategy (pioneer- or shade-tolerant species, according to definitions provided byHartshorn 1978); (ii) seed dispersal syndrome (biotic orabiotic dispersal); and (iii) seed size based on proceduresand criteria adopted by Santos et al. (2008). Based on theabove procedures, we obtained for both habitats: (i) averagecanopy openness, and direct, indirect and total light avail-ability; (ii) average macro, micronutrient contents and soilresistance; (iii) average plant density and species richness;and average percentage of (iv) pioneer species; (v) small-seeded species; and (vi) abiotically dispersed species. Thewhole study was carried out between December 2004 andDecember 2005.

Data analysis

Differences between habitats (nests and controls) wereexamined via paired t-tests or its non-parametriccorrespondent. These statistical procedures are properlydescribed in Sokal and Rohlf (1995) and tests were per-formed with the software Statistica 6.0 (StatSoft, Inc. 2001).We also estimate the total pole species richness (acommunity-level analysis) for nests and their respective con-trols in order to examine plant species richness at landscapescale. Mean species-area accumulation curves, from whichwe obtained the number of species observed (Sobs), wereconstructed by the EstimateS 7.5.0 program (Colwell2005). Species accumulation curves were generated from1000 randomizations within each group of nests – eightrecently abandoned and 10 long-abandoned nests,respectively. Estimators were performed using patchiness 0 inorder to eliminate any bias in the estimation of species rich-ness due to species patchiness (Benítez-Malvido & Martínez-Ramos 2003).

To evaluate whether abandoned nests offer particularregenerating niches and thus support taxonomically distinctregenerating assemblages and nest-dependent plant species,we adopted four independent but largely complementaryprocedures. First, we performed a non-metric multidimen-sional scaling (NMDS) ordination of all surveyed plots usingtheir Bray-Curtis dissimilarity matrix of pole species compo-sition (Krebs 1999). Species abundance data were squareroot-transformed and standardized (sensu Clarke & Gorley

2001) in order to avoid any bias resulting from highly abun-dant species and differences in sample sizes (i.e. stem densityper plot). Second, to examine the relationships betweenhabitat type (nest and undisturbed forest) and species simi-larity between plots, habitat was considered as a factor in anANOSIM test (Clarke & Gorley 2001). Third, we examinedthe level of plot nestedness (sensu Atmar & Patterson 1995)by obtaining the nestedness temperature of a presence–absence matrix containing all pole species across the 36 plots(18 nest plots and their controls). The value obtained wasthen compared with temperature values of 1000 randomlygenerated matrices with the same number of species occur-rence via the Nestedness Temperature Calculator software(Atmar & Patterson 1995). Finally, we performed an indica-tor species analysis (sensu Dufrêne & Legendre 1997) basedon two groups of plant species: one consisting exclusively ofnest plots (n = 18) and another of control plots (n = 18).Thislast group of analyses was carried out using Systat (Wilkin-son 1996), Primer (Clarke & Gorley 2001) and PC-Ord(McCune & Mefford 1999).

RESULTS

In contrast to our expectations, nests and control plotsexhibited very similar, relatively bright light environ-ments irrespective of nest age (Table 1). Briefly,canopy openness ranged from 5.30 � 0.94% (controlplots of recently abandoned nests) to 6.01 � 1.07%(long-term control plots), and light availability reached7–9% across habitats. In general, the soil across allhabitats proved to be relatively acid (pH < 6.0) andnutrient-impoverished in terms of both microand macronutrients considering latosol standards.Recently and long-abandoned nests were not differentfrom controls across the majority of the soil attributesexamined (Appendix S1), but some cross-habitat dif-ferences appeared, particularly in the case of recentlyabandoned nests (Table 2). Relative to its control, thisnest type revealed substantial reductions in contents ofcarbon (1.45 � 0.24% vs. 1.79 � 0.13%) and organicmatter (2.5 � 0.41% vs. 3.08 � 0.23%), and proved

Table 1. Forest structure and light environment in recently abandoned nests (n = 8) and long-abandoned nests (n = 10) of theleaf-cutting ant Atta cephalotes compared with their respective control sites in the Coimbra Forest, north-eastern Brazil

Forest structure and lightenvironment attributes (%) Nests Controls P-values

Recently abandoned nestsCanopy openness 5.72 � 0.90 5.30 � 0.94 P = 0.249Direct light 8.13 � 1.59 8.01 � 2.55 P = 0.930Diffuse light 8.77 � 1.18 7.76 � 1.44 P = 0.099Total light 8.43 � 0.97 7.89 � 1.52 P = 0.466

Long-abandoned nestsCanopy openness 6.01 � 1.07 5.82 � 1.14 P = 0.959Direct light 9.14 � 4.15 9.12 � 2.68 P = 0.991Diffuse light 8.95 � 1.20 8.43 � 1.98 P = 0.193Total light 9.03 � 2.23 8.85 � 1.72 P = 0.753

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to be much more resistant to penetration (30.57 �6.08 mm vs. 39.4 � 7.53 mm). Likewise, soils of long-abandoned nests were more impenetrable than thoseof control plots and also retained lower iron contents.This implied that abandoned nests represent long-term edaphic habitats with a kind of degraded soil/substrate that should result in harsher environmentalconditions for plant recruitment in general, as com-pared with nest-free forest understorey.

A total of 10812 seedlings, saplings and poles wererecorded across the 18 nest plots and their respectivecontrols, with a large predominance of pioneer treeand shrub species in the Melastomataceae, Anacardi-aceae, Euphorbiaceae and Sapindaceae families.Recently abandoned nests exhibited less dense andmore impoverished regenerating plant assemblagesthan their controls, except for the seedling categorythat did not present any difference. For example,sapling density and species richness was less than ahalf of the scores recorded in the control plots, drop-ping from 2.37 � 0.99 stems per square metre and0.70 � 0.27 species per square metre in control plotsto 0.72 � 0.48 stems per square metre and0.40 � 0.23 species per square metre in nest plots(t7 = 4.27, P = 0.0037 and Z8 = 2.52, P = 0.0117,respectively; Fig. 1). Robust differences emerged inthe case of pole assemblages as well: pole density andspecies richness was reduced by one-third in recentlyabandoned nests relative to their respective controlplots (t7 = 3.56, P = 0.0091 and t7 = 4.87, P = 0.0018,respectively; Fig. 1). Long-abandoned nests andcontrol plots exclusively differed in pole assemblageattributes, but differences were biologically expressiveas well: stem density and species richness varied from0.30 � 0.18 stems per square metre and 0.23 � 0.14species per square metre in ant nests up to0.64 � 0.23 stems per square metre and 0.49 � 0.22species per square metre in control plots, respectively(t9 = 5.67, P = 0.0003; t9 = 6.23, P = 0.0002; Fig. 2).Collectively, a total of 155 poles belonging to 65woody plant species were recorded across long-termcontrol plots, in contrast to only 38 species (71 poles)

in the respective abandoned nests. At the communitylevel, species–richness estimators based on species–area accumulation curves identified marked differ-ences across habitats. Adopting a 95% confidenceinterval, recently abandoned nests reached 12.29–21.71 observed pole species (Sobs), whereas controlswere expected to host 28.35–43.65 pole species(Fig. 3a). The same patterns emerged for long-abandoned nests and their control plots (Fig. 3b),implying that 10–15 years after abandonment, antnests had still not achieved the regeneration patterns(e.g. plant density, local- and landscape-scale speciesrichness) recorded across undisturbed forestunderstorey.

The functional signature of plant assemblages in thepole class exhibited little variation across habitats. Interms of regeneration strategies, both recently andlong-abandoned nests and their respective controlplots were dominated by pioneer species, reaching90.10 � 15.10% of the individuals sampled acrossnests (Figs. 4,5). Contrary to our expectations,however, the average relative abundance of abioticallydispersed plants dropped to one quarter (5.03%,Fig. 4) in recently abandoned nests as compared withcontrols (t7 = 2.34, P = 0.05). Finally, small-seededpole species were predominant in abandoned A. cepha-lotes nests of both ages as well as in the control plots(Figs. 4,5).

Differences in plant assemblage structure, particu-larly in the pole class, paralleled slight floristic differ-ences between nest and control plots. NMDSordination of plots based on species similarity andabundance partially segregated nest and control plots(Fig. 6); plot ordination was well supported by a lowstress level of 0.14. In fact, abandoned nests exhibitedhigher floristic homogeneity than adjacent controlplots with older nests more scattered than thoserecently abandoned. In addition, ANOSIM uncovereda slight effect of habitat type (R = 0.19, P = 0.001) onthe similarity of plant assemblages across plots. Suchreduced floristic differences were also reinforced bythe nestedness analysis, which provided a matrix

Table 2. Soil parameters of recently abandoned nests (n = 8) and long-abandoned nests (n = 10) of the leaf-cutting ant Attacephalotes compared with their respective control sites in the Coimbra Forest, north-eastern Brazil

Soil parameters Unit Nests Controls P-values

Recently abandoned nestsC % 1.45 � 0.24 1.79 � 0.13 P = 0.007Na ppm 9.69 � 1.62 11.13 � 0.64 P = 0.040Organic matter % 2.50 � 0.41 3.08 � 0.23 P = 0.006Resistance to penetration mm 30.57 � 6.08 39.48 � 7.53 P = 0.017

Long-abandoned nestsFe ppm 225 � 49.72 268 � 31.90 P = 0.003Resistance to penetration mm 32.12 � 6.83 41.70 � 4.39 P = 0.003

Non-significant differences in soil parameters are available as supplementary material.

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temperature of 13.45°, much colder than expectedfrom random species distribution (P < 0.0001). Thispattern resulted from the fact that plant assemblagesover ant nests were largely impoverished relative tocontrols (9.5 � 4.19 species in control plots vs.4.4 � 2.4 in nests plots) and therefore consisted ofsubsets of the surrounding understorey flora. Finally,the indicator species analysis identified the occurrenceof three indicator tree species (Brosimum rubescens,Lacistema robustum and Lecythis lurida, P � 0.05) inthe floristic group formed by the 18 control plots, butno indicators were detected in the second samplegroup formed by the ant nest flora, that is, no evidenceof nest-dependent species.

DISCUSSION

Our results suggest that both recently and long-abandoned nests represent a distinct habitat not in

terms of light availability, but regarding soil attributes.In contrast to previous findings and expectations,abandoned A. cephalotes nests soils retained lowerlevels of organic matter and were far less penetrablethan soils in nest-free tracts of forest, and they prob-ably provide long-lasting harsh/unsuitable environ-ments for plant recruitment. Moreover, abandonednests appear to support largely impoverished (at localand landscape scale) and low-density assemblages ofregenerating forest plants for periods of a decade ormore. Such assemblages, although functionally similarto forest understorey in terms of plant light require-ments and seed dispersal strategy, represent floristicsubsets of the surrounding forest vegetation, ratherthan distinct units with particular nest-dependentspecies (i.e. indicator species). In synthesis, the pat-terns uncovered during this study reinforce the notionthat abandoned Atta nests offer a distinct substrate forplant regeneration, which affects or even drives plantabundance and species richness of regenerating

Fig. 1. Mean (+SD) number of individuals (a) and species (b) of seedlings (plant individuals with less than 50 cm in height),saplings (plant individuals with more than 50 cm height and less than 1.5 cm d.b.h.) and poles (individuals with d.b.h. between1.5 and 10 cm) occurring in recently abandoned nests (n = 8) of the leaf-cutting ant Atta cephalotes and their respective controlplots, in the Coimbra Forest, north-eastern Brazil. Asterisks above bars denote significant differences (P < 0.05).

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assemblages as frequently proposed (Garrettson et al.1998; Farji-Brener & Illes 2000; Farji-Brener 2005;Hudson et al. 2009). However, by being opportunisti-cally colonized by impoverished subsets, ant nestsapparently fail to represent a special regenerationniche that promotes species coexistence and plantdiversity.

Such pattern of nest colonization and forest regen-eration is consistent with the behaviour of A. cephalotes(at least in the Serra Grande landscape) and itsimpacts on nest mound attributes. These ants disposetheir refuse in subterraneous chambers (usually a fewmetres below the soil surface) rather than above-ground dumps, and promote active and regularremoval of leaf litter, resulting in bare ground (Weber1972; Meyer 2008). Ants also keep the mound free ofvegetation by repeated plant defoliation, and thisresults in canopy and understorey gaps above thenests (Meyer 2008; Corrêa et al. 2010). Thereby, ant

nest mounds, although immense, tend to primarilyconsist of mineral soil at the top layer (see also Meyer2008, who demonstrated lower nutrient contents inthe top soil of active ant nests compared with adjacentforest soil) and remain exposed to extremes of light,temperature and leaching by direct rain whereas colo-nies are active (8–10 years; Meyer et al. 2009). Previ-ous studies along the edges of Coimbra Forestdocumented a 5% increment of light availability aboveactive ant nests (Corrêa et al. 2010) due to nest-related canopy gaps sized over 87 m2. As we assessedabandoned nests, where forest regeneration alreadytook place, light levels were indistinguishable from theundisturbed forest understory, showing that this con-dition is rather ephemeral as already suggested byCorrêa et al. (2010). In contrast, the range of nest agesincluded in this study was long enough to demon-strate that nest soil remains compacted over manyyears despite active forest regeneration. Probably, ant

Fig. 2. Mean (+SD) number of individuals (a) and species (b) of seedlings (plant individuals with less than 50 cm in height),saplings (plant individuals with more than 50 cm height and less than 1.5 cm d.b.h.) and poles (individuals with d.b.h. between1.5 and 10 cm) occurring in long-abandoned nests (n = 10) of the leaf-cutting ant Atta cephalotes and their respective controlplots, in the Coimbra Forest, north-eastern Brazil. Asterisks above bars denote significant differences (P < 0.05).

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nest soils, as areas free of vegetation above and belowground and with higher proportion of clay, receivemore impact of rainfall and reach higher temperature,getting compacted through time. The lack of a con-tinuous litter layer on the nest surface, previouslyreferred by several Atta nests studies and systemati-cally showed by Meyer (2008), and soil compactionhave been demonstrated to affect seed germination/seedling recruitment in many ways. A thick litter layeron the forest ground is able to inhibit germination ofsmall seeds (Pinard et al. 1996; Benítez-Malvido &

Kossmann-Ferraz 1999), but litter occasionally oper-ates as a buffering layer, reducing both temperatureand humidity extremes and consequently preventingsoil and seed/seedling desiccation (Pinard et al. 1996;Benítez-Malvido & Kossmann-Ferraz 1999; Santos &Válio 2002). Likewise, increased soil compaction inant nests (Nkem et al. 2000), termite nests (Bandeira& Harada 1991) and logging activity areas (Pinardet al. 1996; Howlett & Davidson 2003) has been fre-quently identified as the force leading to reducedplant recruitment.

Fig. 3. Accumulated number of species observed in the recently abandoned nests (n = 8; a) and long-abandoned nests (n = 10;b) of the leaf-cutting ant Atta cephalotes and their respective control plots, in the Coimbra Forest, north-eastern Brazil.Non-overlapping bars denote significantly different species richness.

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In fact, by dying A. cephalotes colonies may representa kind of temporary recruitment bottleneck and func-tion as a particular but rather unsuitable habitat forplant recruitment, in which forest regeneration pro-ceeds slowly or is retarded (at least compared withregular regeneration speed in undisturbed areas), assuggested by the degraded regenerating assemblageswe have documented for abandoned nests, regardlessof their ages. As A.cephalotes ants present undergroundwaste disposal and cut almost everything growing ontheir nests, such assemblages should largely resultfrom seed arrival and seedling recruitment followingthe death of colonies and the definitive cessation ofant-mediated nest clearing (Farji-Brener & Illes 2000;Wirth et al. 2003; Farji-Brener 2005). Although wewere unable to detect evidence for selective plantestablishment among the nests of Coimbra Forest inboth taxonomic and ecological terms (i.e. no occur-

rence of nest-dependent species), this does not excludethe possibility that particular taxa inhabiting undis-turbed tracts of forest are less negatively affected bythe described effects of abandoned nests and, thus,have a higher chance of successful recruitment irre-spective of plant light requirements, seed size and seeddispersal mode. Such nest-mediated filtering processeshave been proposed to play a major role in active nestsof A. cephalotes across the edges of Coimbra Forest(Meyer 2008; Corrêa et al. 2010). By documentingincreased light availability and ant-promoted seedlingdefoliation on and around ant nests, paralleled byimpoverished sapling stands, Corrêa et al. (2010)argued that nest-affected habitats may represent aninhospitable environment for: (i) light-sensitive,shade-tolerant species; (ii) plant species bearing ant-targeted seedlings; and (iii) plant species whose seedsrequire undisturbed habitats for better germination.

Fig. 4. Mean (+SD) percentage of individuals (a) and species (b) of poles (individuals with d.b.h. between 1.5 and 10 cm)assigned into mutually exclusive categories of: (i) regeneration strategy; (ii) seed dispersal syndrome; and (iii) seed size (cf.Hartshorn 1978 and Santos et al. 2008) occurring in recently abandoned nests (n = 8) of the leaf-cutting ant Atta cephalotes andtheir respective control plots, in the Coimbra Forest, north-eastern Brazil. Asterisks above bars denote significant differences(P < 0.05).

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Apparently, such filtering alters species abundance,but it is not strong enough to promote the establish-ment of taxonomically distinct plant assemblages asabandoned nests host subsets of the surrounding flora.

It is now clear that Atta leaf-cutting ants representimpacting organisms across neotropical forests byimposing small-scaled disturbance regimes (via her-bivory and nest activities), which frequently increaseenvironmental heterogeneity in ecosystems (Wirthet al. 2003). By achieving increased densities inhuman-disturbed forests (Terborgh et al. 2001; Wirthet al. 2007; Meyer et al. 2009; Silva et al. 2009), thisengineering role played by Atta species may be largelyamplified and impose unanticipated effects on forestdynamic, particularly in terms of filtering processesand forest successional trajectory (Corrêa et al. 2010).In the Coimbra Forest, abandoned A. cephalotes nestsas initially illuminated spots covered by degraded soils

clearly extend forest heterogeneity, but the empiricalevidence accumulated so far contradicts the notionthat such small-scale disturbance provides, in general,particular or even irreplaceable regeneration niches fora sort of nest-dependent species, resembling the roleplayed by small treefall gaps (Farji-Brener & Illes2000). Actually, by exclusively altering the relativeabundance of some plant species and hosting highlynested and patchily impoverished assemblages, onecan argue that Atta nests interfere with forest succes-sional trajectories and consequently enhance forestheterogeneity.This perspective is probably also appliedto those Atta species, which locate their nutrient-enriched refuse in internal waste chambers (all Attaspecies except A. colombica and A. mexicana) andpromote intensive and regular nest clearing (e.g. A.cephalotes, A.colombica, A. laevigata, A. sexdens cf. Farji-Brener & Illes 2000; I.R. Leal and R.Wirth pers. obs.),

Fig. 5. Mean (+SD) percentage of individuals (a) and species (b) of poles (individuals with d.b.h. between 1.5 and 10 cm)assigned to mutually exclusive categories of: (i) regeneration strategy; (ii) seed dispersal syndrome; and (iii) seed size (cf.Hartshorn 1978 and Santos et al. 2008) in long-abandoned nests (n = 10) of the leaf-cutting ant Atta cephalotes and theirrespective control plots, in the Coimbra Forest, north-eastern Brazil.

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the forces controlling nest soil attributes. Furtherstudies should examine the generality of the patternsdocumented in the Coimbra Forest in order to betterunderstand the role played by Atta-related distur-bances on the dynamics of neotropical vegetation.

ACKNOWLEDGEMENTS

The study was supported by the Brazilian–Germancollaboration project (PROBRAL CAPES/DAAD,project 257/07), Conselho Nacional de Desenvolvi-mento Científico e Tecnológico (CNPq, processes540322/01-6 and 243000/02), the Deutsche Fors-chungsgemeinschaft (DFG, process WI 1959/1-2)and Schimper Foundation. We are grateful for theinfrastructure and logistical support given by theUsina Serra Grande, Centro de Pesquisas Ambientaisdo Nordeste and Conservação Internacional doBrazil. AGDB and MAO received master andPhD fellowships, respectively, from CAPES. IRLand MT received research grants from CNPq theConselho Nacional de Desenvolvimento Científicoe Tecnológico. Mr Eleno Araújo (in memoriam)helped us in many field activities, including plantidentification. H. Vasconcelos provided helpful criti-cisms on the first draft of this manuscript. The Asso-ciate Editor and an anonymous referee offered usefulcomments.

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SUPPORTING INFORMATION

Additional Supporting Information may be found inthe online version of this article:

Appendix S1. Non-significant results for soil param-eters of recently abandoned nests (n = 8) and long-abandoned nests (n = 10) of the leaf-cutting ant Attacephalotes compared against their respective controlsites in the Coimbra Forest, north-eastern Brazil.

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