Rev. BiolTrop., lS(l): 89-99, 1977

Sorne factors influencing colony spacing and surviYal in the grass-cutting Acrornyrmex landolti fracticomis (Forel) (Forrnicidae : Attini) in Paraguay '

by

H. G. Fowler"

(Received for publicalion January 31, 1977)

Abstract: Variou. a.pecl. of Ihe ecology of Ihe Paraguayan grau·cutting ant, Iando/tifra cticornis (Forel) , were .Iudied in Ihe field. Colonie. were found lo linearly orienl their foraging lerritories, partilioning Ihe habilat in a mosaic pattem. The di.tance. foraged by workers were found lo be a funclion of

= �.sIS6 0.4121 \ colony .ize: (Logl O maxirnum di.lance foraged) + (Log O area of ne.I). Colony .pacing, however, wa. found lO be nol immediate y dependenl on colony size. Colony .pacing patterns are influenced by intra-specific aggression, and perhaps by Ihe predalion or execulion of founding queens. Bolh Ihe mound and .uperficial delrilus heap of nesls were fo und lo favor lhe survivorship of fo rbs. Colony numbeu were found lo decrease significantly as gra.. .coverage decreased and forb ,pecies becarne dominanl.

Leaf-cutting of the genera and Acromyrmex are important of the neotropics (Weber, 1972). Wheeler (1907) considered them to be the "dominant" invertebrates of those areas in which they occur. The Acromyrmex has bcen shown to be "dominant" in terms of its numetical abundance (Kusnezoy, 1963), while indirect evidence exists that Acromyrmex /ando/ti tra cticomis (Forel) may be "dominant" by regulating the grazing of other invertebrates (Fowter & Robinson, 1977). However, factors governing the spatial distribution of leaf-cutting ant colonies are not well understood. Wilson (1971) considered that colon y spacing in ants may be achieved through inter- and intra-specific aggression between mature queens, and by the execution of founding queens by established colonies. In leaf-cutting ants, colony displacement also serves as spacing mechanism (RoclMood, 1973). As leaf-cutting ants may employ a conservational foraging strategy (Cherrett, 1968; Rockwood, 1975), spacing mechanisms should maximize long-term resource exploitation. The relationships between leaf-cutting ants and vegetation have been studied

• InstllulO

from the standpoint of tlle ants' exploitation ofplants (Cherrett, 1968; Rockwood, 1975). Thc nest rnaintenance activities of some of leaf-cutting ants are well known to inOuence vegetatianal patterns (Bucher & Zuccardi, ] 967; Haines, 1971 ; Jonkman, 1976). This paper presents data on the grass-cutting A. lando lti fr acticornis (Forel), a dominant component of Paraguayan range lands (Fowler. 1977a). Foraging te rritories, intraspecific relations, aggression, and ant-plant interactions are cxamincd, and their relative importance to colony spacing is discussed.

METHODS

Ant-ant interactions: In March, 1975, the following data were recorded for cach nes! in a lOa m2 plot of grasses ocar Asunción, Paraguay: (a) the number of nest entranees; (b) the maximum height of the mound; (e) the distance between the nearest eolonies, as measured between entranee holes; and (d) the vegetative eovcr 01' the mound, reeorded as eJasses: 1 =0- 20%; 2=21 -40%; 3=41 -60%; 4 =61 - 80%; 5 =81 -100%. Factors (al, (b), and (d) were used as an index of colony size to determine their effeet on eolony spacing. Foraging territories were studied in a 30 m2 sub-plot of the same pIot in September, 1975. Foragers were aspirated into vials as they left their coJonies, and were then eooled on ice until motionless. The ants were then marked with paint on the dorsal surface of the gastcr, with a distinctive color or color combination used for individuals of diffe rent eolonies. A minimum of 20 workers from eaeh eoJony were marked ayer a 2 day periodo During the next five days, the area was searched earefully for marked ants, whose loeation was then reeorded on a refe renee map to determine the ranges of their foraging aetivity. Behavioral observations from more than 500 hr of field observation over a one-year period (Fowler & Robinson, 1975) are also inc1uded. Ant-vegetation interactions: In rebruary, 1975, the number of oests was recarded far two 100 m2 plots of Bermuda grass, (Cynodon dactylon). At the same time, the pereentage of grass eover was estimated for eaeh plot using the loop method (Parker, 1954), which was modified for small plots. A 2 cm diameter wire loop was placed to the right of the 10, 30, 60 and 90 cm points of eaeh meter of a 10 m tape, and the pereentage eover was estimated for eaeh reading. In all, 100 loop readings were taken for ea eh pIoL rhe grass eoverage was taken as the mean percentage eover of the summed individual loop readings. To determine the effeets of pasture rnanagemeot 00 nest densities, one plot was left untouched to allow vegetational sueeession to proeeed, while the other plot was cut monthly to retaío a fairly high and uniform grass stand. Ayear later, nest densities and grass eoverage were reassessed. In May, 1975, the numbers and hcights of forb seedJings per m2 werc detcnnined for open ground, the nest mound surfaee, and the superficial detritus heap, the !atter being fonn ed of exhausted material aeeurnlllated to one side of the nest. These rneasurements were takcn in the naturally succeeding plot. RESULTS

Colonv size and spacing: The signifieant positive correIations found between the nllmber of nest entranees, the height of the rnound, and the pereentage of vegetational eover tends to indieate that all three may be va lid indexes of colony size (Table 1 l. If high values of these are assumed lo indicate older colonies, and low values young eolonies, then no signifieant refation was found betwecn colony size and the distance to the nearest colony (Table 1 l. Physica/ relations of Acrornyrmex landolti fracticornis neS1s and colony spacing (co"ela.tion coefficients) based on measurements of 65 neSlS

Surface area NO of entrances Max. height % vegeta tive CQver Nearest nest (cm) Surface area 0.770* 0.648* 0.258' 0.032 NO of entrances 0.739* 0.305' -0.051 Max. height 0.165 0.082 %vegetative cover 0.119

• P< 0.05 92 REVISTA DE BIOLOGIA TROPICAL

Foraging territories and colony densities: The pooled total5 foc the tWQ plots gave 117 nestsj200 m " or an equivalent 5,850 neSls/ha. As this value was quite high, a detailed, short-term study of the foraging territories was undertaken to determine if averlap of foraging territories was minimized. Fram behavioral observations, foraging territories were thought to be linear. The results of the marking experiments showed this to be true over a short term and the habitat was partitioned in a mosaic pattern (Fig. 1). The mean length of lhe foraging lcrritories of the individual calanies was 1.03 m (s = 7.17). However, during the period of study not all the habitat was being exploited, allhough it was assumed that these foraging territories would change somewhat with time. More significantly, foraging territories were found to orient away from the superficial detritus heap (Fig. 1). The distance of furthest sighting of marked foragers was also found to ¡ncrease with colony size. The cquation (Log maximum observed cutting , o = - og distan ce) 0.51 56 + 0.4121 (L ¡ o area of nest surface) accounted for 92% of the variation. Ants from larger nests forage farther than ants from smaller nests, orten by displaccment of thc fomler's foraging territories, while smaller colonies tend lo forage c10ser to their nests. Intra-specific aggression: During 500 hs of obscrvation, aggression between colonics was seen on 5 occasions, and each tirneoccurred on or near a nest mound. During aggression, a higher than normal number of workers was seen on the surface, bul actual aggressive contacts wcre few. Mutual avoidance between workers of different colonies was cammon. Aggression lasted from 3 to 6 he. Four of the five attacked colonics lived as long as six months following aggression. In the fifth however, the attacking workers were seen entering lhe nest of the attacked colony, and were later observcd transporting larvac and pupae to their own nest and 15 days later no activity was observed in the attacked nest. Aggression was never seen occurring away from the nest surfacc. t-lowevcr, joint use of foraging trails was observed (Hg. 1), suggesting Ihat only the nest itself is physically protected. Foraging by founding queens: Incipient qucens were observed foraging on four occasions in January, 1976. AII were followed to their freshIy excavated chambers and then collected. AII chambers were excavated, revealing small (0.2 cm) fu ngus gardens in which small pieccs of grass were visible, small egg masses were also present in ea eh chamber.

TABLE 2

Anlll iOl el/ange i" lIegelatille COila and Acr9mynncx landolti fracticornis nest numbers in naturalJy suceeedillg"ánd artifi eially grazed paslllres 2 % grass cover Colony numbcrs/lOO m Natural Artificially Natural Artificially Date succe:tision grazcd liuccession grazed

11/ ¡ 975 62.8 68.7 65 52 11/ ¡ 976 ¡ 1.4 73.2 27 46 2 X2 26.834 X 5.479 p < 0.001 P< 0.02 FOWLI::R: Colony spacing and survival of Acromynnex landolti in Paraguay 93

Ant-vegetation relations: When allowed to proceed a natural successional course, a significant decrease in grass coverage occurred al the end of ayear (Table 2). Forb coverage, though not measured, Iikewise increased in this plot. The managed plot remained practically unchanged (Table 2).

The nesting activities of A. l. lracticornis wcre found to innuence forb survivorship. When forb heights are plotted against forb numbers, assuming height to be proportional with age, both the nest mound and the superficial detritus hcap were found to greatly influence forb survival (Fig. 2). Although more forbs were found in undisturbed ground, a higher percentage of the forbs on the nest mound and superficial detritus heap attained greater height. The reduction of colony numbers aftcr one ycar in the naturally succeeding plot was significant (P 0.02) (Table 2). Although colony numbe" also dropped in the artificially maintained plot, these were not significant.

D1SCUSSION

In species of A Ita, colony age, and therefore populations, can be estimated by extern�1 nest characteristics (Bitancourt, 1941). Data presented here suggested that tltismay also be tme of A. l. fra cticarnis. However, the data suggest no relationsltip betwecn colony size and· colony spacing. Nevertheless, the distance foraged by workers of A. l. lracticornis in creases with ¡ncrcasing colooy size,. as is also true for Atta ceplwlales (L.) (Lewis et al., 1974). The foraging territories observed were linear, which differs them from the foraging territories of other grass-cutting, range-dwelling ants. Foraging territories of Atta capiguara, Atta vollenweideri, Atta laevigata, and Acromyrmex lundi are approximately circular (unpub. pers. observ.), as is probably true for tropical forest species. This linear orientation of foraging territories of A. l. fracticornis may be attributable to minimizing territorial overlap and thus reducing competitiop. However, solitary colonies were also found to forage along a linear orientation, and thus would appear to be species-specific behavior. Even though joint utilization of foraging trails and territories was observed, tltis behavior is rareo Elton (1932), Brían el al. (J 965) and Wilson el al., (J 971) have snown that Sume ant species maintain separate foraging territories, which is also the general rule �or A. l. lr acticornis. Howcver, A. l. fractic.:ornis was not observed to physically defehd theseterritories, althougn these may be maintained by chemical or other means. As aggression was observed 5 times during approximately 500 hr of observation, or ooe aggressive contact/lOO hr, this may be important in regulating colony populations or foraging territories. lntra-specific aggression is common in Atta sexdens (Autuori, 1941; Mariconi, 1970) and also occu" in Atta colambica ' (Rockwood, 1973). ¡ntra- and inter-specific aggression may determine, in part, spacing patterns of leaf-cutting anl calonies (Rockwood, 1973). The theft af brood and subsequent colooy death in one observed aggressive encounter, lend added support to this theory. Brood theft also occurs in laboratory colonies of Acramyrmex actospinasu$ (S. W. Robinson, pers. comm .). This may help to explain why only the nest itself is defended, which is also true for Acromyrmex crassispillus (Fowler, 1977b). As Acromyrmex queens possess a complete repertory of species-specific behavioral acts (totipotency) (Wilson. 1971), foraging by incipient queens was to be 94 REVISTA DE BIOLOGIA TROPICAL expeeted. Foraging has previously been reported for ineipient queens of A. octospinosus (Cordero, 1963), A. hispidus (Weber, 1972) and A. slria lus (Montenegro, 1973); it also oecurs in A. rugosus (unpub. pers. observ.). As Montenegro (1973) points out, foraging by founding queens ¡nereases thcir o probability of predation, due to inere sed superficial exposure. This superficial exposure may also ¡ncrease thcir chanees of being cxecuted by workers from near-by eolonies. Conspecific execution of founding quecns occurs in A. colom bica (Parker, 1954), A. capiguara and A. vol/enweideri (unpub. pers. observ.), and is probably a regulator of spacing pattcrns and colony densities. Thc selective herbivory and nest maintenance activitíes of leaf-cutting ants have becn thought to innucnce vegetational patterns. The detritus rcmoval activities of Atta cephalotes ¡n creases seedling survival in tropical rain rorests (Haines, 1971). Tite nesting activities of A. l'ollemveideri initially inhibit vegetalional growth (Bucher & Zuecardi. 1967), but later serve as the focal points uf woody planl growth in lhe �vanna uf !he Paraguayan Chaco (Jonkman. 1976). Tamayo (1964) suggests thal A. landolti m.y be responsible for shrub succession in Venezuelan rangc lanus. It has been shown here that A. l. fra ctü:ornis infl uenccs fo rb survivorship, though it is difficul t to assess its real importance. The innucnce of vegctation on A. l. fracticornis is more dear·cut. I\s grass i l covcrage signif cant y dccrcl.Ised, so did colony numbers. As the forb : grass balance r is usually detcrmined by grazing p essure, the rcduction of grass coverage may have bccn due to competition with forbs in the naturally succeeding ploL As colon)' density is significantly correlated with basal grass coverage {Fowler & Robinson, 1 ()77). a reduction of colony numbers was expecled. This reduction may be the reslJlt of a short�lgc of available, harvestable substrate, which must meet certain l physical and d CJl1 ical specirications to be fo rage d by A. /. fracticomis (Fowler & Robinson, 1977).

RESUMEN

Algunos factores que afectan los patrones espaciales de las colonias de la hormiga gramnívora Ac.:romyrmex lando/ti !r acticomis (Forel) son. considerados bajo condiciones naturales. La agresión entre co lonias Y. quizá, la ejecución de las reinas incipientcs podrán tener mucha importancia. Los territorios de forrajeo se orientan linealmente, dividiendo los recursos disponibles en un mosaico. La distancia forrajci.lda por las hormigas es fu nción del tamaño de la colonia. Las actividades del mantenimiento de la colonia, tales como la construcción del túmulo y la formación de un basurero superficial, afectan la supervivencia de malezas. Con la disminución de la cobertura de pastos y el aumento de la de malezas, las dcnsidades dc coloni

ACKNOWLEOGMENTS

o Acknowledgment is due to the British Ministry of Overscas Devel pm cnt, the Universidad Nacional de Asunción, the Ministerio de Agricul tura y Ganadería del

Paraguay, and Ihe U. S. Peacc Corps/!,araguay for lheir supporl ofthiswork.S.W Robinson, J . Diehl and E. f'owler helpe d wilh Ihe field work, and Iheir k n eollaboration is gralefully ac nowledged. S. W. Robinson, 1. M. Chcrre and R. 13. Roberts offered many useful criticisms. LlTERATURE ClTED

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Fig. 1. F oraging territories of colonies of Acromyrmex lando/ti fra cticornis in ficld near Asunción. Circle =mound, Star = detritus hcap. , 1 m I 1 Fig. 2. Forb seedling survival on pasture, nest, and detritus heap. Seedling dcnsily is plottcd against seedling height. Vertical lines show 9S%confidcnce levels lo means from 30 replica tes. 150

.... UND1STURBED ...... 1 + 100 + l ····. N� 50

"- ·t .. V> O '" W Z 50

DUMP 100 ...... f. ..." + . . +o" .l ... 50

fORB HEIGHT [cm]