Leaf-Cutting Ant Herbivory in Successional and Agricultural Tropical Ecosystems Author(s): Chantal M. Blanton and John J. Ewel Reviewed work(s): Source: Ecology, Vol. 66, No. 3 (Jun., 1985), pp. 861-869 Published by: Ecological Society of America Stable URL: http://www.jstor.org/stable/1940548 . Accessed: 01/11/2012 15:22 Your use of the JSTOR archive indicates your acceptance of the Terms & Conditions of Use, available at . http://www.jstor.org/page/info/about/policies/terms.jsp . JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact [email protected]. Ecological Society of America is collaborating with JSTOR to digitize, preserve and extend access to Ecology. http://www.jstor.org Ecology,66(3), 1985, pp. 861-869 ? 1985by the Ecological Society of America LEAF-CUTTING ANT HERBIVORY IN SUCCESSIONAL AND AGRICULTURAL TROPICAL ECOSYSTEMS1 Chantal M. Blanton and John J. Ewel Departmentof Botany, Universityof Florida, Gainesville,Florida 32611 USA Abstract. Herbivory by Atta cephalotes was measured in four plant communities of different complexityin Costa Rica. The fourcommunities were a monocultureof cassava {Manihot esculenta) and three diverse assemblages, each 1.5 yr old: (1) successional vegetation,unmodified by the in- vestigators;(2) imitationof succession, a communityof investigator-introducedspecies designed to mimic the unmodified succession; and (3) enriched succession, a successional vegetation that the investigatorshad augmentedby propagule inputs. Each ant colony had access to all fourcommunity typessimultaneously. The average herbivoryrate (daily cuttingof leaf per square metreof ground,all treatmentscombined) was ~ 150 mg (or 38 cm2). In communitieswith greater leaf area index, structuralcomplexity, and species richness,A. ceph? alotes cut lower portionsof total leaf area. Beforeharvest of the cassava monoculture,the herbivory rate of A. cephalotes was 87.9 cm2m~2d_1 in the monoculture,21.4 in the imitation, 14.7 in the succession, and 6.8 in the enrichedsuccession community.These amounts represented0.3% of total leaf area in the monocultureand a mean of 0.03% of total leaf area in the threecomplex ecosystems. Cassava, which occurred in threeof the fourcommunities, was attacked most heavily (per unit leaf area) in the imitationsuccessional community,least heavily in the enrichedsuccession, and at inter- mediate intensityin the monoculture.In response to loss of theirpreferred forage (cassava) through harvestingby humans, the ants cut more leaf tissue in the threespecies-rich communities, especially the imitation. As cassava resproutedin the monoculture,A. cephalotes' rate of attack on the three diverse treatmentsreurned to preharvestlevels. Atta cephalotes cut only 17 of 332 available plant species. They cut proportionallymore woody than herbaceous species, more introducedspecies than natural colonizers, and species with below- average water contents. Plant relative abundance alone did not determinehost plant selection, but most of the attacked species were cut in proportionto theirtotal leaf area. Key words: ants; Atta cephalotes; Costa Rica; herbivory;Manihot esculenta; succession; tropical agriculture;tropical succession. Introduction ants had an opportunity to select forage from a broad of all within easy access of their Developing nations, particularly those with limited range ecosystems, nests: old forest, forest, pasture, plantations of fossil fuel resources, face difficultdecisions in selecting young exotic trees and Pinus), and four kinds of appropriate agricultural ecosystems to keep pace with {Gmelina established as part of the re? food and fiber needs of their rapidly expanding pop? experimental ecosystems search The latter from monocultures ulations. Monocultures are notorious for pest out- project. ranged to even more diverse than successional breaks (Gibson and Jones 1977), whereas mixed crop- ecosystems ping systems sometimes do and sometimes do not afford vegetation. Atta a member ofthe more protection to their component plants (Perrin and cephalotes, myrmicine fungus- tribe Attini, is a widespread her? Phillips 1978, Risch 1981, Ewel et al. 1982). One growing conspicuous, bivore whose increase under human stew- promising line of research suggests that useful char? populations 1967,Haines 1975). acteristics of natural ecosystems may be incorporated ardshipof ecosystems (Goncalves This forest-dweller invades sub- into agroecosystems (Hart 1980). We are part ofa team gap-loving frequently sistence and from Mexico to Bra- of researchers that has been working in Costa Rica since plantation agriculture zil and Cherrett Over 1978, testing this idea. An integral part of such research (Cherrett Peregrine 1976, 1981). this A. is a is to study how herbivores interact with host plants in geographic range cephalotes generalist (sensu it attacks of several chemi- simple and diverse ecosystems (van Emden and Way Feeny 1976); i.e., species unrelated local 1972, Root 1973, Feeny 1976, Rausher 1981). cally plant families, although popula? tions have a diet From the time our study plots were cleared and may very specialized (Rockwood When substrate for their planted in early 1979 (Ewel et al. 1981), it was obvious 1976). harvesting fungal gar- leaf-cutters remove a select of the that the leaf-cutting ant Atta cephalotes L. was one of den, very portion total of leaves, flowers, and fruits the most important herbivores on the site. This impres- standing crop (Lugo et al. Rockwood by A. ceph? sion was reenforced by observations made by Brown 1973, 1978). Herbivory alotes can reduce potential (Rock? (1982) while measuring overall herbivory rates. The plant reproductive wood 1972) and undoubtedly affects net primary pro? 1 Manuscript received 16 February 1984; revised 25 June ductivity as well. Furthermore, the ants' refuse dumps 1984; accepted 26 June 1984. concentrate nutrients and their nest-building disturbs 862 CHANTAL M. BLANTON AND JOHN J. EWEL Ecology, Vol. 66, No. 3 soil profiles, creating point sources or sinks of nutrients tuber. After the first2 mo of this study it was harvested (Haines 1975, 1978, 1983, Alvarado et al. 1981). (September 1980), but the aboveground parts were re- Many agricultural ecologists propose that structural turned to the plots after weighing and subsampling. and floristic complexity can reduce the probability of The twigs continued to sprout for several months. severe pest outbreaks in plant communities (see review Six blocks were established, each containing the four by Altieri and Letourneau 1982). Because of our in- treatments in randomized block design (Fig. 1). Each terest in the attributes, including losses to herbivores, treatment occupied one quadrant of each block and of successional and agricultural communities of differ- thus had two sides contiguous with other treatments ing degrees of complexity, the following question was and two sides contiguous with the second-growth forest of key importance to our research: Do the ants pref- that surrounded each block. Because the 16 x 16 m erentially forage in certain of the four experimental quadrants were far smaller that the area foraged by a ecosystems? A related question was of agricultural sig? A. cephalotes colony, each colony had access to all four nificance: How do the ants respond to a dramatic re? experimental ecosystems. The ants' preferred forage duction in availability of their preferred forage? The species, cassava, was available in all six replications of answer to this question was obtained by measuring the monoculture, imitation, and enriched treatments. herbivory before, during, and after harvest of the Measurement monoculture, which contained >85% (by leaf area) of offoraging the ants' preferred forage species, cassava. Finally, we From late July through mid-November 1980 (rainy asked a third question: What are the characteristics of season months), experimental plots were searched sev? those species and ecosystems that are especially at- eral times weekly for leaf-cutting ants. Plots were vis? tractive to the ants? This question was answered by ited in a predetermined random order to ensure that comparing grazed and ungrazed plant species in terms all received an equal number {n = 33) of surveys, about of their importance in each plant community, their life half during the day and half at night. Because daytime forms, and the moisture content of their leaves. surveys normally began after dawn and ended in late afternoon, and nighttime surveys began after dusk and Methods ended before daybreak, dawn and dusk were under- sampled. Foraging on a given plot was measured only site Study on trails leading from plants on that plot. To determine This study was carried out in the Florencia Norte the area and mass of leaf tissue removed, laden ants Forest, near Turrialba, Cartago Province, Costa Rica. passing an observation point on each active trail were The site lies on lightly undulating terrain located at counted for 5 min. The mean area and mass of each 9?53' N, 83?40' W, and at 650 m elevation. Mean an? ant's burden were estimated by collecting >50 leaf nual rainfall approaching 2700 mm is concentrated fragments from each active trail during each obser? between April and December. The area is in the Trop? vation. The combined area of these fragments