Ecological Applications, 15(4), 2005, pp. 1478-1485 © 2005 by the Ecological Society of America NEST-SITE LIMITATION IN COFFEE AGROECOSYSTEMS: ARTIFICIAL NESTS MAINTAIN DIVERSITY OF ARBOREAL ANTS STACY M. PHILPOTT' AND PAUL F. FOSTER Department of Ecology and Evolutionary Biology, 830 N. University Avenue, University of Michigan, Ann Arbor, Michigan 48109-1048 USA Abstract. Nest sites are a limiting resource for arboreal twig-nesting ants, and nest sites may be increasingly limited with habitat modification. One such habitat modification is the conversion of traditional coffee farms, where coffee is cultivated under a dense, diverse shade canopy, to more intensive production systems with reduced canopy cover and lower diversity, height, and density of shade trees. As a result of such management intensification, ant diversity declines. We ask here if: (1) nest sites are a limiting resource for arboreal twig-nesting ants in coffee farms, especially in intensively managed systems and (2) nest-site limitation is a mechanism causing loss of ant diversity with coffee man- agement intensification. During 2000-2003, we investigated occupancy, species richness, and species composition of arboreal twig-nesting ants using natural (hollow coffee twig) and artificial (bamboo stem) nests in farms either with high or low diversity and density of shade trees. In both high- and low-shade sites ants occupied a majority (>55%) of natural nests and occupied some (>15%) artificial nests, and significantly more artificial nests were occupied in low-shade sites. In both high- and low-shade sites, ant richness was higher in artificial than in natural nests. More species occupied natural nests in low-shade sites, and more species occupied artificial nests in high-shade sites. Furthermore, species composition differed between nest types, with more ant species found more often or only in artificial nests. These results indicate that, although ants are not strongly nest-site limited in coffee agroecosystems, nest limitation increases somewhat with increasing management intensi- fication. Reductions in numbers of nest sites may be a mechanism causing ant diversity loss with coffee management intensification. Interestingly, because relatively fewer species colonized artificial nests in the low-shade site, ants may be recruitment limited in the low- shade sites, possibly maintaining low ant richness in these sites. Key words: biodiversity loss; biological control; Chiapas, Mexico; coffee agroecosystem; nest- site limitation; recruitment limitation; twig-nesting ants. INTRODUCTION nest-site limitation is suggested when ants: (1) occupy Ants are diverse and abundant in tropical habitats, "^^^"^ proportions of available nests (Torres 1984, Byrne sometimes making up >85% of arthropod biomass in 1^94), (2) occupy artificial nests implying natural nests forest canopies (Hölldobler and Wilson 1990, Davidson ^""^ "«t abundant enough (Berbers 1986, Perfecto and 1997, Davidson et al. 2003). Ants use many nest sub- Vandermeer 1994, Andresen 2003), (3) compete for strates including soil, leaf litter, tree stumps, plant dom- "^st sites (Davidson et al. 1989, Way and Bolton 1997, atia, and dry or fallen twigs (Hölldobler and Wilson Stanton et al. 2002, Solano et al. 2003), and (4) ag- 1990) and are frequently assumed to be nest-site lim- gressively take over nests occupied by other ants (Brian ited. Although litter twig-nesting ants may be limited 1952, Yamaguchi 1992, Hasegawa 1993, Vasconcelos by either size or availability of twig nests (Berbers 1993, Cerda and Retana 1998). Although distributions 1986, Kaspari 1996, Foitzik and Heinze 1998, Fonseca of some species are not determined by competitive in- 1999), relatively less is known about nest Kmitation in teractions (Fernandez-Escudero and Tinaut 1999, Ribas arboreal twig-nesters (Carroll 1979). Nest-space limi- and Schoereder 2002) and nest colonization by some tation may mean that only larger ants can occupy larger ants is not affected by ant presence on the same plants nests (i.e., those with greater volume or larger entrañe- (Byrne 1994), nest-site limitation is arguably the stron- es); (Fonseca 1999), and smaller ants dominate smaller gest factor by which the number and size of arboreal nests (Carroll 1979, but see Byrne 1994). Evidence for twig-nesting ant colonies are limited (Carroll 1979), a factor which may be exacerbated by habitat modifi- Manuscript received 28 September 2004; accepted 8 Novem- cations, ber 2004; Final version received 21 January 2005. Corresponding Converting natural ecosystems to agroecosystems or Editor: J. A. Logan. • ^ -c • ^ • 4- -t ,• ,? • ., • ... •• ,^ •T intensifying agroecosystems may increase nest-site ' Present address: Smithsonian Migratory Bird Center, Na- ./ o 0 tional Zoological Park, 3001 Connecticut Avenue NW, Wash- limitation resulting in lower ant abundance and diver- ington, D.C. 20008 USA. E-mail: [email protected] sity. Normally, as natural ecosystems are converted to 1478 August 2005 ARTIFICIAL NESTS PROMOTE ANT DIVERSITY 1479 agroecosystems (or as agroecosystems are intensified), density, and percent cover (Belen Rustic [BR] and Ir- trees are eliminated or pruned. Coffee was traditionally landa Restoration [IR]) and another area of low shade cultivated under a diverse, dense shade canopy, but tree diversity, height, density, and percent cover (Belen recent production is characterized by increased man- Production [BP] and Irlanda Production [IP]) (Mas and agement intensity•namely removing or reducing the Dietsch 2003, Perfecto et al. 2003). We thus examined density and diversity of shade trees, shade tree pruning, natural and artificial nest densities and occupation in and intensive use of agrochemicals and fertilizers (Mo- two high- and two low-shade sites. guel and Toledo 1999). Coffee intensification is also We first examined abundance and ant occupation of associated with lower ant diversity (Nestel and available twig nests defined as naturally occurring hol- Dickschen 1990, Perfecto and Snelling 1995, Perfecto low coffee twigs. On 13 dates, approximately every et al. 1996, 1997, Roberts et al. 2000, Perfecto and two months from July 2000 to June 2001 and again Vandermeer 2002, Armbrecht and Perfecto 2003). Ar- from December 2001 to March 2003, we broke off all boreal twig-nesting ants use dry hollow twigs in trees dry twigs from 10 previously unsampled coffee bushes and other vegetation as nests. As shade trees are pruned in each site for a total of 560 coffee plants (101 in BR, or removed, the total number of available nest sites 140 in IR, 120 in BP, and 199 in IP) and 9323 individual decreases, and arboreal ants may be increasingly forced twigs. For each coffee bush sampled, we counted total into the coffee layer where ant nest-site limitation numbers of dry twigs, hollow twigs, ant-occupied should increase. Furthermore, as the number of avail- twigs, and recorded the identity of each ant species able nest sites declines, ant diversity and abundance found. We considered a nest to be occupied if it con- may decline because inferior competitors may not per- tained alates (males or queens), brood, and/ or workers. sist in intensively managed farms. Thus one mechanism We calculated proportion of available nests (hollow underlying the loss of ant diversity may be increased twigs) occupied by ants for each shade type (high and nest-site limitation due to a decrease in the number of low) and sampling date. available nest sites. To study artificial nest use by twig-nesting ants, we We investigated if arboreal twig-nesting ants in cof- attached hollow bamboo twigs in coffee bushes with fee agroecosystems are nest-site limited by measuring twist ties and later checked artificial nests for ant oc- ant use of natural and artificial nests. We specifically cupants. Every two months between July 2000 and June tested if (1) nest sites are a limiting resource for twig- 2001 and December 2001 to January 2003, we placed nesting ants, (2) nest sites become more limiting with five bamboo twigs (10-20 cm long with 3-10 mm di- coffee management intensification, and (3) nest-site ameter openings at one end) on each of 20 coffee bush- limitation is a mechanism for loss of ant diversity with es, flush with branches, 0.5-2.0 m above ground. We coffee intensification. We predicted that (1) ants would used bamboo of these varying sizes to best approximate occupy most natural and artificial nests sites and (2) a the size and size variation of nests in hollow coffee higher proportion of natural and artificial nests would twigs. After two months, for a total of 14 harvests, we be occupied in more intensively managed (low-shade) removed and opened all nests and recorded the number sites because of a lower availability of nest sites in of nests recovered, number occupied, and the identity low-shade sites. Furthermore, it is possible that some of the species occupying the nest. Voucher specimens ant species (inferior competitors) will have been ex- of all ants that could not be identified to species will cluded from natural nests due to competition such that be deposited in collections at the Smithsonian Insti- natural nests are primarily occupied by a few species tution. Some nests (7.6%) fell or were lost during the of competitive dominants. We thus expected that in- two-month period, but we harvested a total of 4605 creasing nest availability would increase ant diversity artificial nests (1122 in BR, 1213 in IR, 1094 in BP, so that (1) ant richness would be greater in artificial and 1176 in IP). Again, all nests with alates, brood, nests (i.e., after less time for competitive exclusion), and/or workers were included as occupied. We calcu- and (2) ant species composition would differ between lated occupancy as the total number of occupied arti- nest types where more species occur in a higher pro- ficial nests per total nests recovered, standardized as portion of artificial nests than natural nests (i.e., natural nest occupancy per 60 days. For both natural and ar- nests are dominated by a few species).