BIOTROPICA 33(1): 110±120 2001 The Macrofauna of Water-®lled Tree Holes on Barro Colorado Island, Panama1 Stephen P. Yanoviak2 Department of Zoology, University of Oklahoma, Norman, Oklahoma 73019, U.S.A. ABSTRACT The fauna of water-®lled tree holes in Neotropical forests is not well documented. Cumulatively, 54 macroinvertebrate and 5 vertebrate taxa were found in arti®cial and natural tree holes censused over four wet seasons on Barro Colorado Island, Panama. Most of the species were in the insect order Diptera, occurred as aquatic larvae in tree holes, and were detritivore/omnivores. Half (49%) of the collected species are considered specialists in this and similar container habitats, and three invertebrate taxa were previously unknown from tree holes. Successional patterns were weak in tree holes, but some taxa predictably colonized holes shortly after they were ®lled. The mosquito Culex urichii was more common and abundant in arti®cial than in natural tree holes; occurrence frequencies and densities of most other taxa were similar between hole types. RESUMEN La macrofauna de reservorios de agua en los huecos de los aÂrboles de bosques neotropicales no ha sido bien docu- mentada. En total, 54 grupos de macroinvertebrados y 5 grupos de vertebrados fueron encontrados en huecos arti®- ciales y naturales examinados durante cuatro estaciones lluviosas en la Isla Barro Colorado, PanamaÂ. De las especies encontradas, la mayorõÂa pertenecen al orden Diptera. TambieÂn, la mayorõÂa ocurren como larvas acuaÂticas y son detrõÂvoros/omnõÂvoros. Cerca de la mitad (49%) de las especies son consideradas como especialistas en este tipo de haÂbitat y reservorios similares, y tres grupos de invertebrados no han sido reportados en los huecos de los aÂrboles. No existen patrones de sucesioÂn fuertes en los huecos de los aÂrboles, pero algunos grupos colonizan los reservorios poco tiempo despueÂs que estaÂn llenos. El mosquito Culex urichii fue la especie maÂs comuÂn y maÂs abundante en los huecos arti®ciales que en los huecos naturales; las frequencias y densidades de otras especies fueron similares entre los dos tipos de huecos. Key words: Diptera; macroinvertebrates; microcosm; mosquitoes; Panama; phytotelmata; succession; tropical moist forest. TREE HOLES ARE PHYTOTELMATA (i.e., plant-held wa- atively well studied in North America (e.g., Jenkins ters; Varga 1928) formed by the collection of rain- & Carpenter 1946, Snow 1949, Copeland 1989, fall in rot holes or natural convolutions in the Barrera 1996, Paradise 1997), Europe (Rohnert aboveground woody portions of trees (Kitching 1950, Kitching 1971), Asia (e.g., Kitching 1987; 1971). They are among the most abundant stand- Tsuda et al. 1994; Sota 1996, 1998), and Australia ing-water habitats in many tropical and temperate (e.g., Kitching & Callaghan 1982, Kitching 1983). forests. A variety of macroorganisms use tree holes In Africa and the Neotropics, most tree hole sur- as breeding sites, and many species breed exclu- veys have focused on mosquitoes and other poten- sively in this habitat. Larvae of true ¯ies (Diptera) tial disease vectors (e.g., Dunn 1927; Galindo et al. are generally the most common and diverse inhab- 1950, 1951; Vitale 1977; Lounibos 1981; Louni- itants of this system. Because they are primary bos et al. 1987). Aside from the work of Snow breeding sites for many disease vectors, especially (1949) in Guatemala and Fincke (1992a, 1998, mosquitoes (Diptera: Culicidae) and biting midges 1999) in Panama and Costa Rica, the non-mos- (Diptera: Ceratopogonidae), tree holes are econom- quito fauna of Neotropical tree holes is poorly ically important habitats. known. The macrofauna of water-®lled tree holes is rel- Water-®lled tree holes are common in lowland, seasonally moist forests of central Panama. They are ®lled each wet season (May±December), and 1 Received 26 July 1999; revision accepted 25 November typically dry up completely by late dry season (i.e., 1999. 2 Current address: Evergreen State College, Lab 1, Olym- March; Fincke 1992b). Tree holes on Barro Colo- pia, Washington 98505, U.S.A. E-mail: yanoviak@ rado Island (BCI) range in volume from ,0.05 to racsa.co.cr .50 liters (Fincke 1992b, Yanoviak 1999a) and 110 Panama Tree Hole Fauna 111 cupied) was determined for each species or higher taxon in natural holes. For holes censused more than once, I used data from the most species-rich census. Mean density (individuals per 0.5 liters where they occurred) was calculated for each spe- cies present in a subset of 100 natural holes $0.2 and #2.0 liters total volume. This range includes the mean (1.67 liters 6 0.34 SE) and median (0.46 liters) total volumes for the complete natural hole data set; holes outside this range were not used for density calculations due to problems associated with extrapolation. FIGURE 1. Size (volume) distributions of natural and arti®cial tree holes examined in this study. N 5 201 nat- The percent occurrence and mean density (as ural holes (5 of 206 holes not measured) and 153 arti®cial above) of each taxon were calculated in arti®cial holes. holes using data from 20 repeated censuses of 40 containers used in a vertical strati®cation study (Ya- noviak 1999b). Only data from small (0.65 liter) occur from ground level to the forest canopy (Ya- arti®cial holes were used for the density calculations noviak 1999b). The goal of this study was to cat- (N 5 360 censuses divided among 18 holes). I did alog the common macroorganisms associated with not use the entire arti®cial tree hole data set (1684 water-®lled tree holes on BCI and summarize their censuses) for density and frequency calculations be- ecological roles within the tree hole ecosystem. cause many of the 153 containers were in place for relatively short periods of time (creating a bias for METHODS early successional species), or were manipulated for experiments. Arti®cial holes used in the vertical This study was conducted in the lowland moist strati®cation experiment remained in place for forest of BCI, Panama (site description in Leigh et more than three years and were not experimentally al. 1996) and included several tree holes on the manipulated. The damsel¯y Megaloprepus coerula- mainland (Gigante Peninsula and Pipeline Road in tus is a common tree hole inhabitant, but was Parque Metropolitano). The same tree hole fauna found only in understory holes (Yanoviak 1999b); occurred at all sites; hereafter, I will refer only to therefore, I calculated the frequencies for this spe- tree holes on BCI. cies using data only from understory-level natural A total of 206 natural water-®lled tree holes and arti®cial holes. and 153 arti®cial tree holes (black plastic cups or Insect larvae and pupae collected in tree hole pans tied to trees and containing leaf litter, rain subsamples were returned to the insectary or lab water, and an emergent section of tree bark or balsa for rearing and identi®cation. Immature organisms wood) were sampled on BCI during the wet sea- were ®xed and stored in 70 percent ethanol, and sons 1994±1997 (1995±1997 for natural holes). adults were killed by freezing and mounted on pins The size distributions of natural and arti®cial holes or points. Larvae of chironomid midges and other examined in this study are shown in Figure 1. Tree non-mosquito Diptera were mounted on micro- holes located .2 m above the ground were accessed scope slides. These specimens were cleared in warm using various modi®cations of the single-rope 85 percent lactic acid for 15 to 30 min (Cumming climbing technique (Perry 1978). I censused the 1992), rinsed in distilled water, dehydrated in eth- macrofauna of most holes more than once in a year anol (from 70 to 100 percent in three steps), and or in successive years; 657 censuses were from nat- mounted in euparal. ural holes and 1684 from arti®cial holes. Each hole Invertebrates were identi®ed to genus and high- was censused by removing its contents to a plastic er taxonomic levels with general references (Stehr pan and recording the abundance of all species (or 1987, 1991) and specialized references listed in morphospecies) present. Except for subsamples Hurlbert and Villalobos-Figueroa (1982). Mosqui- needed to con®rm species identi®cations, all con- to species were determined with keys of Darsie tents were returned to the holes. Additional meth- (1993) and an unpublished key to the mosquitoes odological details are summarized in Yanoviak of Panama (Gorgas Mem. Lab., Ancon, Panama). (1999b). Taxonomic assistance was provided by H. P. Brown The occurrence frequency (percent of holes oc- (Coleoptera), J. Gelhaus (Diptera: Tipulidae), P. 112 Yanoviak Gnaspini (Opiliones), D. Penney (Araneae), R. including hole size and location, nutrient quality, Pinto da Rocha (Opiliones), G. Poinar (Nemato- nutrient quantity, and the presence or absence of da), J. Polhemus (Hemiptera), N. Powers (Diptera: top predators (Yanoviak 1999a, b, c, 2001). Culicidae), A. S. Rand (Vertebrata), P. Spangler Based on the association classes of Rohnert (Coleoptera), F. C. Thompson (Diptera: Syrphi- (1950), 15 percent of tree hole-inhabiting species dae), S. A. Vanan (Coleoptera), and N. Woodley on BCI are accidental, 36 percent are facultative, (Diptera: Stratiomyidae). Vouchers were deposited and 49 percent are tree hole specialists (Table 1). at the Smithsonian Tropical Research Institute, Some species (e.g., of Chironomidae and Syrphi- Panama, the U.S. National Museum, Washington dae) were tentatively classed as specialists, but too DC, the Museu de Zoologia da Universidade de little is known of their habitat speci®city to be cer- SaÄo Paulo, Brazil, and my private collection. tain of this designation. All taxa were classi®ed based on their degree of As in temperate forests (Kitching 1971, Fish & association with the tree hole habitat following Carpenter 1982), tree holes on BCI are detritus- Rohnert's (1950) scheme as translated by Kitching based systems.