In an old-growth tropical wet forest at La Selva, Costa Rica, we combined radiocarbon (¹⁴C) dating and tree-ring analysis to estimate the ages of large trees of canopy and emergent species spanning a broad range of wood densities and growth rates. We collected samples from the trunks of 29 fallen, dead individuals. We found that all eight sampled species formed visible growth rings, which varied considerably in distinctiveness. For five of the six species for which we combined wood anatomical studies with ¹⁴C-dates (ring ages), the analyses demonstrated that growth rings were of annual formation. The oldest tree we found by direct ring counting was a Hymenolobium mesoamericanum Lima (Papilionaceae) specimen, with an age of ca. 530 years at the time of death. All other sampled individuals, including very large trees of slow-growing species, had died at ages between 200 and 300 years. These results show that, even in an everwet tropical rain forest, tree growth of many species can be rhythmic, with an annual periodicity. This study thus raises the possibility of extending tree-ring analyses throughout the tropical forest types lacking a strong dry season or annual flooding. Our findings and similar measurements from other tropical forests indicate that the maximum ages of tropical emergent trees are unlikely to be much greater than 600 years, and that these trees often die earlier from various natural causes.

I analyzed patterns of variation in root mass allocation and root morphology among seedlings of woody species in relation to environmental factors in four Neotropical forests. Among forests, I explored the response of root traits to sites varying in water or nutrient availability. Within each forest, I explored the plastic response of species to different microhabitats: gaps and understory. Additionally, I explored evidence for life history correlation of root and shoot traits by comparing species differing in their successional group (light-demanding [22 spp.] or shade tolerant [27 spp.]) and germination type (species with photosynthetic cotyledons or species with reserve cotyledons). At each forest site, young seedlings from 10 to 20 species were excavated. A total of 55 species was collected in understory conditions and 31 of them were also collected in gaps. From each seedling, six morphological ratios were determined. Allocation to roots was higher in forest sites with the lowest soil resources. Roots were finer and longer in the most infertile site, while roots were deeper in the site with the longest dry season. Seedling traits did not differ between germination types. Shade tolerant species allocated more to roots and developed thicker roots than light-demanding species. Light-demanding species showed stronger plastic responses to habitat than shade tolerant species, and species with photosynthetic cotyledons showed lower plasticity than species with reserve cotyledons. Overall, these results suggest that among Neotropical species, root allocation and root morphology of seedlings reflect plant adjustments to water or nutrient availability at geographic and microhabitat scales. In addition, life history specialization to light environments is suggested by differences among groups of species in their allocation to roots and in their root morphology.

Natural hybrids between Ficus septica and two closely related dioecious species, F. fistulosa and F. hispida, were confirmed using amplified fragment length polymorphisms (AFLP) and chloroplast DNA markers. Ficus species have a highly species-specific pollination mutualism with agaonid wasps. Therefore, the identification of cases in which breakdown in this sophisticated system occurs and the circumstances under which it happens is of interest. Various studies have confirmed that Ficus species are able to hybridize and that pollinator-specificity breakdown can occur under certain conditions. This study is the first example in which hybrid identity and the presence of hybrids in the natural distribution of parental species for Ficus have been confirmed with molecular markers. Hybrid individuals were identified on three island locations in the Sunda Strait region of Indonesia. These findings support Janzen's (1979) hypothesis that breakdown in pollinator specificity is more likely to occur on islands. We hypothesized that hybrid events could occur when the population size of pollinator wasps was small or had been small in one of the parental species. Later generation hybrids were identified, indicating that backcrossing and introgression did occur to some extent and that therefore, hybrids could be fertile. The small number of hybrids found indicated that there was little effect of hybridization on parental species integrity over the study area. Although hybrid individuals were not common, their presence at multiple sites indicated that the hybridization events reported here were not isolated incidences. Chloroplast DNA haplotypes of hybrids were not derived solely from one species, suggesting that the seed donor was not of the same parental species in all hybridization events.

Pollinator visual systems differ considerably among broad groupings such as bees, bats, and birds, and it has been proposed that factors shaping the diversity of flower color in tropical plants include differences in pollinator perceptual abilities. Within the pollinators of the Neotropics, one major difference between taxa is that hummingbirds perceive color well across a broad range of wavelengths from 300–660 nm whereas most bees perceive color well over a narrower range spanning 300–550 nm. Thus, hummingbirds can see red and other long-wavelength reflection much better than bees. Another factor that could potentially influence flower color in tropical forests is the difference in light availability among habitats such as gaps, canopy, and forest understory. I conducted a survey of floral color in four Neotropical forests using a portable spectroradiometer to provide an unbiased measure of color reflectance. The primary pollinator agents and light habitats of each plant species were classified using primary literature or accounts of direct observations by experts. Flower color was not influenced by differences in light availability between open and closed habitats but was influenced by pollinator visual systems. Specifically, insect flowers reflected across a broad range of the spectrum but hummingbird flowers reflected mostly long-wavelength light (typically median wavelength >585 nm). Thus, hummingbird-pollinated flowers are not tuned specifically to hummingbird color sensitivity but instead may decrease conspicuousness to bees and other insects that have poor visual sensitivity to long-wavelength color.

The selective advantage of Müllerian mimicry in nature was investigated by releasing live mimetic and nonmimetic butterflies close to wild, aerial-hunting tropical kingbirds (Tyrannus melancholicus) and cliff-flycatchers (Hirundinea ferruginea) in three Amazon habitats (rain forest, a city, and “canga” vegetation). Only mimetic butterflies elicited sight-rejections by birds, but protection conferred by mimicry was restricted to sites in which both predators and mimics co-occurred, as in the case of six mimicry rings at a forest site and two at a city site. Most other Müllerian mimics released at city and canga vegetation were heavily attacked and consumed by birds. These results appear to reflect the birds' previous experiences with resident butterfly faunas and illustrate how birds' discriminatory behavior varied among habitats that differed in butterfly species and mimicry ring composition.

The effect of the fossorial land crab Gecarcinus quadratus (Gecarcinidae) on patterns of accumulation and distribution of leaf litter was studied for two years in the coastal primary forests of Costa Rica's Corcovado National Park. Within this mainland forest, G. quadratus achieve densities up to 6 crabs/m² in populations extending along the Park's Pacific coastline and inland for ca 600 m. Crabs selectively forage for fallen leaf litter and relocate what they collect to burrow chambers that extend from 15 to 150 cm deep (N = 44), averaging (±SE) 48.9 ± 3.0 cm. Preference trials suggested that leaf choice by crabs may be species-specific. Excavated crab burrows revealed maximum leaf collections of 11.75 g dry mass—2.5 times more leaf litter than collected by square-meter leaf fall traps over several seven-day sampling periods. Additionally, experimental crab exclosures (25 m²) were established using a repeated measures randomized block design to test for changes in leaf litter as a function of reduced crab density. Exclosures accumulated significantly more (5.6 ± 3.9 times) leaf litter than did control treatments during the wet, but not the dry, seasons over this two-year study. Such extensive litter relocation by land crabs may affect profiles of soil organic carbon, rooting, and seedling distributions.

We examined genetic diversity of howler monkeys (Alouatta palliata) from Costa Rica. Blood samples of howler monkeys were collected at various locations in Costa Rica, and electrophoresis of total plasma proteins yielded no variation. We also conducted starch gel electrophoresis of red cell isozymes and did not find variation for any of the 14 loci analyzed (i.e., ACP, ADA, CA2, EST, GPI, IDH, LDH-1, LDH-2, MDH, PGD, PGM-1, PGM-2, SOD, and TPI). These findings were compared with the levels of genetic variation for A. seniculus and A. belzebul from one Brazilian population. Four of the 14 isozymes (ADA, GPI, PGD, and SOD) showed more than one allele for these species. Both A. seniculus and A. belzebul from Brazil showed similar levels of genetic variation. The potential causes of the low genetic variation in A. palliata from Costa Rica are discussed.

Given current accelerated trends of tropical land conversion, forest fragments are being incorporated into many conservation programs. For investing in fragments to be a viable conservation strategy, forest fragments must maintain their ecological integrity over the long term. Based on fieldwork in 22 forest fragments in the crater lakes region of western Uganda and in the continuous forest of Kibale National Park, we examined (1) seed predation on experimentally dispersed seeds, (2) abundance and composition of the dung beetle community that may play a major role in removing seeds from sites of high seed predation, and (3) compared the fragments' seedling community composition to adult tree community composition and the seedling community in continuous forest. First, the rate of seed removal at experimental stations was lower in forest fragments (85% remaining after 1 day) than at stations in the continuous forest (79% remaining) and the probability of stations being discovered by seed predators was lower in fragments (23%) than in the intact forest (41%). Second, there was a 62 percent decline in fragment dung beetle abundance. The magnitude of this decline varied among dung beetle guilds that process dung and seeds in different fashions. The abundance of large rollers that move large seeds away from sites of defecation did not differ, but medium and smaller rollers and burying beetles that process small and medium-sized seeds were less common in the fragments than in the intact forest. Finally, we compared the seedling community composition relative to adult tree community composition by identifying all adult trees in each fragment and by sampling the composition of the seedling community. We found some evidence to suggest that there was movement of seeds among forest fragments by large-bodied dispersers, particularly chimpanzees (Pan troglodytes) and hornbills (Ceratogymna subcylindricus).

Habitat fragmentation and the widespread creation of habitat edges have recently stimulated interest in assessing the effects of ecotones on biodiversity. Ecotones, natural or anthropogenic, can greatly affect faunal movement, population dynamics, species interactions, and community structure. Few data exist, however, on insect community response to forest–savanna ecotones, a natural analog to anthropogenically cleared areas adjacent to forest. In this study, the abundance, total biomass, average individual biomass, and distribution of scarabaeine dung beetles were examined at a sharp tropical evergreen forest–savanna ecotone in Santa Cruz, Bolivia. The abundance, total biomass, and average individual biomass of dung beetles varied significantly across the forest, edge, and savanna habitats. Species richness (S_obs) also varied significantly across the three habitats, but statistical estimations of true species richness (S_est) did not. Habitat specificity of the dung beetles in this study was extremely high. Of the 50 most common species collected during the study, only 2 species were collected in both the forest and savanna habitats, signaling nearly complete community turnover in just a few meters. Strong edge effects were evidenced by the decline in abundance, total biomass, and species richness at the forest–savanna boundary.

As tropical forest fragmentation accelerates, scientists are concerned with the loss of species, particularly those that play important ecological roles. Because bats play a vital role as the primary seed dispersers in cleared areas, maintaining healthy bat populations is critical to natural forest regeneration. Observations of foraging bats suggest that many Neotropical fruit-eating species have fairly general habitat requirements and can forage in many different kinds of disturbed vegetation; however, their roosting requirements may be quite different. To test whether or not general foraging requirements are matched by equally broad roosting requirements, we used radiotelemetry to locate roost sites of two common frugivorous bat species (Sturnira lilium and Artibeus intermedius) in a fragmented forest in southeastern Mexico. Sturnira lilium roosted inside tree cavities and selected large-diameter roost trees in remnant patches of mature forest. Fewer than 2 percent of trees surveyed had a mean diameter equal to or greater than roost trees used by S. lilium. Artibeus intermedius roosted externally on branches and vines and under palm leaves and selected roost trees of much smaller diameter. Compared to random trees, roost trees chosen by A. intermedius were closer to neighboring taller trees and also closer in height to these trees. Such trees likely provide cryptic roosts beneath multiple overlapping crowns, with sufficient shelter from predators and the elements. While males of A. intermedius generally roosted alone in small trees within secondary forest, females roosted in small groups in larger trees within mature forest and commuted more than three times farther than males to reach their roost sites. Loss of mature forest could impair the ability of frugivorous bats to locate suitable roost sites. This could have a negative impact on bat populations, which in turn could decrease forest regeneration in impacted areas.

Litterfall was collected in a 0.64 ha area for three months, using 25 litter traps. Resampling methods were used to calculate the total litterfall mass grand mean and coefficient of variation (CV) for each month using different numbers of litter traps (2–25). Based on the variation of those two statistics, it was possible to determine the minimum number of litter traps necessary to achieve accuracy (variation in grand mean) and precision (magnitude of CV).

I set up a 250 m² plot and found after five years that (1) grass dominated by ferns and woody species was gradually increasing over time; (2) woody genera (Syzygium, Calophyllum, and Clidemia) common here have not been found elsewhere on the island and “successional” trees, such as Cecropia sp. and Shefflera sp., were completely absent; (3) smaller, earlier trees were also found away from the forest; and (4) the plot had multiple strata and was recovering forest structural characteristics such as productivity and richness.

The relative importance of cotyledons and leaves for seedling survival was evaluated using a factorial field experiment on three neotropical tree species with contrasting cotyledon functional morphologies (photosynthetic, epigeal reserve vs. hypogeal reserve). In all species, cotyledon and leaf removal shortly after leaf expansion had additive negative effects on seedling survival over 7 weeks. Carbon supplies from cotyledons and other carbohydrate reserves apparently enhanced ability of seedlings to cope with herbivory and disease.

Radiocarbon dating and ⁴⁰Ar/³⁹Ar analysis of overlying tephra indicate that plant fossil assemblages exposed by stream erosion and well construction in and near La Selva Biological Station in eastern lowland Costa Rica are Pleistocene in age. We identified plant taxa based on wood, leaves, fruits, seeds, pollen, and spores examined from three sites at ca 30 m elevation. Extrapolating from modern ranges and surface temperature lapse rates suggests paleotemperatures 2.5–3.1°C cooler than at present.

We quantified patterns of vegetation removal and light availability above Atta colombica nests on Barro Colorado Island, Panama. Ants cleared vegetation less than 1 cm in diameter from an area of 77 m², and up to 3 m above ground level. Overall light availability 1.5 m above ground level was 49 percent greater at ant nest sites than at sites in undisturbed understory. These higher light levels fell within the range known to enhance growth of both shade tolerant and pioneer species.