Epiphytic bromeliad communities in secondary and mature forest in a tropical premontane area

We analyzed the differences in species richness, community composition, population structure and within-tree location of epiphytic bromeliads in contiguous secondary and mature forests in a premontane area in Costa Rica. Diversity in the mature forest was highest, and the communities differed in their composition as well as in the recruitment rates of the dominant species. Guzmania monostachia and Catopsis nutans dominated the secondary forests, whereas Tillandsia fasciculata and T. tricolor were more abundant in the mature forest. The secondary forest species showed high rates of seedling recruitment while the opposite was found for the mature forest species. Species presence and abundance among and within habitats did not correlate with their physiological (i.e. CAM vs. C₃ photosynthesis) or morphological attributes. The spatial distribution patterns were similar among habitats; bromeliads tended to aggregate on a few relatively large phorophytes. The species shared a similar vertical stratification within habitats, except for the two dominant species in the early and mid-successional stages, although its ecological implication is not clear. With some exceptions, conspecifics of different ages were located on similar substrate types (i.e. stems, primary, secondary, or tertiary branches) within the tree-crowns, which suggests limited within-tree dispersion. Differences in species composition and rates of seedling recruitment among secondary and mature forest may arise from ecophysiological differences among species; however, the combined effect of seed availability and dispersal differences may have a larger influence. Thus, epiphyte community assembly can only be understood when the differences in habitat conditions, the availability of propagules, their dispersal characteristics and requirements for seedling establishment are known.     

Microclimatic factors associated with elevational changes in army ant density in tropical montane forest

Abstract.  1. The density (rate of encountering foraging raids) and species richness of army ants (Formicidae: Ecitoninae, and behaviourally convergent Ponerinae) was measured in montane tropical forest. Above-ground and subterranean army ant raids were sampled using standard protocols at four sites across an elevational gradient (1200–1650 m above mean sea level) in and near cloud forest in the area of Monteverde, Costa Rica.
2. Mean ambient temperature differed among sites, and decreased with elevation. For the above-ground foraging army ant species, raid rates also declined with elevation. Surface army ant raid rates, however, were not affected by day to day weather variation within sites (temperature, cloud cover, or precipitation).
3. For the underground foraging army ant species, raid rates did not vary directionally with elevation, and subterranean raid rates were not affected by day to day weather variation within sites.
4. Army ant species richness was not directionally related to elevation, and species sharing among sites was generally high.
5. Army ant community structure changes with elevation in Neotropical montane forest, and the results suggest that the strongest effects are of temperature regimes on the density of raids. These findings provide a baseline against which to detect changes in army ant communities that may accompany directional climate change in tropical cloud forests.     

Microclimatic parameters influencing nitrogen fixation in the phyllosphere in a Costa Rican premontane rain forest

The acetylene reduction method was used to measure nitrogen fixation in the phyllosphere of attached leaves of different phorophytes under natural conditions in a premontane rain forest in Costa Rica. Maximum rates of nitrogen fixation (26 ng N · cm⁻² leaf area · h⁻¹) – mainly due to the activity of two species of Scytonema (Cyanobacteria) – were measured in the rainy season in bright sunlight. Rates of nitrogen fixation were correlated with the leaf area covered by Scytonema. In periods without precipitation the fixation activity decreased to zero within 2–3 days. As long as the epiphylls were sufficiently supplied with water, other microclimatic factors like temperature and light intensity also influenced nitrogen fixation rates, but to a lesser extent. Relative humidity and species of phorophyte showed no direct influence. It was concluded that the most important factor for nitrogen fixation in the phyllosphere was the availability of liquid water. Linking these results to meteorological data, the input of nitrogen by biological nitrogen fixation in the phyllosphere in the investigation area was estimated to be as much as 1.6 ± 0.8 kg N · ha⁻¹ · year⁻¹ per unit of leaf area index (LAI). For an LAI of 2 for the understory the nitrogen input would vary between 2 and 5 kg N · ha⁻¹ · year⁻¹. This work also demonstrates the importance of detailed knowledge of variation in microclimate throughout the year as a basis for extrapolation of the annual nitrogen input. Received: 21 December 1997 / Accepted: 14 June 1998     

Wasp florivory decreases reproductive success in an epiphytic bromeliad

Flower predation or florivory may alter the sexual expression, floral display, and reduce the reproductive success in plants. In this article, we estimated the effect of florivory on the reproductive success of the epiphytic bromeliad Werauhia gladioliflora during 2001–2003 in a premontane area in Costa Rica. Floral buds of W. gladioliflora are attacked by the wasp Eurytoma werauhia (Eurytomidae), which inhibits flower anthesis. Nearly, a quarter of the sampled population flowers were lost due to wasp infection and it accounted for nearly 70% of the reduction in fruit set. Flowers located on the inflorescence upper segment had a higher attack incidence. In the studied population, florivory has a major role in the reproductive success of W. gladioliflora. Florivory on epiphytic plants is scarcely reported in the literature, but evidence suggests that this phenomenon is more spread in the epiphytic community. Whether florivory in W. gladioliflora is a selective pressure influencing the reproductive success or a localized factor operating at the studied population is a subject requiring additional data.     

Conservation of tropical forest birds in countryside habitats

The pressing need to increase agricultural production often seems at odds with conserving biodiversity. We find that if managed properly, the tropical countryside may provide a substantial opportunity for tropical bird conservation. We detected 144 bird species from 29 families in agricultural areas outside of extensive native forest in southern Costa Rica. The majority of the species detected were observed foraging, often kilometres from extensive native forest. We estimate that 46% of those native to this region (excluding nocturnal species and waterfowl) are utilizing the countryside in some manner. Forecasts of biodiversity change under various land-use scenarios indicate that policies that affect habitat composition could greatly impact the persistence of these species in the countryside. In particular, if tall trees and edge habitats were removed from this landscape, we predict that bird richness in the countryside would decline by approximately 40%.     

Mating system dynamics of Ocotea tenera (Lauraceae), a gynodioecious tropical tree

Progeny arrays of Ocotea tenera (Lauraceae), a gynodioecious tree endemic to Costa Rica, were electrophoretically surveyed for allozyme variation to estimate the outcrossing rate in the overall population and to test for differences in outcrossing rates between hermaphroditic and female trees. Multilocus outcrossing rate estimates across 3 yr indicated O. tenera predominantly outcrosses. However, significant heterogeneity in single-locus outcrossing rates was found among loci. Two loci (Fe1, Fe2) gave high outcrossing estimates, and a third locus (Gdh) gave much lower outcrossing estimates. Heterogeneity in Gdh pollen allele frequencies, consanguineous matings, and selection against homozygous zygote genotypes at the Fe1 and Fe2 loci are factors contributing to the discrepancy in outcrossing rate estimates among loci. There were no differences in the mating systems of hermaphroditic and female trees, which suggests that factors beyond prevention of self-fertilization may have also promoted the evolution of gynodioecy in O. tenera.     

Mechanisms driving plant functional trait variation in a tropical forest

Plant functional trait variation in tropical forests results from taxonomic differences in phylogeny and associated genetic differences, as well as, phenotypic plastic responses to the environment. Accounting for the underlying mechanisms driving plant functional trait variation is important for understanding the potential rate of change of ecosystems since trait acclimation via phenotypic plasticity is very fast compared to shifts in community composition and genetic adaptation. We here applied a statistical technique to decompose the relative roles of phenotypic plasticity, genetic adaptation, and phylogenetic constraints. We examined typically obtained plant functional traits, such as wood density, plant height, specific leaf area, leaf area, leaf thickness, leaf dry mass content, leaf nitrogen content, and leaf phosphorus content. We assumed that genetic differences in plant functional traits between species and genotypes increase with environmental heterogeneity and geographic distance, whereas trait variation due to plastic acclimation to the local environment is independent of spatial distance between sampling sites. Results suggest that most of the observed trait variation could not be explained by the measured environmental variables, thus indicating a limited potential to predict individual plant traits from commonly assessed parameters. However, we found a difference in the response of plant functional traits, such that leaf traits varied in response to canopy‐light regime and nutrient availability, whereas wood traits were related to topoedaphic factors and water availability. Our analysis furthermore revealed differences in the functional response of coexisting neotropical tree species, which suggests that endemic species with conservative ecological strategies might be especially prone to competitive exclusion under projected climate change.     

Differences in vital demographic rates in three populations of the epiphytic bromeliad,Werauhia sanguinolenta

Since the response to differences in resource availability is most pronounced in smaller individuals of vascular epiphytes such as Werauhia sanguinolenta Cogn. et Marchal (Bromeliaceae), I expected variation in growth and survival of small individuals to play an important role in the dynamics of entire populations. Four annual censuses (2002–2005) of three study populations, which were located across the isthmus of Panama, allowed me to construct stage transition matrices, and to conduct growth analysis and elasticity analysis. Differences between populations were highly consistent through time, but, contrary to expectations, hardly related to the comportment of smaller plants. For example, although average mortality rates were highest at the driest site, close to the Pacific, small plants were not predominantly affected. Similarly, although the highest relative growth rates (RGR) of individuals and the highest population growth rates (λ) were found in the population with the highest moisture input, which was located close to the Atlantic coast, this was not due to a particularly strong stimulation of RGR in small plants. Elasticity analysis indicated rather small differences in the importance of the three demographic processes growth, survival, and reproduction for population growth in the three populations, but invariably identified the survival of large tanks as the single most important process determining λ.     

Light microhabitats,growth and photosynthesis of an epiphytic bromeliad in a tropical dry forest

In the tropical dry forest of Dzibilchaltún, Yucatan, Mexico, light microhabitats, tissue acidity changes, chlorophyll fluorescence and growth were investigated for the epiphytic bromeliad Tillandsia brachycaulos Schltdl. Most individuals occurred in sites between 20 and 60% of the height of the tree, and directly on the main trunk or branches closed to the main trunk. During the dry season, individuals received about nine times more photon flux density (PFD) than during the rainy season. Individuals that occurred under 30–59% of the daily ambient PFD showed greater leaf length increase and monthly leaf production, and produced more flowers than individuals in the brighter (>60% of ambient PFD) and the darker (11–29% of ambient PFD) light microhabitats. Well-watered plants acclimated to deep shade (6% of ambient PFD) showed values of tissue acidity similar to plants under higher light treatments. Well-watered plants under 60% of daily ambient PFD showed low values of maximum quantum efficiency at midday during most of the year but showed similar values of tissue acidity than plants under 6 and 30% of ambient PFD. Leaf temperatures of plants in low sites within the canopy and on the forest floor were significantly higher than the air during about 1 h, indicating that those plants could not dissipate the excess of heat. Individuals located in the most shaded microhabitats could also receive less amount of water from rainfall and dew. Therefore, it would be less damaging for this epiphytic bromeliad to occur in the most exposed microhabitats than in the most shaded microhabitats in this tropical dry forest.     

Sandfly distribution and abundance in a tropical rain forest

1. The distribution patterns of sandflies (Diptera: Psychodidae) upon tree buttresses were studied in tropical rain forest at Finca la Selva in the Caribbean lowlands of Costa Rica. 2. Four species of sandfly, Lutzomyia shannoni Dyar, L. trapidoi F. & H., L. ylephiletor F. & H. and L. vespertilionis F. & H. comprising 97% of those caught, used tree buttresses as diurnal resting sites. Their distribution on the buttresses was aggregated. 3. During the dry season tree species had no significant effect upon the distribution of the sandflies. However, during the wet season the distributions of two of the species, L. trapidoi and L. ylephiletor, were significantly affected by the species of tree; it is suggested that some species of tree may provide greater protection from rainfall than others. 4. L. vespertilionis was restricted to a single buttress on each positive tree. Distribution of this species is evidently determined by the distribution of its host animal, the bat (Emballonuridae). Female flies feed upon the bat's blood and male flies may be attracted to the bat as it provides a source of female sandflies. It is suggested, therefore, that tree buttresses serve as sandfly swarming sites. 5. Within a large buttress the sandflies are not randomly distributed but are aggregated in particular areas. Within these aggregations, the sandflies are vertically zoned upon the buttress with a shift in species composition with height. Two hypothesis were suggested to account for this distribution pattern: a response to an environmental gradient or an interaction between the four species of fly.     

Patterns of sandfly distribution in tropical forest: a causal hypothesis

1. In tropical rain forest, phlebotomine sandflies (Diptera: Psychodidae), such as Lutzomyia vespertilionis and L.ylephiletor, have an aggregated distribution on their tree buttress diurnal resting sites, as studied during 1987-88 at Finca la Selva in the Caribbean lowlands of Costa Rica. 2. Experimental transfer of flies to trees not used as resting sites indicated that many apparently suitable sites remain unoccupied. 3. Observations of sandflies on the buttresses revealed that males and females are juxtaposed more frequently than expected by chance alone. Courtship behaviour by three of the four species of sandfly studied was observed on the buttresses. 4. It is suggested that the use of buttresses as swarming sites for mating behaviour is more likely to account for the observed distribution patterns of sandflies than their use of buttresses simply as diurnal resting sites.     

Clothing color mediates lizard responses to humans in a tropical forest

Identifying how ecotourism affects wildlife can lower its environmental impact. Human presence is an inherent component of ecotourism, which can impact animal behavior because animals often perceive humans as predators and, consequently, spend more time on human-directed antipredator behaviors and less on other fitness-relevant activities. We tested whether human clothing color affects water anole (Anolis aquaticus) behavior at a popular ecotourism destination in Costa Rica, testing the hypothesis that animals are more tolerant of humans wearing their sexually selected signaling color. We examined whether clothing resembling the primary signaling color (orange) of water anoles increases number of anole sightings and ease of capture. Research teams mimicked an ecotourism group by searching for anoles wearing one of three shirt treatments: orange, green, or blue. We conducted surveys at three different sites: a primary forest, secondary forest, and abandoned pasture. Wearing orange clothing resulted in more sightings and greater capture rates compared with blue or green. A higher proportion of males were captured when wearing orange whereas sex ratios of captured anoles were more equally proportional in the surveys when observers wore green or blue. We also found that capture success was greater when more people were present during a capture attempt. We demonstrate that colors “displayed” by perceived predators (i.e., humans) alter antipredator behaviors in water anoles. Clothing choice could have unintended impacts on wildlife, and wearing colors resembling the sexually selected signaling color might enhance tolerance toward humans.     

Reproductive biology of Lactoris fernandeziana (Lactoridaceae)

Lactoris fernandeziana, monotypic in its family, is endemic to the cloud forests of Robinson Crusoe Island. Although there has been considerable study of the relationships of Lactoris, as a rare species and as a putative primitive paleoherb, little is known of its reproductive biology. Knowledge of the latter is essential for effective conservation programs. The species is gynomonoecious. The overall proportion of flowers is ∼1 female:1 hermaphrodite. The inconspicuous semipendulous green flowers, usually in mixed-gender inflorescences, do not produce rewards. Hermaphrodite flowers are herkogamous and protogynous. Pollen grains are shed from the extrorse anthers in permanent dry tetrads. There is a mean of 12879 tetrads per hermaphrodite flower. Both flower types bear an average of ∼18 ovules. The P/O (pollen/ovule) ratios imply facultative or obligate xenogamy, but hand pollinations show that Lactoris is self-compatible. No floral visitors were ever observed, but stigmata of open-pollinated flowers bore tetrads, and 64% of such styles had pollen tubes. Flowers enclosed in large mesh (1 mm) bags bore similar numbers of tetrads and pollen tubes. Thus, we conclude that Lactoris is anemophilous, a syndrome perhaps reflected by the P/O ratio. Low genetic diversity (isozymes and DNA) supports selfing and implies limited distance wind pollen dispersal. The small size of the island, the ± 1000 extant Lactoris plants, coupled with anemophily, self-compatibility, and pendant flower position, have yielded a geitonogamous system with high seed set and low genetic diversity. If inbreeding depression is expressed, it is in seed germination and seedling vigor, for Lactoris is very difficult to cultivate. For this species, effective conservation practices need to focus on habitat preservation and promotion of outcrossing.     

Bioassays of nutrient limitation in a tropical rain forest soil

Summary Six speices of shrubs and one large herb with contrasting life history patterns were used as bioassays of nutrient availability in a Costa Rican lowland rain forest soil. Growth responses of the herb (Phytolacca rivinoides, Phytolaccaceae) confirmed soil measurements indicating high availability of N and potentially limiting levels of P, K, Mg and Ca. Growth responses of the shrub species (Miconia spp., Melastomataceae and Piper spp., Piperaceae) to a complete nutrient fertilizer were generally less than that of Phytolacca. Lack of a strong shrub response to +P fertilization is probably due to mycorrhizal associations and slower growth rates of woody species. In general, increased growth did not occur at the expense of phenolic production in the leaves. The results emphasize that assessment of specific nutrient limitations to plant growth vary depending on species selected for the bioassay, even among species from the same community.     

Seasonal insectivory by Black‐headed Trogons,a tropical dry forest frugivore

ABSTRACT Fruit‐eating birds are important seed dispersers in tropical forests, but little is known about the extent to which they rely on insects or how their diets vary seasonally. We used field observations of focal adults to quantify the diets of adult and nestling Black‐headed Trogons (Trogon melanocephalus) at nine nests in a lowland dry forest in Costa Rica. From May 2004 to August 2004, we documented 540 food deliveries to nests and 1080 food items consumed by adults. Adult and nestling trogons were largely insectivorous, feeding mainly on moth caterpillars (Lepidoptera). Fruit accounted for only 10.5% of items consumed by adults and 2.2% of items delivered to nestlings (6.1% and 0.6% of estimated dry mass, respectively). Adult and nestling diets differed significantly in both composition and prey size, with adults consuming more fruit and fewer large insects (Phasmatodea and Mantodea) than nestlings and eating more types of arthropods and fruit. Although both adults and nestlings relied heavily on moth larvae, adults preferentially consumed small caterpillars and delivered large ones to their nestlings. In addition, the proportion of large caterpillars delivered to nests remained constant throughout the nestling period, whereas the proportion of large caterpillars eaten by adults declined significantly with nestling age. Overall, arthropods delivered to nests averaged 70% heavier than those consumed by adults (estimated dry mass). Our results suggest that Black‐headed Trogons time reproduction to coincide with arthropod rather than fruit abundance, a pattern that may be more common among omnivorous forest birds than previously recognized.