ABSTRACT. Seeds with ‘imitation arils’ appear wholly or partially covered by pulp or aril but actually carry no fleshy material. The mimetic seed hypothesis to explain this phenomenon proposes a parasitic relationship in which birds are deceived into dispersing seeds that resemble bird-dispersed fruits, without receiving a nutrient reward. The hard-seed for grit hypothesis proposes a mutualistic relationship in which large, terrestrial birds swallow the exceptionally hard mimetic seeds as grit for grinding the softer seeds on which they feed. They defecate, dispersing the seeds, and abrade the seed surface, enhancing germination. Any fruit mimicry is incidental. Fruiting trees of Ormosia spp. (Leguminosae: Papilionoideae) were observed to ascertain mechanisms of seed dispersal and the role of seemingly mimetic characteristics of the seeds in that dispersal. Seed predation and seed germination were also examined. Ormosia isthamensis and O. macrocalyx (but not O. bopiensis) deceived arboreally-foraging frugivorous birds into taking their mimetic seeds, although rates of seed dispersal were low. These results are consistent with the mimetic seed hypothesis. On the other hand, the rates of disappearance of seeds from the ground under the Ormosia trees, hardness of the seeds, and enhancement of germination with the abrasion of the seed coat are all consistent with the hard-seed for grit hypothesis.

RESUMEN. Semillas con arilos falsos aparecen estar cubiertas en parte o completamente por pulpa o arilo, pero en realidad no llevan ninguna materia carnosa. El hipótesis semilla mímica propone que las semillas parecen frutos carnosos cuyas semillas están dispersadas por aves y que engañan las aves a dispersar sus semillas sin recibir una recompensa nutritiva — una relación parasítica. El hipótesis semilla dura para arenisca propone que aves grandes y terrestres tragan las semillas mímicas y excepcionalmente duras como arenisca para moler las semillas más suaves en que se alimentan; las aves defecan y dispersan las semillas, y las rascan, lo cual mejora la germinación — una relación mutua. Cualquier mimetismo es incidente. Se observaron árboles de Ormosia espp. (Leguminosae: Papilionoideae) con frutos para averiguar los mecanismos de dispersión de semillas y el papel que hacen las características aparentemente mímicas de sus semillas en esa dispersión. Se examinaron también la depredación y germinación de semillas. Las semillas mímicas de Ormosia isthamensis y O. macrocalyx (pero no O. bopiensis), engañaron aves frugivoras y arbóreas en comerlas, aunque las tasas de dispersión eran bajas. Estos resultados son consistente con el hipótesis semilla mímica. En cambio, las tasas de desaparición de semillas caídas de Ormosia, dureza de las semillas, y mejoramiento de germinación con la raedura de las capas de las semillas son consistente con el hipótesis semilla dura para arenisca.

Dual stable isotope analyses (δ13C and δ15N) of fiddler crabs from a forest-fringed, land-locked lagoon in Puerto Rico indicated the differential assimilation of material from ingested sediments. Fiddler crabs preferentially selected for nitrogen-fixing benthic microalgae (cyanobacteria) over vascular plant detritus. These results question the assumption that mangrove detritus is always the principal source of energy to estuarine consumers. Previous research from this lagoon as well as from Amazonia suggests that the magnitude of land-water ecotonal coupling may be low for these particular tropical systems where benthic algal productivity is presumably high.

Myrmecophytic species of the Paleotropical plant genus Macaranga (Euphorbiaceae) have hollow stems that are almost always occupied by ants of the genus Crematogaster and scale insects of the family Coccidae (Hemiptera: Coccoidea). The coccids have a cryptic endophytic lifestyle and are confined to this microhabitat. They are much more diverse than previously recognised. First data are presented on the diversity, prevalence, specificity and distribution of the coccids associated with myrmecophytic Macaranga species. Twenty-two species of Coccidae in total, including 15 previously unknown from Macaranga, were discovered from 19 species of Macaranga in Peninsular Malaysia and Borneo. The original describers tentatively assigned the known coccid species to Coccus (Coccinae) but the Macaranga coccids still require taxonomic research to establish their correct placing. The coccids varied in their host-plant specificity from species that occurred in most of the sampled Macaranga to one species that was found almost exclusively only on a single host species. In addition to their occurrence on Macaranga, only three species, C. macarangae and C. secretus and morphospecies C. 214 were found on rare occasions in the stem interior of a few other myrmecophytes and in a non-myrmecophytic liana, but did not regularly colonise these plants. Most of the coccids can be regarded as highly specific at the plant genus level.

ABSTRACT. Factors contributing to the resistance of gallery forests in savannas to the entry of fire were investigated using field observations and manipulation experiments. Mass of savanna fuels did not decrease close to forest boundaries, and in some instances increased, while savanna fuels adjacent to forests were moister than in the savanna beyond for only 1 d after rainfall. A fuel drying experiment conducted in both forest and savanna microclimates indicated that both fuel type and microclimate contributed to the resistance of forests to fire entry, although the former played a larger role. While savanna fuels in a savanna microclimate became ignitable in c. 1 d after rain, forest fuels in a forest microclimate required 4 wk to achieve ignitability. A further experiment juxtaposing forest fuels to burning savanna indicated that fire entry into forests was faciliated by deep root mats and the presence of a superficial litter layer, both of which become attenuated at the forest/savanna contact. It is concluded that fuels in these forests can reach an ignitable state late in the dry season, but that frequent fire entry is probably precluded by the tendency of savanna fires to occur earlier in the dry season and by discontinuities in fuels at the savanna/forest contact.

RESUMEN. Se investigaron los factores que contribuyen a la resistencia de la entrada del fuego a los bosques en galería de las savanas mediante observaciones de campo y experimentos de manipulación. La masa de los combustibles de la savana no decrecieron cerca de los bordes del bosque; por el contrario, en algunos casos se incrementaron, mientras que los combustibles de la savana cercanos al bosque eran mas húmedos que en la savana restante, solamente por un día despues de un evento de lluvia. Un experimento de secado de combustible, que fue conducido en los microclimas de bosque y de savana, indica que tanto el tipo de combustible como el microclima contribuyen a la resistencia de los bosques a la entrada del fuego, aunque el primer factor tuvo un papel de mayor importancia. Mientras que los combustibles de la savana en el microclima de la savana se volvieron ‘encendibles’ cerca de un día despues de lluvia, los combustibles del bosque dentro del microlima del bosque tomaron cuatro semanas para llegar a dicho estado. Un experimento que sobrepuso combustibles del bosque a la savana ardiente indicó que la entrada del fuego a los bosques se facilitó por la presencia de mantillos orgánicos profundos y de una capa superficial de hojarasca, presencia que se ve atenuada en la zona de contacto bosque/savana. Se concluye que los combustibles en estos bosques pueden alcanzar un estado de ignición tarde en la estación de sequía, pero que la entrada frecuente del fuego es probablemente evitada por la tendencia que tienen los fuegos de savana a presentarse temprano en la estación seca y por las discontinuidades de combustibles en la zona de contacto savana/bosque.

The contribution of sprouts to the long-term survival of Chlorocardium rodiei, following logging was investigated. Differently aged sprout populations on cut stumps were surveyed to determine the numbers of sprouts produced, probability of survival and growth rates. Fifty-five to 70% of the stumps produced sprouts. The proportion of stumps that sprouted was lower for the hollow stumps than for the intact ones and the number of sprouts produced per stump was reduced. No significant correlation was found between stump diameter and the number of sprouts. Some sprouts survived for at least 15 y (usually one per stump), and these had a significantly larger average diameter relative to the sprouts at younger sites. The number of sprouts and the diameter of the largest sprout showed a significant positive correlation, suggesting that the maintenance of the inherited stump root system is shared among the sprouts so that the costs per sprout are reduced. We conclude that sprouting on cut stumps may have the potential to contribute to the maintenance of C. rodiei in the canopy at logged sites.

ABSTRACT. The spatial heterogeneity of element fluxes was quantified by measuring litterfall, throughfall and litter decomposition for 1 y in 30 randomly located sampling areas in a lowland dipterocarp rain forest. The idea tested was that turnover of elements is more variable than turnover of dry matter in a forest with extremely high tree species diversity. In spite of the low fertility of the soil (an ultisol), total litter production (leaves, trash, and wood <2 cm in diameter) was high (1105 g m−2 y−1) with inputs to the forest floor of carbon, nitrogen, phosphorus, calcium, magnesium, potassium, manganese and iron of 550, 15.3, 0.47, 6.26, 2.49, 4.75, 0.95 and 0.14 g m−2 y−1 respectively. Throughfall was 81% of the annual rainfall and transferred 22.2, 1.37, 0.14, 1.07, 0.67, 0.39, 7.92, <0.06, and <0.06 g m−2 y−1 of organic carbon, nitrogen (all forms), phosphorus, sulphur, calcium, magnesium, potassium, manganese and iron, respectively. Average turnover rates of nutrients in litter were highest for potassium and decreased in the sequence calcium, magnesium, carbon, nitrogen and phosphorus. Concentrations of phosphorus, nitrogen and potassium in litterfall, litter mass and topsoil were closely correlated with each other. Concentrations of calcium and manganese were positively correlated with each other and with elevation. Variations in leaf chemistry and total litterfall caused the spatial heterogeneity of element input to the forest floor to have a coefficient of variation of 30 — 70%, depending on the element. Due to the strong positive correlation between element fluxes and pools, the spatial variability of turnover rates (CV c. 20%) was lower than that of element input. Turnover rates for K varied by a factor of 4, and for Ca by a factor of 2.8 when the different sites were compared. The results strongly suggest that the most important factor determining spatial heterogeneity of organic matter and element dynamics on the forest floor is the site-specific amount of leaf fall, rather than spatially variable decomposition rates.

Many organisms create or alter resource flows that affect the composition and spatial arrangement of current and future organismal diversity. The phenomenon called ecosystem engineering is considered with a case study of the mound building termite Macrotermes michaelseni. It is argued that this species acts as an ecosystem engineer across a range of spatial scales, from alteration of local infiltration rates to the creation of landscape mosaics, and that its impacts accrue because of the initiation of biophysical processes that often include feedback mechanisms. These changes to resource flows are likely to persist for long periods and constrain the biological structure of the habitat. The value of ecosystem engineering is discussed as a holistic way of understanding the complexity of tropical ecology.

Throughfall and soil solution chemistry were studied for 1 y in a tropical montane forest in the Luquillo Mountains of Puerto Rico. Passage of precipitation through the forest canopy resulted in an increase in the concentration and flux of all solutes except H+ and NO3−. Throughfall chemistry showed no strong seasonal patterns, but concentrations of many solutes declined during weeks of high rainfall. Enrichment in throughfall relative to precipitation was similar to values reported recently for several other tropical sites, with the exception of NH4+, which was particularly high at this site. Based on net throughfall deposition of Cl−, dry deposition of marine aerosols appears to be a relatively minor component (c. 15%) of total deposition. In soil solution, no seasonal patterns in concentration were evident and variability was highest for elements with high biological activity (especially N). Concentrations and fluxes of K+ and dissolved organic carbon (DOC) showed the largest declines as throughfall passed through the soil profile; concentrations of most other elements increased or were relatively constant. Declining DOC flux through the soil profile appears to be due to sorption processes similar to those observed in many temperate forests. Concentrations and fluxes of HCO3− and SiO2 increased substantially in soil solution, but never approached those observed in the stream. This suggests that additional weathering must occur as groundwater moves from this ridgetop site to the stream.

The diets of 17 lizard species (seven families) studied simultaneously in a Caribbean lowland forest of Nicaragua were compared. Lizards varied in body size over nearly one order of magnitude. Twelve species for which there were adequate samples separated by prey types and most diet overlaps were low. A pseudocommunity analysis on volumetric diet data revealed significant guild structure in the assemblage. At each nearest neighbour rank in niche space, observed overlaps were higher than expected based on chance alone when all values in the consumer-resource matrix were randomized. There was no difference between observed and pseudocommunity overlaps with zero positions in the consumer-resource matrix retained (conserved-zero overlaps) indicating that the zero structure of the community matrix was important in maintaining structure and that lizards were converging on key resources. Individual prey size varied among species and mean prey size was significantly correlated with body size of lizard species. A phylogenetic analysis revealed no relationship between similarity in prey use (dietary overlap) and evolutionary relationships — more closely related species did not eat more similar prey types. Based on this analysis of Nicaraguan lizard diets and comparisons with other New World tropical lizard assemblages, it is suggested that factors contributing to the organization of tropical lizard assemblages are complex including historical differences in morphology (size), prey types and sizes, habitat structure and species interactions.