Abundance and Leaf Damage Caused by Gall‐Inducing Insects in a Mexican Tropical Dry Forest1

We evaluated the temporal and spatial patterns of abundance and the amount of damage caused by gall‐inducing insects (GII) in deciduous and riparian habitats in a seasonal tropical dry forest in Mexico. Plants occurring in these habitats differ in their phenology and moisture availability. Deciduous habitats are seasonal and xeric, while riparian habitats are aseasonal and mesic. We found 37 GII species and each one was associated with a specific plant species. In total, 19 species (51.3%) were present in deciduous habitats, 13 species (35.2%) in riparian habitats, and only 5 species (13.5%) occurred in both. Abundance and leaf damage by GII were greater in deciduous than in riparian habitats during the wet season. For each GII species that occurred in both habitats, host plant species supported greater abundance and leaf damage by GII in deciduous habitats during the wet season. These results indicate a greater association of GII species with host plants in deciduous than in riparian habitats during the wet season. In riparian habitats, 11 plant species (61.1%) had greater density of GII in the dry than in the wet season. Similarly, leaf damage by GII was significantly greater in the dry than in the wet season in riparian habitats for 12 plant species (66.7%). Dry forest plants of riparian habitats presented two peaks of leaf‐flushing: GII colonized leaves produced in the first peak at the beginning of the wet season, and accumulated or recolonized leaves in the second peak at the beginning of the dry season. The levels of leaf damage by GII detected in this study in the rainy season were considerably higher than those obtained for folivorous insects in other neotropical forests, suggesting that this GII guild might have an important impact on their host plant species in this tropical community.     

The Influence of Hurricane Winds on Caribbean Dry Forest Structure and Nutrient Pools1

In 1998, we measured the effects of Hurricane Georges after it passed over long‐term research sites in Puerto Rican dry forest. Our primary objectives were to quantify hurricane effects on forest structure, to compare effects in a large tract of forest versus a series of nearby forest fragments, to evaluate short‐term response to hurricane disturbance in terms of mortality and sprouting, and to assess the ability of hurricanes to maintain forest structure. We sampled damage from 33 plots (1.3 ha) across a 3000‐ha tract of forest as well as in 19 fragments. For stems with 2.5‐cm minimum diameter, 1004 stems/ha (12.4%) suffered structural damage, while 69 percent of the undamaged stems were at least 50 percent defoliated. Basal area lost to structural damage equaled 4.0 m²/ha (22%) in south‐facing native forests. Structural damage and defoliation increased with stem diameter and were more common in certain dry forest species. South‐facing forests and those on ridgetops incurred more damage than north‐facing forests or those comprised primarily of introduced species. Stem mortality was only 2 percent of all stems after 9 mo. Structural damage did not necessarily result in stem mortality. Hurricane‐induced mortality was not associated with stem height or diameter, but was ten times greater than background mortality. Basal sprouting was proportional to the amount of structural damage incurred in a stand. Forest fragments experienced the same patterns of hurricane effects as the reference forest. The low, dense structure of Caribbean dry forest can be maintained by hurricane damage to larger stems and induction of basal sprouting to generate multistemmed trees.     

Litter Interception on Astrocaryum mexicanum Liebm. (Palmae) in a Tropical Rain Forest1

The goal of this study was to quantify litter interception on Astrocaryum mexicanum Liebm., an abundant and efficient litter–capturing palm. The study was done in a forest at Los Tuxtlas, Veracruz, on the Gulf of Mexico coast. Palms were randomly selected and total litter retention was quantified by determining dry weight over two years. Litter interception by palms was 4.4 and 27.1 Mg/ha/yr for the first and second years, respectively, and accounted for 47.9 and 239.4 percent of the reported annual litterfall in the same area. Results showed that litter interception by plants is related to litterfall and should therefore be added to total litterfall in order to calculate the net primary productivity.     

Wood Decomposition of Cyrilla racemiflora (Cyrillaceae) in Puerto Rican Dry and Wet Forests: A 13‐year Case Study1

We studied the decomposition of Cyrilla racemiflora logs over a 13‐yr period in tropical dry and wet forests in Puerto Rico. The mean mass loss, ratio of soft to hard wood, nutrient concentrations, and the diversity of wood‐inhabiting organisms were greater in logs decomposing in the dry forest than in the wet forest. Termites were also more abundant in the logs collected from the tropical dry forest than the tropical wet forest. High moisture content and a low animal diversity on the logs in the wet forest seem to retard wood decay in this habitat. Wood decay rates in the tropical dry forest can be related to the high diversity of species and functional groups of wood‐inhabiting organisms.     

Spatial Variability in Litterfall,Litter Standing Crop and Litter Quality in a Tropical Rain Forest Region

Understanding the spatial variability in plant litter processes is essential for accurate comprehension of biogeochemical cycles and ecosystem function. We assessed spatial patterns in litter processes from local to regional scales, at sites throughout the wet tropical rain forests of northern Australia. We aimed to determine the controls (e.g., climate, soil, plant community composition) on annual litter standing crop, annual litterfall rate and in situ leaf litter decomposability. The level of spatial variance in these components, and leaf litter N, P, Ca, lignin, α‐cellulose and total phenolics, was determined from within the scale of subregion, to site (1 km transects) to local/plot (~30 m²). Overall, standing crop was modeled with litterfall and its chemical composition, in situ decomposability, soil Na, and topography (r² = 0.69, 36 plots). Litterfall was most closely aligned with plant species richness and stem density (negative correlation); leaf decomposability with leaf‐P and lignin, soil Na, and dry season moisture (r² = 0.89, 40 plots). The predominant scale of variability in litterfall rates was local (plot), while litter standing crop and α‐cellulose variability was more evenly distributed across spatial scales. Litter decomposability, N, P and phenolics were more aligned with subregional differences. Leaf litter C, lignin and Ca varied most at the site level, suggesting more local controls. We show that variability in litter quality and decomposability are more easily accounted for spatially than litterfall rates, which vary widely over short distances possibly in response to idiosyncratic patterns of disturbance.     

The Bromeliad Microcosm and the Assessment of Faunal Diversity in a Neotropical Forest1

The faunas of tank bromeliads were sampled over two years in three forest types at different elevations in the Luquillo Experimental Forest, Puerto Rico, and the diversity of their animal communities compared. Bromeliad plants behaved as islands in that, within forests, the species richness and abundance of their animal communities were significantly and positively correlated with increase in plant size. The amount of canopy debris they accumulated was similarly correlated with increase in plant size. Overall diversity was lowest in the dwarf forest, where plants were uniformly small. Animal communities were stable from year to year, and could be characterised for each forest type and for compartments within the plant. They showed a pattern of high dominance, which increased with elevation (Mc-Naughton index 37, 54, and 73, respectively, for the tabonuco, palo Colorado, and dwarf forest). Alpha-diversity for sites sampled in each year reflected net primary productivity (NPP) of the forest, declining with increasing elevation when animal abundance measures were used (jackknife estimates of Simpson's diversity index 6.54 & 11.04 [tabonuco], 3.53 & 6.22 [palo Colorado], and 2.75 & 2.17 [dwarf forest]). Species richness over the two years, however, was highest in the intermediate palo Colorado forest (187 species), compared to 146 and 88 in the tabonuco and dwarf forests, respectively. These figures were close to jackknife estimates of maximum species richness. The difference in species richness between tabonuco and palo Colorado forests was significant in one year only. In addition to NPP, other factors, such as litter quality and the structural complexity of the habitat in the palo Colorado forest, may have influenced species richness. The most abundant species in individual plants were also the most widely occurring, confirming known patterns of abundance and distribution in other functional groups. Diversity within bromeliad microcosms at different elevations supported known relationships between diversity, productivity, and habitat complexity along gradients and was not related to differences in the total bromeliad habitat available for colonization.     

Phenology of Plumeria alba and its Herbivores in a Tropical Dry Forest

Understanding phenology in plant populations requires distinction between proximate mechanisms and ultimate (evolutionary) causation. Leaf production and abscission, flower production, and herbivory were monitored for 2 yr in a population of the stem succulent tree, Plumeria alba L. in the Guánica State Forest in southwest Puerto Rico. Dependence of phenological events on abiotic (rainfall and day length) and biotic factors (herbivore abundance/damage) was quantified to discern potential relationships. Leaf flush and flowering were not associated with periods of highest rainfall as might be expected in a dry tropical forest. Rather, these events were highly correlated with day length. We observed that most leaf flush began in March and April, which was several months before the wettest period of the year (August to November). This result is consistent with other studies that show that leaf flush in Plumeria is under photoperiodic control and that the plants initiate growth and reproduction when cloudiness is low and seasonal light availability is greatest. Herbivore damage by caterpillars of the sphinx moth Pseudosphinx tetrio is restricted primarily to the wettest season, consistent with the hypothesis that early leaf flush and reproduction has been selected to avoid herbivory. It is not clear whether photoperiodic control of leaf flush and reproduction serves to maximize seasonal light availability, minimize the impact of herbivores, or both. However, it is clear that peak rainfall is not likely to have been the sole selective factor determining leaf flush and flowering in P. alba .     

Food Web Structure and Basal Resource Utilization along a Tropical Island Stream Continuum, Puerto Rico

Tropical stream food webs are thought to be based primarily on terrestrial resources (leaf litter) in small forested headwater streams and algal resources in larger, wider streams. In tropical island streams, the dominant consumers are often omnivorous freshwater shrimps that consume algae, leaf litter, insects, and other shrimps. We used stable isotope analysis to examine (1) the relative importance of terrestrial and algal‐based food resources to shrimps and other consumers and determine (2) if the relative importance of these food resources changed along the stream continuum. We examined δ¹⁵N and δ¹³C signatures of leaves, algae, macrophytes, biofilm, insects, snails, fishes, and shrimps at three sites (300, 90, and 10 m elev.) along the Río Espíritu Santo, which drains the Caribbean National Forest, Puerto Rico. Isotope signatures of basal resources were distinct at all sites. Results of two‐source δ¹³C mixing models suggest that shrimps relied more on algal‐based carbon resources than terrestrially derived resources at all three sites along the continuum. This study supports other recent findings in tropical streams, demonstrating that algal‐based resources are very important to stream consumers, even in small forested headwater streams. This study also demonstrates the importance of doing assimilation‐based analysis (i.e., stable isotope or trophic basis of production) when studying food webs.     

Short-Term Disappearance of Foliar Litter in Three Species Before and After a Hurricane1

Litter disappearance was examined before (1989) and after (1990) Hurricane Hugo in the Luquillo Experimental Forest, Puerto Rico using mesh litterbags containing abscised Cyrilla racemiflont or Dacryodes excelsa leaves or fresh Prestoea montana leaves. Biomass and nitrogen dynamics were compared among: (i) species; (ii) mid- and high elevation forest types; (iii) riparian and upland sites; and (iv) pre- and post-hurricane disturbed environments. Biomass disappearance was compared using multiple regression and negative exponential models in which the slopes were estimates of the decomposition rates subsequent to apparent leaching losses and the y-intercepts were indices of initial mass losses (leaching). Cyrilla racemiflora leaves with low nitrogen (0.39%) and high lignin (22.1%) content decayed at a low rate and immobilized available nitrogen. Dacryodes excelsa leaves had moderate nitrogen (0.67%) and lignin (16.6%) content, decayed at moderate rates, and maintained the initial nitrogen mass. Prestoea montana foliage had high nitrogen (1.76%) and moderate lignin (16.7%) content and rapidly lost both mass and nitrogen. There were no significant differences in litter disappearance and nitrogen dynamics among forest types and slope positions. Initial mass loss of C. racemiflora leaves was lower in 1990 but the subsequent decomposition rate did not change. Initial mass losses and the overall decomposition rates were lower in 1990 than in 1989 for Dacryodes excelsa. Dacryodes excelsa and C. racemiflora litter immobilized nitrogen in 1990 but released 10-15 percent of their initial nitrogen in 1989, whereas P. montana released nitrogen in both years (25-40%). Observed differences in litter disappearance rates between years may have been due to differences in the timing of precipitation. Foliar litter inputs during post-hurricane recovery of vegetation in Puerto Rico may serve to immobilize and conserve site nitrogen.     

Plant controls on Late Quaternary whole ecosystem structure and function

Plants and animals influence biomass production and nutrient cycling in terrestrial ecosystems; however, their relative importance remains unclear. We assessed the extent to which mega‐herbivore species controlled plant community composition and nutrient cycling, relative to other factors during and after the Late Quaternary extinction event in Britain and Ireland, when two‐thirds of the region's mega‐herbivore species went extinct. Warmer temperatures, plant–soil and plant–plant interactions, and reduced burning contributed to the expansion of woody plants and declining nitrogen availability in our five study ecosystems. Shrub biomass was consistently one of the strongest predictors of ecosystem change, equalling or exceeding the effects of other biotic and abiotic factors. In contrast, there was relatively little evidence for mega‐herbivore control on plant community composition and nitrogen availability. The ability of plants to determine the fate of terrestrial ecosystems during periods of global environmental change may therefore be greater than previously thought.     

Plant Community Structure in Tropical Rain Forest Fragments of the Western Ghats,India1

Changes in tree, liana, and understory plant diversity and community composition in five tropical rain forest fragments varying in area (18–2600 ha) and disturbance levels were studied on the Valparai plateau, Western Ghats. Systematic sampling using small quadrats (totaling 4 ha for trees and lianas, 0.16 ha for understory plants) enumerated 312 species in 103 families: 1968 trees (144 species), 2250 lianas (60 species), and 6123 understory plants (108 species). Tree species density, stem density, and basal area were higher in the three larger (> 100 ha) rain forest fragments but were negatively correlated with disturbance scores rather than area per se. Liana species density, stem density, and basal area were higher in moderately disturbed and lower in heavily disturbed fragments than in the three larger fragments. Understory species density was highest in the highly disturbed 18‐ha fragment, due to weedy invasive species occurring with rain forest plants. Nonmetric multidimensional scaling and Mantel tests revealed significant and similar patterns of floristic variation suggesting similar effects of disturbance on community compositional change for the three life‐forms. The five fragments encompassed substantial plant diversity in the regional landscape, harbored at least 70 endemic species (3.21% of the endemic flora of the Western Ghats–Sri Lanka biodiversity hotspot), and supported many endemic and threatened animals. The study indicates the significant conservation value of rain forest fragments in the Western Ghats, signals the need to protect them from further disturbances, and provides useful benchmarks for restoration and monitoring efforts.     

Phyllostomid Bat Community Structure and Abundance in Two Contrasting Tropical Dry Forests1

Although tropical wet forests are generally more diverse than dry forests for many faunal groups, few studies have compared bat diversity among dry forests. I compared ground level phyllostomid bat community structure between two tropical dry forests with different precipitation regimes. Parque National Palo Verde in northwestern Costa Rica represents one of the wettest tropical dry forests (rainfall 1.5 m/yr), whereas the Chamela‐Cuixmala Biosphere Reserve on the Pacific coast of central Mexico represents one of the driest (750 mm/yr). Mist net sampling was conducted at the two study sites to compare changes in ground level phyllostomid bat community structure between regions and seasons. Palo Verde was more diverse than Chamela and phyllostomid species showed low similarity between sites (Classic Jaccard = 0.263). The distinct phyllostomid communities observed at these two dry forest sites demonstrates that variants of tropical dry forest can be sufficiently different in structure and composition to affect phyllostomid communities. At both dry forest sites, abundance of the two most common foraging guilds (frugivores and nectarivores) differed between seasons, with greatest numbers of individuals captured coinciding with highest chiropterophilic resource abundance.     

The Effect of Macroinvertebrate Exclusion on Leaf Breakdown Rates in a Tropical Headwater Stream1

The role of macroinvertebrates in the process of leaf breakdown is well studied in temperate streams, but less is known about their role in the tropics. We investigated the effect of reducing macroinvertebrate access to leaf material on leaf breakdown rates in a forested headwater stream in the Luquillo Experimental Forest, Puerto Rico. We measured leaf mass loss using fine and coarse mesh bags over 12 weeks for two common riparian species: Cecropia schreberiana (Moraceae) and Dacryodes excelsa (Burseraceae). Coarse mesh allowed freshwater shrimp and other macroinvertebrates to access leaf material, while fine mesh did not. Leaf breakdown rates did not differ between C. schreberiana and D. excelsa in coarse mesh bags (?0.0375/day vs. ?0.0395/day, respectively), but C. schreberiana breakdown was significantly slower than D. excelsa in fine mesh bags (?0.0159/day vs. ?0.0266/day). Additionally, breakdown in fine mesh bags was significantly slower compared to coarse mesh bags for C. schreberiana, but less so for D. excelsa. Breakdown rates for all treatments were fast relative to those in temperate‐zone streams indicating that both macroinvertebrates and macroinvertebrate‐independent processing can strongly influence leaf decomposition in tropical streams. The difference between C. schreberiana and D. excelsa indicates that the effect of macroinvertebrate exclusion can change with leaf type.     

Arthropod Abundance,Canopy Structure,and Microclimate in a Bornean Lowland Tropical Rain Forest1

This study applies a novel, vertically stratified fogging protocol to document arthropod abundance, density, and biomass across a vertical gradient in a primary, lowland dipterocarp forest canopy in Borneo. We fogged arthropods at 5 m vertical intervals and 20 m horizontal intervals along six full‐canopy transects and measured leaf surface areas along the same transects. The results show that arthropod biomass in the aboveground regions was 23.6 kg/ha, the abundance was 23.9 million individuals/ha, and the density on leaf surfaces was 280 individuals/m² leaf area. All three numbers are five to ten times higher than estimated by previous surveys of tropical lowland rain forest canopies using mass‐collection techniques. Arthropod abundance and biomass were analyzed in relation to canopy structure, composition, vapor pressure deficit (VPD), photosynthetic photon flux density (PPFD), and height. Using stepwise regression we found that 13 of 14 arthropod groups had significant positive relationships with one‐sided leaf area, 11 had significant negative relationships with VPD, 3 had significant relationships with height, and none showed positive relationships with light. Classifying the 14 taxa based on their responses to leaf area and VPD created three groups that corresponded roughly to the biology of these taxa. This study suggests that the biomass and abundance, and perhaps therefore—by extrapolation—the biodiversity, of tropical canopy arthropods may be very much higher than previously estimated.     

Cross-Scale Responses of Biodiversity to Hurricane and Anthropogenic Disturbance in a Tropical Forest

Abstract In studies of biodiversity, considerations of scale—the spatial or temporal domain to which data provide inference—are important because of the non-arithmetic manner in which species richness increases with area (and total abundance) and because fine-scale mechanisms (for example, recruitment, growth, and mortality of species) can interact with broad scale patterns (for example, habitat patch configuration) to influence dynamics in space and time. The key to understanding these dynamics is to consider patterns of environmental heterogeneity, including patterns produced by natural and anthropogenic disturbance. We studied how spatial variation in three aspects of biodiversity of terrestrial gastropods (species richness, species diversity, and nestedness) on the 16-ha Luquillo Forest Dynamics Plot (LFDP) in a tropical forest of Puerto Rico was affected by disturbance caused by Hurricanes Hugo and Georges, as well as by patterns of historic land use. Hurricane-induced changes in spatial organization of species richness differed from those for species diversity. The gamma components of species richness changed after the hurricanes and were significantly different between Hurricanes Hugo and Georges. Alpha and two beta components of species richness, one related to turnover among sites within areas of similar land use and one related to variation among areas of different land use, varied randomly over time after both hurricanes. In contrast, gamma components of species diversity decreased in indistinguishable manners after both hurricanes, whereas the rates of change in the alpha component of species diversity differed between hurricanes. Beta components of diversity related to turnover among sites declined after both hurricanes in a consistent fashion. Those related to turnover among areas with different historic land uses varied stochastically. The immediate effect of hurricanes was to reduce nestedness of gastropod assemblages. Thereafter, nestedness increased during post-hurricane secondary succession, and did so in the same way, regardless of patterns of historic land use. The rates of change in degree of nestedness during secondary succession were different after each hurricane as a result of differences in the severity and extent of the hurricane-induced damage. Our analyses quantified temporal changes in the spatial organization of biodiversity of gastropod assemblages during forest recovery from hurricane-induced damage in areas that had experienced different patterns of historic human land use, and documented the dependence of biodiversity on spatial scale. We hypothesize that cross-scale interactions, likely those between the local demographics of species at the fine scale and the landscape configuration of patches at the broad scale, play a dominant role in affecting critical transfer processes, such as dispersal, and its interrelationship with aspects of biodiversity. Cross-scale interactions have significant implications for the conservation of biodiversity, as the greatest threats to biodiversity arise from habitat modification and fragmentation associated with disturbance arising from human activities.     

Soil Effects on Forest Structure and Diversity in a Moist and a Dry Tropical Forest

Soil characteristics are important drivers of variation in wet tropical forest structure and diversity, but few studies have evaluated these relationships in drier forest types. Using tree and soil data from 48 and 32 1 ha plots, respectively, in a Bolivian moist and dry forest, we asked how soil conditions affect forest structure and diversity within each of the two forest types. After correcting for spatial effects, soil‐vegetation relationships differed between the dry and the moist forest, being strongest in the dry forest. Furthermore, we hypothesized that soil nutrients would play a more important role in the moist forest than in the dry forest because vegetation in the moist forest is less constrained by water availability and thus can show its full potential response to soil fertility. However, contrary to our expectations, we found that soil fertility explained a larger number of forest variables in the dry forest (50 percent) than in the moist forest (17 percent). Shannon diversity declined with soil fertility at both sites, probably because the most dominant, shade‐tolerant species strongly increased in abundance as soil fertility increased.     

Difference in Intensity of Ant Defense among Three Species of Macaranga Myrmecophytes in a Southeast Asian Dipterocarp Forest1

To examine interspecific variation in the intensity of ant defense among three sympatric species of obligate myrme‐cophytes of Macaranga (Euphorbiaceae), we measured the ratio of ant biomass to plant biomass, ant aggressiveness to artificial damage on host plants, and increase in herbivore damage on host plants when symbiont ants were removed. Increase in herbivore damage from two‐ and four‐week ant exclusion varied significantly among the three species. The decreasing order of vulnerability to herbivory was M. winkleri, M. trachyphylla, and M. beccariana. The antip/ant biomass ratio (= rate of the dry weight of whole ant colonies to the dry weight of whole aboveground plant parts) and ant agressiveness also varied significantly among the three species; the orders of both the ant/plant biomass ratio and ant aggressiveness were the same as in the herbivory increase. These results indicated that the intensity of ant defense differs predictably among sympatric species of obligate myrmecophytes on Macaranga. In addition to the interspecific difference in the total intensity of ant defense, when symbiont ants were excluded, both patterns of within‐plant variation in the amount of herbivore damage and compositions of herbivore species that caused the damage differed among species. This suggests that the three Macaranga species have different systems of ant defense with reference to what parts of plant tissue are protected and what herbivorous species are avoided by ant defense. Thus, it is important to consider the interspecific variation in ant defense among Macaranga species to understand the herbivore community on Macaranga plants and the mechanisms that promote the coexistence of multiple Macaranga myrmecophytes.     

Survival, Growth, and Ecosystem Dynamics of Displaced Bromeliads in a Montane Tropical Forest1

Epiphytes generally occupy arboreal perches, which are inherently unstable environments due to periodic windstorms, branch falls, and treefalls. During high wind events, arboreal bromeliads are often knocked from the canopy and deposited on the forest floor. In this study, we used a common epiphytic tank bromeliad, Guzmania berteroniana (R. & S.) Mez, to determine if fallen bromeliads can survive, grow, and reproduce on the forest floor and evaluate the potential impact of adult dispersal on plant and soil nutrient pools. Bromeliads were transplanted to and from tree stems and the forest floor and monitored intensively for six months; survival, growth, and impacts on ecosystem nutrient pools were followed on a subset of plants for 16 months. Six months after transplanting, bromeliad mortality was low (3%), and 19 percent of study individuals had flowered and produced new juvenile shoots. Mortality on the subset of plants followed for 16 months was 14–30 percent. Although survival rates were relatively high in all habitats, bromeliads transplanted to trees grew significantly more root length (x?± SE: 189 ± 43 cm) than those moved to the forest floor (53 ±15 cm) and experienced lower rates of leaf area loss. All transplanted bromeliads rapidly altered the substrate they occupied. Individuals transplanted to and among trees rapidly decreased base cation concentrations but significantly increased P concentrations of their underlying substrate. On the ground, bromeliads increased C, N, and P concentrations within nine months of placement. Our results suggest that in this montane tropical forest, bromeliads respond rapidly to displacement, locally modify their substrates, and can access the resources needed for survival regardless of habitat.