Efectos de la Extracción de Maderas sobre la Diversidad de Mamiferos Pequeños en Bosques de Tierras Bajas de la Guayana Venezolana1

The responses of small mammal communities to forest disturbance by logging were evaluated. The study area was located in the Imataca Forest Reserve (Venezuelan Guayana Region), where vegetation was predominantly lowland rain forest. Field analyses were based on a comparative inventory of species inhabiting primary forests and areas disturbed by selective logging. The taxonomic groups used as indicators of die ecological impact of logging belonged to the orders Didelphimorphia, Chiroptera, and Rodentia (families Sciuridae, Muridae, and Echimyidae). The following sampling methods were used: (1) mist nets; (2) traps in 2.4 ha grids (each with 120 stations: 60 at ground level and 60 in trees); and (3) diurnal and nocturnal sight surveys. Total sampling effort consisted of 1904 net‐hours, 10,320 trap‐nights, and 567 h of direct observations. At least 83 mammalian species inhabited die evaluated forests (74.7 percent corresponding to Chiroptera), of which 15.3 percent were restricted to primary forest. In logged areas, small mammal communities were characterized by: (1) higher abundances of individuals; (2) lower proportions of carnivorous and gleaning insectivorous bats; (3) increases in the relative abundance of frugivorous bat species that eat the fruits of colonizing plants; (4) higher proportions of aerial insectivores (Molossidae) at die lowest levels of die forest; (5) simplification in trophic structure of non‐volant species, widi semiarboreal predator‐omnivores being die dominant guild (followed by terrestrial frugivore‐omnivores); and (6) reduction in the relative abundance of mainly canopy‐associated species. These results are explained by: (1) lower availability of key resources associated with primary forest (e.g., roosts in hollow trunks of mature trees, canopy fruits, and tree‐crown continuity); (2) higher relative abundance of some food resources, such as terrestrial invertebrates and saprophytic plants, principally in areas where primary production is limited by low‐fertility soils; (3) increase of roosting sites under fallen trunks; (4) modification of microclimatic conditions at die understory level as a consequence of a greater incident sunlight after canopy opening; and (5) increase in density of early successional plants. The implications of these results to the conservation of biodiversity in forests managed for timber extraction in die Venezuelan Guayana Region are discussed.     

Tree species diversity alters plant defense investment in an experimental forest plantation in southern Mexico

How plant species diversity affects traits conferring herbivore resistance (e.g., chemical defenses), as well as the mechanisms underlying such effects, has received little attention. One potential mechanism for the effect of diversity on plant defenses is that increased plant growth at high diversity could lead to reduced investment in defenses via growth–defense trade‐offs. We measured tree growth (diameter at breast height) and collected leaves to quantify total phenolics in 2.5‐year‐old plants of six tropical tree species (N = 597 plants) in a young experimental plantation in southern Mexico. Selected plants were classified as monocultures or as polycultures represented by mixtures of four of the six species examined. Tree species diversity had a significant negative effect on total phenolics, where polycultures exhibited a 13 percent lower mean concentration than monocultures. However, there was marked variation in the effects of diversity on defenses among tree species, with some species exhibiting strong reductions in phenolic levels in mixtures, whereas others were unresponsive. In addition, tree species diversity had no effect on growth, nor was the negative effect of diversity on chemical defenses mediated by a growth–defense trade‐off. These results demonstrate that tree diversity can alter investment in chemical defenses in long‐lived tree species but that such effect may not always be under strong control by plant endogenous resource allocation trade‐offs. Regardless of the underlying mechanism, these findings have important implications for predicting effects on consumers and ecosystem function.     

Diurnal foraging ant–tree co-occurrence networks are similar between canopy and understorey in a Neotropical rain forest

Discussion of the vertical stratification of organisms in tropical forests has traditionally focused on species distribution. Most studies have shown that, due to differences in abiotic conditions and resource distribution, species can be distributed along the vertical gradient according to their ecophysiological needs. However, the network structure between distinct vertical strata remains little-explored. To fill this gap in knowledge, we used baits to sample ants in the canopy and understorey trees of a Mexican tropical rain forest to record the ant–tree co-occurrences. We examined the ant–tree co-occurrences in the canopy and understorey using complementary network metrics (i.e., specialization, interaction diversity, modularity, and nestedness). In addition, we evaluated co-occurrence patterns between ant species on trees, using C-score analysis. In general, we found no differences in the network structure, although the interaction diversity was greater in the understorey than in the canopy networks. We also observed that co-occurrence networks of each vertical stratum featured four ant species in the central core of highly co-occurring species, with three species unique to each stratum. Moreover, we found a similar trend toward ant species segregation in the both strata. These findings reveal a similar pattern of ant–ant co-occurrences in both vertical strata, probably due to the presence of arboreal-nesting ants in the understorey. Overall, we showed that despite the marked differences in species composition and environmental conditions between understorey and canopy strata, ant–tree co-occurrences in these habitats could be governed by similar mechanisms, related to dominance and resource monopolization by ants.     

Spatial Patterns of Haplotype Variation in the Epiphytic Bromeliad Catopsis nutans

Identifying factors governing the origin, distribution, and maintenance of Neotropical plant diversity is an enduring challenge. To explore the complex and dynamic historical processes that shaped contemporary genetic patterns for a Central American plant species, we investigated the spatial distribution of chloroplast haplotypes of a geographically and environmentally widespread epiphytic bromeliad with wind‐dispersed seeds, Catopsis nutans, in Costa Rica. We hypothesized that genetic discontinuities occur between northwestern and southwestern Pacific slope populations, resembling patterns reported for other plant taxa in the region. Using non‐coding chloroplast DNA from 469 individuals and 23 populations, we assessed the influences of geographic and environmental distance as well as historical climatic variation on the genetic structure of populations spanning >1200 m in elevation. Catopsis nutans revealed seven haplotypes with low within‐population diversity (mean haplotype richness = 1.2) and moderate genetic structure (F_ST = 0.699). Pairwise F_ST was significantly correlated with both geographic and environmental distance. The frequency of dominant haplotypes was significantly correlated with elevation. A cluster of nine Pacific lowland populations exhibited a distinct haplotype profile and contained five of the seven haplotypes, suggesting historical isolation and limited seed‐mediated gene flow with other populations. Paleodistribution models indicated lowland and upland habitats in this region were contiguous through past climatic oscillations. Based on our paleodistribution analysis and comparable prior phylogeographic studies, the genetic signature of recent climatic oscillations are likely superimposed upon the distribution of anciently divergent lineages. Our study highlights the unique phylogeographic history of a Neotropical plant species spanning an elevation gradient.     

Landscape‐scale Variation in Pathogen‐suppressive Bacteria in Tropical Dry Forest Soils of Costa Rica

Bacteria in the genus Streptomyces are ubiquitous in soil and are well‐known for their production of diverse secondary metabolites, including antibiotics that can inhibit soil‐borne plant pathogens and suppress disease. Pathogen‐suppressive soil bacteria have the potential to influence plant community composition and diversity, but remain relatively unexplored in tropical forest soils. To estimate the potential for disease suppression among Streptomyces communities in tropical dry forests, we cultured soil‐borne Streptomyces from plots in two forests in northwestern Costa Rica (Santa Rosa and Palo Verde) and quantified antibiotic‐mediated pathogen inhibition against three plant pathogens. The potential for pathogen inhibition and disease suppression by Streptomyces was highly variable across the landscape. Densities of pathogen‐suppressive Streptomyces varied by over ten‐fold and were correlated with soil nutrients across the plots. In particular, Streptomyces communities became more pathogen‐suppressive as labile soil P decreased. Inhibitor densities were significantly higher in Santa Rosa than Palo Verde, which may be related to differences in soil texture and/or plant community composition between the two forests. Our findings suggest potential differences in the degree and specificity of antibiotic‐mediated disease suppression in tropical dry forest soils of Costa Rica, and highlight the need for further studies on the drivers of pathogen‐suppressive phenotypes as well as the consequences of spatially variable pathogen inhibition for plant community composition in tropical forest ecosystems.