Summary 1We hypothesized that the survival of shade-tolerant juvenile trees in the understorey depends on allocation strategies that slow their growth but enhance recovery from herbivore damage. In contrast, allocation patterns should maximize height growth in gaps where competitors grow rapidly. We tested the interactive effects of canopy gaps and simulated herbivory (by removing the apical meristem, or 10%, 50% or 90% of tissue from all leaves) on juvenile (< 1 cm diameter at breast height) Shorea quadrinervis Sloot (Dipterocarpaceae), a dominant canopy tree in Borneo. 2 Damage did not diminish survival over 8 months, except for 90% foliage removal from understorey plants.Height growth in the understorey was negligible in both control and damaged plants. In gaps, height growth was stimulated by removal of either the apical meristem or 10% of leaf tissue; growth was reduced only after 90% foliage removal. 3 Higher damage levels increased subsequent net leaf loss. Leaf production was much greater, but leaf retention much lower, in gaps than in the understorey. Ninety per cent foliage removal drastically reduced production in gaps and retention in the understorey. After 10% defoliation, height growth was enhanced in gaps despite approximately 30% net leaf loss. 4 In a separate experiment, juveniles were moved from a shade house to gaps. After 8 months, gap plants had more leaves and more total biomass, but only half the root : shoot ratio of plants left in the shade. 5 Allocation patterns, together with the survival of understorey plants after all but the highest levels of damage, indicate that root : shoot allocation there tends to reduce the risk of mortality following herbivore or mechanical damage at the expense of growth. In contrast, in gaps, where interference competition for light is high, resource allocation apparently maximizes height growth.The interaction between resource availability and herbivore damage provides the basis for a more synthetic theory of resource allocation and growth strategies than one based on resource availability alone.
Intraspecific, negative density dependence may contribute to the maintenance of diversity by limiting the dominance of common species. Shorea quadrinervis Slooten (Dipterocarpaceae) is one of the dominant canopy trees in a species‐rich tropical rain forest in Southeast Asia. We test whether juvenile density and performance and overall population growth rate of S. quadrinervis decline with increasing local abundance of conspecific adult trees. We mapped the 357 S. quadrinervis adults (≥15 cm dbh) in 75 ha at Gunung Palung National Park, Indonesian Borneo, and compared the dynamics of juveniles <1 cm dbh between eight areas (80 m diameter plots) with high densities of conspecific adults and eight containing low densities. Within these 16 areas, we mapped 5215 S. quadrinervis juveniles and followed their growth and survival for two years. Plots with high and low density of S. quadrinervis adults did not differ in understory light, slope, aspect, or soil properties nor in the total density (all species combined) of either adults or juveniles. We evaluated four kinds of evidence. All were consistent with the hypothesis of negatively density‐dependent population dynamics: (1) Density and size: Although the density of juveniles <25 cm tall per capita adult increased with local adult density, the pattern reversed for juveniles >75 cm tall. This suggests positive density‐dependent seedling recruitment followed by negative density‐dependent juvenile dynamics. (2) Foliar condition: Juveniles had more leaves per unit height where adult density was low. Growth and survival increased more with leaf number than with height. (3) Juvenile dynamics: Growth and survival were higher where adult density was low. Previous work suggests that herbivores may drive this negative density dependence in juvenile performance. (4) Population dynamics: We modeled population growth rate (λ) for each of the 16 areas using separate matrix models parameterized with empirical juvenile transition rates. We found that λ declined with local adult density (r2 = 0.40), and this trend was robust to the inclusion of El Niño conditions. We conclude that replacement of adults in the canopy is less likely in areas where conspecific adults are more abundant, tending to limit the dominance of S. quadrinervis in the canopy.
The international wildlife trade is a principal cause of biodiversity loss, involving hundreds of millions of plants and animals each year, yet wildlife trade records are notoriously unreliable. We assessed the precision of wildlife trade reports for the United States, the world's largest consumer of endangered wildlife, by comparing data from the Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES) with U.S. Customs data. For both U.S. imports and exports, CITES and Customs reported substantially different trade volumes for all taxa in all years. Discrepancies ranged from a CITES-reported volume 376% greater than that reported by Customs (live coral imports, 2000) to a Customs' report 5202% greater than CITES (conch exports, 2000). These widely divergent data suggest widespread inaccuracies that may distort the perceived risk of targeted wildlife exploitation, leading to misallocation of management resources and less effective conservation strategies. Conservation scientists and practitioners should reexamine assumptions regarding the significance of the international wildlife trade.Resumen: El comercio internacional de vida silvestre es una causa principal de la pérdida de biodiversidad, ya que involucra a cientos de millones de plantas y animales cada año; no obstante eso, los registros del comercio son notoriamente poco confiables. Evaluamos la precisión de los registros de comercio de vida silvestre de Estados Unidos, el mayor consumidor de vida silvestre en peligro en el mundo, mediante la comparación de datos del Convenio Internacional para el Comercio de Especies de Flora y Fauna Silvestre en Peligro (CITES) con datos de la Aduana de E.U.A. Tanto para importaciones como exportaciones, CITES y Aduana reportaron volúmenes de comercio de todos los taxa sustancialmente diferentes en todos los años. Las discrepancias abarcaron desde un volumen reportado por CITES 376% más grande que el reportado por la Aduana (importaciones de coral vivo, 2000) hasta un reporte de la Aduana 5202% mayor que el de CITES (exportaciones de caracol, 2000). Estos datos ampliamente divergentes sugieren imprecisiones generalizadas que pueden distorsionar el riesgo percibido por la explotación de vida silvestre, lo que conducirá a la incorrecta asignación de recursos para la gestión y a estrategias de conservación menos efectivas. Los científicos y profesionales de la conservación deberían reexaminar sus suposiciones respecto al significado del comercio internacional de vida silvestre.Palabras Clave: aduanas, CITES, especies en peligro, programa de aranceles armonizados Paper
We assessed density- and distance-dependence in herbivore effects and juvenile condition for four species of Shorea, the most speciose genus in the dominant canopy family of southeast Asian rain forest trees (Dipterocarpaceae). Herbivore damage was quantified as partial leaf loss on young leaves, and whole plant foliar condition as the product of the fraction of leaf nodes containing leaves and the fraction of tissue remaining on extant leaves. Adults of the four species were centers of high total, as well as conspecific, density of juveniles (<1 m tall). For two species, S. hopeifolia and S. pinanga, herbivore damage declined significantly with distance, decreasing by 40% and 51% respectively, between 5 m and 35 m from the parent. For the same two species, foliar condition improved significantly between 5 m and 35 m, increasing by 45% for S. hopeifolia and 24% for S. pinanga. If foliar condition influences juvenile survival and growth, more widely dispersed seeds of these species are more likely to recruit to the canopy. In contrast, there was no significant distance-dependence for S. parvifolia or S. longisperma. Among species, herbivore damage was greatest in those species with greatest local juvenile abundances, i.e., those with highest densities, leaf size, juvenile foliar mass and/ or foliar mass/m ground area, but was unrelated to the toughness of mature leaves. However, distance was a better predictor of herbivore damage than was conspecific juvenile density, as evaluated by backward elimination regressions, for both S. hopeifolia and S. pinanga. For foliar condition, the best predictor was distance for S. pinanga, but conspecific density for S. hopeifolia, whose juveniles were smallest and occurred at the highest densities. Total juvenile density (all woody plants) was eliminated as a factor in all cases. The species-specificity of effects (i.e., their dependence on conspecific distance or density), together with the marked differences among congeneric species, caution against generalizations regarding distance-dependent effects in diverse forests.
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