Spatiotemporal variation in predispersal seed predation intensity

The effect of predispersal seed predation by Bruchus atomarius (Bruchidae, Coleoptera) on individual performance and population dynamics of the perennial forest herb, Lathyrus vernus (Leguminosae), was investigated in 11 permanent plots over 4 years. Seed predation and parameters describing intra-specific neighbour distance, plant size, inflorescence size, flowering phenology and current and previous herbivore damage were measured on all plants. In addition, demographic information from all plots was analysed using transition matrix population models in order to estimate the influence of seed predation on population growth rates. Predispersal seed predation rates differed significantly among years. Plot averages ranged from 0 to 83.7%. However, most of the variation occurred among individuals. Within individuals there was no consistency in predation rates among years. Exposure to herbivory, plant size and flowering phenology did not affect predation rates but individuals with larger inflorescences suffered from significantly higher predation. Seed predation in L. vernus was not influenced by neighbour distances of individual plants but it was positively correlated with the average density of seeds within plots, suggesting that seed predation is density dependent at the patch level. The reduction in population growth rate due to seed predation ranged from 0 to 7.6%. The sensitivity of population growth rate to reductions in seed production varied considerably among years and plots. This variation was mainly due to differences in the reproductive value of seeds and seedlings. The intensity of seed predation over the range found was not correlated with changes in population growth rate. The results of this study suggest that the influence of external factors, like seed predation, on population growth rate largely depends on the demographic transition rates in the investigated population.     

Differential seed and seedling predation by crabs: impacts on tropical coastal forest composition

Recently, the importance of seed predation by crabs on mangrove species distributions and densities has been established by several studies. In a tropical coastal terrestrial forest in Costa Rica, we investigated the relative importance of predation by land crabs, Gecarcinus quadratus, and hermit crabs, Coenobita compressus, on measured forest composition through a series of seed removal and seedling establishment experiments. We also used natural light-gaps and adjacent non-gap sites to test how canopy cover affects crab predation (seed removal) and seedling establishment. We found fewer tree species (S=18) and lower densities (seedlings, saplings, and adults) in the coastal zone within 100 m of coastline, than in the inland zone (S=59). Land crab densities were higher in the coastal zone (3.03±1.44 crabs m^–2) than in the inland zone (0.76±0.78 crabs m^–2), and hermit crabs were not present in the inland zone. Seed removal and seedling mortality also were higher in the coastal zone than in the inland zone, and in the open controls than in the crab exclosures. Mortality of seeds and seedlings was two to six times higher in the controls than exclosures for four of the five experiments. Crabs preferred seeds and younger seedlings over older seedlings but showed no species preferences in the seed (Anacardium excelsum, Enterolobium cyclocarpum, and Terminalia oblonga) and seedling (Pachira quinata and E. cyclocarpum) stages. We conclude that the observed differences in tree densities were caused by differential crab predation pressure along the coastal gradient, while the differences in species composition were due to predator escape (satiation) by seed quantity. Canopy cover did not affect seed removal rates, but did affect seedling survival with higher mortality in the non-gap versus gap environments. In summary, crab predation of seeds and seedlings, and secondarily canopy cover, are important factors affecting tree establishment in terrestrial coastal forests.     

Predispersal seed predation in Bartsia alpina

Summary A northern Swedish population of Bartsia alpina, an arctic-alpine perennial herb, was found to suffer high levels of predispersal seed predation by larvae of two insect species, both specialists on rhinanthoid Scrophulariaceae hosts. The primary predator is Aethes deutschiana (Lepidoptera-Tortricidae), the host of which was previously unknown. The other predator is Gimnomera dorsata (Diptera-Scatophagidae), which is basically a Pedicularis specialist. Both predators are attacked by larvae of Scambus brevicorais (Hymenoptera-Parasitica-Ichneumonidae). Total predation pressure was more or less constant during 1985–1987, but in 1988 the level was doubled, the possible reasons of which are discussed. Large inflorescences of B. alpina suffer significantly higher predation pressures than small ones. It is shown that predation is most intense in the middle of the inflorescences. The same floral nodes are known to produce more selfed seeds than distal and basal nodes. Seed predation in B. alpina thus results in an increased proportion of outcrossed seeds entering the seed pool. Selection pressures on host plant and predator fauna are discussed.     

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.     

Shell strength and primate seed predation of nontoxic species in eastern and southern Africa

The large African primates that eat fruit destroy the seeds of a number of fruiting species. This paper addresses several questions about seed-eating: What is the nature of the dietary niche provided by the nonpoisonous seeds of eastern and southern Africa? How well are these seeds mechanically protected? What other means of reducing seed predation are employed by the plants? and is the niche ecologically stable? Measurements of seed shell strength on 37 species from 17 families reveal a range of values, from <100-kg (numerous species) to over 2000-kg (palm nuts) breaking load. Primates crack open with their teeth seed shells from species exhibiting test strengths less than 600 kg. Variation in shell strength appears to increase dramatically for average species strengths above 100 kg. Plant species are not characterized by specific shell strengths but instead, display envelopes of shell strength overlapping broadly with other species. Taking this into account, adult male baboons (Papio spp.) appear to be dentally capable of preying upon most of the seed species of eastern and southern Africa. The possibility for predation of nonpoisonous seeds exists primarily because the plants periodically produce large crops in synchrony and the hard-shelled seeds are effectively dispersed, sometimes explosively but more often by means of edible fruits. The concomitant primate seed predation is a facultative specialization, of little apparent threat to the community of plants that support it.     

Leaf water relations and anatomy of a tropical rainforest tree species vary with crown position

Summary Leaf water potentials, osmotic properties and structural characteristics were examined in the Australian tropical rainforest tree species, Castanospermum australe. These features were compared for individuals growing in the understorey and canopy of the undisturbed forest and in an open pasture from which the forest had been cleared. Leaf water potentials during the day declined to significantly lower values in the open-grown and canopy trees than in the understorey trees. During most of the day the opengrown tree experienced the lowest water potentials. These differences were paralleled by significant differences in tissue osmotic properties. The tissue osmotic potential at full hydration was lowest in the open-grown tree (-1.80 MPa), intermediate in the canopy trees (-1.38 MPa), and highest in the understorey trees (-0.80 MPa). As a result, the degree to which high and positive turgor pressures were maintained as water potentials declined was highest in the open-grown tree, intermediate in the canopy trees, and lowest in the understorey trees. The differences in tissue osmotic properties between individuals in the three crown positions were paralleled, in turn, by differences in leaf structual characteristics. Relative to leaves of the canopy and open-grown trees, leaves of the understorey trees had significantly larger epidermal cells with thinner cell walls, larger specific leaf areas and turgid weight: dry weight ratios, and a higher proportion of intercellular air space.Abbreviations ₁ Leaf tissue water potential - _min Lowest value of ₁ during the day ( noon) - _{P=0 1} zero turgor - R Relative water content - P Tissue turgor pressure - Tissue osmotic potential - ₀ at full hydration     

Effects of light quantity and quality and soil nitrogen status on nitrate reductase activity in rainforest species of the genus Piper

Summary We studied nitrate reductase (NR) activity in six species of the genus Piper (Piperaceae) growing under a broad range of light availabilities. Field measurements were made on plants growing naturally in rainforest at the Los Tuxtlas Tropical Biological Preserve, Veracruz, Mexico at high- and lowlight extremes for each species. Foliar nitrogen on an area basis was positively related to the average daily photosynthetically active photon flux density (PFD) received by the leaf (r=0.76, p<0.01). In vivo NR activity was highly correlated with PFD (r=0.95, p<0.001) and less so with total leaf nitrogen (r=0.68, p<0.05). In vivo NR activity was always higher in high-light plants than in low-light plants within a species. Similarly, gap species such as P. auritum had much higher in vivo NR activities than shade species such as P. aequale. Soil NO ₃ ^– and NH ₄ ⁺ pools and nitrogen-mineralization rates at Los Tuxtlas were similar between high- and low-light sites, indicating that the elevated NR activities in high-light plants were not the result of higher NO ₃ ^– availabilities in high-light microsites. We performed additional experiments at Stanford, California, USA on Piper plants grown at high- and low-light. Foliar NR was highly inducible by nitrate in the gap species (auritum) but not in the generalist (hispidum) or shade (aequale) species. Root NR activities were, in general, an order of magnitude lower than foliar activities. In total, these studies suggest that Piper gap species are inherently more competent to assimilate NO ₃ ^– and are better able to respond to sudden increases in NO ₃ ^– availability than are shade species.CJWDPB publication # 1097     

Pre-dispersal seed predation reduces the reproductive compensatory advantage of thrum individuals in Erythroxylum havanense (Erythroxylaceae)

Male-sterility mutations in hermaphroditic species represent the first step in the evolution of gender specialization. Male-sterile individuals commonly compensate the loss of the male function by increasing the number or quality of seeds. Because the magnitude of compensation determines the maintenance of females within populations and the evolution of sexual specialization, plant-animal interactions may affect these evolutionary processes if animals are sensitive to such reproductive asymmetries. Here we explore the effect of seed predation on the reproductive compensation of Erythroxylum havanense, a distylous shrub with morph-biased partial male sterility, during two consecutive years. Seed predation reduced the compensatory advantage of thrums in 1987, but not in 1988. Annual differences in the intensity of seed predation seem to be accounted for differences in the onset and synchrony of flowering. Thus, although seed predators may reduce the compensatory advantage of thrums, their impact is modulated by the environmental cues triggering flowering and insect emergence.     

Factors affecting post-dispersal seed survival in a tropical forest

Summary Using the subcanopy tree Faramea occidentalis in Panama, I studied post-dispersal seed survival as a function of five characteristics describing seed locations. By simultaneously considering distance from a conspecific adult, size of the nearest conspecific adult, leaf litter quantity, proximity to logs or tree trunks, and whether or not the seed was in a gap, I was able to analyze the influences of individual factors, as well as the interactions among factors. Seed survival was significantly less in treefall gaps than in the forest understory. Seed survival was also influenced by the size of the nearest adult but in a complex interaction with distance to an adult. For seeds beneath adults, survival decreased with increasing tree size, while for seeds away from adults, survival was independent of the size of the nearest conspecific adult. Distance did not directly affect seed survival, nor did the quantity of leaf litter or the proximity to a tree trunk or a log. In a separate analysis, the relationship between distance and seed survival was consistent over four years, suggesting that single cohort studies may provide accurate insights into the consequences of dispersal. In contrast, the spatial locations of surviving seeds were not consistent over the four-year period. Transects with high survival one year did not tend to have high survival in other years, and the locations of surviving seeds in any particular year could not be predicted from the knowledge of where seeds survived in other years. While survival is patchy within a year, the locations of patches shift from year to year.     

Leaf-cutting ant preferences for five native tropical plantation tree species growing under different light conditions

To examine the susceptibility of five Costa Rican tree species to leaf-cutter ants (Atta cephalotes L.: Formicidae, Attini), young and mature leaves from trees that were grown in a plantation under full sun and partial shade were offered to six leaf-cutter colonies located in full sun and six in patial shade. In addition to offering leaf disks to the ants, we offered large pieces of leaves to assess the effect of cutting leaves on food choices. Leaf-cutters responded differently to each plant species, preferring Virola koschyni Warburg (Myristicaceae) and Hyeronima alchorneoides Allemao (Euphorbiaceae) over Stryphnodendrum microstachyum Poeppig & Endlicher (Mimosoideae), Pentaclethra macroloba Willdenow (Fabaceae) and Vochysia ferruginea Martius (Vochysiaceae). In agreement with previous studies, interspecific differences among the tree species in water and saponin content appeared to account for the observed ant preferences among the five tree species: leaf-cutter ants preferred leaves and disks with more water and less saponins and the five tree species varied significantly in these traits. An observed positive correlation between preference and phenolic/nitrogen ratio appears to be spurious, and is instead due to a negative correlation between water content and nitrogen content. For the first time, Atta nest location has been shown to affect consumption: nests located in partial shade removed more leaf material than those from the sun plots. However, nest location had no effect on preference ranking of the tree species tested. Pieces of leaves from all the tree species grown in partial shade were significantly more removed than those trees grown in full sun. Leaf age, toughness, leaf specific weights, ant activity, and colony were not correlated with food choices. Because of the observed preferences, the five tree species should not be considered as equal candidates for plantation purposes. However, complete characterization of the candidate status of tree species for plantations in the neotropics must include information on the ability of such species to tolerate pest attacks in addition to their natural defenses to attack.     

Survivorship and growth of seedlings of four dipterocarp species in a tropical rainforest of Sarawak,East Malaysia

Survivorship and growth of seedlings of four dipterocarp species (Dipterocarpus actangulus, D. globosus, Dryobalanops aromatica, Dryobalanops lanceolata) were studied for 2.5 years in a mixed dipterocarp forest in Sarawak, East Malaysia. Predispersal seed predation rates were larger forD. globosus (75%) thanD. lanceolata (27–34%) andD. aromatica (18–26%). Less than 20% of the twoDryobalanops seeds were damaged by vertebrates after seed dispersal. During the period from seed dispersal to the time when the seedlings had shed cotyledons, more dispersed seeds died in the twoDipterocarpus (ca 90%) than the twoDryobalanops (ca 60–70%). The major mortality factors during this period were uprooted and seed/seedling predation by insects or vertebrates. After the seedlings shed cotyledons, all species showed constant mortality rates of 34, 15–16, 17 and 6%/year forD. actangulus, D. lanceolata, D. aromatica andD. globosus, respectively, in the forest understorey. Mortality was lower in less shaded conditions than in more shaded ones forD. aromatica andD. actangulus, but not significantly different forD. lanceolata andD. globosus. A majority of dead seedlings were killed by fallen branches or were found standing with wilted leaves, probably due to water stress. No significant correlation was found between seed/seedling mortality and distance from mother trees or the initial density of seeds/seedings for all species. The mean leaf production was positively correlated with the estimated diffuse light factor of their habitats for each species.     

Diversity,composition, and structure of tropical dry forests in Central America

Tropical dry forests have been reduced to less than 0.1% of their original expanse on the Pacific side of Central America and are considered by some to be the most endangered ecosystem in the lowland tropics. Plots 1000 m² were established in seven tropical dry forests in Costa Rica and Nicaragua in order to compare levels of species richness to other Neotropical dry forest sites and to identify environmental variables associated with species richness and abundance. A total of 204 species and 1484 individuals 2.5 cm were encountered. Santa Rosa National Park was the richest site with the highest family (33), genera (69), and species (75) diversity of all sites. Species richness and forest structure were significantly different between sites. Fabaceae was the dominant tree and shrub family at most sites, but no species was repeatably dominant based on number of stems in all fragments of tropical dry forest. Central American dry forests had similar species richness when compared to other Neotropical forests. There was no correlation between forest cover within reserves, or precipitation and plant species richness. There was a significant correlation between anthropogenic disturbance (intensity and frequency of fire, wood collection, grazing) and total species richness, tree and shrub species richness, and liana abundance. These results suggest controlling levels on anthropogenic disturbance within reserves should be a high priority for resource managers in Central America. Further research in forest fragments which examine individual and a combination of disturbance agents would help clarify the importance of anthropogenic disturbance on species richness and abundance.     

Photosynthetic light acclimation in two rainforest Piper species with different ecological amplitudes

Summary Piper auritum (H.B. & K.), a pioneer tree restricted to open sites and Piper hispidum (Swartz), a shrub common in sites ranging from recent clearings to shaded understory, both adjust photosynthetic characteristics in response to light availability during growth. The sensitivity of photosynthetic capacity to light availability during growth was indistinguishable for the two species growing in their natural habitat. Photosynthetic capacity was strongly correlated with leaf nitrogen in both species, and the relationship was similar between species. Dark respiration and leaf specific mass were more sensitive to light during growth in P. hispidum, the species with the broad habitat ange, than in P. auritum. In general, similarities between the species were more striking than differences between them. The differences in dark respiration could have important implications for carbon balance. The difference in the responsiveness of leaf specific mass to light indicates that the broad-ranging species maintains access to modes of response little utilized by the open-site specialist. We did not and, in the gas exchange characteristics, any evidence that the open site specialist is better suited than the generalist to high-light sites.This CIW DPB publication number 962     

The role of pre- and post-dispersal seed predation in determining total seed loss

Most seed predation studies focus on either pre- or post-dispersal predation and may therefore underestimate the role of predation in regulating plant populations. We therefore estimated total seed predation of an invasive tree, mesquite (Leguminoseae: Prosopis spp.), by examining the entire seed pool from tree to seed bank. The spatio-temporal dynamics of total seed predation was examined by sampling across its Australian distribution and through time. The main predator was a host-specialist multivoltine beetle, Algarobius prosopis L. (Bruchidae), previously introduced as a biocontrol agent. Seed predation exceeded 20% in all seed stages (in pods on and off the tree, and seeds within woody endocarps (capsules) and free seeds on and in the ground) but was consistently highest in capsules on the ground (up to 90%). Pre-dispersal predation contributed little. Total seed predation rates were primarily determined by predation rates on the most persistent seed stage, in this case fallen pods if only pods are considered and seeds in capsules for the total seed pool. This pattern was consistent across the surveyed taxa, regions, years and seasonally. Predation rate was relatively unaffected by seed density, potentially because densities were always low (<150 seeds m⁻²). Average total seed predation within a region reached 55%, but we conclude that any population regulation of mesquite by seed predation will principally be through reduced seed bank persistence. Our results highlight the need to consider the entire seed pool, especially the often cryptic and overlooked long-lived stages, when determining seed loss to predation and its likely population consequences.     

食种子动物对三种锦鸡儿属植物繁殖更新的影响

锦鸡儿属(Caragana)植物是黄土高原植被的重要组分,具有固氮、耐旱的特性,对黄土丘陵沟壑区的植被恢复具有重要意义。作者于2002–2003年对该属本地种白毛锦鸡儿(C.licentiana)和甘蒙锦鸡儿(C.opulens)及外来种中间锦鸡儿(C.intermedia)种子被动物取食情况进行了比较研究。2003年白毛锦鸡儿种子散落前的虫害率为88.2%,甘蒙锦鸡儿为29.7%±1.7%,中间锦鸡儿为43.2%±4.8%。种子落地后,白毛锦鸡儿和甘蒙锦鸡儿的种子被小啮齿动物取食的比例在各种微生境下均为100%,且取食速度很快,在种子放置后4d内即全部被取食。中间锦鸡儿种子被取食率则因微生境的不同而不同:在灌丛下被取食率为100%,在半阳坡的开阔草地上为85.0%±10.0%,在种植植物未成功的水平阶地上为31.0%±8.7%,且取食速度相对较慢。研究结果表明,食种子动物的取食大大减少了白毛锦鸡儿和甘蒙锦鸡儿的种子量以及种子传播萌发的机会。但食种子动物可能充当了白毛锦鸡儿4.0%硬实种子的传播者,从而有助于白毛锦鸡儿实现种子途径的更新;中间锦鸡儿种子无论在散落前还是散落后受食种子动物的影响均很小。     

Assessing ant seed predation in threatened plants: a case study

Erodium paularense is a threatened plant species that is subject to seed predation by the granivorous ant Messor capitatus. In this paper we assessed the intensity and pattern of ant seed predation and looked for possible adaptive strategies at the seed and plant levels to cope with this predation. Seed predation was estimated in 1997 and 1998 at the population level by comparing total seed production and ant consumption, assessed by counting seed hulls in refuse piles. According to this method, ant seed predation ranged between 18% and 28%. A more detailed and direct assessment conducted in 1997 raised this estimate to 43%. In this assessment spatial and temporal patterns of seed predation by ants were studied by mapping all nest entrances in the studied area and marking the mature fruits of 109 reproductive plants with a specific colour code throughout the seed dispersal period. Intact fruit coats were later recovered from the refuse piles, and their mother plants and time of dispersal were identified. Seeds dispersed at the end of the dispersal period had a greater probability of escaping from ant seed predation. Similarly, in plants with late dispersal a greater percentage of seeds escaped from ant predation. Optimum dispersal time coincided with the maximum activity of granivorous ants because, at this time, ants focused their harvest on other plant species of the community. It was also observed that within-individual seed dispersal asynchrony minimised seed predation. From a conservation perspective, results show that the granivorous ant–plant interaction cannot be assessed in isolation and that the intensity of its effects basically depends on the seed dispersal pattern of the other members of the plant community. Furthermore, this threat must be assessed by considering the overall situation of the target population. Thus, in E. paularense, the strong limitation of safe-sites for seedling establishment reduces the importance of seed predation.     

Host-plant selection,diet diversity,and optimal foraging in a tropical leafcutting ant

Summary A month-long study was conducted on the comparative foraging behavior of 20 colonies of the leafcutting ant, Atta cephalotes L. in Santa Rosa National Park, Guanacaste Province, Costa Rica. The study was conducted during the middle of the wet season, when trees had mature foliage and the ants were maximally selective among species of potential host plants. The colonies always gathered leaves from more than a single tree species but on average one species constituted almost half the diet with the remaining species being of geometrically decreasing importance. Colonies exhibited greater diversity in their choice of leaves and lower constancy of foraging when the average quality of resource trees was lower, as predicted by elementary optimal foraging theory. Furthermore, the ants were more selective of the species they attacked at greater distances from the nest. However, the ants sometimes did not attack apparently palatable species, and often did not attack nearby individuals of species they were exploiting at greater distances.A classical explanation for why leafcutting ants exploit distant host trees when apparently equally good trees are nearer, is that the ants are pursuing a strategy of conserving resources to avoid long-term overgrazing pressure on nearby trees. We prefer a simpler hypothesis: (1) Trees of exploited species exhibit individual variation in the acceptability of their leaves to the ants. (2) The abundance of a species will generally increase with area and radial distance from the nest, so the probability that at least one tree of the species will be acceptable to the ants also increases with distance. (3) The ants forage using a system of trunk-trails cleared of leaf litter, which significantly reduces their travel time to previously discovered, high-quality resource trees (by a factor of 4- to 10-fold). (4) Foragers are unware of the total pool of resources available to the colony. Therefore once scouts have chanced upon a tree which is acceptable, the colony will concentrate on harvesting from that tree rather than searching for additional sources of leaves distant from the established trail.