Herbivores Rule

Directed seed dispersal of Piper by Carollia perspicillata and its effect on understory plant diversity and folivory

Directed dispersal occurs when seeds are differentially deposited to sites where offspring survivorship is higher than at randomly chosen sites. Traditionally, characteristics of the dispersal target sites that could increase survivorship of the dispersed plants are thought to be intrinsic to the sites. If directed dispersal is constant over extended periods of time, however, it is likely that nonrandom patterns of dispersal could modify the ecological characteristics of the target site in ways that could increase survivorship and fitness of the dispersed plants. Here we report patterns of Piper diversity (richness, equitability, and similarity) and Piper folivory within plots near natural or artificial roosts of Carollia perspicillata vs. similar plots without bat roosts. Plots with bat roosts, both natural and artificial, had significantly higher Piper species diversity. Additionally, we found that plots with a higher Piper species diversity showed less specialist folivory, higher generalist folivory, and lower total herbivore leaf damage than plots with low Piper diversity. Finally, plots with bat roosts also showed less specialist folivory, lower generalist folivory, and lower total folivory when compared to plots without roosts. We propose that long-lasting nonrandom patterns of seed dispersal can change the local ecological characteristics of target sites via changes in plant diversity, and that these changes are likely to reduce the local rates of folivory and, therefore, increase seed and adult plant survivorship.

Top-down control of herbivory by birds and bats in the canopy of temperate broad-leaved oaks (Quercus robur)

The intensive foraging of insectivorous birds and bats is well known to reduce the density of arboreal herbivorous arthropods but quantification of collateral leaf damage remains limited for temperate forest canopies. We conducted exclusion experiments with nets in the crowns of young and mature oaks, Quercus robur, in south and central Germany to investigate the extent to which aerial vertebrates reduce herbivory through predation. We repeatedly estimated leaf damage throughout the vegetation period. Exclusion of birds and bats led to a distinct increase in arthropod herbivory, emphasizing the prominent role of vertebrate predators in controlling arthropods. Leaf damage (e.g., number of holes) differed strongly between sites and was 59% higher in south Germany, where species richness of vertebrate predators and relative oak density were lower compared with our other study site in central Germany. The effects of bird and bat exclusion on herbivory were 19% greater on young than on mature trees in south Germany. Our results support previous studies that have demonstrated clear effects of insectivorous vertebrates on leaf damage through the control of herbivorous arthropods. Moreover, our comparative approach on quantification of leaf damage highlights the importance of local attributes such as tree age, forest composition and species richness of vertebrate predators for control of arthropod herbivory.

Effects of sap-feeding insect herbivores on growth and reproduction of woody plants: a meta-analysis of experimental studies

The majority of generalisations concerning plant responses to herbivory are based on studies of natural or simulated defoliation. However, effects caused by insects feeding on plant sap are likely to differ from the effects of folivory. We assessed the general patterns and sources of variation in the effects of sap feeding on growth, photosynthesis, and reproduction of woody plants through a meta-analysis of 272 effect sizes calculated from 52 papers. Sap-feeders significantly reduced growth (-29%), reproduction (-17%), and photosynthesis (-27%); seedlings suffered more than saplings and mature trees. Deciduous and evergreen woody plants did not differ in their abilities to tolerate damage imposed by sap-feeders. Different plant parts, in particular below- and above-ground organs, responded similarly to damage, indicating that sap-feeders did not change the resource allocation in plants. The strongest effects were caused by mesophyll and phloem feeders, and the weakest by xylem feeders. Generalist sap-feeders reduced plant performance to a greater extent than did specialists. Methodology substantially influenced the outcomes of the primary studies; experiments conducted in greenhouses yielded stronger negative effects than field experiments; shorter (<12 months) experiments showed bigger growth reduction in response to sap feeding than longer experiments; natural levels of herbivory caused weaker effects than infestation of experimental plants by sap-feeders. Studies conducted at higher temperatures yielded stronger detrimental effects of sap-feeders on their hosts. We conclude that sap-feeders impose a more severe overall negative impact on plant performance than do defoliators, mostly due to the lower abilities of woody plants to compensate for sap-feeders' damage in terms of both growth and photosynthesis.

Differential effects of mammalian seed predators on the regeneration of five Papua New Guinean tree species and implications for sapling recruitment

Although herbivores may account for a significant source of seed and seedling mortality in many tropical tree species, plant species differ in their response to seed damage. Here we investigate the relative effects of seed predation on the regeneration of five tree species in a mid-elevation Papua New Guinean rain forest. Exclosure treatments and shade-house experiments were monitored from November 2004 to March 2006 to assess the differential effects of seed predation on seed viability and seedling growth. Results indicate that although seed predators attack all five focal species, they influence the seedling populations in two, Cerbera floribunda and Microcos grandiflora, and minimally affect the seedling populations of Terminalia impediens, Pandanus penicillus and Endiandra latifolia in the years measured. Predation and germination frequencies were compared to the abundance of focal species at several life stage classes to explore potential correlations between species-specific seed mortality patterns and life stage distributions. We found that the species-specific influence of mammalian seed predators correlated with abundance distributions in three life stages. Species with high survivorship after seed predator attacks displayed a significant decrease in abundance from the seedling-to-sapling transition, while those species with high seed mortality demonstrated relatively even distributions of seedlings, saplings and adults (> 10 cm dbh). These contrasting patterns suggest that differential seed predation effects on regeneration may play a key role in the recruitment of individuals to the sapling stage.