Host specificity of insect herbivores in tropical forests

Studies of host specificity in tropical insect herbivores are evolving from a focus on insect distribution data obtained by canopy fogging and other mass collecting methods, to a focus on obtaining data on insect rearing and experimentally verified feeding patterns. We review this transition and identify persisting methodological problems. Replicated quantitative surveys of plant-herbivore food webs, based on sampling efforts of an order of magnitude greater than is customary at present, may be cost-effectively achieved by small research teams supported by local assistants. Survey designs that separate historical and ecological determinants of host specificity by studying herbivores feeding on the same plant species exposed to different environmental or experimental conditions are rare. Further, we advocate the use of host-specificity measures based on plant phylogeny. Existing data suggest that a minority of species in herbivore communities feed on a single plant species when alternative congeneric hosts are available. Thus, host plant range limits tend to coincide with those of plant genera, rather than species or suprageneric taxa. Host specificity among tropical herbivore guilds decreases in the sequence: granivores > leaf-miners > fructivore > leaf-chewers = sap-suckers > xylophages > root-feeders, thus paralleling patterns observed in temperate forests. Differences in host specificity between temperate and tropical forests are difficult to assess since data on tropical herbivores originate from recent field studies, whereas their temperate counterparts derive from regional host species lists, assembled over many years. No major increase in host specificity from temperate to tropical communities is evident. This conclusion, together with the recent downward revisions of extremely high estimates of tropical species richness, suggest that tropical ecosystems may not be as biodiverse as previously thought.     

Above- and belowground insect herbivores differentially affect soil nematode communities in species-rich plant communities

Interactions between above‐ and belowground invertebrate herbivores alter plant diversity, however, little is known on how these effects may influence higher trophic level organisms belowground. Here we explore whether above‐ and belowground invertebrate herbivores which alter plant community diversity and biomass, in turn affect soil nematode communities. We test the hypotheses that insect herbivores 1) alter soil nematode diversity, 2) stimulate bacterial‐feeding and 3) reduce plant‐feeding nematode abundances. In a full factorial outdoor mesocosm experiment we introduced grasshoppers (aboveground herbivores), wireworms (belowground herbivores) and a diverse soil nematode community to species‐rich model plant communities. After two years, insect herbivore effects on nematode diversity and on abundance of herbivorous, bacterivorous, fungivorous and omni‐carnivorous nematodes were evaluated in relation to plant community composition. Wireworms did not affect nematode diversity despite enhanced plant diversity, while grasshoppers, which did not affect plant diversity, reduced nematode diversity. Although grasshoppers and wireworms caused contrasting shifts in plant species dominance, they did not affect abundances of decomposer nematodes at any trophic level. Primary consumer nematodes were, however, strongly promoted by wireworms, while community root biomass was not altered by the insect herbivores. Overall, interaction effects of wireworms and grasshoppers on the soil nematodes were not observed, and we found no support for bottom‐up control of the nematodes. However, our results show that above‐ and belowground insect herbivores may facilitate root‐feeding rather than decomposer nematodes and that this facilitation appears to be driven by shifts in plant species composition. Moreover, the addition of nematodes strongly suppressed shoot biomass of several forb species and reduced grasshopper abundance. Thus, our results suggest that nematode feedback effects on plant community composition, due to plant and herbivore parasitism, may strongly depend on the presence of insect herbivores.     

Canopy insect herbivores in the Azorean Laurisilva forests: key host plant species in a highly generalist insect community

This article explores patterns of insect herbivore distribution in the canopy of the Laurisilva forests on seven islands in the Azores archipelago. To our knowledge, this is one of the first extensive study of this type in tree or shrub canopies of oceanic island ecosystems. One of the most frequently debated characteristics of such ecosystems is the likely prevalence of vague, ill‐defined niches due to taxonomic disharmony, which may have implications for insect‐plant interactions. For instance, an increase in ecological opportunities for generalist species is expected due to the lack of predator groups and reduced selection for chemical defence in host plants. The following two questions were addressed: 1) Are specialists species rare, and insect herbivore species randomly distributed among host plant species in the Azores? 2) Are the variances in insect herbivore species composition, frequency and richness explained by host plants or by regional island effects? We expect a proportional distribution of herbivore species between host plants, influenced by host frequency and distinct island effects; otherwise, deviation from expectation might suggest habitat preference for specific host tree crowns. Canopy beating tray samples were performed on seven islands, comprising 50 transects with 1 to 3 plant species each (10 replicates per species), giving 1320 samples from ten host species trees or shrubs in total. From a total of 129 insect herbivore species, a greater number of herbivore species was found on Juniperus brevifolia (s=65) and Erica azorica (s=53). However, the number of herbivore species per individual tree crown was higher for E. azorica than for any other host, on all islands, despite the fact that it was only the fourth more abundant plant. In addition, higher insect species richness and greater insect abundance were found on the trees of Santa Maria Island, the oldest in the archipelago. Insect species composition was strongly influenced by the presence of E. azorica, which was the only host plant with a characteristic fauna across the archipelago, whereas the fauna of other plant crowns was grouped by islands. The great insect occurrence on E. azorica reflects strong habitat fidelity, but only four species were clearly specialists. Our findings indicate a broadly generalist fauna. The simplicity of Azorean Laurisilva contributed to the understanding of insect‐plant mechanisms in canopy forest habitats.     

Linking climate change and species invasion: an illustration using insect herbivores

Climate change and invasive species are two of the most important ecological issues facing the world today. Yet, to date these two factors have largely been viewed independently. In order to prevent large-scale economic and environmental damage and as a first step towards predicting and preventing invasions, it is important to understand the factors affecting invasions. Here, we focus on insect herbivores and link the climate change and invasive research fields. We illustrate using existing published research that life history traits can be useful indicators of future invasive potential. However, climate change will also affect propagule pressure and the communities into which invaders will arrive. With the aid of a meta-analysis we show that climate-induced community changes are likely to increase niche-availability in the future, further exacerbating the problem of invasive species. It is timely and important that further research linking these two important ecological threats is undertaken.     

The role of indirect interactions in structuring tropical insect communities

Apparent competition is a form of indirect interaction among species that can potentially structure biological communities. In insect communities, parasitoid-mediated apparent competition has been proposed as a particularly important structuring force. We argue that short-term apparent competition may be less important in structuring insect communities in tropical regions, compared with temperate regions. This prediction arises because, compared with temperate insects, tropical insects that share natural enemies are more likely to be isolated in both space and time.     

Seasonal and diel variations in the activity of canopy insect herbivores differ between deciduous and evergreen plant species in a tropical dry forest

This study evaluated whether herbivorous insects can be expected to have particular adaptations to withstand the harsh dry season in tropical dry forests (TDFs). We specifically investigated a possible escape in space, with herbivorous insects moving to the few evergreen trees that occur in this ecosystem; and escape in time, with herbivores presenting an increased nocturnal rather than diurnal activity during the dry season. We determined the variation in the free-feeding herbivorous insects (sap-sucking and leaf chewing) between seasons (beginning and middle of both rainy and dry seasons), plant phenological groups (deciduous and evergreen trees) and diel period (diurnal and nocturnal) in a Brazilian TDF. We sampled a total of 5827 insect herbivores in 72 flight-interception traps. Contrary to our expectations, we found a greater herbivore diversity during the dry season, with low species overlap among seasons. In the dry season, evergreen trees supported greater richness and abundance of herbivores as compared to deciduous trees. Insects were also more active at night during the dry season, but no diel differences in insect abundance were detected during the rainy season. These results indicate that the strategies used by insect herbivores to withstand the severe climatic conditions of TDFs during the dry season include both small-scale escape in space and time, with evergreen trees playing a key role in maintaining resident insect herbivore populations in TDFs. Relatively more nocturnal activity during the dry season may be related to the avoidance of harsh climatic conditions during the day. We suggest that the few evergreen tree species occurring in the TDF landscape should be especially targeted for protection in this threatened ecosystem, given their importance for insect conservation.     

Nutrient recycling by insect and fish communities in high-elevation tropical streams

Hydrobiologia - High- to mid-elevation streams are often oligotrophic, but harbor diverse groups of aquatic animals that can satisfy a substantial proportion of nutrient demand. Therefore, we...     

Evaluation of intraguild interactions between two species of insect herbivores on Pistia stratiotes

Pistia stratiotes L. (Araceae) is an important weed in many waterways around the world. The South American weevil Neohydronomus affinis Hustache (Coleoptera: Curculionidae) is a successful biocontrol agent for this weed but additional agents are needed for some areas. The planthopper Lepidelphax pistiae Remes Lenicov (Hemiptera: Delphacidae) is a specific herbivore of P. stratiotes and is highly damaging in laboratory conditions. A field experiment was designed to evaluate the damage potential of L. pistiae compared to N. affinis and to assess the potential for competition between the two herbivores. Both herbivore species were reared inside floating cages on a P. stratiotes-infested lake. Plant growth indices assessed were clonation levels, biomass and plant diameter. The growth indices of plants exposed to the herbivores, individually and combined, were comparable, and significantly lower than in the control treatments. Population levels of N. affinis inside the cages were not significantly different when alone or together with L. pistae. Conversely, the density of L. pistiae was significantly lower in combination with the weevil. This suggests that L. pistiae is a damaging herbivore that will not reduce the biological control effectiveness of N. affinis.     

Plant tannins and insect herbivores: an appraisal

Abstract. 1. The bioassays with tannins and insects, and the ecological studies on insects implicating tannins, are summarized and discussed.
2. Because of the great variation now shown in all aspects of the insect-tannin relationship, the difficulty of making generalizations is stressed.
3. The significance of plant tannins for insect herbivores is reconsidered in the light of recent work and little-known older work, which illustrate the very varied nature of its effects.     

Mechanisms of plant defense against insect herbivores

Plants respond to herbivory through various morphological, biochemicals, and molecular mechanisms to counter/offset the effects of herbivore attack. The biochemical mechanisms of defense against the herbivores are wide-ranging, highly dynamic, and are mediated both by direct and indirect defenses. The defensive compounds are either produced constitutively or in response to plant damage, and affect feeding, growth, and survival of herbivores. In addition, plants also release volatile organic compounds that attract the natural enemies of the herbivores. These strategies either act independently or in conjunction with each other. However, our understanding of these defensive mechanisms is still limited. Induced resistance could be exploited as an important tool for the pest management to minimize the amounts of insecticides used for pest control. Host plant resistance to insects, particularly, induced resistance, can also be manipulated with the use of chemical elicitors of secondary metabolites, which confer resistance to insects. By understanding the mechanisms of induced resistance, we can predict the herbivores that are likely to be affected by induced responses. The elicitors of induced responses can be sprayed on crop plants to build up the natural defense system against damage caused by herbivores. The induced responses can also be engineered genetically, so that the defensive compounds are constitutively produced in plants against are challenged by the herbivory. Induced resistance can be exploited for developing crop cultivars, which readily produce the inducible response upon mild infestation, and can act as one of components of integrated pest management for sustainable crop production.     

Indirect plant defense against insect herbivores: a review

Plants respond to herbivore attack by launching 2 types of defenses: direct defense and indirect defense. Direct defense includes all plant traits that increase the resistance of host plants to insect herbivores by affecting the physiology and/or behavior of the attackers. Indirect defense includes all traits that by themselves do not have significant direct impact on the attacking herbivores, but can attract natural enemies of the herbivores and thus reduce plant loss. When plants recognize herbivore‐associated elicitors, they produce and release a blend of volatiles that can attract predators, parasites, and other natural enemies. Known herbivore‐associated elicitors include fatty acid–amino acid conjugates, sulfur‐containing fatty acids, fragments of cell walls, peptides, esters, and enzymes. Identified plant volatiles include terpenes, nitrogenous compounds, and indoles. In addition, constitive traits including extrafloral nectars, food bodies, and domatia can be further induced to higher levels and attract natural enemies as well as provide food and shelter to carnivores. A better understanding of indirect plant defense at global and componential levels via advanced high throughput technologies may lead to utilization of indirect defense in suppression of herbivore damage to plants.     

Comparison of tree genotypic diversity and species diversity effects on different guilds of insect herbivores

Although the effects of plant diversity on herbivores are contingent upon herbivore traits and the source of plant diversity (e.g. intra‐ and interspecific), most studies have analyzed these effects separately. We compared the effects of genotypic diversity of big‐leaf mahogany Swietenia macrophylla with that of tree species diversity on two specialist caterpillars (Hypsipyla grandella stem borers and Phyllocnistis meliacella leaf miners) and three generalist leafhoppers (Cicadellidae) feeding on mahogany in a large‐scale (7.2 ha) forest diversity experiment in southern Mexico. The experiment consisted of fifty‐nine 21 × 21‐m plots, with 64 tree saplings each (3‐m spacing between plants). Plots were either mahogany monocultures or species polycultures of four species (including mahogany) and – within each of these two plot types – mahogany was represented by either one or four genotypes. Throughout a five‐month period, beginning six months after planting, we measured mahogany growth and monitored herbivore and predator (spider) abundance. We found no effect of mahogany genotypic diversity on either specialist caterpillars or generalist leafhoppers, and this result was consistent across levels of tree species diversity. In contrast, species diversity had significant effects on both specialists but neither of the generalist herbivores. Specifically, species diversity lowered H. grandella attack at the middle of the sampling season, but increased attack at the end of the season, whereas P. meliacella abundance was consistently reduced. Such effects were not mediated by effects of species diversity on plant growth (of which there were none), but rather through resource heterogeneity. Diversity did not influence spider abundance. This study is one of few to directly compare sources of plant diversity, and uniquely compares such effects among herbivores with contrasting life histories (e.g. diet breadths). Overall, we demonstrate that plant species diversity effects outweigh those of genotypes, and our results suggest that such effects are stronger on specialist than generalist herbivores.     

From species to communities: the signature of recreational use on a tropical river ecosystem

Disturbance can impact natural communities in multiple ways. However, there has been a tendency to focus on single indicators of change when examining the effects of disturbance. This is problematic as classical diversity measures, such as Shannon and Simpson indices, do not always detect the effects of disturbance. Here, we instead take a multilevel, hierarchical approach, looking for signatures of disturbance in the capacity and diversity of the community, and also in allocation and demography at the population level. Using recreational use as an example of disturbance, and the freshwater streams of Trinidad as a model ecosystem, we repeatedly sampled the fish communities and physical parameters of eight pairs of recreational and nonrecreational sites every 3 months over a 28‐month period. We also chose the Trinidadian guppy (Poecilia reticulata) as the subject of our population‐level analyses. Regression tree analysis, together with analysis of deviance, revealed that community capacity and community species richness were greater at sites with higher levels of recreational use. Interestingly, measures of community diversity that took into account the proportional abundance of each species were not significantly associated with recreational use. Neither did we find any direct association between recreational use and proportion of guppy biomass in the community. However, population‐level differences were detected in the guppy: Sex ratio was significantly more female‐biased at more disturbed sites. Our findings emphasize the importance of considering multiple levels when asking how disturbance impacts a community. We advocate the use of a multilevel approach when monitoring the effects of disturbance, and highlight gaps in our knowledge when it comes to interpreting these effects.     

Contrasting impacts of highly invasive plant species on flower-visiting insect communities

Invasive alien plants threaten biodiversity, ecosystems and service provision worldwide. They can have positive and negative direct and indirect effects on herbivorous insects, including those that provide pollination services. Here, we quantify how three highly invasive plant species (Heracleum mantegazzianum, Impatiens glandulifera and Fallopia japonica) influence the availability of floral resources and flower-visiting insect communities. We compared invaded with comparable uninvaded areas to assess floral resources and used pan-trapping to quantify insect communities. Only F. japonica influenced floral resource availability: sites invaded by this species had a higher flowering plant species richness and abundance of open floral units than uninvaded sites, probably due to its late flowering and the paucity of other flowering species at this time of year. Fallopia japonica was also associated with higher abundances of bumblebees, higher overall insect diversity and higher hoverfly diversity than uninvaded areas. Differences in pollinator communities were also associated with I. glandulifera and H. mantegazzianum, despite there being no detectable differences in floral resources at these sites. Specifically, there were more bumblebees and solitary bees in I. glandulifera sites, and a higher overall diversity of insects, particularly hoverflies. By contrast, H. mantegazzianum sites had a lower abundance of solitary bees and hoverflies. These findings confirm that invasive plant species have a range of species-specific effects on ecological communities. This supports the emerging view that control of invasive species, as required under international obligations, is not simple and that potential losses and gains for biodiversity must be carefully evaluated on a case-by-case basis.     

Non‐random correlation of species dynamics in tropical tree communities

The importance of environmental stochasticity for tropical tree dynamics has been recently stressed by several studies. This has spurred the development of a ‘time‐averaged neutral model’ of community dynamics by Kalyuzhny and colleagues that extends the neutral model by incorporating environmental stochasticity. We here show that this framework can be used to assess the presence of non‐random correlations between species dynamics. Indeed, the time‐averaged neutral model makes the simplifying assumption that species responses to environmental variation are uncorrelated. We therefore propose to use this model as a null hypothesis against which observed community dynamics can be compared. This study makes five contributions. First, we describe a novel time‐averaged neutral model of community dynamics that is close to, but more flexible than the one previously proposed by Kalyuzhny and colleagues. Second, we develop an inference method based on approximate Bayesian computation (ABC) and demonstrate the identifiability of the model parameters from community time series data. Third, we develop a test of the significance of environmental stochasticity, and a method to quantify its contribution to population variance. Fourth, we develop a test of non‐random correlation between species dynamics. Fifth, we apply these developments to three datasets of tropical tree dynamics. We evidence both a strong contribution of environmental stochasticity to population variance in the three datasets, and a non‐random correlation of species dynamics in one of them. We finally discuss the implications of these results for the modelling of tropical tree community dynamics.     

Possible causes of high species diversity in tropical Australian freshwater macrobenthic communities

A total of 133 benthic macroinvertebrate species was recorded in four permanent freshwater lagoons (billabongs) and a stream riffle in the Magela Creek catchment, Northern Territory, Australia. 101 species were lentic in occurrence. Each billabong supported at least as many species as larger tropical and Australian temperate lakes, and was an exception to the species-poor status of Australian lentic macrobenthic communities. > 98% of the total benthic biomass was contributed by a mussel species; the remaining species' combined standing crop was an order of magnitude lower than in other lentic systems, further highlighting the diversity of the Magela benthos. Rarefaction (removal of organisms) and predictable environmental heterogeneity, both related to monsoonal variations in flow and water quality, were probably important determinants of the high diversity of these communities.