Contrasting effects of insect and molluscan herbivores on plant diversity in a long-term field experiment

The importance of invertebrate herbivores in regulating plant communities remains unclear, due to the absence of long-term exclusion experiments. An experiment in an English grassland involving long-term exclusions of insect and mollusc herbivores, along with rabbit fencing, showed strong, but opposing, effects of the invertebrate herbivores. Plant species richness declined and biomass increased following insect exclusion, due to increased dominance by a grass species, whereas mollusc exclusion led to increased herbs abundance. The two herbivores had a compensatory interaction: molluscs had no effects in the absence of insects and large insect effects depended on the absence of molluscs. The effects of invertebrate exclusion became apparent only after 8 years, and would have been seriously underestimated in shorter studies. Our results suggest that theorists and conservation managers need to shift from their historic focus on vertebrate herbivory, to a recognition that invertebrates can be equally important drivers of plant community structure.     

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.     

Comparing galling insect richness among Neotropical savannas: effects of plant richness, vegetation structure and super-host presence

Plant species diversity maintains the stability of ecosystems and the diversity of consumer species such as insect herbivores. Considering that gall-inducing insects are highly specialized on their host plants and dependent on the occurrence, abundance and distribution of plants, we evaluated the diversity patterns of gall-inducing insect along Brazilian Neotropical savannas and the potential role of plant species richness, vegetation structure and super-host presence on determining these patterns. We found 1,882 individual plants that belonged to 64 different host plant species grouped in 31 families, associated to 112 galling insect species. The galling richness was positively influenced by plant species richness and the presence of the super-host genus Qualea (Vochysiaceae). Plant species richness explained 48 % of the galling richness and areas with presence of super-hosts had more than twice of galling species than areas where they were absent. On the other hand, we found no evidence that larger plants hosted more species of galling insects. We observed that for the diversity of galling insects in the Brazilian Cerrado, vegetation structure explained almost the same portion as plant richness, because structural variables did not have an effect on residuals of galling richness and plant richness regression. Our findings suggests that plant richness has a more important role on the mitigation of natural enemies and adaptive radiation of galling species, while structural aspects of the vegetation does not seem to have that effect. Furthermore, we show that the super-host taxa provide an increment in local galling richness because they present a great diversity of local number of gall morphospecies (i.e. alpha diversity) and the high turnover of morphospecies among different localities (i.e. beta diversity). Therefore we argue that the quality of resources (richness and super host presence) appears to be a most important factor for the diversity of galling insects in Neotropical systems, than the amount of resources.     

Foliage-dwelling ants in a neotropical savanna: effects of plant and insect exudates on ant communities

Ant dominance in tropical ecosystems can be explained by a capacity to exploit liquid foods such as extrafloral nectaries (EFNs) and secretions from honeydew-producing hemipterans (HPHs). Such nutritious exudates may determine ant distribution in space and shape specialization in ant–plant interactions. We provide a first assessment of how EFNs and HPHs mediate the structure of ant assemblages, ant visitation intensity, and characteristics of ant–plant interaction networks across space in Brazilian “cerrado” savanna. We used arboreal pitfall traps to sample visiting ants in four cerrado localities and recorded the presence of lepidopteran larvae to determine their possible response to ant visitation. Ant species composition and richness did not differ regardless of the presence of liquid rewards on plants, and most network patterns did not show consistent differences. However, in two of the four sites, ant densities were higher on plants with HPHs or EFNs due to increased activity by Camponotus and Pseudomyrmex ants. At these two sites, plants with liquid food sources had a more specific ant assemblage (higher specialization d′) than did plants without resources, and caterpillars were more frequently found on plants with fewer workers of Camponotus and Pseudomyrmex. Plants with HPHs had increased ant visitation and accumulated more ant species than did plants with EFNs or without liquid foods. Ant response to such food sources may thus depend on local conditions and identity of ant species, and may determine how ant assemblages are structured. Results highlight how different patterns of ant visitation to liquid resources can produce distinctive effects on herbivore infestation.     

Effects of plant species richness and composition on epifaunal colonization in brackish water angiosperm communities

Studies conducted in terrestrial ecosystems have shown that increasing plant diversity enhances ecosystem processes such as primary production. In marine systems, knowledge of how plant diversity influences ecosystem processes and higher trophic levels is still limited. To examine how plant richness and composition influence recruitment and colonization processes, defaunated eelgrass (Zostera marina), sago pondweed (Potamogeton pectinatus) and perfoliate pondweed (Potamogeton perfoliatus) were planted on an unvegetated, sandy bottom in all possible combinations in July and August. Both field experiments lasted 1 week. Our results showed that in these plant assemblages plant richness had a negative effect on faunal diversity (H′) and evenness (J′), while total abundance showed strong temporal variation with weak, positive relationships with plant species richness in both August and July. Plant species composition had strong effects on amphipods (Gammarus spp.), thus both the abundance and biomass were higher in treatments containing P. perfoliatus. The colonization process was significantly influenced by the numerical dominance of a few faunal species, e.g. the settlement of lagoon cockles (Cerastoderma glaucum), and by the timing of the experiments. The results indicate that faunal colonization in these communities is rapid and significantly influenced by the traits of particular plant species.     

Linking plant genes to insect communities: Identifying the genetic bases of plant traits and community composition

Community genetics aims to understand the effects of intraspecific genetic variation on community composition and diversity, thereby connecting community ecology with evolutionary biology. Thus far, research has shown that plant genetics can underlie variation in the composition of associated communities (e.g., insects, lichen and endophytes), and those communities can therefore be considered as extended phenotypes. This work, however, has been conducted primarily at the plant genotype level and has not identified the key underlying genes. To address this gap, we used genome‐wide association mapping with a population of 445 aspen (Populus tremuloides) genets to identify the genes governing variation in plant traits (defence chemistry, bud phenology, leaf morphology, growth) and insect community composition. We found 49 significant SNP associations in 13 Populus genes that are correlated with chemical defence compounds and insect community traits. Most notably, we identified an early nodulin‐like protein that was associated with insect community diversity and the abundance of interacting foundation species (ants and aphids). These findings support the concept that particular plant traits are the mechanistic link between plant genes and the composition of associated insect communities. In putting the “genes” into “genes to ecosystems ecology”, this work enhances understanding of the molecular genetic mechanisms that underlie plant–insect associations and the consequences thereof for the structure of ecological communities.     

Feedback effects between plant and flower-visiting insect communities along a primary succession gradient

Primary successions of glacier forelands are unique model systems to investigate community dynamics and assembly processes. However, successional changes of plant and insect communities have been mainly analysed separately. Therefore, changes in plant–insect interactions along successional gradients on glacier forelands remain unknown, despite their relevance to ecosystem functioning. This study assessed how successional changes of the vegetation influenced the composition of the flower-visiting insect assemblages of two plant species, Leucanthemopsis alpina (L.) Heyw. and Saxifraga bryoides L., selected as the only two insect-pollinated species occurring along the whole succession. In addition, we investigated the links between reproductive output of these plants and pollinator abundance through experimental exclusion of pollinators. Plant community structure changed along the succession, affecting the distribution and the abundance of insects via idiosyncratic responses of different insect functional groups. L. alpina interacted with ubiquitously distributed pollinators, while S. bryoides pollinators were positively associated with insect-pollinated plant species density and S. bryoides abundance. With succession proceeding, insect assemblages became more functionally diverse, with the abundance of parasitoids, predators and opportunists positively related to an increase in plant cover and diversity. The reproductive output of both plant species varied among successional stages. Contrary to our expectation, the obligate insect-pollinated L. alpina showed a reproductive output rather independent from pollinator abundance, while the reproductive output of the self-fertile S. bryoides seemed linked to pollinator abundance. Observing ecological interactions and using functional traits, we provided a mechanistic understanding of community assembly processes along a successional gradient. Plant community diversity and cover likely influenced insect community assembly through bottom-up effects. In turn, pollinators regulate plant reproductive output through top-down control. We emphasise that dynamics of alpine plant and insect communities may be structured by biotic interactions and feedback processes, rather than only be influenced by harsh abiotic conditions and stochastic events.     

Relative abundance distributions in plant communities: effects of species richness and of spatial scale

Abstract. Relative abundance distributions (RADs) are an important feature of community structure, but little is known of the factors affecting which type of RAD is observed in a particular community. We examined the influences of species richness and of spatial scale on the RAD of plant communities. The effect of species richness was examined by analysing simulated communities generated under the Broken stick model, the Sequential breakage model, and a randomized version of Niche pre-emption model. In all cases, when there were few species in the community the data only occasionally gave the best fit to the model that was used to generate it. With 40–65 species, a best fit was obtained for the correct model in about 75 % of cases, almost irrespective of the model. Effects of spatial scale were examined in data from four dune slacks and from two semi-arid grasslands, by analysing biomass values at a range of sample sizes. The model that best fitted the whole sample differed between the four slacks and between the slacks and the semi-arid grasslands. The change in which model of RAD fitted best, as sample size was reduced, varied between sites and between habitat types. At the smallest sample sizes, the Zipf-Mandelbrot model often fitted, and in the slack sites the Broken stick also, though neither fitted (in the vegetation examined) at larger spatial scales. It is concluded the RAD is affected by species richness and by spatial scale, in ways that currently do not enable simple prediction. RADs can theoretically give information on the processes such as resource partitioning, immigration and competition that have structured the community, but they are a blunt tool for this purpose.     

Effect of plant species richness on invasibility of experimental plant communities

Invasion of unsown species to artificially created assemblages of grassland species was investigated in a 3-year field experiment. In the experiment, assemblages varying in species richness (1, 2, 4, 8, and 16 species) and functional group richness (1–4, grasses, legumes, rosette forbs, and creeping forbs) were grown in control and fertilized plots, without any attempt to prevent the invasion of unsown species or to weed them. The relationship between species and functional group diversity and above-ground biomass was positive for sown species in all study years (2003, 2004 and 2005). In the latter 2 years, weed invader biomass decreased significantly with increasing biomass of sown species and their functional group richness, but not with number of species. However, no suppressive effect of species or functional group richness beyond that by increased biomass of residents was found. In fact, slight but significant positive partial effect of species richness was found, suggesting that the negative effect of the same amount of resident biomass on invaders is stronger when the biomass is composed of fewer species. The negative relationship between the number of functional groups of residents and invader biomass suggested that better coverage of functional trait space could be a mechanism promoting the resistance to invasion. In Addition, species composition of invaders were significantly related to initial composition of sown residents.     

Contrasting cascade effects of carnivores on plant fitness: a meta-analysis

1. Although carnivores indirectly improve plant fitness by decreasing herbivory, they may also decrease plant reproduction by disrupting plant-pollinator mutualism. The overall magnitude of the resulting net effect of carnivores on plant fitness and the factors responsible for the variations in strength and direction of this effect have not been explored quantitatively to date. 2. We performed a meta-analysis of 67 studies containing 163 estimates of the effects of carnivores on plant fitness and examined the relative importance of several potential sources of variation in carnivore effects. 3. Carnivores significantly increased plant fitness via suppression of herbivores and decreased fitness by consuming pollinators. The overall net effect of carnivores on plant fitness was positive (32% increase), indicating that effects via herbivores were stronger than effects via pollinators. 4. Parasitoids had stronger positive effect on plant fitness than predators. Active hunters increased plant fitness, whereas stationary predators had no significant effect, presumably because they were more prone to disrupt plant-pollinator mutualism. Carnivores with broader habitat domain had negative effects on plant fitness, whereas those with narrow habitat domain had positive effects. 5. Predator effects were positive for plants which offered rewards (e.g. extrafloral nectaries) and negative for plants which lacked any attractors. 6. This study adds new knowledge on the factors that determine the strength of terrestrial trophic cascades and highlights the importance of considering simultaneous contrasting interactions in the same study system.     

The effects of trophic structure and soil fertility on the assembly of plant communities: a microcosm experiment

Using closed, ventilated outdoor microcosms, plant communities were allowed to assemble from a pool of 48 herbaceous species comprising a wide range of plant functional types. The experiment involved factorial manipulation of soil fertility, invertebrate herbivores (slugs, grass aphids) and their predators (ground beetles, ladybirds). In the absence of herbivores the vegetation on soils of high, moderate and low fertility was dominated by plant species which under natural field conditions are restricted to fertile soils. At high fertility, the rate of competitive exclusion was rapid compared with communities persisting at lower fertility which remained species‐rich with high densities of individuals.
The effects of herbivory were profound and attributable mainly to the slug Deroceras reticulatum. At moderate and low soil fertility the main effect of herbivores was to change the ranking of plant species; palatable grasses (e.g. Poa annua, Poa trivialis and Lolium perenne) were suppressed and plant species of slower growth rate (e.g. Festuca rubra, Anthoxanthum odoratum, Festuca ovina) were promoted. More subtle impacts of herbivory altered the size structure of plant populations suggesting the preference, by slugs, for seedlings and small (suppressed) plants. Although ladybirds and their larvae persisted in low numbers in some microcosms, carnivory was more strongly in evidence (lower densities of D. reticulatum and reduced consumption of lettuce discs) in relation to the activities of the ground beetle Pterostichus melanarius. Only in the low fertility treatment, however, was this potential translated into effects on vegetation; for several plant species the frequency of large individuals increased in the presence of P. melanarius.
We conclude that outdoor microcosms provide a valuable ecological tool and, in particular, allow investigation of (a) the role of herbivores in promoting slow‐growing plant species in the vegetation of infertile habitats and (b) the protection of palatable plants against herbivores by carnivores.     

Species richness and canopy productivity of Australian plant communities

The species richness (number of vascular plants per hectare) of Australian plant communities (containing a mosaic of gap, regeneration, maturation and senescent phases) is correlated with the annual biomass productivity of the overstorey canopy.The annual production of leaves and stem in the canopy of the plant community is shown to be limited by the requirements of photosynthesis (particularly light and the availability of water) and the length of the growing season.The species richness of Australian plant communities is the product of the blance between the dominance of the overstorey and the response of the understorey to the shading of the overstorey. For all climatic regions and zones the species richness of the overstorey of the plant community is shown to be exponentially related to the annual shoot growth of the overstorey canopy, until the latitudinal or altitudinal tree line is reached. With latitudinal increase outside the tropics, overstorey canopies of forest communities absorb increasingly more of the incident solar radiation. markedly reducing the species richness of the understorey strata. In contrast, in these latitudes the overstorey of plant communities with widely spaced trees or tall shrubs will absorb far less solar radiation, thus enabling the species richness of the understorey to be maintained.     

Size-dependent species richness: trends within plant communities and across latitude

We examine how species richness and species‐specific plant density (number of species and number of individuals per species, respectively) vary within community size frequency distributions and across latitude. Communities from Asia, Africa, Europe, and North, Central and South America were studied (60°4′N–41°4′S latitude) using the Gentry data base. Log–log linear stem size (diameter) frequency distributions were constructed for each community and the species richness and species‐specific plant density within each size class were determined for each frequency distribution. Species richness in the smallest stem size class correlated with the Y‐intercepts (β‐values) of the regression curves describing each log–log linear size distributions. Two extreme community types were identified (designated as type A and type B). Type A communities had steep size distributions (i.e. large β‐values), log–log linear species‐richness size distributions, low species‐specific plant density distributions, and a small size class (2–4 cm) containing the majority of all species but rarely conspecifics of the dominant tree species. Type B communities had shallow size distributions (i.e. small β‐values), more or less uniform (and low) size class species‐ richness and species‐specific density distributions and size‐dominant species resident in the smallest size class. Type A communities were absent in the higher latitudes but increased in number towards the equator, i.e. in the smallest size class, species richness increased (and species‐specific density decreased) towards the tropics. Based on our survey of type A and type B communities (and their intermediates), species richness evinces size‐dependent and latitudinal trends, i.e. species richness increased with decreasing body size and most species increasingly reside in the smallest plant size class towards the tropics. Across all latitudes, a trade‐off exists between the number of species and the number of individuals per species residing in the smaller size classes.     

Species richness of insect herbivore communities on Ficus in Papua New Guinea

Insect herbivores were sampled from the foliage of 15 species of Ficus (Moraceae) in rainforest and coastal habitats in the Madang area, Papua New Guinea. The collection included 13 193 individuals representing 349 species of leaf-chewing insects and 44 900 individuals representing 430 species of sap-sucking insects. Despite a high sampling intensity, the species accumulation curve did not reach an asymptote. This pattern was attributed to the highly aggregated distribution of insects on individual host trees. The number of insect species collected on a particular Ficus species ranged from 34 to 129 for leaf-chewing and from 51 to 219 for sap-sucking insects. Two Ficus species growing on the seashore sustained less speciose insect communities than their counterparts growing in forest. For the forest figs, significant predictors of insect species richness included leaf palatability and leaf production for leaf-chewing insects (40% of the variance explained), and tree density and leaf expansion for sap-sucking insects (75%). The high faunal overlap among Ficus communities and the importance of local resources for insect herbivores suggest that highly specialized interactions between insect herbivores and Ficus in Papua New Guinea have not been conserved in evolutionary time. This is at variance with the dogma of old, extremely specialized and conservative interactions between insect herbivores and their hosts, providing numerous ecological niches in the floristically rich tropics.     

Riparian bryophyte communities on Madeira: patterns and determinants of species richness and composition

Abstract

We studied 16 streams evenly distributed over the northern and southern slopes of Madeira in order to investigate the riparian bryoflora. Within each stream, three positions (upper, middle and lower reaches) were delimited and within each position two areas were selected. Within each area two plots (each composed of six microplots of 0.2 m²) were sampled, one in the within-stream habitat (submerged all year round), and the other in the stream-border habitat (submerged only in winter). We found that species composition of the riparian bryophytes is affected by the habitat and position in the stream, but not by the main aspect (northern versus southern slope). Concerning species richness, we found that the stream-border community was clearly richer than the within-stream community, upstream plots were richer than plots downstream, and plots on the northern slope of the island were richer than plots on the southern slope. Habitat type was the most significant factor in determining the richness of the threatened species with more species present in the stream-border habitat. Additive partitioning showed that the between-stream component contributed most to total species richness, especially to the richness of the infrequent and threatened species. However, for the common species, the lowest level, i.e. the within-area component, was the most important. Although northern upstream areas are climatically favourable for many bryophyte species due to their higher humidity, the clear effects found may not only be climate-induced, as these areas are also less disturbed and mostly covered by the natural laurel forest. In the southern, downstream parts only a few species were present. Human impacts are largest in the latter situations and probably contributed to the low species richness. As the streams differed considerably in terms of their bryophyte flora, and most of the species were rare, changes in the riparian areas can greatly affect the bryoflora. Therefore, in order to protect the riparian bryophytes as comprehensively as possible, we emphasize the need for careful monitoring of any changes.     

Functional redundancy in a clipping experiment on grassland plant communities

Functional redundancy predicts that some species may play equivalent roles in ecosystem functioning therefore conferring a kind of ‘insurance’ to perturbation when species richness is reduced, by the compensation of species of the same functional group on ecosystem processes. We evaluate functional redundancy on grassland plant communities by a removal experiment in which the evaluated treatments were: GG – clipping two graminoid species, FF – clipping two forb species, GF – clipping one graminoid and one forb species and Control – no removal. We tested the hypothesis that the above‐ground biomass removal of one species of each functional group would cause less change in the community composition (community persistence) and less decrease in biomass production than the above‐ground biomass removal of two species of the same functional group. Functional redundancy was corroborated for community persistence since treatments FG and C caused less change in community composition than treatments GG and FF, although no differences were found between treatments for above‐ground biomass. We verified that clipped species tend to be compensated by an increase in the percent cover of the remaining species of the same functional group. This work provides experimental evidence of early responses after plant clipping in small spatial scale of functional redundancy in naturally established grassland plant communities. We highlight redundancy as an intrinsic feature of communities insuring their reliability, as a consequence of species compensation within functional groups.