Can aphid-induced plant signals be transmitted aerially and through the rhizosphere?

Aphids, through their close association with plants, cause systemic release of semiochemicals. These may have negative effects on subsequent aphid colonisation and can also have positive roles with insects that are antagonistic to aphid development, for example parasitoids. One of the semiochemicals involved in host selection by aphids is methyl salicylate, and since this compound was shown to have a role as a plant stress signal, the hypothesis that aphids might facilitate identification of new plant signals was examined. Confirmation was obtained during an investigation of avoidance of unsuitable hosts by the lettuce aphid, Nasonovia ribis-nigri. (Z)-Jasmone was identified as a plant-derived semiochemical acting negatively for a number of aphid species, and positively for insect antagonists such as parasitoids and predators. However, when the compound was employed at 0.1 ppm in air above intact plants, these plants then attracted aphid parasitoids long after the (Z)-jasmone itself was no longer detectable. A specific interaction was proposed, since the (Z)-jasmone appeared to be selectively taken up by the plants. Aerial interactions between intact barley plants from different cultivars, which may be differentially releasing stress associated signals, can also influence acceptability to aphids. Furthermore, it has been shown that exudates from the roots of aphid-infested plants, grown hydroponically or in soil, cause intact plants to become more attractive to parasitoids.     

Are plant communities on the Canary Islands resistant to plant invasion?

Oceanic islands are renowned for their unique flora and high levels of endemism. Native island plants, however, are imperilled by non-native species that can become invasive by outcompeting natives. The threat of native island assemblages generally increases with isolation and the number of endemics featured, but also with human-associated disturbance and land use. Based on this, the Canary Island native plant systems should be highly threatened by invasives, similar to other oceanic islands globally. However, Canarian native plant systems are only weakly infiltrated and are rarely directly threatened by invasive plants. Further, highly disturbed areas, usually among the first colonized by invasives on islands, are recolonized here by natives. Based on this, we postulate four hypotheses (climatic filter, well-preservation status, human legacy and permanent colonization) for explaining this unusual behaviour of plant systems on the Canary Islands, providing an opportunity to understand the drivers and processes behind invasion into plant communities on islands.     

Factors determining plant–neighbour interactions on different spatial scales in young species-rich grassland communities

In naturally colonised species-rich grassland communities, we examined the properties of a plant’s aboveground neighbourhood that affect its performance (aboveground biomass). To this end a range of neighbourhood parameters were measured: number, biomass and species richness of the neighbours, number and biomass of the conspecific neighbours, and light availability at the base of the target plant. We also determined at which neighbourhood size the strongest target plant–neighbour interactions occurred, and whether conspecific neighbours affected competitively stronger or weaker target species differently. Target plant performance varied with target identity, and was significantly affected by light availability and the number of neighbouring plants (neighbourhood density). Depending on the target species, there was also an effect of total neighbour biomass on plant performance. The target plants were most strongly affected by their neighbours within a 3-cm distance, which could account for 78% of the variance in target biomass. Number or biomass of the conspecific neighbours did not contribute to the explanation of target performance in any of the target species. Whereas in an 8-cm neighbourhood the amount of light penetration was the strongest predictor of target performance, the number of neighbours was more important in a 3-cm neighbourhood. These experimental results might be useful to extend existing neighbourhood competition models for one or two species to multi-species competition models.     

Can ecosystem services be integrated with conservation? A case study of breeding waders on grassland

We review published studies to show that changes in soil moisture levels have significant impacts on a range of wading bird species that use UK lowland grassland, including wet grassland, and obtain their food predominantly by probing the soil. We examine both the hydrological and the ecological literature and assess how management options could alter (1) ecosystem services (via water quality and flooding) and (2) habitat quality for wading birds. The combination of biodiversity goals with broader ecosystem services has been widely advocated and we suggest that appropriate management at multiple scales (e.g. small‐scale: ponds; large‐scale: integrated washlands) could potentially provide both ecosystem services and habitat for wading grassland birds. However, there is only a limited base of evidence on which to assess the potential linkage between these two areas, particularly for non‐wading bird species. Future work should be directed at identifying (1) how crop yield, ecosystem services and biodiversity relate to each other, (2) the extent of land needed to be managed to benefit these multiple purposes and bring about measurable gain (e.g. one or two ponds may make significant inroads in reducing run‐off and pollution but make little difference to wading birds) and (3) solutions to the challenges of setting up management options at large spatial scales (e.g. catchments).     

Can herbivores sharing the same host plant be mutualists?

Resource partitioning is considered to be a prerequisite for coexisting species to evolve from competition to mutualism. This is uniquely different for two major pest insects of rice. These herbivores preferentially opt to coinfest the same host plants, and through plant-mediated mechanisms, cooperatively utilize these plants in a mutualistic manner.     

Can soil temperature direct the composition of high arctic plant communities?

Abstract. Low temperatures exert a primary constraint on the growth of high arctic vascular plants. However, investigations into the impact of temperature on high arctic plants rarely separate out the role of air and soil temperatures, and few data exist to indicate whether soil temperatures alone can significantly influence the growth of high arctic vascular plants in a manner that might direct community composition. We examined the response of high arctic plants of three functional types (grasses, sedges/rushes and non‐graminoids) to manipulated soil temperature under common air temperature conditions. Target plants, within intact soil cores, were placed in water baths at a range of temperatures between 4.9 and 15.3 °C for one growing season. Grasses responded most rapidly to increased soil temperature, with increased total live plant mass, above‐ground live mass and total below‐ground live mass, with non‐graminoids having the lowest, and sedges/rushes an intermediate degree of response. The ratio of above‐ground live mass to total live mass increased in all growth forms. Grasses, in particular, responded to enhanced soil temperatures by increasing shoot size rather than shoot number. In all growth forms the mass of root tissue beneath the moss layer increased significantly and to a similar extent with increasing soil temperature. These results clearly indicate that different growth forms, although collected from the same plant community, respond differently to changes in soil temperature. As a consequence, factors influencing soil temperature in high arctic ecosystems, such as global climate change or herbivory (which leads to reduced moss depth and increased soil temperatures), may also direct changes in vascular plant community composition.     

Removal of livestock alters native plant and invasive mammal communities in a dry grassland–shrubland ecosystem

The impacts of domesticated herbivores on ecosystems that did not evolve with mammalian grazing can profoundly influence community composition and trophic interactions. Also, such impacts can occur over long time frames by altering successional vegetation trajectories. Removal of domesticated herbivores to protect native biota can therefore lead to unexpected consequences at multiple trophic levels for native and non-native species. In the eastern South Island of New Zealand large areas of seral grassland–shrubland have had livestock (sheep and cattle) removed following changes in land tenure. The long-term (>10 years) outcomes for these communities are complex and difficult to predict: land may return to a native-dominated woody plant community or be invaded by exotic plants and mammals. We quantified direct and indirect effects of livestock removal on this ecosystem by comparing plant and invasive mammal communities at sites where grazing by livestock ceased c.10–35 years ago (conservation sites) with paired sites where pastoralism has continued to the present (pastoral sites). There was higher total native plant richness and reduced richness of exotic plants on conservation sites compared with pastoral sites. Further, there were differences in the use of conservation and pastoral sites by invasive mammals: rabbits and hedgehogs favoured sites grazed by livestock whereas house mice, brushtail possums and hares favoured conservation sites. Changes in the relative abundance of invasive mammal species after removal of domesticated livestock may compromise positive outcomes for conservation in successional plant communities with no evolutionary history of mammalian grazing.     

Can orchid mycorrhizal fungi be persistently harbored by the plant host?

The environmental distribution of non-obligate orchid mycorrhizal (OM) symbionts belonging to the ‘rhizoctonia’ complex remains elusive. Some of these fungi, indeed, are undetectable in soil outside the host rhizosphere. A manipulation experiment was performed to assess the importance of neighbouring non-orchid plants and soil as possible reservoirs of OM fungi for Spiranthes spiralis, a widespread photosynthetic European terrestrial orchid species. Fungi of S. spiralis roots were identified by DNA metabarcoding before and 4 months after the removal of the surrounding vegetation and soil. Although such a treatment significantly affected fungal colonization of newly-formed orchid roots, most OM fungi were consistently associated with the host roots. Frequency patterns in differently aged roots suggest that these fungi colonize new orchid roots from either older roots or other parts of the same plant, which may thus represent an environmental source for the subsequent establishment of the OM symbiosis.     

Can species distribution models be used to describe plant abundance patterns?

In recent years, there has been increasing interest in modelling of species abundance data in addition to presence data. In this study, we assessed the similarities and differences between presence‐absence distributions and abundance distributions along similar environmental gradients, derived, respectively, from presence‐absence and abundance data. Moreover, we examined the possibility of using presence‐absence distribution models to derive abundance distributions. For this purpose, we used Braun‐Blanquet abundance scores for 243 vascular species at 10 996 French forest sites. Species distribution models were used to analyse the link between the patterns of occurrence, low abundance and high abundance for each species with regard to mean annual temperature, June water balance, and soil pH. For each species, differences in the modelled distributions were characterised by the ecological optimum and ecological amplitude. A comparison of the presence‐absence and abundance distributions for all species revealed similar optima and different amplitudes along the three ecological factors. An abundant‐centre distribution was observed in environmental space, with species abundance being greatest at the optimal conditions and lower at less favourable conditions of the species occurrence response. Geographical habitat mapping also shows centred, high‐abundance suitability within the presence habitat of each species. We conclude that species distribution models derived from presence‐absence data provide useful information about the ecological optima of abundance distributions but overestimate the range of habitats suitable for high species abundance. This study demonstrates the utility of presence‐absence data for ecologist and conservation biologist when they are interested in the optimal conditions of high species abundance.     

Can plant resistance to specialist herbivores be explained by plant chemistry or resource use strategy?

At both a macro- and micro-evolutionary level, selection of and performance on host plants by specialist herbivores are thought to be governed partially by host plant chemistry. Thus far, there is little evidence to suggest that specialists can detect small structural differences in secondary metabolites of their hosts, or that such differences affect host choice or performance of specialists. We tested whether phytochemical differences between closely related plant species are correlated with specialist host choice. We conducted no-choice feeding trials using 17 plant species of three genera of tribe Senecioneae (Jacobaea, Packera, and Senecio; Asteraceae) and a more distantly related species (Cynoglossum officinale; Boraginaceae) containing pyrrolizidine alkaloids (PAs), and four PA-sequestering specialist herbivores of the genus Longitarsus (Chrysomelidae). We also assessed whether variation in feeding by specialist herbivores is attributable to different resource use strategies of the tested plant species. Plant resource use strategy was quantified by measuring leaf dry matter content, which is related to both plant nutritive value and to plant investment in quantitative defences. We found no evidence that intra-generic differences in PA profiles affect feeding by specialist herbivores. Instead, our results indicate that decisions to begin feeding are related to plant resource use strategy, while decisions to continue feeding are not based on any plant characteristics measured in this study. These findings imply that PA composition does not significantly affect host choice by these specialist herbivores. Leaf dry matter content is somewhat phylogenetically conserved, indicating that plants may have difficulty altering resource use strategy in response to selection pressure by herbivores and other environmental factors on an evolutionary time scale.     

Soil fertility alters the nature of plant–resource interactions in invaded grassland communities

Resource competition theory suggests that the nature of diversity–resource–invasibility interactions will vary along fertility gradients, concurrent with changes in the relative availability of limiting above- versus below-ground resources. Experimental support for this contingency is lacking. Here, we manipulated resident diversity, baseline fertility, and the availabilities of light and soil nitrogen in grassland communities invaded by two functionally distinct non-native plant species (Lolium arundinaceum and Melilotus alba). We tested the hypotheses that increased resident diversity reduces community invasibility and dampens the effects of light and soil nitrogen pulses, and that the relative effects of light versus soil nitrogen additions on diversity–invasibility relationships depend on the baseline fertility of the study system. Our results reveal an overall weak negative effect of resident diversity on Lolium performance, but in contrast to our expectations, this diversity effect did not vary with light or soil nitrogen additions or with baseline fertility. However, the relative effects of above- versus below-ground resource additions on invader performance varied with baseline fertility as expected: Lolium responded most strongly to soil nitrogen additions in low-fertility mesocosms and most strongly to increased light availability in high-fertility mesocosms. In contrast to Lolium, nitrogen-fixing Melilotus was overall less responsive to diversity and resource manipulations. Together, these patterns do not lend support for the dependence of diversity–resource–invasibility relationships on either baseline fertility or invasive species identity, but they do highlight the dominant role of resources over diversity in determining invader performance, as well as the manner in which fertility alters the relative importance of above- versus below-ground resource pulses in promoting invasions.     

Does phosphorus limitation promote species-rich plant communities?

It is known that the number of limiting nutrients may affect the species richness of plant communities, but it is unclear whether the type of nutrient limitation is also important. I place the results from a study in Patagonia (elsewhere in this issue) in the context of the number and types of nutrients that are limiting. I present four mechanisms through which N or P limitation may potentially influence species richness. These mechanisms are related to: (i) the number of forms in which P or N are present in soil and the plant traits needed to acquire them, (ii) the mechanisms and traits that control species competition and coexistence under N or P limitation, (iii) the regional species pools of plants capable of growing under N- and P-limited conditions, and (iv) the interaction between the type of nutrient limitation and community productivity. It appears likely that P limitation can favour a higher species richness than N limitation, in at least in a variety of low productive plant communities, but evidence to support this conclusion is so far lacking. The four mechanisms proposed here offer a framework for exploring whether the type of nutrient limitation per se, or an interaction with productivity, is a potential driver for variation in species diversity.     

Does species diversity limit productivity in natural grassland communities?

Theoretical analyses and experimental studies of synthesized assemblages indicate that under particular circumstances species diversity can enhance community productivity through niche complementarity. It remains unclear whether this process has important effects in mature natural ecosystems where competitive feedbacks and complex environmental influences affect diversity–productivity relationships. In this study, we evaluated diversity–productivity relationships while statistically controlling for environmental influences in 12 natural grassland ecosystems. Because diversity–productivity relationships are conspicuously nonlinear, we developed a nonlinear structural equation modeling (SEM) methodology to separate the effects of diversity on productivity from the effects of productivity on diversity. Meta-analysis was used to summarize the SEM findings across studies. While competitive effects were readily detected, enhancement of production by diversity was not. These results suggest that the influence of small-scale diversity on productivity in mature natural systems is a weak force, both in absolute terms and relative to the effects of other controls on productivity.     

Can network metrics predict vulnerability and species roles in bird‐dispersed plant communities? Not without behaviour

Network metrics are widely used to infer the roles of mutualistic animals in plant communities and to predict the effect of species' loss. However, their empirical validation is scarce. Here we parameterized a joint species model of frugivory and seed dispersal with bird movement and foraging data from tropical and temperate communities. With this model, we investigate the effect of frugivore loss on seed rain, and compare our predictions to those of standard coextinction models and network metrics. Topological coextinction models underestimated species loss after the removal of highly linked frugivores with unique foraging behaviours. Network metrics informed about changes in seed rain quantity after frugivore loss. However, changes in seed rain composition were only predicted by partner diversity. Nestedness, closeness, and d’ specialisation could not anticipate the effects of rearrangements in plant–frugivore communities following species loss. Accounting for behavioural differences among mutualists is critical to improve predictions from network models.     

Can the IUCN criteria be effectively applied to peripheral isolated plant populations?

The IUCN criteria 2001 are considered one of the best methods to evaluate species extinction risk at the global and regional levels. The aim of this work is to test the applicability of the IUCN criteria to peripheral isolated plant populations (PIPPs). PIPPs have been a topic of scientific debate in Conservation Biology for about 15 years and international conventions such as ESPC address the issue of their conservation. Conservation measures often rely on Red Lists based on IUCN criteria, but there is little evidence supporting their application to PIPPs. In this work, we tested the hypothesis that PIPPs’ intrinsic restricted range and rarity lead to the overestimation of their extinction risk. We compared and analyzed four IUCN criteria (A, B, C, D), considering 17 species with PIPPs in different Italian administrative regions. Special attention must be given to the spatial scale at which PIPPs are assessed, the evaluation of the threats affecting the populations, and their decline. PIPPs should not be assessed within political boundaries and we propose a new area designation that better corresponds to the characteristics of PIPPs. Criterion B is strongly biased by restricted range and overestimates the extinction risk of PIPPs, particularly when the population decline is only suspected and not observed. In this case, criterion D more accurately assesses the status of PIPPs. Criterion A is also suitable for assessing PIPPs, because it is not affected by their phytogeographic rarity. The proposed statements could also be valid for the global assessment of narrow endemic species.     

Can MTT be used to quantify the antioxidant activity of plant extracts?

Antioxidant properties of three different medicinal plant extracts from northern Nigeria were evaluated using the traditional qualitative 2,2, diphenyl-picrylhydrazyl (DPPH) method and a new 3-(4,5-dimethylthiazole-2-yl)-2,5-diphenyltetrazolium bromide (MTT) microdilution method described here. The results were in agreement. It is a simple, rapid and inexpensive method compared with many other methods for determining quantitative antioxidant activity of plant extracts.