Does plant phylogenetic diversity increase invertebrate herbivory in managed grasslands?

Plant diversity and land-use intensity have been shown to affect invertebrate herbivory. Several hypotheses predict positive (e.g. associational susceptibility) or negative (e.g. associational resistance) relationships of herbivory with plant species richness. Also, the strength and direction of reported relationships vary greatly between studies leading to the conclusion that relationships either depend on the specific system studied or that other unconsidered factors are more important. Here, we hypothesized that plant phylogenetic diversity is a stronger predictor of invertebrate herbivory than plant species richness because it integrates additional information about the phenotypical and functional composition of communities. We assessed the community-wide invertebrate herbivory, plant species richness and phylogenetic diversity across a range of land-use intensities including a total of 145 managed grasslands in three regions in Germany. Increasing land-use intensity decreased plant species richness and phylogenetic diversity. Plant species richness did not predict invertebrate herbivory. By contrast herbivory moderately increased with increasing plant phylogenetic diversity even after accounting for the effects of region and land use. The strength of direct effects of land-use intensity and indirect effects via altered phylogenetic diversity on herbivory, however, varied among regions. Our results suggest that increasing phylogenetic diversity of plant communities increases invertebrate herbivory probably by providing higher resource diversity. Differences between regions underline the need to account for regional peculiarities when attempting to generalize land-use effects on invertebrate herbivory.     

Does plant community plasticity mediate microbial homeostasis?

Microbial homeostasis—constant microbial element ratios along resource gradients—is a core ecological tenet, yet not all systems display homeostasis. We suggest investigations of homeostasis mechanisms must also consider plant–microbial interactions. Specifically, we hypothesized that ecosystems with strong plant community plasticity to changing resources will have homeostatic microbial communities, with less microbial resource cost, because plants reduce variance in resource stoichiometry. Using long‐term nutrient additions in two ecosystems with differing plant response, we fail to support our hypothesis because although homeostasis appears stronger in the system with stronger plant response, microbial mechanisms were also stronger. However, our conclusions were undermined by high heterogeneity in resources, which may be common in ecosystem‐level studies, and methodological assumptions may be exacerbated by shifting plant communities. We propose our study as a starting point for further ecosystem‐scale investigations, with higher replication to address microbial and soil variability, and improved insight into microbial assimilable resources.     

Does fire increase the spatial heterogeneity of bird communities in Mediterranean landscapes?

The occurrence of large burnt areas has increased considerably in southern Europe in recent years. In order to design management plans to prevent large wildfires while preserving biodiversity, understanding of the ways in which birds respond to these fires is required. We investigated the spatial variability of both avifauna and habitat structure in three zones: unburnt, burnt in 1982, and burnt in 1994. The habitat structure of the unburnt zone was the most variable spatially. However, bird species composition between sampling points was very homogeneous in space. In contrast, the bird communities inhabiting burnt zones were more spatially heterogeneous. This pattern was caused by distinct specific responses to variations in habitat structure. Open-space species responded to small changes in habitat structure with large changes in local abundance, whereas the response of forest species to these structural variations was much less. We suggest that land managers should select specific zones with limited vegetation recovery within large burnt areas and maintain them as open space to keep combustibility low and provide an appropriate habitat for several open space species that are of conservation concern.     

Biotic resistance to plant invasion in grassland: Does seed predation increase with resident plant diversity?

Seed predation impacts heavily on plant populations and community composition in grasslands. In particular, generalist seed predators may contribute to biotic resistance, i.e. the ability of resident species in a community to reduce the success of non-indigenous plant invaders. However, little is known of predators’ preferences for seeds of indigenous or non-indigenous plant species or how seed predation varies across communities. We hypothesize that seed predation does not differ between indigenous and non-indigenous plant species and that seed predation is positively related to plant species diversity in the resident community. The seed removal of 36 indigenous and non-indigenous grassland species in seven extensively or intensively managed hay meadows across Switzerland covering a species-richness gradient of 18–50 plant species per unit area (c. 2 m²) was studied. In mid-summer 2011, c. 24,000 seeds were exposed to predators in Petri dishes filled with sterilized soil, and the proportions of seeds removed were determined after three days’ exposure. These proportions varied among species (9.2–62.5%) and hay meadows (17.8–48.6%). Seed removal was not related to seed size. Moreover, it did not differ between indigenous and non-indigenous species, suggesting that mainly generalist seed predators were active. However, seed predation was positively related to plant species richness across a gradient in the range of 18–38 species per unit area, representing common hay meadows in Switzerland. Our results suggest that generalist post-dispersal seed predation contributes to biotic resistance and may act as a filter to plant invasion by reducing the propagule pressure of non-local plant species.     

Does plasmolysis increase the drought tolerance of plant cells?

Summary Iljin's experiments withRhoeo discolor, cabbage and red beet which seemed to demonstrate increased drought resistance of plasmolysed tissue have been repeated, but his results could not be confirmed. The tissue plasmolysed in sucrose solutions, died either during stepwise plasmolysis and deplasmolysis or else later on, during exposure to unsaturated air within one or two days, even at the highest humidities. Iljin's error was apparently produced by a wrong interpretation of his tests of viability: the plasma of his dead cells did not disintegrate and retained anthocyanin by tonoplast plasmolysis.Plasmolysis proved harmful to all three objects investigated.In view of these results and earlier criticism of Iljin's analogous experiments on frost resistance by others, all experimental evidence produced in support of Iljin's mechanical theory of drought resistance should be reexamined.     

Does hydrological fluctuation alter impacts of species richness on biomass in wetland plant communities?

Aims The diversity–productivity relationship is one of the most critical questions in ecology and can be altered by environmental factors. Hydrological fluctuation affects growth of wetland plants, and such effects vary with plant species. Therefore, we hypothesized that hydrological fluctuation changes effects of species richness on productivity of wetland plant communities.Methods We constructed wetland plant communities consisting of three or six wetland plant species and subjected them to hydrological fluctuation (i.e. gradually changing water level) of two frequencies and two ranges, with unchanged water level as the control. We measured height, root and shoot dry mass of each plant at harvest.Important findings Hydrological fluctuation significantly decreased biomass of wetland plant communities, which was due to impacts of fluctuation range, but not those of fluctuation frequency. Community biomass was significantly higher when species richness was higher, and such an effect did not depend on hydrological fluctuation. Therefore, hydrological fluctuation can decrease the productivity of wetland plant communities but may not alter the diversity–productivity relationship.     

Does disturbance regime change community assembly of angiosperm plant communities in the boreal forest?

Aims To examine if and how species and phylogenetic diversity change in relation to disturbance, we conducted a review of ecological literature by testing the consistency of the relationship between phylogenetic diversity and disturbance and compared taxonomic groups, type of disturbance and ecosystem/habitat context. We provide a case study of the phylogenetic diversity–disturbance relationship in angiosperm plant communities of a boreal forest region, compared with types of natural and anthropogenic disturbances and plant growth forms.Methods Using a large-scale sampling plot network along a complete (0–100%) anthropogenic disturbance gradient in the boreal biome, we compared the changes of angiosperm plant community structure and composition across plots. We estimated natural disturbance with historical records of major fires. We then calculated phylogenetic diversity indexes and determined species richness in order to compare linear and polynomial trends along disturbance gradients. We also compared the changes of community structure for different types of anthropogenic disturbances and examined how the relationships between species and phylogenetic diversity and disturbance regimes vary among three different life forms (i.e. forbs, graminoids and woody plants).Important findings Phylogenetic diversity was inconsistently related to disturbance in previous studies, regardless of taxon, disturbance type or ecosystem context. In the understudied boreal ecosystem, angiosperm plant communities varied greatly in species richness and phylogenetic diversity along anthropogenic disturbance gradients and among different disturbance types. In general, a quadratic curve described the relationship between species richness and anthropogenic disturbance, with the highest richness at intermediate anthropogenic disturbance levels. However, phylogenetic diversity was not related to disturbance in any consistent manner and species richness was not correlated with phylogenetic diversity. Phylogenetic relatedness was also inconsistent across plant growth forms and different anthropogenic disturbance types. Unlike the inconsistent patterns observed for anthropogenic disturbance, community assembly among localities varying in time since natural disturbance exhibited a distinct signature of phylogenetic relatedness, although those trends varied among plant growth forms.     

Does root glutamine synthetase control plant biomass production in Lotus japonicus L.?

To investigate the contribution of root cytosolic glutamine synthetase (GS) activity in plant biomass production, two different approaches were conducted using the model legume Lotus japonicus. In the first series of experiments, it was found that overexpressing GS activity in roots of transgenic plants leads to a decrease in plant biomass production. Using ¹⁵N labelling it was shown that this decrease is likely to be due to a lower nitrate uptake accompanied by a redistribution to the shoots of the newly absorbed nitrogen which cannot be reduced due to the lack of nitrate reductase activity in this organ. In the second series of experiments, the relationship between plant growth and root GS activity was analysed using a series of recombinant inbred lines issued from the crossing of two different Lotus ecotypes, Gifu and Funakura. It was confirmed that a negative relationship exists between root GS expression and plant biomass production in both the two parental lines and their progeny. Statistical analysis allowed it to be estimated that at least 13% of plant growth variation can be accounted for by variation in GS activity. Received: 24 September 1998 / Accepted: 14 April 1999     

Are restricted species checklists or ant communities useful for assessing plant community composition and biodiversity in grazed pastures?

Semi-natural grasslands in Sweden are species-rich, and their natural values are strongly dependent on continuous management, mainly by grazing. However, the large heterogeneity in vegetation within and between grassland sites must be taken into account when designing management and preservation schemes, calling for precise field monitoring and assessment of habitat type and land use history. We have evaluated different surrogate measures to assess community composition and biodiversity of the most common vegetation types in grazed semi-natural pastures. We compared the complete plant community, two reduced checklists intended for quick surveys of the plant community, and the ant community. The results suggest that the taxonomic resolution in a plant inventory is important for both biodiversity assessment and recognition of vegetation types. The extent of a reduced species checklist was of greater importance than its quality for describing the plant community. Reduced checklists should only be used if they comprise species with known affinity to the studied vegetation types. We also found that plants and ants experience grazed semi-natural grasslands in different ways. Ant communities did not resemble the communities deduced from plant inventories, or vegetation types recognised by field staff.     

Does plant diversity increase top–down control of herbivorous insects in tropical forest?

Higher trophic level interactions are key mediators of ecosystem functioning in tropical forests. A rich body of theory has been developed to predict the effects of plant diversity on communities at higher trophic levels and the mechanisms underlying such effects. The 'enemies hypothesis’ states that predators exert more effective top–down control of herbivorous insects with increasing plant diversity. Support for this hypothesis has been found in temperate forests and agroecosystems, but remains understudied in tropical forests. We compared incidence of attacks of different natural enemies using artificial caterpillars in a tropical forest landscape and investigated the role of plant community structure (i.e. species richness, composition and density), and the role of forest fragmentation (i.e. patch size, edge distance and canopy openness) on predation intensity. Plant community effects were tested with respect to three vegetation strata: trees, saplings and herbs. Observed predation was substantially due to ants. Predation rates increased with plant species richness for trees and herbs. Density of saplings, herb cover and herb species composition were important factors for predation. No significant patterns were found for fragmentation parameters, suggesting that forest fragmentation has not altered predation intensity. We conclude that in tropical forests, top–down control of herbivorous insects in the understory vegetation is affected by a combination of plant diversity, plant species composition and structural features of the plant community.     

Do plant modules describe community structure better than biomass? A comparison of three abundance measures

Abstract. Plants are composed of modules, such as shoots and leaves. However, modules have been overlooked as potential abundance measures in describing plant communities. We sampled 8 communities, to examine whether module counts gave different conclusions from biomass on community structure, and better discriminated between communities. Different abundance measures – number of leaves, number of shoots and photosynthetic biomass – gave different results for between‐site comparisons of evenness and rank consistency. Since evenness is intended to represent a feature of the whole community, it should vary more between communities than within; module‐based abundance achieved this better than biomass. It is speculated that the restrictions on species co‐occurrences when plant communities assemble may sometimes be based on the number of modules rather than on biomass.     

Does the disturbance hypothesis explain the biomass increase in basin‐wide Amazon forest plot data?

Positive aboveground biomass trends have been reported from old-growth forests across the Amazon basin and hypothesized to reflect a large-scale response to exterior forcing. The result could, however, be an artefact due to a sampling bias induced by the nature of forest growth dynamics. Here, we characterize statistically the disturbance process in Amazon old-growth forests as recorded in 135 forest plots of the RAINFOR network up to 2006, and other independent research programmes, and explore the consequences of sampling artefacts using a data-based stochastic simulator. Over the observed range of annual aboveground biomass losses, standard statistical tests show that the distribution of biomass losses through mortality follow an exponential or near-identical Weibull probability distribution and not a power law as assumed by others. The simulator was parameterized using both an exponential disturbance probability distribution as well as a mixed exponential–power law distribution to account for potential large-scale blowdown events. In both cases, sampling biases turn out to be too small to explain the gains detected by the extended RAINFOR plot network. This result lends further support to the notion that currently observed biomass gains for intact forests across the Amazon are actually occurring over large scales at the current time, presumably as a response to climate change.     

Does sun glare increase antipredator behaviour in prey?

As the sun gradually lowers over the horizon, prey species with more sun in their eyes should have more difficulty in visually monitoring their surroundings for threats and thus experience a higher predation risk. In a unique setting, I could examine changes in antipredator behaviour in a prey species, the semipalmated sandpiper Calidris pusilla, facing attacks by peregrine falcons Falco peregrinus, which originated from the general direction of the lowering sun. I predicted gradual changes in antipredator behaviour as sun glare becomes more problematic later in the day. As the day progressed, sandpipers occurred in sparser groups when the sun glared but not when clouds obscured the sun, suggesting that fewer individuals engaged in risky foraging. Pecking rate and foraging success decreased later in the day when the sun glared but not otherwise implying an increase in vigilance at the expense of foraging. When more sun hit their eyes, sandpipers also moved faster suggesting increased skittishness. The sun glare effect might be relevant to any species foraging in open areas not only when the sun sets but also when it rises especially if predators can target prey species at these vulnerable times. The temporal gradient in predation risk that the sun glare effect creates might thus apply broadly and have important consequences for antipredator vigilance, foraging efficiency, and habitat use.     

Does UV-B influence biomass growth in lichens deficient in sun-screening pigments?

This study aimed to assess biomass growth as a response variable in lichens during short-term laboratory experiments. To do this, we studied the influence of UV-B and temperature on lichen performance including the synthesis of solar radiation screening cortical compounds. The pioneer lichen Xanthoria aureola from exposed sea cliffs and the old forest lichen Lobaria pulmonaria were cultivated for 15 days in the laboratory in a factorial experiments with temperature (12 and 21 °C) and UV-B (0, 0.1, 0.3 and 1.0 W m^?2) as treatments. Prior to the experiment, the cortical pigment parietin was non-destructively extracted from X. aureola, whereas the sampled shade-adapted thalli of L. pulmonaria lacked cortical melanic compounds. Therefore both lichens were deficient in cortical sun-screening compounds when the UV-B exposure started. At 12 °C, the relative growth rate was 7.2 ± 0.6 and 3.0 ± 0.8 mg g^?1 day^?1 in L. pulmonaria and X. aureola, respectively, reduced to 1.8 ± 0.5 and ?2.6 ± 0.9 mg g^?1 day^?1, at 21 °C. These figures showed that lichen growth is a useful response variable in short-term laboratory experiments. Growth was not influenced by UV-B alone in these pigment-deficient transplants, suggesting that UV-B had little adverse effects on either of the lichen bionts. The cortical sun screens (parietin and melanic compounds) were synthesized in the presence of UV-B, and increased statistically significantly with increasing UV-B at both cultivation temperatures. However, in X. aureola the synthesis was highest at the lowest temperature (12 °C). At 12 °C, changes in chlorophylls, F_v/F_m and NPQ during cultivation were consistent with a substantial level of acclimation to the growth chamber conditions for both species, whereas strong reductions in photosynthetic pigments, F_v/F_m and Ф_II at 21 °C indicated serious damage and chlorophyll degradation at high temperature. In conclusion, lichen growth and the synthesis of protective compounds are highly responsive lichen processes in short-term experiments.     

Is reproduction of endemic plant species particularly pollen limited in biodiversity hotspots?

Current evidence suggests that plants in biodiversity hotspots suffer more from pollen limitation of reproduction than those in lower diversity regions, primarily due to the response of self‐incompatible species. Species in biodiversity hotspots may thus be more at risk of limited reproduction and subsequent population decline. Should these species have restricted ranges (i.e. be endemics to a certain region), pollen limitation within highly diverse regions may pose an important threat to global plant biodiversity. We further dissect the global pattern by exploring whether pollen limitation of range‐restricted (endemic) species is distinctive and/or relates differently to species diversity than that of widespread (non‐endemic) species. To provide a preliminary test of this prediction we conducted both cross‐species and comparative phylogenetic meta‐analyses to determine the effect of endemism on the magnitude of pollen limitation and its relationship with regional species richness. Our data set included 287 plant species belonging to 78 families distributed world‐wide. Our results revealed that endemism and self‐compatibility contribute to the global association between pollen limitation and species richness. Self‐incompatible species were more pollen limited than self‐compatible ones, and the PICs analysis indicated that transitions to endemism were associated with transitions to self‐compatibility. The relationship between pollen limitation and species richness was significant only for the self‐incompatible species, and was monotonically increasing in non‐endemic species but accelerating in the endemic species. Thus, self‐incompatible endemic species from biodiversity hotspots are at the greatest risk of pollination failure, a previously unknown aspect suggesting this group of species as a top priority for future development of conservation strategies. In contrast, reproduction of self‐compatible species appears to be unrelated to plant diversity, although we caution that current data do not account for the reproductive limitation due to the quality of pollen received. Understanding the mechanisms underlying these patterns requires further investigation into plant–plant pollinator mediated interactions and the dynamics of pollen transfer in communities differing in species diversity.     

Reciprocal subsidies in ponds: does leaf input increase frog biomass export?

Reciprocal subsidies occur when ecosystems are paired, both importing and exporting resources to each other. The input of subsidies increases reciprocal subsidy export, but it is unclear how this changes with other important factors, such as ambient resources. We provide a conceptual framework for reciprocal subsidies and empirical data testing this framework using a pond–forest system in Missouri, USA. Our experiment used in situ pond mesocosms and three species of anurans: wood frogs, American toads, and southern leopard frogs. We predicted that increases in ambient resources (primary productivity) and detrital subsidy input (deciduous tree leaves) into pond mesocosms would increase reciprocal export (frog biomass) to the surrounding terrestrial ecosystem. In contrast, we found that increases in primary productivity consistently decreased frog biomass, except with leaf litter inputs. With leaf inputs, primary productivity did not affect the export of frogs, indicating that leaf detritus and associated microbial communities may be more important than algae for frog production. We found that subsidy inputs tended to increase reciprocal exports, and thus partial concordance with our conceptual framework.