Floral trait expression and plant fitness in response to below- and aboveground plant–animal interactions

Although plant–animal interactions like pollination and herbivory are obviously interdependent, ecological investigations focus mainly on one kind of interaction ignoring the possible significance of the others. Plants with flowers offer an extraordinary possibility to study such mutualistic and antagonistic interactions since it is possible to measure changes in floral traits and fitness components in response to different organisms or combinations of them. In a three factorial common garden experiment we investigated single and combined effects of root herbivores, leaf herbivores and decomposers on flowering traits and plant fitness of Sinapis arvensis. Leaf herbivory negatively affected flowering traits indicating that it could significantly affect plant attractiveness to pollinators. Decomposers increased total plant biomass and seed mass indicating that plants use the nutrients liberated by decomposers to increase seed production. We suggest that S. arvensis faced no strong selection pressure from pollen limitation, for two reasons. First, reduced nutrient availability through leaf herbivory affected primarily floral traits that could be important for pollinator attraction. Second, improved nutrient supply through decomposer activity was invested in seed production and not in floral traits. This study indicates the importance of considering multiple plant–animal interactions simultaneously to understand selection pressures underlying plant traits and fitness.     

Foraging theory predicts predator-prey energy fluxes

1. In natural communities, populations are linked by feeding interactions that make up complex food webs. The stability of these complex networks is critically dependent on the distribution of energy fluxes across these feeding links. 2. In laboratory experiments with predatory beetles and spiders, we studied the allometric scaling (body-mass dependence) of metabolism and per capita consumption at the level of predator individuals and per link energy fluxes at the level of feeding links. 3. Despite clear power-law scaling of the metabolic and per capita consumption rates with predator body mass, the per link predation rates on individual prey followed hump-shaped relationships with the predator-prey body mass ratios. These results contrast with the current metabolic paradigm, and find better support in foraging theory. 4. This suggests that per link energy fluxes from prey populations to predator individuals peak at intermediate body mass ratios, and total energy fluxes from prey to predator populations decrease monotonically with predator and prey mass. Surprisingly, contrary to predictions of metabolic models, this suggests that for any prey species, the per link and total energy fluxes to its largest predators are smaller than those to predators of intermediate body size. 5. An integration of metabolic and foraging theory may enable a quantitative and predictive understanding of energy flux distributions in natural food webs.     

A molecular interpretation of genetic interactions in yeast

Gene deletion studies in yeast have shown that only approximately 18% of its genes are essential for survival under standard laboratory conditions. This unexpectedly high fraction of genes with apparently no deletion effect has many practical and fundamental implications, and it is subject of considerable interest. Here, we briefly review some of the complementary models proposed to explain the robustness observed in biological networks. We also present and analyse a collection of well-documented cases of gene pairs with capacity to compensate the deleterious effects caused by the inactivation of one of the partner genes, and suggest the molecular bases of how these functional compensations might occur at the protein level.     

Generalist predators in organically and conventionally managed grass‐clover fields: implications for conservation biological control

Organically managed agroecosystems rely in part on biological control to prevent pest outbreaks. Generalist predators (Araneae, Carabidae and Staphylinidae) are a major component of the natural enemy community in agroecosystems. We assessed the seasonal dynamics of major generalist predator groups in conventionally and organically managed grass–clover fields that primarily differed by fertilisation strategy. We further established an experiment, manipulating the abundant wolf spider genus Pardosa, to identify the importance of these predators for herbivore suppression in the same system and growth period. Organic management significantly enhanced ground‐active spider numbers early and late in the growing season, with potentially positive effects of plant cover and non‐pest decomposer prey. However, enhancing spider numbers in the field experiment did not improve biological control in organically managed grass–clover fields. Similar to the survey results, reduced densities of Pardosa had no short‐term effect on any prey taxa; however, spider guild structure changed in response to Pardosa manipulation. In the presence of fewer Pardosa, other ground‐running spiders were more abundant; therefore, their impact on herbivore numbers may have been elevated, possibly cancelling increases in herbivore numbers because of reduced predation by Pardosa. Our results indicate positive effects of organic farming on spider activity density; however, our survey data and the predator manipulation experiment failed to find evidence that ground‐running spiders reduced herbivore numbers. We therefore suggest that a positive impact of organic fertilisers on wolf spiders in grass–clover agroecosystems may not necessarily improve biological control when compared with conventional farming.     

Effects of plant diversity on Collembola in an experimental grassland ecosystem

The response of species numbers and density of Collembola to manipulation of plant species richness (1, 2, 4, 8, 32 species) and number of plant functional groups (grasses, legumes and non-legume herbs) was studied in an experimental grassland at the Swiss BIODEPTH site (Lupsingen, Switzerland) in October 1997. Plant species richness or number of plant functional groups did not affect total diversity of Collembola, however, the number of Collembola species increased in the presence of legumes and the grass Trisetum flavescens . The abundance of Protaphorura armata increased but that of Hypogastruridae/Neanuridae significantly decreased with increasing number of plant functional groups. Other groups including the herbivorous Symphypleona did not respond to plant species richness and plant functional groups. Possibly, Hypogastruridae/Neanuridae species are weak competitors declining in density if the density of other Collembola groups increase. In general, the effect of the number of plant functional groups on the densities of collembolan taxa was stronger than that of plant species richness. Changes in Collembola density and diversity in part was likely caused by increased soil microbial and fine root biomass in treatments with higher plant functional group diversity. The presence of legumes resulted in an increase in the densities of total Collembola, Symphypleona/Neelipleona and Isotomidae indicating that they benefited from the high litter quality and the increased microbial biomass in the rhizosphere of legumes. The results suggest that microbivorous soil invertebrates are controlled by food quality rather than quantity. Furthermore, they indicate that presence of certain plant species and functional groups may be more important for collembolan community structure than the diversity of plant species and functional groups per se.     

Animal ecosystem engineers modulate the diversity-invasibility relationship

Background

Invasions of natural communities by non-indigenous species are currently rated as one of the most important global-scale threats to biodiversity. Biodiversity itself is known to reduce invasions and increase stability. Disturbances by ecosystem engineers affect the distribution, establishment, and abundance of species but this has been ignored in studies on diversity-invasibility relationships.

Methodology/Principal Findings

We determined natural plant invasion into 46 plots varying in the number of plant species (1, 4, and 16) and plant functional groups (1, 2, 3, and 4) for three years beginning two years after the establishment of the Jena Experiment. We sampled subplots where earthworms were artificially added and others where earthworm abundance was reduced. We also performed a seed-dummy experiment to investigate the role of earthworms as secondary seed dispersers along a plant diversity gradient. Horizontal dispersal and burial of seed dummies were significantly reduced in subplots where earthworms were reduced in abundance. Seed dispersal by earthworms decreased with increasing plant species richness and presence of grasses but increased in presence of small herbs. These results suggest that dense vegetation inhibits the surface activity of earthworms. Further, there was a positive relationship between the number of earthworms and the number and diversity of invasive plants. Hence, earthworms decreased the stability of grassland communities against plant invasion.

Conclusions/Significance

Invasibility decreased and stability increased with increasing plant diversity and, most remarkably, earthworms modulated the diversity-invasibility relationship. While the impacts of earthworms were unimportant in low diverse (low earthworm densities) and high diverse (high floral structural complexity) plant communities, earthworms decreased the stability of intermediate diverse plant communities against plant invasion. Overall, the results document that fundamental processes in plant communities like plant seed burial and invader establishment are modulated by soil fauna calling for closer cooperation between soil animal and plant ecologists.     

Both Functional LTβ Receptor and TNF Receptor 2 Are Required for the Development of Experimental Cerebral Malaria

Background

TNF-related lymphotoxin α (LTα) is essential for the development of Plasmodium berghei ANKA (PbA)-induced experimental cerebral malaria (ECM). The pathway involved has been attributed to TNFR2. Here we show a second arm of LTα-signaling essential for ECM development through LTβ-R, receptor of LTα1β2 heterotrimer.

Methodology/Principal Findings

LTβR deficient mice did not develop the neurological signs seen in PbA induced ECM but died at three weeks with high parasitaemia and severe anemia like LTαβ deficient mice. Resistance of LTαβ or LTβR deficient mice correlated with unaltered cerebral microcirculation and absence of ischemia, as documented by magnetic resonance imaging and angiography, associated with lack of microvascular obstruction, while wild-type mice developed distinct microvascular pathology. Recruitment and activation of perforin⁺ CD8⁺ T cells, and their ICAM-1 expression were clearly attenuated in the brain of resistant mice. An essential contribution of LIGHT, another LTβR ligand, could be excluded, as LIGHT deficient mice rapidly succumbed to ECM.

Conclusions/Significance

LTβR expressed on radioresistant resident stromal, probably endothelial cells, rather than hematopoietic cells, are essential for the development of ECM, as assessed by hematopoietic reconstitution experiment. Therefore, the data suggest that both functional LTβR and TNFR2 signaling are required and non-redundant for the development of microvascular pathology resulting in fatal ECM.