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Grazing and fire are disturbances integral to the evolution and maintenance of savanna grasslands. Humans are altering or completely eliminating these disturbance regimes at a global scale, with important consequences for savanna ecosystem structure and function. It is unknown whether the alteration of these disturbance regimes will have similar effects on grass communities of savanna grasslands in different geographic regions that vary in their biogeographic and evolutionary histories, as well as in the diversity of extant grazers., Here, we examined the effects of large herbivore loss on different aspects of grass community structure - taxonomic, phylogenetic and functional - across a range of fire frequencies in C4-dominated mesic savanna grassland sites of North America (Konza Prairie Biological Station, Kansas, USA) and South Africa (Kruger National Park). The goal of the study was to determine whether the loss of large herbivores exerted a consistent effect on the grass communities of two physiognomically similar grasslands with different biogeographic and grazing histories., The removal of large herbivores resulted in divergent responses in the grass communities at Konza and Kruger that was consistent across fire treatments. At Konza, there was a rapid and significant response to grazing exclusion while the response was muted and transient at Kruger. Functional syndromes associated with grazing resistance were generally conserved across sites, and it was the functional strategies of the dominant species at each site that drove the divergent responses. Further, our study supports the hypothesis that grazing and aridity may be selective forces that act in parallel as those species that were grazing resistant also occupied drier niches., Synthesis. Our study demonstrates that savanna grassland communities with different biogeographic and grazing histories respond differently to the removal of large herbivores and that climate, fire and grazing are interactive forces in maintaining savanna grassland diversity and function. We show that the functional attributes of the dominant grasses, which are in part driven by the biogeographic and grazing history experienced, are the most relevant in predicting the response of savanna ecosystems to the loss of large herbivores. [ABSTRACT FROM AUTHOR]

An index measuring interaction importance ( Iimp) was proposed in a paper published on 2010 by the Journal of Ecology. This index, now widely used by researchers, has been claimed to symmetrically range in the interval (−1, 1), with negative values for competition and positive values for facilitation., Here, the index properties are carefully analysed, and it is demonstrated that Iimp can never exceed the value of 0.5 in facilitation., Several years have passed since its publication, yet nobody has corrected the inexact index range, and several researchers continue to inaccurately refer to it. Overlooking this lack of symmetry could potentially lead to biased conclusions., Synthesis. Iimp cannot be treated as a symmetric index for negative and positive interactions. This is only the last of several concerns discussed in literature: current approaches to measuring interaction importance still need to be debated and analysed in depth. [ABSTRACT FROM AUTHOR]

Although plants and herbivores interact under varying soil resources and natural enemy effects, little is known about how these factors influence plant-herbivore interactions and shape the evolution of plant and herbivore traits. Here, we ask whether soil fertility and parasitoids shape selection on fruit number imposed by a seed predator ( SP) on the perennial herb Ruellia nudiflora., We used a common garden where half the plants of 14 genetic families were fertilized and recorded the abundance of cleistogamous ( CL) fruits and seeds, SPs and parasitoids. We calculated relative fitness per family based on CL seed number under the following three scenarios: three trophic levels (accounting for SP and parasitoid effects), two trophic levels (accounting for SP but not parasitoid effects), and one trophic level (fitness in absence of SPs), and compared selection strength on fruit number between trophic scenarios and fertility environments., In unfertilized conditions, SPs selected for increased CL fruit number, whereas parasitoids dampened (but did not eliminate) this selective impact. With fertilization, however, selection by SPs was reduced and unaffected by parasitoids., Synthesis. Overall, we show that parasitoids can shape herbivore selection on plants, but that both herbivore and parasitoid selective impacts depend upon the abiotic environment. These findings underscore how linkages between abiotic factors and trophic complexity influence the ecological and evolutionary outcomes of species interactions. [ABSTRACT FROM AUTHOR]

The concept of stability in ecological theory has been a subject of debate for more than 40 years. Its value lies in being the explanatory principle underlying equilibrium in nature. Until now, the possibility of spatial stability in trophic interactions, defined as resilience of trophic activity (e.g. herbivore/algae or predator/prey) within patches after a disturbance, has not been explored. This concept requires the demonstration of two properties: (i) the return after a strong disturbance to a reference level of intensity of the trophic interaction (intensity criterion), and (ii) positive spatial correlation between pre- and post-disturbance intensity of the trophic interaction (spatial determinism criterion)., The first criterion can be assessed by checking the consistency of intensities of grazing effects between years and the second criterion by correlating the intensities of grazing between years on the same set of patches. We used a spatially explicit test, repeated after experimental disturbance of the same plots in two consecutive years, to test for the existence of spatial stability in the effects of grazing on rocky shore algal assemblages., The detection of spatial stability depended on the level of resolution at which the prey assemblages were examined (species, functional group and trophic level) and allowed us to demonstrate stability in grazing effects, which shape the mosaic of patches on the high rocky shore at scales of 10s of meters., The community analysed was highly resilient due to its domination by species that can quickly re-colonize and recover to pre-disturbance levels and showed spatial determinism because of the tendency of both grazers and algae to colonize particularly favourable areas., Synthesis. The most important findings are as follows: (i) spatial stability was confirmed for a trophic interaction, and (ii) spatial determinism in these systems was detectable. We suggest that patches exhibiting predictability in trophic interaction strength can function as initiators of recovery after disturbance and at the same time represent the deterministic initial condition that might shape long-term spatial configurations within the landscape. [ABSTRACT FROM AUTHOR]

Assessing the demographic effects of leaf area losses in perennial plants is important to determine population resilience to natural and anthropogenic disturbances. Yet, while impacts of defoliation on vital rates of adult plants have been well documented, consequences for seed dispersal and seedling recruitment have been barely explored., Here, we assessed the effects of defoliation on fruit production, fruit/seed size, seed dispersal and seedling recruitment in populations of Chamaedorea ernesti-augustii, a tropical rain forest, understorey palm from Mesoamerica, whose leaves are exploited as a highly valuable non-timber forest product ( NTFP)., Fruit size and seed production were quantified in mature palms that were subjected to 0% (control), 25%, 50%, 75% and 100% experimental removal of standing leaves, applied every 6 months over 2 years. Seed dispersal by birds and gravity, and seed predation by small vertebrates on the ground were also quantified. Rates of seedling recruitment were recorded in non-defoliated and 'sterile' populations (75% sustained defoliation and periodical removal of all produced fruits). Finally, a stochastic model was used to quantify the overall effect of defoliation on seedlings recruited from locally (i.e. dispersed by gravity) and immigrant produced seeds (i.e. dispersed by birds)., Increasing defoliation strongly reduced seed production. The probability of bird dispersal was positively correlated with fruit/seed size. Isolated (i.e. bird-dispersed) seeds endured lower predation rates than grouped (i.e. gravity-dispersed) ones. Modelling showed that seedling recruitment rate is severely reduced in highly defoliated populations due to a strong decline in the number of seedlings coming from local seed sources. Surrounding non-defoliated populations could partly compensate for this effect via seedlings coming from immigrant seeds., Synthesis. Chronic and intense defoliation negatively affects seed production and dispersal, which reduces the probability of seedling recruitment. Such effects may have a profound impact on the dynamics and genetic variability of populations, which should be taken into account when considering the effects of natural defoliation and sustainability of leaf-harvesting regimes. [ABSTRACT FROM AUTHOR]

In many arctic herbivores, the growth of young depends upon a synchrony between hatching date and seasonal change in plant nutritive quality. If plants respond more quickly than herbivores to climate warming, this may cause a mismatch between the availability of high-quality food and the hatching of young. This study examines the impact of experimental warming on the main food plants of an arctic herbivore, the greater snow goose ( Chen caerulescens atlantica L.) breeding on Bylot Island, Nunavut, Canada., During summers 2007-2009, we increased the temperature using small glasshouses (open-top chambers, OTC) in two habitats, wetlands and mesic tundra. Every 10 days, we measured above-ground plant biomass and a proxy of nutritive quality, nitrogen concentration, of graminoid plants in warmed and control plots from snowmelt in June until late July., Open-top chambers increased mean maximum temperature by up to 2.0 °C in wetlands and 4.6 °C in mesic tundra. Annual warming significantly increased biomass of graminoids by up to 29% in wetlands and 20% in mesic tundra. There was no difference in nitrogen concentration of the four plant species sampled ( Dupontia fisheri, Eriophorum scheuchzeri, Arctagrostis latifolia and Luzula spp.) early in the season, but the seasonal decline in nitrogen occurred more rapidly in warmed than in control plots (10% to 14% less nitrogen in warmed plots in July). This effect was consistent across the 3 years of the experiment and independent of annual variation in plant phenology. There was either a weak positive effect or no effect of the warming treatment on the nitrogen biomass of plants depending on species or period of the season., Synthesis. Our results show that warming speeds up plant phenology and the seasonal decline in nutritive quality for arctic herbivores. Because young herbivores like geese are highly sensitive to the nitrogen concentration of their food, a warmer climate will likely reduce their growth. Climate warming may therefore have a negative impact on the population dynamic of arctic herbivores by reducing the quality of their summer forage. [ABSTRACT FROM AUTHOR]

Despite their ubiquitous nature, interactions between multiple micro-organisms and their effects on host growth and each other's success have received limited scientific attention. In particular, grasses can be commonly infected by both endophytic fungi and viruses, which are typically transmitted by aphids. In this study, we investigated how an aphid-transmitted viral pathogen and a symbiotic endophytic fungus altered host growth and allocation. We hypothesized that, by reducing aphid feeding, endophyte infection would lower viral titre and consequently decrease the negative effects of virus infection on host biomass production., In a greenhouse experiment, we manipulated fungal endophyte status and virus infection ( Barley Yellow Dwarf Virus - PAV) of two tall fescue cultivars with different genetic backgrounds [KY 31 and pasture demonstration farm (PDF)]. In one cultivar (PDF), we also manipulated endophyte strain, using two strains that had been selected for differences in alkaloid production. We assessed host, virus and vector responses., As hypothesized, endophyte infection decreased reproduction and abundance of aphid vectors; however, in contrast to our hypothesis, this response by aphids did not impact viral titre. For both tall fescue cultivars, endophyte infection alleviated the negative effect of virus infection on the proportion of total plant biomass allocated to roots. On the other hand, for the KY 31 cultivar only, virus infection decreased tillering in endophyte-infected individuals, but not endophyte-free individuals. Within the PDF cultivar, both endophyte strains produced similar effects on host, virus and vector responses., Synthesis These results indicate that some of the beneficial effects provided by endophyte infection, particularly alleviating the negative effect of virus infection on the proportion of total plant biomass allocated to roots, do not arise strictly from altering host interactions with herbivores (aphids), but also occur by changing host responses to viral infection. Furthermore, these results emphasize the importance of exploring multispecies microbial interactions and genetic controls on these interactions in order to more fully understand their role in community- and ecosystem-level dynamics. [ABSTRACT FROM AUTHOR]

Past studies have shown that the strength of top-down herbivore control on plant physiological performance, abundance and distribution patterns can shift with abiotic stress, but it is still unclear whether herbivores generally exert stronger effects on plants in stressful or in nonstressful environments., One hypothesis suggests that herbivores' effects on plant biomass and fitness should be strongest in stressful areas, because stressed plants are less able to compensate for herbivore damage. Alternatively, herbivores may reduce plant biomass and fitness more substantially in nonstressful areas, either because plant growth rates in the absence of herbivory are higher and/or because herbivores are more abundant and diverse in nonstressful areas., We test these predictions of where herbivores should exert stronger effects by measuring individual performance, population size structure and densities of a common subshrub, Hibiscus meyeri, in a large-scale herbivore exclosure experiment arrayed across an aridity gradient in East Africa., We find support for both predictions, with herbivores exerting stronger effects on individual-level performance in arid (stressful) areas, but exerting stronger effects on population size structure and abundance in mesic (nonstressful) areas. We suggest that this discrepancy arises from higher potential growth rates in mesic areas, where alleviation of herbivory leads to substantially more growth and thus large changes in population size structure. Differences in herbivore abundance do not appear to contribute to our results., Synthesis. Our work suggests that understanding the multiple facets of plant response to herbivores (e.g. both individual performance and abundance) may be necessary to predict how plant species' abundance and distribution patterns will shift in response to changing climate and herbivore numbers. [ABSTRACT FROM AUTHOR]