Functional identity versus species richness: herbivory resistance in plant communities

Effects of plant diversity on species-specific herbivory: patterns and mechanisms

Invertebrate herbivory can shape plant communities when impacting growth and fitness of some plant species more than other species. Previous studies showed that herbivory varies among plant species and that species-specific herbivory is affected by the diversity of the surrounding plant community. However, mechanisms underlying this variation are still poorly understood. In this study, we investigate how plant traits and plant apparency explain differences in herbivory among plant species and we explore the effect of plant community diversity on these species-specific relationships. We found that species differed in the herbivory they experienced. Forbs were three times more damaged by herbivores than grasses. Variability within grasses was caused by differences in leaf dry matter content (LDMC). Furthermore, higher plant diversity increased herbivory on 15 plant species and decreased herbivory on nine species. Variation within forb and grass species in their response to changing plant diversity was best explained by species' physical resistance (LDMC, forbs) and biomass (grasses). Overall, our results show that herbivory and diversity effects on herbivory differ among species, and that, depending on the plant functional group, either species-specific traits or apparency are driving those differences. Thus, herbivores might selectively consume palatable forbs or abundant grasses with contrasting consequences for plant community composition in grasslands dominated by either forbs or grasses.

Functional trait diversity across trophic levels determines herbivore impact on plant community biomass

Understanding the consequences of trophic interactions for ecosystem functioning is challenging, as contrasting effects of species and functional diversity can be expected across trophic levels. We experimentally manipulated functional identity and diversity of grassland insect herbivores and tested their impact on plant community biomass. Herbivore resource acquisition traits, i.e. mandible strength and the diversity of mandibular traits, had more important effects on plant biomass than body size. Higher herbivore functional diversity increased overall impact on plant biomass due to feeding niche complementarity. Higher plant functional diversity limited biomass pre-emption by herbivores. The functional diversity within and across trophic levels therefore regulates the impact of functionally contrasting consumers on primary producers. By experimentally manipulating the functional diversity across trophic levels, our study illustrates how trait-based approaches constitute a promising way to tackle existing links between trophic interactions and ecosystem functioning.

Ecosystem carbon exchange in response to locust outbreaks in a temperate steppe

It is predicted that locust outbreaks will occur more frequently under future climate change scenarios, with consequent effects on ecological goods and services. A field manipulative experiment was conducted to examine the responses of gross ecosystem productivity (GEP), net ecosystem carbon dioxide (CO2) exchange (NEE), ecosystem respiration (ER), and soil respiration (SR) to locust outbreaks in a temperate steppe of northern China from 2010 to 2011. Two processes related to locust outbreaks, natural locust feeding and carcass deposition, were mimicked by clipping 80 % of aboveground biomass and adding locust carcasses, respectively. Ecosystem carbon (C) exchange (i.e., GEP, NEE, ER, and SR) was suppressed by locust feeding in 2010, but stimulated by locust carcass deposition in both years (except SR in 2011). Experimental locust outbreaks (i.e., clipping plus locust carcass addition) decreased GEP and NEE in 2010 whereas they increased GEP, NEE, and ER in 2011, leading to neutral changes in GEP, NEE, and SR across the 2 years. The responses of ecosystem C exchange could have been due to the changes in soil ammonium nitrogen, community cover, and aboveground net primary productivity. Our findings of the transient and neutral changes in ecosystem C cycling under locust outbreaks highlight the importance of resistance, resilience, and stability of the temperate steppe in maintaining reliable ecosystem services, and facilitate the projections of ecosystem functioning in response to natural disturbance and climate change.

Impacts of an invasive non-native annual weed, Impatiens glandulifera, on above- and below-ground invertebrate communities in the United Kingdom

Vegetation community composition and the above- and below-ground invertebrate communities are linked intrinsically, though few studies have assessed the impact of non-native plants on both these parts of the community together. We evaluated the differences in the above- (foliage- and ground-dwelling) and below-ground invertebrate communities in nine uninvaded plots and nine plots invaded by the annual invasive species Impatiens glandulifera, in the UK during 2007 and 2008. Over 139,000 invertebrates were identified into distinct taxa and categorised into functional feeding groups. The impact of I. glandulifera on the vegetation and invertebrate community composition was evaluated using multivariate statistics including principal response curves (PRC) and redundancy analysis (RDA). In the foliage-dwelling community, all functional feeding groups were less abundant in the invaded plots, and the species richness of Coleoptera and Heteroptera was significantly reduced. In the ground-dwelling community, herbivores, detritivores, and predators were all significantly less abundant in the invaded plots. In contrast, these functional groups in the below-ground community appeared to be largely unaffected, and even positively associated with the presence of I. glandulifera. Although the cover of I. glandulifera decreased in the invaded plots in the second year of the study, only the below-ground invertebrate community showed a significant response. These results indicate that the above- and below-ground invertebrate communities respond differently to the presence of I. glandulifera, and these community shifts can potentially lead to a habitat less biologically diverse than surrounding native communities; which could have negative impacts on higher trophic levels and ecosystem functioning.

Phylogenetic diversity of plants alters the effect of species richness on invertebrate herbivory

Long-standing ecological theory proposes that diverse communities of plants should experience a decrease in herbivory. Yet previous empirical examinations of this hypothesis have revealed that plant species richness increases herbivory in just as many systems as it decreases it. In this study, I ask whether more insight into the role of plant diversity in promoting or suppressing herbivory can be gained by incorporating information about the evolutionary history of species in a community. In an old field system in southern Ontario, I surveyed communities of plants and measured levels of leaf damage on 27 species in 38 plots. I calculated a measure of phylogenetic diversity (PSE) that encapsulates information about the amount of evolutionary history represented in each of the plots and looked for a relationship between levels of herbivory and both species richness and phylogenetic diversity using a generalized linear mixed model (GLMM) that could account for variation in herbivory levels between species. I found that species richness was positively associated with herbivore damage at the plot-level, in keeping with the results from several other recent studies on this question. On the other hand, phylogenetic diversity was associated with decreased herbivory. Importantly, there was also an interaction between species richness and phylogenetic diversity, such that plots with the highest levels of herbivory were plots which had many species but only if those species tended to be closely related to one another. I propose that these results are the consequence of interactions with herbivores whose diets are phylogenetically specialized (for which I introduce the term cladophage), and how phylogenetic diversity may alter their realized host ranges. These results suggest that incorporating a phylogenetic perspective can add valuable additional insight into the role of plant diversity in explaining or predicting levels of herbivory at a whole-community scale.

Diversity protects plant communities against generalist molluscan herbivores

Wildflower strips are used to increase natural enemies of crop pests and to conserve insect diversity on farmland. Mollusks, especially slugs, can affect the vegetation development in these strips considerably. Although recent theoretical work suggests that more diverse plant communities will exhibit greater resistance against herbivore pressure, empirical studies are scarce. We conducted a semi-natural experiment in wildflower strips, manipulating trophic structure (reduction in herbivorous mollusks and reduction in major predators) and plant diversity (2, 6, 12, 20 and 24 sown species). This design allowed us to assess the effect of plant diversity, biomass and composition on mollusks, and vice versa, the effect of mollusc abundance on vegetation. Seven species of mollusks were found in the strips, with the slugs Arion lusitanicus, Deroceras reticulatum and Deroceras panormitanum being most frequent. We found a negative relationship between plant diversity and mollusk abundance, which was due predominantly to a decrease in the agricultural pest species A. lusitanicus. These results are consistent with the hypothesis that plant diversity can reduce the impact of herbivores. However, plant identity also had an effect on mollusks, and accounted for a much larger fraction of the variation in mollusk communities than biodiversity effects. While overall plant diversity decreased during the 3 years of the study, in the final year the highest plant diversity was found in the plots where mollusk populations were experimentally reduced. We conclude that selective feeding by generalist herbivores leads to changes in plant community composition and hence reduced plant diversity. Our results highlight the importance of plant biodiversity as protection against generalist herbivores, which if abundant can in the long term negatively impact plant diversity, driving the system along a “low plant diversity – high mollusk abundance” trajectory.

Functional diversity, soil features and community functioning: a test in a cerrado site

Community functioning may be affected by functional diversity, which measures the extent of complementarity in resource use. We tested whether there was a relationship between functional diversity of woody species and community functioning on a fine scale, using FD as a measure of functional diversity and litter decomposition rate as a surrogate for community functioning. We measured eight functional traits from a woodland cerrado community in southeastern Brazil. Then, we tested the correlation between FD and the decomposition rate taking into account differences in soil features and between decomposition rate and each trait separately. The decomposition rate was related to the aluminium and phosphorus concentration in soil, but not to FD, pointing out that functional diversity was not a good predictor of community functioning. There was a non-significant relationship between FD and the decomposition rate even when we considered each trait separately. Most studies in the relationships between biodiversity and community functioning on fine scales were carried out by experimental manipulation of diversity and in temperate regions. We carried out this fine scale study as a mensurative experiment and in a tropical savanna. Our findings indicated that the relationship between biodiversity and community functioning is not as straightforward as usually assumed.

Grassland resistance and resilience after drought depends on management intensity and species richness

The degree to which biodiversity may promote the stability of grasslands in the light of climatic variability, such as prolonged summer drought, has attracted considerable interest. Studies so far yielded inconsistent results and in addition, the effect of different grassland management practices on their response to drought remains an open question. We experimentally combined the manipulation of prolonged summer drought (sheltered vs. unsheltered sites), plant species loss (6 levels of 60 down to 1 species) and management intensity (4 levels varying in mowing frequency and amount of fertilizer application). Stability was measured as resistance and resilience of aboveground biomass production in grasslands against decreased summer precipitation, where resistance is the difference between drought treatments directly after drought induction and resilience is the difference between drought treatments in spring of the following year. We hypothesized that (i) management intensification amplifies biomass decrease under drought, (ii) resistance decreases with increasing species richness and with management intensification and (iii) resilience increases with increasing species richness and with management intensification.We found that resistance and resilience of grasslands to summer drought are highly dependent on management intensity and partly on species richness. Frequent mowing reduced the resistance of grasslands against drought and increasing species richness decreased resistance in one of our two study years. Resilience was positively related to species richness only under the highest management treatment. We conclude that low mowing frequency is more important for high resistance against drought than species richness. Nevertheless, species richness increased aboveground productivity in all management treatments both under drought and ambient conditions and should therefore be maintained under future climates.