Herbivore Competition Research Articles

Wildfire history and savanna expansion across southern Africa since the late Miocene

The origin of the African savanna has been traced back to 10–6 million years (Ma) ago, but the mechanisms driving its evolution are hotly debated, and include global atmospheric CO 2 , regional fire activity, herbivore competition, and hydrological climate change. Here, we present the first microcharcoal-based fire activity records covering the last ~7 Ma at four International Ocean Discovery Program (IODP) sites near southern Africa. The records show that fire activities in both savanna and non-savanna regions were stable during this interval. Grass vegetation in burnt biomass continued to expand from 6 Ma into the present savanna region, whereas no grass expansion was observed in non-savanna regions. A compilation of regional data suggests asynchronous C4 grass expansion since 6 Ma on the African Continent. We consider that CO 2 concentrations and wildfires might have caused the first appearance of C4 plants at ~10 Ma and ~ 7–6 Ma, respectively. Since 6 Ma, the regional climate promoted expansion of C4 plants into the present-day savanna habitat. • Microcharcoals in four IODP boreholes surrounding the southern Africa are studied. • Microcharcoal influxes show the stable wildfire trends since 6 Ma ago. • Microcharcoal morphologies show the C4 vegetation expanded along specific routes. • The C4 expansion is attributed to the regional climate rather than CO 2 or wildfire.

Insecticides promote viral outbreaks by altering herbivore competition.

While the management of biological invasions is often characterized by a series of single-specieg decisions, invasive species exist within larger food webs. These biotic interactions can alter the impact of control/eradication programs and may cause suppression efforts to inadvertently facilitate invasion spread and impact. We document the rapid replacement of the invasive Bemisia Middle East-Asia Minor I (MEAM1) cryptic biotype by the cryptic Mediterranean (MED) biotype throughout China and demonstrate that MED is more tolerant of insecticides and a better vector of tomato yellow leaf curl virus (TYLCV) than MEAMJ. While MEAM1 usually excludes MED under natural conditions, insecticide application reverses the MEAM1-MED competitive hierarchy and allows MED to exclude MEAMI. The insecticide-mediated success of MED has led to TYLCV outbreaks throughout China. Our work strongly supports the hypothesis that insecticide use in China reverses the MEAMl-MED competitive hierarchy and allows MED to displace MEAM1 in managed landscapes. By promoting the dominance of a Bemisia species that is a competent viral vector, insecticides thus increase the spread and impact of TYLCV in heterogeneous agroecosystems.

Are exotic herbivores better competitors? A meta‐analysis

Competition plays an important role in structuring the community dynamics of phytophagous insects. As the number and impact of biological invasions increase, it has become increasingly important to determine whether competitive differences exist between native and exotic insects. We conducted a meta-analysis to test the hypothesis that native/ exotic status affects the outcome of herbivore competition. Specifically, we used data from 160 published studies to assess plant-mediated competition in phytophagous insects. For each pair of competing herbivores, we determined the native range and coevolutionary history of each herbivore and host plant. Plant-mediated competition occurred frequently, but neither native nor exotic insects were consistently better competitors. Spatial separation reduced competition in native insects but showed little effect on exotics. Temporal separation negatively impacted native insects but did not affect competition in exotics. Insects that coevolved with their host plant were more affected by interspecific competition than herbivores that lacked a coevolutionary history. Insects that have not coevolved with their host plant may be at a competitive advantage if they overcome plant defenses. As native/exotic status does not consistently predict outcomes of competitive interactions, plant-insect coevolutionary history should be considered in studies of competition.

Modeling herbivore competition mediated by inducible changes in plant quality

Competition between herbivorous insects often occurs as a trait mediated indirect effect mediated by inducible changes in plant quality rather than a direct effect mediated by plant biomass. While plant‐mediated competition likely influences many herbivores, progress linking studies of plant‐mediated competition in terrestrial phytophagous insects to longer‐term consequences for herbivore communities has been elusive, and there is little relevant theory to guide this effort. We present simple models describing plant‐mediated interactions between two herbivorous insects or other functionally equivalent organisms. These models consider general features of plant‐mediated competition including specificity of elicitation by and effects on herbivores, positive and negative interactions among herbivores, competition independent of changes in plant biomass, and the existence of multiple relevant plant traits. Our analyses generate four important conclusions. First, herbivores competing strongly via only one plant quality phenotype exhibit a limited range of outcomes. These include coexistence and competitive exclusion of either herbivore, but do not include initial condition dependence. Second, when the outcome of competition is competitive exclusion, the herbivore that persists is the one that can do so under the highest inducible reductions in plant quality. Third, competition via more than one inducible phenotype can exhibit a wider range of outcomes including multiple equilibria and initial condition dependence. Finally, transient dynamics may not predict the eventual outcome of competition when changes in plant quality are slow relative to herbivore population growth, especially when herbivores compete through multiple phenotypes. We interpret our results in terms of competition outcomes reported in the literature, and suggest directions for the future empirical study of herbivore competition mediated by inducible changes in plant quality.

Early season herbivore differentially affects plant defence responses to subsequently colonizing herbivores and their abundance in the field

Induction of plant defences by early season herbivores can mediate interspecific herbivore competition. We have investigated plant-mediated competition between three herbivorous insects through studies at different levels of biological integration. We have addressed (i) gene expression; (ii) insect behaviour and performance under laboratory conditions; and (iii) population dynamics under field conditions. We studied the expression of genes encoding a trypsin inhibitor and genes that are involved in glucosinolate biosynthesis in response to early season herbivory by Pieris rapae caterpillars in Brassica oleracea plants. Furthermore, we studied the interaction of these transcriptional responses with responses to secondary herbivory by the two specialist herbivores, P. rapae and Plutella xylostella, and the generalist Mamestra brassicae. P. rapae-induced responses strongly interacted with plant responses to secondary herbivory. Sequential feeding by specialist herbivores resulted in enhanced or similar expression levels of defence-related genes compared to primary herbivory by specialists. Secondary herbivory by the generalist M. brassicae resulted in lower gene expression levels than in response to primary herbivory by this generalist. Larval performance of both specialist and generalist herbivores was negatively affected by P. rapae-induced plant responses. However, in the field the specialist P. xylostella was more abundant on P. rapae-induced plants and preferred these plants over undamaged plants in oviposition experiments. In contrast, the generalist M. brassicae was more abundant on control plants and preferred undamaged plants for oviposition. P. rapae did not discriminate between plants damaged by conspecifics or undamaged plants. Our study shows that early season herbivory differentially affects transcriptional responses involved in plant defence to secondary herbivores and their population development dependent upon their degree of host plant specialization.

Predators induce interspecific herbivore competition for food in refuge space

Resource competition among herbivorous arthropods has long been viewed as unimportant because herbivore populations are controlled by predators. Although recently resurrected as an organizing force in arthropod communities on plants, there is still general agreement that resource competition among herbivores is reduced by predators. Here we show the reverse: predators induce interspecific resource competi‐tion among herbivores. We found that thrips larvae (Frankliniella occidentalis) use the web produced and inhabited by the spider mite Tetranychus urticae as a refuge from predation by the phytoseiid mite Neoseiulus cucumeris. Thrips larvae prefer clean plant parts, but move into the web upon perceiving volatile cues associated with thrips and predators. This behaviour leads to lower predation risk, but also to reduced developmental rate and lower production of thrips larvae due to competition with spider mites. In addition, thrips larvae consume spider‐mite eggs. Thus, predators induce interspecific competition and intraguild predation among herbivores within refuge space, even when host plants have an overall green appearance.

Induced resistance in mountain birch: defence against leaf-chewing insect guild and herbivore competition

The effect of leaf damage simulating the feeding of early season insect herbivore species, e.g. Epirrita autumnata, to mountain birch, Betula pubescens ssp. tortuosa, on the performance of insect larvae was studied with eleven leaf-chewing sawfly species. I found variation in the results that was due to short- and long-term inducible responses and to the phenology of herbivore species. In general, early and mid-season species were more strongly affected by induced reactions than late-season species. This finding is in accordance with earlier results but I could show that the persistance of induced reactions rather than the influence of timing of damage is responsible for the result. The growth of the larvae of mid-season sawfly species was affected by both short- and long-term induced reactions. This result shows that early season species may escape short-term induced reactions of mountain birch in current year but may not avoid long-term effects. It is supposed that seasonal deterioration of leaf quality either masks the effects of induced defences or late-season species are better adapted to low-quality leaves. Some species show variation in their response to induced defence in different years. This may be due to yearly differences in induced reactions as well as to species-specific responses. Induced defence reactions may play a role in competitive interactions between herbivore species in leaf-chewing guild of mountain birch.