Multitrophic effects of experimental changes in plant diversity on cavity-nesting bees, wasps, and their parasitoids

Abstract

Plant diversity changes can impact the abundance, diversity, and functioning of species at higher trophic levels. We used an experimental gradient in grassland plant diversity ranging from 1 to 16 plant species to study multitrophic interactions among plants, cavity-nesting bees and wasps, and their natural enemies, and analysed brood cell density, insect diversity (species richness), and bee and wasp community similarity over two consecutive years. The bee and wasp communities were more similar among the high (16 species) diversity plots than among plots of the lower diversity levels (up to 8 species), and a more similar community of bees and wasps resulted in a more similar community of their parasitoids. Plant diversity, which was closely related to flower diversity, positively and indirectly affected bee diversity and the diversity of their parasitoids via increasing brood cell density of bees. Increasing plant diversity directly led to higher wasp diversity. Parasitism rates of bees and wasps (hosts) were not affected by plant diversity, but increased with the diversity of their respective parasitoids. Decreases in parasitism rates of bees arose from increasing brood cell density of bees (hosts), whereas decreasing parasitism rates of wasps arose from increasing wasp diversity (hosts). In conclusion, decreases in plant diversity propagated through different trophic levels: from plants to insect hosts to their parasitoids, decreasing density and diversity. The positive relationship between plant diversity and the community similarity of higher trophic levels indicates a community-stabilising effect of high plant diversity.

Plant diversity in a nutshell: testing for small‐scale effects on trap nesting wild bees and wasps

“…Thus our results support the resource heterogeneity and more individuals hypotheses for wasps, but not for bees. Ebeling et al (2012) also found no relationship between plant diversity and brood cell densities Notes: Values of parameters from REML models, with strips as random factors and the plant composition in the different subplots included as a correlation structure. Abbreviations as in Table 1. v www.esajournals.org of bees.…”

“…Thus, the architecture of the vegetation can override the effects of plant diversity on food-web complexity. In contrast to other analyses that simply treated species diversity (e.g., Albrecht et al 2007, Tylianakis et al 2007, Haddad et al 2009, Scherber et al 2010a, Ebeling et al 2012, we accounted for plant composition by including a correlation structure in the residuals of the mixed-effect models. This is important because a change in plant diversity cannot be achieved without altering plant composition.…”

“…Thus declining plant species richness can have cascading effects, releasing herbivores from this control and leading to reduced ecosystem functioning (Knops et al 1999, Balvanera et al 2006. The relationships between abundance and diversity of different trophic levels is therefore difficult to predict, and it is not surprising that recent studies showed varying effects of plant diversity on the abundance or diversity of higher trophic levels, yielding positive (Albrecht et al 2007, Dinnage 2013, neutral (Ebeling et al 2012) or negative (Schuldt et al 2011) relationships. However, positive effects dominate, with a reported decrease in the strength of the relationship for higher trophic levels (Scherber et al 2010a).…”

“…Usually, plant composition is analyzed separately from plant diversity and structure, either by using multivariate approaches , by considering different functional groups (e.g., grasses, herbs, legumes; Hector et al 1999, Haddad et al 2009, Scherber et al 2010b, or by using dissimilarity matrices in Mantel tests (Ebeling et al 2012). To our knowledge, studies that simultaneously analyzed the effect of plant diversity, plant composition and structure on insect diversity and abundance and ultimately food-web structure are rare.…”

Plant diversity has contrasting effects on herbivore and parasitoid abundance in Centaurea jacea flower heads

“…Tephritid infestation rate and insect load both decreased with increasing plant diversity (functional group or species richness), which means the increase in parasitism could also be driven by decreasing host density. This response was reported in a study on trap nesting bees on the same field site (Ebeling et al., 2012). However, such an effect seems less likely as an explanation of our results because we found that tephritid infestation rate did not affect parasitism rate.…”