Determinants of parasitoid communities of willow‐galling sawflies: habitat overrides physiology, host plant and space

Abstract

Studies on the determinants of plant-herbivore and herbivore-parasitoid associations provide important insights into the origin and maintenance of global and local species richness. If parasitoids are specialists on herbivore niches rather than on herbivore taxa, then alternating escape of herbivores into novel niches and delayed resource tracking by parasitoids could fuel diversification at both trophic levels. We used DNA barcoding to identify parasitoids that attack larvae of seven Pontania sawfly species that induce leaf galls on eight willow species growing in subarctic and arctic-alpine habitats in three geographic locations in northern Fennoscandia, and then applied distance- and model-based multivariate analyses and phylogenetic regression methods to evaluate the hierarchical importance of location, phylogeny and different galler niche dimensions on parasitoid host use. We found statistically significant variation in parasitoid communities across geographic locations and willow host species, but the differences were mainly quantitative due to extensive sharing of enemies among gallers within habitat types. By contrast, the divide between habitats defined two qualitatively different network compartments, because many common parasitoids exhibited strong habitat preference. Galler and parasitoid phylogenies did not explain associations, because distantly related arctic-alpine gallers were attacked by a species-poor enemy community dominated by two parasitoid species that most likely have independently tracked the gallers' evolutionary shifts into the novel habitat. Our results indicate that barcode- and phylogeny-based analyses of food webs that span forested vs. tundra or grassland environments could improve our understanding of vertical diversification effects in complex plant-herbivore-parasitoid networks.

Bringing Elton and Grinnell together: a quantitative framework to represent the biogeography of ecological interaction networks

“…Our model comparison of host-parasite interactions revealed that most of the turnover is driven by species-specific responses to the environment, impacting species richness, and that co-occurrence was mostly independent. Indeed, a previous study focusing on a subset of the system studied here identified habitat characteristics as the primary drivers of interactions (Nyman et al 2015). We know that interactions vary with the environment in other systems, for instance, herbivory (Shurin et al 2012, Baskett et al 2018) and predation (McKinnon et al 2010, Legagneux et al 2014 are often found to increase with temperature, resulting in spatial variation of trophic cascades (Gray et al 2016).…”

The use of DNA barcodes in food web construction—terrestrial and aquatic ecologists unite!

“…By comparing food web structure to phylogenetic signal, we may explore how interacting species coevolve (Nyman et al 2007(Nyman et al , 2015Vazquez et al 2009;Rafferty and Ives 2013;Fontaine and Thébault 2015). As another line of investigation, comparisons of food webs along sites of different glacial or geological history will help us understand the rules for how communities disassemble and reassemble over time (Stone et al 2012).…”

“…Aggregating units of observation for subsequent analysis is easy, whereas disaggregating them is often impossible". In some cases, we should also be particularly determined in aiming for the highest end of resolution: in particular when asking questions on population-level dynamics, including the potential for indirect interactions such as apparent competition or apparent mutualism (Müller et al 1999;Morris et al 2004Morris et al , 2005van Veen et al 2006;Tack et al 2011), or questions on coevolution and phylogenetic structuring (Nyman et al 2007(Nyman et al , 2015Vazquez et al 2009;Rafferty and Ives 2013;Fontaine and Thébault 2015). Composite taxa may not share the same population dynamics, let alone the same evolutionary trajectories.…”

“…Where previous approaches have inferred a phylogenetic signal in interaction structure by combining interaction webs with phylogeny inferred from external sources, molecular information will allow the reconstruction of phylogenies from the samples themselves (Elias et al 2013;Evans et al 2016). Here, genomic approaches will add manifold to the information content as compared to classic DNA barcodes only (Papadopoulou et al 2015;Andújar et al 2015;Linard et al 2015)-again building a natural extension from previous studies advancing from one to multiple (Nyman et al 2007(Nyman et al , 2015Kress et al 2009Kress et al , 2015 gene regions. Thus, it is evident how the use of single-locus DNA barcodes have laid a solid fundament for making full use of the information fountains now emerging.…”

Tracking the elusive history of diversification in plant–herbivorous insect–parasitoid food webs: insights from figs and fig wasps

“…They play a central role in the functioning of all terrestrial ecosystems, and the number of species involved is mind‐blowing (Nyman et al . ). Nevertheless, our understanding of the evolutionary and ecological determinants of their diversity is still in its infancy.…”