Molecular analyses reveal consistent food web structure with elevation in rainforest Drosophila - parasitoid communities

Abstract

The analysis of interaction networks across spatial environmental gradients is a powerful approach to investigate the responses of communities to global change. Using a combination of DNA metabarcoding and traditional molecular methods we built bipartite Drosophila-parasitoid food webs from six Australian rainforest sites across gradients spanning 850 m in elevation and 5° Celsius in mean temperature. Our cost-effective hierarchical approach to network reconstruction separated the determination of host frequencies from the detection and quantification of interactions. The food webs comprised 5-9 host and 5-11 parasitoid species at each site, and showed a lower incidence of parasitism at high elevation. Despite considerable turnover in the relative abundance of host Drosophila species, and contrary to some previous results, fundamental metrics of network structure including nestedness and specialisation did not change significantly with elevation. Advances in community ecology depend on data from a combination of methodological approaches. It is therefore especially valuable to develop model study systems for sets of closely-interacting species that are diverse enough to be representative, yet still amenable to field and laboratory experiments.

Natural enemies have inconsistent impacts on the coexistence of competing species

“…The Drosophila species we investigated co-occur in their natural habitat of Australian rainforest (Jeffs et al 2020), although it is important to note that this is not necessarily evidence that they can coexist in the technical sense (Siepielski & McPeek 2010). Furthermore, pairwise coexistence is not the same as community-level coexistence, and coexistence or otherwise between pairs does not necessarily translate to the multispecies community (Song et al 2019), although they are likely to be at least somewhat related (Friedman et al 2017;Broekman et al 2019).…”

“…We investigated a system of six naturally co-occurring Drosophila vinegar flies (D. birchii (BIR), D. pallidifrons (PAL), D. pandora (PAN), D. pseudoananassae (PAN), D. simulans (SIM), and D. sulfurigaster (SUL)) from a well-characterised Australian rainforest community (Hangartner et al 2015;O'Brien et al 2017;Jeffs et al 2020), along with a co-occurring generalist parasitoid wasp from the genus Trichopria (Hymenoptera: Diapriidae, lab strain 66LD, SI 1). This parasitoid attacks Drosophila species at their immobilized pupal stage, and development of the larvae results in the death of the host.…”

“…Drosophila species have long been used in experimental ecology since they are straightforward to culture under laboratory conditions, allowing high levels of replication and standardisation. The set of Drosophila species used here represents a significant fraction of the diversity of Drosophila species co-occurring at rainforest sites in Queensland, Australia (Jeffs et al 2020). The six species span a range of body sizes (Table S1.1), have similar generation times, and can be distinguished visually (at least in the case of males).…”

“…The six species span a range of body sizes (Table S1.1), have similar generation times, and can be distinguished visually (at least in the case of males). Drosophila are generalist feeders (Valadão et al 2019) and in natural populations they experience high levels of parasitism, including from a suite of highly generalist natural enemies (Jeffs et al 2020).…”

“…Equally, our experiment uses a highly simplified representation of the consumer pressure experienced by natural communities of Drosophila. We included a single pupal parasitoid species; in their natural habitats these species exist in a complex trophic network (Jeffs et al 2020) including specialist parasitoids and larval parasitoids (which may directly influence larval competitive performance). Defences against parasitism may be specific to particular enemies, or behaviour-based and not observable in the simplified environment of the fly vials.…”