Barcoding a quantified food web: crypsis, concepts, ecology and hypotheses

Abstract

The efficient and effective monitoring of individuals and populations is critically dependent on correct species identification. While this point may seem obvious, identifying the majority of the more than 100 natural enemies involved in the spruce budworm (Choristoneura fumiferana--SBW) food web remains a non-trivial endeavor. Insect parasitoids play a major role in the processes governing the population dynamics of SBW throughout eastern North America. However, these species are at the leading edge of the taxonomic impediment and integrating standardized identification capacity into existing field programs would provide clear benefits. We asked to what extent DNA barcoding the SBW food web would alter our understanding of the diversity and connectence of the food web and the frequency of generalists vs. specialists in different forest habitats. We DNA barcoded over 10% of the insects collected from the SBW food web in three New Brunswick forest plots from 1983 to 1993. For 30% of these specimens, we amplified at least one additional nuclear region. When the nodes of the food web were estimated based on barcode divergences (using molecular operational taxonomic units (MOTU) or phylogenetic diversity (PD)--the food web became much more diverse and connectence was reduced. We tested one measure of food web structure (the "bird feeder effect") and found no difference compared to the morphologically based predictions. Many, but not all, of the presumably polyphagous parasitoids now appear to be morphologically-cryptic host-specialists. To our knowledge, this project is the first to barcode a food web in which interactions have already been well-documented and described in space, time and abundance. It is poised to be a system in which field-based methods permit the identification capacity required by forestry scientists. Food web barcoding provided an effective tool for the accurate identification of all species involved in the cascading effects of future budworm outbreaks. Integrating standardized barcodes within food webs may ultimately change the face of community ecology. This will be most poignantly felt in food webs that have not yet been quantified. Here, more accurate and precise connections will be within the grasp of any researcher for the first time.

Figure 1. Proportions of cryptic diversity revealed through barcoding the SBW quantified food web.

A) Proportion of barcoding MOTU (provisional species) uncovered within each morphologically described species. B) Proportional representation of the identification of cryptic diversity within each level of the SBW food web. The number above each bar represents the absolute number of cases of cryptic diversity within each category.

Figure 2. Diversity accumulation curves of specimens measured using traditional morphology (species) or using single CO1 variation (barcodes).

Accumulation curves calculated using BOLD following 20 randomizations. Blue lines represent barcode diversity, red lines represent morphologically named taxonomic diversity.

Figure 3. Food web representation where nodes are described morphologically as in (BEFORE), and where nodes were delineated using barcodes (AFTER).

Diversity has increased (Nodes (N) increased from 110 to 156 (a 41% increase)) and links (L) from 336 – to 449) and the connectence (Connectance (L/N²)) has been reduced (from 0.03 to 0.02). Nodes are unlabelled, and the SBW is the center of the fan. Image produced with FoodWeb3D, written by R.J. Williams and provided by the Pacific Ecoinformatics and Computational Ecology Lab (www.foodwebs.org, [73]).

Figure 4. Food-web diversity measured using DNA barcodes and budworm density.

Here diversity was calculated as MOTU (A&B), or PD (C&D) and separate trends were plotted for the nearly homogenous (98% balsam fir), intermediate (77% balsam fir) and the most heterogeneous site (50% balsam fir) (C&D). The ‘bird feeder” effect predicts that the diversity of parasitoids will increase with the budworm density. As was found in , the slopes of these lines (in the 50% and 98% balsam fir plots) are significantly different from the null hypothesis of 0 – supporting the bird feeder effect. As in , the data were de-trended to remove potential temporal autocorrelations but as this yielded consistent results, the original comparisons are illustrated here.

Figure 5. Neighbour-Joining tree of specimens from the parasitoid Scambus.

(Ichneumonidae; Pimplinae) [i.e. a secondary parasitoid whose host records include both primary parasitoids and consumers] (determined morphologically). Tip labels morphological species|Sample ID|Host species|Barcode MOTU. Upon barcoding, the species split into 6 provisional species. Of these, 5 are trophic specialists, one appears to be a trophic specialist, while 4 are species specialists and two are species generalists. One of the provisional species appears to remain a trophic and species generalist.

Figure 6. Species concepts and criteria as applied to one member of the SBW food web, Glypta fumiferanae.

Evidence that this primary parasitoid (Ichneumonidae) is between 1 and 4 species is presented depending on which set of operational criteria one uses.