Riikka Kaartinen

Complementary molecular information changes our perception of food web structure

Significance Understanding the interaction structure of ecological assemblages is the basis for understanding how they vary in space and time. To reconstruct interactions in the High Arctic, we draw on three sources of information: two based on DNA sequence data and one on the rearing of parasitoids from their hosts. Overall, we show that a combination of all three techniques will not only provide high resolution for describing feeding associations among individual species, but also revamp our view of the overall structure of the target network. Thus, our findings suggest that combining several types of information will fundamentally change our impression of both how local interaction webs are structured, and how biotic interactions are patterned across the globe. How networks of ecological interactions are structured has a major impact on their functioning. However, accurately resolving both the nodes of the webs and the links between them is fraught with difficulties. We ask whether the new resolution conferred by molecular information changes perceptions of network structure. To probe a network of antagonistic interactions in the High Arctic, we use two complementary sources of molecular data: parasitoid DNA sequenced from the tissues of their hosts and host DNA sequenced from the gut of adult parasitoids. The information added by molecular analysis radically changes the properties of interaction structure. Overall, three times as many interaction types were revealed by combining molecular information from parasitoids and hosts with rearing data, versus rearing data alone. At the species level, our results alter the perceived host specificity of parasitoids, the parasitoid load of host species, and the web-wide role of predators with a cryptic lifestyle. As the northernmost network of host–parasitoid interactions quantified, our data point exerts high leverage on global comparisons of food web structure. However, how we view its structure will depend on what information we use: compared with variation among networks quantified at other sites, the properties of our web vary as much or much more depending on the techniques used to reconstruct it. We thus urge ecologists to combine multiple pieces of evidence in assessing the structure of interaction webs, and suggest that current perceptions of interaction structure may be strongly affected by the methods used to construct them.

Revealing secret liaisons: DNA barcoding changes our understanding of food webs

1. How food webs are structured will affect how species dynamically interact. To date, the construction of quantitative food webs has largely been based on morphological species characters. Yet, recent work suggests that the use of molecular characters may change our perception of both species limits and species identity.

A parasitoid wasp uses landmarks while monitoring potential resources

Social insects and insects that provision nests are well known to have complex foraging behaviour involving repeated visits to learned locations. Other insects do not forage from a central location and are generally assumed to respond to resources by simple attraction without spatial memory. This simple response to resource cues is generally taken as giving rise to patterns of resource use that correspond directly to resource distribution. By contrast, the solitary parasitoid wasp Hyposoter horticola monitors the locations of multiple potential hosts (butterfly eggs) for up to several weeks, until the hosts become susceptible to parasitism. Essentially all hosts in the landscape are found, and one-third of them are parasitized, independent of host density. Here, we show that the wasps do not relocate hosts using odour markers previously left by themselves or other foragers, nor do they find the eggs anew repeatedly. Instead, the wasps relocate host eggs by learning the position of the eggs relative to visual landmarks. The anticipatory foraging behaviour presented here is a key to the wasps exceptionally stable population dynamics.

Shrinking by numbers: landscape context affects the species composition but not the quantitative structure of local food webs.

1. With habitat fragmentation spreading around the world, there is a pressing need to understand its impacts on local food webs. To date, few studies have examined the effects of landscape context on multiple local communities in a quantitative, spatially realistic setting. 2. To examine how the isolation of a food web affects its structure, we construct local food webs of specialist herbivores and their natural enemies on 82 individual oaks (Quercus robur) growing in different landscape contexts. 3. Across this set of webs, we find that communities in isolated habitat patches not only contained fewer species than did well-connected ones, but also differed in species composition. 4. Surprisingly, the effects observed in terms of species composition were not reflected in the quantitative interaction structure of local food webs: landscape context had no detectable effect on either the interaction evenness, linkage density, connectance, generality or vulnerability of local webs. 5. We conclude that the quantitative structure of food webs may be stable in the face of habitat fragmentation, despite clear-cut impacts on individual species. This finding offers hope-inspiring news for conservation, but should clearly be verified by empirical studies across both naturally and more recently fragmented systems.

Establishing a community‐wide DNA barcode library as a new tool for arctic research

DNA sequences offer powerful tools for describing the members and interactions of natural communities. In this study, we establish the to‐date most comprehensive library of DNA barcodes for a terrestrial site, including all known macroscopic animals and vascular plants of an intensively studied area of the High Arctic, the Zackenberg Valley in Northeast Greenland. To demonstrate its utility, we apply the library to identify nearly 20 000 arthropod individuals from two Malaise traps, each operated for two summers. Drawing on this material, we estimate the coverage of previous morphology‐based species inventories, derive a snapshot of faunal turnover in space and time and describe the abundance and phenology of species in the rapidly changing arctic environment. Overall, 403 terrestrial animal and 160 vascular plant species were recorded by morphology‐based techniques. DNA barcodes (CO1) offered high resolution in discriminating among the local animal taxa, with 92% of morphologically distinguishable taxa assigned to unique Barcode Index Numbers (BINs) and 93% to monophyletic clusters. For vascular plants, resolution was lower, with 54% of species forming monophyletic clusters based on barcode regions rbcLa and ITS2. Malaise catches revealed 122 BINs not detected by previous sampling and DNA barcoding. The insect community was dominated by a few highly abundant taxa. Even closely related taxa differed in phenology, emphasizing the need for species‐level resolution when describing ongoing shifts in arctic communities and ecosystems. The DNA barcode library now established for Zackenberg offers new scope for such explorations, and for the detailed dissection of interspecific interactions throughout the community.

Relationships between natural enemy diversity and biological control

Natural enemy diversity generally strengthens biological control, but individual studies have found everything from positive to negative effects. We discuss the factors that promote these different outcomes. We argue that a trait-based approach is helpful to improve our understanding of the relationship between enemy diversity and biological control, and suggest that enemy diversity is likely to be particularly important as an insurance against effects of climate change. Future research should increase the scale and ecological realism of enemy diversity studies, and consider both the strength and stability of biological control. Such research is likely to reveal even stronger evidence that conserving enemy biodiversity will improve biological pest control.

Apparent competition leaves no detectable imprint on patterns of community composition: observations from a natural experiment

Indirect interactions mediated by natural enemies shared among herbivorous insects have recently attracted much interest. While many studies have predicted a high potential for apparent competition, only a few have rigorously tested predictions derived from the food web structure in terms of realised population and community dynamics. In this study, a quantified food web was used to identify pairs of herbivore species potentially tied by strong parasitoid‐mediated interactions. The host populations and their parasitism rates over two consecutive years were then followed, during which time the abundance of one dominant host crashed. Following this natural experiment, imprints of asymmetrical, long‐term apparent competition among hosts were sought in population growth rates. The population growth revealed no signs of apparent competition as mediated by parasitoids: the abundance and parasitism rates of less abundant herbivores were uncorrelated with the relative growth rate of four dominant herbivore species with which they shared a major part of their parasitoids. Likewise, the population crash of the dominant herbivore, Cynips longiventris, did not detectably affect the abundances or parasitism rates of other herbivores with which it shared a major fraction of its parasitoids. Overall, this case study fails to add evidence to previous suggestions that apparent competition may constitute a major force in structuring natural communities. However, it suggests that dynamics should not be inferred from static food web structure alone, but rather that predictions based on structure should be verified by empirical observations of realised population dynamics.

A revision of the parasitic wasp genus Bathyaulax Szepligeti (Hymenoptera: Braconidae: Braconinae) from Africa and the Arabian Peninsula

The predominantly Afrotropical braconine wasp genus Bathyaulax Szepligeti is revised and an identification key provided. A total of 51 species are recognized as valid; 27 species are described as new. andrewi sp. nov., erythropus sp. nov., kvisti sp. nov., larjuskini sp. nov. vannouhuysie sp. nov. and williami sp. nov. from Kenya, artoi sp. nov., heinie sp. nov. and jimii sp. nov. from D. R. Congo, buntikae sp. nov. from Sierra Leone and D. R. Congo, nigroconus sp. nov. from D. R. Congo and Uganda, atrox sp. nov. from Uganda, bifoveae sp. nov. and varkonyii sp. nov. from Tanzania, ikonenie sp. nov. from Senegal, Kenya and Uganda, fritzeni sp. nov., juhai sp. nov. and ollilae sp. nov. from Saudi Arabia, kossui sp. nov. and marjae sp. nov. from Yemen, kupariensis sp. nov. and raunoi sp. nov. from Mozambique, nirgitarsus sp. nov. from Madagascar, ramosus sp. nov. and suvie sp. nov. from South Africa, pickeri sp. nov. from South Africa and Namibia, and pippolaensis sp. nov. from north East Africa, possibly Somaliland. Bicentra Achterberg & Sigwalt is synonymized with Bathyaulax, hence Bathyaulax concavitarsis (van Achterberg & Sigwalt 1987) comb. nov.; Bathyaulax atriceps (Kriechbaumer 1894), Bathyaulax gutta (Enderlein (1918) 1920) and Bathyaulax pectinatus (Enderlein (1918) 1920) are each synonymized with Bathyaulax martinii (Gribodo 1879) syn. nov.; Bathyaulax laeviventris Enderlein (1918) 1920 is synonymized with Bathyaulax foveiventris (Roman 1912), syn. nov.; Bathyaulax krebsii (Cameron 1909) is synonymized with Bathyaulax hirticeps (Cameron 1909) syn. nov.; Goniobracon areolatus Szépligeti 1913 and Goniobracon abdominalis Szépligeti 1914 are synonymized with Bathyaulax kersteni (Gerstaecker 1871) comb. et syn. nov.; Bathyaulax nigriceps Enderlein 1920 is synonymized with Bathyaulax lucidus (Szépligeti 1911) syn. nov.; Goniobracon seminiger Szépligeti 1914 is synonymized with Bathyaulax perspicax (Szépligeti 1905) syn. nov.; Bathyaulax transitus (Szépligeti 1913) is synonymized with Bathyaulax rufa (Szépligeti 1906) syn. nov.

Shade trees decrease pest abundances on brassica crops in Kenya

Agroforestry practices may mitigate the current loss of biodiversity and ecosystem services due to deforestation and agricultural intensification. To examine the effects of agroforestry on the ecosystem service of pest regulation, we assessed pest abundances and biological control potential in shaded and open kale (Brassica oleracea L. acephala) fields in Western Kenya. Specifically, we compared the abundance of pest aphids and caterpillars, ground-dwelling ants, spiders and predatory beetles, and examined aphid parasitism rates, predation rates on diamondback moth eggs, attack rates on surrogate caterpillars and bird predation on aphids. Shade trees effectively reduced abundances of aphids, caterpillars and increased abundances of spiders and predatory beetles, but neither affected ant abundances, or predation and parasitism rates. Our results suggest that presence of shade trees can decrease pest abundances, but that this is not only due to improved biological control by natural enemies but also due to microclimatic conditions affecting pest performance and bottom-up processes such as changes in plant quality and soil conditions. We encourage studies exploring simultaneously how top-down and bottom-up processes affect pest regulation in agroforestry settings.

Related herbivore species show similar temporal dynamics

Within natural communities, different taxa display different dynamics in time. Why this is the case we do not fully know. This thwarts our ability to predict changes in community structure, which is important for both the conservation of rare species in natural communities and for the prediction of pest outbreaks in agriculture. Species sharing phylogeny, natural enemies and/or life-history traits have been hypothesized to share similar temporal dynamics. We operationalized these concepts into testing whether feeding guild, voltinism, similarity in parasitoid community and/or phylogenetic relatedness explained similarities in temporal dynamics among herbivorous community members. Focusing on two similar datasets from different geographical regions (Finland and Japan), we used asymmetric eigenvector maps as temporal variables to characterize species- and community-level dynamics of specialist insect herbivores on oak (Quercus). We then assessed whether feeding guild, voltinism, similarity in parasitoid community and/or phylogenetic relatedness explained similarities in temporal dynamics among taxa. Species-specific temporal dynamics varied widely, ranging from directional decline or increase to more complex patterns. Phylogeny was a clear predictor of similarity in temporal dynamics at the Finnish site, whereas for the Japanese site, the data were uninformative regarding a phylogenetic imprint. Voltinism, feeding guild and parasitoid overlap explained little variation at either location. Despite the rapid temporal dynamics observed at the level of individual species, these changes did not translate into any consistent temporal changes at the community level in either Finland or Japan. Overall, our findings offer no direct support for the notion that species sharing natural enemies and/or life-history traits would be characterized by similar temporal dynamics, but reveal a strong imprint of phylogenetic relatedness. As this phylogenetic signal cannot be attributed to guild, voltinism or parasitoids, it will likely derive from shared microhabitat, microclimate, anatomy, physiology or behaviour. This has important implications for predicting insect outbreaks and for informing insect conservation. We hope that future studies will assess the generality of our findings across plant-feeding insect communities and beyond, and establish the more precise mechanism(s) underlying the phylogenetic imprint.