Food web structure and biocontrol in a four-trophic level system across a landscape complexity gradient

Abstract

Decline in landscape complexity owing to agricultural intensification may affect biodiversity, food web complexity and associated ecological processes such as biological control, but such relationships are poorly understood. Here, we analysed food webs of cereal aphids, their primary parasitoids and hyperparasitoids in 18 agricultural landscapes differing in structural complexity (42 -93% arable land). Despite little variation in the richness of each trophic group, we found considerable changes in trophic link properties across the landscape complexity gradient. Unexpectedly, aphid -parasitoid food webs exhibited a lower complexity (lower linkage density, interaction diversity and generality) in structurally complex landscapes, which was related to the dominance of one aphid species in complex landscapes. Nevertheless, primary parasitism, as well as hyperparasitism, was higher in complex landscapes, with primary parasitism reaching levels for potentially successful biological control. In conclusion, landscape complexity appeared to foster higher parasitism rates, but simpler food webs, thereby casting doubt on the general importance of food web complexity for ecosystem functioning.

Landscape complexity is not a major trigger of species richness and food web structure of European cereal aphid parasitoids

“…These hypotheses state that high primary parasitoid abundances in complex landscapes should increase parasitism rates and reduce pest numbers (Altieri and Letourneau 1982). For example Gagic et al (2011) reported a higher rate of aphid parasitism in areas with more diverse vegetation. However some authors challenged this view, showing that complex landscape (Menalled et al 1999) as well as the species richness of the food web (Montoya et al Salvo et al (2005) who showed that parasitism and parasitoid species richness were higher in simple than in complex habitats.…”

“…Another explanation could be tied with temporal variation in the dynamics of parasitoids (Menalled et al 2003), or simply the short period of the study. However, most studies exploring this issue (Marino and Landis 1996;Menalled et al 1999;Costamagna et al 2004;Salvo et al 2005;Monmany and Aide 2009;Gagic et al 2011;Rand et al 2012) lasted only one season. Moreover, some of them have tested pests that are usually controlled by generalists, rather, than more specialized parasitoids with few alternative hosts (Marino and Landis 1996;Costamagna et al 2004;Monmany and Aide 2009).…”

Landscape scale study of diversity and interaction in cereal aphids - parasitoids system (Homoptera; Hymenoptera)

“…Ovakav sastav vrsta i velika zastupljenost velike žitne vaši je u skladu sa rezultatima nekoliko prethodnih studija (Petrović, 1996;Sigsgaard, 2002;Schmidt et al, 2003;Roschewitz et al, 2005;Thies et al, 2005;Gagic et al, 2011Gagic et al, , 2012Kos et al, 2012;Al Hassan et al, 2013), ali i u suprotnosti sa studijama koje su zabeležile drugačije odnose relativne zastupljenosti (Tomanović et al, 2008;Caballero-López et al, 2012;Lohaus et al, 2012) kao i drugačiji sastav vrsta (Brewer & Elliott, 2004;Brewer et al, 2008;Caballero-López et al, 2011). Najmanje zastupljena vrsta je bila S. maydis koja je bila prisutna samo u dva kompleksna predela u toku jedne godine.…”

“…Pregledom rezultata sličnih studija (Tomanović & Brajković, 2001;Sigsgaard, 2002;Roschewitz et al, 2005;Vollhardt et al, 2008;Tomanović et al, 2008;Caballero-López et al, 2011, 2012Gagic et al, 2011Gagic et al, , 2012Lohaus et al, 2012;Kos et al, 2012) može se zaključiti da sastav i struktura gilde parazitoida biljnih vašiju značajno varira ne samo između regiona, već i između godina u okviru istog regiona. Imajući u vidu različito ponašanje porazitoida i njihovu preferenciju ka biljnim vašima (Powell & Zhi-Li, 1983;Mackauer & Völkl, 1993;Bueno et al, 2003;Lo Pinto et al, 2004;Langley et al, 2006;Rehman & Powell, 2010) kao i biotičke odnose, pre svega kompeticiju, unutar gilde parazitoida (McBrien & Mackauer, 1990;Brodeur & Rosenheim, 2000), prisustvo ili odsustvo određenih vrsta parazitoida, kao i njihova relativna zastupljenost mogu imati veliki uticaj na uspešnost biološke kontrole biljnih vaši.…”

“…Objašnjenje ovog fenomena se može naći u biologiji prisutnih vrsta Hipoteza da će u kompleksnim predelima procenat parazitiranost biti veći je potvrđena iako efekat nije bio konstantan tokom godina. Ovo je u skladu sa rezultatima nekoliko sličnih studija Thies et al, 2005;Gagic et al, 2011Gagic et al, , 2012Rand et al, 2012‚ i delom u Thies et al, 2011 koje ukazuju da u kompleksnim predelima postoje elementi koji pozitivno utiču na paraziotoide. Veći diverzitet vrsta biljnih vašiju koje žive na prirodnim i poluprirodnim staništima može pozitivno uticati na parazitoide obezbeđujući veći izbor alternativnih domaćina.…”