Direct and indirect effects of landscape structure on a tri‐trophic system within agricultural lands

Abstract

Citation: Daoust, S., M. Bélisle, J. Savage, A. Robillard, R. Baeta, and J. Brodeur. 2012. Direct and indirect effects of landscape structure on a tri-trophic system within agricultural lands. Ecosphere 3(11):94. http://dx.doi.org/10.1890/ ES12-00300.1Abstract. Although several studies have examined the influence of landscape structure and agricultural intensification on species abundance and diversity, few have addressed how these impact populations across multiple trophic levels. We investigated the effects of landscape structure on the tri-trophic interactions between a bird host (the Tree Swallow Tachycineta bicolor (Vieillot)), its blowfly ectoparasites (Protocalliphora Hough), and their parasitoid wasps (Nasonia Ashmead) across 13 spatial scales, along a gradient of agricultural intensification covering 10,200 km 2 in southern Québec, Canada. We showed that the three taxa responded to landscape structure at distinctive spatial scales that are relative to their size rather than their trophic rank. This response, however, differed according to habitat type. The three organisms responded to the amount of intensive cultures (maize, soybean and other cereals) at smaller spatial scales than to the amount of extensive cultures (hayfields and pastures). Although the number of Tree Swallow fledglings, the number of Protocalliphora sialia Shannon & Dobroscky pupae and the number of P. sialia pupae parasitized by Nasonia sp. per nest were negatively affected by agricultural intensification, our data do not support the prediction that organisms at the higher trophic levels are more susceptible to habitat degradation. Here, ectoparasites at the second trophic level were disproportionally affected by agricultural intensification; the abundance of P. sialia decreased by 80% along the gradient of agricultural intensification compared to a 20% and 35% reduction in the number of Tree Swallow fledglings and in the level of parasitism by Nasonia sp., respectively. Our work highlights the importance of designing protocols that take spatial aspects of trophic interactions into account when studying the impact of habitat loss and fragmentation on populations and communities, as these interactions dictate local biodiversity and community function. Furthermore, our results highlight the importance of considering multiple landscape parameters when identifying the functional spatial scales of an organism, as a failure to do so could lead to an underestimation of the area it uses.

Urban Land Use Decouples Plant-Herbivore-Parasitoid Interactions at Multiple Spatial Scales

“…Because dispersal distances are often positively correlated with body size, actively dispersing insects are expected to respond to landcover patterns at finer spatial scales as body sizes decrease [5] . Predators generally experience landscapes at a larger spatial scale than herbivores [15] ; however, because most parasitoid wasps (Hymenoptera that fatally parasitize other insect hosts) are smaller in size than their hosts, these natural enemies may have lower dispersal abilities and smaller effective spatial scales than their hosts [16] . Thus, while specialist insect herbivores may be impacted by land use patterns, impacts may be even greater for their specialist parasitoids.…”

Additive negative effects of Philornis nest parasitism on small and declining Neotropical bird populations

“…The second study followed the tri-trophic interactions among the same Tree Swallow species, its Protocalliphora parasites and their Nasonia parasitoid wasps along a gradient of agricultural intensification. The number of swallow fledglings, the abundance of P. sialia , and the level of Nasonia wasp parasitism were all negatively affected by the habitat loss, fragmentation and degradation associated with the intensification of agricultural practices (Daoust et al 2012). In this case however, lower fledging rate in the presence of human disturbance could not be attributed to nest parasitism.…”

Environmental heterogeneity effects on predator and parasitoid insects vary across spatial scales and seasons: a multi‐taxon approach

“…The 'niche theory' (Hutchinson, 1957) predicts that structurally complex environments are likely to provide more niches and diverse ways of exploiting the environmental resources and thus can contribute to increased species diversity (Tews et al, 2004;Weisberg et al, 2014;Stein & Kreft, 2015). Environmental heterogeneity (EH) is expected to be particularly relevant for the diversity of predators and parasitoids as they depend on the availability of multiple resources such as nectar and pollen and on a variety of prey or hosts (Landis et al, 2005;Tscharntke et al, 2007;Daoust et al, 2012). Considering both spatial and temporal dynamics of EH is hence necessary to fully understand the impacts of habitat fragmentation and habitat loss on the diversity of predator and parasitoid insects (Aranda & Graciolli, 2015).…”