Threshold Effects of Landscape Structure on Biological Control in Agroecosystems

Abstract

Habitat fragmentation may adversely affect the ability of natural enemies to control pest outbreaks in agricultural landscapes by interfering with their search behavior and ability to aggregate in response to prey. We determined how landscape structure affected the ability of two ladybird beetles (Coleoptera: Coccinellidae) to track aphid populations in experimental landscapes that differed in the abundance and degree of fragmentation of red clover (Trifolium pratense). One coccinellid was a native species (Coleomegilla maculata Pallas) and the other (Harmonia axyridis Timberlake) was introduced specifically for the biological control of crop pests such as pea aphids (Acyrthosiphon pisum Harris; Homoptera: Aphididae).Landscape structure exhibited a threshold in lacunarity (a measure of interpatch distances) below 20% habitat, at which point clover patches became significantly more isolated. This threshold in landscape structure was mirrored by a similar threshold in the distribution of pea aphid populations. The distribution of the biocontrol agent, H. axyridis, tracked this threshold in aphid distribution, but the native coccinellid, C. maculata, was unable to do so in fragmented clover landscapes. Although C. maculata was a more active forager within clover cells, overall it was less mobile and moved significantly less among clover cells and between landscapes than H. axyridis, which may have contributed to its inability to track aphid populations in fragmented landscapes. The two coccinellids did not differ in their search success within fragmented landscapes, however, and it was only in clumped landscapes that H. axyridis maximized search success and foraged within clover cells that had 2.5-3 times more aphids than those in which C. maculata occurred.Thus, the potential of predators to control pest populations in fragmented landscapes may ultimately reflect the extent to which thresholds in landscape structure interfere with the aggregative response of predators. In this system, the aggregative response of coccinellids was more closely tied to thresholds in the distribution of clover than aphids. With its greater mobility, H. axyridis was more effective than the indigenous C. maculata at tracking aphids when they occurred at low patch occupancy (below the threshold in landscape structure), which is a requisite for successful biocontrol. If native insect predators are generally more sensitive to habitat fragmentation, greater reliance may be placed on the introduction of exotic species for biocontrol, which is not without economic cost and potential ecological impacts to native insect communities. Our study demonstrates that, in addition to economic thresholds, there are also ecological thresholds that must be surmounted if biocontrol measures are to be successful. In addition to enhancing vegetational diversity within agroecosystems, conservation biological control should also strive to mitigate fragmentation effects on natural enemies, especially if thresholds in landscape structure disrupt predator-prey int...

Intersection between biodiversity conservation, agroecology, and ecosystem services

“…Accordingly, the strength and direction of landscape quality effects on parasitism rates are contingent on parasitoid traits such as search behavior and dispersal ability (Bianchi, Ives, and Schellhorn 2013). This is complicated by the fact that the scale of these responses can greatly vary from species to species depending on their dispersal capabilities and even closely related species often lumped into the same functional group can perceive landscape fragmentation differently (Doak, Marino, and Kareiva 1992;Gardiner et al 2009b;With and Crist 1995;With et al 2002). For example, Kareiva and Odell (1987) found that since two ladybeetle species have different abilities to track patches of high prey density, habitat fragmentation had a different effect on each species.…”

Toward the Development of Novel Long-Term Pest Control Strategies Based on Insect Ecological and Evolutionary Dynamics

“…Recently some scholars incorporated temporal changes in landscape structure (Solbreck, 1995;Onstad et al, 2001), genetic change in insect populations (Singer & Thomas, 1996;Ronce & Kirkpatrick, 2001), and differential responses of predators and prey (Kruess & Tscharntke, 1994;With et al, 2002) into their understanding of the spatial ecology of insects (Hunter, 2002). Roderick and Navajas (2003) suggested that identifying the origin of specific genotypes in an invasive pest population might assist in the identification of natural enemies in the native range, thus facilitating the design of effective biological control programs (Rollins et al, 2006).…”

Effects of habitat loss and fragmentation on the abundance and species richness of aphidophagous beetles and aphids in experimental alfalfa landscapes

“…The size of the experimental plots (landscapes) was selected based on previous studies of similar questions and organisms (Kareiva, 1987;Banks, 1999;Grez et al, 2004a, b;2005;Zaviezo et al, 2006). Previous theoretical and empirical studies suggest that fragmentation effects on population abundance and searching efficiency by natural enemies should be apparent only at high levels of habitat loss (i.e., over 70-80%, Andrén, 1994;Fahrig, 1997;With & King, 1999;Thies & Tscharntke, 1999;Flather & Bevers, 2002;With et al, 2002). Therefore, in our experiments we used percentages of habitat loss both above and below this threshold.…”

“…It has been shown that those species belonging to higher trophic levels, such as parasitoids and predators, are more affected by habitat loss and fragmentation than are their prey, the herbivores (Hunter, 2002;Braschler et al, 2003;van Nouhuys, 2005;Ewers & Didham, 2006). Habitat fragmentation may adversely affect the ability of natural enemies to control pest outbreaks in agricultural landscapes by interfering with their searching behaviour and their aggregative numerical response to prey (Kareiva, 1987;With et al, 2002). Therefore, from the perspective of biological control, it is important to study whether changes in the agricultural landscape due to fragmentation and/or habitat loss, can affect the abundance of natural enemies, resulting in a failure to maintain pests at low populations levels.…”

“…Although habitat loss and fragmentation have been considered of great importance in conservation biology, these processes are also relevant for pest management in agroecosystems because of their potential effects on predatoryprey dynamics (Kruess & Tscharntke, 1994;Thies & Tscharntke, 1999;Tscharntke & Kruess, 1999;Hunter, 2002;With et al, 2002). It has been shown that those species belonging to higher trophic levels, such as parasitoids and predators, are more affected by habitat loss and fragmentation than are their prey, the herbivores (Hunter, 2002;Braschler et al, 2003;van Nouhuys, 2005;Ewers & Didham, 2006).…”