Predicting Landscape Configuration Effects on Agricultural Pest Suppression
Authors: Nathan L. Haan, Yajun Zhang, Douglas A. Landis
Journal: Trends in Ecology & Evolution (2019)

Understanding how landscape structure influences pest suppression in crop fields is critical for the design of sustainable agricultural landscapes.New research shows that landscape configuration (spatial arrangement), in addition to composition, strongly affects natural enemy and pest populations, ultimately affecting crop yield.Natural enemies tend to be more abundant in fine-grained landscapes (comprising smaller fields and habitat patches) and are influenced by the connectivity of crop fields to other habitat types.Configuration effects on pest suppression depend on organismal traits and the relationships between spatial scales at which arthropods disperse and those of underlying landscape structure.Landscape configuration can affect pest suppression through multiple indirect effect pathways, which need more investigation. Arthropod predators and parasitoids attack crop pests, providing a valuable ecosystem service. The amount of noncrop habitat surrounding crop fields influences pest suppression, but synthesis of new studies suggests that the spatial configuration of crops and other habitats is similarly important. Natural enemies are often more abundant in fine-grained agricultural landscapes comprising smaller patches and can increase or decrease with the connectivity of crop fields to other habitats. Partitioning organisms by traits has emerged as a promising way to predict the strength and direction of these effects. Furthermore, our ability to predict configurational effects will depend on understanding the potential for indirect effects among trophic levels and the relationship between arthropod dispersal capability and the spatial scale of underlying landscape structure. Arthropod predators and parasitoids attack crop pests, providing a valuable ecosystem service. The amount of noncrop habitat surrounding crop fields influences pest suppression, but synthesis of new studies suggests that the spatial configuration of crops and other habitats is similarly important. Natural enemies are often more abundant in fine-grained agricultural landscapes comprising smaller patches and can increase or decrease with the connectivity of crop fields to other habitats. Partitioning organisms by traits has emerged as a promising way to predict the strength and direction of these effects. Furthermore, our ability to predict configurational effects will depend on understanding the potential for indirect effects among trophic levels and the relationship between arthropod dispersal capability and the spatial scale of underlying landscape structure. In agricultural landscapes, predatory and parasitic arthropods suppress herbivorous arthropod populations, providing an essential ecosystem service valued at billions of dollars annually [1Losey J.E. Vaughan M. 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In general, seminatural habitats can provide natural enemies with resources including food, overwintering habitat, nest sites, and refuge from agricultural disturbance, allowing them to survive and then colonize crop fields to exploit the herbivores that accumulate there [3Landis D.A. et al.Habitat management to conserve enemies of arthropod pests in agriculture.Ann. Rev. Entomol. 2000; 45: 175-201Crossref PubMed Scopus (1821) Google Scholar]. Pest suppression is generally thought to increase when crop fields are surrounded by more noncrop or seminatural habitat. However, while this occurs in some circumstances, the effects of landscape composition on pest suppression overall are inconsistent, varying among systems and organisms [4Karp D.S. et al.Crop pests and predators exhibit inconsistent responses to surrounding landscape composition.Proc. Nat. Acad. Sci. U. S. A. 2018; 115: E7863-E7870Crossref PubMed Scopus (191) Google Scholar]. Beyond composition, there is also variation in landscape configuration [6Fahrig L. et al.Functional landscape heterogeneity and animal biodiversity in agricultural landscapes.Ecol. Lett. 2011; 14: 101-112Crossref PubMed Scopus (895) Google Scholar]. Multiple lines of reasoning suggest that configurational aspects of landscape structure should affect pest suppression. First, since there is spillover of beneficial organisms along interfaces between habitat patches [7Rand T.A. et al.Spillover edge effects: the dispersal of agriculturally subsidized insect natural enemies into adjacent natural habitats.Ecol. Lett. 2006; 9: 603-614Crossref PubMed Scopus (404) Google Scholar], configurational variables such as patch size, shape, amount of shared edge, and connectivity should influence the amount of spillover that occurs and how far into fields natural enemies can penetrate. Second, some organisms require resources from multiple land-cover types and are thought to benefit from landscape complementation [8Dunning J.B. et al.Ecological processes that affect populations in complex landscapes.Oikos. 1992; 65: 169-175Crossref Google Scholar]. Evidence now strongly suggests that landscape configuration is an important predictor of pest suppression [5Chaplin-Kramer R. et al.A meta-analysis of crop pest and natural enemy response to landscape complexity.Ecol. Lett. 2011; 14: 922-932Crossref PubMed Scopus (534) Google Scholar, 9Duarte G.T. et al.The effects of landscape patterns on ecosystem services: meta-analyses of landscape services.Landscape Ecol. 2018; 33: 1247-1257Crossref Scopus (56) Google Scholar], and the rate of publication on this topic has been accelerating. Therefore, here we highlight recent advances in knowledge of how landscape configuration affects natural enemy abundance and pest suppression, identifying gaps in our understanding and suggesting frameworks for future configuration studies. Thirty-three recent studies, 70% of which were published since 2014, provide evidence as to how landscape configuration affects pest suppression (Figure 1). All but two of these reported significant effects of landscape configuration on at least one variable related to pest suppression, but the strength and direction of these effects were highly context dependent (see Supplements S1 and S2 in the supplemental information online for details of each study). Configuration is multifaceted, describing spatial characteristics (i.e., size, shape, and spatial arrangement) of habitat patches in a landscape. Therefore, it is difficult to encapsulate succinctly and can be quantified using dozens of intercorrelated and sometimes redundant landscape metrics [10McGarigal K. Marks B.J. FRAGSTATS: Spatial Pattern Analysis Program for Quantifying Landscape Structure. US Department of Agriculture, Forest Service Pacific Northwest Research Station, 1995Crossref Google Scholar, 11Kupfer J.A. Landscape ecology and biogeography: rethinking landscape metrics in a post-FRAGSTATS landscape.Prog. Phys. Geog. 2012; 36: 400-420Crossref Scopus (164) Google Scholar, 12Li H. Wu J. Use and misuse of landscape indices.Lands. Ecol. 2004; 19: 389-399Crossref Scopus (566) Google Scholar]. The range of metrics researchers use can obscure broader patterns; therefore, to generalize, we group configurational metrics into three families: (i) grain size; (ii) shape complexity; and (iii) connectivity. Landscapes fall along a spectrum of complexity from coarse-grained with large patches and a low density of edges to fine-grained with small patches and relatively more edges. Pest suppression is generally expected to be greater in fine-grained agricultural landscapes because, in smaller fields, enemies that emerge from field margins or nearby seminatural habitats can reach crop field edges and interiors more easily. The potential for complementation should also be higher in fine-grained landscapes since more cover types are likely to be within the foraging range of generalist natural enemies. Some studies focus on edge density (i.e., length of patch edges per unit area) as a useful index of configurational complexity; although edge density can relate to shape complexity, in general it is used to describe grain size rather than shape [13Martin E.A. et al.Scale-dependent effects of landscape composition and configuration on natural enemy diversity, crop herbivory, and yields.Ecol. 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In one of the most comprehensive studies to date, Martin et al. [13Martin E.A. et al.Scale-dependent effects of landscape composition and configuration on natural enemy diversity, crop herbivory, and yields.Ecol. App. 2016; 26: 448-462Crossref PubMed Scopus (58) Google Scholar] found that natural enemy abundance varied strongly with edge density of landscapes surrounding 35 South Korean crop fields of various types. Syrphids, parasitoids, predatory wasps, and staphylinids were more abundant in fields surrounded by landscapes with higher edge densities, and these configurational effects dwarfed the effects of the amount of seminatural habitat. The same has been found for insectivorous wasps in wheat [17Holzschuh A. et al.How do landscape composition and configuration, organic farming and fallow strips affect the diversity of bees, wasps and their parasitoids?.J. Anim. 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Ecol. 2018; 56: 63-72Crossref Scopus (37) Google Scholar, 27Al Hassan D. et al.Does the presence of grassy strips and landscape grain affect spatial distribution of aphids and their carabid predators?.Agricult. Forest Entomol. 2013; 15: 24-33Crossref Scopus (24) Google Scholar] in wheat and other crops. Compelling trends emerge when natural enemies are partitioned according to their traits, particularly overwintering habitat use and dispersal mode. Recently, data from over 1500 European landscapes (49 studies) were analyzed together, revealing that predators overwintering in habitats outside crop fields were more abundant in fine-grained landscapes with higher edge density. This pattern held true for flying and ground-dispersing enemies, but not for wind-dispersers. For taxa that overwinter in crop fields, the effect was opposite: they tended to be more abundant when edge density was low and/or when surrounded by crops rather than by seminatural habitat. Importantly, these effects of configuration were masked when natural enemies were considered as a group; the pattern only emerged when they were partitioned by traits related to habitat use and dispersal mode [14Martin E.A. et al.The interplay of landscape composition and configuration: new pathways to manage functional biodiversity and agroecosystem services across Europe.Ecol. Lett. 2019; 22: 1083-1094Crossref PubMed Scopus (105) Google Scholar]. We need more information on how grain size affects herbivores, rates of suppression, crop damage, and yield. Evidence on herbivores is biased toward aphids (Figure 1), and variously suggests that finer-grained landscapes can have more [18Ulina E.S. et al.Does composition of tropical agricultural landscape affect parasitoid diversity and their host-parasitoid interactions?.Agricult. Forest Entomol. 2019; 21: 318-325Google Scholar, 19Dominik C. et al.Landscape composition, configuration, and trophic interactions shape arthropod communities in rice agroecosystems.J. Appl. Ecol. 2018; 55: 2461-2472Crossref Scopus (28) Google Scholar, 23Schmidt J.M. et al.Local and landscape-scale heterogeneity shape spotted wing drosophila (Drosophila suzukii) activity and natural enemy abundance: implications for trophic interactions.Agric. Ecosyst. Environ. 2019; 272: 86-94Crossref Scopus (16) Google Scholar] or fewer [27Al Hassan D. et al.Does the presence of grassy strips and landscape grain affect spatial distribution of aphids and their carabid predators?.Agricult. Forest Entomol. 2013; 15: 24-33Crossref Scopus (24) Google Scholar] herbivores, or the results were inconsistent [28Baillod A.B. et al.Landscape-scale interactions of spatial and temporal cropland heterogeneity drive biological control of cereal aphids.J. Appl. 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Lett. 2019; 22: 1083-1094Crossref PubMed Scopus (105) Google Scholar] showed that edge density had mixed effects on yield, with results depending on the amount of seminatural habitat present. Habitat patches range in shape from simple to complex. In some contexts, elements are mostly rectangular, while in others, cover-type boundaries follow tortuous paths, resulting in irregular or convoluted shapes. Shape complexity could feasibly affect pest suppression: fields with more edge per unit area should have more interface with other habitats, allowing natural enemies to reach field interiors more easily. Firm evidence for how patch shape complexity affects pest suppression has yet to emerge. Models suggest that the shape of seminatural areas and hedgerows would influence the control of aphids by coccinellids in adjacent fields [32Bianchi F.J.J.A. Van Der Werf W. 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In rice agroecosystems in the Philippines, natural enemies, including coccinellids and linyphiid spiders, decreased with the fractal dimension of rice fields, the abundance of trichogrammatid wasps increased, but herbivores and several other taxa showed no response and, overall, neither predator nor parasitoid abundance was affected [19Dominik C. et al.Landscape composition, configuration, and trophic interactions shape arthropod communities in rice agroecosystems.J. Appl. Ecol. 2018; 55: 2461-2472Crossref Scopus (28) Google Scholar]. More research will be needed to determine whether there are predictable effects of patch shape complexity on pest suppression. The ways in which landscape connectivity shapes the movement of organisms have been the subject of keen interest and debate for some time [35Haddad N.M. et al.Corridor use by diverse taxa.Ecology. 2003; 84: 609-615Crossref Scopus (287) Google Scholar, 36MacArthur R.H. Wilson E.O. The Theory of Island Biogeography. Princeton University Press, 1967Google Scholar]. The simplest form of connectivity, as it relates to pest suppression, is the distance from a crop field to specific habitats in its surroundings. There is clear evidence that this type of connectivity influences natural enemy abundance, and that the direction of the effect depends on the type of habitat and its utility to the organism in question. Natural enemies on woody crops appear to benefit from proximity to forest: in apple orchards, spider richness and, in some cases, abundance increased with proximity to woody vegetation [37Bailey D. et al.Effects of habitat amount and isolation on biodiversity in fragmented traditional orchards.J. Appl. 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In apple orchards, herbivorous beetle richness, but not abundance, decreased with distance to forest [37Bailey D. et al.Effects of habitat amount and isolation on biodiversity in fragmented traditional orchards.J. Appl. Ecol. 2010; 47: 1003-1013Crossref Scopus (86) Google Scholar]. Effects in coffee systems were mixed, with aphids increasing with distance to forest but coccid and pseudococcid bugs decreasing [38Karungi J. et al.Relating shading levels and distance from natural vegetation with hemipteran pests and predators occurrence on coffee.J. Appl. Entomol. 2014; 139: 669-678Crossref Scopus (10) Google Scholar]. On cherry trees, herbivorous beetles increased with distance to forest, but aphids were unaffected [39Schuepp C. et al.Habitat isolation affects plant-herbivore-enemy interactions on cherry trees.Biol. Control. 2014; 71: 56-64Crossref Scopus (19) Google Scholar]. Pest suppression in crop fields can also change with the spatial arrangement of other habitats without regard to their proximity to the focal field. For example, if seminatural habitat provides natural enemies to nearby crop fields, it is possible that, when patches of that habitat are more interconnected to one another, they sustain larger populations of natural enemies overall. In one study, syrphid flies in oilseed rape fields were more abundant when adjacent hedgerows were connected to forest [43Haenke S. et al.Landscape configuration of crops and hedgerows drives local syrphid fly abundance.J. Appl. Ecol. 2014; 51: 505-513Crossref Scopus (69) Google Scholar]. In irrigated rice agroecosystems, fields are flooded and impounded by vegetated embankments (bunds) harboring natural enemies. In a recent study, parasitoids in rice fields increased with the physical connectivity of the network of bunds [19Dominik C. et al.Landscape composition, configuration, and trophic interactions shape arthropod communities in rice agroecosystems.J. Appl. Ecol. 2018; 55: 2461-2472Crossref Scopus (28) Google Scholar]. Similarly, predatory mirid bugs that attack tomato pests were enhanced when fallow areas in their surroundings were more connected; however, their response to orchard connectivity was opposite, perhaps related to insecticide use in these crops [44Aviron S. et al.Local landscape heterogeneity affects crop colonization by natural enemies of pests in protected horticultural cropping systems.Agric. Ecosyst. Environ. 2016; 227: 1-10Crossref Scopus (20) Google Scholar]. Finally, carabids in cereal grains and maize responded positively to the connectivity of grasslands and winter crops, but not forests or maize [45Aviron S. et al.Connectivity of cropped vs. semi-natural habitats mediates biodiversity: a case study of carabid beetles communities.Agric. Ecosyst. Environ. 2018; 268: 34-43Crossref Scopus (11) Google Scholar]. Landscape configuration is constrained by, and often correlated with, landscape composition. Therefore, the two types of variable are confounded with one another in many landscapes, and their respective effects are difficult to disentangle [6Fahrig L. et al.Functional landscape heterogeneity and animal biodiversity in agricultural landscapes.Ecol. Lett. 2011; 14: 101-112Crossref PubMed