Denno, R. F., Lewis, D. & Gratton, C. 2005: Spatial variation in the relative strength of top-down and bottom-up forces: causes and consequences for phytophagous insect populations. — Ann. Zool. Fennici 42: 295–311.

Most ecologists acknowledge that both natural enemies and host-plant resources act in concert to influence populations of insect herbivores. What is poorly known is how the strength of top-down and bottom-up impacts on insect herbivores vary spatially and what factors dictate the relative strength of these forces. We examined spatially-explicit change in top-down and bottom-up impacts along a tidal gradient on the inter-tidal marshes that fringe the Atlantic coast of North America. These marshes are dominated by extensive stands of Spartina cordgrass and its associated arthropod food web, comprised mostly of herbivorous planthoppers (Prokelisia) and their spider predators. Using "effect sizes" generated from experiments, we show that bottom-up forces dominate in low-marsh habitats and that top-down impacts increase up the elevational gradient. We develop a graphical model that integrates spatial variation in top-down (predation) and bottom-up variables (plant nutrition and vegetation complexity) to make habitat-related predictions concerning herbivore abundance.

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Haukioja, E. 2005: Plant defenses and population fluctuations of forest defoliators: mechanism-based scenarios. — Ann. Zool. Fennici 42: 313–325.

Among the factors contributing to defoliator population fluctuations the possible role of plant defenses depends on how defenses (secondary compounds, primary compounds, and defense cascades) are assumed to function. Delayed inducible production of secondary compounds does not seem to explain cyclic fluctuations in population density. Instead, properties of general plant defense cascades, especially of the octadecanoid pathway, suggest new potential mechanisms. Insect damage, and sunspot-related high ultraviolet radiation (UV-B), trigger the octadecanoid pathway. In birch it also contributes to defoliator immunocompetence. Therefore, the low rate of parasitism in the increase phase of an outbreak has potential connections to inducible plant defense. The octadecanoid pathway is also involved in the production of volatile plant compounds, known to modify defenses of other plants and the behavior of parasitoids. Accordingly, the octadecanoid pathway and other plant defense cascades suggest unappreciated plant-mediated connections between the environment, defoliator performance, parasitism, and spatial expansion of outbreaks.

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Mopper, S. 2005: Phenology — how time creates spatial structure in endophagous insect populations. — Ann. Zool. Fennici 42: 327–333.

The deme formation hypothesis predicts that short-lived insects with long-lived hosts can adapt to the traits of individual plants and form genetically differentiated sub-populations. Several life-history traits have been suggested that could predispose insects to deme formation, the most important being dispersal, feeding mode, and mating system. There is little evidence that adult dispersal ability or mating system promote fine-scale adaptation, but feeding mode appears to be important to the evolution of herbivore demes. Phenology (defined as the timing of life cycles) is now known to be individually variable and highly heritable; even a slight mismatch between an herbivore and its host can reduce egg and larval survival, or isolate adults from potential mates. This is particularly critical for internally-feeding insects that are in close and continuous contact with plants. Here I argue that endophagy, combined with insect and plant variation in phenology, are important predictors of deme formation, and a driving force behind fine scale genetic structure in herbivore populations.

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Gripenberg, S. & Roslin, T. 2005: Host plants as islands: Resource quality and spatial setting as determinants of insect distribution. — Ann. Zool. Fennici 42: 335–345.

Both the quality and spatial configuration of a habitat can affect the distribution of its inhabitants. However, few studies have directly compared the relative effects of the two. In this paper, we focus on spatial patterns in the distribution of an oak-specific moth, Tischeria ekebladella. At the landscape level, the species was more often present on well-connected trees than on isolated trees. Experimental transplants revealed pronounced variation in larval survival among individual leaves within trees. In fact, survival on a "good" and a "bad" leaf within a tree differed almost as much as survival between a "good" and a "bad" leaf chosen randomly on two different trees. Qualitative differences among trees did not explain the distribution of the species across the landscape, as average larval performance did not differ between trees naturally occupied and unoccupied by the species. Hence, spatial effects seem to dominate over host tree quality in determining the regional distribution of Tischeria ekebladella.

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Singer, M. C. & Wee, B. 2005: Spatial pattern in checkerspot butterfly–host plant association at local, metapopulation and regional scales. — Ann. Zool. Fennici 42: 347–361.

Host search behaviour and population dynamics of butterflies interact with host quality and dispersion to generate spatial distributions of insects at local, metapopulation and regional scales. At a local scale, search behaviour causes isolated plants to be more attacked than well-connected individuals. As scale increases this pattern is reversed and hosts in isolated habitat patches are less attacked than those in well-connected patches. In Melitaea cinxia, spatially variable host preferences generated biased colonization of habitat patches containing different hosts. In Euphydryas editha, a metapopulation in anthropogenic evolutionary disequilibrium used a novel host in disturbed patches and the traditional host in intervening habitat. The novel host was less preferred but supported higher fitness. When habitat patches were small, insects achieved higher densities in patches of the preferred host because of biased migration into those patches. When patches were large, density was higher on the less-preferred host because of high survival. Because biased movement of insects among hosts affects gene flow, it should also affect genetic differentiation among insects using different hosts. We investigate this question by describing host-associated genetic differentiation at local and regional scales.

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Weisser, W. W. & Härri, S. A. 2005: Colonisations and extinctions at multiple spatial scales: a case study in monophagous herbivores. — Ann. Zool. Fennici 42: 363–377.

Many plant–insect interactions show spatial structure at a hierarchy of scales. We investigate population turnover at different spatial scales in two specialist insect herbivores that feed on tansy, Tanacetum vulgare, in the Tvärminne archipelago in south-western Finland. Colonies of both aphid species form on individual ramets and in addition to the level of ramets, individuals cluster at the levels of the genet, groups of plants, and islands in the archipelago. Colonisations and extinctions were observed at all spatial levels studied. Mean persistence times in both species ranged from less than five weeks at the level of the ramet to less than 20 weeks at the level of islands, such that aphids over-wintered on only a fraction of the islands in the archipelago. Our field study suggests that aphids on tansy form a hierarchically structured metapopulation where longer-term persistence is only possible at the level of the archipelago.

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Hanski, I. & Meyke, E. 2005: Large-scale dynamics of the Glanville fritillary butterfly: landscape structure, population processes, and weather. — Ann. Zool. Fennici 42: 379–395.

The Glanville fritillary (Melitaea cinxia) has been studied in Finland within an area of 50 by 70 km since 1993. We analyse 11-yr time series for aggregate populations in 20 squares of 4 by 4 km. Different aggregate populations exhibit dissimilar long-term trends, including significant increasing and decreasing trends as well as stable population sizes, and their average size is significantly related to the amount of habitat but not of host plants. Precipitation shows spatially correlated variation in the study area, recorded with high-resolution weather radar. Spatial variation in August and June precipitation explains a significant amount of spatial variation in the dynamics of aggregate populations. Many processes operating in local populations have strong effects, but these effects are not consistent in time and space and they often counter each other, making it less likely that one would detect a signal of local processes in large-scale data for aggregate populations.

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Roland, J. 2005: Are the "seeds" of spatial variation in cyclic dynamics apparent in spatially-replicated short time-series? An example from the forest tent caterpillar. — Ann. Zool. Fennici 42: 397–407.

Variation in the pattern of dynamics of the forest tent caterpillar was assessed over a 13-year interval of population collapse and increase among 68 sites within a 420 km² region. Patterns of population change were compared with the level of forest fragmentation among sites, and interpreted in light of previous studies on the effect of forest fragmentation on the impact of natural enemies of forest tent caterpillar. Outbreaks peaked earlier in contiguous forests than in fragmented forests, and collapsed more rapidly. Parameters of population change for populations in contiguous forests were more characteristic of strongly cyclic dynamics than were those in more fragmented forests, reflecting a significant effect of forest structure on the lagged density-dependent component of tent caterpillar dynamics. Despite the relative shortness of these time series the patterns identified are consistent with a reduction in efficacy of natural enemies in more fragmented forest habitats.

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Harrison, S., Hastings, A. & Strong, D. R. 2005: Spatial and temporal dynamics of insect outbreaks in a complex multitrophic system: tussock moths, ghost moths, and their natural enemies on bush lupines. — Ann. Zool. Fennici 42: 409–419.

To understand the spatial and temporal behavior of outbreaks, it may be necessary to go beyond simple plant–herbivore dynamics and consider trophic complexity, environmental heterogeneity, limited dispersal, and their possible interactions. We examine many of these factors in a system consisting of one plant, two insect herbivores, and multiple natural enemies. Bush lupines (Lupinus arboreus) in coastal California support intense but spatially localized outbreaks of the tussock moth (Orgyia vetusta), a flightless defoliator. Bush lupines are also attacked by the stem-boring larvae of the ghost moth (Hepialis californicus), which may cause periodic mass dieoffs of lupine stands. Here we extend previous models of the tussock moth and its natural enemies to include changes in the carrying capacity (i.e., lupine abundance) caused by the ghost moth and its primary natural enemy. By superimposing these two strongly interactive subsystems, we generate new questions and predictions.

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Tscharntke, T., Rand, T. A. & Bianchi, F. J. J. A. 2005: The landscape context of trophic interactions: insect spillover across the crop–noncrop interface. — Ann. Zool. Fennici 42: 421–432.

Landscape structure influences local diversity and ecosystem processes, including cross-habitat fluxes of organisms coupling the dynamics of different habitats. The flow of organisms across system boundaries is known to occur between different natural habitats as well as across the crop–noncrop interface. Several studies show how field boundaries can enhance predator populations invading arable crops and controlling pest populations. However, generalist arthropods may also spill over from land-use systems to natural areas (mainly grassland) modifying interactions therein. A view of land-use systems as sources and natural habitats as sinks is consistent with the idea that the direction of the organisms’ fluxes is from high to low productivity systems, while noncrop habitats are important sources for recolonization of arable fields after they are cleared for harvest. From the perspective of landscape management, enhancement of population exchanges between crop and noncrop areas may include beneficial as well as unwelcome interactions.

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van Nouhuys, S. 2005: Effects of habitat fragmentation at different trophic levels in insect communities. — Ann. Zool. Fennici 42: 433–447.

Species experience landscapes differently depending on their needs and behaviors, and on their trophic level. We expect species at high trophic levels in a community to be more sensitive to habitat fragmentation than species at lower trophic levels. But this depends on attributes such as resource breadth, dispersiveness, reproductive rate, and longevity, which may not be related to trophic level. I address the association of fragmentation with trophic level using a literature review of 31 studies of herbivores and their natural enemies, and a case study of the parasitoids associated with the Glanville fritillary butterfly. Measures of species richness or total parasitism in an entire insect community provide the strongest support for the idea that negative effects of fragmentation amplify at higher trophic levels. Generally though, there is great variation among studies, due to variation among species, as well as in designs of both experimental and observational studies.

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Morris, R. J., Lewis, O. T. & Godfray, H. C. J. 2005: Apparent competition and insect community structure: towards a spatial perspective. — Ann. Zool. Fennici 42: 449–462.

Competition has been widely discussed as a process that may structure communities of plants and animals. Its role in insect communities is less clear, especially as many insect species do not appear to compete for resources. However, such communities could still be structured by "apparent competition" where the species interact through shared natural enemies. We explore recent attempts to assess whether apparent competition may structure herbivorous insect communities. Communities can be described by quantitative food webs from which the potential for apparent competition can be inferred. We illustrate both the construction of a diverse quantitative food web and a field experimental test of apparent competition using our work on leaf-miner communities in Belize. We consider how a spatial perspective may be incorporated into our leaf-miner community research, and speculate about the shape of apparent competition kernels and their relevance for the structure of herbivorous insect communities.

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Novotny, V. & Weiblen, G. D. 2005: From communities to continents: beta diversity of herbivorous insects. — Ann. Zool. Fennici 42: 463–—475.

Recent progress in molecular systematics that assists species identifications, and in on-line databases of ecological and museum collections that enable the integration of insect distribution data represent important developments facilitating beta diversity studies. The increase in alpha and gamma diversities of insect herbivores from temperate to tropical communities is driven largely by a parallel increase in plant diversity while the diversity of insect herbivores per plant species remains constant. Likewise, the high beta diversity of insect herbivores along altitudinal gradients is only partially explained by changes in plant diversity, while abiotic factors and the abundance of natural enemies may also be important. The high alpha diversity of insect herbivores in lowland tropical forests is not matched by beta diversity as locally co-existing species represent a large proportion of regional species pools. The role of dispersal limitation in the distribution of herbivorous insects in tropical forests could be minor, as short-lived insects are efficient colonisers of their mostly long-lived woody hosts.

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