Population and community ecology need a large-scale perspective because local patterns (of biodiversity) and processes (trophic interactions) are influenced by the regional setting. The ratio of the foraging range and/or dispersal ability to the distance between landscape elements influences local population dynamics. The spatial scale experienced by a species may be linked to its trophic level and also to traits such as body size, resource specialization, rarity, and population size variability. Hence, communities are assemblages of species with different spatial strategies. Effects of habitat loss and habitat fragmentation on plant-herbivore, herbivore-enemy, as well as plant-pollinator interactions are contingent on species and landscape. Metapopulation theory provides a unifying frame to approach plant-insect systems across fragmented landscape, although the landscape context is often ignored. In some cases theory is far ahead of empirical research. We call for more population data on large spatial and temporal scales to better understand plant-insect populations across fragmented landscapes.

Plant-insect interactions in fragmented landscapes.

Multi-function agricultural biodiversity: pest management and other benefits

The relative importance of bottom-up or top-down forces has been mainly studied for herbivores but rarely for pollinators. Habitat fragmentation might change driving forces of population dynamics by reducing the area of resource-providing habitats, disrupting habitat connectivity, and affecting natural enemies more than their host species. We studied spatial and temporal population dynamics of the solitary bee Osmia rufa (Hymenoptera: Megachilidae) in 30 fragmented orchard meadows ranging in size from 0.08 to 5.8 ha in an agricultural landscape in central Germany. From 1998 to 2003, we monitored local bee population size, rate of parasitism, and rate of larval and pupal mortality in reed trap nests as an accessible and standardized nesting resource. Experimentally enhanced nest site availability resulted in a steady increase of mean local population size from 80 to 2740 brood cells between 1998 and 2002. Population size and species richness of natural enemies increased with habitat area, whereas rate of parasitism and mortality only varied among years. Inverse density-dependent parasitism in three study years with highest population size suggests rather destabilizing instead of regulating effects of top-down forces. Accordingly, an analysis of independent time series showed on average a negative impact of population size on population growth rates but provides no support for top-down regulation by natural enemies. We conclude that population dynamics of O. rufa are mainly driven by bottom-up forces, primarily nest site availability.

/pdf/do-resources-or-natural-enemies-drive-bee-population-yjchl8ub4b.pdf

Do resources or natural enemies drive bee population dynamics in fragmented habitats

We measured holly leaf-miner (Phytomyza ilicis) densities, and mortalities imposed by a guild of specific parasitoids and birds, using nested quadrats varying in size from 0.03 to 1 m2. Mortalities imposed by three species of pupal parasitoids, one of which behaves as a facultative hyperparasitoid, were independent of host density at all spatial scales. The larval parasitoid, Chrysocharis gemma, in contrast, apparently aggregated strongly in regions of high host density, imposing spatially density-dependent mortality. The effect was most obvious at the smallest sampling-scale (0.03 m2) and became progressively weaker as the data were aggregated into larger sampling units. It could not be detected at all in samples of 1 m2. Within the smallest sampling units, aggregative responses (as revealed by host mortality imposed by C. gemma) were strongest in regions of low overall host abundance, and vice-versa. These results are in accord with predictions of parasitoid behaviour based on optimum foraging theory. Predation by birds, mainly blue tits Parus caeruleus, was inversely density-dependent. The possibility that birds may aggregate in response to very large patches of the holly leaf-miner (>1 m2) is discussed. Consistent with recent theoretical predictions, the species imposing spatially density dependent mortality (C. gemma) has been used as an effective biological control agent against the holly leaf-miner in Canada. Pupal parasitoids, which do not impose spatially density-dependent mortality, failed as biocontrol agents. Our results are discussed in relation to two problems, namely that of detecting an aggregative response by natural enemies in the field, and what constitutes a 'patch'. Prey grouped on the scale evoking an aggregative response by predators constitute 'patches'. Patches of the same species will be much larger for large active predators than for small, inactive ones. If the wrong spatial scale is chosen, it may be impossible to find evidence for aggregative responses in the field, even though they are present.

Studies on the natural enemy complex of the holly leaf-miner; the effects of scale on the detection of aggregative responses and the implications for biological control [Phytomyza ilicis, Ilex aquifolium, Parus caeruleus, UK]

A growing body of literature points to a large-scale research approach as essential for understanding population and community ecology. Many of our advances regarding the spatial ecology of predators and prey can be attributed to research with insect parasitoids and their hosts. In this review, we focus on the progress that has been made in the study of the movement and population dynamics of hosts and their parasitoids in heterogeneous landscapes, and how this research approach may be beneficial to pest management programs. To date, few studies have quantified prey and predator rates and ranges of dispersal and population dynamics at the patch level—the minimum of information needed to characterize population structure. From host–parasitoid studies with sufficient data, it is clear that the spatial scale of dispersal can differ significantly between a prey and its predators, local prey extinctions can be attributed to predators and predator extinction risk at the patch level often exceeds that of the prey. It is also evident that populations can be organized as a single, highly connected (patchy) population or as semi-independent extinction-prone local populations that collectively form a persistent metapopulation. A prey and its predators can also differ in population structure. At the landscape level, agricultural studies indicate that predator effects on its prey often spill over between the crop and surrounding area (matrix) and can depend strongly on landscape structure (e.g. the proportion of suitable habitat) at scales extending well beyond the crop margins. In light of existing empirical data, predator–prey models are typically spatially unrealistic, lacking important details on boundary responses and movement behaviour within and among patches. The tools exist for conducting empirical and theoretical research at the landscape level and we hope that this review calls attention to fertile areas for future exploration.

Host–parasitoid spatial ecology: a plea for a landscape-level synthesis

Summary
1The holly leaf-miner Phytomyza ilicis Curtis shows density patterns typical for non-eruptive leaf-miners, with stable mean densities, but consistently different infestation rates on individual trees.
2To identify the mechanisms responsible for these patterns the population dynamics of P. ilicis were studied during an 8-year period on 92 host trees in 15 holly patches. The patterns of oviposition, mining and survival and the dominant sources and rates of mortality were monitored. The impact of tree and patch parameters on leaf-miner population dynamics was analysed, and the dispersal ability of P. ilicis was evaluated experimentally.
3Infestation differences among trees were due to host selection, which determined egg numbers on individual trees and was explained partly by holly variety and leaf availability.
4Egg distributions on medium to highly infested host trees were aggregated, suggesting limits in the availability of young leaves for oviposition.
5Due to a suite of mortality factors mean survival rates of P. ilicis were below 10% and did not show a relationship with host choice.
6There was strongly density-dependent pre-mine mortality most probably caused by intraspecific competition in the leaf midrib. This lead to the formation of a single mine on most leaves. There was a weak density-dependent effect of mine density on miscellaneous larval death and on the attack rates of the larval parasitoid Chrysocaris gemma.
7In the holly leaf-miner system phenology effects set a population ceiling delimited by the availability of oviposition sites. The resulting egg distribution determined the impact of density-dependent intraspecific competition in the leaf midrib. The incidence of larval competition was therefore not a function of density per se, but density-relative to the availability of a limiting resource, namely suitable young leaves for oviposition.

Bottom‐up density regulation in the holly leaf‐miner Phytomyza ilicis

Multitrophic interactions: The population dynamics of spatially structured plant-herbivore-parasitoid systems

Summary

1. Some herbivore species periodically undergo damaging, high-density outbreak phases followed by less damaging low-density phases. Others maintain steady, low to moderate density levels that do little damage to their hosts.



2. Two closely related holly leaf-miner species were compared that share many ecological traits and have very similar life cycles, but only one of which exhibits outbreaks. Phytomyza ilicicola in the eastern U.S.A. varied widely in mortality and infestation levels, reaching local densities of over 10 mines per leaf. In contrast, Phytomyza ilicis in the U.K. showed low infestation and high mortality at all sites. Using data from the literature and from field studies, the factors responsible for these contrasting dynamics were sought.



3. Phytomyza ilicicola oviposits into the leaf lamina, and experiences weak larval competition only at high densities. Phytomyza ilicis oviposits into the leaf midrib, which leads to high mortality of young larvae before mine formation. Multiply mined leaves were therefore very common in P. ilicicola but rare in P. ilicis.



4. Differences in the parasitoid complexes of the two systems accounted for further differences in survival to adulthood. The main (larval) parasitoid, which was found to impose high, density-dependent mortality on P. ilicis, is missing on P. ilicicola. It is replaced by an egg–pupal parasitoid, which varies in its impact at differe∼t sites. Multiple emergence of adults from multiply mined leaves is therefore widespread in P. ilicicola but does not occur in P. ilicis.



5. The differences in oviposition behaviour and in the parasitoid complexes are likely to allow P. ilicicola to outbreak when habitat conditions are favourable, while P. ilicis is always tightly regulated.

Holly leaf‐miners on two continents: what makes an outbreak species?

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Bottom‐up density regulation in the holly leaf‐miner Phytomyza ilicis

Multitrophic interactions: The population dynamics of spatially structured plant-herbivore-parasitoid systems

Holly leaf‐miners on two continents: what makes an outbreak species?