Paul G. Craze

Do differences in food web structure between organic and conventional farms affect the ecosystem service of pest control

While many studies have demonstrated that organic farms support greater levels of biodiversity, it is not known whether this translates into better provision of ecosystem services. Here we use a food-web approach to analyse the community structure and function at the whole-farm scale. Quantitative food webs from 10 replicate pairs of organic and conventional farms showed that organic farms have significantly more species at three trophic levels (plant, herbivore and parasitoid) and significantly different network structure. Herbivores on organic farms were attacked by more parasitoid species on organic farms than on conventional farms. However, differences in network structure did not translate into differences in robustness to simulated species loss and we found no difference in percentage parasitism (natural pest control) across a variety of host species. Furthermore, a manipulative field experiment demonstrated that the higher species richness of parasitoids on the organic farms did not increase mortality of a novel herbivore used to bioassay ecosystem service. The explanation for these differences is likely to include inherent differences in management strategies and landscape structure between the two farming systems.

Understanding and planning ecological restoration of plant–pollinator networks

Theory developed from studying changes in the structure and function of communities during natural or managed succession can guide the restoration of particular communities. We constructed 30 quantitative plant-flower visitor networks along a managed successional gradient to identify the main drivers of change in network structure. We then applied two alternative restoration strategies in silico (restoring for functional complementarity or redundancy) to data from our early successional plots to examine whether different strategies affected the restoration trajectories. Changes in network structure were explained by a combination of age, tree density and variation in tree diameter, even when variance explained by undergrowth structure was accounted for first. A combination of field data, a network approach and numerical simulations helped to identify which species should be given restoration priority in the context of different restoration targets. This combined approach provides a powerful tool for directing management decisions, particularly when management seeks to restore or conserve ecosystem function.

Can epidemic models describe the diffusion of topics across disciplines

This paper introduces a new approach to describe the spread of research topics across disciplines using epidemic models. The approach is based on applying individual-based models from mathematical epidemiology to the diffusion of a research topic over a contact network that represents knowledge flows over the map of science—as obtained from citations between ISI Subject Categories. Using research publications on the protein class kinesin as a case study, we report a better fit between model and empirical data when using the citation-based contact network. Incubation periods on the order of 4–15.5 years support the view that, whilst research topics may grow very quickly, they face difficulties to overcome disciplinary boundaries.

The restoration of parasites, parasitoids, and pathogens to heathland communities

Higher trophic level species such as parasites, parasitoids, and pathogens are frequently ignored in community studies, despite playing key roles in the structure, function, and stability of ecological communities. Furthermore, such species are typically among the last in a community to reestablish due to their reliance upon lower trophic level resources and a requirement for persistent, stable ecological conditions. Consequently their presence alone can be indicative of healthy ecosystems. Using replicated, quantitative food webs we studied the impacts of a restoration treatment upon the interactions of a tri-trophic community consisting of plants, their bumble bee pollinators, and the parasites, parasitoids, and pathogens of the bumble bees at heathland sites. We found the lower trophic levels of the community successfully reinstated at restored relative to control sites. However the abundance, load per host, prevalence of parasitism, prevalence of superparasitism, and host range of a key dipteran parasitoid of the family Conopidae were all significantly reduced in restored heathlands. Potential causes for this incomplete reestablishment at restored sites include the lag in floral resources due to differences in floral species composition, and the reduced ability of this parasitoid species in accessing host resources relative to other natural enemy species present in these communities. Moreover the incomplete reinstatement of the natural enemy community was found to significantly reduce levels of network vulnerability (a measure of how vulnerable prey is to being consumed) at restored sites relative to ancient, control networks.

The potential impact of global warming on the efficacy of field margins sown for the conservation of bumble-bees

Climate change is expected to drive species extinct by reducing their survival, reproduction and habitat. Less well appreciated is the possibility that climate change could cause extinction by changing the ecological interactions between species. If ecologists, land managers and policy makers are to manage farmland biodiversity sustainably under global climate change, they need to understand the ways in which species interact with each other as this will affect the way they respond to climate change. Here, we consider the ability of nectar flower mixtures used in field margins to provide sufficient forage for bumble-bees under future climate change. We simulated the effect of global warming on the network of plant–pollinator interactions in two types of field margin: a four-species pollen and nectar mix and a six-species wildflower mix. While periods without flowering resources and periods with no food were rare, curtailment of the field season was very common for the bumble-bees in both mixtures. The effect of this, however, could be ameliorated by adding extra species at the start and end of the flowering season. The plant species that could be used to future-proof margins against global warming are discussed.

Parasitoid diversity reduces the variability in pest control services across time on farms

Recent declines in biodiversity have increased interest in the link between biodiversity and the provision and sustainability of ecosystem services across space and time. We mapped the complex network of interactions between herbivores and parasitoids to examine the relationship between parasitoid species richness, functional group diversity and the provision of natural pest control services. Quantitative food webs were constructed for 10 organic and 10 conventional farms. Parasitoid species richness varied from 26 to 58 species and we found a significant positive relationship between parasitoid species richness and temporal stability in parasitism rates. Higher species richness was associated with lower variation in parasitism rate. A functional group analysis showed significantly greater parasitoid species complementarity on organic farms, with on average more species in each functional group. We simulated parasitoid removal to predict whether organic farms experienced greater robustness of parasitism in the face of local extinctions. This analysis showed no consistent differences between the organic and conventional farm pairs in terms of loss of pest control service. Finally, it was found that the different habitats that make up each farm do not contribute equally to parasitoid species diversity, and that hedgerows produced more parasitoid species, significantly more so on organic farms.

Population structure and coil dimorphism in a tropical land snail

Tree snails of the subgenus Amphidromus s. str. are unusual because of the chiral dimorphism that exists in many species, with clockwise (dextrally) and counter-clockwise (sinistrally) coiled individuals co-occurring in the same population. Given that mating in snails is normally impeded when the two partners have opposite coil, positive frequency-dependent selection should prevent such dimorphism from persisting. We test the hypothesis that a strong population structure with little movement between tree-based demes may result in the fixation of coiling morphs at a very small spatial scale, but apparent dimorphism at all larger scales. To do so, we describe the spatial structure in a Malaysian population of A. inversus (Müller, 1774) with 36% dextrals. We marked almost 700 juvenile and adult snails in a piece of forest consisting of 92 separate trees, and recorded dispersal and the proportions of dextrals and sinistrals in all trees over a 7-day period. We observed frequent movement between trees (155 events), and found that no trees had snail populations with proportions of dextrals and sinistrals that were significantly different from random. Upon recapture 1 year later, almost two-thirds of the snails had moved away from their original tree. We conclude that population structure alone cannot stabilise the coil dimorphism in Amphidromus.

The fate of balanced, phenotypic polymorphisms in fragmented metapopulations

In large populations, genetically distinct phenotypic morphs can be maintained in equilibrium (at a 1 : 1 ratio in the simplest case) by frequency‐dependent selection, as shown by Sewall Wright. The consequences of population fragmentation on this equilibrium are not widely appreciated. Here, I use a simple computational model to emphasize that severe fragmentation biases the morph ratio towards the homozygous recessive genotype through drift in very small populations favouring the more common recessive allele. This model generalizes those developed elsewhere for heterostylous plants and major histocompatibility complex alleles, emphasizes one particular outcome and avoids the restricting assumptions of more analytical models. There are important implications for both fundamental evolutionary biology and conservation genetics. I illustrate this with a range of examples but refer particularly to shell polymorphism in snails. These examples show how habitat fragmentation could have a direct and often unappreciated effect on species at the level of their population genetics.

Spatial ecology, habitat and speciation in the Porto Santan land snail genus Heterostoma

The Madeiran land snail genus Heterostoma expresses two forms of genital anatomy. One form (hemiphallic) lacks the distal male organs while the other (euphallic) has fully developed hermaphroditic genitalia. Recent evidence suggests this genital variation characterizes two sibling or incipient species. The spatial ecology and habitat associations of these taxa are described in the light of recent ideas on the role of ecology in speciation. Hemiphallic snails are found at a lower density and show a tendency to be more separated from each other compared to euphallic snails. Both taxa are more likely to be closer to other members of the same taxon. The genus as a whole is not found in the presence of sand or pale rock. However, when only areas actually containing snails are examined, hemiphallics show a positive association with sand and rock while euphallics continue to show a negative association. The differences in population density and spacing may suggest an adaptive explanation for reduction of male organs in hemiphallics since this taxon would be expected to have significantly fewer mating encounters. The observed differences in habitat association may suggest a mechanism for sympatric or parapatric divergence in keeping with current models of habitat-driven adaptive speciation. Both of these factors (selection on the mating system in a marginal environment and small scale separation based on habitat) may have been significant in the divergence of hemiphallic and euphallic taxa.

Differences in physiological tolerance between coexisting taxa of the Madeiran land snail genus Heterostoma under controlled humidity and simulated rainfall

The Madeiran land snail genus Heterostoma contains two taxa distinguished by differences in genital anatomy: one has full hermaphroditic genitalia (euphallic) while the other lacks the distal male organs (hemiphallic). Snails from the Madeiran islands of Porto Santo and Ilheu de Cima were kept under controlled conditions differing in humidity and simulated rainfall. There were four treatments: (1) low humidity, no rainfall; (2) low humidity, rainfall; (3) high humidity, no rainfall; (4) high humidity, rainfall. For snails from Porto Santo there were viability differences between taxa under all treatments such that hemiphallics were most viable under treatment 1 and euphallics were more viable under all other treatments. There were no signicant differences between taxa taken from Ilheu de Cima. This may be a real effect or the result of smaller sample size or a combination of both. Under all conditions, both taxa from both islands show a time-dependent reduction in viability in response to rainfall. Taxa from Porto Santo also respond differently to humidity with hemiphallics surviving for less time than euphallics at high humidity. These results are interpreted as showing a difference in physiological tolerance between taxa from Porto Santo with respect to humidity. It is suggested that this ecological difference may have been a factor in the divergence of the taxa, perhaps through interaction with genital anatomy variation. No rm conclusions can yet be drawn regarding samples taken from Ilheu de Cima.