Heterogeneous Host Susceptibility Enhances Prevalence of Mixed-Genotype Micro-Parasite Infections

Dose response in micro-parasite infections is usually shallower than predicted by the independent action model, which assumes that each infectious unit has a probability of infection that is independent of the presence of other infectious units. Moreover, the prevalence of mixed-genotype infections was greater than predicted by this model. No probabilistic infection model has been proposed to account for the higher prevalence of mixed-genotype infections. We use model selection within a set of four alternative models to explain high prevalence of mixed-genotype infections in combination with a shallow dose response. These models contrast dependent versus independent action of micro-parasite infectious units, and homogeneous versus heterogeneous host susceptibility. We specifically consider a situation in which genome differences between genotypes are minimal, and highly unlikely to result in genotype-genotype interactions. Data on dose response and mixed-genotype infection prevalence were collected by challenging fifth instar Spodoptera exigua larvae with two genotypes of Autographa californica multicapsid nucleopolyhedrovirus (AcMNPV), differing only in a 100 bp PCR marker sequence. We show that an independent action model that includes heterogeneity in host susceptibility can explain both the shallow dose response and the high prevalence of mixed-genotype infections. Theoretical results indicate that variation in host susceptibility is inextricably linked to increased prevalence of mixed-genotype infections. We have shown, to our knowledge for the first time, how heterogeneity in host susceptibility affects mixed-genotype infection prevalence. No evidence was found that virions operate dependently. While it has been recognized that heterogeneity in host susceptibility must be included in models of micro-parasite transmission and epidemiology to account for dose response, here we show that heterogeneity in susceptibility is also a fundamental principle explaining patterns of pathogen genetic diversity among hosts in a population. This principle has potentially wide implications for the monitoring, modeling and management of infectious diseases

The Market Instinct: The Demise of Social Preferences for Self-Interest

Environmental policy design has much to gain from a better understanding of existing voluntary behaviour and motivations. In laboratory experiments, participants often exhibit social preferences such as altruism, spite, reciprocity and notions of fairness. In contrast, traditional neoclassical theory assumes that people act rationally in a way that maximises their self-interest. In environmental markets, social preferences and self-interest interact. We apply experimental economics to test the hypothesis that social preferences are not maintained in the presence of a competitive market institution. In the initial public goods game, many participants were prepared to make costly voluntary contributions. However the introduction of the market institution triggered a 'market instinct' in experimental participants. They abandoned the social preferences they were previously expressing and became self-interested profit maximisers. This self-interested behaviour persisted even after the market institution was discontinued. These findings are important to understanding the role and impact of markets for environmental policy. © 2010 Springer Science+Business Media B.V

Trust, reputation and relationships in grazing rights markets: An experimental economic study

Trust is frequently a requirement for economic exchanges and the management of natural resources. Providing public informaiton on past actions can promote trust through the formation of reputations. We developed an economic experiment to test whether a formal reputation mechanism could facilitate trusting relationships in the tradeable grazing rights markets. Providing informaiton to create formal public reputations for market participants did not increase the overall efficiency of the market. However, it did result in greater equality of income between partners, suggesting the participants showed more concern for their partners when they knew they would be rated. Even with public reputation informaiton, bilateral relationships remained central to the market. Market failures in exisitng grazing rights markets may be better addressed by measures to increase communication between partners rather than simply relying on a formal reputation mechanism

Economic Behavior in the Face of Resource Variability and Uncertainty

Policy design is largely informed by the traditional economic viewpoint that humans behave rationally in the pursuit of their own economic welfare, with little consideration of other regarding behavior or reciprocal altruism. New paradigms of economic behavior theory are emerging that build an imprical basis for understanding how humans respond to specific contexts. Our interest is in the role of human relationships in managing natural resources (forage and livestock) in semiarid systems, where spatial and temporal variability and uncertainty in resource availability are fundamental system drivers. In this paper we present the results of an economic experiment designed to explore how reciprocity interacts with variability and uncertainty. This behavior underpins the Australian tradable grazing rights, or agistment, market, which facilitates livestock mobility as a human response to a situaiton where rainfall is so variable in time and space that it is difficult to maintain an economically viable livestock herd on a single management unit. Contrary to expectations,we found that variability and uncertainty significantly increased transfers and gains from trade within our experiment. When participants faced variability and uncertainty, trust and reciprocity took time to build. When variability and uncertainty were part of the experiment trust was evident from the onset. Given resource variability and uncertainty are key drivers in semiarid systems, new paradigms for undertstanding how variability shapes behavior have special importance

Acorn consumption improves the immune response of the dung beetle Thorectes lusitanicus

Thorectes lusitanicus, a typically coprophagous species is also actively attracted to oak acorns, consuming, burying them, and conferring ecophysiological and reproductive advantages to both the beetle and the tree. In this study, we explored the possible relation between diet shift and the health status of T. lusitanicus using a generalist entomopathogenic fungus (Metarhizium anisopliae) as a natural pathogen. To measure the health condition and immune response of beetles, we analysed the protein content in the haemolymph, prophenoloxidase (proPO) content, phenoloxidase (PO) activity and mortality of beetles with diets based on either acorns or cow dung. Protein content, proPO levels and PO levels in the haemolymph of T. lusitanicus were found to be dependent on the type of diet. Furthermore, the beetles fed with acorns developed a more effective proPO-PO system than the beetles fed with cow dung. Furthermore, a significant decrease in mortality was observed when infected individuals were submitted to an acorn-based diet. In addition to enhancing an understanding of the relevance of dietary change to the evolutionary biology of dung beetles, these results provide a more general understanding of the ecophysiological implications of differential dietary selection in the context of fitness.Financial support was provided by the Projects CGL2008/03878/BOS and CGL2011-25544 of the Secretaría de Estado de Investigación-Ministerio de Educación, Ciencia e Innovación (http://www.mecd.gob.es/portada-mecd/) and OAPN 762/2012, Ministerio de Agricultura, Alimentación y Medio Ambiente (http://www.magrama.gob.es/es/)

Adapting auctions for the provision of ecosystem services at the landscape scale

Auctions, or competitive tenders, are capable of overcoming information asymmetries to efficiently allocate limited funding for the provision of ecosystem services. Most auctions focus on ecosystem services on individual properties to maximise the total amount provided across the landscape. However, for many services it is not just the total quantity but their location in the landscape relative to other sites that matters. For example, biodiversity conservation may be much more effective if conserved sites are connected to other conserved areas. Adapting auctions to address ecosystem services at the landscape scale requires a good scientific understanding of the biophysical system. It also requires an auction mechanism which can promote coordination while maintaining the competition required to overcome information asymmetries. Iterated auctions, in which bidding is spread out over a number of rounds, with information provided between rounds on the location of other bids in the landscape, offers an approach to cost effectively deliver landscape-scale ecosystem services outcomes. Experimental economic testing shows that these auctions deliver the most cost effective environmental outcomes when the number of rounds is unknown in advance, which minimises rent seeking behaviour. It also shows that a bid improvement rule facilitates coordination and reduces rent seeking. Where the biophysical science is well developed, such auctions should be relatively straight forward to implement and participate in, and have the potential to provide significantly better outcomes than standard ‘one-shot’ tenders

The Market Instinct: The Demise of Social Preferences for Self-Interest

Experimental economics, Environmental markets, Public goods, Environmental policy,

Heritability of immune function in the caterpillar Spodoptera littoralis.

Phenoloxidase (PO) is believed to be a key mediator of immune function in insects and has been implicated both in non-self recognition and in resistance to a variety of parasites and pathogens, including baculoviruses and parasitoids. Using larvae of the Egyptian cotton leafworm, Spodoptera littoralis, we found that despite its apparent importance, haemolymph PO activity varied markedly between individuals, even amongst insects reared under apparently identical conditions. Sib-analysis methods were used to determine whether individuals varied genetically in their PO activity, and hence in one aspect of immune function. The heritability estimate of haemolymph PO activity was high (h2 = 0.690 0.069), and PO activity in the haemolymph was strongly correlated with PO activity in both the cuticle and midgut; the sites of entry for most parasites and pathogens. Haemolymph PO activity was also strongly correlated with the degree to which a synthetic parasite (a small piece of nylon monofilament) was encapsulated and melanized (r = 0.622 0.142), suggesting that the encapsulation response is also heritable. The mechanism maintaining this genetic variation has yet to be elucidated