Andrew P. Dobson

Introduced species and their missing parasites

Damage caused by introduced species results from the high population densities and large body sizes that they attain in their new location. Escape from the effects of natural enemies is a frequent explanation given for the success of introduced species. Because some parasites can reduce host density and decrease body size, an invader that leaves parasites behind and encounters few new parasites can experience a demographic release and become a pest. To test whether introduced species are less parasitized, we have compared the parasites of exotic species in their native and introduced ranges, using 26 host species of molluscs, crustaceans, fishes, birds, mammals, amphibians and reptiles. Here we report that the number of parasite species found in native populations is twice that found in exotic populations. In addition, introduced populations are less heavily parasitized (in terms of percentage infected) than are native populations. Reduced parasitization of introduced species has several causes, including reduced probability of the introduction of parasites with exotic species (or early extinction after host establishment), absence of other required hosts in the new location, and the host-specific limitations of native parasites adapting to new hosts.

Impacts of biodiversity on the emergence and transmission of infectious diseases

Current unprecedented declines in biodiversity reduce the ability of ecological communities to provide many fundamental ecosystem services. Here we evaluate evidence that reduced biodiversity affects the transmission of infectious diseases of humans, other animals and plants. In principle, loss of biodiversity could either increase or decrease disease transmission. However, mounting evidence indicates that biodiversity loss frequently increases disease transmission. In contrast, areas of naturally high biodiversity may serve as a source pool for new pathogens. Overall, despite many remaining questions, current evidence indicates that preserving intact ecosystems and their endemic biodiversity should generally reduce the prevalence of infectious diseases.

Trade-offs across Space, Time, and Ecosystem Services

Ecosystem service (ES) trade-offs arise from management choices made by humans, which can change the type, magnitude, and relative mix of services provided by ecosystems. Trade-offs occur when the provision of one ES is reduced as a consequence of increased use of another ES. In some cases, a trade-off may be an explicit choice; but in others, trade-offs arise without premeditation or even awareness that they are taking place. Trade-offs in ES can be classified along three axes: spatial scale, temporal scale, and reversibility. Spatial scale refers to whether the effects of the trade-off are felt locally or at a distant location. Temporal scale refers to whether the effects take place relatively rapidly or slowly. Reversibility expresses the likelihood that the perturbed ES may return to its original state if the perturbation ceases. Across all four Millennium Ecosystem Assessment scenarios and selected case study examples, trade-off decisions show a preference for provisioning, regulating, or cultural services (in that order). Supporting services are more likely to be “taken for granted.” Cultural ES are almost entirely unquantified in scenario modeling; therefore, the calculated model results do not fully capture losses of these services that occur in the scenarios. The quantitative scenario models primarily capture the services that are perceived by society as more important—provisioning and regulating ecosystem services—and thus do not fully capture tradeoffs of cultural and supporting services. Successful management policies will be those that incorporate lessons learned from prior decisions into future management actions. Managers should complement their actions with monitoring programs that, in addition to monitoring the short-term provisions of services, also monitor the long-term evolution of slowly changing variables. Policies can then be developed to take into account ES trade-offs at multiple spatial and temporal scales. Successful strategies will recognize the inherent complexities of ecosystem management and will work to develop policies that minimize the effects of ES trade-offs.

Ecology of infectious diseases in natural populations

List of participants Introduction Part I. Broad Patterns and Processes: 1. Impact of infectious diseases on wild animal populations: a review F. M. D. Gulland 2. Microparasites: observed patterns A. P. Dobson and P. J. Hudson 3. Mathematical models for microparasites of wildlife J. A. P. Heesterbeek and M. G. Roberts 4. Microparasite group report C. Dye 5. Macroparasites: observed patterns P. J. Hudson and A. P. Dobson 6. Mathematical models for macroparasites of wildlife M. G. Roberts, G. Smith and B. T. Grenfell 7. Macroparasite group report G. Smith 8. Critical evaluation of wildlife disease models N. D. Barlow Part II. Insects and Plants: 9. Nonlinearities in the dynamics of indirectly-transmitted infections (or, does having a vector make a difference?) C. Dye and B. G. Williams 10. Model frameworks for plant-pathogen interactions J. Swinton and R. M. Anderson 11. The dynamics of insect-pathogen interactions C. J. Briggs, R. S. Hails, N. D. Barlow and H. C. J. Godfray Part III. Impact of Ecological and Genetic Heterogeneity: 12. Environmental influences on host immunity S. Lloyd 13. Modelling the immuno-epidemiology of macroparasites in wildlife host populations B. T. Grenfell, K. Dietz and M. G. Roberts 14. Spatial dynamics of parasitism D. Mollinson and S. A. Levin 15. Spatial dynamics group report B. M. Bolker 16. Genetic diversity in host-parasite interactions C. M. Lively and V. Apanius 17. Genetics and evolution of infectious diseases in natural populations group report A. P. Read 18. Beyond host-pathogen dynamics M. Begon and R. G. Bowers 19. Glossary C. Watt, A. P. Dobson and B. T. Grenfell.

Parasites in food webs: the ultimate missing links

Parasitism is the most common consumer strategy among organisms, yet only recently has there been a call for the inclusion of infectious disease agents in food webs. The value of this effort hinges on whether parasites affect food-web properties. Increasing evidence suggests that parasites have the potential to uniquely alter food-web topology in terms of chain length, connectance and robustness. In addition, parasites might affect food-web stability, interaction strength and energy flow. Food-web structure also affects infectious disease dynamics because parasites depend on the ecological networks in which they live. Empirically, incorporating parasites into food webs is straightforward. We may start with existing food webs and add parasites as nodes, or we may try to build food webs around systems for which we already have a good understanding of infectious processes. In the future, perhaps researchers will add parasites while they construct food webs. Less clear is how food-web theory can accommodate parasites. This is a deep and central problem in theoretical biology and applied mathematics. For instance, is representing parasites with complex life cycles as a single node equivalent to representing other species with ontogenetic niche shifts as a single node? Can parasitism fit into fundamental frameworks such as the niche model? Can we integrate infectious disease models into the emerging field of dynamic food-web modelling? Future progress will benefit from interdisciplinary collaborations between ecologists and infectious disease biologists.

Detecting disease and parasite threats to endangered species and ecosystems

Ecologists have recently begun to acknowledge the importance of disease and parasites in the dynamics of populations. Diseases and parasites have probably been responsible for a number of extinctions on islands and on large land masses, but the problem has only been identified in retrospect. In contrast, endemic pathogens and parasites may operate as keystone species, playing a crucial role in maintaining the diversity of ecological communities and ecosystems. Will recent advances in the understanding of parasite population biology allow us to predict threats to endangered species and communities?

Patterns of macroparasite abundance and aggregation in wildlife populations: a quantitative review

In this paper we review the published literature on patterns of abundance and aggregation of macroparasites in wildlife host populations. We base this survey on quantitative analyses of mean burden and a number of measures of the degree of aggregation of parasite burdens between hosts. All major parasite and vertebrate host taxa were represented in the database. Mean parasite burden was found to be log-normally distributed, indicating that all parasite burdens are regulated to some degree. In addition, all but one of the parasitic infections were aggregated with respect to their hosts, and the relationship between log mean parasite burden and log variance was found to be very strong (R 2 = 0.87). That is, for a given mean parasite burden there are constraints on the degree of variation in individual host burdens. The aggregated nature of the parasitic infections is also apparent from other measures of the degree of aggregation : prevalence - mean relationships, and the negative binomial parameter, k. Using a relatively new technique for parasitological infection data - tree-based models, as well as traditional linear models - a number of the parasitic infections was found to be associated with systematically lower or higher parasite burdens. Possible biological explanations for these and other patterns are proposed.

Regulation and stability of a free-living host - parasite system : Trichostrongylus tenuis in red grouse. II: Population models

Summary 1. The population dynamics of red grouse, Lagopus lagopus scoticus and the parasitic nematode, Trichostrongylus tenuis were explored to determine whether interactions between the parasite and host were sufficient to generate cycles in grouse abundance. Two alternative models were used that explicitly consider the dynamics of either the free-living, or arrested larval stages of the parasite. 2. Providing that the life expectancy of the free-living larvae is more than 2-4 weeks, the parasite can readily establish in grouse populations. Larval arrestment tends to reduce the intrinsic growth rate of the parasite and thus increases the size of the host population required for the parasite to establish. 3. Grouse numbers will tend to cycle when the parasites exhibit low degrees of aggregation and parasite-induced reductions in host fecundity are greater than parasite-induced increases in host mortality. The population cycles produced in the model have the slow increase followed by a rapid decline characteristic of the grouse population studied at Gunnerside. 4. The period of the cycles is determined by the intrinsic growth rate of the grouse population and either larval life expectancy (Model I), or the duration of larval arrestment (Model II). Cycle periods decrease as host population growth rate increases, and lengthen with increases in either free-living larval life expectancy, or the duration of larval arrestment. If the duration of larval arrestment is sufficiently long (>6 months), the cycles die out and the dynamics of the grouse-nematode system are very stable. 5. Estimates of all of the models parameters may be made from long-term records of grouse populations. Numerical analysis of the models behaviour suggest that a model with limited arrested larval stages more closely corresponds to the grouse populations in the North of England. The 4-5 year cycles exhibited by these populations will be more sporadic, or absent on estates where the parasite is unable to establish. 6. The analysis shows that the empirical data collected on T. tenuis are consistent with it being the cause of the cycles observed in grouse populations in the North of England.

Population Dynamics of Pathogens with Multiple Host Species

Pathogens that can infect multiple host species will have different dynamics than pathogens that are restricted to a single species of host. This article examines the conditions for establishment and long‐term population dynamic behavior of pathogens that infect multiple species of hosts. The article attempts to address three major questions in this area: First, under which conditions will increases in the diversity of host species buffer infectious disease outbreaks, and under which conditions will host diversity amplify disease outbreaks? Second, under what conditions is it possible to control an infectious agent by focusing control against only one host species? Third, what role does host species diversity play in maintaining pathogen persistence? The answers to these questions supply some important general insights into the role that biodiversity plays in buffering humans and other species against new and emerging pathogens.

Human health effects of a changing global nitrogen cycle

Changes to the global nitrogen cycle affect human health well beyond the associated benefits of increased food production. Many intensively fertilized crops become animal feed, helping to create disparities in world food distribution and leading to unbalanced diets, even in wealthy nations. Excessive air- and water-borne nitrogen are linked to respiratory ailments, cardiac disease, and several cancers. Ecological feedbacks to excess nitrogen can inhibit crop growth, increase allergenic pollen production, and potentially affect the dynamics of several vector-borne diseases, including West Nile virus, malaria, and cholera. These and other examples suggest that our increasing production and use of fixed nitrogen poses a growing public health risk.