Funded Grants

Allometry of parasites and foodwebs

Understanding the factors that determine the structure, dynamics, and assembly rules for food webs is a defining problem for complexity science. Food webs are assembled from multiple sets of non-linear interactions between populations of organisms with widely differing birth and death rates; these create layered networks of interactions that are many steps removed from the single species or pair-wise inter-specific interactions that are the traditional domain of theoretical ecology. The level of complexity has only been made more challenging by the recent empirical realization that parasites and pathogens are ubiquitous features of all food-webs; their presence increases the species diversity by at least fifty percent and the density of links between species by a factor of around five (1-3). One would almost be prepared to abandon these problems as intractable, were it not for the fact that food-webs are the ultimate manifestation of the complex interactive systems whose emerging properties are fundamentally aligned with the persistence of natural ecosystems and the ecosystem services they supply to human health and economic welfare. There is a disconcerting urgency to our desire to understand how food-webs are assembled; most of the world's food-webs are being rapidly eroded and we need a better understanding of how the sequential loss of species through local extinction will lead to changes in food-web structure and declines in the rates of delivery of ecosystem services. This is particularly true in parts of the world where human health and economic welfare are intimately linked to services supplied by natural ecosystems and where pathogens and parasites are frequently shared between humans and a large reservoir of other host species.

The recent renaissance in 'macroecology' has described many interesting relationships between body size and life history for a large diversity of animals and plants (4-6). Although we are beginning to develop a fuller understanding of the mechanistic underpinnings of these 'allometric scalings' (both in ecology and biology in general), less has been done on the consequences of these scalings for the way in which food-webs are assembled. Similarly, considerably less has been done on scaling rules for species with parasitic life styles, most of the focus has been on free-living species. This is an important omission as best estimates suggest that around 40% of all known species are parasitic on the 60% of species that are free-living (1). Parasitic species are ubiquitous features of all natural ecosystems, they are often orders of magnitude more fecund than free-living species as their energy requirements are independent of their need to search for food; essentially they live in an environment where their host constantly supplies all their nutritional needs. However, most host species effectively resent this and devote considerable energy into developing an immunological response that recognizes the parasite as being foreign and then tries to kill or remove it. Does this cause the vital rates (birth and death rates) of parasites and pathogens to scale in different ways from those of free-living species? Or do parasitic species simply require their excess fecundity in order to ensure their offspring locate a viable host that they can infect and exploit in ways that maximize their fitness while ensuring the hosts they exploit live long enough to allow the parasite's offspring to colonize and exploit subsequent generations of hosts? Do any body size scaling underlie the dynamics and efficiency of the hosts immunological response? Ultimately the host's immune response has the dynamics of a predator-prey relationship with all the inherent constraints of reproductive rates and search and suppress times that characterize any exploiter-victim relationship.

Although ecologists and evolutionary biologists have made significant advances in our understanding of the dynamics and coevolutionary biology of parasite-host relationships (7-9), most of these studies have focused on models, or experimental systems, that are constrained to examine a single species of pathogen and a single host species. Although some work has been done on pathogens that infect multiple hosts (10-12), and on communities of parasites that can coexist within a single host population (13, 14), there is a significant need to expand our understanding of the role that host and parasite diversity play in determining the dynamics of more complex and diverse host and parasite communities. In many ways parasites and pathogens are the 'dark matter' that lies beneath the more readily perceived and quantified consumer-resource relationships of free-living species (2, 3). Although each individual parasite is small in mass, its dynamics work on a much faster timescale than those of the free-living vertebrates, insects and plants that serve as hosts; this allows parasites and pathogens do evolve much faster and have significantly better potential to regulate the abundance of their hosts than can predators and herbivores whose population dynamics are constrained by digestion and the need to find mates and raise and defend young. Ultimately, we will not be able to make realistic models of food webs and realistically functioning natural ecosystems until we understand the dynamics of pathogens in realistically complex multi-host communities.

Over the last ten years we have developed a framework that uses underlying allometric scalings between body-size and demographic rates to rescale mathematical models for a diversity of parasite-host relationships(10, 15, 16). We have also collaborated with a group of food-web ecologists to assemble data-sets for a number of natural food-webs that include extensive data on how parasite diversity and abundance is distributed in marine and terrestrial food-web networks (www:NCEAS Working Group on Parasites and Food-Webs). In this project we will build on our analytical initial work on scaling in host-parasite population dynamics and develop allometrically scaled models for complex food-webs that include parasites (and mutualists) as well as the more traditional predator-prey, plant-herbivore, and competitive interactions. Our work will proceed in a hierarchical manner, initially developing models for hosts that are parasitized by multiple species of parasites, and pathogens that can infect multiple species of hosts (both sequentially and simultaneously). Once the properties that allow coexistence (and coevolution) within theses systems have been elucidated for these 'community modules' of food web structure, we will combine them to examine how helminth (worm) parasites, and viral and bacterial pathogens, couple together layered networks of predator-prey (resource-consumer) relationships within large food-webs. Our ultimate goal is not only to understand the role that parasites and pathogens play in regulating the abundance of free-living species in natural ecosystems, but also to more finely delineate the roles that pathogens play in modifying rates of nutrient and energy flow through the complex ecological food-webs of natural ecosystems. Our work proceeds from the ironic, but empirically justifiable position, that a healthy ecosystem is one that contains a diverse and abundant parasitic fauna. Understanding the dynamics of processes that disrupt this balance and cause some parasitic species to become overabundant will provide both important insights into how natural ecosystems have evolved as complex adaptive systems. It will also provide insight into how natural ecosystems can be better conserved and managed in ways that both reduce disease risk to humans while also optimizing the benefits obtained from the ecosystem services they supply to the human economy.