Biological Sciences

Parasites as the missing links in natural ecosystems


Andy Dobson, Professor
Ecology and Evolutionary Biology, Princeton University
and  and Twitter Andy2Dobson


Understanding the structure and dynamics of food webs is one of the central scientific challenges of the 21st Century; parasites and pathognes play a potentially huge role in linking free-living species together and regulating their abundance.  Preliminary estimates suggest that considering parasites increase the number of species in food webs by 40% and the density of links between species by a factor of 3.  My research focuses on developing a quantitative and qualitative understanding of the abundance and diversity of parasites and pathogens in natural ecosystems.  The work is undertaken in salt marshes along the coast of California, in Serengeti in Tanzania and in Yellowstone NP, Wyomimg.  All of the work is collaborative and seeks to develop a mathematical understanding of the way that the ‘dark matter’ of parasites and pathogens determines the rates at which free-living species interact to drive natural ecosystems and the services they provide to humans.


My research is conducted in collaboration with a really super set of colleagues, I remain convinced that the only way to address big scientific questions is to collaborate with insightful and energetic people who are socially unified by their quest for new knowledge.  My own skills are as a mathematical modeler, a data analyst and support person who enthuses others, cooks meals, pours drinks, and writes grants to keep everyone else busy.  If time allows I take as many photograhs as possible of people, plants, predators, prey and parasites in action.

So my principal tools are pen, pencil and notebook, computer, binoculars and a camera.  A lot of time is spent reading through old papers to identify feeding links between predator and prey and hosts and parasites, but significant time is also spent in the field, occaasionally dissecting out these links.

Occasionaly I’ll result to a well stocked kitchen and wine cellar to enhance team cohesion or to celebrate publication of significant results.


Our research has shown that parasites and pathogens play a major role in the lives of all free-living species and that there is  a huge diversity of species living in and upon free-living species that are traditionally assumed to determine the structure of food webs.   In the simplest sense we have shown that consideration of parasites converts the basic underlying structure of food webs from a pyramid, with the predators at the apex, to an inverted rhomboid, with an increasing diversity of parasite species feeding on predators, than on herbivores, or on plants.

We have also shown that the ‘body size and energetic efficiency rules’ that determine the abunace and biomass of free-living species of different sizes, also apply to parasites and pathogens.  As they are often very tiny, there is a huge abundance of parasites (eg more biomass of parasitic worms than birds in salt marshes), but because they feed so high in the food chain, they are less abundant than free-living microbes of equivalent size.

Our major present goals are to collect data from different ecosystems and to fill out the data bases we have for the systems we have initially focused upon.  Along the coast of California we are examining salt marshes that have been perturbed by  humans in different ways: eutrophication, invasive species and asking how the parasites in these food webs differ from those of more natural, or less perturned webs.  In Serengeti and Yellowstone we are searching a large data base of research publications and assembling data on which parasites utilize which hosts and is there any information on historical changes in the abundance of different species in the system.  For example, the introduction of rinderpest virus to East Africa in 1890 hugely changed the abundance of all free-living species in Serengeti, it’s subsequent eradication again led to huge changes in the abundance of most plants, herbivores and carnivores – very few of which were actually hosts to the rinderpest virus.

Ultimately if we are to have any hope of understanding how the natural world is assembled and how it will respond to repeated human perturbations, we need a deep quantitative and empirical understanding of hwo food webs work.  It’s arguably  the most challanging and important tsk for 21st century science  and it has a very urgent timeline as most natural food webs are hugely threatened by a diversity of human actvities.

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