Where is my home? – Finding a place for parasites in food web representations
When we think about food webs, it is rare that parasites are the first group of species that comes to mind; most likely because of their small size and their cryptic nature. But parasites are a ubiquitous component of all ecosystems, account for a substantial share of the earth’s biomass and represent approximately half of all species.
Food webs are key to understanding ecosystem function and health and predicting how a community would react to an extreme stressor (e.g. climate change). They are a man-made mapping tool, constructed using the natural feeding interactions between two species: predator-prey, herbivore-plant, detritivore-detritus, and parasite-host. Wait, host-parasites? But how do we know the food/nutrient exchanges happening between a parasite and its host? Isn’t it negligible? Well, no. Researchers have demonstrated that parasites are involved in 50-75 % of food web links, and can represent up to 50% of the biomass in certain ecosystems.
So, how can we track the nutrient exchanges between a parasite and its host, and ultimately find a home for parasites in our food web representations?
A possible answer could be isotopes. Isotopes are different variants of the same atom, which differ very slightly in mass. Often, one isotope is less abundant, and can be isolated and used to trace the flow of the dominant variant. Common examples are carbon (12C and 13C) and nitrogen (14N and 15N). These stable (non-radioactive) isotopes are expressed as a ratio of the heavier isotope over the lighter (e.g. 13C/12C). Indeed, naturally occurring reactions within animal’s tissues tend to preferentially react with lighter isotopes, leaving behind the heavier variant, thus affecting its ratio. The changing ratios of these stable isotopes as they progress through a food web, as nutrients, provide a unique tracer of the flow of energy within a system. And while stable isotopes have been widely applied in food web ecology, studies that applied this technique to a host-parasite system are very rare. Moreover, previous research utilising the so-called “bulk” approach to tackle some of the issues surrounding the incorporation of parasites and host-parasite interactions into food webs, found their results to be inconsistent at best. A potential explanation is that parasites often feed on specific compounds within their hosts, which have a unique composition of such compounds in their tissues, and the bulk approach is not sensitive enough to detect changes caused by such selective feeding.
Recent analytical developments enable the stable isotope technique to be applied to specific molecules within tissue e.g. amino acids and fatty acids, providing a valuable tool to trace molecular exchanges between the host and parasite. This approach has the potential to improve our fundamental understanding of food webs in general, and to facilitate the addition of parasites into these webs by utilising very small samples that can focus on specific molecules that are targeted by parasite species. It will also allow us to reveal the specific energy source of a parasite, the quantity of biomass transferred, and the flow-on repercussions to the food web at unprecedented detail. To date, this approach has never been tested.
All the work done in this field was summarised here: Sabadel, A.J.M., A.D. Stumbo and C.D. MacLeod: Stable isotope analysis: a neglected tool for placing parasites in food webs. Journal of Helminthology 93, 1, 1-7 (2019).