To begin we would like to help define parasitism using the classic analogy of the guy in his mid-thirties with no job who still lives in his parents basement playing video games... a nester. Parasites are users, they grow at the expense of another organism, the host, in this case the parents. A host can be many different species, a plant, an animal, even you or me and parasites come in many forms. Most parasites are worms (classified under the phylums Nematoda and Annelida), small “bugs” (phylum Arthropoda) or bacterial infections. Parasites can live on the surface and within their host and can have multiple hosts due to different life cycle stages. Some parasites begin in one host, need to be ingested by another and only then can grow and reproduce. How can a parasite do this? How can the lifestyle of these small, harmful, highly specialized and environmentally isolated organisms be the most common on Earth?
Three scientific articles conducted by Lafferty and colleagues, used experiments demonstrating profound parasitic interactions within a habitat and will be outlined to provide you with a new outlook on just how delicate, yet advanced parasites are.
In the first study conducted by Lafferty (Philosophical Transactions of the Royal Society, 2009), food web strength (robustness) in the presence and without the presence of parasites was compared. Food webs are basically constructed between producers and consumers but have never generally included parasites or the influence of parasites. Non-parasitic (free-living) species were removed from the Carpinteria Salt Marsh estuary to determine the effect of parasites on the food web. Most parasites depend on a single host species at some point during their life cycle yet few predators depend on a single prey, so the effect of removing one free-living organism should be greater on parasites leading to more frequent secondary extinctions, which is an extinction event resulting from the removal of another species. This effect was clearly demonstrated when the California red snail was replaced by the Japanese mud snail which caused no change in predator-prey interactions but showed elimination of 17 native parastic worm species greatly reducing the connections linking the food web. It is clear from this experiment that parasites on average are more sensitive to secondary extinction most likely due to their high degree of specialization and host specificity. Therefore adding extinction-prone parasites to a food web would reduce its entire strength and robustness, a big impact for such a small organism. This fact could make parasites a strong indicator of food web integrity.
A second study by Lafferty and colleagues (Ecology, 2011) was designed to investigate if manipulative parasites, those who chemically mind control their host, can alter energy flow through an ecosystem. The experiment was conducted in a Japanese river ecosystem and looked at crickets and grasshoppers and the parasitic worms who manipulated their behaviour. It was observed that the insects would be twenty times more 
likely to enter the stream water when infected by the worms, greatly increasing the consumption risk by river trout. This behavioural drive to the water was noticed primarily in the summer and fall, which correlated with high trout feeding frequencies. The diet of the trout during this period is highly subsidized by terrestrial insects and reduces the feeding pressure on the entire river bottom invertebrate community. This vast quantity of infected insects eaten by the trout population is the first evidence that manipulative parasites can substantially alter the energy flow within an ecosystem and between ecosystems. It is clear from this experiment that terrestrial energy in the form of the grasshoppers and crickets, can be easily manipulated by parasites to enter aquatic ecosystems from terrestrial ecosystems. The energy movement within the aquatic ecosystem is also affected by the parasites as trout predation on bottom feeders is highly subsidized by the terrestrial bugs. It is very apparent that parasites alter energy flow and balance within an ecosystem and environment.
The final article investigated by Lafferty and colleagues (Ecology, 2010) dealt with large scale marine fishing impacts on parasite levels. Evidence gathered in other studies suggested that fishing leads to decreasing parasite a
bundance and diversity, as humans selectively fish for larger individuals. Older, larger fish populations will have higher parasite ratios then younger as they have higher interaction rates with parasites and stronger spatial patterns leading to infection. The removal of these hosts was shown to drastically alter parasite levels and as suggested in the previous study by Lafferty, parasites have a broad impact on food web complexity and integrity. Their decrease in ocean ecosystems would have large scale impacts on biodiversity but the implications are still poorly understood. If fishing reduces a primary predator of a parasitic host, the parasite population will increase as the host population does, but if the host of the parasite is over fished, the parasite populations will decrease. This has positive effects on fishery yields but also has negative effects as this reduces food web complexity by reducing the number of pathways.
It is clear from these three studies that parasites can have drastic impacts on global ecosystems. These organisms are highly specialized yet abundant through many ecosystems. They lead to their hosts decline yet they persist over generations. Parasites clearly are influential in any place they are found as these studies have demonstrated parasites altering food web integrity, ecosystem energy flow and ecosystem biodiversity across a variety of environments. Hopefully this blog has provided some insight on just how large the impact of parasites is on global ecosystems and diversity. Like they say, big things come in small packages!
