What is a parasite? Are parasites inherently ‘bad’? What are the consequences of removing them? Are there unintended consequences? Can parasites even be ‘good’?
It is a widespread belief that parasites are inherently ‘bad’. By definition, they harm the host, to the benefit of the infecting parasite. Sounds pretty bad to me! But there may be more to the picture than meets the eye. Just because something seems ‘bad’ does not exclude it from being necessary for a variety of reasons. There are obvious limitations to this way of thinking, some parasites can be extremely harmful, even deadly, but others can survive with relatively little impact to the host. Because of parasites being designated to ‘bad’ status, very little thought was ever given to what role they play in a larger context, they were always something that needed to be eliminated, until recently. In recent years scientists have finally started to look at the role that parasites might play in ecosystems.
In order to discuss the implications of parasites beyond there immediate effects we need tools to detect and measure them in some manner. The first way that scientists have begun measure the presence of a parasite (or any biological matter) is by looking at its ‘biomass’. This is the total mass of the parasite compared to all of the mass in a particular ecosystem. Researchers (Kuris et al, 2008) have assessed the role of parasites in terms of their biomass in a particular ecosystem (in this case estuaries or inlets). These researchers found that parasites are responsible for a significant portion of the biomass in the ecosystem, up to 3.2-13.2% depending on the location. This amount of biomass is often larger than the combined biomass of all birds in the ecosystem! The ability for these parasites to gain such an incredible amount of biomass is due to the nature of their lifecycles. Accounting only for the biomass of the parasites themselves, they will only account for between 0.2%-1.2% of the total biomass. However, some parasites are able to castrate their host (often snails), effectively turning the biomass of the host into that of the parasite. This calculation is valid as the parasite effectively turns the biomass of these hosts entirely into a means for reproductive of the parasite. This staggering amount of biomass attributed to parasites suggests that parasites must be included in assessments of ecosystems, or else you may be missing a big part of the overall picture.
Another set of researchers (Lafferty et al, 2008) have furthered explored the inclusion of parasites in ecological assessments (specifically looking at the inclusion of parasites in food webs). It is theorized that parasites might affect food-web stability, interaction strength as well as energy flow in ecosystems. However the complexities of adding parasites to ecosystem are numerous. In a typical food-web predators consume prey, in a pyramid like fashion. The prey (producers) are most prevalent (bottom of the pyramid), while top predators (consumers) eat the prey beneath them and are the least prevalent (top of the pyramid).
Parasites throw this relationship off, as they are capable of parasitizing all levels of the pyramid. In essence, parasites can even feed on the ‘top predators’ (tertiary consumers) making them very unique in that regard. The same parasite may be placed at both the top of the food pyramid as well as the bottom of the pyramid depending on which stage of its life-cycle it is at. A lot of parasites have complex lifecycles, going through intermediate hosts, before arriving at its final host, feeding on all levels of the pyramid along the way. In addition, certain parasites may act as food source for some animals (instead of parasitizing them), further adding complexity. A consumer who is feeding on something else often consumes these parasites inadvertently. It is these types of interactions that alter our current understandings of food web dynamics, and predator-prey relationships. Typically, when one aspect of a food web is altered there is a distinct and often obvious result. For example, the removal of a prey species, means less food for the predator, and subsequently a decrease in the abundance of the predator species. However, a decrease or absence of a parasite species can have a wide range of ramifications, making the outcomes far harder to predict.
The first figure (a) shows interactions between predator and prey with out parasites (the arrows indicate what is eaten by what, eg. B is eaten by G1). We can see in (b) that the addition of 2 parasite species (P [with Adult (A), L1 and L2 life-stages] and HP [a hyperparasite that can ‘consume’ another parasite]) will add a lot more complexity to the interactions observed. (Link)
Research is now starting to focus on the exact effects these parasites have or will have on particular ecosystems. This is often accomplished through the use of complex algorithms to approximate their impact. Scientists want to study these kinds of interactions so that we can predict how these ecosystems will change in the future. With the topic of climate change becoming increasingly important in recent years, our assessment of how it will affect ecosystems will be highly inaccurate unless we take the role of parasites into account.
Another set of researchers (Dobson et al, 2008) have taken these ideas another step further, by suggesting that up to 75% of all links in food webs involve parasitic species. In addition is theorized that ~40% of all known species on the globe are considered parasitic (this number may actually be higher due to the presence of ‘cryptic species’ that closely resemble known species). The researchers refer to parasites as the hidden ‘dark matter’ of food webs that hold the structure of the food web together. This can be viewed visually below:
The red balls represent non-parasitic species, while the yellow balls represent the parasitic species in a food-web. We can observe the food web links between the various species visually. Parasitic species seem to play a role in a significant amount of the observed links, as well they seem to interact with all levels of the food web. (Link)
Parasites can play a crucial role in maintaining certain animal populations. For example some parasitized hosts have their behavior modified to make them more likely to be preyed upon, which can obviously alter food-web dynamics. As a result of parasites playing such role in ecosystem interactions this raises the question of their importance. As mentioned earlier we typically see parasites and think of them as something that needs to be eradicated. However, given their prevalence, the removal of a parasitic species could have devastating effect on ecosystem balance. Ecosystem dynamics as we know them could fall apart! Okay, that may be taking it too far, but it would most definitely significantly change the interactions taking place. Current research is indicating that we are entering a period of mass extinction of particular parasite species. This loss of parasites is often associated with a loss or decline of the host species that the parasite predominately infects.
With the loss of these parasitic species, we are losing a lot of the links in food webs, but it may also lead to the loss of certain ‘Ecosystem Services’ that the parasites provide. Aside from the role that parasites play in food-webs and ecosystem dynamics, they also fulfill unique niches that cannot be filled by other species. As mentioned, parasites can act as regulators of host abundance throughout the host community. However, parasites (particularly parasitic worms) can play a major role in buffering levels of pollution in the environment. This is accomplished by the parasites absorbing heavy metals and other pollutants from the guts of their hosts. Parasitic worms can survive heavy metals levels that are 2000% above background levels. Without parasites ‘soaking up’ these toxins, most of them would become concentrated in top predators, the effects of which could be devastating. It is estimated that parasites can super-concentrate pollutants soaking up 30-50% of the mass of the pollutants in an ecosystem! This idea relates back to the ‘hygiene hypothesis’, which is an explanation for why we see higher levels of allergies and sensitivities in human populations that have successfully reduced their parasite burdens. It is theorized that the parasites can absorb the allergens and pollutants before they can cause an immune response. This definitely seems like an interesting hypothesis! While it is most likely a consensus that the con’s of having a parasitic infection far outweighs the benefits, it is interesting that they may have this type of impact, and in some cases actually be beneficial.
Given this information we can observe that parasites seem to play a vital role in food-webs and ecosystems, and can provide irreplaceable ‘ecosystem services’. While often seen as undesirable, parasites seem necessary to maintain the delicate balance of the planets ecosystems. A lot of the problems we see with parasitic infections (especially in livestock animals) are most likely due to human interference, throwing off the natural balance between parasite and host. (For example herding and confining animals together, can lead to increased parasite burdens that would not otherwise be seen ‘in the wild’.) Without human manipulation, parasites may remain as hidden ‘dark matter’, that hold food web dynamics together without causing much harm. As we continue to develop a deeper understanding of the structure of food webs and resulting dynamics, it seems increasingly likely that parasites play a major and crucial role. I think we can all agree that these parasites are well worth trying to preserve, rather than eradicate.