Ecology is the study of interactions among organisms and their environment. Two particularly interesting interactions occur between Predators and Preys and a slightly different between Hosts and Parasites.
Predator-prey interactions are a typical study topic for ecological and evolutionary dynamics. It’s quite inuitive for us to understand that predator and prey exert a selective pressure on one another. For example if the prey can run fast, is toxic or can hide very well, the predator should evolve higher speed, higher resistance or very good eyesight. And the evolution of the predator will induce improving on prey’s traits. This is called coevolution: evolution of both driven by each species on one another. Thus, predators want to evolve an extreme advantage to always get a prey, while prey want to escape all the time. This is referred to an “arm race” between the two species and it means that they are running a race where each species want to surpass the other.
Host-parasites interactions are really similar. A parasite can be defines as an organism that grows, feeds and is sheltered on or in a different organism while contributing nothing to the survival of its host. Most of the time the parasite will actually cause damage or diseases to the host. So in this coevolution hosts want to have the best defence to impede the parasites from getting in. While the parasite wants to evolve a better attack mechanism to infect the host. It’s interesting to notice that most parasites need to infect the host to reproduce otherwise they will just die.
There is a subtle difference between predator-prey and host-parasite dynamics. In the former prey and predators would like to escape the arm race. In the sense that a predator which is super fast will always eat every prey and always get a benefit. The same for super eyesight or super resistant. Eating the prey is always convenient and will bring the predator advantages that will help it reproducing more and thus increasing the frequency of the “good” genes in the future generations. The same holds for the prey. The prey always want to escape, so it would like to become super fast, super toxic or super mimetic. Predators and preys want to push their traits to an extreme to beat the other.
Why is that different in host-parasite interactions? While the same is true for the host, this is often not true for the parasite. Let me explain. The host always wants to escape infection, so it really wants to escape the arm race by developing super defenses and thus being immune to infections. This would not give the host any disadvantages (in theory), but only advantage like the predator eating all the preys and the prey escaping all the predators.
On the other hand, the parasites can’t always be super aggressive. If it always wants to develop a good way to infect the host, it can’t increase its virulence at the maximum. Think about it. An infected host usually is diseased and weakened. Potentially the infected host will die after some time. So if the parasite wants to reproduce it can’t kill its host immediately because it wouldn’t have the time to spread to other hosts. And if the parasites is confined on one host it will just die in that host (even if it managed to infect it). The goal of a parasite is to infect the host and then spread to other hosts.
In summary, the virulence of a parasite should depend on its capacity of dispersal: if it can spread from host to host very easily than it can develop a very high virulence/toxicity and kill the host immediately. But if it can’t spread easily then it should allow the host to survive and be in a relatively good shape so that it could help the parasite to spread. For example, if you can count on a mosquito to deliver you around from host to host, then you can be pretty aggressive and kill everything. If you need your host to transmit you through sex for example, you need to leave your host almost healthy for him to do the job.
Now let me say this: We were mutualists with Earth. Mutualists are in a interaction where both members get benefits. In the past this was true. We, as any other species on this planet would strive on our home and give everything back becoming part of the natural life-cycle. Now we became parasites. We are literally an organism that grows, feeds and is sheltered on or in a different organism while contributing nothing to the survival of its host; where the organism we’re infecting is Earth.
Now, according to this ecology theory if a parasite has difficulties in spreading to new hosts, it should guarantee a good survival to its host. Think about it. We are parasites of the only host we can have: Earth. We have immense difficulties in finding another host. The chances that we could spread to new hosts are so low that we should let our host very healthy and alive. We should be almost neutral, because if we kill our host we kill ourselves. This is NOT what we are doing. Nature always find its balance between host’s survival and parasite’s virulence. If a parasite don’t follow that rule there’s only one destiny: extinction.
The only solution is to go back from parasites to mutualists again. Then we will benefit again from this host, and we’ll keep living successfully according to ecological nature.
Help us become less parasites. Don’t let us go extinct. Let me tell you another story…
P.S I’m aware that Earth is not necessarily an organism. But I want to point out that our planet is as complex as any organism can be and many expert described it as Gaia, “the superorganism”. Thus, I think the comparison can be made. Even though you think this is not appropriate, then for the sake of the argument we can assume that this ecological model can be applied to any host-parasite-like interaction, in which the human-Earth category would fall, once we accept the parasitic comparison of humans’ lifestyle.