Why do invading hosts leave their parasites behind?

Earlier this week we visited Mark Torchin at the Naos research station where he delivered an excellent talk on the importance of parasites to ecosystem processes. A portion of his talk focused on the “Enemy Release Hypothesis”, a fundamental theory in invasive species ecology that has generated over 1800 publications and greater than 40,000 citations.

The Enemy Release Hypothesis posits that as non-native species enter new habitats they leave their enemies behind (Keane & Crawley 2002). In their natural habitats these species’ enemies (predators, parasites, and pathogens) typically regulate their populations, allowing for coexistence with their natural competitors (via “apparent competition”). Without these enemies, however, introduced species lose that form of regulation and can then outcompete the native species.

At the end of Mark’s talk, Patrick Jansen raised a valuable question about this theory:

Why aren’t enemies introduced with the invaders? 
Why are enemies “left behind”?

When species are introduced they tend not to take their predators with them for obvious enough reasons. But parasites that live on or within their hosts can easily make the journey because of their tight physical connection. So why aren’t all of a host’s parasites introduced with the host? There are many factors that may prevent parasite introduction, and one of the simplest is the life cycle of the parasite itself.

As parasite trophic strategies vary, so do their life cycles and their modes of transmission. Some parasites have one-host life cycles and are transmitted horizontally, meaning they only require one host species to reproduce and are transmitted from one individual to another. This is the dominant strategy of the feather lice that infect birds, as well as the head lice that infect us.
Head Lice Life Cycle

One-host parasites can also be vertically transmitted from parent to offspring. Indeed, there are some parasites of humans that utilize the breast milk pathway in order to move from mother to child. In addition to horizontal and vertical transmission, there is transmission via autoinfection wherein the host can reinfect itself. A classic example of autoinfection is the human pinworm. If a person (often a child) consumes a pin worm egg, the adult pin worm becomes gravid with eggs as she moves toward the rectum. At night while the person is sleeping, the pin worm exits the anus and lays eggs around it that are encased in a substance that causes intense itching. When the person scratches the area, those eggs are easily moved to the fingers where they can then reinfect the person or any person they may come into contact with.



One-host parasites can probably follow invaders efficiently, given that the introduced population contains enough infected individuals to establish. Pinworms, for instance, are thought to have followed humans during their migration into the new world (Araujo et al. 2008)

If we go up a level in complexity, we find parasites that spend a portion of their lives on their hosts, and a portion of their lives in the environment. This is the case for the human hookworm which spends part of its life molting and maturing in the soil. Humans acquire infections by walking barefoot in high hookworm areas- the hookworms can penetrate the foot tissue and then migrate through the body. Because of their dependence on environmental factors, abiotic filtering may prevent establishment of parasites with this form of life cycle.


And then we get to the parasites with multi-host life cycles, like the trematodes we examined earlier this week. Complex life cycle parasites have 2 or more hosts that are critical for completion of their life cycles. If, during an introduction, any of these hosts are missing, the parasite will not be able to establish. Parasites with multi-host life cycles may vary in how generalist or specialist they are, but typically have at least one life history stage where they are highly host-specific. The composition of the invaded community and the parasite’s degree of specificity will then determine in part whether or not the parasite can be introduced.


So is the capacity for a parasite to invade correlated with the simplicity of of its life cycle? This is one idea in the field, but there are certainly additional factors that may play important roles, including how the invasion occurs, the size of the invading population, the prevalence of the parasite, and its pathogenicity.



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