Today I am going to be talking about a very controversial topic, or at least a controversial topic in the field of ecology: the dilution effect (I know, it’s not as familiar as gun rights). The dilution effect is a fairly new theory in the field that involves exploring the relationship between population dynamics and disease patterns. It was initially used to explain the relationship between ticks and white-footed mice and the transmission of Lyme’s disease, but now it is being applied to a variety of systems. In fact, I just heard a talk by Dr. Julia Buck at the American Parasitology conference that discussed parasite infections in sunfish.
Here’s the basics of the theory: high host diversity dilutes the impact of pathogens on a host and lowers disease risk for a species overall. My undergraduate parasitology professor, the brilliant Dr. Valerie McKenzie, explained it best. Imagine there is a forest that contains two white-footed mice (Mouses? Meeces? Mices?), and ten deer ticks, all of which are infected with the bacteria that causes Lyme’s disease (Borrelia burgdorferi). Ticks gain necessary nutrients for reproduction and growth from multiple blood meals from a mammal. While this blood meal is occurring the bacteria can be transferred from the tick to the mammal causing the mammal to then become infected. In our imaginary forest these ten ticks only have two hosts to choose from for their blood meals so each mouse is going to have to be the host for around five ticks. Having five ticks take a blood meal from a 20g (.05 lb) organism is probably going to cause pathology in the host (anemia, organ failure, and even death) and having five transmission events will most likely lead to the mouse contracting Lyme’s disease. Now, if the forest had ten white footed mice, or ten different species (a deer, a badger, etc.), in it the tick load of each host would be much lower and the pathology would be much lesser.
Okay, so mice and ticks are a little hard to get excited about if you aren’t into science, but you know what’s not boring? Vampires. Let’s imagine you live in the small Southern town of Hellsville (because everyone knows Vampires love those) with a population of 100 people. It also turns out that these 100 people really like to hunt and loathe rabbits eating their gardens so the wildlife population of Hellsville is extremely low. Little do they know that their small town is home to 50 vampires (the vampires just love southern home cooking and the small town charm). With no deer or other mammals to prey on the vampires must begin to feed on the people of Hellsville daily. Some of the 100 people are tastier than others (I suppose vampires prefer O- blood types) so their ‘vampire load’ (same as pathogen load in this case, sorry vamps) are especially high. So in this town you have some people that are completely vampire free (mostly the garlic enthusiasts), some that are being fed on only once or twice, and those that are being fed on by multiple vampires multiple times a day. The people with high vampire loads are most likely going to die, sending the already small population of Hellsville crashing. When those individuals are gone the moderately fed on individuals will then become the focus of the vampire’s attention and eventually the population will decrease even more. Despite the garlic blood being disgusting to the vampires, in order to survive, they will begin to feed off the last human survivors as well. Thus the town of Hellsville is now home only to hungry vampires that will most likely move on to the next town.
So what did Hellsville do wrong? Why are some towns able to sustain a human population and a vampire population without large population crashes of either? The answer lies in the number and variety of hosts available for the pathogen (vampires). If Hellsville had a high deer/rabbit/squirrel population perhaps the vampires would be able to subsidize some human meals with animal meals, thus allowing their human hosts time to recover from blood loss and survive another day to be meals. Also, if Hellsville had a higher population of people in their town that could have also helped them. That way each individual could have only had one or two vampires feeding off of them, thus lowering their vampire load and spreading the vampires across the whole population.