One thing I’ve learned over the past 10+ years of being in wildlife science education and research, things are WAY more complicated than they appear at face value. I suppose most things in life are that way.
Lyme’s disease is a major concern to human health and well being, and millions upon millions of dollars have been spent to understand its epidemiology and cycling patterns in nature.
There’s a lot going on this figure, but for the sake of this post, let the emphasis rest on the 2 o’clock to 4 o’clock portion of the cycle. Tick larvae emerge from tick eggs and need blood. Deer mice are a destination food source for the tick larvae, and deer mice are the main reservoir for the Lyme-disease causing bacteria.
Put simply, tick eggs hatch into hungry larvae which seek a blood-pumping host. If that host turns out to be a deer mouse, there is a good chance that the tick larvae will contract the Lyme’s disease bacteria.
There is a great example of ecosystem and trophic level interconnectedness in the next statement: In fragmented (patchy) habitats, humans are 5 times more likely to contract Lyme’s disease than comparable landscapes with more contiguous habitat.
Let’s explore this further…
Some animal species are adapted to living in urbanizing landscapes characterized by more fragmented habitats, some are not.
For the different patches of forest in the preceding aerial image, mesocarnivores (e.g., foxes, raccoons, coyotes) and birds-of-prey – both predators of deer mice – are more likely to be found in larger habitat patches. Most deer mouse competitors (e.g., other mouse-sized rodents) are also more likely to be found in more contiguous blocks (bigger patches) of habitat.
Deer mice, enjoying an absence of predators and competitors, are found in extremely high densities within small patches of habitat.
Picture these 3 species groups – deer mice, predators, and competitors – interacting in the following graph.
Back to the hungry tick larvae and let’s pretend we’re following “Joe” on his mission to find blood.
In a landscape characterized by larger habitat patches, “Joe” is seeking blood and could really care less about the supplier of that blood. “Joe” is likely to find himself fastened to any number of species – a fox, a coyote, a vole, a mole, a deer mouse, a chipmunk, a squirrel. Let’s pretend the probability that Joe finds himself attached to a deer mouse is 5%.
Switch over to a fragmented landscape, “Joe” is still hungry for blood but now the pool of critters wherewith to find blood is noticeably less diverse. In fact, 50% of the critters with four legs are deer mice. Now “Joe” has a 50% probability of finding himself attached to a deer mouse.
Remember that deer mice are the primary source by which a growing tick becomes a carrier of Lyme’s disease…
So long story short – deer mice are a greater proportion of the available blood donors within fragmented forest patches…ticks are more likely to latch on to deer mice in those habitats…ticks are more likely to contract the Lyme’s disease bacteria in fragmented forest patches because they are more frequently in contact with the primary reservoir…people are more likely to contract Lyme’s disease from tick bites suffered from within more fragmented forest patches.
It’s a complicated world we live in.