What bike crashes and tarantulas have in common

Here I am prepping for a night lap in a 24 hour race in February with my great big headlight. It's important to be a responsible mountain biker: stay on trails, don't ride them if they're wet and you will destroy them, brake for wildlife. Biking can have a negative impact on the environment, though nowhere near the impact motorized vehicles have off-road.

I recently went out into the desert after dark and bashed myself against the rocks for a few hours. By this I mean I went on a mountain bike ride. I had a good light; the problem really lay in my not having ridden on trails for six months and being off balance. I failed to hit any saguaro or other types of cactus, so I considered the night a major win.

Over the course of the ride, my riding companions and I stopped several times. Once was to observe from a safe distance the Mojave rattlesnake coiled at the side of the trail. Another stop or two were to observe male tarantulas crossing our path. I assume they were male because the males crawl out from their holes in force during late summer to wander in search of a mate. The third and longest break was to fix a broken bike chain. As I watched the others struggle with the chain tool that was produced, I tried and failed to discern what they were doing with it. Much like the tarantula with a nice deep hole and plenty of prey, if my chain were to break when I was out alone, having a chain tool would be insufficient to succeed in life. Just as he must find one more thing – a mate – in order to pass on his genes, were I to find myself with a broken chain and a tool to fix it, I would still need to hunt down someone who knew how to use the tool.

Okay, so that’s a bad analogy. But at least give me this: both of us are stranded in the Arizona desert with little to no water, and if we stay stranded alone for too long it can be bad for what biologists call “fitness” (and everyone else calls “the chance to have babies”). And both of us need more than one scarce resource to get de-stranded. In fact, if you break it down, we both need many resources over the course of our lives to survive and pass on our genes.

Pierson and Turner (1998) showed saguaro cactus populations grown in spurts when conditions are good. "Good" for a saguaro mean rain, temperatures above freezing, plenty of seeds landing under other plants ("nurse plants"). You can think of conditions being "not bad," meaning no freezing or drought, but that implies some kind of maximum that might not be realistic.

Tarantulas, people, and other animals all need shelter, need water, need energy and need nutrients. The iconic saguaro cacti I rode past last night need water and need nutrients, and when they are little baby cacti, they need shelter, too, from another plant (a nurse plant). Basically, plants and animals almost always compete for more than one resource. Elizabeth Pierson and Ray Turner found in a 1998 paper in the journal Ecology that saguaro populations have massive numbers of seedlings surviving in wet years without bad freezes. Some ecologists, like David Tilman, have looked at coexistence of several species as results of their limiting factors (Tilman 1990 in Oikos), where limiting factors might be water, or nurse plants. It’s sort of analogous to cooking: you have enough of everything except flour to make a full batch of cookies. So you have to limit yourself to a half batch.

However, the number of environmental factors (like temperature or humidity) that an animal or plant experiences, and the importance of each one, can make one daunting laundry list. That is why I prefer the approach Peter Chesson (as in his Annual Review of Ecology and Systematics article in 2000) takes in describing the “environmental response” of a plant or animal without specifying the environment necessarily. His models deal with the way growth rate changes based on the environment with some probability of a “favorable” year, whatever that may be, coming up so often, like your odds on a slot machine.

In the meantime, I think I will learn to repair a chain. Fun as slot machines may be, I’d rather have greater certainty in my ability to return from a bike ride than the gamble I can find that other individual out in the desert who can help me.

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