Invaders and Recluses

Do these defenses even work?

Nature, it has been said, is red in tooth and claw. An endless struggle. When two species go at it for long enough, they even begin to adapt to each other. Each new weapon prompts a new counter-weapon in your enemy. This constant escalation, as illustrated in this Futurama gif, is called a coevolutionary arms race. The term originated back when the USA and the USSR were busy building missiles and anti-missiles, and it was easily to see evolution in the same light. But it’s still just a metaphor, and perhaps not the best one. After all, the goal of a nuclear arms race is Mutual Assured Destruction: peace because the consequences of war would be unthinkable to either side. An analogous situation might exist in a face-off between two heavily-armored members of the same species, but interspecies interactions are rarely so symmetrical. They are less like The Butter Battle Book and more like Green Eggs and Ham: a persistent and dynamic chase. That matters, because sometimes one of the species is us. Humans often seek to control nature: to kill germs, to eliminate weeds and pests, to stop vectors from spreading disease. If we’re going to get our teeth and claws bloody, we should at least know what kind of fight we’re joining.

Species with antagonistic relationships might be prey and predator, herb and herbivore, or host and parasite. In such pairings, the first party just wants to be left alone. The second wants intimate access. A reclusive isolationist has two basic options. First, it could bolster its defenses. That is, to pick a metaphor with no particular relevance to current events, it could build a wall. But battlements are expensive and often get in the way of everyday non-defensive activities. Porcupines must endure cumbersome spines. Wild parsnips churn out blister-inducing juices using energy they would otherwise apply to seed production. Snails kill parasitic flukes with harsh oxidizing chemicals that can damage their own tissues. A second strategy is to become sophisticated at recognizing your enemy, so you can just pull out the big guns when they’re really needed. As another questionably relevant metaphor, you can use profiling. Organisms put a lot of effort into recognizing adversaries, from eyes and ears down to the molecular level. But though surveillance is cheap, it’s easily evaded. Appearances, as it turns out, say little about threat. We see some of the fastest known evolutionary rates for genes involved in recognition of invaders, from the MHC of vertebrates to the R genes of plants. That’s because it’s easy to change the proteins being detected, and the detectors must constantly change themselves keep up. One of the snail genes I study comes in three flavors. Only one can detect the parasite strain we have in our lab, but it doesn’t replace the other two in the wild. Presumably the other gene variants recognize other parasite strains, which keep alternating depending on which snail genotypes are prevalent. There is no permanent victory.

So neither walls nor profiling, even when used in combination, seem to work all that well. Hungry fishers go for a porcupine’s soft underbelly. Webworm caterpillars drink parsnip juice all day with nary a blister thank to detoxifying enzymes. Flukes have been infecting snails since the days of Pangaea. Of course, if a recluse has ever won outright, we wouldn’t know about it, since the invader would be long gone. Conversely, a universally successful killer would eliminate the target upon which it depends. But I suspect such conclusive finales have been rare, given the immense diversity of life and its thick snarls of interspecies dependencies. I’ve never met a perfectly reclusive recluse.

Yet these feuds don’t end in armageddon, as the “arms race” metaphor might imply. Invader and recluse inevitably reach an uneasy compromise. A coexistence that isn’t ideal for anyone. Take the case of newts and snakes. Rough-skinned newts here on the west coast contain tetrodotoxin, the nerve decimator famous for making pufferfish such a dangerous meal. The daintiest bite of flesh from the right newt would kill you. Newts have become so poisonous because garter snakes keep evolving increased resistance to the toxin. In some places the snakes are now so immune that newts could not possibly produce enough brew to be lethal. Yet serpentine gastronomy hasn’t made newts endangered. Newt populations absorb the losses, secure that no other carnivore will touch them. Meanwhile, processing the toxin makes the snakes sluggish and vulnerable themselves. As they lie there in recovery watching herons or raccoons gobble up harmless fish and frogs, they may have mixed feelings about their own choices. In the end, some invader is always going to get in. But the recluses deal with it.

How do the recluses hold on? There are several factors that maintain a somewhat stable equilibrium between pursuer and pursued, and they all involve diversity. First, molecular diversity in the genes underlying the recluse’s defense arsenal. Immunity genes often harbor a lot of variation, which means that when one variant loses its effectiveness, another one is ready to go. Second, diversity of other recluse species. If you’re not the only option, if the invader also has other victims to go after, that takes the pressure off. Third, a diversity of potential invaders. If invaders find themselves competing for space in a tangled bank, side effects like newt-induced lethargy might not be worth the effort.

What does this all mean for us? We are the ultimate recluses. We want to live in sterile houses free from bacteria, mold, and vermin. We kill invaders, and they keep evolving resistance to our weapons. Are our efforts doomed? Well, yes and no. We do succeed sometimes. We’ve (potentially) eradicated smallpox, and we are on track to eliminate Guinea worm disease. We clearly do know how to drive species extinct. But we can’t plan on always winning. Invasive life finds a way. We can minimize its victories, though, and even the most hardened superbug can be managed so it doesn’t cause a pandemic. Keep changing its environment with different combinations of drugs. Give it benign places to grow where the cost of antibiotic-resistance outweighs the benefit. Surround it with a rich ecosystem of competitor microbes that don’t waste energy on resistance mechanisms. Do all that wisely, and it will remain rare. Similarly, we can engineer snails to be fluke-proof, but we can’t think of them as unbeatable. They are useful tools for disease management in combination with other tactics, even when some worms do find a way in.

This post is about science. It’s not a political allegory. But it’s hard not to see political lessons here, whether you are talking about transnational alliances or immigration policy. I want to emphasize that we can’t just derive ethical precepts from the behavior of non-human organisms. Human society need not be red in booth and law. We can be better. Still, I might indulge in speculating that isolationism is futile. Sealed borders are destined to fail. Accept that you share the world with others, and that you are going to influence each other. Embrace your existence as part of a diverse community. Indeed, acknowledge that your very survival depends on that diversity. Anything else is just delusional.

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