Word of the day: irrupt

Today’s word is irrupt. It’s not just an alternative spelling of “erupt”; it has a specific meaning in ecology. Merriam-Webster’s 11 team defines it as follows:

ir•rupt vi [L irruptus, pp. of irrumpere, fr. in– + rumpere to break — more at REAVE] of a natural population : to undergo a sudden upsurge in numbers esp. when the natural ecological balances and checks are disturbed.

While the etymology seems sound (to break into, basically), this is an unsatisfying definition on several levels. For one, it implies that the word applies especially or more strongly when there has been an ecological disturbance, presumably caused by man (notice the “natural” in the mention of ecological balances; would a disturbance like a hurricane count? Or would it take something like the disaster that created Yellowstone to disturb the checks and balances enough to cause population irruptions?).

I don’t like tying the meaning of this word to something manmade, like the DDT contamination that caused many populations of predatory birds to crash in the 1960s and 1970s. After all, many northern bird species demonstrate irruptive winter movements that, as far as we know, have nothing to do with human behavior, as atrocious as that behavior sometimes is for the environment.

To imply that these irregular and abundant population shifts are caused by ecological disturbance posits that said disturbance is somehow unnatural. For instance, it has been well documented that Snowy Owls winter farther south in years when the vole population in the north crashes. These crashes in turn seem to coincide with a fairly regular failure of the mast crop that sustains their populations. To say that this is a disturbance in ecological balances and checks is disturbing. An irruption of a species may be attributed to non-natural causes, but to write it into the definition is somewhat disappointing.

The American Heritage team takes a simpler tack in their definition, but their example also implies some hand-of-man work going on in the background

To increase rapidly and irregularly in number: In the absence of predators, the island’s rodent population irrupted

In their example, the predators seem to have somehow been removed from the equation; therefore the subsequent increase in rodent populations is unnatural. Either this is just a bad example of the word, or something is wrong with ecological science.

After all, if there had been rodents on the island to begin with (as implied by the phrase “the island’s rodent population”), any increase in their numbers could not be an irruption, because the population would already have attained an equilibrium state: there would be no room for an irruption, just a slight increase or decrease in a relatively stable number. On the other hand, if there were no rodents on the island to begin with, and then they were accidentally introduced to an island where they had no predators, then yes, any subsequent population explosion would indeed be an irruption. And that, presumably, would be more likely to be caused by human behavior than by “rafting” from the mainland, as happens on Trinidad and Tobago when the Oronoco is in flood.

So, OK, I’m willing to cut both dictionary teams some slack, as long as they admit that there’s a lot of context missing from their examples…

With all of that dictionary-searching, I almost lost sight of the reason I was interested in this word: we were hoping for an irruption of American Robins on our Christmas Bird Count last weekend, but we didn’t get it. Very few were seen by any of the parties; I had none myself. Oh, well.