Someone Is Happy about Population Decline

By now, most of you are aware that the loon population in northern Wisconsin is falling. Since my last report on this topic, we have made two separate formal calculations of λ (“lambda”), which estimates the number of adults in the population in year 2 divided by the number in year 1. Lambda is convenient and intuitive; if λ equals one, there are as many loons in the population this year as there were last year, and we are okay. λ greater than one tells us that the population is growing; λ less than one tells us that it is in decline. Our two separate calculations generated λ values of 0.96 and 0.94, which indicate that the loon population in Oneida County is currently falling at a rate of 4% to 6% per year. The picture is somewhat worse, it seems, than we had thought a few months ago.

This rate of decline — if it is correct, and if it persists — is grave news for humans who love loons. If these numbers are accurate, we will notice the effects of the decline within the next several years. Territory vacancies will go unfilled. Pair members that lose their mates will struggle to re-pair with new ones. Still fewer surviving young will fledge than do now. And our loons will have entered the dizzying downward vortex of a dwindling population.

In the short term, though, one cohort of the loon population benefits from falling floater numbers. The sharp downturn in floater abundance has territorial pairs breathing a sigh of relief. For breeding males and females, you see, fewer intruders — fewer scenes like that depicted in Linda Grenzer’s photo above — means fewer young upstarts seeking to evict them from their territories and a higher rate of territory tenure. How much better off are breeders? As the plot below shows, they are a good deal better off.

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The threat of being evicted from a territory in a given year is now only 1/4 to 1/5 what it was only two decades ago. Small and/or old loons that were lucky to hold a good territory for a year or two in 1998 can kick back and relax nowadays, because the eviction rate is trivial. The decreasing floater population is making the prospect of lifelong breeding on a single territory look like a reasonable expectation for both sexes. People who have become familiar with the breeding pair on their lake might feel better off in the short term. They can be much more confident now that the birds they greet each April are the same two from the previous year.

Though I feel that I know several dozen of my study animals reasonably well and look forward to seeing them each spring, I cannot celebrate the fact that I now stand an even better chance than before of doing so. To me, the dynamism of the system — the likelihood that a breeding female or male might have to accept eviction, lick its wounds, and find a new territory with a new mate nearby — was part of its beauty. Knowing what I do now, each reunion with a familiar breeder for me will be a reminder of the new normal: unnaturally long breeding tenure made possible by the drastic decrease in territorial challengers.