2024, the centenary of the publication of Charles Elton’s traditional paper, Periodic fluctuations within the numbers of animals: their causes and results, in Journal of Experimental Biology, additionally noticed a flurry of curiosity on the identical matter and the identical mammals that occupied a big a part of Elton’s evaluate, lemmings.
 |
| Norway Lemming, Lemmus lemmus Kevo Strict Nature Reserve, Finland, 2011 by Lakahillo (from Wikipedia) |
The spectacular periodic fluctuations in lemming numbers—every cycle lasting 3-4 years—described by Elton from proof gathered from across the Arctic have excited renewed curiosity as a result of there had been options that the massive cyclical fluctuations in abundance are being worn out by current local weather change, with penalties for the populations of predators and the entire ‘ecosystem’. Nonetheless, twenty-four authors from Canada, France, Sweden, Finland, Norway, Russia, USA, Germany and Denmark analysed 24 units of current information gathered throughout the Arctic that recorded inhabitants adjustments over time.
That is the authors’ abstract:
Reviews of fading vole and lemming inhabitants cycles and persisting low populations in some elements of the Arctic have raised issues in regards to the unfold of those elementary adjustments to tundra meals internet dynamics. By compiling 24 distinctive time collection of lemming inhabitants fluctuations throughout the circumpolar area, we present that just about all populations displayed alternating intervals of cyclic/non-cyclic fluctuations over the previous 4 many years. Cyclic patterns have been detected 55% of the time (n = 649 years pooled throughout websites) with a median periodicity of three.7 years, and non-cyclic intervals weren’t extra frequent in recent times. General, there was a sign for a unfavorable impact of heat spells occurring throughout the snow onset interval of the previous 12 months on lemming abundance. Nonetheless, winter length or early winter weather conditions didn’t differ on common between cyclic and non-cyclic intervals. Evaluation of the time collection reveals that there’s presently no Arctic-wide collapse of lemming cycles, although cycles have been sporadic at most websites over the past many years. Though non-stationary dynamics seems a standard characteristic of lemming populations additionally prior to now, continued warming in early winter could lower the frequency of periodic irruptions with unfavorable penalties for tundra ecosystems.
Two commentaries have been printed on the paper by Charles Krebs and by Rudy Boonstra, well-known ecologists who work on inhabitants adjustments. These commentaries spotlight the truth that regardless of many years of analysis the underlying trigger of those inhabitants cycles in lemmings and different rodents stays unknown. Equally, the physiological processes concerned are additionally unknown. I’ve written beforehand on Dennis Chitty’s (1912-2010) makes an attempt to establish the reason for inhabitants cycles in voles (see right here).
What is evident is that the mathematical modelling method pioneered by Alfred Lotka in 1925 primarily based on adjustments in numbers of predators and prey over time and which have been pursued actively ever since are, as is so typically the case with mathematical fashions, insufficient. Experimental approaches to lemming and vole cycles have additionally not, normally, been profitable, with different attainable explanations of any adjustments noticed and doubt whether or not the experimental circumstances imposed are ones that really acquire within the wild.
The significance of making an attempt to know what occurs below the snow—the habitat of lemmings in winter the place they feed and breed—was additionally confused within the commentaries. These observing lemmings solely see the outcomes of that exercise after they rely the inhabitants in spring and summer time.
It’s simple to overlook when discussing the reason for cyclicity in inhabitants measurement and the adjustments in reproductive fee essential to drive such adjustments that the the incidence of ‘lemming years’ is a serious wildlife phenomenon. Elton wrote of the Norway Lemming, Lemmus lemmus:
For a few years the lemmings have periodically compelled themselves upon public consideration in Southern Norway by migrating down in swarms into the lowland in autumn, and in lots of instances marching with nice velocity and dedication into the ocean, in trying to swim throughout which they perish. The small print of the destiny of the migrants don’t concern us right here and are absolutely described by [R] Collett [1911-1912]….Lemming-years in Norway have the standing of nice floods or horrible winters.
and of lemmings in northern Scandinavia:
It’s apparent that the phenomenon of migration is much extra hanging than a mere improve within the numbers. The spectacle of processions of lemmings ecstatically throwing themselves over the ends of railway bridges, and falling to an apparently ineffective loss of life beneath; the ocean strewn with lifeless lemmings like leaves on the bottom after a storm; lemmings making a bee-line throughout crowded visitors oblivious to hazard; all this stuff are certain to make folks speak.
Is was descriptions like this that gave rise to the favored notion generated and churned by the information media that lemmings commit suicide fairly than merely die en masse when presumably transferring in giant numbers searching for meals (Elton wrote not of meals however of ‘relieving congestion in an space’). The Walt Disney movie White Wilderness (1958) confirmed the form of scenes described by Elton and others, with lemmings tumbling over the sting of a cliff. Many years after I noticed the movie at a gathering of the college pure historical past society in 1960 it emerged that captured lemmings had been pushed over the sting of synthetic rockwork. The footage could have been faux and the lemmings poorly handled however movie definitely conveyed what occurs within the wild.
There are questions in my thoughts on this associated phenomenon of migration, just like the distances concerned and whether or not they’re performing like a plague of locusts, stripping one space of meals earlier than having to maneuver on, provided that their physique measurement and elevated exercise would recommend they might starve to loss of life inside every week with out meals. Nonetheless, it’s the response to overcrowding view that dominated analysis on this space for many years. However that’s one other story.
To conclude, 100 years after Elton, the trigger(s) and mechanism(s) of inhabitants cycles in lemmings—and voles and so forth—stay unsolved. However analysis exercise and monitoring continues. We are able to solely hope for progress.
– – – – – – – – – – – – – – – – – – – – –
As an afternote I discovered studying Elton’s paper from 1924 effectively price whereas. Not solely did he take into account mechanisms that will account for the phenomenon in lemmings however he additionally reviewed the incidence of periodic booms in inhabitants of different rodents in some years. Within the backyard we had a big inhabitants of Financial institution Voles within the summers of 2018 and 2019, however not within the years earlier than or since. Are the identical or completely different mechanisms at play in numerous species is one other abiding query.
Boonstra R. 2024. Inhabitants regulation and limitation—insights from lemming cycles: previous, current and future. Proceedings of the Royal Society B 291: 20240660. doi.org/10.1098/rspb.2024.0660
Elton CS. 1924. Periodic fluctuations within the numbers of animals: their causes and results. Journal of Experimental Biology 2, 119–163. doi.org/10.1242/jeb.2.1.119
Gauthier G, Ehrich D, Belke-Brea M, Domine F, Alisauskas R, Clark Okay, Ecke F, Eide NE, Framstad E, Frandsen J, Gilg O, Henttonen H, Hörnfeldt B, Kataev GD, Menyushina IE, Oksanen L, Oksanen T, Olofsson J, Samelius G, Sittler B, Smith PA, Sokolov AA, Sokolova NA, Schmidt NM. 2024. Taking the beat of the Arctic: are lemming inhabitants cycles altering attributable to winter local weather? Proceedings of the Royal Society B 291: 20232361. doi.org/10.1098/rspb.2023.2361
Krebs CJ. 2024. Lemming inhabitants fluctuations across the Arctic. Proceedings of the Royal Society B 291: 20240399. doi: 10.1098/rspb.2024.0399