The Evolutionary Trap That Wiped Out Thousands Of Butterflies
Changing land use in an isolated Nevada meadow has driven the extinction — and subsequent recolonization — of a local population of checkerspot butterflies
As humans have spread across the planet, we have earned our label, “ubiquitous keystone pest,” in recognition of our dramatic, diverse and far-reaching influences upon the survival and evolution of other groups of organisms that we come into contact with (ref). Despite these mostly negative effects, quite a few wild species have adapted to the habitats that we’ve created along the way, and they manage — somehow — to co-exist alongside us. As an example, according to a study published today in the journal, Nature, evolution of a large, isolated checkerspot butterfly population in Nevada successfully adapted the insects to cattle-grazing. However, this adaptation contained a hidden risk: when we humans abruptly changed our land use practices and removed our cattle, the butterflies suffered local extinction.
The butterflies had fallen into is an example of an “ecological trap” or “evolutionary trap”. Such a “trap” consists of an environment that is altered suddenly by human activities, so that local species are deceived into making poor habitat choices based upon formerly reliable environmental cues — even when higher quality habitat or resources are still available (ref). One such example that I’ve written about before is the population crash experienced by Australian monitor lizards after they began preying upon introduced cane toads, a highly toxic invasive species (read more).
The subject of today’s Nature study is the butterfly, Edith’s checkerspot, Euphydryas editha. Edith’s checkerspot is a handsome medium-sized butterfly species found throughout western North America. This sedentary butterfly seems to have an inherent gift for rapid adaptation to the idiosyncrasies of their local habitat: different populations show a lot of variation in measurable traits, especially in coloration, wing length and body size. This evolutionary flexibility makes them particularly vulnerable to falling into evolutionary traps.
Edith’s checkerspots readily evolve dependence upon alternative host plants
“I’ve spent 50 years studying the interconnectedness of Edith’s checkerspot with its host plants,” said the study’s co-author, behavioral ecologist Michael Singer, a professor at the University of Plymouth, in email. Decades of research has shown that different populations of Edith’s checkerspot butterflies have evolved preferences for different host plants throughout their range.
“Females choose to lay their eggs on different plants at different sites,” Professor Singer elaborated. “[E]ven when those sites support apparently identical plant communities.”
The traditional host plant where this study population of Edith’s checkerspots lay their eggs, and upon which their caterpillars (larvae) feed, grow and pupate, is Collinsia parviflora, commonly known as the maiden blue-eyed Mary. In Nevada, this native wildflower is found in semidesert scrublands and pine forests.
The butterflies lay their eggs in the spring and two weeks later, the eggs hatch. The young caterpillars have only a few weeks to feed and grow before the plant dies back for the summer. At this point, the caterpillars become dormant. The following spring, when blue-eyed Mary seeds germinate, the caterpillars wake up, gorge themselves, and spin a cocoon. A few weeks later, the adult butterflies emerge, mate and the cycle begins again.
According to Professor Singer, he and the study’s co-author, Camille Parmesan, who also is at the University of Plymouth, along with their collaborator, Chris Thomas, noted in 1982 that the population of Edith’s checkerspot living on Schneider’s Ranch in Carson City was evolving a host preference for an invasive plant, Plantago lanceolata(ref), known by a collection of common names, including the English plantain. (I refer to this plant as Plantago.)
For more than 100 years, this alpine meadow was owned by the Schneider family, who ranched cattle there. The Schneiders inadvertently set an ecological trap by introducing a novel resource, the exotic Plantago. Unlike the blue-eyed Mary, Plantago does not die back in summer, so caterpillars can feed on it longer before entering their dormant state, and thus, more of them survive to adulthood. Since a butterfly’s host plant preference is largely inherited from its mother, as this small group grew and flourished, their host plant preference trait spread quickly through the entire population living in Schneider’s Meadow. But when the Schneiders abruptly stopped cattle ranching in 2005, the novel resource was suddenly snatched away because the grass overgrew the Plantago, thereby hiding it from the butterflies, and cooling it, slowing the growth of the insects. This sudden change in land use — removal of the cattle — is how this particular evolutionary trap was sprung.
“Evolutionary athletes though these butterflies be, they can’t evolve as fast as humans can alter habitats,” Professor Singer explained in email.
By this time, the Schneider’s Meadow butterflies had abandoned Collinsia entirely: their host plant preference for Plantago was absolute, but Plantago was suddenly unavailable. Predictably, the Schneider’s Meadow checkerspots crashed and the entire population winked out between 2007–2008.
People change habitats faster than evolution can react
For at least the next four years, Edith’s checkerspot butterflies were completely absent from Schneider’s Meadow. However, when Professors Singer and Parmesan returned in 2014 to confirm the extinction of this population of butterflies, they were astonished to discover …. Edith’s checkerspot caterpillars! And these caterpillars were feeding on their original host plant, Collinsia!
Where had this now-recovering population come from? The team later discovered that the ‘Carter Springs’ forest fire in September 2012 had greatly enhanced the size and lifespan of Collinsia in the area around Simee Dimeh Summit, located 37.7 kilometers (23.4 miles) away. This was enough to support a population explosion of Edith’s checkerspots there, and this, in turn, could possibly be the source for the Collinsia-feeding butterflies that popped up in Schneider’s Meadow, despite the vast distance that this sedentary species would have to cross. Although this is a hopeful sign, this recolonization of Schneider’s Meadow has also sets the stage for a potential repeat of this anthropogenic evolutionary cycle.
The rapid evolution of Edith’s checkerspot butterflies serves as a cautionary tale for conservation efforts. It illustrates the process whereby species becomes dependent upon, say, traditional farming and land use methods, and how this dependence then renders them vulnerable to extinction when those practices are suddenly abandoned, as in this case. This story also serves as a powerful warning regarding the many, often subtle, ways that human activities affect flora and fauna, as we drive evolutionary adaptation, however unwittingly.
Changing land uses affects entire species communities, even species that are not directly exploited by humans. Although this is the first time that an evolutionary trap like this has been reported, more traps like it could already be set around the world, just waiting to go off. Edith’s checkerspot clearly illustrates the potentially lethal evolutionary traps that human activities can and do create for natural populations, and it documents the dynamic nature of this relationship.
Michael C. Singer and Camille Parmesan (2018). Lethal trap created by adaptive evolutionary response to an exotic resource, Nature, published online on 9 May 2018 ahead of print | doi:doi:10.1038/s41586–018–0074–6
Andrew P. Hendry, Kiyoko M. Gotanda, and Erik I. Svensson (2017). Human influences on evolution, and the ecological and societal consequences, Philosophical Transactions of the Royal Society B, Biological Sciences, 372(1712):0160028 | doi:10.1098/rstb.2016.0028
Martin A. Schlaepfer, Martin A. Schlaepfer, Michael C. Runge, and Paul W. Sherman (2002). Review: Ecological and evolutionary traps, TRENDS in Ecology & Evolution, 17(10):474–480 | doi:10.1016/S0169–5347(02)02580–6
Michael C. Singer, Chris D. Thomas, and Camille Parmesan (1993). Rapid human-induced evolution of insect–host associations, Nature, 366:681–683 | doi:10.1038/366681a0
Read more butterfly science:
GrrlScientist. “DNA Barcodes Reveal Two Distinct Butterflies Are Male And Female Of Same Species”, Forbes, 4 August 2017. (Medium link.)
GrrlScientist. “Fragmented Habitats Accelerate Butterfly Evolution — And Extinction”, Forbes, 25 February 2016. (Medium link.)
GrrlScientist. “Birth of the blue morphos”, The Guardian, 13 April 2015. (Medium link.)
GrrlScientist. “Why does this butterfly hiss?”, The Guardian, 3 February 2014. (Medium link.)
GrrlScientist. “Watch: A century of butterflies and moths”, The Guardian, 14 August 2012. (Medium link.)
GrrlScientist. “Watch: Weird and wonderful butterflies in the Cal Academy Collection”, The Guardian, 26 April 2012. (Medium link.)
GrrlScientist. “Watch: How to fix a broken butterfly wing”, The Guardian, 30 November 2011. (Medium link.)
GrrlScientist. “Watch: Supergene controls butterfly mimicry”, The Guardian, 18 August 2011. (Medium link.)
GrrlScientist. “The hot and cold of butterfly dancing”, The Guardian, 1 February 2011. (Medium link.)
Originally published at Forbes on 9 May 2018.