Butterflies fall off anthropogenic adaptive peak and meet their doom in anthropogenic ecological trap

Background/Question/Methods The study documents final outcomes of two independent anthropogenic host shifts by the North American butterfly Euphydryas editha. The first, at Schneider’s Meadow, Carson City, involved inclusion of an exotic, Plantago lanceolata, into the diet in addition to the traditional host, Collinsia parviflora.   Field experiments in the 1970’s involving manipulated oviposition showed strong natural selection for using the exotic.  Heritability of oviposition preference was around 0.9, so anthropogenic evolution was expected.  At the time of the previous published report (in 1993), the proportion of field-caught insects preferring the exotic had risen from 5% in 1982 to 52% in 1990.  This change was reproduced among lab-raised offspring of field-caught insects, so it was genetic.  The second host shift occurred at Rabbit Meadow, Sequoia NP, in the 1970’s when loggers burned their trash and fertilized habitat patches in which the lifespan of Collinsia torreyi was increased.  This generated natural selection for butterflies to oviposit on this host, and C. torreyi was rapidly colonized in logged patches, supporting high insect densities there.  In neighboring unlogged patches the traditional diet of Pedicularis semibarbata was retained.
  

Results/Conclusions By 2005 all tested insects at Schneider preferred the exotic and field censuses showed monophagy on it.  In 2007 cattle-grazing ceased.  Within months Plantago were shaded by grasses, thermophilic larvae were unable to bask in sunlight on their host, and the butterfly population became extinct.  No adult insects were found in 2008, 2009 or 2010.    At Rabbit, insects using C. torreyi evolved preference for this species over P. sembarbata but failed to evolve preference for young over old C. torreyi plants.  As a result, when post-fire succession rendered host quality variable, fitness was reduced on C. torreyi and this host was abandoned around 1999.  Manipulated oviposition experiments performed in the field in the 2000’s allowed insects to choose their hosts and so to express adaptive or maladaptive discrimination among C. torreyi individuals.  In these experiments the highest fitness was achieved by insects ovipositing on C. torreyi but imported from a site where this host was used and the butterflies were adapted to it.  The second highest fitness was of local insects using P. semibarbata.  The lowest fitness was of local insects using C. torreyi.  Local butterflies had abandoned C. torreyi at a time when it would still have supported the highest fitness, IF the insects had been well-adapted to it.  They had fallen off an anthropogenic adaptive peak.