The Clodius Parnassian butterfly is more common at the top of its elevation range in the California mountains than in the past.
Climate change is making things rough for many vertebrate and invertebrate species. But add to this a steady loss of habitat, and many species just can’t adapt successfully to the combined stresses.
From the coastal lowlands to the coniferous tree lines of Northern California’s Sierra Nevada Mountains, scores of species of butterfly are in an existential fight for their evolutionary futures. The survival challenge seems to be most impacting those species whose preferred habitats lay in the lower elevations, but the effects are being felt further up as well, as more butterfly species are moving into higher-elevated habitats. This evolutionary struggle might have gone unnoticed but for the diligent work of one research team, lead by butterfly expert Arthur Shapiro of the University of California at Davis.
[Note to reader: The following article contains extensive quotes from a recent interview with University of California at Davis professor of Evolution & Ecology Arthur Shapiro, lead researcher]
For the past 35 years, Dr. Shapiro and his field team (often comprised of his graduate students), have been collecting data on some 159 species of butterfly at ten locations scattered throughout this biological “hot spot” that is the Sierra Nevadas. Mention butterfly collecting, and one might conjure up a comical image from some old movie of a net-wielding ‘Lepidopterist’ (a moth or butterfly collector) in pith helmet happily hunting his inconsequential quarry. But for Shapiro and his team, lepidoptera (the taxonomic order that includes butterflies) are serious science. No mere, showy examples of Nature’s handy work, butterflies are in fact important “harbinger” or “indicator” species. Their colonizing, migratory and mating behaviors offer scientists valuable clues to the ecological health and robustness of the ecosystems they are an integral part of. The team’s findings were dramatic: half of the monitored sites showed declines in “species richness” (the total biodiversity or numbers of species of a certain type). The monitored sites range in elevation from sea-level to nearly one and half miles above sea-level–with lower elevation sites showing the greatest reduction in richness.
“Individual species responses to climate change are guaranteed to be idiosyncratic.” – Arthur M. Shapiro, Professor of Evolution & Ecology, UC Davis
Some species are apparently adapting to the effects of warming by shifting their ranges higher up the mountain sides (where temperature are somewhat cooler), while a few others were expanding “horizontally”, moving northwards and east. Naturally, there will be ecological winners and losers in this “faunal shift”. Currently, treeline (montane) habitats are the only niches observed in the study that showed an actual increase in species richness (as more species shift to the higher, coniferous habitat). It would be natural to deduce that this colonizing shift is a consequence of warming. But the range shifts and overall declines could not be attributed solely to warming trends–only when adding the contribution of habitat loss (via human development) were the researches able to account for the declines. As noted earlier, species declines were greatest at low elevations–precisely where habitat destruction was greatest. Dr. Shapiro offered more detail on this trend: “The heaviest impacts in our study clearly are at low elevations at this time, and our statistical analysis indicates they are being driven more by land-use changes than by climate (though climate is contributing). There has been little change in land use at the higher elevations, though massive urbanization/suburbanization has begun in the foothills along the Highway 49 corridor and we fully expect to see impacts from it appearing in the next decade, regardless of what the climate does.”
Lorquin’s Admiral (Limenitis lorquini), a species in decline in the central valley but not in the Sierra (photo credit: Ian Wright)
In the past several years, field biologists and ecologists have been focusing more and more on the vital role in species declines played by “habitat splitting” or habitat fragmenting–a factor that, apart from pathogens (such as the BD fungus), has led to amphibian declines (i.e., in frogs and salamanders) over the last couple of decades. Many invertebrate species start out their existence in one type of habitat (such as in or near water, as larvae) and then move to other habitats (e.g., dryer or “woodsy” locales) to mate and live out their adult stages. When these variable habitats are split up–mostly due to development–many species are unable to transition to their adapted mating habitats. So, how is this breaking up of habitat impacting the butterflies? According to Shapiro: “Land use contributes to faunal decline in butterflies both by removing habitat and by fragmenting it such that colonization of remaining patches by randomly dispersing females becomes increasingly unlikely.” And, regarding the larger scale effects of this colonizing dynamic, Shapiro elaborated further: “Most butterflies display “metapopulation” behavior, with frequent turnover of local populations, which are temporary manifestations of the larger “meta” unit. This type of dynamic is seriously impacted when dispersing colonizers are forced to traverse extensive, butterfly-unfriendly habitat in an essentially random search for suitable sites.” Many animal species live out their life-cycles in the same geographic regions or elevations–a function of natural adaptation to an ecological niche. But various environmental changes can induce whole populations to shift. As noted, many low-elevation butterfly species are being forced to seek out higher climes in which to colonize; butterfly species that once only inhabited middle-range elevations are now moving into much higher locales. Unfortunately, these shifting species do not always find sufficient host plants, which can impact their reproductive cycles. “Basically, we are seeing more “7000′ [feet] – and – below” species at 9000′ than we used to. In many cases they are unable to breed at 9000′ because their host plants do not (yet) occur there. We appear to be seeing upslope movement in response to warming, and there is no reason to think this is temporary. As vegetation, which cannot fly, responds too and host plants colonize upslope, what are now dispersing butterflies will in many cases become breeding residents.”
The Great Purple Hairstreak (Atlides halesus), generally considered endemic to the warm-winter climates of the Pacific slope, has recently become established as a breeding resident at Sierra Valley, apparently capable of overwintering.
One might deduce that eventually, as host plants shift upslope, their adapted butterflies might recover, in time. But apparently, this ecological over-crowding is leaving few options for the higher-elevation species. ‘The most “alpine” species have nowhere higher to go on Castle and Basin Peaks. If the climate warms beyond their ability to adapt, they will go extinct there. The Sierra Nevada is much higher farther south, where there are species (such as Hesperia miriamae and Chlosyne damoetas malcolmi (its taxonomy is polemical, by the way) that do not exist as low as the northernmost Sierran alpine islands. Someone should be tracking them. They could go globally extinct.” This writer was curious about this upslope, multi-species crowding and asked Dr. Shapiro to what extent this is increasing competition over resources, and whether this might drive interbreeding amongst butterfly species and/or result in new species. “Data on competitive interactions between plant-feeding insects are hard to come by, and despite several excellent reviews there is still controversy over the importance of interspecific competition among leaf-feeders (flower, fruit and seed eaters are better documented). We do not have data bearing on these questions. We have seen two instances of subspecies replacement on the transect (not discussed in the PNAS paper) in which one taxonomic subspecies went extinct and after a short interval was replaced by another. Both of these occurred at Sierra Valley. Neither appears to have had any element of competition involved. In one of the cases the lost entity was in fact of apparent hybrid origin, and was replaced by one of the presumed parents!” Shapiro continues:
“As far as hybridization is concerned, the most interesting case is the Hesperia colorado complex, in which Castle Peak receives strays of both the West- and East-slope entities, which are very different-looking subspecies; sometimes both in one year but sometimes not. The two entities hybridize in the Feather River canyon, which is a very low pass across the Sierra, and we have documented this molecularly (and published it). In theory, at least, the current climate change might trigger new interactions of this sort in somewhat higher passes, such as Yuba and Donner, but we have not seen it yet. As for changes in reproductive isolation [note: a condition required for generating new species] they would not appear “overnight!” but might require hundreds or thousands of generations to become detectable. Most of these critters have one generation a year.”
It is not uncommon with ecological “inventory” studies that findings vary significantly from study to study, and from region to region, due to many factors including strong, local effects. This writer asked Shapiro how his findings compare to other faunal research. [Referring to the Grinnell Project at the Museum of Vertebrate Zoology, Berkeley] “It is doing a 100-year comparison of the Sierran mammal fauna. My sites fit the pattern they observe: We have lost the pika [note: a small chinchilla-like animal, related to rabbits] completely from the Donner Summit area and the Golden-Mantled Ground Squirrel is extremely reduced in abundance there. At the same time, we are now recording the Belding Ground Squirrel at Donner for the first time. On my transect, it formerly occurred only at Sierra Valley, 2000′ lower. The butterfly patterns are consistent with other groups.”
The Fiery Skipper (Hylephila phyleus), one of the few “ruderal” species of butterfly that is adapting to suburban sprawl.
The team published its long-term study results earlier this month on the website Proceedings of the National Academies of Science (‘Compounded effects of climate change and habitat alteration shift patterns of butterfly diversity’). One of the most surprising discoveries by the team was the fact that the more critical species (from a conservation viewpoint) were not the most impacted by this compounded situation. Instead, it was the common, ruderal (or “weedy”) species–ones with less specialized habitats–that were experiencing the greatest declines. Says Shapiro: “We did not anticipate this finding. It just came out of the analysis…These species made their accommodations with us a long time ago, with most of them now dependent on non-native host plants. The fact that they have declined precipitously in recent years–specifically, since the late 1990s–and on a regional basis points to a threshold phenomenon. We have done very detailed analyses (in another paper, in the pipeline) that show that although their declines are correlated with climate change, the contribution of different climatic variables to that decline varies from site to site, even though the declines are uniform and simultaneous across sites. In a nutshell, that says that climate is not the main driver. We think land use–habitat availability and especially habitat connectivity–is.” Apparently, urban and suburban “sprawl”–decried by environmentalists and over-development critics for years–is especially bad for butterflies as well. But are any able to adapt to this land-use surge? “Except for the Fiery Skipper, Hylephila phyleus, which eats Bermudagrass in lawns, few of these species can sustain breeding in extensive urban and suburban landscapes. Vacant lots, roadside ditches, and such are essential to their persistence. Tract sprawl is lethal.” In the paper’s concluding thoughts, the team calls for expanding the scope of butterfly conservation efforts to include these ruderal species when attempting to estimate the effect of compounded, environmental stresses. As far as the long-term impact of butterfly declines on the ecosystem(s) of this region, Shapiro added: “The crystal ball is cloudy. I am a student of paleoclimatology and paleovegetation dynamics and I know that “communities” are an illusion–as I put it in my book, freeze-frames from a very long movie. Change is the norm (though the rate varies); stasis is the exception. When the American Chestnut disappeared as a forest dominant in the east, the system did not collapse–and as far as we know, not one species went extinct (note: see comments at the end of this article). I doubt that butterfly faunistics will trigger a catastrophe, but I’d love to come back from the dead in a couple of hundred years (that is, two or three conifer generations!) to see what the Sierra looks like.” Shapiro’s thirty-five year old (and growing) database is somewhat of a rarity in science; it is long-term, has had the same collector/team, and covers multiple sites and multiple species. Such an important data resource will no doubt prove even more invaluable in the coming years as the effects of climate change accumulate. But it seems that with butterflies, as with so much other flora and fauna, accurate prediction of long-range ecological impacts from loss of species is beyond Science’s abilities–even if you’re an expert with a prized database.
Research team members and additional listed authors of the paper include Matthew L. Forister, Andrew C. McCallb, Nathan J. Sanders, James A. Fordyce, James H. Thorne, Joshua O’Brien, and David P. Waetjen.
Visit Arthur Shapiro’s Butterfly Site
top photo credit: Heather Dwyer / UC Davis
1st middle photo: Lorqhin’s Admiral – Ian Wright
2nd middle photo: Purple Hairstreak – Art Shapiro’s Butterfly Site image galleryt
bottom photo: ‘Regular Daddy’ on wikipedia.org (cc-by-sa)