Local Adaptation in Widespread Species

Common garden experiments, a type of study in which individuals from distinct populations of a species are grown side-by-side, have provided a great deal of evidence to support the position that populations adapt their phenology and resource allocation strategy to local conditions. Environmental conditions that may vary across a species’ range include average annual temperature, minimum and maximum temperatures, timing and amount of precipitation, ratio of precipitation to evaporative losses, associated plants and animals, and soil characteristics. Some aspects of phenology and resource allocation strategies that may evolve include timing of germination, flowering, fruiting, or senescing, number of flowers, seed number and mass, and ratio of aboveground to belowground biomass. Adaptation to a given set of conditions may hinder survival under a different set of conditions, hence the need to collect and make available to restorationists seed from a wide variety of locally adapted populations of a given species.

 

[Lewisia rediviva, the bitterroot]

The bitterroot, Lewisia rediviva, offers a good case study of a plant that lives in widely separated areas which receive precipitation in very different amounts and at very different times. While the plant is reported from all western states and British Columbia and Alberta, I’ll make a short study of only three reported populations (Coconino co., AZ, Humboldt co., NV, San Joaquin co., CA) that experience a range of conditions across the southwestern United States. The Arizona population, reported from near the south rim of the Grand Canyon, receives roughly 16 inches of rain per year spread over a winter and a late summer rainy season. The Nevada population, reported from near Winnemucca in Humboldt Co., receives roughly 8 inches of rain per year, almost entirely in the winter. The California population, reported from northeast of Stockton in San Joaquin co., receives roughly 14 inches of rain per year, almost entirely in the winter. While it does not appear that any common-garden studies of this species have been conducted, it does seems unlikely that an individual from a population that is adapted to high rainfall (i.e. an individual from the south rim of the Grand Canyon) would prosper under the much more arid conditions around Winnemucca, NV. In the same vein, an individual transplanted from San Joaquin co., CA, which lacks a summer monsoon, might not prosper when planted among the south rim population due to a lack of adaptation to the local timing of rainfall.

[L. rediviva on high, dry ridge – the small white forms in the center are Lewisia flowers]

A note on the value of genetic diversity within a species

Within the S.O.S. protocol it is noted that “each seed collection should comprise of a significant representation of the genetic variation within the sampled population.” This statement reflects a recognition, stated explicitly elsewhere in the protocol, that the capture and storage of genetic diversity within a species or within a population of a species is a goal nearly so worthwhile as the collection of seeds from a large number of species. I will use this blog post to first relate my team’s recent experience collecting seed of Juniperus osteosperma from two distinct populations and then to examine an ongoing story in which a rare, naturally occurring genotype may play a role in future ecosystem-level restoration.

My crew travelled to Washoe co., NV, on two occasions over the last week to collect cones from two stands of Juniperus osteosperma. By collecting many tens of thousands of viable seeds from a large geographic area we increased the odds of collecting genes that will allow the species to persist in an era of changing climate and novel pathogens. While it remains unknown which genes, if any, collected by my team will be of use to J. osteosperma in the future, I will offer an example of how genetic diversity may play into a future large-scale reintroduction effort in the eastern United States.

[A juniper woodland in Washoe co., NV]

Castanea dentata, the American chestnut, was driven to the edge of extinction by a fungal disease in the early 1900s. Some individuals, however, show varying degrees of genetic resistance to the pathogen. While several organizations are attempting to develop resistance to this fungal pathogen in American chestnuts by means such as the insertion of a gene found in wheat into the chestnut genome and cross-breeding with the naturally resistant C. sativa of eastern Asia, the American Chestnut Cooperators Foundation is actively cross-breeding these resistant strains of C. dentata to a degree of success. This may, in the future, allow for a reintroduction of the species into the forests of which it was once a part and a restoration of lost aspects of those forests’ ecology.

[Good job crew- that oughta do it]

While the story of C. dentata and the American Chestnut Cooperators Foundation revolves around genes that were preserved in situ in the eastern American hardwood forests, similar stories may in the future be told about a great many species which were unable to persist in their historic range under the combined stresses of habitat fragmentation, climate change, and novel pathogens and which, consequently, will revolve around the use of genetics preserved in seed banks around the world.

Concerning the need for a distant horizon in a biologist’s education

Freedom, though not necessarily ease, of movement over expanses of land allows for a far more visceral understanding of the lives of wild animals and plants than could any amount of reading. The two ways of knowing are, of course, deeply and necessarily complimentary, but not until a student of nature has moved for a time across a landscape wide enough to allow them to experience and negotiate a variety of environments and conditions will they have more that an abstract understanding of the lives and histories of the place’s inhabitants.

(The Clan Alpine Mountains, Churchill co., NV, from the floor of Dixie Valley)

A student of biogeography, one interested in the peculiar distribution of a montane species in the the basin and range province offers a useful illustration of this complimentarity. A review of the literature concerning this hypothetical species shows that it hasn’t been recorded beneath about 7000 feet in elevation. Strangely, though, not all areas above 7000 feet in the region harbor this species. Why? Clearly the hot, alkaline basins surrounding montane areas in this region are insurmountable barriers to migration.

(Equisetum sp., in Ash Canyon, Carson City, Nevada)

A long hike or drive on dirt roads from ridge to ridge via an intervening basin, taken with plenty of water but with no books, lays the foundation for this abstract answer to become a visceral understanding of what is, of how things operate. The mountain air where the species is found cools the sweat on the brow while the basin’s hot wind leaves skin dry and with an accumulation of gritty salt and dust. The distances involved, easily laid out on a map, are more comprehensible from the point of view of a wild thing after half an hour of bouncing across a dirt road brings only a small change to one’s view of a distant ridge.

(A violet in a a sunny ponderosa pine woodland)