Photo by Southwest Desert Flora: http://southwestdesertflora.com/WebsiteFolders/All_Species/Loasaceae/Mentzelia%20multiflora,%20Adonis%20Blazingstar.html

Islands in the Sand

*Featured image of Mentzelia multiflora, owned by Southwest Desert Flora

For organisms that lack the ability to move (i.e. plants), dispersal is critical for evolutionary success. Whether it’s the effusion of spores, pollen, seeds, or asexual propagules, plants generally try to obtain as wide a berth as possible from their offspring.  At a broad scale, this ensures that plants can shift their ranges with changing climates and maintain a large amount of genetic diversity. On a finer scale, when offspring are dispersed away from the parent plant, it means the two aren’t directly competing for resources, increasing the likelihood of survival for both.

There are some environments, however, in which long-distance dispersal might actually hinder the survival of a species. Plants that grow on islands have generally reduced dispersal capability when compared with their mainland relatives, since most plants are mal-adapted to ocean environments (1). This creates the unique situation in which a species with progeny so well-dispersed that they made it to a remote island must immediately limit its dispersal capacity if it is to maintain its new population. This change can occur so rapidly, in some cases, that differences in dispersal capability can be observed within the span of just a few years (2). This pattern is true not only for plants, but for insects (flightless insects are less likely to get inadvertently blown out to sea) (3) and birds as well. Darwin (4) was the first to note that oceanic islands harbored more species of flightless birds compared to mainlands, likely due to fewer numbers of nearby predators, rendering escape by flight unnecessary (3). This makes their eventual demise by colonizing humans all the more tragic (think the dodo from the island of Mauritius or the moa of New Zealand).

Oceanic islands aren’t the only environments in which limited dispersal is favored. You could easily imagine that any type of discontinuous substrate separated by vast distances might harbor species that try to stay put as much as possible rather than risk the intervening miles. One place you might not expect to find them is in the dried up remains of ancient lagoons throughout deserts around the world. The story of how these environments formed, along with the evolution of the plants that fill them, takes us all the way back to the Eocene, when several environmental changes were just beginning to shape our planet into the world we know today.

 

THE FORMATION OF DESERTS AND THEIR FLORAS

 

50 million years ago, Earth began a gradual cooling process that’s overall continued to the present, resulting in the great ice ages of the last few million years. The cause of this global cooling is up for debate, but there’s several lines of evidence that point to a reduction in the amount of atmospheric CO2 (5). Mountain ranges began forming in several parts of the world, leading to increased erosion, a process that sequesters large amounts of carbon (5). The movement of continents began to slow (15cm per year in the Jurassic compared to the 2-3cm per year in the present), leading to a reduction in the deep sea geothermal venting of CO2 (6). Along with the cold arrived increasingly drier environments, and in order to cope with these new conditions, plants had to completely change the way they obtained energy and stored water. Several plant lineages (at least 66) independently evolved the ability to undergo C4 photosynthesis (7), which is able to incorporate carbon more efficiently than the regular C3 pathway (the type most plants have). The first grasses evolved during this time as well (8), colonizing vast stretches of open plains and savannahs, spurring the evolution of the many grazing mammals we’re familiar with today. And in the deserts, the first cacti evolved, taking photosynthesis a step further by storing carbon only at night rather than during the day.

The origin of various specialized groups of plants throughout the Cenozoic. Gray shading represents atmospheric CO2 as inferred from several proxies. Blue line represents a decline in overall global temperatures. Used with permission from Arakaki et al., 2011 (12).

As conditions became drier, hypersaline lakes that once dotted the southwest portion of North America, western Mexico, eastern Spain, western Asia, and the horn of Africa began to dwindle and disappear, leaving behind calcified rock unsuitable for most plants to grow on (9). These deposits (comprised mostly of gypsum) have high concentrations of salt, which make it difficult for roots to absorb nutrients from the soil.  Gypsum is also tougher than the surrounding desert soils and contains very little organic matter (9), overall a place you wouldn’t expect to find a diverse flora. And yet, beginning in the late Miocene (ca. 8-5.3 mya) (9), several groups of plants evolved to specialize on gypsum islands, to the point where they now can’t survive anywhere else.

 

LIMITED DISPERSAL IN MENTZELIA

 

In the case of soil specialists, species often aren’t restricted by an inability to grow in the surrounding environment, as is the case for island floras; rather, they’ve lost their competitive edge with the plants around them from years of growing in an area free of competition (10), although there’s some evidence to suggest that species can become so specialized to a particular soil type that they can no long grow on any other (10). This appears to be the case for at least a few species of blazingstars (Mentzelia section Bartonia) that grow exclusively on gypsum in the deserts of the southwest United States and northwest Mexico.

As their name suggest, blazingstars have striking yellow flowers, with sharply tapering petals and a profusion of stamens that curve slightly inward. Once mature, the seeds are retained within an open fruit until dislodged by wind. Many species have a peripheral wing surrounding the entire seed, allowing them to ride gusts and eddies, their path unhindered by large trees or thick shrubs common in wetter areas (10). But wind-dispersal is problematic for gypsum specialists. While gypsum outcrops are widespread in several arid regions throughout the world, they’re often small and thinly distributed, forming ‘archipelagos’ that run along the spine of the desert floor (10).  A seed carried even a moderate distance by the wind would therefore be more likely deposited on desert sands than on gypsum soils. The seed wings of gypsum blazingstars have correspondingly decreased in size compared to their non-specialist relatives, ensuring that progeny never get too far.

Gypsum outcrop, with desert sands just beyond, showing the stark difference between the two floras. Photo by John Schenk.

Limiting dispersal can be problematic for multiple reasons, including a reduction in genetic diversity and a higher occurrence of inbreeding, which can uncover deleterious alleles. There hasn’t been much work to determine whether this is the case for blazingstars, but studies on other groups of gypsum endemics have shown the opposite pattern. In one case, a gypsum endemic (Tiquilia hispidissima) had higher levels of variation within populations than any other sand-dwelling species in the genus (11)*. And with approximately 200 species of gypsophiles across 35 plant families in the Chihuahuan Desert alone (11), the diversity of the islands themselves doesn’t appear to have suffered at all. Rather, these islands can act as centers of diversification, not only for new gypsophilic species, but for plants that revert back to non-gypsum soils as well (10).

 

* For the markers matk, ndhF, rps16, ITS, and waxy

Bibliography

  1. Carlquist S (1965) Island Life: The Natural History of the Island of the World (Natural History Press, New York).
  2. Cody ML, Overton JM (1996) Short-Term Evolution of Reduced Dispersal in Island Plant Populations. J Ecol 84(1):53.
  3. Carlquist S (1966) The biota of long-distance dispersal. I. Principles of dispersal and evolution. Q Rev Biol 41(3):247–270.
  4. Darwin C (1959) in Life and Letters of Charles Darwin, ed Darwin F.
  5. Ruddiman WF (2010) Climate. A paleoclimatic enigma? Science 328(5980):838–839.
  6. Graham A (2011) The age and diversification of terrestrial New World ecosystems through Cretaceous and Cenozoic time. Am J Bot 98(3):336–351.
  7. Kellogg EA (2013) C4 photosynthesis. Curr Biol 23(14):R594-9.
  8. Strömberg CAE (2011) Evolution of grasses and grassland ecosystems. Annu Rev Earth Planet Sci 39(1):517–544.
  9. Moore MJ, Mota JF, Douglas NA, Olvera HF, Ochoterena H (2014) The ecology, assembly and evolution of gypsophile floras. Plant ecology and evolution in harsh environments’(Eds N Rajakaruna, RS Boyd, T Harris) pp:97–128.
  10. Schenk JJ (2013) Evolution of limited seed dispersal ability on gypsum islands. Am J Bot 100(9):1811–1822.
  11. Moore MJ, Jansen RK (2007) Origins and Biogeography of Gypsophily in the Chihuahuan Desert Plant Group Tiquilia subg. Eddya (Boraginaceae). Syst Bot 32(2):392–414.
  12. Arakaki M, et al. (2011) Contemporaneous and recent radiations of the world’s major succulent plant lineages. Proc Natl Acad Sci U S A 108(20):8379–8384.

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