Evolving For City Life

Urban environments pose unique challenges to any plant. Cities are generally warmer, have significantly higher CO2 levels, and experience altered levels of disturbance and precipitation patterns than do rural areas nearby. Still, many plants have taken to these concrete jungles, popping up wherever they can eke out an existence. Although we are not reinventing ecological principals in urban areas, they nonetheless present distinct selective pressures on every living thing within their jurisdiction. Evidence now suggests that urban environments are actually shaping the evolution of at least some plant species. 

Motivated by a desire to better understand how urban conditions are influencing evolution, a team of researchers based out of the University of Minnesota decided to take a closer look at a common mustard called Virginia pepperweed (Lepidium virginicum). This hardy little annual is at home wherever disturbance occurs. As such, it can be found throughout most of North America and beyond. Because it self pollinates readily, researchers were able to quantify phenotypic differences between populations growing in dense urban centers and compare them to those growing in more rural areas.

They collected seeds from numerous urban and rural populations and grew them together in a greenhouse experiment. By exposing each population to the same conditions in the greenhouse, the team were able to tease out the true phenotypic differences between these populations. 

What their data revealed were distinct differences between urban and rural populations. For starters, urban plants had larger rosettes but fewer leaves. They also bolted sooner than rural plants but then exhibited a much longer period of time between bolting and flowers. Previous studies have shown that the inflorescence of related species "accounted for 55% of a plants photosynthetic activity but only 25% of water loss." Coupled with the reduction in the number of leaves, these results suggest that urban plants are maximizing photosynthesis under drier conditions. 

Another interesting difference is that urban plants produced far more seed than their rural counterparts. This very well may be due to the fact that urban plants tended to be larger. This could also be due to reduced herbivory in urban environments, though such pressures may vary from city to city. Due to the urban heat island effect, it is likely that this could be a result of more stable temperature conditions than those experienced by their rural counterparts. Taken together, these results show that there is indeed selection for traits that allow plants to not only survive but thrive in urban environments.

Photo Credit: Wikimedia Commons

Further Reading:
[1]

One Badass Moss

Badass and moss are two words that don't find themselves in the same sentence very often, if ever. Today I would like to introduce you to one moss that is certainly worth such a description. Meet Ceratodon purpureus, sometimes referred to as "fire moss." This lowly bryophyte is tough as nails and enjoys a global distribution because of it. From fires and heavy metal pollution to living in our most densely populated urban areas, this moss is a survivor. What's more, its ecology is absolutely fascinating.

Fire moss is truly cosmopolitan. It can be found on every continent and may only lose ground in the tropics where it is replaced by its close relatives. Though we often think of mosses as delicate denizens of shaded forest floors, fire moss is anything but. This is a disturbance-loving species. It gets the name fire moss for its habit of turning up in profusion following wildfires. Cleared of its competition, fire mosses growth can be quite explosive.

Being able to grow on a variety of substrates means that fire moss is equally at home in man-made habitats. It can be found growing in and along sidewalk cracks, old roofs, depressions in asphalt, and on wooden structures. What's more, it can tolerate pollution levels that would normally kill most mosses. One study found that moss grown on mine soils contaminated with toxic levels of heavy metals showed absolutely no decrease in fitness. In fact, they were indistinguishable from moss grown on clean soils.

This moss' lifecycle is ephemeral. Because it needs disturbance to persist, natural succession usually causes it to disappear from a site after a decade or two. Its spores, however, can remain viable for upwards of 16 years in the soil until fire, bulldozer, or any other large-scale disturbance opens the land again.

One of the strangest aspects of this fire moss is how it reproduces. Like all mosses, male gametophytes produce sperm that must make their way to the female gametophyte. They do this by swimming. Whereas moss species living in wet environments can let rain do the work of uniting the sex cells, fire moss has evolved a strategy more familiar to the flowering plants.

It was found that fire moss emits complex volatile scents. What's more, these scents are produced at different rates in the different sexes with females producing much more scent than males. It was found that microarthropods, specifically springtails, are attracted to these scents. Close investigation revealed that springtails significantly increased the fertilization rates in fire moss, hinting at quite a specific reproductive relationship between these organisms, both of which are representatives of some of the first organisms to ever make it onto land.

If this story has not convinced you that fire moss is one badass bryophyte I don't know what will. It is amazing to think that such an incredible organism is probably living out its life a stones throw away from where you are sitting right now.

Photo Credit: Ian Sutton (http://bit.ly/1LqqpMY)

Further Reading:
http://www.publish.csiro.au/?paper=BT9650303

http://www.nature.com/nature/journal/v489/n7416/full/nature11330.html

http://bit.ly/1U4lE2G