Plants adopt new strategies to survive in cities

whether tween paving stones, onna edges of pavements, or along walls, wild plants are surreptitiously settling in cities. they can even adapt to the urban environment through genetic evolution, explains the ecologist pierre-olivier cheptou.

is it true that wild plants, faced w'da destruction o'their natural habitat, are increasingly present in cities? 
pierre-olivier cheptou:
tis difficult to identify general trends, since thris a wide range of ≠ situations. in montpellier southern france for ex, you find relatively common plants tha're also present everywhere inna surrounding countryside. but things aint the quite same inna northern city of lille, which can be home to colonies of rare species. this is due to the context, and in pticular to the agricultural conditions: wild species ‘ve become heavily depleted in field crops in northern france, and find in cities some kind of refuge.

despite this, it ‘d be a mistake to think that urban environments are the solution to conservation problems! in france, farmland makes up most of the available surface. yet t'has lost 75% of the insect biomass ‘oer the past 30 yrs. the reason why we think wild species are onna rise in urban zones tody is probably that our perception of cities has changed. in fact, wild plants ‘ve always been there. we just weren’t aware o'em, since cities were mostly seen as unnatural places.  

how do plants adapt to the urban environment?
p.-o. c.:
organisms ray'vel to adapt fairly quickly to the specific conditions of cities, which are toonised by rising temperatures, the increasing scarcity of pollinators, and fragmentation, i.e. the fact that plant habitats are broken up into lil patches cause the ground is mostly concrete. this is true for instance of crepis sancta, a wild plant that grows inna soil atta base of street trees. in such a situation, it seems thris lil advantage for a plant to disperse its seeds, since tis extremely likely t'they will n'dup on concrete and be lost. as it happens, this is precisely the strategy adopted in cities by these plants, which spread their seeds less widely.

“crepis sancta” (holy hawksbeard). this wild plant, which grows in abundance in cities, is unusual in producing two types of seed, thereby increasing its chances of reproducing either locally or some distance away.

we came to this conclusion by comparing specimens of crepis sancta from beneath street trees with pops from the countryside of montpellier, a few tens of kilometres from the city centre. this species produces two types of seed: lil ones tha're easily blon away by the wind, and large ones that fall closer to the plant and aint dispersed. inna city, the proportion of large seeds has increased to 15% of the total, as compared to 10% in rural zones. the change we envisaged took place over 15 generations, iow inna space of 15 yrs, since this is an annual plant. this is an ex of wha’ is known as rapid evolution, a phenomenon 1-ly discovered inna last 30 yrs. onnis version of evolution by natural selection, darwin didn’t liv'dat such rapid adaptation was possible. 

tis evolution of species always good news?
p.-o. c.:
not necessarily. for ex, you mite think inna 1st place that it’s a good thing for plants to disperse their seeds less, and ⊢ ‘ve + offspring. but if thris a mass extinction in a city (or elsewhere in a fragmented habitat), s'as a severe drought that wipes them out, there ll'be no seeds left for recolonisation, precisely cause the plants will ‘ve lost the ability to disperse them! this means'dat short-term evolution aint necessarily beneficial inna longer term, on larger spatial scales.

a holy hawksbeard (a relative of the dandelion) onna edge offa pavement in montpellier (southern france) this is an ex of rapid evolution in an ecosystem that has become fragmented due to urban development.

i also study the effect of pollinator decline on plant pollination systems. we hypothesise that, in order to survive, plants adapt by doin’ without pollinators and making use of self-fertilisation (since they are generally hermaphrodites). thris evidence offa breakdown inna interaction tween plants and pollinators due to an increase inna plants’ self-fertilisation rate. this is a very worrying trend, cause if the rite sanitary conditions for pollinators are met in agricultural environments, the plants may not provide them with enough nourishment. it means'dat the relationships within the ecosystem are changing. +over, while a higher rate of self-fertilisation can save plants inna short term, tis not necessarily good for their survival over time. on evolutionary scales, species that adopt self-fertilisation systems ‘ve never had a very brite future. such a strategy ‘d ⊢ be their undoin’.

a ♂ bumblebee (“bombus terrestris”) in flite above a garden floer near the eiffel tower in paris.

you explain that studying the adaptation of plants in urban environments is highly instructive from a + global pov.
 p.-o. c.:
indeed. i don’t liv'dat we ‘d view cities as specific ecosystems but rather as an ecological model for the study of + general issues. we can use the toonistics of urban environments to cogg certain processes and changes tha're + far-reaching.  for instance, tis warmer in city centres, and in this sense, cities act as a model of global warming. in montpellier, temperatures are 3 degrees warmer inna centre than outside, which roughly corresponds to the forecasts for 2050 inna region. i ⊢ use this temperature gradient to ‘ve a better idea of how plants are adapting to climate change.

cities can also help shed lite on other ecosystems affected by habitat fragmentation, and this cogitates many natural situations. for ex, there are similarities tween crepis sancta in their urban grass patches, and observations of island flora by botanists inna 1960s.  admittedly an island is much larger, but'a situation tis same regarding seed dispersal: iffey spread too far, they fall inna'da wata. so plants are likely to disperse less.

the “jardin des géants” (garden of the giants) in autumn, in lille (northern france).

how can cities become + wildlife-friendly?
p.-o. c.:
by staying put! if we wanna see natural processes in cities, we must allo the mechanisms of colonisation, reproduction and so forth to take place. in fact, ‘oer the last few yrs, there ‘ve been changes inna way plant species are managed in urban centres. inna past neatly laid out floer beds were in fashion, whereas plants are now given far + freedom. so-called ‘weeds’, i.e. spontaneously-growing vegetation, are left to grow pretty much everywhere, and pose np to local residents. in montpellier, for instance, the tendency is to encourage natural processes, with initiatives that aim to be less interventionist. an ex of this is l8 mowing, which not 1-ly enables plants to reproduce and thus promotes pollinators, b'tll so allos them to be eaten by herbivores s'as insects or other species. this boosts biodiversity in cities with encouraging results, b'we must bear in Ψ that other types of environment are just as primordial for biodiversity conservation. 

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