Scientific results of Sauvages de ma rue
Here are the results obtained from the data collected through the programme Sauvages de ma rue.
The most seen species
Here are the top 5 species seen by our participants:
- Annual bluegrass (Poa Annua)
- Common Dandelion (Taraxacum sp)
- Market Sowthistle (Sonchus oleracerus)
- Common groundsel (Senecio vulgaris)
- The bristly cardamine (Cardamine hirsuta)
In what types of urban gaps are they found?
Half of all plant observations are made in cracks in the pavement, between paving stones or on walls! It has to be said that to colonise these spaces, no provision is made for flowers, it is the wind, animals and humans that transport their seeds without knowing it! The other half of the observations are made in gaps that are more or less intended to host plants: the feet of trees, lawns, etc. All of these urban gaps represent a significant surface area.
And what about the diversity of the streets?
On average, there are 6.5 different species per pavement, and this number increases as one moves away from Paris, especially because pavements are longer on the outskirts of the capital. To compare the data, an average was calculated for a fixed length and 4.5 species were found for every 100 m of pavement.
This number varies little with distance from the centre of Paris.
The figure shows four types of street layout: "no layout", "streets with lawns", "streets with tree stands", "streets with lawns and tree stands". Streets with no special facilities, consisting mainly of impermeable surfaces (asphalt, paving stones, cement, etc.) have almost three times fewer plant species on average than streets with varied vegetated interstitial spaces. Streets with trees and grassed areas are much richer in wild flowers, especially in central Paris. These streets could provide pathways for the dispersal of many species across the city and thus link urban parks and gardens to natural areas generally located on the outskirts of cities.
The design of the street is a determining factor in the richness of the plant life, especially in the centre of the city.
Is the tolerance of urban plants higher than that of rural plants?
We thought that urban plants were more tolerant of harsh urban conditions.
The urban environment is characterised by a limited amount of land, reduced water availability, higher air temperatures than in rural areas, air pollution that accentuates drought, increasingly fragmented green spaces and soil pollution (rich in nitrates and ammonia).
However, after an analysis of lists of species from 138 pavements in the Ile-de-France region, the plants in the city centre seem to have the same tolerance to drought, nitrates and shade as those in the suburbs.
On the graph, the drought tolerance index is given from 1 for the driest to 10 for the wettest.
We can therefore observe an identical tolerance of plant communities to drought whether they are near or far from the centre of Paris.
The plant communities in the city centre would therefore not be more adapted to urban conditions than those in the periphery.
Is the relationship between urban plants and insect pollinators important?
Plants in city centres are often physically isolated by urban constructions. We therefore expect to find plants that are able to pollinate themselves, i.e. self-pollinating.
On this graph, we can see that in the city centre, the communities have more wind-pollinated species (anemophiles) and, as we move away from the heart of Paris, insect-pollinated plant species (entomophiles or entomogamous) are more numerous.
The composition of urban plant communities would therefore be influenced by their mode of pollination.
Thanks to your data, we can observe the opposite phenomenon in provincial cities. In the city centre, plants are more often entomophilous. Why are these phenomena reversed between Paris and the provinces? Is it the impact of peri-urban agriculture? Is it the impact of the structure of the city, which is different between large and small cities?
These results are very interesting because they are relatively unexpected. In-depth analysis of a few more intensively inventoried cities may provide some explanations.
Students interview Nathalie Machon: Questions about the results of Vigie-Nature École.
Positioning your data
The number of species you counted depends on the surrounding conditions (pesticides, vegetation cuttings...), but also on the length of the street in which you made your observations. Use this graph of species diversity as a function of street length to see how good the diversity is.
In addition to the length of the street, other factors will also influence the distribution of wild plants and therefore your results:
- lthe presence of facilities allows plants with different requirements to establish themselves: reclining sow-thistle tolerates trampled areas well, wallflower takes advantage of the cracks in old walls, etc.
- the type of management: ryegrass can tolerate frequent mowing, pesticides systematically limit the number of species...
- environmental factors: e.g. Polycarpon quatrefoil is a Mediterranean species that is now found in cities further north because of the higher temperatures in urban centres.)
Understanding your result :
Diversity is lower than usual: this may be due to a temporary factor (e.g. weather) or to a type of management that does not favour biodiversity. Further surveys will provide more information.
Your results correspond to the norm. This situation may be transitional, either towards an improvement in species diversity or towards a deterioration. Continue your observations to find out!
You are in a rather encouraging situation, the biodiversity is richer than elsewhere! New surveys will make it possible to monitor this specific richness.