Montjuïc: laboratory of biological invasions

• Biological invasions are currently considered one of the main threats to the planet.
• A recent study by an IBB research group has identified over 250 species of exotic plants in the Montjuïc mountain area.
• Montjuïc contains a significant concentration of introduced species, and it is an entry for many species into Catalonia and even Europe.

Over the last few years, the number of invasive species that are distributed around the world has increased. This has led to a loss of global biodiversity due to its negative impact by directly causing or adding to the destruction of natural habitats on one hand, and the displacement or extinction of other native species on the other one. Against a backdrop of strong human impact and a more than evident climate change, biological invasions are currently posed as one of the main threats to the planet.

A recent study led by the XenoPlants research group at the Botanical Institute of Barcelona, and published in the journal Plants, has shown that urban areas in our country, and also natural or semi-natural areas around urban areas, can be a focus for the entry and establishment of exotic species. This research, which has involved researchers from both the Consell Superior de Investigacions Científiques (CSIC) and the Museu de Ciències Naturals de Barcelona (MCNB), has identified around 250 species of exotic flora on the Montjuïc mountain, an area of just over 3 km². Among these species some of the most aggressive invaders of the Mediterranean basin can be found, such as Ailanthus altissima, which already dominates the NE side of the mountain, and Senecio angulatus.

<<The situation is alarming>> states Neus Ibáñez, first author of the study <<in such a small area, there is almost a quarter of all the exotic flora detected in Catalonia (32.108 km²) and the eighth part of that of the Spanish state (505.944 km²)>>. On the other hand, Montjuïc contains more species of exotic plants than, for example, the city of Rome (228 exotic plants in 1287 km²).

The reasons are many and varied for the enormous concentration of species introduced to Montjuïc, but they can be grouped into four main types. The first cause is the large number of gardens that skirt the mountain, many of them built for the Universal Exhibition of 1929, but others much more recently built, such as the Mossèn Costa i Llobera succulent garden or the Botanical Garden of Barcelona. Many of the garden plants that are grown there can escape and become wild in nearby places. The second cause is the proximity of the mountain to various routes of accidental entry of invasive alien species, especially the seaport and the airport. The third cause is the progressive anthropisation of the mountain with the construction of various infrastructures in addition to the aforementioned gardens, such as museums, a multitude of sports facilities (many linked to the 1992 Games) and, finally, an amusement park (now dismantled). It is worth mentioning the development of the mountain shantytown, which had an important effect on the introduction of foreign plants when, at its peak, was home to a population of more than 50,000 people at the end of the 1950s. Even today we can still see the remains of ancient subsistence crops (fruit trees such as the loquat and the peach, both of Asian origin), evidence of those hard times. And, as a third reason, it is not to be overlooked the number of visitors to the mountain (who number several million every year), attracted by its museums, concerts and shows, or simply by the marvellous views of the city from the top of the mountain.

For all these reasons, Montjuïc has not only become a place where many species from other parts of the region have established, but it has also often been the point of entry and expansion of many species to Catalonia and even to the whole of Europe; some of the first sightings of exotic plants (some of them invasive) in these areas have been in this mountain. <<And this is a process that, far from slowing down, will increase, largely due to the strong human impact and the effects of climate change>> says Carlos Gómez-Bellver, specialist in invasive flora and researcher at the IBB. Montjuïc could be considered a kind of natural “laboratory” for biological invasions and, perhaps, a model of what could end up happening to other urban mountains in the metropolitan area, such as the Parc dels Tres Turons or Collserola itself.

As the researcher Neus Ibañez explains <<one possible measure for the containment and control of exotic flora on the Montjuïc mountain would be the implementation of actions to remove, at the very least, the most invasive species, such as the ailanthus>>. The ailanthus, precisely, affects one of the clusters where in recent times a population of the only native palm tree on the Iberian Peninsula, the margalló (Chamaerops humilis), protected by law, has been observed. In the same way, the expansion of exotic flora can affect the Succowia balearica, also protected by law and very rare in Catalonia.

Reference Article:

Ibáñez, N., Gómez-Bellver, C., Farelo, P., Montserrat, J. M., Pyke, S., Nualart, N., López-Pujol, J. 2023. Montjuïc Hill (Barcelona): A hotspot for plant invasions in a Mediterranean city. Plants, 12(14), 2713. Doi: 10.3390/plants12142713