Among the invasive alien species we find many that are introduced accidentally. However, when talking about plants, deliberate introduction is more frequent, and one of the most important ways for this introduction is its use as an ornamental plant in gardens and urban parks.
It is enough to go out to the nearest park to find several invasive species. Knowing how many there are is no easy task, and even less knowing their impact.
Recently, Dr. Montserrat Vilà , from the Doñana Biological Station (CSIC), Dr. Oscar Godoy , from the University of Cádiz, and yours truly analyzed the risks of invasion and the possible environmental impacts of exotic woody plants in the parks urban Spanish, in a work published in the journal Anales del Jardín Botánico de Madrid .
List making
A fundamental part of the work has been based on drawing up lists with the plants that appear in the parks, based on various variables: the state of invasion in which they are found, whether or not they are potential invaders in Spain, and what are the impacts potential that can cause From there, a ” priority list ” was obtained, made up of four species that are not only invasive, but are also regulated by the Catalog of the Ministry of Ecological Transition. This is particularly striking, since the same catalog indicates that the competent administrations “will progressively eliminate, in cases where it is justified, these species.” Something that, obviously, is not being done.
On this ” priority list ” are mimosa wattle ( Acacia dealbata ), butterfly bush ( Buddleja davidii ), prickly pear ( Opuntia ficus-indica ) and tree of heaven ( Ailanthus altissima ). The latter is particularly relevant in the study, because despite being invasive, it appears in 43.5% of the parks and gardens studied.
Another important list of the study is the “ watch list ”, which includes 47 species widely distributed in parks and gardens, which are considered to be either invasive or not yet regulated —such as the eucalyptus ( Eucalyptus globulus ) or the false acacia ( Robinia pseudoacacia )— or species that are not yet invasive, but have the potential to be , and with a high level of possible impacts —such as the Brazilian pepper tree ( Schinus terebinthifolius ) or the Virginia sumac ( Rhus typhina )—.
In addition, a prioritization system developed in 2019 specifically for ornamental plants was carried out on this list, which combines the risks of impact and invasion and the interest shown by the population in the different species. The most representative species in this index were the castor bean ( Ricinus communis ), the false acacia, the espinillo ( Parkinsonia aculeata ) and the eucalyptus.
Not a single park out of all those studied was found in the study that did not have an invasive or potentially invasive species , and almost half have one or more species that are still regulated.
The creation of fashions
One of the most interesting conclusions of the study is the clear correlation between the presence of invasive and potentially invasive plants in parks and the interest they arouse in the population, measured with a standardized system based on Google Trends, called “standard value of interest”. ” (STV). The more common a plant is in a park, the more interest it arouses in the population .
This result is interpreted as a fashion creation effect. When a plant is common, interest in it is generated. There are studies that correlate the search intensity in Google Trends with the increase in trade, which would lead to a vicious circle : more presence of plants increases interest, more interest implies more trade, and the more they are traded, the more present they are . All this would further favor the invasion process.
Measuring Potential Impacts
Invasive species often cause certain impacts on the ecosystems in which they settle and the plants in our parks and gardens, although they seem harmless, are no exception. These impacts are intimately correlated with each other in a positive, but never negative way; there are potential impacts that complement each other, but no impact is reducing the incidence of others.
According to the results of the study, among the most common impacts that they could cause, competition with native species and the physical modification of habitats stand out.
By competing with other plants, invasive species can displace them, which would change the structure of the ecosystem and alter the way in which the vegetation regenerates. In addition, many invaders produce allelopathic substances —chemicals that they emit through their roots or through fallen leaves—, which permeate the soil and inhibit the growth of other plants.
But these potential impacts would not only affect ecosystems. The human species is also a victim of these damages. Plants that produce allelopathic substances or those that transmit diseases frequently generate negative impacts on agriculture ; the toxicity of certain invasive plants, meanwhile, can cause damage to livestock . Some plants, like the tree of heaven, have roots so strong that they are capable of destroying infrastructure. And there is a constant impact on human health, mainly through the entry of allergy-causing pollen, or accidental poisoning. In the study, a total of 31 invasive and potentially invasive species in our parks and gardens can cause health damage .
This article has been written for Very Interesting by Dr. Álvaro Bayón, first author of the referenced scientific article
REFERENCES:
Bayon, A. et al. 2022. Invasion risks and social interest of non-native woody plants in urban parks of Spain [Data set]. In Annals of the Botanical Garden of Madrid. DOI: 10.3989/ajbm.2623
