A non-native shrub invading a dry Andean valley: predicting the spread of Pyracantha angustifolia
Pyracantha angustifolia is an ornamental shrub that produces bright orange berries that, when coupled with its thorns, provides this plant with its suiting common name of “Firethorn”. Most of us have probably come across this shrub and may even have planted it in our gardens! Unfortunately, in some montane areas in South America it has invaded local habitats, where it forms dense thickets (Image 1). Here it may outcompete native vegetation, change the local conditions, and even alter the migratory patterns of the birds that feed on its plentiful supply of berries during the winter. Therefore, there is significant concern about the impacts of its invasion. However, little is known about what causes Pyracantha to grow so successfully in some places, which makes it very difficult to manage and prevent its spread.
So how do we predict where invasions are going to be both rapid and dense in a study system where there is no long-term data, where resources are limited, and where the invasion is occurring rapidly?
These were a few of the issues that I hoped to address as a part of my MSc project back in 2019 as a part of the University of Aberdeen’s Ecology and Conservation Programme. We aimed to first understand how fruit production in Pyracantha was influenced by the local habitat and grazing pressure. Then we wanted to apply computer simulations to see how these changes influence the population density and speed of invasion.
The field study
The first part of the project was to collect information on how local conditions influence fruit production by Pyracantha. For this I was lucky enough to spend six weeks working with local researchers in Tucaman (North-west Argentina).
We set up 30 study plots across the study site (Tafí del Valle). In each plot we recorded the habitat type (shrublands, rocky areas or grasslands), elevation, and the characteristics of the Pyracantha shrubs (damage by grazers, number of fruits, and size of shrubs). Our results suggested that Pyracantha produces more fruit in habitats dominated by native shrubs (shrublands) compared to those in rocky boulder fields and in grasslands. We also found that grazing reduced fruit production.
These results lead us to ask a bigger question: do these changes in fruit prodcution influence the invasion speed and density? In an attempt to answer that, we decided to apply a model to these results.
The computer model
We used computer simulations that incorporate the information about species dynamics that we collected in the field (fruit production in different conditions), as well as data from similar species in published literature (for example, how far seeds move from parent plants and survival) to simulate spread in different habitats and grazing regimes.
Through this model, we found that changes in fruit production may result in a substantially higher rate of spread in shrublands compared with rocky and grassland habitats—by up to 33%. Furthermore, grazing pressure from livestock can substantially depress the rate of spread by up to 53%. This is an especially important takeaway as urbanisation in the valley has resulted in a decline in the number of livestock.
This study provides useful information about Pyracantha spread, which could help to inform management. In addition this acts as a case study for modelling invasions in systems where invasions are rapid, long term data are lacking, and resources are limited.
If you would like to learn more about this research, you can find this study which was recently published in Biological Invasions here.
My take home messages
The time I spent in Argentina was certainly the highlight of my MSc programme. The people I met and the places we visited will stay with me forever. I gained experience in planning and conducting my own project, I was given a new appreciation of challenges of fieldwork (especially when working with thorny plants!), and I was able to apply many of the skills gained during my MSc to analyse this data and to develop a model to predict the spread of this invasive shrub.
Flash forward a few years and I am now a third year PhD student here in the University of Aberdeen’s School of Biological Sciences. My research uses similar approaches to those I was introduced to during my MSc to predict and inform mitigation of the impacts of ash dieback on connectivity for woodland invertebrates. I am thankful to have had the strong foundation and skills from my MSc programme to help prepare me for my continued research.
Plenderleith, F.A., Irrazabal, V.A., Burslem, D.F., Travis, J.M. and Powell, P.A., 2022. Predicting spatially heterogeneous invasive spread: Pyracantha angustifolia invading a dry Andean valley in northern Argentina. Biological Invasions. https://doi.org/10.1007/s10530-022-02769-8