Predicting spatially heterogeneous invasive spread: Pyracantha angustifolia invading a dry Andean valley in northern Argentina

Abstract

Understanding the drivers of invasive species spread is key to designing optimal management programmes for controlling them. Population models, parameterized from demographic and dispersal data, are useful for simulating invasion when long-term observations are lacking, which is particularly important when invasions are occurring rapidly. We aimed to understand how the spread of Pyracantha angustifolia (an invasive ornamental shrub) is influenced by heterogeneity in local conditions within a dry inter-montane valley in northwestern Argentina that is currently experiencing rapid urban expansion and concomitant reduction in cattle farming. Field surveys demonstrated that individuals growing in shrublands have a higher fecundity than those in grassland and rocky habitats. Furthermore, grazing pressure of cattle and horses substantially reduces fecundity and impacts the relationship between age and fecundity. We incorporated our field-estimated habitat and age-specific fecundities into both analytical integro-difference equation models and individual-based models to predict rates of spread across each habitat type in the presence or absence of grazing pressure. Results indicate that the rate of spread would be substantially higher in shrublands (by up to 33%) and highlight that grazing pressure can, through direct consumption of seeds by livestock, substantially depress the rate of spread (by up to 53%). These results suggest that shrubland areas in the valley are most vulnerable to invasion and that grazing may help to reduce the impact of Pyracantha. This suggests that a synergism between urban expansion and reduction in domestic livestock density contributes to the spread of an invasive species in this environment.

Why Are Invasive Plants Successful?

“…Plant invasions are a global phenomenon associated with human activities and socioeconomic drivers (217). Human activities are important in not only transporting non-native plants beyond their native distribution ranges at unprecedented rates (177,246) but also promoting range expansion within the introduced ranges, even in protected or highly biodiverse systems (13,71,210,233,282). Moreover, invasions are mitigated or exacerbated by negative and positive feedback loops resulting from people's movements, behavior, and response to the collection, transport, introduction, and spread of non-native species (251).…”

“…Changes in land use and uses of non-native plants in agriculture and forestry that result in landscape fragmentation and degradation represent further disturbances at the landscape level. These phenomena have contributed to the range expansion of many non-native species (210,282), including invasion into ecosystems traditionally regarded as less invasible, such as forest remnants (282), by creating habitat edges that can be colonized by non-native species (106,173).…”

Reproductive Ecology of the Invasive Alien Shrub Pyracantha angustifolia in the Grassland Biome, South Africa

“…Empirical data published on P. angustifolia includes distribution and occurrence [ 13 , 15 , 31 , 32 , 33 , 34 , 35 , 36 ], germination [ 22 , 37 ], plant recruitment [ 38 , 39 , 40 ], uses by humans [ 41 ], impact on frugivore populations in association with P. angustifolia fruit abundance [ 42 , 43 , 44 ], and seed viability after ingestion by mammals and birds [ 37 , 45 ]. Although soil seed banks have been investigated, soil seed bank viability and distance from the source plant have not been assessed [ 38 ].…”