Colonization pressure: a second null model for invasion biology

Abstract

Understanding the causes of spatial variation in the distribution and richness of alien species is a key goal of invasion biology. Thanks to the increasing availability of geographical compendia of alien species it is also the subject of a burgeoning scientific literature. Here, we review elements of this literature to argue that understanding the causes of variation in alien species richness cannot be achieved without knowledge of the number of species introduced to an area-termed colonization pressure. The implications of a lack of information on colonization pressure are widespread and, we believe, poorly recognised. We start by discussing a recent general model for alien species richness that demonstrates why colonization pressure is a fundamental determinant of this. We then explore the literature on alien species richness, particularly on islands, to demonstrate how failing to account for colonization pressure affects our perception of richness drivers. We further show that ignoring colonization pressure can bias our understanding of patterns in the distributions of alien species. We finish by discussing situations when colonization pressure may be more or less important for our understanding of alien invasions.

Impact of introduction pathways on the spread and geographical distribution of alien species: Implications for preventive management in mediterranean ecosystems

“…deliberate release). We note that patterns of alien species richness (number of species occupying a site) are interrelated with range size (number of sites occupied by species; Blackburn et al, 2019). Therefore, the insights from the relationship among pathways and range size will also contribute to understanding the processes behind the spatial pattern with low congruence.…”

“…We finally selected a set of environmental variables that have been linked to alien species richness in Catalonia and Spain (Table 1) based on previous studies, at 10‐km UTM scale (Gassó et al., 2009; Giraldo‐Beltrán et al., 2015; Pino et al., 2005). We included proxies of propagule and colonization pressure (urban cover, population density, distance to roads), which are key factors in determining variation in alien species richness across sites (Blackburn et al., 2019; Lockwood et al., 2009). It was not possible to separate the relative contributions of colonization and propagule pressure to pathway‐specific alien species richness, since both are positively related and adequate specific proxies remain uncertain (Blackburn et al., 2019; Lockwood et al., 2009).…”

Applying the Convention on Biological Diversity Pathway Classification to alien species in Europe

“…Some pathways and taxonomic groups contribute disproportionally to the overall risk from IAS (Essl et al 2015;Pergl et al 2017) and these should be the subject of increased attention. However, to fully assess the potential risk of each pathway, not only is the proportion of species with negative impact relevant, but also the propagule pressure (Blackburn et al 2020), climatic match to the source region (Faulkner et al 2017) and other factors like presence/absence of sanitary and phytosanitary measures need to be used. The terminology in the CBD Pathway Classification framework and in other broadly-used schemes is somewhat different.…”

The role of arbuscular mycorrhizal fungi in nonnative plant invasion along mountain roads

“…Apart from mycorrhizal associations, anthropogenic disturbances are another important determinant of nonnative plant invasion (Hobbs & Huenneke, 1992; Jauni et al ., 2015; Lembrechts et al ., 2016). The effects of disturbance on plant competition (Biswas & Mallik, 2010), nutrient availability (Davis et al ., 2000; Blumenthal, 2006) and propagule and colonisation pressure (Vilà & Ibáñez, 2011; Blackburn et al ., 2020) are all known to facilitate the invasion success of nonnative plants. We hypothesise that changes in belowground mutualist interactions caused by disturbances could also play a significant role during the invasion process.…”