Introduction
Biological invasions are a global issue with potentially severe consequences for native communities (Davis et al. 2000; Fauschet al. 2001; O’Dowd et al. 2003; Lake & Leishman 2004). Successful invader colonisations can reduce biodiversity, alter community dynamics and cause large financial costs (Vitousek et al. 1997; Sher & Hyatt 1999; Fausch et al. 2001; Shea & Chesson 2002; Levine et al. 2003; Didham et al. 2005; Altman & Whitlatch 2007; Leishman et al. 2007). Disturbances - events that, through destroying biomass, change the availability of resources and habitats – often promote invader success (Shumway & Bertness 1994; Roxburgh et al. 2004; Altman & Whitlatch 2007; Lear et al. 2020). These can be small (e.g. leaves falling) or large (e.g. wildfires) in scale and facilitate invasions in a number of ways, for example by increasing resource availability, which in turn reduces invader-resident competition (Hobbs & Huenneke 1992; Baldwin & Mitchell 2000; Davis et al. 2000; Tilman 2004; Lear et al.2020). Disturbances may also remove any priority effects, and cause resident maladaptation (Davis et al. 2000; Stachowicz et al. 2002; Fargione et al. 2003; Altman & Whitlatch 2007; Fukami 2015).
Despite a large body of work showing that disturbance increases invader success (Lake & Leishman 2004; Roxburgh et al. 2004; Altman & Whitlatch 2007; Lembrechts et al. 2016; Lear et al. 2020), some studies show no or even a negative effect (Fausch et al.2001; Narimanov et al. 2020). This may be due to disturbance interacting or covarying with other key environmental variables that affect success. Resource abundance is likely to be particularly important in this context (Davis et al. 2000; Lear et al.2020). Where resources are abundant but not easily accessible, disturbance is likely to play an important role in promoting invader establishment. This is because disturbance will lead to an increased availability of resources that would otherwise be stored as biomass and depleted by consumption (Davis et al. 2000). In communities with low resources, the amount of resources released by disturbance will necessarily be low (Davis et al. 2000).
The effects of disturbance and resource abundance on invasion success are likely to depend on the invader’s life-history traits (Roxburghet al. 2004). Specifically, successful invaders are often fast-growing ‘coloniser’ species (van Kleunen et al. 2010) that can quickly convert available resources into biomass (Mächler & Altermatt 2012), and so are expected to excel in high disturbance and resource abundant conditions. However, whether slower-growing ‘competitor’ species invade more successfully at low disturbance and low resource abundance remains unclear.
Disturbance and resource abundance may have additional indirect effects on invasion by altering the composition of the resident community. On the one hand, disturbance frequency and resource abundance can help increase community productivity and biodiversity (Agard et al.1996; Worm et al. 2002; Kassen et al. 2004), which in turn may make the community more resistant to invasion (Levine & D’Antonio 1999; Hodgson et al. 2002; Tilman 2004; Brockhurst et al.2006): productive and diverse communities are more likely to contain dominant species (i.e. species that has a disproportionally large influence on invasion resistance) and have priority effects (i.e. situations where the first species to occupy a niche has a fitness advantage over species arriving subsequently) (Hodgson et al.2002; Fargione et al. 2003; Fukami 2015). These factors increase invasion resistance mainly by reducing invader access to resources (Naeem et al. 2000; Hodgson et al. 2002; Seabloom et al. 2003; Tilman 2004; Emery & Gross 2007; Fukami 2015). On the other hand, there is growing evidence that diversity may facilitate invasions through increased niche dimensionality (Simberloff & Von Holle 1999; Ricciardi 2001; Green et al. 2011), which increases the chance of an invader occupying a niche and leads to a negative relationship between diversity and invasion resistance (Fridley et al. 2007). Disturbance may weaken or eliminate these resident effects by decreasing resident population sizes, causing resource influxes and diminishing niche dimensionality.
The complexity of the potential interacting factors means that a simple, one-at-a-time, experimental investigation cannot unravel casual processes. Here, we experimentally investigate the independent and interactive effects of resource abundance and disturbance on invader success. We do this by invading wildtype populations of the bacteriumPseudomonas fluorescens SBW25 with genetically marked P. fluorescens SBW25 genotypes (Hodgson et al. 2002; Zhang & Buckling 2016; Lear et al. 2020) at different disturbance frequencies and resource abundances in a fully factorial design. The rapid evolutionary diversification of P. fluorescens populations into niche specialists (Rainey & Travisano 1998; Gómez & Buckling 2013) allowed us to determine any additional effects of evolved biodiversity and resident density - caused by disturbance and resource variation - on invasion success (Rainey & Travisano 1998; Kassenet al. 2000; Spiers et al. 2002; Koza et al. 2011; Hall et al. 2012). We invaded resident populations with two distinct genotypes: a fast-growing, coloniser morphotype and a slower growing competitor morphotype (Hall et al. 2012). This allowed us to determine whether the effects of disturbance and resource abundance on invasion success was contingent on the invader’s life history.