Discussion
Here, we used a microbial system to experimentally test how disturbance frequency and resource abundance interact to affect the success of two ecologically different invaders. Both invaders were affected by an interaction between disturbance and resources, however this acted differently on each type of invader. The success of the fast-growing smooth (SM) invader increased with increasing disturbance frequency when resources were abundant, but decreased when resources were low. Conversely, the slower growing wrinkly spreader (WS) suffered decreasing success with increasing disturbance frequency under high resource abundance, but was not affected by disturbance in medium or low resource conditions.
Disturbances are commonly linked with invasion success (Shumway & Bertness 1994; Roxburgh et al. 2004; Altman & Whitlatch 2007; Lear et al. 2020), and the positive relationship between disturbance frequency and SM invasion success in the resource rich treatment supports this view. Disturbances open up resources for the fast growing invaders and reduce biotic resistance (Hodgson et al. 2002; Fargione et al. 2003; Fukami 2015; Lear et al.2020). Moreover, high resource availability allows rapid population growth between disturbances, reducing the chance of small invader populations being stochastically removed by disturbance. That SM invaders had reduced fitness at low disturbance frequency-high resource abundance was likely a consequence of escalating broth toxicity and oxygen depletion. Moreover, surviving residents may have reduced invader access to resources through priority and dominance effects (Hodgsonet al. 2002; Zee & Fukami 2018). These factors (broth toxicity, oxygen depletion and resident effects) will likely be weaker when resources are less abundant as growth will be slower, potentially explaining why success was higher at low disturbance when resource abundance was less. The inability of disturbances to facilitate invasion under lower resources can be explained by disturbances not providing sufficient additional resources to benefit the invader (14). At the lowest resource levels, the inhibitory effect of disturbance on invasion is presumably because invader populations could not grow fast enough between disturbances to recover. These results may offer an explanation as to why disturbance may not always facilitate invasion by fast-growing coloniser species.
It is likely that low disturbance-high resource facilitated WS invasion because of its ecological niche: WS forms a mat at the air-broth interface that provides access to both nutrients and oxygen. Mat formation requires a threshold density to be reached, and low disturbance and high resource abundance will make this more likely (Buckling et al. 2000; Brockhurst et al. 2006; Hallet al. 2012). At higher disturbances and lower resources, the slower growth rate of WS relative to SM (Haddad et al. 2008) also likely increases the importance of stochastic removal of WS invaders, which would have happened less under high resources due to faster growth rates. We therefore demonstrate high resource abundance can reduce the negative effects of disturbance on slower-growing species. That the WS invader had much greater success than the faster growing SM under high resources-low disturbance shows the classical view that invaders are fast growing coloniser species (van Kleunen et al. 2010; Mächler & Altermatt 2012) depends strongly on the new disturbance regime. This suggests the balance between disturbance-induced mortality and growth rate is an important factor deciding invader success, with resource abundance dictating growth rate and disturbance affecting mortality.
As well as invader success, treatments affected resident populations, with disturbance and resources affecting resident biodiversity and total density. That diversity peaked at medium disturbance frequency in our treatments is in line with previous work in this system and supports the Intermediate Disturbance Hypothesis (Connell 1978; Wilkinson 1999; Buckling et al. 2000; Cardinale et al. 2006; Benmayoret al. 2008; Violle et al. 2010). This proposes that diversity is lost at high disturbance due to species being unable to recover between events, lost at low through competitive exclusion and highest when disturbance facilitates a balance between tolerant and competitive species (Huston 1979). In our system we found this pattern to remain the same across resource treatments, but diversity to be lower under the lowest resource abundance. This pattern is consistent with previous findings in this system (Kassen et al. 2004). Our prior work (Miller et al. 2011; Hall et al. 2012) also shows that this result depends on the other disturbance aspects, such as disturbance intensity. Resident density decreasing with increasing disturbance in low and medium (but not high) resource abundances is most is likely explained by resource-limited growth causing slow population recovery between disturbances. Changes to resident populations were, however, found to have little indirect effect on invasion resistance, with their explanatory power non-significant when direct treatment effects were included in the model. This does not rule out a role for resident species, but shows that they were relatively unimportant compared with the direct effects of treatments. Further, we show factors that cause differences in biodiversity (for example disturbance frequency and resource abundance) need to be controlled for when studying the effect of diversity on invasion resistance, as the direct effect of these may be causing the differences in success rather than biodiversity per se (as is the case of the SM invader here).
In conclusion, we find disturbance frequency and resource abundance to both affect the success of two different invaders. Further, we find both invaders to be differently affected by an interaction between these factors: the fast growing SM success is positively associated with disturbance frequency when resources are readily available, but negatively when they are limited whereas the slower-growing WS is only affected by disturbance when resource abundance is high. As this interaction between disturbance and resources acts on two ecologically fundamental processes – growth and mortality – we suggest it is applicable to species outside of this microbial system. Additionally, and contrary to classical theory stating invaders are generally fast growing species, the slower growing WS invader had very high success when disturbance was infrequent and resource abundance high. We therefore demonstrate that when studying invasion ecology multiple factors need to be considered to create an accurate predictive theory of invasibility, with the same disturbance frequency having both positive and negative effects depending on resource abundance and invader life-history. Finally, we show that, by understanding these interactions, it may be possible through ecological manipulations of resource abundance to reduce the effect that disturbances have on invasion resistance.