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.