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\begin{document}

\title{Disturbance-mediated invasions are dependent on community resource
abundance}



\author[1]{Luke Lear}%
\author[2]{Daniel Padfield}%
\author[3]{Hidetoshi Inamine}%
\author[4]{Katriona Shea}%
\author[5]{Angus Buckling}%
\affil[1]{University of Exeter College of Life and Environmental Sciences}%
\affil[2]{Environment and Sustainability Institute}%
\affil[3]{Pennsylvania State University - Main Campus}%
\affil[4]{Penn State University}%
\affil[5]{University of Exeter}%


\vspace{-1em}



  
  \date{June 15, 2024}


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\selectlanguage{english}
\begin{abstract}
Disturbances can facilitate biological invasions, with the associated
increase in resource availability being a proposed cause. Here, we
experimentally test the effects of disturbance regime and resource
abundance on invasion success. We invaded populations of the bacterium
Pseudomonas fluorescens with two invader morphotypes in a factorial
design containing five disturbance frequencies and three resource
levels. As resident populations were altered by the treatments, we
additionally tested their effect on invader success. Disturbance
frequency and resource abundance interacted to affect the success of
both invaders. For one, success was positively affected by disturbance
under high resources but negatively under low. For the other,
disturbance negatively affected success under high resource abundance
but not under low or medium. Resident population changes did not alter
invader success beyond direct treatment effects. Overall, how
disturbance affects invasion success is dependent on community resource
abundance, and this interaction acts differently on invaders with
different life-histories.%
\end{abstract}%



\sloppy


\emph{Introduction}

Biological invasions are a global issue with potentially severe
consequences for native communities (Davis \emph{et al.} 2000;
Fausch\emph{et al.} 2001; O'Dowd \emph{et al.} 2003; Lake \& Leishman
2004). Successful invader colonisations can reduce biodiversity, alter
community dynamics and cause large financial costs (Vitousek \emph{et
al.} 1997; Sher \& Hyatt 1999; Fausch \emph{et al.} 2001; Shea \&
Chesson 2002; Levine \emph{et al.} 2003; Didham \emph{et al.} 2005;
Altman \& Whitlatch 2007; Leishman \emph{et al.} 2007). Disturbances -
events that, through destroying biomass, change the availability of
resources and habitats -- often promote invader success (Shumway \&
Bertness 1994; Roxburgh \emph{et al.} 2004; Altman \& Whitlatch 2007;
Lear \emph{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 \emph{et al.} 2000; Tilman 2004; Lear \emph{et
al.}2020). Disturbances may also remove any priority effects, and cause
resident maladaptation (Davis \emph{et al.} 2000; Stachowicz \emph{et
al.} 2002; Fargione \emph{et al.} 2003; Altman \& Whitlatch 2007; Fukami
2015).

Despite a large body of work showing that disturbance increases invader
success (Lake \& Leishman 2004; Roxburgh \emph{et al.} 2004; Altman \&
Whitlatch 2007; Lembrechts \emph{et al.} 2016; Lear \emph{et al.} 2020),
some studies show no or even a negative effect (Fausch \emph{et
al.}2001; Narimanov \emph{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 \emph{et al.} 2000; Lear \emph{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 \emph{et al.} 2000). In communities with
low resources, the amount of resources released by disturbance will
necessarily be low (Davis \emph{et al.} 2000).

The effects of disturbance and resource abundance on invasion success
are likely to depend on the invader's life-history traits
(Roxburgh\emph{et al.} 2004). Specifically, successful invaders are
often fast-growing `coloniser' species (van Kleunen \emph{et al.} 2010)
that can quickly convert available resources into biomass (M\selectlanguage{ngerman}ächler \&
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 \emph{et
al.}1996; Worm \emph{et al.} 2002; Kassen \emph{et al.} 2004), which in
turn may make the community more resistant to invasion (Levine \&
D'Antonio 1999; Hodgson \emph{et al.} 2002; Tilman 2004; Brockhurst
\emph{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 \emph{et
al.}2002; Fargione \emph{et al.} 2003; Fukami 2015). These factors
increase invasion resistance mainly by reducing invader access to
resources (Naeem \emph{et al.} 2000; Hodgson \emph{et al.} 2002;
Seabloom \emph{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 \emph{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
\emph{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
bacterium\emph{Pseudomonas fluorescens} SBW25 with genetically marked
\emph{P. fluorescens} SBW25 genotypes (Hodgson \emph{et al.} 2002; Zhang
\& Buckling 2016; Lear \emph{et al.} 2020) at different disturbance
frequencies and resource abundances in a fully factorial design. The
rapid evolutionary diversification of \emph{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;
Kassen\emph{et al.} 2000; Spiers \emph{et al.} 2002; Koza \emph{et al.}
2011; Hall \emph{et al.} 2012). We invaded resident populations with two
distinct genotypes: a fast-growing, coloniser morphotype and a slower
growing competitor morphotype (Hall \emph{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.

\emph{Methods}

\emph{Resident populations}

Ancestral \emph{Pseudomonas fluorescens} SBW25 was grown overnight to
carrying capacity in shaken glass vials (microcosms) containing 6mL of
King's medium B (KB) at 28°C. Sixty microliters of this culture was then
transferred into static microcosms containing KB of varying
concentrations (100\% KB, 10\% or 1\%) to create different resource
abundances; KB was diluted with M9 salt solution (3g
KH\textsubscript{2}PO\textsubscript{4}, 6g
Na\textsubscript{2}HPO\textsubscript{4}, 5g NaCl L\textsuperscript{-1}).
Microcosms were disturbed every 1, 2, 4, 8 or 16 days (Fig. 1) by
transferring 1\% of homogenised broth into fresh media (99\% mortality)
for a total of 16 days. Invaders were inoculated at days 4, 8 and 12
(Fig. 1 and see \emph{Invasions} below). In between transfers and
invasions, all microcosms were kept static at 28°C with loose lids to
allow oxygen transfer. We used 12 replicates of each resource abundance
(3) and disturbance frequency (5) combination, for a total of 180
microcosms. Additional microcosms (n=3 per resource abundance x
disturbance combination) were set up to quantify resident density on day
4 -- the first invasion time point. This was necessary as the sampling
microcosms required homogenisation of treatments that would otherwise
not be disturbed.

\emph{Invasions}

\emph{Pseudomonas fluorescens} with a \emph{lacZ} marker was used as the
invader; the \emph{lacZ} marker makes it visually distinguishable from
the wildtype on agar containing X-gal
(5-bromo-4-chloro-3-indolyl-\selectlanguage{greek}β-\selectlanguage{english}D-galactopyranoside) due to a blue colour
change (Zhang \& Rainey 2007). Although originally reported as a neutral
marker (Zhang \& Rainey 2007), the \emph{lacZ} insert has previously
been found to offer a fitness advantage to invaders (Lear \emph{et
al.}2020). The strain was left to diversify for five days in static KB
before being plated and a single smooth (SM) and wrinkly spreader (WS)
colony was selected, grown and stored in 25\% glycerol solution at
-80\textsuperscript{o}C. SM morphotypes (our `coloniser' invader)
inhabit the broth where they grow rapidly, whereas WS (our `competitor'
invader) form biofilms at the air-broth interface: biofilm formation
increases competitive ability for oxygen but at a cost to growth rate
(Hall \emph{et al.} 2012). Before each invasion, these freezer stocks
were used to grow each morphotype overnight in shaken KB as described
above; these cultures were diluted to 1\% with M9 salt buffer before
use. All microcosms were invaded every four days with 60\selectlanguage{greek}µ\selectlanguage{english}L of either SM
or WS invader (total colony forming units (CFU) added over the three
events: SM = 8.1x10\textsuperscript{6}; WS = 6.6x10\textsuperscript{6}).
If a microcosm had been disturbed, invasion would occur
post-disturbance. This resulted in 6 replicates for each disturbance by
resource abundance combination per invader morphotype.

Experiments finished on day 16, when all microcosms were homogenised and
a 900\selectlanguage{greek}µ\selectlanguage{english}L sample was frozen in 25\% glycerol at -80\selectlanguage{ngerman}ºC. After plating on KB
agar containing 100\selectlanguage{greek}µ\selectlanguage{english}g/L of X-gal, wildtype and invader SM, WS and fuzzy
spreader (FS; a rarer bottom-dwelling morph (Rainey \& Travisano 1998))
colonies were counted.

\emph{Statistical Analyses}

All counts were first standardised to colony-forming-units (cfu) per mL.
Invasion success (relative invader fitness) was calculated as
proportional change, \emph{v} , of the proportion of invader to
resident, calculated as: \emph{v = x\textsubscript{2}.} (\emph{1 -
x\textsubscript{1}} )/\emph{x\textsubscript{1}.} (\emph{1 -
x\textsubscript{2}} ), where \emph{x\textsubscript{1}} is the initial
invader proportion and \emph{x\textsubscript{2}} the final
(Ross-Gillespie \emph{et al.} 2007). Initial invader proportion
(\emph{x\textsubscript{1}} ) was calculated as the average frequency of
the introduced invader:

\(x_{1}\ =\ E\left[\frac{I_{t}}{I_{t}\ +\ R_{t}}\right]\ =\ \frac{1}{3}\sum_{t\ =\ \left\{4,\ 8,\ 12\right\}}\frac{I_{t}}{I_{t}\ +\ R_{t}}\)(1)

where \emph{I\textsubscript{t}} is the density of the invader introduced
on day \emph{t} and \emph{R\textsubscript{t}} is the density of the
residents getting invaded on day \emph{t} . We could not measure
resident density on days 8 and 12, because it would require destructive
sampling of undisturbed treatments. We therefore used the resident
density on day 4 and assumed that \emph{R\textsubscript{4,}
R\textsubscript{8}} and\emph{R\textsubscript{12}} were equal for 1-, 2-,
and 4-days disturbance treatments.

We sampled \emph{R\textsubscript{4}} for 1-, 2-, and 4-days disturbance
treatments during their transfers, but we could not
sample\emph{R\textsubscript{4}} for 8- and 16-days disturbance
treatments as it is a destructive process. The disturbance history up to
day 4 for 8- and 16-days treatments is identical to that for 4-days
treatment. We therefore assumed the resident community dynamics are the
same for these three treatments, and used \emph{R\textsubscript{4}} for
4-days treatment (before the disturbance) to
calculate\emph{R\textsubscript{4}} for 8- and 16-days treatments:

\(R_{4,\ 8-days}\ =\ R_{4,\ 16-days}\ =\ \frac{R_{4,\ 4-days}}{\text{Disturbance\ mortality\ rate}}=\ \frac{R_{4,\ 4-days}}{0.01}\)(2)

where \emph{R\textsubscript{i,j}} is the density of the resident on
day\emph{i} under \emph{j-} days disturbance treatment. Based on this
calculation, we further assumed that \emph{R\textsubscript{8,16-days} =
R\textsubscript{12,16-days} = R\textsubscript{4,16-days}} for 16-days
disturbance treatment, where \emph{R\textsubscript{8,16-days}} =\emph{R}
on day 8 in the 16-day disturbance treatment and so forth. For 8-days
disturbance treatment, we assumed\emph{R\textsubscript{12,8-days} =
R\textsubscript{4,8-days}} and\emph{R\textsubscript{8,8-days} = 0.01
R\textsubscript{4,8-days}} to account for the disturbance event on day
8.

In order to eliminate zero inflation, one was added to the final invader
density \emph{v} (post volume standardisation) and was transformed to
log(\emph{v} +1) to normalise the residuals. A value greater than 0.69
(log(1+1)) would indicate that the invader increased in proportion
throughout the experiment, whereas a value below this would suggest that
invasion was unsuccessful.

To analyse the effect of disturbance and resource abundance on invasion
success\emph{,} \emph{v,} a linear model was used to test effects of
disturbance, resource abundance, and invader morphotype, with all
two-way and three-way interactions. As the different morphotypes have
distinct growth strategies, we expected their invasion success to be
markedly different. Given a significant three-way interaction in the
most complex model, we did all further analysis on each invader
morphotype (SM \& WS) separately.

For each invader morph, separate linear models were used to investigate
treatment (disturbance frequency and resource abundance) effects on
invasion success, evolved biodiversity (calculated using Simpson's index
(Simpson 1949)) and total resident density (log10(cfu+1
mL\textsuperscript{-1}). Disturbance frequency was treated as a
continuous predictor, whereas resource abundance was treated as
categorical due to only having three levels. Model selection was done
using likelihood ratio tests.

We then tested whether treatments indirectly affected invasion success
through changes in resident populations. To do this we first used a
model with resident biodiversity and total resident density, plus their
interaction, as predictors of invasion success. We then included
treatment (disturbance, resource abundance, and their interaction),
alongside resident population effects as predictors of success. The
models with both treatment and resident population effects were
initially tested using an ANOVA with type III sums of squares, then with
type II if no significant interactions were found to account for
differences in the ordering of predictors on significance testing.

Post-hoc model comparisons were used to look at significant differences
between levels of resource abundances and disturbance. For pairwise
comparisons of single treatments (e.g. between high, medium and low
resource abundances), model estimates were averaged over other
predictors in the model. Where multiple pairwise comparisons were used,
p values were adjusted using Bonferroni adjustments. When comparing
slopes to 0, confidence intervals overlapping zero indicated no
significant effect. All statistical analyses were carried out in R
version 4.0.3 (R Core Team).

\emph{Results}

\emph{Invasion success (invader proportional change) differed between
invader types}

Invader success was significantly affected by a 3-way interaction
between disturbance frequency, resource abundance and invader morphotype
(F\textsubscript{2,163}=10.2, p\textless{}0.001; Fig. 2). We therefore
analysed treatment effects on each invading morphotype separately. The
fast-growing smooth (SM) invaders were significantly affected by an
interaction between disturbance frequency and resource abundance
(F\textsubscript{2,85}=9.7, p\textless{}0.001, Fig. 2). Greater
disturbance increased invasion success when resources were abundant
(slope = 0.12, 95\% CI {[}0.2, 0.050{]}), but decreased success when
they were of low abundance (slope = -0.089, 95\% CI {[}-0.020,
-0.16{]}). Disturbance had no significant effect under medium resource
abundance (slope = -0.048, 95\% CI {[}0.021, -0.12{]}). This meant the
highest levels of SM invasion occurred when both disturbance frequency
and resource abundance was high.

Wrinkly spreader (WS) success was also affected by an interaction
between disturbance frequency and resource abundance
(F\textsubscript{2,78}=3.31, p=0.042; Fig. 2). Here we found disturbance
to be negative for WS success when resources were high (slope = -0.24,
95\% CI {[}-0.11, -0.37{]}), but to have no effect when they were at
medium or low abundance (medium: slope = -0.033, 95\% CI {[}0.069,
-0.14{]}; low: slope = -0.076, 95\% CI {[}0.026, -0.18{]}).

\emph{Resident biodiversity was affected unimodally by disturbance,
whereas density linearly}

Resident biodiversity (Simpson's index) showed the same unimodal pattern
across disturbance frequencies irrespective of invader type (SM invader:
F\textsubscript{1,86}=10.3, p=0.002, WS: F\textsubscript{1,79}=7.87,
p=0.006) with the least diverse communities at both high and low
disturbance (Fig. 3). Resource abundance also altered resident
biodiversity (SM invader: F\textsubscript{2,86}=3.84, p=0.025, WS:
F\textsubscript{2,79}=33.1, p\textless{}0.001), with diversity being
significantly lower in the low resource treatment than the medium when
invaded by SM (p=0.025) and lower than both the medium and high resource
treatments when invaded by WS (p=\textless{}0.001 for both).

Like biodiversity, resident density showed the same patterns
irrespective of invader type (Fig. 4), with an interaction between
disturbance frequency and resource abundance significantly affecting
density (SM invader: F\textsubscript{2,85}=49.4, p\textless{}0.001, WS:
F\textsubscript{1,79}=47.0, p\textless{}0.001; Fig. 4). Resident density
increased with disturbance under high resources, but disturbance
negatively impacted density at low and medium resources (Fig. 4).

\emph{Resident population changes did not alter success above the direct
effects of treatments}

To test if these changes to the resident populations impacted invasion
success, we first analysed a model with resident biodiversity and total
resident density, plus their interaction, as sole predictors of invasion
success. Once again this was done separately for each invader morph. SM
invaders were significantly affected by resident density
(F\textsubscript{1,88}=5.03, p=0.028), but not by biodiversity
(F\textsubscript{1,88}=2.64, p=0.11) or an interaction between density
and diversity (F\textsubscript{1,87}=3.06, p=0.084). Conversely, the WS
invader was only significantly affected by biodiversity
(F\textsubscript{1,81}=7.07, p=0.010), with density having no
significant effect either as a main effect (F\textsubscript{1,81}=0.67,
p=0.42) or as through an interaction with biodiversity
(F\textsubscript{1,80}=1.25, p=0.27). This demonstrates treatments may
have indirectly affected the success of both invaders by manipulating
resident populations. We therefore tested whether the direct effect of
treatments on success remained when these manipulations were considered.
SM invaders were still significantly affected by the interaction between
disturbance and resources (F\textsubscript{2,82}=9.27,
p\textless{}0.001). However, we find the effect of both biodiversity and
total resident density to not be significant (biodiversity:
F\textsubscript{1,82}=2.49, p=0.12; Fig. 5A; density:
F\textsubscript{1,82}=0.24, p=0.63; Fig. 5B). When testing resident
population effects alongside treatments on the success of the WS
invader, we no longer found any significant interactions. Disturbance
and resource abundance both significantly affected WS success
(F\textsubscript{1,75}=8.27, p=0.005 and F\textsubscript{2,75}=27.8,
p\textless{}0.001, respectively). However, resident population effects
did not have a significant effect (biodiversity:
F\textsubscript{1,75}=2.88, p=0.094, total resident density:
F\textsubscript{1,75}=0.006, p=0.94). We therefore show that, although
treatments had a significant effect on resident populations, this did
not have an effect on success above the direct effects of disturbance
and resource abundance.

\emph{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 \emph{et al.} 2004; Altman \& Whitlatch 2007;
Lear \emph{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 \emph{et
al.} 2002; Fargione \emph{et al.} 2003; Fukami 2015; Lear \emph{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
(Hodgson\emph{et 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 \emph{et al.} 2000; Brockhurst \emph{et al.} 2006;
Hall\emph{et al.} 2012). At higher disturbances and lower resources, the
slower growth rate of WS relative to SM (Haddad \emph{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 \emph{et al.} 2010; M\selectlanguage{ngerman}ächler
\& 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 \emph{et al.} 2000; Cardinale \emph{et al.} 2006;
Benmayor\emph{et al.} 2008; Violle \emph{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 \emph{et al.} 2004). Our prior
work (Miller \emph{et al.} 2011; Hall \emph{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 \emph{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.

\emph{Data accessibility}

The full dataset used is accessible from the Zenodo Digital Repository:
10.5281/zenodo.5057319

\emph{Funding}

This work was supported by NSF-NERC award no. DEB-1556444 and NERC award
no. NE/R011524/1.

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\emph{Figures}

\emph{Fig. 1}\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image1/image1}

\end{center}
\end{figure}

\emph{Fig. 2}\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image2/image2}

\end{center}
\end{figure}

\emph{Fig. 3}\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image3/image3}

\end{center}
\end{figure}

\emph{Fig. 4}\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image4/image4}

\end{center}
\end{figure}

\emph{Figure legends}

\textbf{Figure 1} Schematic of the experimental design. Microcosms of
either 100\%, 10\% or 1\% resource concentration were disturbed every 1,
2, 4, 8 or 16 days (denoted by an icon of a microcosm) to test for the
effects of both disturbance frequency and resource abundance on invader
success. Disturbances involved 1\% transfer of homogenised broth into
fresh media. All microcosms were invaded every four days (immediately
post-disturbance) with either a smooth (SM) or wrinkly spreader (WS)
invader. Six replicates per treatment were used.

\textbf{Figure 2} Invasion success, log(\emph{v} +1), of (A) the smooth
(SM) invader and (B) the wrinkly Spreader (WS), in response to different
disturbance frequencies and resource abundances (low resources = red
circles and lines, medium = blue, high = black). \emph{v} is the
proportional change in invader density compared to the residents; the
dashed line shows the value of equal population growth rate between
residents and invaders, where invaders would have the same proportion in
the community at the beginning and the end of the experiment. Jittered
points represent individual replicates. Lines show the best model fits
and shaded areas show the 95\% confidence interval.

\textbf{Figure 3} Evolved resident \emph{P. fluorescens} biodiversity
(Simpson's index) in treatments of different disturbance frequencies
(increasing from left to right within panels) and resource abundances
(low resources = red circles and lines, medium = blue, high = black)
when invaded by (A) a smooth (SM) invader and (B) a wrinkly spreader
(WS). Diversity was significantly lower in the low resource treatment
for both invaders. Resource abundance and invader type affected
diversity through an interaction. Jittered points represent individual
replicates. Lines show the best model fits and shaded areas show the
95\% confidence interval.

\textbf{Figure 4} Final resident density (log10(cfu+1)/mL) after sixteen
days in treatments of different resource abundances (low resources = red
circles and lines, medium = blue, high = black and disturbance
frequencies). Panel A shows treatments invaded with a smooth (SM)
morphotype, panel B by a wrinkly spreader (WS). Jittered points
represent individual replicates. Lines show the best model fits and
shaded areas show the 95\% confidence interval.

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