Introduction
Disturbances, defined as events that destroy biomass and change species
interactions, are ubiquitous and fundamental to structuring natural
communities (Connell 1978; Huston 1979; Wilkinson 1999; Bucklinget al. 2000; Kassen et al. 2000; Mackey & Currie 2001).
How disturbances structure communities is dependent on their regime,
which is characterized by their frequency (number of events over a given
time period), severity (proportion of biomass destroyed), extent,
timing, and duration (Miller et al. 2021). Past studies have
shown that a spectrum of disturbance regimes can lead to a wide range of
disturbance-diversity relationships (Connell 1978; Wilkinson 1999;
Buckling et al. 2000; Mackey & Currie 2001; Wilson & Tilman
2002; Cardinale et al. 2006; Violle et al. 2010; Milleret al. 2011). Changes to disturbance regimes can either directly
increase or decrease biodiversity in a community, resulting from the
complex interactions between the historical and novel disturbance
regimes and the life-histories of the component species (Miller et
al. 2021; Lear et al. 2022b). Whilst some changes in disturbance
regimes can increase diversity, the global trend suggests that more
frequent and intense disturbance regimes are likely to decrease
diversity (Seidl et al. 2017). More importantly, disturbances are
increasingly occurring simultaneously due to human activities (e.g.,
extreme fire and drought due to warmer temperatures) (Seidl et
al. 2017; Stockwell et al. 2020), and have been shown to have
additive, antagonistic, or synergistic effects on diversity (Sala &
Knowlton 2006; Darling & Côté 2008). However, despite previous work
testing the effect of multiple disturbance (Seidl & Rammer 2017;
Stockwell et al. 2020; Polazzo et al. 2022), whether the
interactive effect of multiple disturbances on diversity is the same
across different disturbance regimes remains uncertain. Evidence from
the multiple stressor (a perturbation that impairs the function of a
population) literature suggests that a change in interactive effect is
likely, as it shows that the effect of one stressor on the density of a
population can vary from antagonistic to synergistic depending on the
magnitude of a second stressor (King et al. 2022; Turschwellet al. 2022).
Two common and relatively well studied types of disturbances are pulse
mortality events and the introduction of non-native species (invasions)
into communities (Didham et al. 2005). Both disturbance types can
severely reduce the diversity of the resident community: frequent pulses
eliminate organisms before they have a chance to reproduce (Bohnet al. 2014), and aggressive invaders can have a suite of
negative impacts (Pyšek et al. 2020), including affecting
nutrient availability (Zhang et al. 2017) and introducing novel
parasites and pathogens (Prenter et al. 2004).
Importantly, these two different disturbances may interact to affect
biodiversity. For example, pulse mortality may temporarily promote
invasion by reducing both the competition between the invader and the
residents for resources (Davis et al. 2000; Altman & Whitlatch
2007; Lear et al. 2020), and any priority or dominance effects
the residents may have (Fargione et al. 2003; Urban & De Meester
2009). Invaders, in turn, may aggravate pulse mortality by competing
with the residents for resources needed to quickly recover after the
event and thus change the ecological and evolutionary course of the
community (Davis et al. 2000) (Faillace et al. 2022).
Thus, there may often be strong interactions between these two
disturbances, with the specific prediction that frequent pulse mortality
and invasion will synergistically interact to decrease resident
diversity beyond their additive effects. However, it is also plausible
that frequent pulses keep invaders at low density and prevent them from
establishing in the resident community (e.g., due to demographic
stochasticity) (Lear et al. 2022b). Therefore, frequent pulses
and invasion could instead antagonistically interact to neutralise their
additive effects on the resident diversity. Although these two
predictions have opposite outcomes, they both lead to the hypothesis
that the interactive effects of multiple disturbances on diversity
depend on their regimes, such that they may amplify or lessen each
other’s effect as their magnitude increases.
Here, we test this hypothesis by leveraging a stably coexisting
community of bacteria in which we can manipulate pulse disturbance
frequencies, add a defined propagule of an invader and calculate final
diversity to tease apart the independent and interactive effects of
multiple disturbances on resident diversity. Specifically, we expose a
five-species bacterial community to one of four pulse mortality
frequencies, and factorially challenge each with a fast growing
“invasive” species of bacteria. Using this system, we find the
greatest loss of resident diversity in the invaded and most frequently
disturbed communities, and that interactive effects are weak at low
levels of disturbance but strong at high levels. These findings
therefore improve our understanding of how multiple disturbances
interact, and shed light on why we see a range of interaction outcomes
in the current literature.