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.