Disturbance-mediated invasions are dependent on community resource abundance

Abstract

Disturbances can facilitate biological invasions, with the associated increase in resource availability being a proposed cause. Here, we experimentally tested the interactive effects of disturbance regime (different frequencies of biomass removal at equal intensities) and resource abundance on invasion success using a factorial design containing five disturbance frequencies and three resource levels. We invaded populations of the bacterium Pseudomonas fluorescens with two ecologically different invader morphotypes: a fast-growing "colonizer" type and a slower growing "competitor" type. 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, but this interaction differed between the invader types. The success of the colonizer type was positively affected by disturbance under high resources but negatively under low. However, disturbance negatively affected the success of the competitor type under high resource abundance but not under low or medium. Resident population changes did not alter invader success beyond direct treatment effects. We therefore demonstrate that the same disturbance regime can either be beneficial or detrimental for an invader depending on both community resource abundance and its life history. These results may help to explain some of the inconsistencies found in the disturbance-invasion literature.

Keywords: biodiversity; disturbance frequency; invader life history; invasion; invasion success; resource abundance.

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 homogenized broth into fresh media. All microcosms were invaded every 4 days (immediately post‐disturbance) with either a smooth (SM) or wrinkly spreader (WS) invader. Six replicates per treatment were used.

FIGURE 2

Invasion success, log(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). The variable 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.

FIGURE 3

Evolved resident Pseudomonas 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.

FIGURE 4

Final resident density (log₁₀[(cfu/ml) + 1]) after 16 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.