Estimates of microbial community stability using relative invader growth rates are robust across levels of invader species richness.
Citation
Meaghan Castledine, Daniel Padfield, Angus Buckling, Estimates of microbial community stability using relative invader growth rates are robust across levels of invader species richness, ISME Communications, 2025;, ycaf040, https://doi.org/10.1093/ismeco/ycaf040
Abstract
A key feature of natural communities is that the species within them stably coexist. A common metric used to test community stability is the ability of each species to invade from rare. A potential issue with this measurement is that single species are invaded from rare, while in natural communities, multiple species would likely decline simultaneously following perturbations. This is especially common in microbes which can be rapidly disturbed by environmental stressors. If species coexistence is dependent on indirect interactions among community members, multiple species declining may result in community instability. As such, invading a single species into a community may overestimate the stability of a community when multiple species decline. Here, we compare estimates of community stability in a five species microbial community to experimental results in which multiple species are simultaneously invaded. Our results showed that single species invasions were qualitatively predictive of whole community stability when multiple species are invaded simultaneously. However, quantitative values of relative invader growth rate were less comparable, being non-significantly different in most comparisons in 3/5 species. This was emphasised by the lack of correlation between exact values of growth rates under single or multi-species invasion. This work provides experimental support for the robustness of using invasion growth rate of single species to infer qualitative estimates of community stability.
Community stability is usually assessed by invading single species from rare and measuring relative fitness. However does this hold up when multiple species invade from rare? Here, we - led by the talented Meaghan Castledine test this using our super stable 5 species microbial community.
We did separate assays where the resident community was the same, but the number of co-invaders alongside the focal species differed. For example, when Achromobacter and Ochrobactrum are residents, we would have separate assays where Stenotrophomonas, Variovorax and Pseudomonas are invaded on their own, and with each other. The experimental setup can be seen on the right.
First, we used linear models to look at the relationship between estimates of mean relative invader growth rate from single invasions as compared to mean multi-species invasions. These values are from all of the species combinations. If single species invasions perfectly predicted relative invader growth rate with co-invaders, we would expect a 1-1 relationship, whereas our slope is much lower.
This means that higher relative invader growth rate estimates from single invasions do not correspond to high estimates of relative invader fitness estimates from multiple invasions. However, the significant species-specific intercept indicates differences in how the two growth rate measures compare between species.
We also compared relative invader growth rate values within each combination of resident species as mean estimates do not account for within treatment variation. There were 28 instances where there was a significant difference between single and multi-species invasions, which are highlighted by the asterisks. The text shows who the other co-invaders were in those trials.
11 of the significant differences were when the focal species was Stenotrophomonas (78% of all trials for that species), which generally had lower values for relative invader growth rate with co-invaders present.
Of the other significant differences, 9 were when the focal species was Ochrobactrum, 4 were Achromobacter, and 2 each were Pseudomonas and Variovorax.
This suggests that for most species within this community, estimates of relative invader growth rate are robust to the presence of co-invaders when accounting for within-treatment variation.
Overall our work shows that qualitative predictions of community stability can be broadly predictive of stability under community perturbation where multiple species may recover from rare. Quantitative predictions were less reliable.
