Identifying knowledge gaps about context dependency in freshwater species invasions

The abiotic template of freshwater ecosystems is being altered by human activity on a global scale, often promoting the establishment of new invasive taxa and increasing the likelihood of facilitation between invaders. These positive interactions represent a substantive threat to freshwater ecosystems by promoting the establishment and persistence of invasive taxa. To assess the threat posed by freshwater invasive species under global change, we leveraged the existing literature at the intersection of freshwater invasive species and positive biotic interactions using systematic review and meta-analysis. Our findings revealed critical knowledge gaps in the study of interactions between invasive taxa, which may lead to an underestimation of their threat. These gaps included a lack of studies that address indirect interactions, particularly over long temporal scales. Such interactions were understudied for invasive taxa but were often strongly positive. Additionally, almost a third of the publications examined documented context-dependent interactions, but only a single publication did so between invasive organisms. Common forms of anthropogenic change, including alterations to habitat quality, nutrient availability, and hydrology, were among the most frequent drivers of context dependency. Our analysis supports the conclusion that 1) the threat of facilitation between freshwater invaders may be underestimated by the current literature, and 2) global change may drive context-dependent interactions between freshwater invasive species, but there is a lack of research addressing this possibility.

Methods
We based our analysis on a systematic search of the available literature on positive species interactions in freshwater ecosystems. The core of our corpus was formed by publications that had previously been identified by Albertson et al. (2021). These publications were augmented by an updated search conducted in ISI Web of Science on April 1, 2022, using the following search terms:

(“aquatic” OR “stream$” OR “river$” OR “freshwater$” OR “wetland$” OR “lake$” OR “pond$” OR “vernal”) AND ((“positive interact*” OR “mutualis*” OR “commensal*” OR “facilitation” OR “symbios*” OR “habitat ameliorat*” OR “stress ameliorat*” OR “ecosystem engineer*” OR “stress gradient*”) OR ((“positive interact*” OR “mutualis*” OR “commensal*” OR “facilitation” OR “symbios*” OR “habitat ameliorat*” OR “stress ameliorat*” OR “ecosystem engineer*” OR “stress gradient*”) AND (“invas*” OR “non-native” OR “invad*” OR “non-indigenous”))) NOT “blood"

The objectives of the updated search were to expand the previous corpus to include any new articles that had not been published at the time of the Albertson et al. (2021) search, and to conduct a more thorough search for publications that examined interactions between invasive species.

Publication Vetting and Data Collection

Following the literature search, we examined each publication to determine if it met our criteria for inclusion in the meta-analysis. Publications were included if they met the following criteria:

1.      A measure of effect size for a beneficiary species from both a treatment group (with potential facilitator species present) and a control group (no potential facilitator species present), reported as the mean response of the beneficiary in both groups. These means were derived from observations. Effect sizes derived from model estimates (e.g., marginal means obtained from regression parameters) or estimated means arising from extrapolation were not included. We also excluded observations for which one or both group means were zero, since such observations interfered with our calculation of effect size (see Eqs. 1 and 2 below). Although we use the terminology “beneficiary” and “facilitator” here based on the original authors’ treatment assignments, a goal of our paper is to describe how the potential facilitative interaction ultimately took place (see number 5 below).

2.      A sample size (n) for both groups.

3.      A measure of uncertainty for both the treatment and control group means. This could be reported as standard deviation, standard error, sample variance, 95% confidence interval of the mean, etc., but had to be able to be transformed to yield the within-group standard deviation of the mean.

4.      At least one of the participants in the interaction had to be an obligate freshwater species.

5.      At least one interaction documented in the publication had to be positive. We defined positive interactions as those for which the mean value of the response variable in the treatment group exceeded the mean value in the control group, regardless of uncertainty around the mean. We therefore included publications for which the difference between the means did not meet author assigned thresholds for statistical significance.

If a publication met the criteria for inclusion, we collected the group means, uncertainty, and sample size for any species interactions that reported the relevant values.  We collected values when reported in text or tables directly, or in cases for which values of group mean, uncertainty, or moderators were reported only in figures, we obtained values using the Figure Calibration plugin for ImageJ (Schindelin et al., 2012). When available, we collected information on several moderating variables. These moderators included the temporal scale over which the measurement was carried out, the spatial scale of the measurement, whether the measurement was collected from a manipulative or observational study, the interaction pathway (i.e., direct interaction, indirect interaction, or the net result of both types), and the invasion status of the participants.