The impact of anthropogenic disturbances on the genetic diversity of terrestrial species: a global meta-analysis

Human activities are primarily responsible for habitat loss and changes in natural environments around the world. It has been suggested that populations inhabiting human-modified landscapes are subject to reduced gene flow, inbreeding depression, and loss of alleles due to genetic drift. However, empirical evidence shows contradictory effects of anthropogenic disturbances on the genetic diversity of terrestrial species. We performed a meta-analysis of 61 studies that compared the genetic diversity of plant and/or animal populations in disturbed and preserved areas (317 paired comparisons) to investigate general responses to different disturbance type. We found significant negative effects of disturbance on genetic diversity (effect size: -0.45), in which the loss of structural connectivity was the most detrimental disturbance type. The choice of the genetic parameter has an influence on the detection of the effect (direction and magnitude), and consequently the studies using number of effective alleles did not detect genetic erosion, while all other indices, especially allelic richness, revealed negative responses to disturbances. Yet, only studies performed with transferred or both transferred and specific microsatellites showed negative responses to disturbances. The general effect was more detrimental in animal than plant populations. Only plant species with biotic pollination and seed dispersal mode, self-incompatible reproductive system, and shrubs showed negative responses to disturbances. Despite all heterogeneity among studies, we found an overall negative effect of disturbance on genetic diversity of terrestrial populations, which suggests that the remaining populations inhabiting anthropogenic landscapes have a reduced evolutionary potential being more prone to local extinction.

Methods
We searched for studies that evaluated the effects of anthropogenic disturbances on plant and animal genetic diversity published until February 07th, 2019. The search was performed in the Scopus database using the following term sequences: [("Genetic diversity") AND (Microsatellite OR SSR) AND ("fragmented habitat" OR "habitat fragmentation" OR deforest* OR logging OR "habitat disturbance" OR "disturbed habitat" OR "habitat loss")]. These terms were searched in the title, abstract, and keyword sections of the manuscripts, except for the sequence of terms related to the genetic marker that we searched along the entire manuscript.