Stressor-driven emigration and recolonisation patterns in disturbed habitats.

Although essential to conservation, little is known about how stress intensity can provoke emigration from disturbed habitats and allow recolonisation of those same environments. To demonstrate the applicability of laboratory experiments, we tested two hypotheses empirically using zebrafish response to artificially polluted environments that exhibited a linear gradient of stressor (acid mine drainage) levels. We hypothesized that emigration is distance-independent but time-correlated (spacelessness hypothesis). Additionally, we hypothesized that stressor-driven emigration could predict the extent of population growth in recovering habitats (avoidance-recolonisation hypothesis). For example, if half the organisms emigrate at a given stressor level, then the remainder should be able to recolonise a habitat experiencing the same stressor intensity. Comparisons of the small-scale experiment with a larger-scale simulation suggested that controlled laboratory results can be extrapolated to field populations (although time to perceive the contamination gradient may pose differential individual effects) because AC50 (median avoidance concentration) values of an acid mine drainage sample (AMD) were not statistically different when fish were exposed to the same gradient in 3-m long [0.50% (0.43-0.57)] or 30-m long [0.73% (0.30-2.2)] systems. Regarding the avoidance-recolonisation hypothesis, the number of recolonisers was inversely proportional to the number of avoiders (ACx = RC100-x). In particular, the similar distribution of fish along the 0-3% AMD gradient in both avoidance and recolonisation experiments resulted in identical AC50 and RC50 values: 0.55% (0.34-0.87) and 0.55% (0.45-0.67) AMD, respectively. The inclusion of avoidance and recolonisation responses in the environmental risk assessments provides a novel perspective of risk based on the emigration of organisms and contributes to the understanding and prediction of biological invasions and ecosystem recovery after restoration.