Explaining Recruitment Stochasticity at a Species’ Range Margin

Mats Westerbom,Olli Mustonen,Eliecer Díaz,Patrik Kraufvelin

doi:10.3389/fmars.2021.659556

Abstract

Advancing our understanding of how environmental variability affects the distribution of organisms is crucial for ecology and conservation. The exploration of changes in demographic patterns close to species distribution margins is important as populations here may provide a window into future population changes also elsewhere. However, the knowledge of factors causing recruitment variation is still inadequate in many systems and this deficiency is particularly evident close to species’ distribution borders. We studied the spatiotemporal variability in recruit-adult dynamics in a blue mussel, Mytilus trossulus, population to get insights into how environmental variables drive variation in recruitment and how this variability affects adult population growth. Thirty sites along a wave exposure gradient were monitored during four consecutive years. From each site, mussels were collected both from artificial recruitment units and from natural mussel beds. Our results showed high year-to-year variation in recruitment strength with high spatial variation. Mussel recruitment to artificial units and later recruitment to the benthos correlated highly. Juvenile abundances 1 year later paralleled prior recruitment strengths and caused synchronous but time-lagged changes in adult cohorts. Seawater salinity was the strongest predictor for recruitment variation, whereas sea temperature and wave exposure had low predictive power for this early life stage. For juveniles and for adults in the benthos, wave exposure explained the variation best, whereas temperature and especially salinity explained less. The results indicate that (a) the studied blue mussel population is strongly driven by variation in recruitment strength that (b) drives the size of the later cohorts, and the population is possibly even (c) recruitment limited in some years. Our study predicts a challenging future for this range population, resulting from a higher frequency of recruitment failure caused by a deteriorating sea climate. Knowledge about factors underlying variation in recruitment is thus essential for forecasting the future of this range population and for conserving its future state.

Highlights

Questions of why population sizes fluctuate through space and time and how organisms and the environment interact to produce patterns in natural systems are central for population ecology
A commonly accepted view is that marginal populations living near the limit of their geographical range show higher sensitivity to stress, greater probability of local reproductive failure and a higher probability of recruitment limitation (e.g., Karlson and Levitan, 1990)
We show here that recruitment strength varies highly in this marginal population and effects of recruitment variability is seen later in the adult cohorts

Summary

Introduction

Questions of why population sizes fluctuate through space and time and how organisms and the environment interact to produce patterns in natural systems are central for population ecology. The understanding of these mechanisms and how they modulate populations is still poorly known in many systems (Beukema et al, 2010; Shanks et al, 2017; Okamoto et al, 2020). This concerns blue mussel research in the Baltic Sea (Liénart et al, 2021) where there has been a call for studies on how different life stages of blue mussels respond to changes in the environment (Jaatinen et al, 2021). Marginal areas may be especially important for predicting changes occurring elsewhere as responses to changes in the environment are generally seen earlier at species distribution limits (e.g., Gaston, 2003)

Methods

Results

Discussion

Conclusion