Environmental factors affect the condition of critically endangered freshwater pearl mussels by disrupting terrestrial–aquatic resource connectivity

Abstract Freshwater mussels are important in aquatic ecosystems but are among the most imperiled species globally. Freshwater pearl mussel (FPM) is a critically threatened species that probably relies on terrestrial organic matter (OM), which suggests that FPM is susceptible to disruptions in terrestrial–aquatic resource connectivity. We performed a regional‐scale field study covering 29 FPM populations across Finland to investigate whether terrestrial ecosystem modifications indirectly affect FPMs by disrupting OM inputs and impairing ecosystem connectivity. We also examined how the resource connectivity covaries along environmental gradients of water quality and catchment characteristics. We used Bayesian mixing models of hydrogen stable isotope ratios from terrestrial and aquatic OM sources to estimate the proportion of terrestrial inputs in FPM as a proxy for resource connectivity. We tested how environmental factors and terrestrial ecosystem alterations influenced connectivity and examined how connectivity and environmental conditions affected eight metrics of individual and population‐level condition in FPM. We found that FPM substantially relied on terrestrial OM across the region, implying strong connectivity. However, environments with higher anthropogenic pressures were associated with lower degrees of connectivity. Reduced connectivity and environmental change translated into lower relative growth and reduced muscle strength in FPM, implying poorer condition status in more human‐impacted areas. These results suggest that human‐induced environmental changes may alter the degree of resource connectivity, which in turn negatively impacts the condition of FPM. These findings highlight the importance of not only conserving ecosystems but also their connections with adjacent ecosystems to maintain the quality and quantity of energy flows which species may depend upon. Our findings also highlight the importance of catchment‐level conservation measures in preserving mussel habitats. Given the status of riverine mussels worldwide, our results are of importance not only to FPM but may inform conservation and management of threatened mussel species more broadly.