Humans are expanding their environmental impact through habitat fragmentation, climate change, and more. This growing impact increases the likelihood of large, sudden changes in ecosystems that last for long periods of time, known as regime shifts. These shifts often lead to degraded environmental conditions, making them a cause for conservation concern.
The dynamics of the shift have to do in part with how connected systems are. High connectivity can at times aid invading species or encourage population synchrony, but it also can promote the spread of diversity through a system, leading to better resilience to the shift. So far ideas about how connectivity and surrounding environmental factors impact resilience to these shifts are mostly theoretical, with more empirical evidence needed to understand how resilience works in reality.
Olin, A.B., Bergström, U., Bodin, Ö., Sundblad, G., Eriksson, B.K., Erlandsson, M., Fredriksson, R. and Eklöf, J.S. 2024. Predation and spatial connectivity interact to shape ecosystem resilience to an ongoing regime shift. Nature Communications 15(1): 1304.
Recent research addresses this gap, using the Swedish coast of the Baltic Sea to test ideas about ecosystem resilience. Shallow bays across the region support pike and perch, two top fish predators that feed on stickleback fish, which in turn feed on pike and perch eggs and larvae. It’s a delicate balance: bays are dominated by either pike and perch, or by sticklebacks. When sticklebacks dominate, it has cascading effects on lower trophic levels by promoting algae growth and lowering water and habitat quality.
Using over 7,000 fish samples from an island-rich archipelago covering almost 700 km of coast over two decades, researchers looked at what causes the ecosystem to shift from predator-dominated to stickleback-dominated, and which areas are most resilient to the “stickleback wave”. Understanding where and why this shift occurs can help promote and protect a higher quality marine environment and lead to better fisheries management in the region.
The comparison of multiple drivers showed that the complex interactions between predators and prey are mediated by both habitat connectivity and environmental conditions. Higher connectivity for pike and perch leads to higher resilience to the shift to stickleback-dominated, likely because connectivity allows adult fish to move more freely between spawning areas. However, higher connectivity is only beneficial when there are low densities of fish-eating predators like grey seals and great cormorants, who leave behind too few adult pike and perch for connectivity to make an impact.
In addition, pike and perch resilience to the shift increases with temperature, likely because warmer waters shorten and advance the time period when their larvae can be eaten by sticklebacks. While warming water temperatures in general are detrimental to ecosystems, in this case higher temperatures provide some help with resilience to regime shift. These interactions between spatial connectivity, environmental conditions, and trophic levels highlight how complex regime shifts can be to understand and predict.
Habitat connectivity remains a key component of healthy ecosystems, with its many benefits including resilience to regime shifts. Given the numerous ways in which natural areas are changing – through habitat loss, habitat fragmentation, climate change, and more – understanding what drives some of these changes in large, heterogeneous ecosystems can lead to better management and conservation decisions. Keeping habitats connected can ensure that species stay resilient to whatever the future brings.
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