Quantitative biogeography: Decreasing and more variable dynamics of keystone and foundation species in an iconic meta-ecosystem

Ecosystem stability has intrigued ecologists for decades, and the
realization that the global climate was changing sharpened and focused
this interest. Determination of climate change effects on community
stability, however, requires long-term studies of structure and underlying
dynamics, including bottom-up and top-down effects in natural ecosystems.
Although relevant datasets were rare in the early years of community
ecology, such information has increased in recent decades. In a rocky
intertidal system, we investigated the changes in ecological subsidies
(nutrients, phytoplankton, prey colonization), several performance metrics
of the dominant space occupier (mussels) and its primary predator (sea
stars), and rate of predation by sea stars on mussels in relation to
climatic oscillations, temperature, and disease. We focused on
spatio-temporal changes in the mean and variability of these metrics. The
research protocol involved annually repeated multiyear (~1999–2018),
multisite (13 sites nested within 5 regions along ~260 km of the Oregon
coast) observations, measurements, and experiments. We analyzed
associations between environmental variables and ecological performance of
key elements of the sea-star-mussel-dominated mid-intertidal system. We
found that upwelling declined in some regions, but became more variable
across all study regions. Air and water temperatures oscillated, but their
mean and variation increased through time, with peak values coinciding
with the 2014–16 combined El Niño and Marine Heat Wave. Ecological
subsidies generally declined during the study period, and bottom-up
processes increased in variability. Excepting growth rate, mussel (Mytilus
californianus) performance (condition index, reproductive output)
generally decreased and became more variable. Primarily due to a sea star
wasting epidemic, reproductive output of the top predator Pisaster
ochraceus decreased and became more variable, and predation rate on
mussels also dropped abruptly. Analyses indicated that the primary drivers
of these changes were temperature-related environmental factors. Since
declining means and increasing variability of ecological performances are
thought to typify destabilizing ecosystems, and environmental trends are
toward ever more stressful conditions, the outlook for this iconic
ecosystem is discouraging. Immediate and rapid action to mitigate and
ultimately reverse climate change likely is the only option available to
prevent an irreversible shift in the future of this, and most other
ecosystems.