Evidence for an ecosystem state change at the landscape scale: Loss of top-down control of stream macroconsumers over decades significantly alters stream ecosystem processes across the island of Puerto Rico
Losing a key organism or assemblage can be critical for the structure and function of ecosystems potentially causing a complete state change at different scales. In Puerto Rico, migratory shrimps are the most dominant macroconsumer in headwater stream ecosystems and their top-down effects have been shown to regulate benthic algal standing crop and the ecosystem process of leaf decomposition. Large dams without spillway discharge act as impermeable barriers that prevent the upstream migration of shrimps and fishes, resulting in their complete extirpation from upstream reaches. Previous studies have shown that, in the absence of native macroconsumers, headwater streams above large dams have nine-fold more algae and other depositional resources due to the lack of top-down control. In this study, we measured multiple ecosystem processes in dammed and undammed stream reaches across the island of Puerto Rico to examine how shrimp extirpation affects ecosystem function at the landscape scale. We measured leaf decomposition [in 14 stream reaches (seven dammed and seven undammed) and ran experiments in two focal stream reaches using macroconsumer exclosures to isolate shrimp effects. We also measured nitrogen and phosphorus uptake in six streams (three dammed and three undammed) and whole-reach metabolism in two streams (dammed and undammed).
A state change in ecosystem processes occurs above dams where native stream biota have been extirpated for decades. Leaf breakdown rate was significantly higher in undammed streams (p=0.0031) where shrimps are present compared to dammed streams where shrimps have been extirpated. Consumer exclusion experiments showed significantly greater (p<0.001) decomposition rates in consumer access versus exclosure treatments in the undammed stream, but not significantly different in the dammed stream. In streams above large dams, uptake of nitrogen and phosphorus, and community respiration rates were higher and gross primary production was significantly lower relative to undammed streams. Our findings indicate that extirpation of native stream macroconsumers has a significant effect on ecosystem function and that these effects are directly related to the elimination of top-down controls by migratory shrimps. Moreover, other non-migratory consumers have not been able to compensate for this loss of top-down control by macroconsumers although some exemptions exist. In summary, dams with no spillway discharge have pushed stream ecosystems into a new ecosystem statecharacterized by significant differences in ecosystem properties and processes. This is important at the landscape-scale since 23% of the total stream and rivers network in located above large dams without any kind of downstream connectivity.