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Secondary metabolites often function as antipredator defenses, but when bioactive at low concentrations, their off-target effects on other organisms may be overlooked. Candidate “keystone molecules” are proposed to affect community structure and ecosystem functions, generally originating as defenses of primary producers; the broader effects of animal chemistry remain largely unexplored, however. Here, we characterize five previously unreported polyketides (alderenes A to E) biosynthesized by sea slugs reaching exceptional densities (up to 9000 slugs per square meter) in Northern Hemisphere estuaries. Alderenes comprise only 0.1% of slug wet weight, yet rendered live slugs or dead flesh unpalatable to three co-occurring consumers, making a potential food resource unavailable and redirecting energy flow in critical nursery habitat. Alderenes also displaced infauna from the upper sediment of the mudflat but attracted ovipositing snails. By altering communities, such compounds may have unexpected cascading effects on processes ranging from bioturbation to reproduction of species not obviously connected to the producing organisms, warranting greater attention by ecologists. 

Urban landscaping conversions can alter decomposition processes and soil respiration, making it difficult to forecast regional CO2 emissions. Here we explore rates of initial mass loss and net nitrogen (N) mineralization in natural and four common urban land covers (waterwise, waterwise with mulch, shrub, and lawn) from sites across seven colleges in southern California. We found that rates of decomposition and net N mineralization were faster for high-N leaf substrates, and natural habitats exhibited slower rates of decomposition and mineralization than managed urban landcovers, especially lawns and areas with added mulch. These results were consistent across college campuses, suggesting that our findings are robust and can predict decomposition rates across southern California. While mechanisms driving differences in decomposition rates among habitats in the cool-wet spring were difficult to identify, elevated decomposition in urban habitats highlights that conversion of natural areas to urban landscapes enhances greenhouse gas emissions. While perceived as sustainable, elevated decomposition rates in areas with added mulch mean that while these transformations may reduce water inputs, they increase soil carbon (C) flux. Mimicking natural landscapes by reducing water and nutrient (mulch) inputs and planting drought-tolerant native vegetation with recalcitrant litter can slow decomposition and reduce regional C emissions.