Impacts of nutrient subsidies on salt marsh food webs: Do field manipulations approximate long-term anthropogenic disturbance?
Anthropogenic nutrient inputs into native ecosystems cause fluctuations in resources that normally limit plant growth, which has important consequences for associated foodwebs. Such inputs from agricultural and urban habitats into nearby natural systems are increasing globally and can be highly variable, spanning the range from sporadic to continuous. Despite the global increase in anthropogenically-derived nutrient inputs into native ecosystems, the consequences of variation in subsidy duration on native plants and their associated foodwebs are poorly known. Specifically, while some studies have examined the effects of nutrient subsidies on native ecosystems for a single year (a nutrient pulse), repeated introductions of nutrients across multiple years (a nutrient press) may better reflect the persistent nature of anthropogenic nutrient enrichment. We initiated a manipulative experiment to compare the effects of short-term nutrient pulses with a multi-year nutrient press on arthropod community structure in a relatively undisturbed Spartina saltmarsh. Additionally, to test whether the effects of our nutrient press manipulation on the arthropod foodweb approximate arthropod responses in marshes subjected to long-term anthropogenic disturbance, we conducted a survey of Atlantic coastal Spartina marshes. We compared arthropod community structure and diversity of coastal marshes receiving variable amounts of anthropogenic nitrogen to our nutrient press manipulation.
We found that the effects of repeated nutrient inputs on arthropod food web structure were similar at both the plot and landscape scale; our nutrient press manipulation causes changes to arthropod community structure that are similar to what we observed during our survey in marshes that were subjected to significant anthropogenic nutrient enrichment. Nutrient subsidies alter the trophic structure of the arthropod community by changing the relative abundances of various feeding groups; for instance, predators had a significant positive response to nitrogen availability in both our manipulative experiment and in the marsh survey. Salt marshes represent an ideal system to address the differential impacts of nutrient pulses and presses on ecosystem and community dynamics because human development and other anthropogenic activities lead to recurrent introductions of nutrients into these natural systems. Our research will help us to understand how long-term nutrient enrichment of native ecosystems by human activities affects arthropod community structure and foodweb dynamics.