Hello everyone, my name is Ross Boucek and I am PhD student at Florida International University. Oikos has asked me to write about our Early View paper “No free lunch: displaced marsh consumers regulate a prey subsidy to an estuarine consumer” where we investigate the value of food subsidies to recipient consumers as well as what controls the amount of food subsidies available to them. Food subsidies are resources that enter an ecosystem from another place, and add on to the resource base that is already available within the system. Subsidies in some instances can be almost considered a bonus, like getting cash for your birthday in the mail. If you are on a graduate student stipend, birthday cash subsidies can go a long way! Because of the predictability of birthday cash, many graduate students budget these cash bonuses into their spending well before they arrive. Therefore, if for some reason these checks get lost in the mail, or they are for less money, we could be in some trouble!
Graduate students and their reliance on birthday money is very similar to how some animals rely on food subsidies to survive. One particularly charismatic example of consumer-subsidy interaction is bears and salmon in the U.S. Pacific Northwest. In the fall, these bears congregate around rivers and streams to gorge themselves on salmon that predictably migrate from the oceans to spawn. These salmon subsidies help bears build necessary fat reserves that play a major role in their survival and reproduction over the winter. Because of the importance of food subsidies to consumers, biologists and ecologists have gone to great lengths to identify where subsidies occur in nature, and more importantly what controls how much of the food subsidies reach recipient consumers.
Diagram of the relationship between marine salmon subsidies and bears
Moving back to the bear-salmon subsidy interaction, in some areas of the U.S. Pacific Northwest, sea lions have figured out salmon are easy prey during their spawning migration; such that some sea lions track salmon up rivers, and pig out on these spawning fish, preventing them from reaching bears. The interception of these salmon subsidies by sea lions can be so intense, that they can reduce salmon available to hungry bears waiting upstream by as much as 65%.
Diagram showing how sea lions reduce salmon subsidies to bears
This brings me to my research. I work in the Florida Everglades, more specifically, where the iconic Everglades marshes or the “river of grass” joins the beautiful tropical mangrove estuary.
The study area of our research, located at the marsh-estuary interface in the Southwest Everglades
At this interface between the marsh and the estuary, during the winter and spring, rainfall decreases, causing freshwater marshes to dry. When these marshes dry, large numbers of small-bodied freshwater fishes are forced into the estuary. At the same time that these prey enter the estuary, estuarine fish predators triple in abundance presumably to gorge on the marsh fish prey forced into the estuary. Thus at the Everglades ecotone, estuarine predators function similarly to the bears in the Pacific Northwest, and small bodied marsh prey are like the salmon. However, similar to the sea lions, these small bodied fishes are accompanied by large-bodied fish predators that also live in the marsh that can intercept and remove or reduce fish prey subsidies to the estuarine consumers. With all of this in mind, my research questions were 1) how important are marsh prey subsidies to estuarine consumers? And 2) how much of this subsidy is being removed by marsh predators?
Diagram showing the relationsship between estuarine and marsh predators and marsh prey subsidies at the marsh-estuarine interface of the southwest Everglades
Our results show, like the salmon-bear example, that estuarine predators gorge themselves on these marsh prey subsidies. Consequently, the consumption of this subsidy makes estuarine predators roughly 15% fatter than they were before.
However, like the sea lion example, freshwater marsh predators consume about 60% of the fish prey that enter the estuary, leaving only 40% for the estuarine consumers. The regulation of this subsidy to snook, the estuarine predator, could influence how much energy snook allocate to reproduction each year. Snook spawn in the early summer to mid fall. Therefore the weight snook gain from this subsidy in the spring could go to egg production in the summer, thus increasing reproductive output. If marsh predators decrease in abundance, then the amount of subsidy available to snook could increase, which may allow snook to invest more energy into reproduction and increase spawning success. My current research is investigating just this question, whether or not larger prey subsidies, facilitated by the loss of marsh predators, result in increased spawning effort by snook.
Understanding the dynamics behind this subsidy could have important implications for South Florida. In South Florida, snook are an important recreationally sought after fish. In fact, snook are the 5th most targeted fish in the entire east coast of the United States, despite only occurring in South Florida. The money spent from anglers fishing for snook generates substantial amount of revenue for local businesses. Therefore, knowing the drivers behind snook population dynamics like the regulation of food subsidies will help us better understand the provisioning of ecosystem services such as fisheries in the Everglades.