Yeager,LA, Stoner,EW, Peters,JR, Layman,CA. A terrestrial-aquatic food web subsidy is potentially mediated by multiple predator effects on an arboreal crab. Journal of Experimental Marine Biology and Ecology, 475: 73-79. doi:10.1016/j.jembe.2015.10.017
Generally speaking, food webs are thought of as closed systems. For example, a terrestrial food web consisting of plants, insects, birds, snakes, and raccoons are often thought of as closed loop. Closed loop suggests that the food web is not connected to any other, such as an aquatic food web consisting of algae, mollusks, small herbivorous fish and large predatory fish (Figure 1). This is a reasonable and often necessary assumption in many cases. However, the reality of interconnected food webs can be both fascinating and essential to our understanding of true ecosystem dynamics.
One such example of interconnected food webs is the terrestrial food webs of mangrove forests and their neighboring estuarine aquatic environment. For instance, the mangrove tree crab (Aratus pisonii) (Figure 2) spends most of its time on the mangrove tree roots during high tides and foraging for tree leaves on the ground during low tides. This species is a rare example of an organism that can tolerate both the aquatic and terrestrial environments and has the mobility to move freely between the two. These two attributes make the mangrove tree crab a direct link between the aquatic and terrestrial systems.
The authors of this study were investigating the behavior of the mangrove tree crab and the impacts that the crab’s behavior has on the terrestrial and aquatic ecosystems. Specifically, the authors hypothesized that predatory birds, such as the Great Blue Heron would drive crabs into the water in a desperate attempt to avoid predation from the birds. In turn, this would expose the crabs to greater predation risk by fish. They also hypothesized that a potential impact of the environment shift by the crabs would be the transfer of nutrient sources from one environment to the other. For instance, mangrove tree crabs feed largely on mangrove tree leaves, which is a land-based nutrient source. If fish where to eat a greater number of crabs, then the aquatic food web would be provided with a supply of terrestrial-based nutrients via the crabs. To test these hypotheses, the researchers split the project up into three experiments.
First: Crab Behavior
The first step in the puzzle is determining if mangrove tree crabs do in fact respond to threats of bird predation by fleeing to the water, ultimately spending more time in the water when birds are present than when they are not. To test this, the researchers conducted a “mesocosm” experiment – essentially a mini-ecosystem in a big bucket (Figure 3)! They created four different mesocosm environments: 1. A system with no predators, 2. A system that only had fish predators (a predatory fish swimming in the water at the bottom of the tank), 3. A system that had only simulated bird predators (pecking with a plastic model great blue heron), and 4. A system that had both fish and simulated bird predators. Then, Yeager and her colleagues examined how much time the crabs spent in the water vs. out of the water in each of the four scenarios before and after simulated bird attacks. For scenarios 1&2 which did not involve simulated bird strikes, the researchers still approached the tank and conducted the simulated pecking out of sight of the crabs to be sure the crabs weren’t responding to the presence of humans.
As you might expect, in the tank with no predators, crabs spent a fair amount of time in and out of the water; when only fish were present they spent little to no time in the water; when only birds were present they spent more time in the water, particularly after simulated bird strikes; and when both fish and birds were present, the crabs would initially flee from simulated bird attacks into the water, but then quickly exit the water to avoid fish.
Step two: Tether experiments
The scientists now knew that crabs would change their behavior and habitat preference depending on which predators were present, they needed to determine if changing the time that they spent either on land or in the water changed how likely they were to be eaten by each respective predator. To do this, they tethered one set of crabs to a set of mangrove roots below the tide line (unable to escape the water) and another set of crabs to a similar set of mangrove roots above the tide line (unable to enter the water). This experiment showed that crabs were at much greater risk of predation in the water (over 80% lost to predation) than crabs on land (23% lost to predation). This demonstrates why crabs may prefer to spend most of their time on land.
Final step: Ecosystem dynamics
Once the researchers knew that crabs will change which habitat they spend time in based on predators and the likelihood of a crab being lost to predation, the final step was to determine what the predation of crabs by fish meant to the aquatic ecosystem. They sought to answer the following questions: How many fish are eating crabs? How important are the crabs to the overall scheme of the fish’s diet? And, finally, can the presence of predatory birds drive a substantial amount of land-based nutrients into the neighboring aquatic ecosystem via a change in mangrove tree crab behavior?
Throughout this study, between 2 and 22% of predatory fish sampled were found to have tree crabs in their stomachs. Furthermore, tree crabs were found to make up nearly 30% of the diet by volume of some fish. Interestingly, the areas with greater levels of tree crabs in fish diets coincide with areas of greater wading bird populations. Naturally, the final and broadest scale question is the most difficult to answer. Just how much energy and nutrients are delivered to the aquatic system by mangrove tree crabs fleeing bird predation? That’s hard to say. But the authors of this study provide compelling evidence for this as a plausible mechanism of nutrient and energy transport between two distinct food webs (Figure 4).
This study highlights how the behavior of individuals can influence larger scale ecosystem dynamics. It also demonstrates how reductions in wading bird populations like the great blue heron might result in a reduction in the flow of terrestrial nutrients into the aquatic food web.