Shelter or Buffet? The Predator Paradox In Mangrove Communities

Samoan crab (Scylla serrata), a predator associated with mangrove forests in Hawaii and other parts of the Pacific. Though alien species in Hawaii, they are not considered a threatening invasive, because they grow slowly and are a popular fishery.

What's in a mangrove? Anoxic, tannin-rich sediments, yes. But there are also predators... quite a few of them. Brian Nakahara, a recently graduated masters student from the Oceanography department at UH found that there are higher abundances of predators inside mangrove forests than on adjacent bare sediments. He also found that predators with smaller sizes tended to inhabit the mangrove canopy, suggesting that even in Hawaii, where mangroves are invasive, predators might use the canopy as nursery habitat. Why is this exciting, you ask?

Well, for one, predators are exciting. They are big and scary and sometimes delicious (see Scylla serrata, above). But they also present an opportunity to challenge a common conception about the functional role of mangroves.

Mangroves provide carbon in the form of leaf litter, and are an efficient sink for heavy metals and nutrient runoff. Though the sediments underneath mangroves are indeed anoxic, they contain a hefty load of organic material, which enters the food web where mangroves are native (Demopoulos 2007). But because of their unique structure, there is also a strong physical component to their influence on marine systems: the surfaces of prop roots are habitat for fouling organisms and prop root structure protects organisms inside the mangrove from large predators. Though mangroves provide habitat for some organisms, they provide food for others.

In terms of predator-prey interactions, the case for mangrove function can be made two ways:

"Mangrove As Shelter": mangrove prop roots increase habitat complexity, which, according to ecological theory, allows more species to co-exist in an area (Levin 1992). It may also impede the ability of larger predators to forage, depending on the density of roots.

"Mangrove As Buffet": On the other hand, the same complex microhabitats in mangroves may provide habitat for many small prey, making mangroves a choicy foraging habitat for predators (Nagelkerken 2004), enhancing predation in mangroves.

The difference between Buffet and Shelter may be one of scale: smaller predators can access more microhabitats within the root structures, while bigger ones can't fit through the door to the buffet. This is an oversimplification, but an important one to bear in mind when examining predator function in the mangroves. My current project seeks to test the relationship between mangroves and the presence of mangrove-associated predators like Samoan crab and tilapia. My primary goal is to characterize the effects of these predators on the infaunal community-- what do these predators eat from the buffet, and how much? This type of question can be answered with a caging experiment in the existing mangroves. Cages that exclude predators will be "predator-free" zones, where infauna are protected from the predators they usually experience. I'll take sediment cores (see "Little Shop of Cores") in caged areas and uncaged areas, and comparisons between them will reveal what predators are eating at the buffet (biodiversity) and how much (biomass).

No experiment would be informative without replication, of course. The LAIP interns and I are finishing the last of 48 predator exclusion cages this week.

An exclusion cage without its lid. The metal band along the bottom sinks into the sediment, anchoring the cage and preventing burrowers from digging under the cage to get inside.

Interns carrying trial cages into the mangrove area.

Demopoulos AW, Fry B, & Smith CR (2007). Food web structure in exotic and native mangroves: a Hawaii-Puerto Rico comparison. Oecologia, 153 (3), 675-86 PMID: 17587064

Levin, S. (1992). The Problem of Pattern and Scale in Ecology: The Robert H. MacArthur Award Lecture Ecology, 73 (6) DOI: 10.2307/1941447

Nagelkerken, I., & van der Velde, G. (2004). Relative importance of interlinked mangroves and seagrass beds as feeding habitats for juvenile reef fish on a Caribbean island Marine Ecology Progress Series, 274, 153-159 DOI: 10.3354/meps274153

Kon, K., Kurokura, H., & Tongnunui, P. (2009). Do mangrove root structures function to shelter benthic macrofauna from predators? Journal of Experimental Marine Biology and Ecology, 370 (1-2), 1-8 DOI: 10.1016/j.jembe.2008.11.001

Little Shop of Cores: What Lives in He'eia's Sediments

Behold: The World Beneath Our Tabis! An assortment of worms and amphipods found in sediment cores from mangrove removal areas. The plant fragments are mangrove rhizome fibers.

These invertebrates were found in sediment cores from the edge of the pond, all of them areas were mangroves had been removed (See "Old Scourge, New Questions," January 30th). Some organisms may have been living a few centimeters underneath the sediment surface, while others may have had shallower burrows-- since these samples were depth-integrated, we don't know where these organisms dwell on a finer scale (This can be resolved by sectioning cores in the field; more on this later). While parts of the pond with low salinity are likely to be less species rich, the infauna collected today were collected on the makai side of the pond, closer to the ocean. Whether or not they are more diverse than infauna from the fresher areas of the pond is unknown at this point. If this infaunal community has changed since the mangroves were removed, we may be looking at more "pre-invasion" species which returned when the low-oxygen high-tannin environment of the mangrove sediments was ameliorated by removal. Alternatively, they could be "leftover" anoxia-tolerant species that remained even after mangrove overstory was removed.

Processing a sediment core involves sieving material through 500 µm mesh several times, fixing with formalin, staining with Rose Bengal dye overnight (hence the brilliant magenta of the worms above), and picking through a mixture of mangrove bark, algae fragments, and rhizome pieces to find brightly stained organisms. The search alone takes at least 30 minutes per core. Sorting and identification will be the next step.