What is an ecosystem engineer?

Contents of one clump of G. salicornia from a shallow reef at Ala Moana. These include sponges, ascidians, larval fish, invasive and native algae, and crabs. Other clumps contained juvenile sea cucumbers and other fish species. A recent introduction to the islands has not prevented G. salicornia from becoming both an effective invader and a new habitat for benthic species.

Though I use the term frequently, deciding whether an organism is an ecosystem engineer is difficult. The term "ecosystem engineer" itself is problematic: almost every organism modifies its environment in some way, and in the face of indirect effects, quantifying this modification is nearly impossible. However, this category is useful because it can help us distinguish species which have strong physical impacts on ecosystems from those who affect the community mostly through competition, predation, or other biological pathways. Jones et al. (1997) define ecosystem engineers as "organisms that directly or indirectly control the availability of resources to other organisms by causing physical state changes in biotic or abiotic materials. Physical ecosystem engineering by organisms is the physical modification, maintenance, or creation of habitats." Essentially, ecosystem engineers create, modify, or destroy physical habitat.

Famous examples of ecosystem engineers include beavers, which fell trees and build dams, creating habitat for other organisms and altering patterns of water flow, and prairie dogs, whose burrows create nest habitat for birds. Plant examples abound: terrestrial forests are ecosystem engineers, as are many invasive plants. The cordgrass Spartina anglica has converted soft-bottom nearshore communities in the northeastern US to poorly drained swamps. In Hawai'i, the nitrogen-fixing shrub Morella faya has taken over areas of native forest, and because it fixes nitrogen, has significantly increased nitrogen concentrations in the areas where it has taken over. Mangroves have invaded much of Hawai'i's nearshore habitats and are expected to have significant and varied community impacts (Simberloff 2011). The Invasive alga Gracilaria salicornia alters nutrient concentration and sedimentation and flow rates. Okay, organisms can have physical effects on ecosystems. Why do we need to know whether or not they are engineers, or how much engineering they can do, exactly?

There are at least two reasons: 1) determining the extent of an organisms physical impact on a system is key in deciding whether or not the species will flourish and how it will affect the invaded community. This is particularly important in Hawai'i, which has endured a number of invasions and continues to be on the lookout for new, dangerous potential invasive species. 2) If we study these systems we may be able to build predictive models that tell us not only whether a species will be successful but where it is likely to colonize (Cuddington and Hastings 2004).

Information about invasive engineers can be difficult to sort, and sometimes difficult to find in the first place. But understanding their impacts can be a useful tool for management, and an ecological lesson.

Dinoflagellates and diatoms removed from the surface of a frond of Acanthophora spicifera, another structure-forming alga. A. spicifera is a physical host for microalgae, which grow on its surface and take advantage of localized high nutrient concentrations. In the Caribbean, it hosts Gambierdiscus toxicus, the dinoflagellate that causes Ciguatera Fish Poisoning (CFP). Interactions like these are important and can be vital to human health. (Note: G. toxicus does not grow well in waters with low salinity, so it's unlikely to show up in the fishpond).

Jones, C., Lawton, J., & Shachak, M. (1997). Positive and Negative Effects of Organisms as Physical Ecosystem Engineers Ecology, 78 (7) DOI: 10.2307/2265935

Jones, C., Lawton, J., & Shachak, M. (1994). Organisms as Ecosystem Engineers Oikos, 69 (3) DOI: 10.2307/3545850

Daniel Simberloff (2011). How common are invasion-induced ecosystem impacts? Biological Invasions : 10.1007/s10530-011-9956-3

Cuddington, K. (2004). Invasive engineers Ecological Modelling DOI: 10.1016/S0304-3800(04)00152-8

Homemaker, Lovemaker, Engineer

A blue pincher emerging from its burrow in the rhizome mat. The green at the edge is likely MPB, which might collect in this relatively protected area.

While they are not the most attractive of crabs, nor of interest to any fishery, Thalamita crenata, or the "blue pincher" is numerically the most dominant crab in the fishpond. I am interested in it because it seems to have no problem living in concert with the toughest invaders. Blue pinchers can be found burrowing in the mangrove rhizome mat, or feeding within the Gracilaria canopy. They are an excellent example of how structure-forming species (mangrove and Gracilaria) can influence community structure, and also a lesson: species can be ecosystem engineers not just by increasing habitat complexity by building structure, but also by decreasing it.

Two blue pincher crabs mating at the ocean break.

Generally, invasive species that increase habitat complexity also increase abundance and/or diversity in invaded areas. Unlike invasive pigs (Sus scrofa) and other terrestrial invaders which decrease habitat complexity in native forests, organisms like limu and burrowing crabs create more diverse habitat, which can make room for more native or alien species. Knowledge of these community effects can be helpful for predicting responses to new invasions, but the scales of invasion and community interaction are important in determining what these responses are (Crooks 2002). Additionally, the life histories of the involved species can also affect community responses (e.g., the snail Littorina littorea transforms muddy habitat into what is essentially a rocky shore, but native species which favor rocky habitat do quite well with the modification). In a place like Hawaii where the preservation of native diversity is a high priority, knowing the effects of certain invaders on the ecosystem is a powerful conservation tool.

Crooks, J. (2002). Characterizing ecosystem-level consequences of biological invasions: the role of ecosystem engineers Oikos, 97 (2), 153-166 DOI: 10.1034/j.1600-0706.2002.970201.x

Alien Babies: To'au in He'eia Mangroves

While I was seining with the LAIP interns this past summer, we came across some interesting fish living near the mangroves. While some of the fish and nearly all the invertebrates we've seen are species that may spend their entire lives in the pond (the half-spotted goby, for example, or Podopthalmus vigil, the Hawai'ian swimming crab), others are transient. Many Hawai'ian fishponds are strategically placed at the mouths of streams because highly productive, protected estuaries are important nursing grounds. As the fishpond wisdom goes, tiny fish and food go in, larger fish can't escape, and voilà: a productive and low-impact fishery. Fishponds like He'eia can attract juvenile reef fish in addition to traditional food fish, making for a diverse and colorful community of juveniles. Above is a juvenile to'au (Lutjanus fulvus) we caught in the mangroves by one of the river makaha. To'au are an alien snapper species found in abundance on Hawai'ian reefs. They are efficient at colonizing new areas (genetic evidence here) though they are not necessarily invasive; more on this important distinction later.

Mahalo nui to Jack Randall, Senior Ichthyologist at the Bishop Museum for help with the ID.

More on L. fulvus and its range in the Indo-Pacific:
Gaither, M., Toonen, R., Robertson, D., Planes, S., & Bowen, B. (2009). Genetic evaluation of marine biogeographical barriers: perspectives from two widespread Indo-Pacific snappers (Lutjanus kasmira and Lutjanus fulvus) Journal of Biogeography, 37 (1), 133-147 DOI: 10.1111/j.1365-2699.2009.02188.x

Back to Bacteria: A "Big Rotten Loofah"

Mangrove detritus pulled out of a sediment core at the south edge of the pond.

The surface of a rhizome mat where mangrove overstory was removed four years ago.

The surface is soft, and decomposing root fibers protrude into the water. The stringy fragments in the foreground are worm waste.

More on the mangrove story: This Tuesday we took sediment cores from two areas where mangrove overstory (prop roots and trunks) were cut down in 2007 and 2008. In these areas, dead stumps still stick out of the mud, and a thick, fibrous root mat stabilizes the sediments. Even though the overstory was removed four years ago, the rhizome mat is still intact, making it relatively easy to walk on. In most areas, however, it's riddled with burrows. This brings up an important question-- what is the role of these burrowing and other bioturbators in breaking down the rhizome mat? Importantly, does mangrove-based carbon enter the food web more readily when mangrove mat is decomposing, and exposed to wind-mixed water, than when mangroves are actively building fresh (tough, tannin-rich) rhizome? This is a food web approach to mangrove community effects, and a very interesting one. Previous studies suggest that short term carbon cycling in mangrove sediments is dominated by bacteria at first, then macrofauna (crustaceans, worms, mollusks, nematodes), and then back to bacteria (Sweetman et al. 2010). Additionally, in living mangroves in Kaneohe Bay, mangrove-based carbon isn't taken up into the food web (Demopoulos et al. 2007). Perhaps when crabs and other burrowers are more actively breaking down mangrove material, there are more opportunities for mangrove carbon to subsidize the local food web. This is a question for stable isotopes.

Sweetman, A., Middelburg, J., Berle, A., Bernardino, A., Schander, C., Demopoulos, A., & Smith, C. (2010). Impacts of exotic mangrove forests and mangrove deforestation on carbon remineralization and ecosystem functioning in marine sediments Biogeosciences Discussions, 7 (2), 2631-2671 DOI: 10.5194/bgd-7-2631-2010

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

What's In A Sponge?

This weekend we found a sort of sponge raft drifting along the bottom of the pond. Composed of Gracilaria and the orange sponge Mycale sp., the piece was weighed down with sediment and tiny organisms living inside. The contents included two brittle stars, several polychaete worms, a few amphipods, and some mysterious organism (pictured below). Whether the sponge originally settled on Gracilaria or grew separately and managed to incorporate a few branches, these two species have the ability to get together and host organisms... lots of them.

The picture doesn't show them in great detail, but if you know what those little gelatinous white things are, please comment.

Old Scourge, New Questions

Mangrove seedlings collected in muddy shallows in the pond. Clumps of Gracilaria have also floated in, and some collect near the new roots.

There are some new project developments at hand! In addition to examining invasive algal canopies, I've also started some preliminary work on a new structure-forming alien species: red mangrove (Rhizophora mangle). Mangroves, while important nutrient sources and nursery grounds in their native habitats, are alien species in Hawai'i, with quite different effects on the native ecosystem. Though mangroves are a vascular plant, they have some similarities to the Gracilaria canopy. Like Gracilaria, mangroves slow down water flow. They are also incredibly effective at trapping fine sediments and drawing down nutrients and heavy metals. This ability to trap particles is what led people to introduce them to the fishpond in 1922: upstream agriculture and land development was releasing large amounts of sediment and excess nutrients into the stream, and mangroves seemed to be a good biological solution. However, now the mangroves have grown over many parts of the pond, and their negative impacts on pond function are more obvious. As mangrove roots trap fine sediment and shed leaves, they make the underlying sediment anoxic, so that worms and other sediment-dwelling organisms can't survive. The mangrove canopy provides habitat for other alien species (like tilapia; more on these later) which threaten fish stocks. Currently I'm examining the distribution of infauna (organisms that live inside the sediment) in the pond in areas where mangroves have invaded and areas where mangroves have been removed.

A sediment core before being sieved. After a core is taken, the contents are run through a 500 µm sieve to remove fine sediments. Macrofauna (organisms visible to the naked eye) are retained on the mesh, counted and classified.

Straight Lines In A Curved World

This weekend we had the great privilege of visiting Auntie Donnie's property mauka of the fishpond. This was an amazing cultural experience and a great opportunity to see the watershed at all its stages. The water condenses at the mountaintop and drips down into pools, where it is fresh enough to drink. The stream runs through a bamboo forest, where high flow and small rocks and sand have carved out some of the larger rocks. Once the stream gets to actual cleared forest, it empties into taro patches, where it brings nutrients to the taro and flushes the roots. The taro pull some sediment and extra nutrients out of the water, and clean water continues downstream. Historically, this tiered system of taro production was an important part of land and water use in this area, and many community and conservation groups would like to see it return to the He'eia ahupua'a. The patches provide fish as well as taro, since freshwater fish live among the stems and roots. Currently, the wetland area immediately upstream of the pond is filled with California grass (Brachiara mutica), and the water remains in pools, which flood and connect during heavy rains in the winter. Some experimental taro patches exist in this area, but removal of B. mutica and mangrove (which has extended back from the pond into the wetlands) is costly and difficult. One hope is that taro patches will be able to handle a sediment load comparable to what the mangroves are currently holding, so that mangrove removal will not overload the pond with sediment.