Nothing says “welcome” to a newly arrived marine species wanting to settle in Hawai`i as much as a nice, sheltered reef. Other factors play a role, such as proximity to a harbor or boat ramp and, to a lesser extent, the absence of a rich diversity of native species.
But, according to Steve Coles of the Bishop Museum, lead author of a recent study on this subject,* “it was the degree of exposure to open-ocean conditions that seemed to be the determining factor;” the less the exposure, the greater the likelihood that a site would foster growth of introduced species and what Coles and his colleagues termed “cryptogenetic” species – those suspected of being recent introductions to the Main Hawaiian Islands, but for which conclusive documentation is unavailable.
“As you get into harbors,” Coles said in a recent phone interview, “introduced species tend to become dominant. Even when you’re within harbors or bays, you see the same trend: the more exposed the reef-type environment, the less likely or common it is to see marine introductions.”
Coles and his colleagues conducted rapid assessments of 41 reefs off Kaua`i, O`ahu, Moloka`i, Maui, and Hawai`i Island, looking for introduced species. The surveyed sites included a variety of diverse reef conditions and were varying distances from harbors and other areas that could serve as sources for introductions. All totaled, they found 26 introduced species (three algae, 19 invertebrates, and four fish species) among the 486 taxa they identified.
Coles offered an explanation for the apparent preference of introduced species for the relatively still waters of embayments. “It probably has to do with two things. First, they tend to be closer to possible sources, such as harbors and small-boat harbors. The other thing is, most of the introduced species, with a few exceptions, appear to be those things that require high levels of food in the water – particulates, organic matter… Also, there was some sort of counter-signal if the area happened to be more species diverse; in those instances, there seemed to be fewer introductions.”
Proximity to harbors does not always correlate with a high presence of introduced species, though. Coles and his co-authors found that “some stations near harbors or boat ramps had some of the lowest values for nonindigenous and cryptogenic species…. Many sites within a few hundred meters from harbors and docks showed few or no nonindigenous and cryptogenic species.”
Introduced invertebrates were generally not abundant at the surveyed sites. “In contrast to the usual circumstances in Hawaiian harbors, where invertebrate nonindigenous and cryptogenic species are often among the most common or dominant species, this was not the case at any reef site,” the authors wrote. The only nonindigenous invertebrate evident in abundance was Carijoa riisei, or snowflake coral, found on the undersides of collapsed structures adjacent to the reef at Mala Wharf, on Maui. “All other invertebrate nonindigenous and cryptogenic species were rare at all sites and usually consisted of a single or few individuals or colonies in cryptic locations,” they found.
With algae, it was a somewhat different story: Acanthophora alga was plentiful at several sites on Moloka`i and O`ahu, while Kappaphycus was abundant at two sites in Kane`ohe Bay.
One of the challenges faced by Coles and his colleagues was simply knowing which species are indigenous (that is, found naturally in Hawai`i) and which are introduced.
“Unless there’s clear, historical documentation of something having been purposely brought here, there’s always a chance it’s not an introduction,” Coles said. “The evidence is purely circumstantial for most of the 340 plus species we identify as non-native. Of those, we have documentation on introductions for about a dozen – four or five species of fish, three or four of algae, and a few invertebrates for which there were comprehensive surveys before their first detection.” For this reason, most of the species Coles and his co-authors suspect of being nonindigenous they refer to as “cryptogenic,” or of uncertain geographic origin.
And so far as distinguishing between natives and new arrivals is concerned, Hawai`i is better off than lots of other island areas. “We’re probably in the best situation here, since marine biota have been pretty well studied in Hawai`i for more than a century,” Coles said. “It’s possible to go to places like Pearl Harbor and do a chronological record of when things were first detected… If you do work in a place where there is not such a long record, such as American Samoa, it is much more challenging.”
Among the most ubiquitous of the introduced species were three fish intentionally introduced in the 1950s: roi, ta`ape, and blacktail snapper. When these three species are not counted, more than half of the sites surveyed (22 of 41, or 54 percent) were found to have fewer than two introduced species, leading Coles and his co-authors to write: “Overall, few introduced marine invertebrates have colonized Hawai`i’s coral reefs and even fewer are invasive.”
Does this mean that concerns over marine invasions are exaggerated?
“I’m glad you ask,” Cole replied. “In terms of the raw numbers or percentage of the total community that introduced species represent, they seem to be a fairly small and seemingly not very important portion. However, the problem is that any environment can give you surprises. The best example of that is a sponge I’ve been working on for a couple of years. In Kane`ohe Bay, this has become one of the most dominant and invasive organisms – the most invasive invertebrate in the state after Carijoa,” or snowflake coral.
“I would not want to say that marine introduced species do not constitute an important issue worthy of continued concern and vigilance,” he continued. “You can never predict what the troublesome species would be. Who would have ever thought Gracilaria” – an invasive species of algae – “could ever become this troublesome thing it is now, the biggest mess we have.” Gracilaria, Coles pointed out, was known to be present on the Big Island as early as the 1950s, but did not create a problem until it “was brought to O`ahu and took off.
“So even though we found introductions to be a small component of reef communities and not very prominent, there is legitimate reason for continued concern.”
Coles speaks with the voice of experience. In 2002, he said, he wrote in a review article “that I considered Carijoa riisei a ‘relatively benign introduction.’ Clearly I was wrong, emphasizing the need for caution in making such judgments, even when all seems well enough at the time.”
Did Coles and his colleagues find any common trait among the introduced species that might lead them to a better understanding of what to look for in assessing whether a given species will take on the characteristics of an unwelcome invader?
“From my viewpoint, it’s impossible to say what group is the most important to look out for,” he replied. “Fouling organisms and algae tend to be the ones that do most damage when they get established.” As an example he pointed to Chthamalus proteus, a barnacle introduced to Hawai`i from the Caribbean. “It’s invasive in the inter-tidal area and is everywhere in harbors across the state. It wasn’t even here in 1973, but between then and 1996, when we found it in Pearl Harbor and Kane`ohe Bay, it achieved incredible densities. That’s the hallmark of something that is unexpected. Now it has spread itself as far west as Guam and Midway.
“We’re not particularly concerned with it, because it doesn’t interfere with anything we want to protect, but it is a good example of the problem.”
*S.L. Coles, F.L.M. Kandel, P.A. Reath, K. Longenecker, and L.G. Eldredge, “Rapid Assessment of Nonindigenous Marine Species on Coral Reefs in the Main Hawaiian Islands,” Pacific Science (2006), vol. 60, no. 4:483-507.
— Patricia Tummons
Volume 17, Number 5 November 2006