The annual Hawai`i Conservation Conference, held this year in August at the University of Hawai`i-Hilo, offered hundreds of discussions on a wide range of topics.
This month, we continue to report on selected presentations.
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When Looking for Snakes,
When Is Enough Really Enough?
There’s good news and bad news on the brown treesnake front.
The bad news practically everyone in Hawai`i already knows. Should the snake find its way to Hawai`i and become established here, it would be a disaster – for forest birds, for power lines, for babes in their cribs… The list of troubles Guam has seen from the snake’s introduction there could fill several books.
The good news?
At Andersen Air Force Base, on the northern tip of Guam, the U.S. Geological Survey has established a geographically closed population of the snake. With no new snake entry possible – thanks to a snake-proof fence – and no snakes able to leave, researchers have the ability to study the efficacy of control tools at known snake densities. And from that, reported Amy Yackel Adams, it has been possible to develop a means of determining when the rapid response teams established to respond to reports of snake invasions on other islands can say, with certainty, that their work there is done.
Yackel Adams, with the U.S. Geological Survey in Fort Collins, Colorado, along with Bjorn Lardner stationed on Guam from Colorado State University, tackled the difficult subject of how you prove a negative. How, that is, you can state, with a high degree of confidence, that something (the brown treesnake) doesn’t exist in a given area.
And their research was put to the test a year ago September on the island of Rota, in the Commonwealth of the Northern Mariana Islands, when a brown treesnake was caught in an interdiction trap there.
The event triggered “a litany of questions, ranging from how did that snake get to Rota, presumably a snake-free island? to was it a lone individual, or part of an incipient population? And how long should our rapid response continue?”
The event marked the first time a snake had been caught in an interdiction trap on a Mariana island, she noted, “and I was tasked with figuring out how long we respond. The answer needed to be justified – hence, quantifiable. But how much uncertainty is tolerable?”
The BTS Rapid Response Team jumped into action. “Successful eradication is best implemented when it’s ready and prompt, with a team ready to deploy the day a sighting occurs,” she said.
“To detect whether a colonization has occurred is difficult,” she went on to say, “especially for cryptic species like the brown treesnake and ones that occur at low density.”
Furthermore, “rapid response is costly and cannot continue indefinitely. So when do you call it enough?” she said. “If you don’t search long enough, you might say there’s no incipient population, which allows the population to expand.” While you can confirm the presence of a population through detection, she continued, “the absence of a population can only be inferred probabilistically.”
To determine the probability of encountering a snake in an incipient population, Yackel Adams turned to the closed population of snakes in the fenced-in area of five hectares at Andersen AFB. Researchers had established 27 transects through the area, and each snake had been individually tagged. While it was “geographically closed,” she said, it remained “demographically open,” with the snakes reproducing naturally.
By conducting surveys along transects far enough apart to ensure “we won’t see the same snake twice” in the course of an evening, Yackel Adams and her colleagues were able to determine snake densities and the probabilities associated with detecting a snake at a given level of effort.
In the closed BTS area, with a population of 117 snakes in five hectares, the average density was 24 snakes per hectare, she said. At that density, she and her fellow researchers “would expect to find 8.2 snakes on a given night” when they walked 5.94 kilometers of transects, for a detection probability of 0.07 percent.
But there was a confounding factor that prevented this formula from being applied in Rota – or anywhere else, for that matter: what Yackel Adams called satiation.
“On the island of Rota, there’s a lot more prey than exists on Guam. Guam has rodents at densities of one to 16 rats per hectare,” she noted. On Rota, “there are 9 to 96 rats per hectare.”
“So this will have a bit of an impact on the detection probability. A lot more prey is available.” The tools developed in Guam, a “prey-limited situation,” had to be adjusted downward in a prey-rich situation.
“Poisson distribution provides the answer,” she said. “It provides the probability of a given number of events in a fixed interval of space and time, provided you have a known average rate and independence between events. Simply put, you can determine the probability of finding at least one snake.”
On Rota, personnel with the Rapid Response Team walked no fewer than 639 kilometers, without finding a single snake. “With 95 percent certainty,” Yackel Adams said, “we would have detected at least one snake given that level of effort, if the density was .16 snakes per hectare or more.” If the density was as high as one snake per four hectares, the likelihood of detecting at least one snake, with that same level of effort, would be 99 percent, she noted.
“This is the first time rapid response has ever quantified the level of certainty associated with a given effort,” she concluded. With more work, she said, the method could potentially be transferred to invasive species other than the brown treesnake.
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Parrotfish Need More Protection
Talks on fisheries management rarely are quite as straightforward as the report that Edward DeMartini gave on parrotfish in Hawai`i.
The state, he said at the end of his talk, “should seriously consider banning nighttime scuba spearfishing.”
Just as rare was the enthusiastic applause that greeted his blunt recommendation. (That and other recommendations he made were his own, DeMartini took pains to note, and not necessarily those of his employer, the National Marine Fisheries Service.)
DeMartini looked at the sizes and ages of five species of parrotfish that are commonly taken by fishers.
“There are dozens of important reef fish,” he said, “and not much is known about their life history. Especially lacking are rates of growth and sexual maturity.”
When determining which species to study, he continued, “you have to consider whether it’s economically or culturally important, and, ideally, also whether it has an important function within the ecosystem.”
“One of the groups of fishes which have great ecological importance are parrotfish,” he said, calling out their role as “habitat engineers.”
Of the seven species of parrotfish in Hawai`i, five are commonly caught by fishers, and all are recorded under the single name of uhu. Given the differences among the five species in their sizes at maturity, DeMartini said, “We should be hesitant to manage uhu as a single-species taxon.”
“The two large-bodied species greatly dominate commercial landings, in terms of biomass,” DeMartini noted, with smaller-bodied species making up a smaller portion of the overall catch.
“The fishery is highly selective, with most of the take occurring through the use of nighttime scuba spearfishing,” he said.
All of the parrotfish are “female first sex-changers,” he pointed out, which creates some special challenges for managing the fish.
“Both of the large species” – spectacled and red-lipped parrotfish – “mature on average at a size that’s several inches greater than the minimum legal size” of 12 inches, he said, adding that the current legal minimum size protects less than 20 percent of redlip parrotfish.
DeMartini concluded with a series of personal recommendations for state fishery managers, including:
- Improve catch records;
- Change minimum size limits – notably by increasing the minimum size for the two largest species to 14 inches;
- Conduct more thorough censuses and improve creel and market surveys for size and species composition for all species, but “especially for multi-species groups like parrotfishes;” and, finally
- Seriously consider banning nighttime scuba spearfishing.
Last year, the state Board of Land and Natural Resources passed new rules for the island of Maui that prohibit the take of spectacled and red-lipped parrotfish altogether and set the minimum take size of two other species of uhu at 14 inches. Elsewhere, such takes continue to be legal. The Land Board has also banned scuba spearfishing in West Hawaii.
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Homeless and Sick?
Of Hawai`i’s many rare and endangered birds, the `akohekohe (Palmeria dolei) has had one of the more stable populations. Found now only on the upper slopes of windward Haleakala, its population has held steady at around 3,800 individuals for years.
The bird is relatively long lived, with a life expectancy of at least 12 years. And it has a high nest success rate relative to other forest canopy birds.
“But with a high survival rate and high nest success, why is its population not increasing?” asked Alex Wang, a graduate student at the University of Hawai`i-Hilo who has been working with the birds for several years.
One of the problems Wang has identified is the presence of disease – specifically, avian malaria – at the lower range of the `akohekohe’s habitat, around 1,700 meters elevation.
“Because of the introduction of mosquitoes and disease,” Wang said, “the birds are restricted to high-elevation refugia.”
But `akohekohe are nectivores, feeding primarily on the nectar of `ohi`a blossoms. `Ohi`a nectar makes up between 50 and 75 percent of the birds’ diet, Wang said.
Wang developed a hypothesis: `Akohekohe juveniles move to lower elevations in the summer months, following `ohi`a blooms, where they are exposed to malaria. This, he continued, could explain the bird’s static population.
To test this, he outfitted several birds with transmitters and for two months, the life of the batteries, he followed them around the forest with a handheld antenna.
He then quantified `ohi`a blossoms by elevation, from 1,450 meters up to 1,950, on two transects. “I counted `ohi`a blooms at all stations,” he said – to the gasps of his astonished audience.
The blooms “decreased dramatically at high elevation sites during the summer,” he found, “but there was no corresponding increase in low-elevation sites.”
He found no support for the idea that juveniles emigrated to lower elevations for food. However, he added, “`akohekohe pairs are aggressive and territorial. Their home ranges don’t overlap.”
While Wang didn’t find any adults at lower elevations, he said, he did find juvenile birds covering wider areas and “descending into potential malaria zones.”
“What I found corroborates the high adult survival and high nest success already documented,” he explained in a follow-up email, “but what I found different was that there is likely low juvenile recruitment back into the probably saturated population.”
“The high adult survival,” he concluded, “fits with the `akohekohe’s high philopatry” – its fidelity to a small home range at high elevation. The “high juvenile output,” he said, will contribute to the bird’s reproductive success “only if the juveniles don’t go to lower elevations.”
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Of Earthworms and Pigs
While there are plenty of studies on the effects of feral pigs on native ecosystems in Hawai`i, Noa Kekuewa Lincoln, a researcher with the University of Hawai`i’s College of Tropical Agriculture and Human Resources, has investigated the way those pigs interact with the ecosystems – specifically, the relationship between pigs and earthworms.
Earthworms, Lincoln pointed out, are not native to Hawai`i but were here by the early 1800s. About 35 species are now found in the islands and they make up the largest fraction of macrofauna found in Hawaiian soils.
Pigs eat them, and although earthworms make up somewhere between 1 and 4 percent of gut content in pigs on Hawai`i island, they are the primary source of protein for the pigs, he said. “Hence, the vast majority of rooting in forests may be … pigs looking for earthworms.”
Lincoln examined earthworm biomass under a range of canopy species. “Within the same site,” he reported, “earthworm density can vary dramatically.” Under some species of eucalyptus and redwood, he continued, “there are no earthworms. But you could walk two feet and go from no worms to choke!” Tropical ash canopy is apparently one of the most preferred areas for earthworms.
Koa also shelters few earthworms, but native forests generally fall “in the middle of the spectrum.” Pasture, especially kikuyu grass, has more.
“So I looked at this in terms of pig rooting,” Lincoln said. “Almost all the rooting we were seeing is in direct correlation to the amount of earthworms in the soil.”
Also, he went on to say, “the impacts of rooting aren’t equal under different forest canopies. … If you have a tropical ash forest on a steep slope, expect lots of rooting and erosion.”
“If we think about pig rooting correlating to time spent in forest areas, this has big implications for how we designate and manage forests,” he said. “If we set up eucalyptus forests as hunting areas, the hunters will be disappointed.” On the other hand, “if we’re going to try to lower pig populations in an area,” he said, it’s a waste of time if those efforts are focused on areas invaded by tropical ash or other trees that enhance rooting.
“There’s lots of speculation that before earthworms were introduced, pigs in Hawai`i were much smaller, and were limited by protein availability in their diet,” he said. “As everything has moved toward pasture grasses, we’ve greatly increased earthworm habitat, presumably increasing the abundance and vitality of pigs as well.”
Conversely, he concluded, if pastureland is converted back to native forest, “it may lower pig vitality by altering overall land use.”
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Hawai`i’s Bees: An Experiment
Hawai`i has some 60 species of native Hylaeus yellow-faced bees, seven of which are candidates for protection under the federal Endangered Species Act. In an effort to boost the population of one candidate species, Hylaeus anthracinus, found on the Kona coast of the Big Island, Karl Magnacca undertook to translocate several hundred of them from Puako, where they are relatively abundant, to three sites at Pu`uhonua o Honaunau National Historic Park in South Kona.
In a poster presentation at the Hawai`i Conservation Conference, Magnacca noted that a century ago, the bee occurred widely along the leeward coasts of Hawai`i island, from South Point, in Ka`u, up to South Kohala. Now, the isolated South Point population “is extremely small,” he wrote, and “Recent extensive surveys have found no surviving populations in South Kona and only a few suitable sites.”
“This combination of large-scale declines and extirpation from historic sites with continued high numbers in some areas makes H. anthracinus ideal for testing translocation as a conservation tactic,” Magnacca wrote. And although translocation has been done for birds and plants, until Magnacca translocated the bees, it had not been attempted for any native insect.
Magnacca, supported by the O`ahu Army Natural Resources Program, chose three sites in Pu`uhonua o Honaunau to release the bees captured at Puako, beginning last January. One site, Alahaka Bay, had a good mix of native vegetation and relatively few ants. Another site, at the start of the coastal trail, had vegetation quality described by Magnacca as “medium” and abundant big-headed ants (Pheidole megacephala). The third site, at the royal grounds, had “low” quality vegetation and also many big-headed ants.
Six months later, the 100 bees that had been released at Alahaka had become an established population, with nesting observed in coral rocks, Magnacca reported.
“Bees were not able to establish at the other two sites even after a second, larger release in April 2015,” Magnacca wrote. “This suggests that the presence of large numbers of aggressive ants is the biggest barrier to [the bees’] existence in their historic range.”
Could ant control help out the bees? Magnacca was asked.
“Unfortunately, there doesn’t seem to be a good way of controlling ants,” he replied by email. “Sheldon Plentovich did some experiments eradicating them from offshore islets on O`ahu and found that they tend to just be replaced by other species, which are sometimes even worse. If big-headed ants are in one area, if you get rid of them, it might be ant-free for a couple of years, but then long-legged ants show up and soon they’ve taken over.
“And of course, on the main islands, it’s much more difficult to get rid of them, because there’s usually a nearby population they can move back in from over land. You could keep them under control, but that would require a huge effort of baiting – and, of course, as soon as you stop they come back.”
— Patricia Tummons
Volume 26, Number 4 October 2015