Bird Study Reveals, Dispels Overlaps in Diets of Native, Introduced Species
Over the past year or so, research on the endangered `akepa (Loxops coccineus) by the husband-and-wife team of University of Hawai`i zoology professor Leonard Freed and Rebecca Cann, also a UH professor, has garnered publicity in the prestigious scientific journals Science and Nature. Articles in those journals reported Freed’s work suggesting that `akepa within the Hakalau Forest National Wildlife Refuge are declining as a result of food-competition from the introduced Japanese white-eye (Zosterops japonicus).
But a recent study by Robert Peck and Paul Banko of the U.S. Geological Survey and David Leonard of the state Department of Land and Natural Resources on the diets of endangered and introduced forest birds at Hakalau tells a different story.
During the mid-1990s, in the Maulua and Nauhi sections of the 32,733-acre refuge, Peck and his colleagues collected `akiapola`au (Hemignathus monroi), `akepa, Hawai`i creepers (Oreomystis mana), Hawai`i `amakihi (Hemignathus virens), `elepaio (Chasiempis sandwichensis), and Japanese white eye (JWE) in mist nets and collected fecal samples. They studied the identifiable body parts – fangs, mandibles, legs – from those samples and determined to a large degree what those birds ate. They presented their results in a poster at the Hawai`i Conservation Conference held last July in Honolulu.
Although the researchers were unable to determine about 10 to 16 percent of the birds’ diets, they were able to identify 41 categories of prey items and found that caterpillars are key food items, which is consistent with observations made by naturalist R.C.L. Perkins 100 years ago, the poster states.
Caterpillars make up 67 percent of all prey for the `akiapola`au, 48 percent for the Hawai`i `amakihi, 42 percent for the `akepa, and 41 percent for the Hawai`i creeper. Caterpillars made up only 16 percent of the white-eye’s diet, the researchers found. (The researchers were unable to determine any particular species of caterpillar. Rearing moths from caterpillars with the same 16 different mandible types found in the fecal samples may allow for identification at the family or genus level, the poster states.)
Spiders, the second most popular prey item, comprised 18 percent of the diet of the Hawai`i creeper, 16 percent of the `akepa’s diet, 15 percent of the `elepaio’s, and only 3 percent of the `akiapola`au’s. For the white-eye, spiders accounted for 23 percent of its diet.
The second-most common food item found in the `akiapola`au’s diet was a cerambycid beetle, which accounted for 12 percent.
The researchers also found differences in the types of homopteran prey – plant-hoppers, lice, aphids, scales, etc. – preferred by the birds. Delphacid plant-hoppers were favored by `elepaio, white-eye, and `amakihi, but made up just 29 percent of homopteran prey consumed by the `akepa. The `akepa, they found, preferred Psyllids, a type of plant lice.
“These results suggest that diet overlap between Japanese white-eye and the endangered birds is relatively small,” the poster’s abstract states. “A more substantial threat to caterpillars likely comes from alien parasitoids, which kill about 25 percent of native Scotorythra caterpillars at Hakalau.”
The native honeycreeper whose diet overlapped most with the white-eye’s was the Hawai`i creeper, which consumed a lot of the same type of caterpillar. Among the native birds, the greatest overlap in diet was between the `akepa and the `amakihi. The `akepa and the `akiapola`au also ate many of the same things.
The study notes that the influence of sex, age, and season on the diets of these birds has not yet been studied and that detailed studies on foraging behavior are needed to understand how native and introduced birds partition resources.
Still, Freed and Cann are not swayed by the dietary study. Their most recent article is to appear this month in Current Biology, titled, “Negative Effects of an Introduced Bird Species on Growth and Survival in a Native Bird Community.” In this paper, they argue that as a result of an explosive growth in numbers of white-eye at Hakalau, `akepa “became nonviable during 2000-2006,” the last year Freed was allowed to work at the refuge. Once more, Freed and Cann propose “intensive management” measures to control white-eye populations at Hakalau.
Peck says that he and his team are trying to get more detailed diet information by identifying the prey samples to the highest taxanomic level possible. Using the caterpillar mandibles, for example, they are trying to identify the species, or at least family they belong to, which will help narrow down the microhabitats where the caterpillars live, he says.
Regarding efforts to determine the age of the birds sampled, Peck says for species like the `akepa, the sample size was limited to only a dozen.
“There is very little we’re going to be able to say about seasonality and sex” with regard to the `akepa, he says. He adds that they did collect enough `amakihi and `elepaio to make some determinations and that the group hopes to have a manuscript on the additional analysis prepared by the end of the year.
As for future research, Peck says, “We would like to do more field work and get data we don’t have now….We would really like to get foraging observations on birds, which is really critical. You can only surmise so much using diet samples.”
For Further Reading
Environment Hawai`i has reported extensively on the controversy over the work of Freed and Cann. See:
“Is Hawai`i `Akepa on the Brink of Collapse? Alone among Peers, UH Professor says Yes,” November 2006; and
“UH Professor Takes Long-Running Feud with Feds into Court of Public Opinion,” April 2009.
Ant Control Results
For Offshore Islets
As part of a larger strategy to control pests on offshore islets, University of Hawai`i graduate student Sheldon Plentovich has attempted to control and eradicate two species of ants on islets off O`ahu’s eastern shore. Ant poison can effectively eradicate certain species, she’s found, and while that might seem to be an ideal result, it can also open the door to new invasions. Even so, merely suppressing ants allows native seabirds and plants to thrive, she concluded.
In her presentation (for which won the award for best student presentation) at this year’s Hawai`i Conservation Conference, Plentovich stated that more than 50 species of ants have been introduced to the Hawaiian islands, where native ecosystems evolved without them. Ants can reduce or extirpate arthropods, increase unwanted bugs, harm seabirds, and change forest structure, among other things, she said, noting as an example that the yellow crazy ant (Anoplolepis gracilipes) killed off the red land crab on Australia’s Christmas Island, and as a result, altered the island’s forest structure.
While ant eradication has been accomplished in some places, such as the Galapagos, there is very little information on the ecological effects of ant control, she said.
In her experiment, Plentovich tested the effects of the ant poison Amdro (hydramethylnon) on the big-headed ant (Pheidole megacephala) and the tropical fire ant (Solenopsis geminata), which can inflict crippling stings on shearwater chicks. In 2002, the first year of her study, the big-headed ant was the most abundant arthropod on her first set of study sites, the islets of Mokuauia and Popia off O`ahu’s east coast. The tropical fire ant was most abundant on her second pair of study sites, the twin islands known as the Mokuluas. All four islands are seabird sanctuaries and host a variety of rare native species.
On each islet, Plentovich randomly selected 15 monitoring sites. She set out cards baited with peanut butter, honey, and SPAM to monitor ants; collected arthropods in pitfall traps; and monitored the seabirds there for three years. She treated one islet from each pair — Mokuauia and Moku Nui – with Amdro.
Following treatment, big-headed ant numbers on Mokuauia dropped to zero from 2003-2008, while on the control island, Popia, big-headed ant numbers grew.
In the long term, the big-headed ant remained the dominant species on Popoia. On Mokuauia, however, the eradication of the big-headed ant was followed by a significant change in the ant species on that island. The tropical fire ant showed up, and later disappeared; pavement ants (Tetramorium bicarinatum) also came and went, and the yellow crazy ants eventually arrived.
“We were really concerned about this because the yellow crazy ant is the species that’s causing problems on Christmas Island, we know it attacks seabirds, and in some situations, it can cause colony abandonment,” Plentovich said. The invasion made her reexamine her seabird data which revealed that the yellow crazy ant invasion of Mokuauia coincided with declines in the number of wedge-tail shearwater chicks.
On the Mokuluas, “things were a little less clear cut,” she said. Tropical fire ant numbers decreased on both islands (Moku Nui and Moku Iki). On Moku Nui, the tropical fire ant population was diminished but not eradicated.
“We have this period of suppression…but we were not able to achieve eradication on that island,” she said.
Although Plentovich was unable to eradicate the tropical fire ant from Moku Nui, reduced densities “resulted in increased weight and fledging success of wedge-tailed shearwater (Puffinus pacificus) chicks and increased leaf cover in the native plant ilima (Sida fallax),” she wrote in the abstract of her talk.
Also, from her pitfall traps, Plentovich found that the pesticide also seemed to have affected alien cockroaches. “I’m sure everyone is devastated by all this,” Plentovich joked. Because the islands had no native detritivores, she said she doesn’t know how they might be affected by Amdro.
As a result of her experiment, Plentovich concluded that hydramethylnon can eradicate big-headed ants but, with monitoring, should be used cautiously in an adaptive management plan.
Shearwater chicks, like this one, have been crippled by stings from invasive tropical fire ants.
Do Aliens Evolve
To be More Invasive?
Do individuals from a Hawaiian population of an invasive species grow faster than individuals of the same species from their home ranges? A study by National Park Service botanist David Benitez suggests that the answer is yes.
“It’s long been noted that invasive plants appear larger and more aggressive in their new ranges…but only recently have researchers begun to explore the possibility that genetic differences contribute to this larger appearance and hence invasiveness,” Benitez said in his conference presentation.
When invasive plants enter new habitats that lack natural enemies, those species can put more energy into growing larger and eventually evolve to stay that way, leading to genetic differences between populations, Benitez explained.
In his study, Benitez set out to test two hypotheses: 1) that plants from invasive populations here grow faster than their counterparts from South America, and 2) that plants from invasive populations here will have fewer chemical and structural defenses than their South American counterparts. As subjects, he chose some of Hawai`i’s most invasive weeds – strawberry guava (Psidium cattleianum), Koster’s curse (Clidemia hirta), and herb cane tibouchina (Tibouchina herbacea). All three species have been in the islands anywhere from 40 to 150 years and two are on the state Department of Agriculture’s noxious weed list, he said.
With seeds collected in both Hawai`i and the plants’ home ranges in Brazil and Venezuela, Benitez grew some 1,200 plants at a quarantine lab in Volcano, Hawai`i, and monitored them for 120 to 180 days. Benitez said the data he collected on plant height, mass, and growth rates support his hypothesis that Hawaiian plants grow larger than the South Americans. (The Hawai`i Department of Agriculture’s import permits prohibited him from conducting experiments on their ability to reproduce.)
The tibouchina exhibited the greatest differences. Based on 365 individuals, Benitez found that plants from Hawai`i grew significantly taller, faster, and more massive than those from South America. He also found that the leaf shape of those from South America varied more than those from Hawai`i, and that the Hawai`i plants grew more upright.
The trends for strawberry guava were similar, but not as strong, Benitez continued. Based on 465 individuals, he found that the Hawai`i plants grew significantly taller and more massive. They also grew faster, but not significantly so. He found that while the seed size of strawberry guava varied in Hawai`i, it varied even more for those from South America.
With clidemia, Benitez said he faced significant challenges: 60 percent of his plants died from volcanic gases and pest infestation. As a result, he could not identify any clear or statistically significant trends. However, his general observations were that there were no height differences, but there was greater survival among the Hawai`i plants.
“These findings suggest that the plants are more competitive here than in their native South America,” he said.
In response to a question from the audience about whether his South American and Hawaiian plants were really the same taxa, Benitez said that “depends on which botanist you talk to” since some do not recognize varieties. When asked whether the differences could be due to chromosome number, Benitez said he was interested in pursuing that kind of research, but was legally limited in his ability to do genetic work.
Regarding the Hawai`i tibouchina, he added that some say it evolved here, while another possibility is that it is a mutant variety from South America. If that’s the case, he said, “We need to locate it.”
Although Benitez did not share his results regarding plant defenses, a couple of his clidemia pictures suggested that the leaves of the South American individuals were hairier.
The Hawai`i Conservation Alliance webpage contains links to more than 70 talks and presentations given at this year’s Hawai`i Conservation Conference: [url=http://hawaiiconservation.org/2009hcc_presentations.asp]http://hawaiiconservation.org/2009hcc_presentations.asp[/url]
— Teresa Dawson
Volume 20, Number 5 November 2009
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