The Enemy of my Enemy…: Ants are relative newcomers to Hawai`i, and generally not welcome ones at that. But in an article published in the July issue of Pacific Science, three German scientists discuss an unusual case of mutualism involving ants and the endemic shrub Vaccinium reticulatum, better known as ohelo.
The authors – Richard Bleil, Nico Blüthgen, and Robert Junker – studied ohelo at two sites in Hawai`i Volcanoes National Park. One site was infested with three species of ants (Pheidole megacephala, Paratrechina bourbonica, and Plagiolepis alluaudi), while another had just a single species, Linepithema humile.
What they found was that plants where only ants were allowed to visit had more berries set than plants where all insects (including flying ones) were allowed. The least fruit was set on plants where ants were excluded. “On average,” write the authors, “the presence of ants increased fruit set more than fivefold compared with the fruit set on ant-excluded branches,” regardless of whether flying insects were allowed on the plant. The presence of ants discouraged predation on ohelo flowers by caterpillars of an introduced plume moth, Stenoptilodes littoralis, allowing more flowers to set fruit.
Unfortunately, only 10 percent of all seeds produced by plants in the study were viable, whether ants were present or not. A reason for this, the authors speculate, is the decline of populations of natural pollinators, such as the native yellow-faced bees (Hylaeus spp.). Bearing out this hypothesis, the authors note, is a study of ohelo back in 1993, looking at seed set in Hawai`i Volcanoes National Park and Haleakala National Park. That study found 32.6 percent of the seeds were viable, “four times more than what we found,” they write. “Lower predatory pressure by ants … may have supported a denser population of Hylaeus spp. bees.”
The authors conclude: “Despite the fact that ants effectively protected the flowers of V. reticulatum in our study, their strong resource exploitation and their negative effect on native pollinator populations may outweigh this positive effect.”
Whale Families in Hawai`i: Over the last several years, studies by the Cascadia Research Collective have pretty well confirmed the presence of at least two discrete populations of false killer whales in waters around Hawai`i. There’s the insular population, numbering only around 120, and the pelagic population, found generally further out to sea.
Last month, scientists with the CRC, Hawai`i Pacific University, the Wild Whale Research Foundation, and the National Marine Fisheries Service published a paper noting a similar population structure for melon-headed whales (Peponocephala electra) in Hawaiian waters. There’s the population, consisting of around 866 individuals, that is distributed around all the main Hawaiian islands, and there’s another, smaller one, made up of around 180 individuals, that limits itself to the waters northwest of the island of Hawai`i. The paper was published in Marine Mammal Science, a journal of the Society for Marine Mammalogy.
The Hawai`i island resident population inhabits shallower water (averaging about 381 meters) than the Main Hawaiian Islands population (1,844 meters). It hangs out with humpback whales when they overwinter here, while the MHI population associates with a broad range of other species, including rough-toothed dolphins and short-finned pilot whales.
Sightings of melon-headed whales are rare events, amounting to just 2.6 percent of all sightings by the authors in surveys conducted over a 10-year period. On average, there was one sighting for every 14 days on the water. While the whales are distributed from Hawai`i island to Kaua`i, in surveys around the four islands that make up Maui Nei, no melon-headed whales were sighted at all.
The authors conclude with a discussion of the management implications of their work. “The habitat for these individuals overlaps with popular recreational fishing grounds, increasing the possibility for fisheries interactions. The biennial RIMPAC naval sonar training exercises as well as other naval exercises may also occur in waters adjacent to or overlapping with the habitats … and given the evidence of susceptibility to sound impacts, this may pose additional concern.”
A copy of the paper may be found at the Cascadia Research Collective’s website: www.cascadiaresearch.org
Correction: In our September edition, in an item concerning predation by a Peregrine falcon on birds in the Northwestern Hawaiian Islands, we reported that transplanted Laysan finches on Midway escaped the predation. As Sheila Conant has graciously pointed out, “that must be because there aren’t any there right now!”
Conant noted that Laysan finches were introduced to Midway in the 1890s, but became extinct about a year and a half after black rats were introduced to the atoll during World War II. Thank you, Sheila.
Volume 22, Number 4 — October 2011