A Cycas micronesica tree in Guam, right, shows the extent of damage done by invasive insect herbivores, a scale called Aulacaspis yasumatsui, as compared to a healthy Cycas micronesica tree, left, found in Yap. In a 15-year study, published in the May 2020 issue of Diversity journal, researchers studied the rate and influences of mortality in Guam cycads affected by invasive species. Photos courtesy of University of Guam
A long-term study on Guam and Micronesia’s native and threatened species of cycad plant has revealed factors that contribute to the plant’s ability to survive in the face of invasive species.
Research out of the Western Pacific Tropical Research Center at the University of Guam and College of Micronesia-FSM shows that the Cycas micronesica population in Guam declined to 4 percent of its population size from 2005 to 2020 following the introduction of the non-native scale insect, Aulacaspis yasumatsui, in 2003, and the cycad-eating butterfly, Chilades pandava, shortly thereafter. Of the few cycads that survived or survived the longest, plant size and habitat traits were key.
“Guam’s Cycas micronesica was the most abundant tree on Guam two decades ago, but then an onslaught of non-native insect herbivores invaded the island and initiated sweeping mortality,” said Adrian Ares, associate director of the Western Pacific Tropical Research Center.
The plant variety is also found on the Northern Mariana Island of Rota, the Federated States of Micronesia island of Yap, and Palau, though the two invasive herbivores are not present in Yap or Palau.
Excerpts from the report
The effect of plant size on speed of mortality following A. yasumatsui damage among 12 Guam localities corroborated our earlier report . The seedlings reached 100 percent mortality first, the juveniles reached 100 percent mortality second, and the persisting plants were restricted to adults.
These demographic controls over speed of mortality were founded in the resource depletion methods that A. yasumatsui employs to kill its host. Larger individuals began their infestation with a substantive pool of resources, smaller individuals began their infestation with a limited pool of resources.
However, the speed of seedling mortality occurred for three other known reasons. First, the damage by A. yasumatsui to the seedlings was not restricted to infestation of petiole and leaflet surfaces. Seeds of this and other Cycas species are platyspermic, and germinate by opening a longitudinal cleft to expose the gametophyte and allow the coleorhizae to extend. Direct A. yasumatsui infestation of exposed coleorhizae, radicle, and gametophyte surfaces added to the speed of seedling resource depletion.
Second, for unknown reasons the R. lophanthae predator avoided the lowest strata in Guam’s forests, so predation of the armored scale was less effective in the stratum where the seedlings were restricted. Third, the olfactory signals from seedling leaves infested with A. yasumatsui were less attractive to R. lophanthae adults than were the signals from adult leaves infested with A. yasumatsui.
In establishing 120 permanent plots where the plant grows in Guam in 2005 and examining them over 15 years, the researchers found that 100 percent of seedlings were killed by 2006 and 100 percent of juvenile plants were killed by 2014. The persisting plants were larger individuals that possessed substantial stored resources.
“The means by which the invasion of an island by a non-native herbivore can wreak such havoc is actually two-fold,” said Murukesan V. Krishnapillai, a research scientist at the College of Micronesia-FSM and one of the authors of the study.
“First, the native tree that is attacked by the new pest has spent millennia evolving without a comparable herbivore and, therefore, possesses few defensive strategies. Second, the pest has escaped from its own native habitat, where it co-evolved with natural enemies, and, therefore, the new island environment contains no natural enemies to tamp down the pests’ population growth.”
The study also illuminated two other spatial factors that have influenced the plant population’s response to the non-native insect herbivore threats. First, the populations in Western localities have exhibited much higher mortality than the populations in Eastern localities. Second, the plants within large contiguous forests have exhibited less damage than plants within forest fragments.
The long-term nature of the study, which Ares said is rare in contemporary academia, and the use of benchmarking starting in 2005 were crucial in understanding how rapidly threats can devastate island tree populations.
Knowing that plant size and habitat traits influence cycad susceptibility to non-native threats can now inform conservation management decisions.
“Conservationists need developing knowledge like this to craft the most appropriate mitigation strategies and identify where available funds should be spent,” Ares said.
The study concluded that the only high priority activity to conserve the Cycas micronesica species where the scale and butterfly are present is to establish a complex integrated biological control program under the direction of scientists with international expertise.