Abstract
Simple SummaryThe armored scale known as cycad aulacaspis scale invaded Guam in 2003 and began killing individuals of the host tree Cycas micronesica by 2005. The plant mortality dynamics of this case study have been reported in detail. However, the influence of this invasion on reproductive behaviors of the host tree has not been reported. We took a look at how the male cones of this gymnosperm tree were influenced by the invasion over many years. As expected, male cones decreased substantially in size after the invasion, but then began recovering shortly thereafter. As of 2021, the cones remain only 57% as large as the pre-invasion cones. The mutualist pollinator relies on these cones as brood sites, and the ability of male cones to produce offspring of the mutualist pollinator was similarly reduced by the invasion. Conservationists need new knowledge such as this to inform species recovery efforts.Aulacaspis yasumatsui Takagi invaded Guam in 2003, and the influence on survival and demography of the host Cycas micronesica K.D. Hill population has been well-studied. To more fully understand how A. yasumatsui has threatened the host cycad species, we determined the microstrobilus size and number of pollinators per microstrobilus from 2001 to 2021. The microstrobilus height and diameter were measured directly, and the volume was calculated. Microstrobili were 58 cm in height, 13 cm in diameter, and 4740 cm3 in volume prior to direct A. yasumatsui infestations. Microstrobili decreased in size immediately after direct infestations by A. yasumatsui, and then began to slowly increase in size until 2021. For example, the volume was 24% of pre-invasion volume in 2007, and was 57% of pre-invasion volume in 2021. Microstrobili were harvested; then, the number of pollinator pupae were counted after an incubation period. Pollinator pupae counts per microstrobilus declined to 66% of pre-invasion levels by 2007 and have remained similarly constrained through 2021. Our results revealed that A. yasumatsui damage to the host C. micronesica population is not limited to attrition of the extant plant population, but also includes a loss in male reproductive effort and the risk of coextinction of the insular pollinator.
Highlights
The invasion of Guam by A. yasumatsui was documented in 2003 [1,2], and the resulting in situ C. micronesica plant mortality was recognized by 2005 [3]
The habitats were invaded by A. yasumatsui in late 2005, and the scale predator Rhyzobius lophanthae Blaisdell was purposefully released at the same time
We found no microstrobili that were adequately protected from direct A. yasumatsui infestations throughout 2006, so there were no data for that year
Summary
The invasion of Guam by A. yasumatsui was documented in 2003 [1,2], and the resulting in situ C. micronesica plant mortality was recognized by 2005 [3]. The projected plant mortality as a result of this invasion was the basis of an endangered listing by 2006 [4] This arborescent cycad species was the most abundant tree on Guam at the time of the invasion [5], but the mortality due to a coalition of biotic threats reached 96% by 2020 [6]. The infestations of this nonnative specialist armored scale have generated numerous negative outcomes at the organismal and population levels. The tree’s intrinsic resistance to tropical cyclone damage was impaired by the invasion, leading to increased tree mortality during tropical cyclones [10,11,12]
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