Biotic Threats to Cycas micronesica Continue to Expand to Complicate Conservation Decisions

Abstract

Invasions of non-native species can threaten native biodiversity, and island ecosystems are ideal for studying these phenomena. In this article, first, we report on the invasive species that combine to threaten the island cycad Cycas micronesica by reviewing the history of previously reported invasions and providing an update of recent invasions. Then, we prioritize the threat status of each herbivore and the interactions among them. Plant damage was initiated in 2003─2005 by the non-native Aulacaspis yasumatsui Takagi armored scale, Erechthias sp. Meyrick leaf miner, and Luthrodes pandava Horsfield butterfly, which elicited unprecedented irruptions of the native Acalolepta marianarum Aurivillius stem borer and increased herbivory by feral pigs (Sus scrofa L.). The combined impact of these five consumers represents the greatest sustained threat to the cycad tree species. Mitigation of the damage caused by phytophagous non-native species is urgently needed to conserve this unique gymnosperm tree.

Reciprocal Garden Study Reveals Acute Spatial-Edaphic Adaptation for Cycas micronesica

“…Encroaching plants and insect herbivores were controlled with weekly scouting and interventions when necessary. The two ubiquitous non-native insect herbivores which required periodic control were the armored scale Aulacaspis yasumatsui Takagi and the specialist butterfly Luthrodes pandava Horsfield [19]. The scale was controlled with pyriproxyfen and the butterfly was controlled with dinotefuran.…”

“…Resistance to leaf herbivory among closely positioned subpopulations of Quercus rubra L. were shown to occur through reciprocal garden experiments [41]. The insular populations of C. micronesica on the island of Guam are facing risk of extinction due to insect herbivores [19]. However, only one of the leaf or stem consumers is native, and the remainder of the species were very recent invasions to the island.…”

“…Therefore, disparity in herbivory from co-evolved herbivores among various habitats could not explain the differences in north versus south Guam habitats. The two most threatening invasive species are A. yasumatsui and L. pandava, and both of these insects have also invaded the adjacent island of Rota [19]. The native insect herbivore that is among the acute threats to Guam's C. micronesica population is the stem borer Acalolepta marianarum Aurivillius [42].…”

“…The ongoing costly transplantation projects on Guam also ignore the primary threats to species survival. The primary threats that are propelling this arborescent cycad species toward extinction are ubiquitous non-native insect herbivores [19]. Until available conservation funds are re-directed toward mitigation of these primary threats, no amount of continued expenditures for transplantation projects will be of value for species recovery [51].…”

“…The extensive range and extremely dense historical areas of occupancy caused this gymnosperm tree to be the most abundant tree on Guam in 2002 [18]. But plant mortality following invasions of several non-native insects beginning 2003 [19,20] prompted an Endangered status on the Red List of the International Union for Conservation of Nature in 2006 [21] and a Threatened status on the United States Endangered Species Act in 2015 [22].…”