Abstract
Island invasions may cause severe changes in biodiversity, but the factors that influence these changes are not well understood. We established 120 plots in Cycas micronesica habitats throughout Guam in 2005 following the invasion of the armored scale Aulacaspis yasumatsui, then observed plant mortality through 2020. We used transects in Yap as benchmarks, as the Yap C. micronesica population is not threatened. The initial Guam plots contained about 1600 seedlings, 1160 juveniles, and 1240 mature plants per ha. Seedling mortality was 100% by 2006, juvenile mortality was 100% by 2014, and the 2020 census revealed 96% mortality of the plant population. Localities in western Guam and isolated forest fragments exhibited the greatest mortality, with 100% extirpation from two fragmented western localities. The juvenile and mature trees in Yap were unchanged from 2010 to 2018, but the seedling count was heterogeneous among the years. Constrained recruitment from seedlings to juveniles explained these dynamics. Yap transects contained about 6120 seedlings, 3400 juveniles, and 1250 mature plants per ha. Biological control of the invasive insects remains the acute conservation action needed for the Guam population. Lessons learned may be useful in other regions where invasions of non-native pests threaten biodiversity.
Highlights
Invasions of non-native organisms alter biodiversity relationships and ecological processes [1,2,3].The damage to biodiversity and ecosystem processes can be extreme following invasions of non-native insects, especially when the insects are specialist herbivore and a native tree serves as their host [4,5,6,7,8,9,10,11,12]
We monitored the expansion of the A. yasumatsui population from the initial 2003 outbreak site in the urban landscape on the west coast of Guam into the in situ C. micronesica habitats throughout the island
The first island to experience the A. yasumatsui invasion was Guam, and we have shown that the infested C. micronesica plants within habitats on the west of Guam have died at a greater percentage between 2005 and 2020 than plants within habitats on the east of the island
Summary
Invasions of non-native organisms alter biodiversity relationships and ecological processes [1,2,3]. The damage to biodiversity and ecosystem processes can be extreme following invasions of non-native insects, especially when the insects are specialist herbivore and a native tree serves as their host [4,5,6,7,8,9,10,11,12]. A well-studied case has unfolded on the island of Guam in which the native cycad. The native and cultivated Cycas species were the only available hosts for these invasive pests. The combined damage decreased the general health of the trees which caused an increase in herbivory by the native stem borer Dihammus marianarum Aurivillius [13,15]
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