Unexpected High Diversity of Galling Insects in the Amazonian Upper Canopy: The Savanna Out There

Abstract

A relatively large number of studies reassert the strong relationship between galling insect diversity and extreme hydric and thermal status in some habitats, and an overall pattern of a greater number of galling species in the understory of scleromorphic vegetation. We compared galling insect diversity in the forest canopy and its relationship with tree richness among upland terra firme, várzea, and igapó floodplains in Amazonia, Brazil. The soils of these forest types have highly different hydric and nutritional status. Overall, we examined the upper layer of 1,091 tree crowns. Galling species richness and abundance were higher in terra firme forests compared to várzea and igapó forests. GLM-ANCOVA models revealed that the number of tree species sampled in each forest type was determinant in the gall-forming insect diversity. The ratio between galling insect richness and number of tree species sampled (GIR/TSS ratio) was higher in the terra firme forest and in seasonally flooded igapó, while the várzea presented the lowest GIR/TSS ratio. In this study, we recorded unprecedented values of galling species diversity and abundance per sampling point. The GIR/TSS ratio from várzea was approximately 2.5 times higher than the highest value of this ratio ever reported in the literature. Based on this fact, we ascertained that várzea and igapó floodplain forests (with lower GIA and GIR), together with the speciose terra firme galling community emerge as the gall diversity apex landscape among all biogeographic regions already investigated. Contrary to expectation, our results also support the “harsh environment hypothesis”, and unveil the Amazonian upper canopy as similar to Mediterranean vegetation habitats, hygrothermically stressed environments with leaf temperature at lethal limits and high levels of leaf sclerophylly.

Understory host plant and insect gall diversity changes across topographic habitats differing in nutrient and water stress in the Brazilian Amazon rainforest

“…Hence, plant species zonation, richness, and composition in Amazonian forest types seem to represent important explanatory variables for the pattern of galling insect species richness observed among habitats (Fernandes 1992;Veldtman and McGeoch 2003;Julião et al 2014b). While significant differences in the number of tree species between different habitats were not found in Amazonian terra firme forests, differences in taxonomic composition was consistently reported by Kahn and Castro (1985), Oliveira (1997), Oliveira and Amaral (2004), Ribeiro et al (1999), Valencia et al (2004) and by Silva et al (2014).…”

“…Future studies should examine variation in taxonomic composition of plants and its relationship to variation in galling insect species richness in the understory of Amazonian terra firme forests. Another alternative explanation for the differences in galling richness among terra firme habitats is related to bottom-up (via host plant), and top-down (via natural enemies) factors that could result in different mortality rates (Fernandes 1998;Fernandes et al 2003;Julião et al 2014b). Unfortunately, we are not aware of any study dealing with the proximate causes of galling insect mortality in Amazonian forest types (but see Fernandes et al 2012).…”

Insect gall occurrence in savanna and forest remnant sites of Hidrolândia, GO, Brazil Central

“…Galls and galling insects are widely distributed around the globe, especially in the Neotropical region (Espírito-Santo & Fernandes, 2007). In the Neotropics stand out Amazon (Julião et al, 2014), Atlantic Forest and Cerrado (Araújo et al, 2014a) as hotspots of insect gall diversity.…”

Induction, engineering, and hijacking of defensive strategies of the host by a gall‐inducing weevil

“…These variations, dictated by the insect, enable the galling larvae to thrive within even the harshest environmental conditions, such as those of the Cerrado, deserts, and the arid and hostile rainforest canopy (Julião et al. and references therein). Plants that produce costly nectaries induced by gall development, which are then defended by ants that have, in fact, evolved to protect the plant against this very kind of damage, is a phenomenon not previously reported.…”