Plant species dispersed by Galapagos tortoises surf the wave of habitat suitability under anthropogenic climate change

Abstract

Native biodiversity on the Galapagos Archipelago is severely threatened by invasive alien species. On Santa Cruz Island, the abundance of introduced plant species is low in the arid lowlands of the Galapagos National Park, but increases with elevation into unprotected humid highlands. Two common alien plant species, guava (Psidium guajava) and passion fruit (Passiflora edulis) occur at higher elevations yet their seeds are dispersed into the lowlands by migrating Galapagos tortoises (Chelonoidis spp.). Tortoises transport large quantities of seeds over long distances into environments in which they have little or no chance of germination and survival under current climate conditions. However, climate change is projected to modify environmental conditions on Galapagos with unknown consequences for the distribution of native and introduced biodiversity. We quantified seed dispersal of guava and passion fruit in tortoise dung piles and the distribution of adult plants along two elevation gradients on Santa Cruz to assess current levels of ‘wasted’ seed dispersal. We computed species distribution models for both taxa under current and predicted future climate conditions. Assuming that tortoise migratory behaviour continues, current levels of “wasted” seed dispersal in lowlands were projected to decline dramatically in the future for guava but not for passion fruit. Tortoises will facilitate rapid range expansion for guava into lowland areas within the Galapagos National Park where this species is currently absent. Coupled with putative reduction in arid habitat for native species caused by climate change, tortoise driven guava invasion will pose a serious threat to local plant communities.

Land Cover Classification of Complex Agroecosystems in the Non-Protected Highlands of the Galapagos Islands

“…However, by 2009, it allowed the extraction of C. odorata once again, but only within the agricultural zone [92]. The distribution of P. guajava in Santa Cruz is also consistent with our current understanding of how cattle and tortoises are dispersing this invasive plant [51], and the presence of other invasive plants like R. niveus was also expected in all four inhabited islands [93]. It should be noted that plant species like R. niveus and Coffea commonly grow in the understory of a vegetated area.…”

“…Psidium guajava or guava is a small tree (8 m tall) that has escaped cultivation and become common in mesic forests (>150 masl) of several of the large islands [50,51].…”

Identifying Shared Strategies and Solutions to the Human–Giant Tortoise Interactions in Santa Cruz, Galapagos: A Nominal Group Technique Application

“…A recent study retrieved social (perceptions and attitudes), economic (damage costs to crops and fences), and ecological information (seasonal density of giant tortoises in the rural area) to initially map and begin to understand the conflict [71]. However, in order to start a process leading to more complete understanding of the scale of interactions and integrate related complementary research [93][94][95]101] into a comprehensible and applicable management framework, a participatory and transdisciplinary process is needed [16,18,19].…”

“…The workshop was organized in two phases, each encouraging a constructive dialogue between the participants (Table A1 as shown in Supplementary Materials). The first phase aimed at socializing and presenting different results from our social and ecological research in Galapagos [71,91,[93][94][95]101], the current farming production in Santa Cruz Island and Galapagos, and the current GTMEP, which conducts research on the ecology and conservation of giant tortoises on Santa Cruz Island. Workshop participants by sector Informative fact sheets of the different research results/projects were provided to the participants at the end of this phase to provide background knowledge on our research (Factsheets B1-B4), including the ecology of migration and the ecological role of giant tortoises (Factsheet B5).…”

“…), guava (Psidium guajava), and passion fruit (Passiflora edulis), which have already altered Galapagos ecosystem compositions, since they were brought by humans [115,116]. It is likely, however, that under climatic changing scenarios (PM7), guava and passion fruit will be spread over the national park area in Santa Cruz Island [101], where the main seed dispersers are in fact GTs [94]. With regard to climate change, it is important to realize that climate change has a broad range of structural, psychological, social, and cultural factors that shape people's understanding and perceptions [117][118][119].…”