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. 2024 Feb 13;14(2):e10994.
doi: 10.1002/ece3.10994. eCollection 2024 Feb.

Invasion by Cedrela odorata threatens long distance migration of Galapagos tortoises

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Invasion by Cedrela odorata threatens long distance migration of Galapagos tortoises

Stephen Blake et al. Ecol Evol. .

Abstract

Invasive alien species are among the most pervasive threats to biodiversity. Invasive species can cause catastrophic reductions in populations of native and endemic species and the collapse of ecosystem function. A second major global conservation concern is the extirpation of large-bodied mobile animals, including long-distance migrants, which often have keystone ecological roles over extensive spatial extents. Here, we report on a potentially catastrophic synergy between these phenomena that threatens the endemic biota of the Galapagos Archipelago. We used GPS telemetry to track 140 migratory journeys by 25 Western Santa Cruz Island Galapagos tortoises. We plotted the spatial interaction between tortoise migrations and recently established non-native forest dominated by the invasive tree Cedrela odorata (Cedrela forest). We qualified (a) the proportion of migratory journeys that traversed Cedrela forest, and (b) the probability that this observed pattern occurred by chance. Tortoise migrations were overwhelmingly restricted to small corridors between Cedrela forest blocks, indicating clear avoidance of those blocks. Just eight of 140 migrations traversed extensive Cedrela stands. Tortoises avoid Cedrela forest during their migrations. Further expansion of Cedrela forest threatens long-distance migration and population viability of critically endangered Galapagos tortoises. Applied research to determine effective management solutions to mitigate Cedrela invasion is a high priority.

Keywords: GPS telemetry; alien species; conservation; ecosystem engineer; eradication; megaherbivores; silviculture; vegetation mapping.

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Conflict of interest statement

The authors declare no conflict of interest.

Figures

FIGURE 1
FIGURE 1
Conceptual model illustrating hypotheses on why Galapagos tortoises may avoid Cedrela forest on Santa Cruz Island. Edge effects at the Cedrela forest boundary promotes invasion by exotic blackberry, which often creates impenetrable thickets that block migratory Galapagos tortoises. With distance from the edge into the interior of large Cedrela forest patches shade increases, and tortoises have greater difficulty maintaining body temperature. Furthermore, low light levels coupled with the allelopathic nature of Cedrela, eliminates ground vegetation from the forest floor, limiting forage availability for tortoises.
FIGURE 2
FIGURE 2
Map of the southwestern flank of Santa Cruz Island, Galapagos, showing Cedrela forest distribution (dark green) and 140 Western Santa Cruz tortoise migration routes (gray tracks). Black tracks are migration route segments that pass through Cedrela forest. The three gaps in Cedrela forest cover are labeled A, B, and C. Inserts to the right of the main figure illustrate the detail of tortoise tracks in relation to the three gaps in Cedrela forest cover.
FIGURE 3
FIGURE 3
Histogram of percent of overlap of randomized tortoise migration trajectories with Cedrela patches in southwestern Santa Cruz Island, Galapagos. These randomized paths provide a null expectation of the potential overlap with Cedrela if tortoises were not responding to it. Observed overlap (red line) is significantly lower than these expectations, indicating a clear avoidance of Cedrela by tortoises.

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