The Dominance of Introduced Plant Species in the Diets of Migratory Galapagos Tortoises Increases with Elevation on a Human‐Occupied Island

Abstract

The distribution of resources and food selection are fundamental to the ecology, life history, physiology, population dynamics, and conservation of animals. Introduced plants are changing foraging dynamics of herbivores in many ecosystems often with unknown consequences. Galapagos tortoises, like many herbivores, undertake migrations along elevation gradients driven by variability in vegetation productivity which take them into upland areas dominated by introduced plants. We sought to characterize diet composition of two species of Galapagos tortoises, focussing on how the role of introduced forage species changes over space and the implications for tortoise conservation. We quantified the distribution of tortoises with elevation using GPS telemetry. Along the elevation gradient, we quantified the abundance of introduced and native plant species, estimated diet composition by recording foods consumed by tortoises, and assessed tortoise physical condition from body weights and blood parameter values. Tortoises ranged between 0 and 429 m in elevation over which they consumed at least 64 plant species from 26 families, 44 percent of which were introduced species. Cover of introduced species and the proportion of introduced species in tortoise diets increased with elevation. Introduced species were positively selected for by tortoises at all elevations. Tortoise physical condition was either consistent or increased with elevation at the least biologically productive season on Galapagos. Santa Cruz tortoises are generalist herbivores that have adapted their feeding behavior to consume many introduced plant species that has likely made a positive contribution to tortoise nutrition. Some transformed habitats that contain an abundance of introduced forage species are compatible with tortoise conservation.

Benefits of the destinations, not costs of the journeys, shape partial migration patterns

“…We estimate τ y based on two sets of observations: (i) Roughly 60% of the lowland diet and 30% of the highland diet are comprised of forbs, with the remaining portions consisting of lower quality forage, including grasses (Blake et al . ); (ii) the efficiency of energy uptake in a sister taxa, desert tortoises, varies from 0·7 to 0·5 depending on whether they were fed higher quality forage consisting of forbs or lower quality forage consisting of grasses (Nagy, Henen & Vyas ). Taken together, these observations suggest that overall energetic efficiency is 0·62 in lowland habitat and 0·56 in the highland habitat.…”

“…(a) Foraging gains are calculated using a type II functional response (Holling ), where the assimilation efficiency, τ, is estimated from diet observations (Blake et al . ) and studies in related taxa (Nagy, Henen & Vyas ), the handling time varies with body size, M (Yodzis & Innes ; Franz et al . ), the search rate, s , is estimated from the observed growth of three non‐migratory juvenile tortoises in the field, and NDVI is used as a proxy for resource density.…”

“…); (v) diet data are available to estimate forage quality in the two habitats (Nagy, Henen & Vyas ; Blake et al . ); (vi) growth data for three juveniles are available to estimate the search rate in terms of NDVI, and (vii) data on male and female body sizes and timing of migration exist to independently test the reliability of the model (Fig. c).…”

Comparison of the Effect of Three Food Diets Provided to Giant Tortoises (Chelonoidis guntheri and Chelonoidis vicina) Bred in Captivity at Isabela Island – Galápagos

“…In this regard, [17] indicate that resource distribution and food selection are fundamental to ecology, life history, physiology, population dynamics, and conservation of animals; so in Galapagos introduced plants are changing the feeding dynamics of herbivores in many ecosystems often with unknown consequences; Giant tortoises, like many herbivores, undertake migrations along elevation gradients dependent on variability in vegetationandcourse of a year, specifically during periods of motivated [18].…”

Antimicrobial resistance genes present in the faecal microbiota of free‐living Galapagos tortoises (Chelonoidis porteri)

“…Characterized as generalist herbivores, Galapagos tortoises undertake migrations driven by spatiotemporal variability in vegetation productivity (Blake, Guézou, Deem, Yackulic, & Cabrera, ; Yackulic, Blake, & Bastille‐Rousseau, ). These giants move long distances, from protected national park areas to human‐modified landscapes, including tourist farms and areas managed for livestock on human‐inhabited islands such as Santa Cruz (Blake et al, , ). Despite the identification of human–animal cohabitation as one of the most critical factors for the conservation of wildlife species (Daszak, Cunningham, & Hyatt, ; Deem, Parker, & Miller, ), few studies have been performed to assess how this cohabitation may affect tortoise health and the spread of AMR in the archipelago.…”