Plant invasion causes alterations in Darwin's finch feeding patterns in Galápagos cloud forests

Invasive species pose a major threat to forest biodiversity, particularly on islands such as the Galapágos. Here, invasive plants are threatening the remnants of the unique cloud forest and its iconic Darwin's finches. We posit that food web disturbances caused by invasive Rubus niveus (blackberry), have contributed to the rapid decline of the insectivourous green warbler finch (Certhidae olivacea). We compared the birds' dietary changes in long-term management, short-term management and unmanaged areas. We measured C:N ratios, and δ15N‑nitrogen and δ13C‑carbon values in both consumer tissues (bird-blood) and food sources (arthropods), as indicators of resource use change, and collected mass abundance, and arthropod diversity data. We characterised the birds' diets using isotope mixing models. The results revealed that finches in (blackberry-invaded) unmanaged areas foraged more on abundant, yet lower quality, arthropods present in the invaded understory. This suggests that blackberry encroachment leads to a decrease in food source quality with physiological consequences for green warbler finch chicks. Results also implied that blackberry control has a short-term impact on food source quantity, which led to a decrease in chick recruitment that we observed in our previous studies; despite this, in the long-term, these managed systems show signs of recovery within three years of restoration. Schematic of whole system. Each spot in forest represents 10 mg dry mean mass of arthropods/m2, orange-canopy, dark yellow-moss, dark green-understory (nplots = 10, round 2, 2015). Long-term management area (LTM), recently controlled area (RC), and unmanaged area (NC). Bug colour represents arthropod nutritional quality, with dark green high quality, light green lower quality. Eggs in nests indicate mean relative breeding success (Cimadom et al., 2019). Bird size highlighting lower tarsus length in NC area (but not to scale). Bar charts below indicated proportion of bird diet derived from the different strata, orange-canopy, dark yellow-moss, dark green-understory based on isotope mixing model data. [Display omitted] •Blood stoichiometry and stable isotope analysis reveal dietary changes of finches.•Disturbance of Scalesia forest by blackberry invasion caused changes in bird diet.•Analysis of bird's diet & its stoichiometry, disclosed trophic ladder alterations.•Higher C:N ratios of diet, possibly reduced bird's tarsus length in invaded areas.•Stable isotope analysis use