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Testing mechanisms of species invasion in Galápagos: role of resource competition and species traits

Final Report Summary - TEMSPIG (Testing mechanisms of species invasion in Galápagos: role of resource competition and species traits)

Biological invasions are the second most important threat to biodiversity, after habitat loss and habitat transformation. An outstanding example is the red quinine tree, Cinchona pubescens, which threatens unique ecosystems in the Galápagos archipelago by transforming the formerly treeless highlands of Santa Cruz Island into a forest habitat. Previous studies revealed significant negative effects on species richness and abundance of endemic species. As the mechanisms underlying these impacts are completely unknown, the TEMPSIG project aimed at elucidating the invasion processes and associated consequences. Thus two key issues were addressed by experimental approaches: (a) changes in ecosystem functioning due to the “new“ life form “tree” and (b) the role of micorrhizal fungi in the invasion process.

For the first approach, a field experiment with 60 experimental and 20 artificial trees was established in Galápagos in collaboration with the Charles Darwin Foundation (CDF) and the Galápagos National Park Service (GNPS). The manipulated and artificial Cinchona trees successfully simulated shading caused by individual trees and of dense tree stands. Results indicated that air temperature and light intensity were lower and air and soil humidity higher under Cinchona trees, resulting in a microclimate change for the co-occurring species. An alleviation from the shade resulted in a recovery of a plant species composition similar to that of the area surrounding the trees. Frequent clipping of the leaves of the Cinchona trees resulted in a severe damage or even death of the trees so that this method is now under closer evaluation by the GNPS for its suitability as a future control measure for Cinchona in the Gálapagos National Park.

To test whether Cinchona is changing the nutrient cycling in the soil, leaf samples of Cinchona were taken, as well as samples of three dominant native and endemic plant species, along with leaf litter, soil and resin bag samples from invaded areas. Cinchona had a higher specific leaf area, higher leaf nutrient concentrations, and a faster decomposition rate than native species. However, the most surprising result was that the senesced leaves of Cinchona contained significantly more phosphorus than the green leaves. This suggested an enhanced nutrient uptake by Cinchona (especially of phosphorus) and a faster decomposition of its litter, leading to increased nutrient availability in the soil. And indeed, nitrogen, ammonium and phosphorus concentrations were also significantly higher in soils, resin bags and leaf litter from invaded sites. These findings point towards an accelerated nutrient cycling in Cinchona-infested areas, which could promote the spread of Cinchona and other introduced species. As a consequence, this could increase the risk of further displacement of indigenous species of the Santa Cruz highlands. These results have prompted a new project led by the CDF and the GNPS to investigate the impacts of an enhanced nutrient availability in the soil on native and endangered plant species in this area. In addition, this knowledge was incorporated into the management plans for the control of other introduced species. A new research collaboration project between the Chairs of Ecosystem Research/Plant Ecology (I. Kowarik and H. Jäger) and Soil Sciences (M. Kaupenjohann) at TU and the Department for Plant Ecology (M. Rillig) at Freie Universität is now in the planning phase to determine how Cinchona manages to take up so much phosphorus from the phosphorus-poor soil and what the role of mycorrhizal fungi (MF) are in this process.

To shed light on the involvement of these MF in the highly successful invasion, the micorrhization rate of Cinchona roots in Galápagos was compared to roots from the species’ native range in Loja (Ecuador). Results revealed a higher degree of micorrhization in roots from Galápagos. To test the involvement of other abiotic and biotic factors in this invasion process, the effects of nutrients (nitrogen, phosphorus) and MF were tested on the growth of Cinchona from Galápagos and Loja in a greenhouse experiment at the Universidad Nacional de Loja (UNL) under the cooperation of the Lehrstuhl für Waldbau (M. Weber and B. Stimm) at the Technische Universität München. Results obtained from work in collaboration with the department of Plant Ecology (M. Rillig) at Freie Universität Berlin showed higher micorrhization and stronger response to nutrients of plants from Galápagos compared to those from Loja which signifies a growth advantage of the former. Understanding these mechanisms will help inform conservation about general invasion mechanisms in oceanic island ecosystems and elsewhere.

Maria Eugenia Apolo, an Ecuadorian student worked in this project for her Bachelor thesis at the UNL and successfully defended her thesis in November 2012. Maria Eugenia was involved in the establishment of a protocol for germination and growth of Cinchona, which is of great interest to conservation organizations and private land owners in the province of Loja, as well as to the municipality of Loja. Since Cinchona pubescens is now a rare species in this area in Southern Ecuador, there is an increasing demand for the cultivation of this species.

A new collaboration was established with the Universidad Tecnica Particular de Loja (UTPL) and the Universität Tübingen to carry out a molecular analysis of the MF encountered in the roots of the greenhouse plants. An unexpected discovery was made at the Universität Tübingen by identifying fungi of the family Paraglomeraceae. Currently, there is only very limited information available on this family (and its members) and therefore strains obtained from these samples will help to gather more information on their morphology and phylogeny. An Ecuadorian student will be recruited at UTPL to work on this topic for his or her PhD thesis at Freie Universität Berlin.