Bachelor Thesis

Spatial patterns in dispersal mechanisms on islands - a case study of La Palma, Canary Islands

Katharina Müller (08/2014-09/2014)

Support: Manuel Steinbauer, Carl Beierkuhnlein

Aim
To reveal altitudinal and spatial patterns of dispersal mechanisms on La Palma, representing an oceanic island. It is anticipated that due to changing environmental and climatic conditions along an altitudinal gradient, the importance and thus proportion of dispersal mechanisms should alter. It is hypothesized that gradients in precipitation, structural complexity of vegetation and isolation are major factors influencing the proportions of dispersal mechanisms, the so-called dispersal spectrum.
Location
La Palma, the northwestern most island of the Canary Island Archipelago (27.62° N–29.42° N and 13.33° W–18.17° W).
Methods
Data on species occurrence on the entire island of La Palma was known to a resolution of 500 m x 500 m grid cells (Banco de Datos de Biodiversidad de Canarias). Morphological seed traits were systematically transferred into dispersal mechanisms for each species through a literature and internet based research. Due to climatic reasons, the island was divided into a north-eastern and a south-western side for the analysis. Patterns of dispersal mechanisms spatially and along altitude were then statistically tested.
Results
Zoochory, Anemochory and Autochory showed consistent trends along altitude on both island sites, however more distinct for the north-eastern side. Zoochory showed a clear peak in medium altitudes at around 500 m- 1500 m, resulting in a hump-shaped correlation with altitude. Furthermore Zoochory was most prominent on the wet north-eastern side of the island. Anemochory decreased with increasing elevation and showed higher proportions on the dry south-western side. Autochory increased with increasing elevation and showed high proportions in high elevation ecosystems.
Main Conclusions
Climatic conditions result in certain environmental conditions that put evolutionary pressure under ecosystems. According to circumstances some dispersal mechanisms are favored over others as chances of survival for the population increase. For Zoochory it is suggested that high resource availability in medium altitudes (due to north-easterly trade winds) result in complex ecosystems. The production of less but bigger seeds is to enhance survival probability under conditions like thick litter layers, light competition and other disturbances in dense forests. The dispersal by birds is then the most effective dispersal strategy. In the case of Anemochory due to low complex vegetation structure in lower altitudes dispersal by wind is more successful than elsewhere. In addition greatest area and least isolation in lower altitudes could influence the prominence of Anemochory in lower altitudes. Extreme conditions in high elevations might favor species that produce many small seeds, such as many autochorous species do, rather than investing energy in dispersal structures or bigger seeds. The investment in number instead of dispersal strategy can be seen as an evolutionary strategy itself.