In this dissertation I was only able to target certain aspects of research on HEIs and several new and exciting questions have arisen as a result of my research. In the following I would like to present and discuss the eleven most intriguing research questions for future HEI science that reach beyond the issues covered in this thesis. These questions are grouped into three categories: 1) Within-island patterns of species diversity, 2) Disturbance-driven island ecology, and 3) Global patterns.
3.1 Within-island patterns of species diversity
On La Palma, my model HEI, I was able to identify several important environmental drivers of species richness, endemic richness and endemicity using a floristic dataset of unprecedented size (in comparison to the island area) and resolution (100 x 100 m; see Manuscript 6). In my opinion, this dataset allows tackling an array of different questions of great interest to the ecology community owing to the spatial nature and the coverage of many different environmental gradients:
1. Recently, ecology has tried to reach beyond the species concept as the basic unit in biodiversity and ecosystem research (Cadotte et al. 2011). A new focus is being put on the functionality of ecosystems and communities, where specific traits or the combination of traits is of major importance (Schleuter et al. 2010). Therefore, it is important to ask how are plant traits such as life form, plant functional type (e.g. N-fixers, succulence, woodiness, flower color), dispersal mode, functional diversity, but also phylogenetic diversity distributed on the within-island scale and are there differences between natives, archipelago endemics and SIEs?
Furthermore, I ask what are the major environmental determinants of these different trait-based measures?
2. Species turnover, or β-diversity, has been predicted to change along gradients (e.g. Qian &
Ricklefs 2007) but the underlying drivers are still poorly understood, especially for endemic species on HEIs. How is β-diversity distributed within an HEI and what drives these patterns?
Are there differences between natives, archipelago endemics and SIEs?
3. How does climate change influence endemic plant species? Does it threaten spatially restricted and poor dispersing SIEs to extinction? Which SIEs are potentially most likely to go extinct? Where would potential conservation areas be most effective and how should conservation programs be designed if considering climate change and future land use change?
These questions could be targeted using different species distribution modeling (SDM) techniques such as MaxEnt (Phillips et al. 2006), ensembles (Araújo & New 2007) or n-dimensional hypervolumes (Blonder et al. 2014)
4. Do SIEs really occupy smaller ecological niches than archipelago endemics or natives (Vilenkin et al. 2009), or are they rather (super-)generalists (e.g. Lomolino 2010)? How is a
quantification of their ecological niches possible? Novel methods such as quantifications of n-dimensional hypervolumes (Blonder et al. 2014) might be valuable tools in addressing these questions.
3.2 Disturbance-driven island ecology
Disturbances clearly shape the pattern of species richness and endemism on La Palma. However, our understanding of the disturbance-induced process on HEIs is still in an initial phase, leaving room for further questions. In addition, several follow-up questions arise from the three disturbances studied in this dissertation (roads, introduced herbivores and fire):
5. Roads have been shown to be vectors of non-native plant species in mountainous ecosystems (also HEIs) and non-native plant species richness decreases with elevation according to the directional elevational filtering hypothesis (Alexander et al. 2011). However, a currently unstudied question is how non-native plant species behave along other environmental gradients than elevation such as precipitation, climatic rarity or seasonality and if this behavior differs between islands and archipelagos?
6. Introduced herbivores have been shown to be crucial in shaping ecosystems on the HEIs of the Canary Islands such as pine forests (Garzón-Machado et al. 2010) and the summit scrub (Manuscript 3 and 4). However, how do introduced herbivores influence tree rejuvenation in the laurel forest – an iconic ecosystem of the Canaries – and understory diversity? Is it as poor as currently thought or do introduced herbivores depauperate understory diversity, as is the case in the pine forest (Garzón-Machado et al. 2010) and the summit scrub (Manuscript 4)?
7. The leguminous shrub A. viscosus subsp. spartioides is virtually mono-dominant shrub in the summit scrub of La Palma and shows a patchy distribution of living, semi-living, and dead individuals. Do environmental factors such as fire and winter ice storms drive this pattern or are intrinsic drivers responsible, such as cohort dieback?
8. The summit scrub of La Palma has been severely impacted by introduced herbivores, leading to the mono-dominance of A. viscosus subsp. spartioides. However, why does the floristically very similar summit scrub of Tenerife (del Arco-Aguilar et al. 2010) not show the same dominance of A. viscosus subsp. viscosus (i.e. a vicariant subspecies), even though the same introduced herbivores and similar environmental conditions are present?
3.3 Global patterns
Explaining large-scale patterns was an integral part of this dissertation, especially in terms of global treeline research, plant diversity and climate change. In particular, the innovative sampling method applied in Manuscript 1 used to identify island treeline elevations is a promising tool for further treeline research encompassing all landmasses. Climate change is one of the major threats of our times making
Emerging research challenges and frontiers
estimations of future threats and possible extinctions highly valuable for researchers, decision and policy makers, conservation managers and society in general.
9. Can the island biogeographical determinants of island treeline elevation be transferred to all treelines, also on the mainland? Is it possible to quantify the influence of mass elevation effect and continentality on global treeline elevation?
10. Can we assess the climate change-driven extinction threat of island species on a global scale by combining SDMs with sea level rise models and land use change models?
11. Do tropical species really have smaller elevational ranges than temperate species because of lower temperature seasonality in the tropics? By combining elevation-specific species data from tropical, subtropical and temperate regions, a first global-scale quantification of this hypothesis made by Janzen in 1967 can be achieved, which so far has remained untested but highly cited and influential in ecology (Ghalambor et al 2006).
Presented Manuscripts