Disharmony of the world’s island floras

Abstract

Aim: Disharmony is a key concept in island biology that describes the imbalance in the representation of higher taxa on islands compared to their mainland source regions. Although there are strong theoretical arguments for the differential colonization success of different taxa on islands, the empirical evidence for disharmony remains largely anecdotal. Here, we develop a novel method for delineating island source regions and present the first global quantitative assessment of island disharmony.Location: Global. Time period: Recent.Major taxa studied: Vascular plants. Methods:We developed a generalizable method for estimating the source regions of an island flora based on statistical predictions of species turnover. We then designed two metrics to investigate disharmony from an island-and a taxon-centered perspective. First, we used linear mixed effects models to analyse the overall taxonomic bias of 305 island floras (compositional disharmony) as a function of geographical and climatic island features. Second, we applied linear models to examine the over-or under-representation of 450 vascular plant families on islands (representational disharmony) as a function of family size, age, higher taxonomic group and family-specific functional traits. Results:We found that compositional disharmony scales positively with island isolation and negatively with island area, and is strongly modulated by climatic variables. In contrast, the relationship between representational disharmony and family-specific characteristics was weaker. We found significant effects of family species richness and pollination syndrome, whereas family age and all other tested functional traits remained without effect. Conclusions:The taxonomic scope of the disharmony concept has historically limited its wider applicability, because higher taxa are inconsistent ecological proxies. However, our results provide a strong foundation for integrating disharmony with quantitative functional and phylogenetic approaches in order to gain a deeper understanding of assembly processes on islands.

What Will the Future Bring for Biological Invasions on Islands? An Expert-Based Assessment

“…Most respondents agree that effects of increases in alien species richness will occur mainly on oceanic islands, followed by atolls and continental islands. Oceanic islands show a higher degree of endemism and more disharmonic floras and faunas than the other island types (König et al, 2019;Taylor et al, 2019). This disharmony and proportion of endemism increases with island isolation (Kier et al, 2009;König et al, 2017) and at the same time isolation has been shown to increase island invasibility (Moser et al, 2018).…”

GIFT – A Global Inventory of Floras and Traits for macroecology and biogeography

“…The latter may be particularly useful for species distribution modelling where random pseudo-absences are commonly used when true absences are not known (Lobo et al, 2010;Barbet-Massin et al, 2012). Furthermore, GIFT can be used to define regional species pools of local plant communities (Karger et al, 2016), for example, for identifying likely source regions of species that colonize oceanic islands (König et al, 2019). Defining the regional species pool or inferring the floristic status may not only be important for macroecological studies but also for field projects at the local to regional scale.…”

Biogeographic context mediates multifaceted diversity-productivity relationships in island and mainland forests

“…Processes such as rare dispersal events, environmental filtering, and in-situ speciation have generated unique, highly endemic plant assemblages on islands worldwide (Weigelt et al, 2015). The biased representation of higher taxa compared to the source pool on islands due to dispersal, environmental, and biotic filters, i.e., disharmony, (Carlquist, 1974;König et al, 2021;Kraft et al, 2015) may result in closely-related species occupying different ecological niches, potentially increasing ecosystem functioning. Yet, unfilled niche space may persist on oceanic islands due to limited colonization and low species diversity.…”