Disentangling direct and indirect effects of island area on plant functional trait distributions

Julian Schrader*, Mark Westoby, Ian J. Wright, Holger Kreft

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review


Aim: Species diversity on islands generally increases with island area. This might arise either from direct effects of island area via neutral assembly processes or from indirect effects via habitat and structural differences between islands that scale positively with island area. Here, we tested whether community-weighted functional trait means of woody plants are directly or indirectly affected by island area to elucidate how functional traits mediate the assembly on differently sized islands. Location: Twenty-eight tropical islands (25 m2 – 12,000 m2) in the Raja Ampat archipelago, Indonesia. Taxon: Woody angiosperms. Methods: Studied islands had a shared geological history but differed in terms of area, habitat quality expressed by soil depth, forest structure expressed by tree basal area and degree of isolation. Traits studied were seed and fruit mass, tree height, wood density, leaf mass per area, leaf nitrogen concentration and chlorophyll content (estimated from chlorophyll-meter units) and summarised as community-weighted means (CWM) for each island. Using liner regression, we tested whether CWMs were correlated to island area and basal area and structural equation models (SEMs) to test on direct and indirect effects of island area, basal area, soil depth and isolation on trait distributions. Results: CWM of seed mass, tree height and chlorophyll content increased with both island area and basal area, whereas leaf nitrogen concentration decreased with increasing basal area. Fruit mass was not correlated to island area and basal area. SEMs revealed that the shifts in tree height, wood density, leaf nitrogen concentration and chlorophyll content were caused directly by basal area, which in turn was directly and positively affected by both island area and soil depth. Differences in seed mass among islands were explained by combined effects of basal area, island area and isolation, whereas fruit mass was only explained by isolation. Main conclusions: Trait values shifted systematically across islands of different sizes. Being small and having light seeds are prevailing trait combinations for establishing on small islands with simple forest structure. For establishment on larger islands with more complex forest structures, species are taller, have heavier seeds, higher chlorophyll content and lower leaf N concentrations. We conclude that mechanisms affecting CWM on islands directly link to ecological differences between islands like forest structure – and only indirectly to island area.

Original languageEnglish
Number of pages13
JournalJournal of Biogeography
Early online date24 May 2021
Publication statusE-pub ahead of print - 24 May 2021

Bibliographical note

Funding Information:
Funding for this study was provided to JS by the DAAD (Project ID: 57445626), by the German Research Foundation (DFG) with a research scholarship (No: SCHR1672/1‐1) and by a Macquarie University Research Fellowship. HK acknowledges funding of research unit FOR2716 DynaCom from the German Research Foundation (DFG). No collection permits were required for this research.

Publisher Copyright:
© 2021 The Authors. Journal of Biogeography published by John Wiley & Sons Ltd.

Copyright 2021 Elsevier B.V., All rights reserved.


  • community-weighted means
  • dispersal strategy
  • forest structure
  • functional island biogeography
  • habitat quality
  • species competition

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