Life‐history dimensions indicate non‐random assembly processes in tropical island tree communities

Abstract

Community assembly processes on islands are often non‐random. The mechanisms behind non‐random assembly, however, are generally difficult to disentangle. Functional diversity in combination with a null model approach that accounts for differences in species richness among islands can be used to test for non‐random assembly processes, but has been applied rarely to island communities. By linking functional diversity of trees on islands with a null model approach, we bridge this gap and test for the role of stochastic versus non‐random trait‐mediated assembly processes in shaping communities by studying functional diversity–area relationships. We measured 11 plant functional traits linked to species dispersal and resource acquisition strategies of 57 tree species on 40 tropical islands. We grouped traits into four life‐history dimensions representing 1) dispersal ability, 2) growth strategy, 3) light acquisition and 4) nutrient acquisition. To test for non‐random assembly processes, we used null models that account for differences in species richness among the islands. Our results reveal contrasting responses of the four life‐history dimensions to island area. The dispersal and the growth strategy dimensions were underdispersed on smaller islands, whereas the light acquisition dimension was overdispersed. The nutrient acquisition dimension did not deviate from null expectations. With increasing island area, shifts in the strength of non‐random assembly processes increased the diversity of dispersal and acquisition strategies in island communities. Our results suggest that smaller islands may be more difficult to colonize and provide more limited niche space compared to larger islands, whose tree communities are likely determined by stochastic processes and higher niche diversity. Our null model approach highlights that analyzing the functional diversity of different life‐history dimensions provides a powerful framework to unravel community assembly processes on islands. These complex, non‐random assembly processes are masked by measures of functional diversity that do not account for differences in species richness between islands.

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

“…In our island system, seed and fruit mass were directly affected by island isolation. The links of seed and fruit mass to island isolation indicate that (at least some) plant species follow specific dispersal strategies in this island system (see also Schrader et al, 2021).…”

“…We compared small tropical coralline islands in the Raja Ampat archipelago, Indonesia. A related study (Schrader et al, 2021) showed that the diversity of community traits in this island system was affected by non-random assembly processes caused by environmental and 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.…”

“…Resource competition among plants drives communities on larger islands, whereas limited niche space and ocean-borne disturbances structure plant communities on smaller islands (Schrader et al, 2021;; see also Whitehead & Jones, 1969).…”

Sticking around: plant persistence strategies on edaphic islands

“…The study of plant functional traits in insular systems has boosted in recent years, providing important insights into the eco-evolutionary dynamics of these systems (e.g. Biddick et al, 2019;Biddick & Burns, 2021;García-Verdugo et al, 2020;Negoita et al, 2016;Schrader et al, 2021;Spasojevic et al, 2014;Taylor et al, 2019).…”

“…The study of plant functional traits in insular systems has boosted in recent years, providing important insights into the eco-evolutionary dynamics of these systems (e.g. Biddick et al, 2019; Biddick & Burns, 2021; García-Verdugo et al, 2020; Negoita et al, 2016; Schrader et al, 2021; Spasojevic et al, 2014; Taylor et al, 2019). Yet, most of this research targeted dispersal and resource acquisition traits, neglecting other important, non-acquisitive functions (but see Aikio et al, 2020), which are needed for better understanding how plant traits promote persistence and help avoid local extinction (Auffret et al, 2017).…”