Tree diversity on islands: assembly rules, passive sampling and the theory of island biogeography

Abstract

Aim Species diversity is distributed heterogeneously through space, for reasons that are poorly understood. We tested three hypotheses to account for spatial variation in coniferous tree species diversity in a temperate island archipelago. The theory of island biogeography (ToIB) predicts that island area affects species diversity both directly (by increasing habitat diversity) and indirectly (by increasing abundances, which in turn reduce extinction rates). The ToIB also predicts that island isolation directly affects species diversity by reducing immigration rates. The passive sampling hypothesis predicts that island area and isolation both affect species diversity indirectly, by increasing and decreasing abundances, respectively. Community assembly rules (i.e. even partitioning of conifer abundances among islands) might also reduce tree species diversity beyond the core predictions of ToIB and the passive sampling hypothesis.Location Barkley Sound, British Columbia, Canada.Methods The abundances of eight coniferous tree species were quantified on 34 islands and two (1 ha) mainland plots. The predictions of the ToIB and the passive sampling hypothesis were tested using path analysis, and null models were used to test for abundance-based assembly rules and to further test the passive sampling hypothesis.Results Path analysis showed that island area and isolation did not have direct, statistical effects on tree species diversity. Instead, both geographic variables had direct statistical effects on total tree abundances, which in turn predicted tree diversity. Results from several passive sampling null models were correlated with observed patterns in species diversity, but they consistently overestimated the number of tree species inhabiting most islands. A different suite of null models showed support for community assembly rules, or that tree species often reached higher abundances on islands that housed fewer heterospecific trees.Main conclusions Results were inconsistent with the ToIB. Instead, patterns in tree diversity were best explained by a combination of stochastic (passive sampling) and deterministic (assembly rules) processes. Stochastic and deterministic processes are commonly considered to be exclusive explanations for island community structure, but results from this study suggest that they can work synergistically to structure island tree communities.

The theory of island biogeography, the sample‐area effect, and the habitat diversity hypothesis: complementarity in a naturally fragmented landscape of lake islands

“…We therefore used path analysis with correlated causes (a structural equation model) to assess both direct and indirect effects of predictor variables according to an a priori model structure (Grace & Pugesek, , ; Li, ). Path analysis is particularly useful for distinguishing the effects of multiple collinear variables (e.g., habitat diversity and area per se) on multiple response variables (for similar applications, see Kohn & Walsh, ; Triantis, Mylonas, Weiser, Lika, & Vardinoyannis, ; Triantis et al., ; Sfenthourakis & Triantis, ; Burns et al., ). Multigroup path analysis, grouped by small and large island set, permitted comparisons of relationships between the two ranges of island sizes used in SLOSS‐based analyses.…”

“…Contrasting with the theory of island biogeography, the habitat amount hypothesis (Fahrig, ) replaces fragment area and isolation with a single predictor of species richness, total habitat area. Not unlike the passive sampling hypothesis, developed in the context of oceanic islands (Connor & McCoy, ), the habitat amount hypothesis uses the sample‐area effect to explain positive species–area relationships across isolated fragments: larger sample areas generally contain more individuals, belonging to more species (Burns, Berg, Bialynicka‐Birula, Kratchmer, & Shortt, ; Fahrig, ). In SLOSS terms, the sample‐area effect specifically predicts that single large and several small fragments will contain equivalent numbers of species when total area is held constant.…”

“…Nevertheless, it remains controversial whether habitat diversity or area per se is more important in structuring patterns of species richness on fragmented landscapes, and the habitat diversity hypothesis makes no specific predictions of fragmentation effects. While there is support for effects of habitat diversity on species richness independent of area (Burns et al., ; Hortal et al., ; Kohn & Walsh, ), there is also support for direct effects of area per se on demographic processes affecting species richness (Buckley, ; Nilsson et al., ). An important consideration in such investigations is scale, as the relative importance of habitat diversity and area per se has been shown to vary with island or fragment area (Rosenzweig, ; Sfenthourakis & Triantis, ).…”

Environmental filtering underpins the island species—area relationship in a subtropical anthropogenic archipelago

“…all species are not functionally equivalent, as assumed by neutral models and IBT). Therefore, islands will have fewer observed species than expected based on the total number of individuals that they support (Burns et al, 2010).…”

“…A variety of hypotheses have been proposed to explain the ISAR. First, the passive sampling hypothesis states that the number of species increases with island area due to large islands randomly sampling more individuals, and thus accumulating more species, from a pool of colonists than small islands (Burns, Berg, Bialynicka‐Birula, Kratchmer, & Shortt, ; Connor & Mccoy, ). Second, the habitat diversity hypothesis is based on the idea that larger islands have greater habitat diversity, which enables a greater range of habitat specialists to exist on an island (Bracewell, Clark, & Johnston, ; Gaston & Blackburn, ; Hortal, Triantis, Meiri, Thebault, & Sfenthourakis, ).…”