Abstract
AbstractAimAn individual tree resembles a living island, a small spatially distinct unit upon which colonizers maintain populations. However, several differences exist compared to oceanic islands: a tree is relatively young, is composed of numerous differently aged branches, may be phylogenetically isolated from neighbours, and some of its colonizers are specific to particular tree lineages. We suggest that these specificities strongly affect both alpha‐ and beta‐diversity within trees, including positive effects of isolation on the diversity of generalists, and strengthening of the effect of isolation with tree age.LocationRennes, Bretagne, Western FranceTaxonLittle‐dispersive, generalist oribatid mites (Acari) and highly dispersive, specialist gall wasps (Hymenoptera: Cynipidae) on oak (Quercus sp.) trees.MethodsWe tested the effects of tree and branch age, tree and branch habitat diversity, and tree phylogenetic isolation on per‐branch and per‐tree alpha‐diversity, and on within‐tree beta‐diversity of both taxonomic groups.ResultsFor gall wasps, no variable explained diversity patterns at any level. In contrast, for oribatid mites, we found that high phylogenetic isolation of trees and high branch age increased alpha‐diversity per tree and per branch (in young trees) as well as turnover among branches. High tree age decreased alpha‐diversity per branch (in phylogenetically isolated trees) and increased turnover among branches. Increasing habitat diversity increased alpha‐diversity per tree, but decreased alpha‐diversity per branch (in young trees).Main conclusionsFor mites, contrary to common expectation, we suggest that: (a) phylogenetically distant neighbours are a source of immigration of distinct species and (b) with the increase of tree age, species‐sorting results in a few species colonizing and dominating their preferred patches. In gall wasps, strict specialization on oaks, and efficient dispersal may render oak age or isolation unimportant. The positive relationship between isolation and within‐tree turnover is a new contribution to biogeography in general.
Highlights
Studies on oceanic islands have provided key insights into the assembly and structuring of ecological communities (Santos et al, 2016; Whittaker et al, 2017)
Several differences exist compared to oceanic islands: a tree is relatively young, is composed of numerous differently aged branches, may be phylogenetically isolated from neighbours, and some of its colonizers are specific to particular tree lineages
For oribatid mites we found that high phylogenetic isolation of trees and high branch age increased alpha-diversity per tree and per branch as well as turnover among branches
Summary
Studies on oceanic islands have provided key insights into the assembly and structuring of ecological communities (Santos et al, 2016; Whittaker et al, 2017). Island properties have major effects on species diversity. Islands with higher habitat diversity typically harbour larger numbers of species because they can accommodate species with different habitat requirements (Fattorini et al, 2015; Hortal et al, 2009), habitat specialists. Larger islands tend to have higher species diversity, probably because the rate of extinction relative to colonization is lower (MacArthur & Wilson, 1967). Isolated islands typically have lower species diversity, primarily because they can only be reached by few dispersers There is evidence that, all else being equal, young islands are occupied by less species due to little time available for their arrival (Whittaker et al., 2008; Simberloff & Wilson, 1969; Cornell & Harrison, 2014), opposing forces may operate (Whittaker et al, 2017)
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