Phylogenetic information is now commonly employed in analyses of species co-occurrence and community assembly, as a way of explaining patterns of species distributions and diversity [1
]. Community phylogenetic studies have increased steadily in sophistication over the past decade. For example, the effects of spatial and phylogenetic scale have been explored [5
], and null models for phylogenetic community structure have been developed [6
]. More generally, the idea that patterns of phylogenetic attraction (where close relatives are more likely to co-occur than expected by chance) and repulsion (where close relatives are less likely to co-occur) can act as a proxy for ecological processes is now regarded more critically. It is generally agreed that phylogenetic and phenotypic patterns should be analysed simultaneously if informative inferences about processes are to made [6
One assumption that remains common in community phylogenetics, however, is the idea that the structure of a community is governed by an overarching process, such as competition or environmental filtering, that applies to all species in the community. This assumption is implicit in whole-community metrics of phylogenetic community structure such as NRI and NTI [4
]. While some studies provide evidence for the simultaneous operation of competition and environmental filtering, often depending on spatial or phylogenetic scale [5
], the assumption that the same process applies to all species in a community, at a given scale, is typically still made. However, ecological processes such as competitive exclusion occur at the level of individuals, populations or species, not communities. The structure of a community is simply an emergent property that results from the influence of such processes on the combined distributions of multiple species within a region. Therefore, it is possible that whole-community metrics obscure much informative variation in phylogenetic structure by “averaging out” the patterns of occurrence or abundance of different species within a community. An alternative approach is to analyse the phylogenetic and phenotypic signal of pairwise co-occurrences among species across a set of communities (e.g. [9
]. This approach treats species pairs, rather than communities, as the units of analysis, more easily allowing patterns of co-occurrence to be examined separately for species with different demographic and ecological attributes.
In this study I compare the phylogenetic and phenotypic signal of co-occurrence patterns of plant species that differ in fire-regeneration strategy, in the Mediterranean-climate shrublands of southwestern Australia. This ecosystem type is exceptionally species-rich, but is still poorly represented among community phylogenetic studies (but see [11
]. In these shrublands, fires are frequent, with average recurrence intervals around 10–15 years [14
]. Some authors have argued that in these highly disturbed, non-equilibrium communities, classic theories of species coexistence through niche differentiation are less realistic than models of coexistence based on lottery recruitment and the availability of transient niches [14
]. Other studies in Mediterranean shrublands have found evidence for patterns that may be competition-driven, such as niche differentiation [20
] or phylogenetic overdispersion [11
There are two main strategies for coping with fire within Mediterranean-climate floras. Resprouters survive fire and regenerate from lignotubers, epicormic buds or other structures, and reseeders are killed by fire and replaced by seedlings. Although there are different degrees and modes of resprouting, in Mediterranean-climate shrublands regeneration mode is usually considered a simple dichotomous variable [21
]. Essentially, therefore, the flora is divided into two components with fundamentally different demographics: fire-resistant (resprouter) populations are relatively stable, persistent and impervious to disturbance by fire, and fire-killed (reseeder) populations are more variable and susceptible to local extinction after fire [18
Do these ecological differences between resprouters and reseeders generate different patterns of association between phylogenetic relatedness, phenotypic similarity, and co-occurrence? Several predictions can be made:
(1) If resprouters are longer-lived with more stable populations, co-occurrence among resprouter species may be influenced by competition for space, soil moisture or nutrients, and by differentiation of species along niche axes associated with the pre-emption of these resources. Among reseeders, on the other hand, co-occurrence may be mediated by the frequency of fire, and its influence on dispersal and colonization, rather than by niche differentiation [22
]. Under this scenario we would predict phylogenetic and phenotypic repulsion among resprouters, but not among reseeders.
(2) Alternatively, competition may be intense among reseeders, because the pressure to grow rapidly to maturity and set seed within the average fire interval is traded off against the costs of faster growth [24
]. Hence, co-occurrence among reseeders may be controlled by niche differentiation along life-history axes associated with time to maturity, or environmental gradients such as soil fertility, which influences growth rates [22
]. Under this scenario, we would predict phylogenetic and phenotypic repulsion among reseeder species but not among resprouters.
(3) Finally, there may be no difference in the mechanisms of co-occurrence between species differing in regeneration mode, if co-occurrence is determined primarily by processes that apply regardless of regeneration mode.
As a case study for testing these predictions I use the genus Banksia
, one of Australia’s iconic plant genera and a prominent part of the flora of southwestern Australia. The genus includes 170 species that have radiated into a variety of growth forms, from prostrate ground-covers to shrubs and trees >6 m in height, and includes resprouters and reseeders. Local-scale diversity of Banksia
is high: up to twelve species have been recorded from single 10 m x 10 m plots [26
]. The analyses presented here are based on surveys of species within plots of two sizes: 20 m x 20 m (0.04 ha) and 200 m x 200 m (4 ha). I begin by quantifying patterns of co-occurrence among pairs of Banksia
species within these plots. I then test whether co-occurrence among species pairs is associated with phylogenetic relatedness, regeneration mode, ecological similarity, and similarity in soil type preferences. I interpret co-occurrence patterns in the context of the phylogenetic signal in the different niche dimensions.