Species introductions and the phylogenetic and functional structure of California's grasses

Abstract

Many species assemblages represent a nonrandom subset of a larger species pool. When an assemblage tends to contain close evolutionary relatives or species with similar functional traits, it can be described as phylogenetically or functionally clustered. Clustering is often interpreted as evidence for filtering by some combination of environmental and biotic factors. At sufficiently large spatial extents, however, biogeographic barriers can also lead to strong clustering. Here, we suggest that the breakdown of biogeographic barriers associated with human introductions of exotic species can be used as an unintentional experiment to assess their importance in driving phylogenetic and functional structure. An important role of biogeographic barriers would be revealed by a breakdown in clustering, particularly phylogenetic clustering, following species introductions. On the other hand, a role of filtering can be supported by similar patterns of clustering in the native and exotic assemblages along environmental gradients. We test these predictions using the grasses of California, a diverse group including many introduced species. Native grass assemblages in the state are highly clustered with respect to the global grass species pool, both phylogenetically and functionally. Within the state, variation in the strength of clustering is well explained by climatic variables, suggesting an important role for environmental-biotic filtering. Further, subregions within the state with highly clustered native assemblages also contain highly clustered exotic assemblages. Contrary to expectation, though, the introduction of exotic species led to even more strongly clustered assemblages. We conclude that biogeographic barriers have generally not excluded the major grass lineages (e.g., tribes) from the state and likely act only on finer taxonomic scales (for example, excluding particular genera). Our approach should prove broadly applicable and contribute to improved understanding of broad-scale patterns of assemblage structure.

Richness‐dependence of phylogenetic diversity indices

“…Thus, the conclusion from these studies that higher elevations have stronger environmental‐biotic filtering than lower elevations is not in jeopardy from these results. However, the observation that native‐only floras can be less phylogenetically clustered than full native + exotic floras (Lososov et al 2015, Sandel and Tsirogiannis ) could plausibly be generated by this bias.…”

“…In all cases, though, the only factor that could influence community composition was a simple and well‐defined environmental‐biotic filter. In reality, a number of other factors can also lead to phylogenetic clustering, such as dispersal barriers (Pennington et al , Sandel and Tsirogiannis ) and rapid speciation (Li et al ). Simultaneously, competition between close relatives may tend to produce phylogenetic overdispersion, such that gradients of competitive intensity might be incorrectly interpreted as gradients of environmental‐biotic filtering, or the reverse (Gerhold et al ).…”

“…In either case, the range of functional traits present in a community will represent a limited subset of the regional pool, leading to reduced functional diversity. Numerous studies have identified filtering effects on functional diversity across a huge range of spatial scales (Diaz et al , Cornwell and Ackerly , Lebrija‐Trejos et al , Swenson et al , Sandel and Tsirogiannis ).…”

Testing Darwin's Naturalization Conundrum using phylogenetic relationships: Generalizable patterns across disparate communities?

“…The lack of a general pattern appears to be largely due to studies differing in temporal scale (i.e., stage of invasion) (Li et al, 2015;Ma et al, 2016), spatial scale (e.g., local versus regional) (Carboni et al, 2013;Davies, Cavender-Bares, & Deacon, 2011;Ma et al, 2016;Schaefer, Hardy, Silva, Barraclough, & Savolainen, 2011) and/or phylogenetic scale (Procheş, Wilson, Richardson, & Rejmánek, 2008;Thuiller et al, 2010). For example, a number of prior tests of DNC have focused on a single taxonomic group or clade (e.g., Park & Potter, 2015;Sandel & Tsirogiannis, 2016;Strauss et al, 2006). Although this approach illuminates how phylogenetic relatedness influences invasion success among close relatives, it has limited utility for understanding the invasion dynamics and assembly process of a community comprising species from a wider diversity of evolutionary lineages (e.g., see Cadotte, Hamilton, & Murray, 2009;Lososová et al, 2015;Ordonez, 2014;Qian & Sandel, 2017;Schaefer et al, 2011).…”

Phylogenetic relatedness of native and exotic plants along climate gradients in California, USA

“…Climate likely plays a central role in determining large‐scale patterns of phylogenetic structure (e.g., Hawkins, Rueda, Rangel, Field, & Diniz‐Filho, ; Qian, Jin, & Ricklefs, ; Sandel & Tsirogiannis, ; Weigelt et al., ). Many major clades (e.g., orders and families) of plants originated and initially diversified when the Earth's climate was warm and wet.…”