Priority effects can play a fundamental role in the assembly of ecological communities, but how they shape the dynamics of biodiversity over macroevolutionary timescales remains unclear. Here we develop and analyse a metacommunity model combining local priority effects with niche evolution, speciation and extinction. We show that by promoting the persistence of rare species, local priority effects cause the evolution of higher metacommunity diversity as well as major disparities in richness among evolutionary lineages. However, we also show how classic macroevolutionary patterns of niche incumbency—whereby rates of regional diversification and invasion slow down as ecological niches are filled—do not depend on local priority effects, arising even when invading species continuously displace residents. Together, these results clarify the connection between local priority effects and the filling of ecological niche space, and reveal how the impact of species arrival order on competition fundamentally shapes the generation and maintenance of biodiversity.

AimThe exceptional turnover in biota with elevation and number of species coexisting at any elevation makes tropical mountains hotspots of biodiversity. However, understanding the historical processes through which species arising in geographical isolation (i.e. allopatry) assemble along the same mountain slope (i.e. sympatry) remains a major challenge. Multiple models have been proposed including (1) the sorting of already elevationally divergent species, (2) the displacement of elevation upon secondary contact, potentially followed by convergence, or (3) elevational conservatism, in which ancestral elevational ranges are retained. However, the relative contribution of these processes to generating patterns of elevational overlap and turnover is unknown.LocationTropical mountains of Central‐ and South‐America.Time PeriodThe last 12 myr.Major Taxa StudiedBirds.MethodsWe collate a dataset of 165 avian sister pairs containing estimates of phylogenetic age, geographical and regional elevational range overlap. We develop a framework based on continuous‐time Markov models to infer the relative frequency of different historical pathways in explaining present‐day overlap and turnover of sympatric species along elevational gradients.ResultsWe show that turnover of closely related bird species across elevation can predominantly be explained by displacement of elevation ranges upon contact (81%) rather than elevational divergence in allopatry (19%). In contrast, overlap along elevation gradients is primarily (88%) explained by conservatism of elevational ranges rather than displacement followed by elevational expansion (12%).Main ConclusionsBird communities across elevation gradients are assembled through a mix of processes, including the sorting, displacement and conservatism of species elevation ranges. The dominant role of conservatism in explaining co‐occurrence of species on mountain slopes rejects more complex scenarios requiring displacement followed by expansion. The ability of closely related species to coexist without elevational divergence provides a direct and faster pathway to sympatry and helps explain the exceptional species richness of tropical mountains.

Understanding the processes through which ecological communities are assembled remains a major challenge, particularly in hotspots of biodiversity such as tropical mountains. Here, we apply continuous-time Markov models to elevational, geographical and phylogenetic data for 166 avian sister pairs to infer the relative frequency of different historical pathways involved in the build-up of species on Neotropical mountains, the Earth's most biodiverse terrestrial region. We show that sister species arising in geographic isolation but now sympatric (i.e. living on the same mountain slope), have assembled through a mix of different routes. Ecological sorting whereby species diverge to different elevations in allopatry occurs with a similar frequency to ecological displacement where divergence in elevation occurs upon secondary contact. However, both these routes are far less common than ecological niche conservatism, whereby species transition to sympatry without diverging in their elevation range. Our results suggest the predominance of this more direct and thus faster route to sympatry may help explain the exceptional species richness of tropical mountains.

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