Examining the potential impact of assuming continental scale sympatry

Our biogeographical reconstructions add important realism into models of species interactions. Nevertheless, species that occur on the same continent do not necessarily interact with one another. We conducted a simulation analysis to determine how our ability to detect the impact of competition on trait evolution may be impacted by the fact that only a subset of the species occurring in a given continent are actually sympatric.

First, we determined the proportion of species that are sympatric within each continent. We calculated range-wide overlap for all family members that ever coexist on the same continent from BirdLife range maps [77] (S6 Data). We defined sympatry as 20% range overlap according to the Szymkiewicz–Simpson coefficient (i.e., overlap area/min(sp1 area, sp2 area)). We also determined if overall levels of sympatry vary latitudinally; to do so, we subset pairs of taxa whose latitudinal means are separated by less than 25° latitude [36] and calculated the midpoint latitude for each pair.

Next, we conducted a simulation study to determine whether competition unfolding between ‘truly’ sympatric species only (i.e., at a level finer than the course continental scale we employed) would systematically impact the fit (i.e., model selection) or performance (i.e., parameter estimation) of the 2-regime competition (MC) models for which we used continental-level sympatry (as in the empirical analyses). To do this, we selected 3 clades spanning the range of tree sizes, each with some traits best fit by single-regime MC model, but none best fit by 2-regime MC model (Cracidae.0 [N = 50, N_tropical = 38, N_temperate = 12], Nectariniidae.0 [N = 122, N_tropical = 89, N_temperate = 33], Picidae.1 [N = 190, N_tropical = 86, N_temperate = 104]). For each of these clades, we simulated 2 biogeographic scenarios reflecting empirical levels of sympatry (see above). In the first, we downsampled the continental biogeography such that 50% of tropical and 50% of temperate taxa that were estimated to occur in the same continent were sympatric (see S1 Text for more details). In the second scenario, to reflect the observed latitudinal variation in sympatry, we downsampled the continental biogeography such that 33% of tropical and 50% of temperate taxa that were estimated to occur in the same continent were sympatric (see S1 Text for more details).

With these downsampled biogeographic histories, representing hypothetical range overlap that is more realistic than our continental-level assumption of sympatry, we simulated trait evolution under the 2-regime MC model. For each clade, we used the mean σ² value estimated under the single-regime MC model in empirical fits of a trait that was best fit by the single-regime MC model. We then varied the ratio of the S_tropical:S_temperate within the range of values in other trait-by-clade combinations where the 2-regime MC model was the best-fit model (S12 Table). For each clade, parameter combination, and downsampled biogeographic scenario, we simulated 100 datasets, for a total of 3,000 simulated datasets. Finally, we fit the same 12 models that were used in empirical analyses. We conducted model selection to identify the best-fit model for each simulated dataset and assessed whether the estimated ln(|S_tropical|/|S_temperate|) had the sign expected given the simulated ratio of S_tropical:S_temperate (S9 Data).