The role of environment, dispersal and competition in explaining reduced co-occurrence among related species

Abstract

The composition of ecological assemblages depends on a variety of factors including environmental filtering, biotic interactions and dispersal limitation. By evaluating the phylogenetic pattern of assemblages, we gain insight into the relative contribution of these mechanisms to generating observed assemblages. We address some limitations in the field of community phylogenetics by using simulations, biologically relevant null models, and cost distance analysis to evaluate simultaneous mechanisms leading to observed patterns of co-occurrence. Building from past studies of phylogenetic community structure, we applied our approach to hummingbird assemblages in the Northern Andes. We compared the relationship between relatedness and co-occurrence among predicted assemblages, based on estimates of suitable habitat and dispersal limitation, and observed assemblages. Hummingbird co-occurrence peaked at intermediate relatedness and decreased when a closely-related species was present. This result was most similar to simulations that included simultaneous effects of phylogenetic conservatism and repulsion. In addition, we found older sister taxa were only weakly more separated by geographic barriers, suggesting that time since dispersal is unlikely to be the sole factor influencing co-occurrence of closely related species. Our analysis highlights the role of multiple mechanisms acting simultaneously, and provides a hypothesis for the potential importance of competition at regional scales.

Fig 1. Conceptual diagram of four simulated assemblage types given known patterns of trait evolution.

Each combination of phylogenetic conservatism and repulsion in the occurrence trait (a) would lead to a different pattern of co-occurrence among related species (b) leading to a different function representing the relationship between probability of presence and phylogenetic distance to the closest related species (c).

Fig 2

Probability of occurrence based on a thousand draws from the posterior distributions for each of the simulation assemblages (A) and for the observed and predicted assemblages (B). The simulated assemblages modeled hypothetical species occurrence based on a known model of trait evolution incorporating phylogenetic conservatism and repulsion among closely related species. The predicted assemblages are rearrangements of the true observed assemblages based on predicted habitat suitability (environment assemblages) and the addition of a dispersal filter (environment + dispersal assemblages). The hierarchical posteriors were plotted on the scale of the input data to show the estimated relationship between occurrence and the phylogenetic distance to the closest related species. The shaded region is the central 95^th quantile of the posterior distribution. The observed pattern of co-occurrence at first increases with increasing relatedness to a co-occurring species and then decreases in co-occurrence when a very closely related species is present. The overall observed co-occurrence is less than expected given species abiotic tolerances and potential dispersal.

Fig 3. Example species level curves for probability of occurrence and phylogenetic distance to the closest related species.

The graph shows the presence (y = 1) or absence (y = 0) of a given species at each of the geographic assemblages.Since the nearest closest related species are often the same species in multiple assemblages, there are multiple points overlapping. The number of assemblage lists that define each point is shown by the size of the point. Species were selected to show a variety of co-occurrence curves and represent a diversity of phylogenetic lineages within the hummingbird clade.

Fig 4. Average cost distance between localities for each pair of sister taxa.

The cost distances incorporates the change in elevation and Euclidean distance as a measure of geographic barriers between localities. We calculated the average cost distance between each presence locality of one sister taxa to the closest presence locality of the other sister taxa. Assemblages where both taxa co-occur had zero cost distance. The solid regression line is for all species pairs, and the dashed regression line is for only species pairs that diverged less than 10 myr ago. Two example sister pairs are shown. The Phaethornis sister taxa have high cost distance due to presence of the Andes, whereas the Lesbia sister taxa have relatively low cost distance due to their short distance along mountain slopes. In the insets, one sister taxa is shown as a solid triangle, and the other an open circle. The least cost path along the change in elevation frictional surface is shown in black.