Tropical Forests Are Non-Equilibrium Ecosystems Governed by Interspecific Competition Based on Universal 1/6 Niche Width

<div><p>Tropical forests are mega-diverse ecosystems that display complex and non-equilibrium dynamics. However, theoretical approaches have largely focused on explaining steady-state behaviour and fitting snapshots of data. Here we show that local and niche interspecific competition can realistically and parsimoniously explain the observed non-equilibrium regime of permanent plots of nine tropical forests, in eight different countries. Our spatially-explicit model, besides predicting with accuracy the main biodiversity metrics for these plots, can also reproduce their dynamics. A central finding is that tropical tree species have a universal niche width of approximately 1/6 of the niche axis that echoes the observed widespread convergence in their functional traits enabling them to exploit similar resources and to coexist despite of having large niche overlap. This niche width yields an average ratio of 0.25 between interspecific and intraspecific competition that corresponds to an intermediate value between the extreme claims of the neutral model and the classical niche-based model of community assembly (where interspecific competition is dominant). In addition, our model can explain and yield observed spatial patterns that classical niche-based and neutral theories cannot.</p></div

Observed (bold) and predicted species richness for all trees with diameter at breast height (dbh)≥1 cm for the first census in nine tropical forest plots.

<p>Data from Center for Tropical Forest Science <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0082768#pone.0082768-Center1" target="_blank">[11]</a>. The predicted values of species richness correspond to averages ± std of 100 model simulations for the best estimates of model parameters. <i>H</i>₁ is the equitability or standardized Shannon-Weaver diversity for the first census in each forest, <i>L</i> is the lattice size and the parameter values providing the best fit to empirical data where <i>σ</i> is the species' niche width, <i>m</i> is the dispersal rate from outside each neighbourhood and <i>T is</i> the stochasticity parameter.</p

Species-area curves (SAR) and spatial patterns of tree species richness for selected censuses of tropical forests.

<p>Predicted curves correspond to averages over 100 simulations for the best estimates of model parameters (<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0082768#pone-0082768-g002" target="_blank">Fig. 2a</a>), and the error bars correspond to one std. <b>a.</b> Observed and predicted (grey line) number of tree species with dbh≥1 cm for sampling areas of different sizes at Barro Colorado (1990, triangles) and Pasoh (1987, crosses). Estimated curves for Barro Colorado and Pasoh were calculated using data from the Center for Tropical Forest Science <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0082768#pone.0082768-Center1" target="_blank">[11]</a> and dividing the entire plots into non-overlapping quadrats <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0082768#pone.0082768-Harte1" target="_blank">[29]</a>. The calculation of the SAR is explained in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0082768#pone.0082768.s001" target="_blank">Information S1</a>. <b>b.</b> The estimated (triangles) and predicted probability <i>F</i>(<i>r</i>) that two randomly selected trees of dbh≥10 cm located <i>r</i> meters apart for the 1990 census at Barro Colorado plot are conspecific. The curves are shown only for 10≤<i>r</i>≤100 m, a range of distances for which the NTB fails to reproduce the estimated <i>F</i>(<i>r</i>) <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0082768#pone.0082768-Condit1" target="_blank">[6]</a>.</p

Observed (bold) and predicted distribution of relative species abundances (RSA) for all trees with diameter at breast height (dbh)≥1 cm for the first census of nine in nine tropical forest plots.

<p>Data from Center for Tropical Forest Science <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0082768#pone.0082768-Center1" target="_blank">[11]</a>. The predicted (grey) are averages ± std of 100 model simulations for the best estimates of model parameters of each forest (see <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0082768#pone.0082768.s001" target="_blank">Information S1</a> for comparisons with other censuses). The calculation of the RSA is explained in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0082768#pone.0082768.s001" target="_blank">Information S1</a>.</p

Relation between species rarity and spatial aggregation for <i>Spachea membranacea</i> at Barro Colorado in the 1995 and model species # 253.

<p>Both species have the highest spatial aggregation, as measured by the aggregation index Ω_0→10 and the same abundance of 14 individuals. <b>a.</b> The clumpy distribution of relative abundance of species in the finite niche axis, showing that species # 253 (niche position <i>x</i> = 0.8179) lies in a gap between clumps of coexisting species. <b>b.</b> The observed distribution of <i>Spachea membranacea</i> at Barro Colorado in the 1995 census <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0082768#pone.0082768-Center1" target="_blank">[11]</a>, <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0082768#pone.0082768-Hubbell4" target="_blank">[24]</a> yielding Ω_0→10 = 689.3. <b>c.</b> Spatial distribution of a model species # 253 in grey. The selected 9×9 sublattice shows the species identity of individuals in the immediate neighbourhood containing all 14 individuals of model species # 253, yielding Ω_0→10 = 1031.7. <b>d.</b> The set of fitnesses in the same sublattice containing all 14 individuals of model species # 253. Individuals of the rare species # 253 (in grey) are poorer competitors because they have lower fitnesses than most of their immediate neighbours.</p

Observed and predicted compositional changes in tree communities between censuses at Pasoh and Barro Colorado forests.

<p>Compositional changes are measured by the coefficient of determination <i>R</i>² of the regression of the log-transformed, time-lagged population abundance of all species between censuses for all individuals with dbh≥1 cm and species with two or more individuals in first census of each forest <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0082768#pone.0082768-Hubbell5" target="_blank">[25]</a>, <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0082768#pone.0082768-Hubbell6" target="_blank">[26]</a>. At a time lag of zero, no change in community composition can yet have occurred, and thus <i>R</i>² is by definition equal to unity. As time elapses between censuses, the progressive compositional changes are reflected by the decay in <i>R</i>²: empirical results for Barro Colorado (triangles) and Pasoh (crosses), and predicted values, corresponding to averages over 100 simulations with error bars equal to one standard deviation for the best parameters estimates for each forest. The model predicts a nearly perfectly linear decay in average values of <i>R</i>² that is indistinguishable from the prediction from NTB.</p

Mean (±SD)values of δ¹³C (A) and δ¹⁵N (B) for bone from the skulls of male and female sea lions collected in three different regions of the Galapagos archipelago.

<p>Regions with different superscript (lower-case letters) are statistically different in their mean values according to the Scheffe's post hoc test. Vertical bars show standard deviation.</p

genotypic_data

Genotypic data (genepop format) of the 875 individuals sampled in the course of the study. Spatial coordinates (Lambert II), sector of origin, year of sampling and sex are provided for each individua

Predator and potential preys’ stable isotope signal.

<p>Biplot of the isotopic contents of δ¹⁵N and δ¹³C of the South American sea lion (<i>Otaria flavescens</i>), the South American fur seal (<i>Arctocephalus australis</i>) and their main potential preys in Uruguay. Prey species were captured in the pelagic and neritic areas of the Uruguayan continental shelf and their names are fully indicated in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0080019#pone-0080019-t001" target="_blank">Table 1</a>. Error bars correspond to standard deviations. These averages and standard deviations were used as input for the mixing models.</p

Map of the Galapagos archipelago showing the islands where sea lion skulls were collected and the chlorophyll levels.

<p>Chlorophyll values are the cumulative average values of chlorophyll-a concentration (mg/m³) from 1 September 1997 to 31 August 2001 derived from SeaWiFS Project (<a href="http://oceancolor.gsfc.nasa.gov" target="_blank">http://oceancolor.gsfc.nasa.gov</a>). The hydrogeographic regions, in agreement with Ruttenberg et al.[<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0147857#pone.0147857.ref042" target="_blank">42</a>], are denote by the names in brackets.</p