High exposure of global tree diversity to human pressure.

Safeguarding Earth's tree diversity is a conservation priority due to the importance of trees for biodiversity and ecosystem functions and services such as carbon sequestration. Here, we improve the foundation for effective conservation of global tree diversity by analyzing a recently developed database of tree species covering 46,752 species. We quantify range protection and anthropogenic pressures for each species and develop conservation priorities across taxonomic, phylogenetic, and functional diversity dimensions. We also assess the effectiveness of several influential proposed conservation prioritization frameworks to protect the top 17% and top 50% of tree priority areas. We find that an average of 50.2% of a tree species' range occurs in 110-km grid cells without any protected areas (PAs), with 6,377 small-range tree species fully unprotected, and that 83% of tree species experience nonnegligible human pressure across their range on average. Protecting high-priority areas for the top 17% and 50% priority thresholds would increase the average protected proportion of each tree species' range to 65.5% and 82.6%, respectively, leaving many fewer species (2,151 and 2,010) completely unprotected. The priority areas identified for trees match well to the Global 200 Ecoregions framework, revealing that priority areas for trees would in large part also optimize protection for terrestrial biodiversity overall. Based on range estimates for >46,000 tree species, our findings show that a large proportion of tree species receive limited protection by current PAs and are under substantial human pressure. Improved protection of biodiversity overall would also strongly benefit global tree diversity.

Mapping local and global variability in plant trait distributions.

Our ability to understand and predict the response of ecosystems to a changing environment depends on quantifying vegetation functional diversity. However, representing this diversity at the global scale is challenging. Typically, in Earth system models, characterization of plant diversity has been limited to grouping related species into plant functional types (PFTs), with all trait variation in a PFT collapsed into a single mean value that is applied globally. Using the largest global plant trait database and state of the art Bayesian modeling, we created fine-grained global maps of plant trait distributions that can be applied to Earth system models. Focusing on a set of plant traits closely coupled to photosynthesis and foliar respiration-specific leaf area (SLA) and dry mass-based concentrations of leaf nitrogen ([Formula: see text]) and phosphorus ([Formula: see text]), we characterize how traits vary within and among over 50,000 [Formula: see text]-km cells across the entire vegetated land surface. We do this in several ways-without defining the PFT of each grid cell and using 4 or 14 PFTs; each model's predictions are evaluated against out-of-sample data. This endeavor advances prior trait mapping by generating global maps that preserve variability across scales by using modern Bayesian spatial statistical modeling in combination with a database over three times larger than that in previous analyses. Our maps reveal that the most diverse grid cells possess trait variability close to the range of global PFT means.

Evidence of temporal niche separation via low flowering time overlap in an old-field plant community.

Flowering time is a trait that reflects the timing of specific resource requirements by plants. Consequently, several predictions have been made related to how species are assembled within communities according to flowering time. Strong overlap in flowering time among coexisting species may result from clustered abiotic resources, or contribute to improved pollination success. Conversely, low flowering time overlap (asynchrony) among coexisting species may reduce competition for soil, light, or pollinator resources and alleviate interspecific pollen transfer. Here, we present evidence that coexisting species in an old-field community generally overlap less in flowering time than expected under a commonly used and statistically validated null model. Flowering time asynchrony was more pronounced when abundance data were used (compared to presence-absence data), and when analyses focused on species that share bees as pollinators. Control and herbivore-exclusion plots did not differ in flowering time overlap, providing no evidence of the reduction in overlap expected to result from increased competition. Our results varied with the randomization algorithm used, emphasizing that the choice of algorithm can influence the outcome of null models. Our results varied between 2 years, with patterns being less clear in the second year, when both growing season and flowering times were contracted. Finally, we found evidence that further supports a previous finding that higher plot-level flowering time overlap was associated with higher proportions of introduced species. Reduced flowering time overlap among species in our focal community may promote coexistence via temporal niche differentiation and reduced competition for pollinators and other abiotic resources.

Calculating Competitive Intransitivity: Computational Challenges.

Intransitive (or "rock-paper-scissors") competition is compelling because it promotes species coexistence and because recent work suggests that it may be common in natural systems. One class of intransitivity indices works by considering s, the minimum number of competitive reversals to convert a given competitive community (i.e., a "tournament") to a hierarchy. The most straightforward example of such "reversal-based" indices is Petraitis's index, [Formula: see text], where M is the maximum s across all possible n-species tournaments. Using exhaustive searches, we prove that Petraitis's formula for M (and, therefore, t) does not hold for [Formula: see text]. Furthermore, determination of s for even moderate values of n may prove difficult, as the equivalent graph theoretical problem is NP (nondeterministic polynomial time) hard; there is no known computationally feasible way to compute an exact answer for anything but small values of n, let alone a closed-form solution. Petraitis's t is a valuable index of intransitivity; however, at present its use is limited to relatively species-poor systems. More broadly, reversal-based indices, while intuitive, may be problematic because of this computability issue.

A global meta-analysis of the relative extent of intraspecific trait variation in plant communities.

Recent studies have shown that accounting for intraspecific trait variation (ITV) may better address major questions in community ecology. However, a general picture of the relative extent of ITV compared to interspecific trait variation in plant communities is still missing. Here, we conducted a meta-analysis of the relative extent of ITV within and among plant communities worldwide, using a data set encompassing 629 communities (plots) and 36 functional traits. Overall, ITV accounted for 25% of the total trait variation within communities and 32% of the total trait variation among communities on average. The relative extent of ITV tended to be greater for whole-plant (e.g. plant height) vs. organ-level traits and for leaf chemical (e.g. leaf N and P concentration) vs. leaf morphological (e.g. leaf area and thickness) traits. The relative amount of ITV decreased with increasing species richness and spatial extent, but did not vary with plant growth form or climate. These results highlight global patterns in the relative importance of ITV in plant communities, providing practical guidelines for when researchers should include ITV in trait-based community and ecosystem studies.

Dispersal strength influences zooplankton co-occurrence patterns in experimental mesocosms.

Negative co-occurrence patterns are intriguing because they may reflect the outcome of interspecific interactions and therefore signal how competition shapes communities. However, other factors also contribute to these patterns. For example, theoretical studies as well as two survey-based studies have all suggested that dispersal may also impact these patterns. While natural communities commonly have nonrandom patterns of negative co-occurrence, understanding how different processes drive these patterns requires further research. We tested the influence of dispersal on co-occurrence patterns using a zooplankton mesocosm experiment with four different dispersal treatments varying in the number of dispersers delivered into mesocosms on regular intervals. Our dispersal treatments were intended to adjust the relative importance of dispersal and competition experienced within mesocosms (i.e., high dispersal results in a relatively low influence of competition on species composition and vice versa). Higher dispersal translated into increased zooplankton species richness and inter-mesocosm compositional similarity, and also changed species occupancy patterns such that species occurrences were more even across mesocosms in higher-dispersal treatments. Dispersal treatments also differed markedly in species co-occurrence patterns. Negative co-occurrence patterns were significant for all but the lowest-dispersal treatment, peaked in the intermediate-dispersal treatments, and declined in the highest-dispersal treatment. Stability analyses illustrate that co-occurrence differences are robust to the exclusion of any single mesocosm in null model analyses. Dispersal treatments did not significantly differ with respect to abiotic variation, which has been recognized as a potential driver of negative co-occurrence patterns. These results suggest that not only can dispersal influence patterns of negative co-occurrence via changes to species richness and distribution (occupancy patterns among mesocosms), but the degree to which they do so varies nonlinearly with the strength of dispersal. Critically, because negative co-occurrence patterns were nonsignificant when the contribution of dispersal was lowest, it is possible that dispersal contributes strongly to many observed patterns of negative co-occurrence. Consequently, great care should be taken prior to interpreting significant co-occurrence tests as a product of species interactions.

Competitive intransitivity, population interaction structure, and strategy coexistence.

Intransitive competition occurs when competing strategies cannot be listed in a hierarchy, but rather form loops-as in the game rock-paper-scissors. Due to its cyclic competitive replacement, competitive intransitivity promotes strategy coexistence, both in rock-paper-scissors and in higher-richness communities. Previous work has shown that this intransitivity-mediated coexistence is strongly influenced by spatially explicit interactions, compared to when populations are well mixed. Here, we extend and broaden this line of research and examine the impact on coexistence of intransitive competition taking place on a continuum of small-world networks linking spatial lattices and regular random graphs. We use simulations to show that the positive effect of competitive intransitivity on strategy coexistence holds when competition occurs on networks toward the spatial end of the continuum. However, in networks that are sufficiently disordered, increasingly violent fluctuations in strategy frequencies can lead to extinctions and the prevalence of monocultures. We further show that the degree of disorder that leads to the transition between these two regimes is positively dependent on population size; indeed for very large populations, intransitivity-mediated strategy coexistence may even be possible in regular graphs with completely random connections. Our results emphasize the importance of interaction structure in determining strategy dynamics and diversity.

Global beta-diversity of angiosperm trees is shaped by Quaternary climate change.

As Earth's climate has varied strongly through geological time, studying the impacts of past climate change on biodiversity helps to understand the risks from future climate change. However, it remains unclear how paleoclimate shapes spatial variation in biodiversity. Here, we assessed the influence of Quaternary climate change on spatial dissimilarity in taxonomic, phylogenetic, and functional composition among neighboring 200-kilometer cells (beta-diversity) for angiosperm trees worldwide. We found that larger glacial-interglacial temperature change was strongly associated with lower spatial turnover (species replacements) and higher nestedness (richness changes) components of beta-diversity across all three biodiversity facets. Moreover, phylogenetic and functional turnover was lower and nestedness higher than random expectations based on taxonomic beta-diversity in regions that experienced large temperature change, reflecting phylogenetically and functionally selective processes in species replacement, extinction, and colonization during glacial-interglacial oscillations. Our results suggest that future human-driven climate change could cause local homogenization and reduction in taxonomic, phylogenetic, and functional diversity of angiosperm trees worldwide.

Smaller species experience mild adversity under shading in an old-field plant community.

Plant competition experiments commonly suggest that larger species have an advantage, primarily in terms of light acquisition. However, within crowded natural vegetation, where competition evidently impacts fitness, most resident species are relatively small. It remains unclear, therefore, whether the size advantage observed in controlled experiments is normally realized in habitats where competition is most intense. We characterized the light environment and tested for evidence of a size advantage in competition for light in an old-field plant community composed of perennial herbaceous species. We investigated whether larger species contributed to reduced light penetration (i.e., greater shading), and examined the impact of shade on smaller species by testing whether their abundance and richness were lower in plots with less light penetration. Light penetration in plots ranged from 0.3% to 72.4%. Significant effects were more common when analyses focused on small plants that reached reproduction (i.e., flowering rooted units); focusing on only flowering plants (i.e., excluding nonflowering rooted units) can clarify community patterns. Plots with a greater mean species height had significantly lower light penetration, and plots with lower light penetration had significantly lower flowering abundance and richness of small species. However, the impact of shade on the flowering abundance and richness of small species was relatively small (R 2 values between 8% and 15%) and depended on how we defined "small species." Synthesis: Our results confirm that light penetration in herbaceous vegetation can be comparable to levels seen in forests, that plots with taller species cast more shade, and that flowering smaller species are less abundant and diverse in plots where light penetration is low. However, variation in mean plot height explained less than 10% of variation in light penetration, and light penetration explained between 5 and 15% of variation in the flowering abundance and richness of small species. Coupled with the fact that flowering small species were present even within the most heavily shaded plots, our results suggest that any advantage in light competition by large species is limited. One explanation is that at least some small species in these communities are shade-tolerant. Shade tolerance in predominantly herbaceous communities, particularly among small plant species, requires further research.

Examining the relative influence of dispersal and competition on co-occurrence and functional trait patterns in response to disturbance.

Disturbance gradients are particularly useful for understanding the relative influences of competition and dispersal. Shortly after disturbance, plant composition should be influenced more strongly by dispersal than competition; over time, this should reverse, with competition becoming more important. As such, we predicted that plant functional traits associated with high dispersal ability would be over-represented shortly after a disturbance event occurs, while those associated with high competitive ability would have increased representation as time progresses. Additionally, it has been suggested that competitive interactions may contribute to negative co-occurrence patterns; if this is the case, negative co-occurrence patterns should also increase as time-since-disturbance increases. Here, we examine how functional trait and co-occurrence patterns change over time following a herbicide-based disturbance, compared to undisturbed vegetation, in a temperate, old-field grassland dominated by herbaceous perennials. In our study system, negative co-occurrence patterns were most pronounced in disturbed plots one year after herbicide application, consistent with several lines of evidence that dispersal can strongly impact both composition and co-occurrence patterns. Over three years post-disturbance, co-occurrence patterns in disturbed plots decreased, becoming more similar to control plots. This pattern is inconsistent with the expectation that competition contributes to negative co-occurrence patterns, at least over three growing seasons. More pronounced negative co-occurrence patterns were associated with higher species evenness among plots. Functional traits related to increased dispersal (mean seed mass, and proportion of stoloniferous/rhizomatous species) and competitive ability (mean species height, and mean specific leaf area) did not differ significantly across treatments, with the exception of mean height in the third-year post-disturbance; however, the overall trajectory of this trait was inconsistent with theoretical expectations. Overall, co-occurrence patterns changed across the gradient of time-since disturbance, but not as expected; functional trait patterns (trait means, functional diversity measures) were not responsive to our experimental disturbance gradient.

Within- and among-species variation in specific leaf area drive community assembly in a tropical cloud forest.

Specific leaf area (SLA) is a key functional trait reflecting the trade-off between resource capture and conservation, and has been identified as playing an important role in plant community assembly. Mechanistic models of community assembly state that the assemblage of species in a local community is controlled by environment filters operating on functional traits. We measured within- and among-species variation of SLA, and environmental conditions in a tropical cloud forest to explore how variation in this functional trait contributes to community assembly. SLA variation at the species level was also decomposed into alpha (within assemblage variation), and beta (across assemblage variation) values. SLA decreased with increasing solar irradiance (approximated using plant height) within the three study sites, and differed among the three sites both for within- and among-species comparisons. Mean plot SLA, accounting for both within and among species across the three sites, increased significantly in relation to air temperature but not local photosynthetic photon flux density and soil total phosphorus. Alpha SLA decreased with increasing solar irradiance within the three sites and beta SLA differed among the three sites. Our results clearly demonstrate that light and air temperature are key environmental factors involved in organizing plant species within and among communities in tropical cloud forests. The strong relationship between both intra- and interspecific variation in SLA and environmental conditions strongly confirms the role of trait variation in the assembly of plant species in tropical cloud forest communities via environment filtering related to light availability and air temperature.

Opposing community assembly patterns for dominant and nondominant plant species in herbaceous ecosystems globally.

Biotic and abiotic factors interact with dominant plants-the locally most frequent or with the largest coverage-and nondominant plants differently, partially because dominant plants modify the environment where nondominant plants grow. For instance, if dominant plants compete strongly, they will deplete most resources, forcing nondominant plants into a narrower niche space. Conversely, if dominant plants are constrained by the environment, they might not exhaust available resources but instead may ameliorate environmental stressors that usually limit nondominants. Hence, the nature of interactions among nondominant species could be modified by dominant species. Furthermore, these differences could translate into a disparity in the phylogenetic relatedness among dominants compared to the relatedness among nondominants. By estimating phylogenetic dispersion in 78 grasslands across five continents, we found that dominant species were clustered (e.g., co-dominant grasses), suggesting dominant species are likely organized by environmental filtering, and that nondominant species were either randomly assembled or overdispersed. Traits showed similar trends for those sites (<50%) with sufficient trait data. Furthermore, several lineages scattered in the phylogeny had more nondominant species than expected at random, suggesting that traits common in nondominants are phylogenetically conserved and have evolved multiple times. We also explored environmental drivers of the dominant/nondominant disparity. We found different assembly patterns for dominants and nondominants, consistent with asymmetries in assembly mechanisms. Among the different postulated mechanisms, our results suggest two complementary hypotheses seldom explored: (1) Nondominant species include lineages adapted to thrive in the environment generated by dominant species. (2) Even when dominant species reduce resources to nondominant ones, dominant species could have a stronger positive effect on some nondominants by ameliorating environmental stressors affecting them, than by depleting resources and increasing the environmental stress to those nondominants. These results show that the dominant/nondominant asymmetry has ecological and evolutionary consequences fundamental to understand plant communities.

Deterministic assembly of land snail communities according to species size and diet.

1. We investigated whether coexisting snail species in 145 treeless fen communities in the Western Carpathian Mountains differed more in size and diet than would be expected by chance, as predicted for traits commonly associated with competition and differential resource acquisition under limiting similarity theory. 2. Contrary to expectations, coexisting snail species were no more different in body size than expected by chance under a null model. However, variation in body size played a significant role in structuring snail communities: coexisting snail species were significantly more similar with respect to body size. 3. We developed two new test statistics to expand our investigation of limiting similarity to include diet, a nominal trait. We tested whether communities of snails were characterized by a greater richness of diet, and whether different diets were represented more or less evenly within communities. Communities of snails were significantly less evenly distributed than expected by chance, with detritivores being over-represented relative to predatory strategies. 4. We also examined the effect of water pH and conductivity, herbaceous cover, and bryophyte and vascular plant richness, on these trends by examining how the effect size of our tests varied across these gradients. Convergence in species size increased with increasing habitat pH. Specifically, smaller snail species were over-represented in fen communities in general, and this effect was accentuated in increasingly calcareous fens. 5. Theory predicts that traits related strongly to environmental conditions are more likely to be convergent. Our findings support this suggestion, as small snail species have an advantage in tolerating freezing conditions over winter when refuges are limited. 6. These results add to the growing body of literature demonstrating that variation in body size and diet play a strong role in structuring communities, although frequently in ways not predicted by limiting similarity theory. Finally, our results increase our understanding of how species are assembled non-randomly into communities with respect to important traits.

The role of plant species size in invasibility: a field experiment.

Large plant species self-thin to disproportionately lower densities than smaller plant species, and therefore may leave more patches of unused space suitable for invasion. Using experimental monocultures of 11 old-field perennial plant species differing in maximum size, as well as mixtures composed of all monoculture species, we tested our primary hypothesis that monocultures of larger species will be more susceptible to natural invasion. After 3 years, monocultures of larger species were invaded by a significantly greater number of species, and more ramets, from the surrounding vegetation. Invading plant species were significantly smaller than the monoculture species being invaded, suggesting that smaller plant species may be better invaders. Thus, we quantified a trade-off between species size, which is frequently associated with increased competitive ability for light, and invasibility, suggesting one reason why large and small species coexist in virtually all plant communities. Although we expected that invasion would enhance biomass production by more fully capturing available resources, we found that the most highly invaded plots of each species produced significantly less biomass. This suggests that increased diversity resulting from invasion did not result in complementary resource use. Mixture plots containing all experimental species did not admit a significantly different number of invading ramets or species than most monocultures, indicating no obvious role for diversity in resistance to invasion, or complementary resource use. Our results suggest that relatively large species may be limited in their capacity to competitively exclude other, smaller species from communities because pure stands of the former are more susceptible to invasion by the latter.

Species coexistence, intransitivity, and topological variation in competitive tournaments.

Competitive intransitivity occurs when species' competitive abilities cannot be listed in a strict hierarchy, but rather form competitive loops, as in the game 'Rock-Paper-Scissors'. Indices are useful for summarizing intransitivity in communities; however, as with most indices, a great deal of information is compressed into single number. So while recent ecological theory, experiments, and natural history observations demonstrate that competitive intransitivity can promote species coexistence, the consequence of variation in the 'topology' of competitive interactions that is not accounted for by intransitivity indices is much less well understood. We use a continuous analytical model and two complementary discrete lattice models (one spatially explicit, the other aspatial) to demonstrate that such variation does indeed greatly affect species coexistence. Specifically, we show that although intransitivity indices are good at capturing broad patterns of coexistence, communities with different levels of intransitivity can have equal coexistence, and communities with equal intransitivity can have different coexistence, due to underlying variation in competitive network topology.

A comparative evaluation of five common pairwise tests of species association.

Researchers have long viewed patterns of species association as key to understanding the processes that structure communities. Community-level tests of species association have received the most attention; however, pairwise species associations may offer greater opportunity for linking patterns to specific mechanisms. Although several tests of pairwise association have been developed, there remain gaps in our understanding of their performance. Consequently, it is unclear whether these methods reliably detect patterns of association, or if any one method is superior. We maximized association patterns for single species pairs in synthetic community matrices and examined how accurately five pairwise association tests found that pair, while not finding others (i.e., type I and II error rates). All tests are more likely to miss patterns of association than to falsely detect them. When we maximized association for a species pair that included one or more rare or common species, tests were frequently unable to identify that pair as significantly associated. Consequently, these tests are best suited for identifying significant associations between pairs of species that occur in an intermediate number of samples; for such pairs, three of the five tests considered here detected 100% of the pairs for which we maximized associations.

Does local competition increase the coexistence of species in intransitive networks?

Competitive intransitivity, a situation in which species' competitive ranks cannot be listed in a strict hierarchy, promotes species coexistence through "enemy's enemy indirect facilitation." Theory suggests that intransitivity-mediated coexistence is enhanced when competitive interactions occur at local spatial scales, although this hypothesis has not been thoroughly tested. Here, we use a lattice model to investigate the effect of local vs. global competition on intransitivity-mediated coexistence across a range of species richness values and levels of intransitivity. Our simulations show that local competition can enhance intransitivity-mediated coexistence in the short-term, yet hinder it in the long-term, when compared to global competition. This occurs because local competition slows species disaggregation, allowing weaker competitors to persist longer in the shifting spatial refuges of intransitive networks, enhancing short-term coexistence. Conversely, our simulations show that, in the long-term, local competition traps disaggregated species in unfavorable areas of the competitive arena, where they are excluded by superior competitors. As a result, in the long-term, global intransitive competition allows a greater number of species to coexist than local intransitive competition.

Multiple facets of biodiversity drive the diversity-stability relationship.

A substantial body of evidence has demonstrated that biodiversity stabilizes ecosystem functioning over time in grassland ecosystems. However, the relative importance of different facets of biodiversity underlying the diversity-stability relationship remains unclear. Here we use data from 39 grassland biodiversity experiments and structural equation modelling to investigate the roles of species richness, phylogenetic diversity and both the diversity and community-weighted mean of functional traits representing the 'fast-slow' leaf economics spectrum in driving the diversity-stability relationship. We found that high species richness and phylogenetic diversity stabilize biomass production via enhanced asynchrony in the performance of co-occurring species. Contrary to expectations, low phylogenetic diversity enhances ecosystem stability directly, albeit weakly. While the diversity of fast-slow functional traits has a weak effect on ecosystem stability, communities dominated by slow species enhance ecosystem stability by increasing mean biomass production relative to the standard deviation of biomass over time. Our in-depth, integrative assessment of factors influencing the diversity-stability relationship demonstrates a more multicausal relationship than has been previously acknowledged.

Competitive intransitivity promotes species coexistence.

Using a spatially explicit cellular automaton model with local competition, we investigate the potential for varied levels of competitive intransitivity (i.e., nonhierarchical competition) to promote species coexistence. As predicted, on average, increased levels of intransitivity result in more sustained coexistence within simulated communities, although the outcome of competition also becomes increasingly unpredictable. Interestingly, even a moderate degree of intransitivity within a community can promote coexistence, in terms of both the length of time until the first competitive exclusion and the number of species remaining in the community after 500 simulated generations. These results suggest that modest levels of intransitivity in nature, such as those that are thought to be characteristic of plant communities, can contribute to coexistence and, therefore, community-scale biodiversity. We explore a potential connection between competitive intransitivity and neutral theory, whereby competitive intransitivity may represent an important mechanism for "ecological equivalence."

The Influence of Matrix Size on Statistical Properties of Co-Occurrence and Limiting Similarity Null Models.

Null models exploring species co-occurrence and trait-based limiting similarity are increasingly used to explore the influence of competition on community assembly; however, assessments of common models have not thoroughly explored the influence of variation in matrix size on error rates, in spite of the fact that studies have explored community matrices that vary considerably in size. To determine how smaller matrices, which are of greatest concern, perform statistically, we generated biologically realistic presence-absence matrices ranging in size from 3-50 species and sites, as well as associated trait matrices. We examined co-occurrence tests using the C-Score statistic and independent swap algorithm. For trait-based limiting similarity null models, we used the mean nearest neighbour trait distance (NN) and the standard deviation of nearest neighbour distances (SDNN) as test statistics, and considered two common randomization algorithms: abundance independent trait shuffling (AITS), and abundance weighted trait shuffling (AWTS). Matrices as small as three × three resulted in acceptable type I error rates (p < 0.05) for both the co-occurrence and trait-based limiting similarity null models when exclusive p-values were used. The commonly used inclusive p-value (≤ or ≥, as opposed to exclusive p-values; < or >) was associated with increased type I error rates, particularly for matrices with fewer than eight species. Type I error rates increased for limiting similarity tests using the AWTS randomization scheme when community matrices contained more than 35 sites; a similar randomization used in null models of phylogenetic dispersion has previously been viewed as robust. Notwithstanding other potential deficiencies related to the use of small matrices to represent communities, the application of both classes of null model should be restricted to matrices with 10 or more species to avoid the possibility of type II errors. Additionally, researchers should restrict the use of the AWTS randomization to matrices with fewer than 35 sites to avoid type I errors when testing for trait-based limiting similarity. The AITS randomization scheme performed better in terms of type I error rates, and therefore may be more appropriate when considering systems for which traits are not clustered by abundance.