Joseph A. Veech

Partitioning Species Diversity across Landscapes and Regions: A Hierarchical Analysis of α, β, and γ Diversity

Species diversity may be additively partitioned within and among samples (α and β diversity) from hierarchically scaled studies to assess the proportion of the total diversity (γ) found in different habitats, landscapes, or regions. We developed a statistical approach for testing null hypotheses that observed partitions of species richness or diversity indices differed from those expected by chance, and we illustrate these tests using data from a hierarchical study of forest‐canopy beetles. Two null hypotheses were implemented using individual‐ and sample‐based randomization tests to generate null distributions for α and β components of diversity at multiple sampling scales. The two tests differed in their null distributions and power to detect statistically significant diversity components. Individual‐based randomization was more powerful at all hierarchical levels and was sensitive to departures between observed and null partitions due to intraspecific aggregation of individuals. Sample‐based randomization had less power but still may be useful for determining whether different habitats show a higher degree of differentiation in species diversity compared with random samples from the landscape. Null hypothesis tests provide a basis for inferences on partitions of species richness or diversity indices at multiple sampling levels, thereby increasing our understanding of how α and β diversity change across spatial scales.

Diversity partitioning without statistical independence of alpha and beta

Diversity partitioning has become a popular method for analyzing patterns of alpha and beta diversity. A recent evaluation of the method emphasized a distinction between additive and multiplicative partitioning and further advocated the use of multiplicative partitioning based on a presumed independence between alpha and beta. Concurrently, additive partitioning was criticized for producing dependent alpha and beta estimates. Until now, the issue of statistical independence of alpha and beta (in either type of partitioning) has not been thoroughly examined, partly due to confusion about the meaning of statistical independence. Here, we adopted a probability-based definition of statistical independence that is essentially identical to the definition found in any statistics textbook. We used a data simulation approach to show that alpha and beta diversity are not statistically independent in either additive or multiplicative partitioning. However, the extent of the dependence is not so great that it cannot be overcome by using appropriate statistical techniques to control it. Both additive and multiplicative partitioning are statistically valid and logically sound approaches to analyzing diversity patterns.

A null model for detecting nonrandom patterns of species richness along spatial gradients

I present a null model that can be used for detecting nonrandom patterns of species richness along spatial gradients, such as latitude and elevation. Because estimates of species richness along a single spatial gradient are nonindependent, tests of statistical inference should not be applied. The null model described here circumvents this problem of nonindependence by randomly placing the actual range width of each species along the gradient. The simulated richness curve generated in this way can be compared to an “average” random curve composed of many such simulations. The comparison involves measuring the mean displacement (D) of the simulated curve from the average random curve. By repeating these steps a specified number of times, one can obtain a distribution of D values. The displacement of the actual species richness curve from the average random curve can also be determined and compared to the distribution of D values of the simulated random curves. This comparison allows one to determine whether...

INTRASPECIFIC AGGREGATION DECREASES LOCAL SPECIES DIVERSITY OF ARTHROPODS

The aggregation model of species coexistence predicts that insect species diversity within a community is maintained by intraspecific aggregation among resource patches. An untested corollary of this prediction is that diversity within resource patches should decrease with increasing intraspecific aggregation. The recently derived species- aggregation relationship provides a general formulation of this prediction: as intraspecific aggregation increases within a geographic area, the species richness within samples of the area decreases. We tested this prediction by compiling and analyzing 76 data sets of ar- thropod species distribution and abundance. For each data set, we determined the mean amount of intra- and interspecific aggregation and three types of within-sample or local species diversity: species richness, evenness, and dominance. Using regression, we found a negative relationship between intraspecific aggregation and all three types of local di- versity. Intraspecific aggregation explained a significant percentage of the variation in species diversity, typically between 20% and 60%. By comparison, interspecific aggregation usually explained ,1% of the variation in species diversity. Our study provides empirical support for the species-aggregation relationship as a general macroecological pattern that emerges from intraspecific aggregation.

Predator-mediated interactions among the seeds of desert plants

Abstract In theory, seed predators are capable of inducing indirect interactions among the seeds they consume. However, empirical evidence of predator-mediated interactions among seeds is rare. Rodents in the Heteromyidae are highly granivorous and therefore likely to induce indirect interactions among the seeds of desert plants. The indirect interactions may be in the form of apparent competition and apparent mutualism between seeds within a patch. Apparent competition exists when the survival of seeds of a focal species is lessened because of the presence of additional seeds of other species in the patch. Apparent mutualism exists when the presence of the other seeds results in an increase in survival of seeds of the focal species. By measuring seed removal from trays placed in the field, apparent competition between the seeds of several plant species was detected. Apparent mutualism might also exist, but there was no strong evidence of it. Apparent competition appeared most likely to occur among the species whose seeds were the most heavily predated. For instance, predation on seeds of Astragalus cicer, Oryzopsis hymenoides, and Sphaeralcea coccinea was substantial with more than 50% of the seeds removed from the trays, on average. The intensity of apparent competition (measured by the indirect effect, IS) between these species and two others was significant; IS ranged from –0.02 to –0.39 on a scale of 0 to –1. This indicates that, in some communities, indirect effects are most likely to exist when direct effects are strong.

Incorporating socioeconomic factors into the analysis of biodiversity hotspots

Abstract Hotspot analysis is the identification and ranking of countries or other geographic regions on the basis of biodiversity. Hotspots have exceptional biodiversity per unit land area. This paper introduces a new method of hotspot analysis that ranks hotspots on the basis of biodiversity and anthropogenic threats to biodiversity. These threats are represented by socioeconomic factors such as human population size, rural population density, population growth rate, and governmental debt. Residuals from multiple regression models were used to rank the 17 megadiversity countries. In general, ranks based on threats to biodiversity were different from those based on biodiversity alone.

Cheek pouch capacities and loading rates of heteromyid rodents

Abstract Rodents of the family Heteromyidae are proficient gatherers and hoarders of seeds. A major component of their adaptive specialization for harvesting and transporting seeds is their spacious, fur-lined cheek pouches. Precise measurements of cheek pouch capacities are essential if ecologists are to understand the foraging ecology, possible constraints on locomotion patterns, and competitive relationships of heteromyid rodents. To measure the size of these cheek pouches and the rate at which animals load seeds into their pouches during seed harvest, we attracted 56 individuals representing ten species of heteromyid rodents to bait stations in the field and allowed them to fill their cheek pouches with seeds several times while we observed and timed the events with the aid of night-vision equipment. The largest load taken by each individual was used as an estimate of its cheek pouch capacity. At the end of observations, each subject was captured and its mass and other data gathered. The allometric relationship between cheek pouch capacity and body mass for ten species of heteromyids was significant [pouch capacity (ml) = 0.148 body mass (g)0.992, r2=0.91, P<0.0001]. The regression coefficient is ≈1.0, which indicates that the volume of the cheek pouches scales in direct proportion to body size. When the data were subdivided into quadrupeds (Perognathus and Chaetodipus) and bipeds (Dipodomys) (n=5 for each), the relationships between pouch capacity and body mass were significant, but the two regressions were not significantly different from each other. When all loads (full and partial) were considered, subjects filled their cheek pouches an average of 93 ± 10% of pouch capacity (n=185). Cheek pouch capacities from published studies of artificially filled pouches of heteromyids in the laboratory averaged about 40% below the field measurements obtained here. The allometric relationship between mean loading rate and body mass was also significant [seeds/s=1.067 bodymass (g)0.830, r2=0.85,P=0.0011), but when quadrupeds and bipeds were considered separately, the relationships were not significant. Seed densities and bulk densities were used to calculate packing coefficients for seed species, which, when used in conjunction with the allometric relationship between cheek pouch capacity and body size, can be used to estimate the maximum load carried by a heteromyid. Except for the very largest kangaroo rat species, a full pouch load of Indian ricegrass seeds represents less than the daily energy requirements of an active heteromyid.

Significance testing in ecological null models

In the past decade, the use of null models has become widespread in the testing of ecological theory. Along with increasing usage, null models have also become more complex particularly with regard to tests of significance. Despite the complexity, there are essentially only two distinct ways in which tests of significance are conducted. Direct tests derive a p value directly from the null distribution of a test statistic, as the proportion of the distribution more extreme than the observed value of the test statistic. Indirect tests compare an observed value of a parameter to a null distribution by conducting an additional analysis such as a chi-square test, Kolmogorov–Smirnov test, or regression, although in many cases, this additional step is not necessary. Many kinds of indirect tests require that the null distribution is normal whereas direct tests carry no assumptions about the form of the null distribution. Therefore, when assumptions are violated, indirect tests may have higher type I and II error rates than their counterpart direct tests. A review of 108 null model papers revealed that direct tests were used in 56.5% of studies and indirect tests used in 45.5%. A few studies used both types of test. In general, the randomization algorithms used in most null models should produce normal null distributions, but this could not be confirmed because most studies did not present any description of the null distribution. Researchers should be aware of the differences between direct and indirect tests so as to better use, communicate, and evaluate null models. In many cases, direct tests should be favored for their simplicity and parsimony.

LACK OF CALLING SONG DISPLACEMENT BETWEEN TWO CLOSELY RELATED GROUND CRICKETS

Thorough examinations of purported cases of reproductive character displacement are critical for reaching an understanding of the role of reinforcement in the evolution of reproductive barriers between closely related species. In this paper, we report the results of an extensive investigation of male calling song variation in the ground crickets, Allonemobius fasciatus and A. socius. Contrary to the results of an earlier study, we uncovered little evidence of displacement of songs in areas of overlap. We discuss explanations for the lack of displacement as well as for the discrepancies between the results of the current study and those of the earlier study.

A new measure of ecological network structure based on node overlap and segregation

Summary 1. Despite substantial recent progress, ecologists continue to search for methods of measuring the structure of ecological networks. Several studies have focused on nestedness, a pattern reflecting the tendency of network nodes to share interaction partners. Here, we introduce a new statistical procedure to measure both this kind of structure and the opposite one (i.e. species’ tendency against sharing interacting partners) that we call ‘node segregation’. In addition, our procedure provides also a straightforward measure of modularity, that is, the tendency of a network to be compartmented into separated clusters of interacting nodes. 2. This new analytical measure of network structure assesses the average deviation between the observed number of neighbours shared by any pair of nodes (species), and the expected number, that is computed using a probabilistic approach based on simple combinatorics. The measure can be applied to both bipartite networks (such as plant–pollinators) and unimode networks (such as food webs). We tested our approach on several sets of hypothetical and real-world networks. 3. We demonstrate that our approach makes it possible to identify different kinds of non-random network configurations (nestedness, node segregation and modularity). In addition, we show that nestedness in ecological networks is less common than previously thought, and that most ecological networks (including the majority of mutualistic ones) tend towards patterns of segregated associations. 4. Our analyses show that the new measure of node overlap and segregation can efficiently identify different structural patterns. The results of our analyses conducted on real networks highlight the need to carefully reconsider the assumption that ecological networks are stable due to their inherent nestedness.