Null Model

Network analyses of plant-animal interactions hold valuable biological information. They are often used to quantify the degree of specialization between partners, but usually based on qualitative indices such as 'connectance' or number of links. These measures ignore interaction frequencies or sampling intensity, and strongly depend on network size. Here we introduce two quantitative indices using interaction frequencies to describe the degree of specialization, based on information theory. The first measure (d') describes the degree of interaction specialization at the species level, while the second measure (H2') characterizes the degree of specialization or partitioning among two parties in the entire network. Both indices are mathematically related and derived from Shannon entropy. The species-level index d' can be used to analyze variation within networks, while H2' as a network-level index is useful for comparisons across different interaction webs. Ana...

CiteSeerX - Document Details (Isaac Councill, Lee Giles, Pradeep Teregowda): We propose to use two seemingly different R 2 measures of fit in PROC LOGISTIC and PROC GENMOD (SAS/STAT), and we show that they are closely related to each other in terms of the amount of ...

The general principles of scale and coarse and fine filters have been widely accepted, but management agencies and industry are still grappling with the question of what to monitor to detect changes in forest biodiversity following forest management. Part of this problem can be attributed to the lack of focused questions for monitoring including absence of null models and predicted effects, a certain level of disconnect between research and management, and recognition that monitoring can be designed as a research question. Considerable research from the past decade has not been adequately synthesized to answer important questions, such as which species or forest attributes might be the best indicators of change. A disproportionate research emphasis has been placed on community ecology, and mostly on a few groups of organisms including arthropods, amphibians, migratory songbirds, and small mammals, while other species, including soil organisms, lichens, bats, raptors, some carnivores...

A possible role of host tree identity in the structuring of vascular epiphyte communities has attracted scientific attention for decades. Specifically, it has been suggested that each host tree species has a specific subset of the local species pool according to its own set of properties, e.g. physicochemical characteristics of the bark, tree architecture, or leaf phenology patterns. A novel, quantitative approach to this question is presented, taking advantage of a complete census of the vascular epiphyte community in 0.4 ha of undisturbed lowland forest in Panama. For three locally common host-tree species (Socratea exorrhiza, Marila laxiflora, Perebea xanthochyma) null models were created of the expected epiphyte assemblages assuming that epiphyte colonization reflected random distribution of epiphytes in the forest. In all three tree species, abundances of the majority of epiphyte species (69-81 %) were indistinguishable from random, while the remaining species were about equall...

Phylogenetic information is increasingly being used to understand the assembly of biological communities and ecological processes. However, commonly used metrics of phylogenetic diversity (PD) do not incorporate information on the relative abundances of individuals within a community. In this study, we develop three indices of PD that explicitly consider species abundances. First, we present a metric of phylogenetic-abundance evenness that evaluates the relationship between the abundance and the distribution of terminal branch lengths. Second, we calculate an index of hierarchical imbalance of abundances at the clade level encapsulating the distribution of individuals across the nodes in the phylogeny. Third, we develop an index of abundance-weighted evolutionary distinctiveness and generate an entropic index of phylogenetic diversity that captures both information on evolutionary distances and phylogenetic tree topology, and also serves as a basis to evaluate species conservation value. These metrics offer measures of phylogenetic diversity incorporating different community attributes. We compare these new metrics to existing ones, and use them to explore diversity patterns in a typical California annual grassland plant community at the Jasper Ridge biological preserve.Ecology Letters (2010) 13: 96–105

We examined the patterns of tree species richness, density and basal area in the Eastern Himalayan elevation gradient, and evaluated the roles of geometric constraints and environmental factors for the observed pattern. We followed the quadrat sampling method for this study at 23 different locations along a 300–4700 m elevation gradient in Sikkim. We used regression analysis to establish elevational pattern of trees (species richness, density and basal area) and to relate environmental factors with tree species and density. We also tested geometric constraints on species distribution by comparing empirical species richness with null model predictions generated by a Monte Carlo Simulation Programme (McCain, 2004). We observed a total of 3874 individual trees belonging to 171 species and 58 families. Tree species richness depicted unimodal pattern with a peak at 1500 m, whereas basal area peaked at 1900 m. Although density did not follow a uniform pattern, maximum value was observed at 3050 m. Climatic and energy related factors–rainfall, temperature, moisture index (MI) and actual evapotranspiration (AET)-correlated positively with tree species richness. The Mid-domain effect null model showed significant deviation of empirical richness from simulated species richness. Stepwise regression revealed that combined effects of energy (surrogated by temperature), productivity (surrogated by AET) and precipitation (reflected by MI) influenced observed tree species patterns in the Eastern Himalayan elevation gradient although historical and spatial factors might interplay.

The habitat heterogeneity hypothesis states that an increase in habitat heterogeneity leads to an increase in species diversity. We tested this hypothesis for a community of small mammals in the semiarid, sand-shinnery-oak ecosystem of the southwestern United States. We used indices of differentiation diversity to quantify differences between two habitat types (blowouts in a sand-shinnery-oak matrix) in terms of species diversity. The Wilson-Shmida index (beta(T)) considers species composition only, whereas the Morisita-Horn index (C(mH)) also takes species abundances into account. We constructed null models to test the hypothesis that differentiation diversity between habitat types is greater than that produced by stochastic processes. Two models were constructed, one based on the random placement of species and one based on the random placement of individuals. No evidence supported the hypothesis that habitat heterogeneity enhances diversity of a landscape by increasing the number of species in an area. Indeed, paired habitats were more similar than chance alone would dictate in terms of species identities. In contrast, habitat heterogeneity affects diversity by significantly altering the relative proportions of species in contrasting habitat types. Because seeds differentially accumulate at the interface between blowouts and matrix, the high productivity of the edge may actually homogenize habitat types in terms of species richness. Nonetheless, blowouts might best be considered to be microhabitats which enhance or complement the value of the matrix even though the species which use either habitat type are identical.

A supposed hybrid population (Agave peacockii) between Agave marmorata and Agave kerchovei in a semiarid ecosystem at Zapotitlán Salinas, in the Mexican State of Puebla, was investigated, and its hybrid status corroborated in morphometric terms. A Stepwise Discriminant Analysis of the ratios: Number of leaved Rosette diameter, Leaf width/Leaf length, Number of spines/Leaf length, Distance between the upper spine and the tip of the leaf/Leaf length, and Distance between the upper spine and the tip of the leaf/Number of spines, supported our hypothesis of the existence of these naturally occurring hybrids, which are intermediate and distinct from the parental species. A null model was constructed to contrast the real case with a possible artifact, and the results also supported our hypothesis. The possible meaning of this kind of hybridization is discussed.

Community assembly rules are important to help understand the dynamics of biological invasions. The coexistence of native and invasive ant species was examined by litter sampling on six remote islands within the Fijian archipelago, in the Pacific Ocean. The composition of ant assemblages of the islands and also of three different habitats across islands was very similar to each other. Estimates of species richness indicated that the sampling effort had generally captured a large proportion of ant species (60–97%). Analysis at two different spatial scales (regional [islands within an archipelago], and local [plots within an island]) and on two null model data sets (co-occurrence and body size), showed that the majority (10 of 12) of assemblages were not different from randomly assembled communities. Habitat type played an important role in the co-occurrence patterns. Scrub and coconut habitats, which are non-native habitats and frequently disturbed (i.e. harsh environments), strongly influenced the assembly of the ant community. However, two invasive species, Pheidole megacephala and Anoplolepis gracilipes may have also shaped the ant communities through inter-specific competition. These two species excel at both the discovery and domination of resources, and could have ‘disassembled’ the native ant fauna. Recent surveys and ecological studies from other Pacific islands show that a very similar set of invasive species to the Yasawa islands are ubiquitous throughout the region. Thus, similar patterns of competition, co-occurrence and community organisation that exist in the Yasawa islands could be manifested throughout the Pacific Ocean region.

It is widely accepted that species diversity is contingent upon the spatial scale used to analyze patterns and processes. Recent studies using coarse sampling grains over large extents have contributed much to our understanding of factors driving global diversity patterns. This advance is largely unmatched on the level of local to landscape scales despite being critical for our understanding of functional relationships across spatial scales. In our study on West African bat assemblages we employed a spatially explicit and nested design covering local to regional scales. Specifically, we analyzed diversity patterns in two contrasting, largely undisturbed landscapes, comprising a rainforest area and a forest-savanna mosaic in Ivory Coast, West Africa. We employed additive partitioning, rarefaction, and species richness estimation to show that bat diversity increased significantly with habitat heterogeneity on the landscape scale through the effects of beta diversity. Within the extent of our study areas, habitat type rather than geographic distance explained assemblage composition across spatial scales. Null models showed structure of functional groups to be partly filtered on local scales through the effects of vegetation density while on the landscape scale both assemblages represented random draws from regional species pools. We present a mixture model that combines the effects of habitat heterogeneity and complexity on species richness along a biome transect, predicting a unimodal rather than a monotonic relationship with environmental variables related to water. The bat assemblages of our study by far exceed previous figures of species richness in Africa, and refute the notion of low species richness of Afrotropical bat assemblages, which appears to be based largely on sampling biases. Biome transitions should receive increased attention in conservation strategies aiming at the maintenance of ecological and evolutionary processes.