T. Michael Anderson

On the specification of structural equation models for ecological systems

The use of structural equation modeling (SEM) is often motivated by its utility for investigating complex networks of relationships, but also because of its promise as a means of representing theoretical concepts using latent variables. In this paper, we discuss characteristics of ecological theory and some of the challenges for proper specification of theoretical ideas in structural equation models (SE models). In our presentation, we describe some of the requirements for classical latent variable models in which observed variables (indicators) are interpreted as the effects of underlying causes. We also describe alternative model specifications in which indicators are interpreted as having causal influences on the theoretical concepts. We suggest that this latter nonclassical specification (which involves another variable type—the composite) will often be appropriate for ecological studies because of the multifaceted nature of our theoretical concepts. In this paper, we employ the use of meta-models to ...

Landscape-scale analyses suggest both nutrient and antipredator advantages to Serengeti herbivore hotspots

Mechanistic explanations of herbivore spatial distribution have focused largely on either resource-related (bottom-up) or predation-related (top-down) factors. We studied direct and indirect influences on the spatial distributions of Serengeti herbivore hotspots, defined as temporally stable areas inhabited by mixed herds of resident grazers. Remote sensing and variation in landscape features were first used to create a map of the spatial distribution of hotspots, which was tested for accuracy against an independent data set of herbivore observations. Subsequently, we applied structural equation modeling to data on soil fertility and plant quality and quantity across a range of sites. We found that hotspots in Serengeti occur in areas that are relatively flat and located away from rivers, sites where ungulates are less susceptible to predation. Further, hotspots tend to occur in areas where hydrology and rainfall create conditions of relatively low-standing plant biomass, which, coupled with grazing, increases forage quality while decreasing predation risk. Low-standing biomass and higher leaf concentrations of N, Na, and Mg were strong direct predictors of hotspot occurrence. Soil fertility had indirect effects on hotspot occurrence by promoting leaf Na and Mg. The results indicate that landscape features contribute in direct and indirect ways to influence the spatial distribution of hotspots and that the best models incorporated both resource- and predation-related factors. Our study highlights the collective and simultaneous role of bottom-up and top-down factors in determining ungulate spatial distributions.

Forage Nutritive Quality in the Serengeti Ecosystem: The Roles of Fire and Herbivory

Fire and herbivory are important determinants of nutrient availability in savanna ecosystems. Fire and herbivory effects on the nutritive quality of savanna vegetation can occur directly, independent of changes in the plant community, or indirectly, via effects on the plant community. Indirect effects can be further subdivided into those occurring because of changes in plant species composition or plant abundance (i.e., quality versus quantity). We studied relationships between fire, herbivory, rainfall, soil fertility, and leaf nitrogen (N), phosphorus (P), and sodium (Na) at 30 sites inside and outside of Serengeti National Park. Using structural equation modeling, we asked whether fire and herbivory influences were largely direct or indirect and how their signs and strengths differed within the context of natural savanna processes. Herbivory was associated with enhanced leaf N and P through changes in plant biomass and community composition. Fire was associated with reduced leaf nutrient concentrations through changes in plant community composition. Additionally, fire had direct positive effects on Na and nonlinear direct effects on P that partially mitigated the indirect negative effects. Key mechanisms by which fire reduced plant nutritive quality were through reductions of Na‐rich grasses and increased abundance of Themeda triandra, which had below‐average leaf nutrients.

Scale-dependent relationships between the spatial distribution of a limiting resource and plant species diversity in an African grassland ecosystem.

One cornerstone of ecological theory is that nutrient availability limits the number of species that can inhabit a community. However, the relationship between the spatial distribution of limiting nutrients and species diversity is not well established because there is no single scale appropriate for measuring variation in resource distribution. Instead, the correct scale for analyzing resource variation depends on the range of species sizes within the community. To quantify the relationship between nutrient distribution and plant species diversity, we measured NO3- distribution and plant species diversity in 16 paired, modified Whittaker grassland plots in Serengeti National Park, Tanzania. Semivariograms were used to quantify the spatial structure of NO3- from scales of 0.4–26xa0m. Plant species diversity (Shannon-Weiner diversity index; H ′) was quantified in 1-m2 plots, while plant species richness was measured at multiple spatial scales between 1 and 1,000xa0m2. Small-scale variation in NO3- (<0.4xa0m) was positively correlated with 1-m2 H ′, while 1,000-m2 species richness was a log-normal function of average NO3- patch size. Nine of the 16 grassland plots had a fractal (self-similar across scales) NO3- spatial distribution; of the nine fractal plots, five were adjacent to plots that had a non-fractal distribution of NO3-. This finding offered the unique opportunity to test predictions of Ritchie and Olff (1999): when the spatial distribution of limiting resources is fractal, communities should display a left-skewed log-size distribution and a log-normal relationship between net primary production and species richness. These predictions were supported by comparisons of plant size distributions and biomass-richness relationships in paired plots, one with a fractal and one with a non-fractal distribution of NO3-. In addition, fractal plots had greater large-scale richness than paired non-fractal plots (1,0–1000xa0m2), but neither species diversity (H ′) nor richness was significantly different at small scales (1xa0m2). This result is most likely explained by differences in the scale of resource variation among plots: fractal and non-fractal plots had equivalent NO3- variation at small scales but differed in NO3- variation at large scales (as measured by the fractal dimension). We propose that small-scale variation in NO3- is largely due to the direct effects of plants on soil, while patterns of species richness at large scales is controlled by the patch size and fractal dimension of NO3- in the landscape. This study provides an important empirical step in understanding the relationship between the spatial distribution of resources and patterns of species diversity across multiple spatial scales.

RAINFALL AND SOILS MODIFY PLANT COMMUNITY RESPONSE TO GRAZING IN SERENGETI NATIONAL PARK

Terrestrial plant community responses to herbivory depend on resource availability, but the separate influences of different resources are difficult to study because they often correlate across natural environmental gradients. We studied the effects of excluding ungulate herbivores on plant species richness and composition, as well as available soil nitrogen (N) and phosphorus (P), across eight grassland sites in Serengeti National Park (SNP), Tanzania. These sites varied independently in rainfall and available soil N and P. Excluding herbivores decreased plant species richness at all sites and by an average of 5.4 species across all plots. Although plant species richness was a unimodal function of rainfall in both grazed and ungrazed plots, fences caused a greater decrease in plant species richness at sites of intermediate rainfall compared to sites of high or low rainfall. In terms of the relative or proportional decreases in plant species richness, excluding herbivores caused the strongest relative decreases at lower rainfall and where exclusion of herbivores increased available soil P. Herbivore exclusion increased among-plot heterogeneity in species composition but decreased coexistence of congeneric grasses. Compositional similarity between grazed and ungrazed treatments decreased with increasing rainfall due to greater forb richness in exclosures and greater sedge richness outside exclosures and was not related to effects of excluding herbivores on soil nutrients. Our results show that plant resources, especially water and P, appear to modulate the effects of herbivores on tropical grassland plant diversity and composition. We show that herbivore effects on soil P may be an important and previously unappreciated mechanism by which herbivores influence plant diversity, at least in tropical grasslands.

The Relationship of Phylogeny to Community Structure: The Cactus Yeast Community

Species abundances are important properties of ecological communities. Theoretical debate has arisen over whether communities are governed by assembly rules. Some ecologists have suggested that community organization depends on the phylogenetic relatedness of its interacting members. This postulate has not been adequately tested primarily because molecular phylogenies for entire communities are rare. We use the molecular phylogeny and species abundances from the well‐studied yeasts that live in decaying cactus tissue (i.e., cactus yeast community) to demonstrate that community structure is indeed linked to genetic relatedness but that the relationship is different for the same yeasts found in hosts of two subfamilies of the Cactaceae. In the Opuntia cacti, the genetically related and unrelated yeasts have equivalent probability of being similar in abundance. Conversely, in the columnar cacti, the abundant yeast species tend to be genetically distant from one another, whereas the rare species tend to be closely related. The distinctive biochemical differences between the Opuntia and columnar cacti habitats suggest that conditions imposed by the environment modify the relationship between phylogenetic relatedness and species abundances in the cactus yeast community.

Metschnikowia shivogae sp. nov., a yeast species associated with insects of morning glory flowers in East Africa

The novel species Metschnikowia shivogae is described to accommodate three isolates recovered from insects of morning glory flowers at two localities in East Africa. The isolates differ slightly in rDNA ITS and D1/D2 large-subunit sequences and one isolate featured a two-base heterogeneity that might be the result of recombination between two variant rDNAs. M. shivogae is a sister species to Metschnikowia aberdeeniae and shares the same habitat. The reproductive boundaries of M. aberdeeniae, which were not clear in the past, have now been elucidated further. The type strain of Metschnikowia shivogae sp. nov. is strain SUB 04-310.1(T) (h(+); =CBS 10292(T) =NRRL Y-27924(T)) and the allotype is strain UWOPS 07-203.2 (h(-); =CBS 10770 =NRRL Y-48447).