1. Introduction and Overview. 2. Detecting Influential Observations and Outliers. 3. Detecting and Assessing Collinearity. 4. Applications and Remedies. 5. Research Issues and Directions for Extensions. Bibliography. Author Index. Subject Index.

Regression Diagnostics: Identifying Influential Data and Sources of Collinearity

It is often claimed that we do not understand the forces driving the global diversity gradient. However, an extensive literature suggests that contemporary climate constrains terrestrial taxonomic richness over broad geographic extents. Here, we review the empirical literature to examine the nature and form of the relationship between climate and richness. Our goals were to document the support for the climatically based energy hypothesis, and within the constraints imposed by correlative analyses, to evaluate two versions of the hypothesis: the productivity and ambient energy hypotheses. Focusing on studies extending over 800 km, we found that measures of energy, water, or water-energy balance explain spatial variation in richness better than other climatic and non-climatic variables in 82 of 85 cases. Even when considered individually and in isolation, water/ energy variables explain on average over 60% of the variation in the richness of a wide range of plant and animal groups. Further, water variables usually represent the strongest predictors in the tropics, subtropics, and warm temperate zones, whereas energy variables (for animals) or water-energy variables (for plants) dominate in high latitudes. We conclude that the interaction between water and energy, either directly or indirectly (via plant productivity), provides a strong explanation for globally extensive plant and animal diversity gradients, but for animals there also is a latitudinal shift in the relative importance of ambient energy vs. water moving from the poles to the equator. Although contemporary climate is not the only factor influencing species richness and may not explain the diversity pattern for all taxonomic groups, it is clear that understanding water-energy dynamics is critical to future biodiversity research. Analyses that do not include water-energy variables are missing a key component for explaining broad-scale patterns of diversity.

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Energy, water, and broad-scale geographic patterns of species richness

Understanding the relationship between species richness and productivity is fundamental to the management and preservation of biodiversity. Yet despite years of study and intense theoretical interest, this relationship remains controversial. Here, we present the results of a literature survey in which we examined the relationship between species richness and productivity in 171 published studies. We extracted the raw data from published tables and graphs and subjected these data to a standardized analysis, using ordinary least-squares (OLS) regression and generalized linear-model (GLIM) regression to test for significant positive, negative, or curvilinear relationships between productivity and species diversity. If the relationship was curvilinear, we tested whether the maximum (or minimum) of the curve occurred within the range of productivity values observed (i.e., was there evidence of a hump?). A meta-analysis conducted on the distribution of standardized quadratic regression coefficients showed that ...

What is the observed relationship between species richness and productivity

Early studies of nutrient cycling in moist tropical forests described productive forests rich in nutrients (98, 114, 176) in which rates of primary production and the amounts of nutrients cycled clearly exceeded those in temperate zone forests. Reviews of global-scale patterns in biomass, production, and nutrient cycling reported these results as representative of tropical forests (130, 175). At the same time, tropical forest soils were described as acid, infertile clays that harden irreversibly to "laterite" when cleared (106), or as bleached quartz sands low in mineral nutrients (88). This apparent paradox was crystallized by Whittaker (174) in the statement "The tropical rain forest thus has a relatively rich nutrient economy perched on a nutrient-poor substrate" (p. 271). Reviews of more recent research on overall patterns of mineral cycling in the tropics (78, 124), and of important components such as biomass (20, 21), litterfall nutrients (123, 164), and decomposition (5), clearly show that patterns of nutrient cycling in tropical forests are diverse. It makes no more sense to describe a 'typical' tropical forest than a 'typical' temperate forest (33, 151). Variations in mineral cycling nonetheless follow coherent, explicable patterns in tropical forests. Our purposes in undertaking this review are: (a) to illustrate the patterns of nutrient cycling in moist tropical forests; (b) to identify the mechanisms which regulate those patterns; and (c) to show how those patterns affect the productivity, physiology, and population biology of tropical forests and their large-scale linkages with aquatic ecosystems and the atmosphere. We emphasize the cycling of nitrogen, phosphorus, potassium, calcium, and magnesium; these are the best-studied soil-derived nutrients, and they are the nutrients most likely to limit primary production and other ecosystem functions.

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Nutrient cycling in moist tropical forest

Aim Current weaknesses of diversity theory include: a failure to distinguish different biogeographical response variables under the general heading of diversity; and a general failure of ecological theory to deal adequately with geographical scale. Our aim is to articulate the case for a top-down approach to theory building, in which scale is addressed explicitly and in which different response variables are clearly distinguished. Location The article draws upon both theoretical contributions and empirical analyses from all latitudes, focusing on terrestrial ecosystems and with some bias towards (woody) plants. Methods We review current diversity theory and terminology in relation to scale of applicability. As a starting point in developing a general theory, we take the issue of geographical gradients in species richness as a main theme and evaluate the extent to which commonly cited theories are likely to operate at scales from the macro down to the local. Results A degree of confusion surrounds the use of the terms alpha, beta and gamma diversity, and the terms local, landscape and macro-scale are preferred here as a more intuitive framework. The distinction between inventory and differentiation diversity is highlighted as important as, in terms of scale of analysis, are the concepts of focus and extent. The importance of holding area constant in analysis is stressed, as is the notion that different environmental factors exhibit measurable heterogeneity at different scales. Evaluation of several of the most common diversity theories put forward for the grand clines in species richness, indicates that they can be collapsed to dynamic hypotheses based on climate or historical explanations. The importance of the many ecological/ biological mechanisms that have been proposed is evident mainly at local scales of analysis, whilst at the macro-scale they are dependent largely upon climatic controls for their operation. Local communities have often been found not to be saturated, i.e. to be non-equilibrial. This is argued, perhaps counter-intuitively, to be entirely compatible with the persistence through time of macro-scale patterns of richness that are climatically determined. The review also incorporates recent developments in macroecology, Rapoport’s rule, trade-offs, and the importance of isolation, landscape impedance and geometric constraints on richness (the mid-domain effect) in generating richness patterns; highlighting those phenomena that are contributory to the first-order climatic pattern, and those, such as the geometric constraints, that may confound or obscure these patterns. Main conclusions A general theory of diversity must necessarily cover many disparate phenomena, at various scales of analysis, and cannot therefore be expressed in a simple formula, but individual elements of this general theory may be. In particular, it appears possible to capture in a dynamic climate-based model and ‘capacity rule’, the form of the

https://www.jstor.org/stable/pdfplus/827394.pdf

Scale and species richness: towards a general, hierarchical theory of species diversity

We present an analysis of local species richness in neotropical forests, based on a number of 0.1 ha samples of woody plants collected by the late Alwyn Gentry. For each of 69 forests, soils were analysed and climatic data were collated. Using transformed independent variables and interaction terms, multiple regression equations were developed that explained the greatest possible amount of variation in species richness, and the best equations were selected on the basis of regression diagnostics. The best models are presented for (a) all neotropical forests, (b) forests west of the Andes (transandean) and (c) east of the Andes (cisandean), and for various subsets based on elevation and annual rainfall. For the whole dataset, and for most subsets, annual rainfall and rainfall seasonality were the most important variables for explaining species richness. Soil variables were correlated with precipitation — drier forests have more nutrient-rich soils. After the inclusion of rainfall variables, available soil nutrient concentrations contributed little to explaining or accounting for additional variation in species numbers, indicating that tropical forest species richness is surprisingly independent of soil quality. The results are consistent with the hypothesis that plants in mature tropical forests may obtain nutrients through the process of direct cycling, in which mineral nutrients are extracted from litterfall before they enter the soil. The strong relationship between community species richness and rainfall patterns has implications for biodiversity conservation. Wet forests with an ample year-round moisture supply harbour the greatest number of woody plant species and should be a focus of conservation efforts.

Prediction of neotropical tree and liana species richness from soil and climatic data

Surface root mats on oxisols and spodosols near San Carlos de Rio Negro, Venezuela, were sprayed with (45)Ca and (32)P to simulate the addition of nutrients to the soil surface through leaching of decomposing litter, and through precipitation and throughfall. Collections from lysimeters placed below the root mat and humus layer showed that in all but 1 case, <0.1% of the radiotracers leached past the root–organic mat, and leaching stopped completely after 1 to 2 months. Through analysis of root mat samples, the radioisotopes were found to have been taken up and translocated by living roots. This is the first evidence supporting the ability of the root map on the surface of poor, depleted tropical soils to efficiently take up dissolved nutrients before they percolate down to mineral soil. Other studies have shown up that one of the uptake mechanisms is mycorrhizal fungi. See full-text article at JSTOR

Nutrient Retention by the Root Mat of an Amazonian Rain Forest

[3-5]. The present communication suggests the presence of one pathway for this reduction in the microsomal system that does not seem to have been reported previously. The more exact nature and magnitude of this process in various tissues need to be studied further. The carcinogenicity of Cr(VI) compounds-particularly less water-soluble ones-has been documented [6]. Cr(VI) has also been reported to be mutagenic in bacterial systems [7, 8]. The biological effect of Cr(VI) thus certainly depends on where it is reducedwhether in extracellular spaces or in the cytoplasm or the nucleus of various organ cells. The importance of Cr(VI) metabolism does not diminish by the fact that Cr(III) compounds, at sufficiently high concentrations, have been found to be mutagenic in the Salmonella system (unpublished data). This work has been supported by ERDA grant E(04-3)-34-PA156 and carried out in the laboratory of Dr. B.N. Ames, University of California, Berkeley, whom I would like to thank for his support and advice.

Direct phosphorus transfer from leaf litter to roots

Twenty control burns performed with a wide range of fuel loadings and moisture conditions were used to study the effectiveness of old fuel reduction under standing Douglas—fir/larch forest. This paper reports the influence of burning on nutrient retention and loss from the soil. Sixty % of the fires were successful in reducing residual fuels with no accelerated loss of nutrients below the root zone. Net less of Ca+2 and Mg+2 occurred below the root zone when soil surface temperatures exceeded 300°C, but were insignificant when soil surface temperatures remained below 200—300°C. No other elements were loss (net) from the soil as a result of burning. Precipitation on control soils delivers as much Ca+2 as is normally lost below the root zone in the absence of fire. Iron concentration in the soil water is a good indicator of the intensity of burn. The hotter the fire, the less iron in the soil water as a result of the alkaline pH. Ash shows a definite pattern of nutrient release under the influence of precipitation. Homogeneous subsamples of litter showed predictable nutrients losses when ignited at different temperatures. Overland flow and surface erosion are of little significance on this soil type. Decomposition of Douglas—fir litter was only slightly more rapid on hot burned substrates than on control (unburned) substrates. When the biological life concept was applied to this soil, it showed that this soil is young and capable of withstanding many years of cyclic intensive burns.

Fire and Nutrient Cycling in a Douglas-Fir/Larch Forest

Microbial biomass and relative bacterial and fungal percentages were measured in organic forest soil from a Rocky Mountain site subjected to four harvesting treatments: RL, clear-cut and residue left; RR, clear-cut and residue removed; RB, clear-cut and residue burned; C, uncut control Microbial biomass peaked in spring and fall regardless of treatment Biomass in soil from the RB treatment was significantly (p < 005) less than that in soil from the other treatments most of the year; biomass did not significantly differ in soil from the RR and C treatments During summer and winter, microbial biomass in soil from the RL treatment was significantly greater than that in soil from any other treatment, probably because of the large amount of organic residue left after harvest; moreover, this residue insulated the soil, preventing it from drying or freezing At soil temperatures above 5 °C, microbial biomass correlated positively with soil moisture regardless of treatment; at soil temperatures below 25 °C, 

Nellie M. Stark

Papers

Prediction of neotropical tree and liana species richness from soil and climatic data

Nutrient Retention by the Root Mat of an Amazonian Rain Forest

Direct phosphorus transfer from leaf litter to roots

Fire and Nutrient Cycling in a Douglas-Fir/Larch Forest

Effect of timber harvesting on microbial biomass fluxes in a northern Rocky Mountain forest soil