Otto T. Solbrig

The meaning and measurement of size hierarchies in plant populations

SummaryThe term “size hierarchy” has been used frequently by plant population biologists but it has not been defined. Positive skewness of the size distribution, which has been used to evaluate size hierarchies, is inappropriate. We suggest that size hierarchy is equivalent to size inequality. Methods developed by economists to evaluate inequalities in wealth and income, the Lorenz curve and Gini Coefficient, provide a useful quantification of inequality and allow us to compare populations. A measure of inequality such as the Gini Coefficient will usually be more appropriate than a measure of skewness for addressing questions concerning plant population structure.

The role of topkill in the differential response of savanna woody species to fire

Understanding the impact of fire on the demography of savanna trees and shrubs is necessary for understanding human impacts in tropical savannas. In a replicated experiment, we studied the impact of fire and vegetation cover on survival and growth of two subshrubs (Periandra mediterranea and Protium ovatum), two shrubs (Miconia albicans and Rourea induta) and three trees (Myrsine guianensis, Piptocarpha rotundifolia and Roupala montana) of the Brazilian cerrado savannas. Burning increased complete mortality (i.e. death of the individual) of five of the seven species, but primarily among individuals with stem diameters 2 m) caused greater mortality and topkill than fires of lower intensity (flame length <2 m). Pre-burn vegetation density had little effect on survival or resprout size, but did affect subsequent growth rates. Four species had greater growth rates in open sites, whereas only one species had greater growth rates in dense sites. For the three tree species and one shrub, resprouting individuals did not reach the minimum reproductive size within 1 year of burning, while the other shrub and the two subshrubs were able to reach reproductive size during this time, indicating that growth form largely determines the population response to frequent burning.

Determinants of Tropical Savannas

Tropical savannas, defined as ecosystems formed by a continuous layer of graminoids (grasses and sedges) with a discontinuous layer of trees and/or shrubs, are the most common vegetation type (physiognomy) in the tropics. Tropical savannas are found over a wide range of conditions: rainfall from approximately 200 mm to 1500 mm a year, temperature from subtropical regimes such as the South American Chaco and the South-African savannas with temperature seasonality and cold-month average temperatures below 10 °C, to low-latitude savannas with no temperature seasonality, and soils from volcanic soils such as in parts of the Serengueti plains in Tanzania to dystrophic soils such as in the Brazilian cerrados. The one constant climatic characteristic of tropical savannas is rainfall seasonality. Yet the duration of the dry season can vary from 3 to 9 months, with a mode of 5 to 7 months.

Shoot demography in new England populations of Maianthemum canadense desf.

SummaryThe demography of shoots of eight populations of a herbaceous perennial exhibiting clonal growth, is presented. The study was done along an elevational gradient, from a more open secondary mixed forest to a denser, more mature stand. Most shoots lived one to three years on the average, but shoots as old as twelve years were found. Large variation in formation and mortality of shoots was observed among plots and years. Yearly trends in the mortality rates of site replicates showed a higher correlation than rates of shoot formation. Although the density of shoots was highest in the drier sites, the turnover of shoots was highly variable and apparently uncorrelated with site location. Age structures revealed a tendency of longer-lived and higher reproductive activities among shoots from more mesic sites. It is hypothesized that environmental rather than density controls are primary causes of the population dynamics observed in this species.

Savanna Biodiversity and Ecosystem Properties

The overall question addressed here is the effect of different degrees of biodiversity on the function of savanna ecosystems. Function can be interpreted in two different ways. It can refer to the flow of energy and nutrients through an ecosystem or to the flow of species populations through time, i.e., the persistence of species populations and their properties, which we call the structure of the system. Here we discuss the effect of biodiversity on ecosystem function in this second sense. The role of biodiversity in the flow of energy and nutrients is addressed in Chapter 10.

The phylogeny of gutierrezia: An eclectic approach

One approach to the problem of deducing the genealogy of a set of organisms is to propose several hypotheses using different procedures, based on different evolutionary inferences. Such an approach was followed here, and four different phylogenies were constructed, three of them computer built. Consistency with independently obtained phenetic, cytological, and phytogeographical data was used to select the most probable phylogenetic tree among the four. It is shown that the most probable tree is one constructed under the assumption that character states found close to the mean and/or modal values for the genus are primitive. It is also shown that certain phylogenetic conclusions obtain in all four phylogenies.

The Theory and Practice of the Science of Biodiversity: A Personal Assessment

Diversity is a property, not an entity in itself. It refers to the property of a set of objects of not being identical, of varying one from another in one or more characteristics. When applied to organisms, it refers to the universal attribute of all living things that each individual being is unique, that is, no two organisms are identical. The origin of this variability is to be found in the basic and fundamental property of the DNA molecule that the order of the bases does not affect the free energy of the molecule, in other words all combinations of the four bases that form the genetic code are chemically equally viable. This characteristic combined with natural selection allows the acquisition and accumulation of favorable mutations, and in the approximately 3 thousand million years that life has existed on Earth these processes have produced the enormous biological variation that we see today, which is at best only a very small percentage of all the variation that has existed in the past. Today this diversity of life is threatened by human activities, although the exact rate of species loss is difficult to ascertain. And species loss is only one aspect of the profound transformation of the terrestrial landscape that is presently taking place, brought about by the growth of human populations and their economic activities. This transformation extends from genes to ecosystems. The transformation of the worlds habitats by people has many causes, and many effects. As long as there is doubt regarding the exact mechanism underlying this transformation, there will be differences of opinion concerning the impact, and people who always are reluctant to change their behavior will use these varying opinions to rationalize their actions. Only a rigorous, unbiased, and mechanistic science of biodiversity can help resolve these differences.

Principles of demographic analysis applied to natural polyploid populations.

The general principles of demographic analysis apply equally well to diploid or polyploid populations. The question is whether the morphological, physiological, or behavioral characteristic of polyploid populations will affect their demographic development.

Biodiversity and the world's food crisis.

During the next fifty years the world’s population is expected to double. In order to avoid catastrophic famines the world’s food production also must double. Although this is technically possible with present high-input methods, such technology is probably not affordable by poor tropical countries with high population growth. A more environmental friendly, and less demanding but equally productive technology based on time tested, labour intensive, tropical systems must be developed to avoid both famine, landscape deterioration, and biodiversity loss.