The sequence of species observed after a relatively large space is opened up is a consequence of the following mechanisms. "Opportunist" species with broad dispersal powers and rapid growth to maturity usually arrive first and occupy empty space. These species cannot invade and grow in the presence of adults of their own or other species. Several alternative mechanisms may then determine which species replace these early occupants. Three models of such mechanisms have been proposed. The first "facilitation" model suggests that the entry and growth of the later species is dependent upon the earlier species "preparing the ground"; only after this can later species colonize. Evidence in support of this model applies mainly to certain primary successions and in heterotrophic succession. A second "tolerance" model suggests that a predictable sequence is produced by the existence of species that have evolved different strategies for exploiting resources. Later species will be those able to tolerate lower levels...

Mechanisms of succession in natural communities and their role in community stability and organization

Antimicrobial properties of tannins

The Use of Distance Measures in Phytosociological Sampling

Regional differences in species number have puzzled naturalists ince the early 1800's, and explanations account for a large part of modern ecological research. Two venerable observations form the cornerstone of our knowledge on the subject: The number of species within a taxonomic group tends to increase with decreasing latitude (see Fischer 1960; Pianka 1966); and the number of species within a taxonomic group tends to increase with increasing area (see Preston 1960, 1962; Williams 1964; MacArthur and Wilson 1967; Simberloff 1972). Despite early research on the latter trend (the species-area relationship), ecologists have studied it intensely only in the last 50 yr. The relationship was originally envisioned as an empirical tool and used in three principle ways: (1) to determine optimal sample size and sample number, (2) to determine the minimum area of a \"community,\" and (3) to predict the number of species in areas larger than those sampled. All three uses are discussed by Kilburn (1966). More recently interest in the species-area relationship has focused on mechanistic explanations, its precise mathematical descriptions, and interpretations of parameters derived from these mathematical descriptions. Williams (1964) and Preston (1960, 1962) have proposed that the exponential and power function models (\"exponential model\" throughout his paper also refers to the species/log area transformation, and \"power function\" also refers to the log species/log area transformation) of the species-area relationship result from the way in which individuals are distributed among species. Williams' (1964) exponential model, which emphasizes habitat heterogeneity, was considered important by many plant ecologists but is now largely ignored. Preston's (1960, 1962) power function model was based on the assumption of a dynamic equilibrium of species exchanges between islands in an archipelago. This assumption led to the equation of the power function model with the idea of a dynamic equilibrium as expounded by MacArthur and Wilson (1963, 1967), such that an adequate fit of this model to observed species numbers has been viewed as support of the equilibrium hypothesis (Grant 1970; Diamond 1973; Simpson 1974). The interplay of the equilibrium hypothesis and the power function model of the species-area relationship has led to interpretation of the slope and intercept of the power function model exclusively in the context of the equilibrium hypothesis. In particular, specific values of the slope of the power function are often construed to * Order of authorship determined by the toss of a coin. t Present address: Department of Biology, University of South Florida, Tampa, Florida 33620. Am. Nat. 1979. Vol. 113, pp. 791-833. c) 1979 by The University of Chicago. 0003-0147/79/1306-0002$03.26

/pdf/the-statistics-and-biology-of-the-species-area-relationship-442cynzyis.pdf

The statistics and biology of the species-area relationship

Tannins (commonly referred to as tannic acid) are water-soluble polyphenols that are present in many plant foods. They have been reported to be responsible for decreases in feed intake, growth rate, feed efficiency, net metabolizable energy, and protein digestibility in experimental animals. Therefore, foods rich in tannins are considered to be of low nutritional value. However, recent findings indicate that the major effect of tannins was not due to their inhibition on food consumption or digestion but rather the decreased efficiency in converting the absorbed nutrients to new body substances. Incidences of certain cancers, such as esophageal cancer, have been reported to be related to consumption of tannins-rich foods such as betel nuts and herbal teas, suggesting that tannins might be carcinogenic. However, other reports indicated that the carcinogenic activity of tannins might be related to components associated with tannins rather than tannins themselves. Interestingly, many reports indicated negative association between tea consumption and incidences of cancers. Tea polyphenols and many tannin components were suggested to be anticarcinogenic. Many tannin molecules have also been shown to reduce the mutagenic activity of a number of mutagens. Many carcinogens and/or mutagens produce oxygen-free radicals for interaction with cellular macromolecules. The anticarcinogenic and antimutagenic potentials of tannins may be related to their antioxidative property, which is important in protecting cellular oxidative damage, including lipid peroxidation. The generation of superoxide radicals was reported to be inhibited by tannins and related compounds. The antimicrobial activities of tannins are well documented. The growth of many fungi, yeasts, bacteria, and viruses was inhibited by tannins. We have also found that tannic acid and propyl gallate, but not gallic acid, were inhibitory to foodborne bacteria, aquatic bacteria, and off-flavor-producing microorganisms. Their antimicrobial properties seemed to be associated with the hydrolysis of ester linkage between gallic acid and polyols hydrolyzed after ripening of many edible fruits. Tannins in these fruits thus serve as a natural defense mechanism against microbial infections. The antimicrobial property of tannic acid can also be used in food processing to increase the shelf-life of certain foods, such as catfish fillets. Tannins have also been reported to exert other physiological effects, such as to accelerate blood clotting, reduce blood pressure, decrease the serum lipid level, produce liver necrosis, and modulate immunoresponses. The dosage and kind of tannins are critical to these effects. The aim of this review is to summarize and analyze the vast and sometimes conflicting literature on tannins and to provide as accurately as possible the needed information for assessment of the overall effects of tannins on human health.

Tannins and Human Health: A Review

The latest previous comprehensive review of allelopathy (Rice, 1974) covered research done primarily prior to 1973. There have been numerous reviews published in recent years on specific phases of allelopathy (see III), but most of them covered research results published primarily through 1972 also. This review has been restricted, therefore, largely to research conducted subsequent to 1972, except for background information and significant papers overlooked in previous reviews. There has been almost an exponential increase in the rate of publication of papers concerned with allelopathy. Research has been particularly active in relation to the roles of allelopathy in agriculture, forestry, phytopathology, patterning of vegetation, algal succession, and old-field succession. Our increasing knowledge of the conditions under which certain crop residues cause allelopathic effects to subsequent crops should enable us soon to guard against such effects. We are on the threshold of breeding crop plants that will inhibit the chief weeds in a given area through allelopathic action, and thus decrease the need for synthetic weed killers. Our understanding of allelopathic interactions of various plant species has already been used advantageously in reforestation, and future developments are very encouraging. Our increasing knowledge of allelopathy is aiding greatly in our understanding of many ecological phenomena, such as succession and patterning of vegetation. Evidence is mounting that inhibition of nitrification increases as succession progresses toward the climax vegetation, at least in many vegetation types. This leads to a decrease in the loss of nitrogen. Addition of inhibitors to arable lands to prevent nitrification has proved to be valuable in preventing loss of nitrogen and in increasing crop yields. There has been a rapid advance in our knowledge of mechanisms of action of known allelopathic compounds, at increasingly more fundamental levels. We have known for several years, for example, that some of these compounds inhibit uptake of various minerals, but have not known the mechanism of the action. Recent research indicates this may be due, at least in part, to inhibition of a plasma membrane ATPase that is involved with ion absorption. A thorough knowledge of mechanisms of action of allelochemics is essential to our overall understanding and appreciation of the field of allelopathy.

Allelopathy—An update

AB STRA CT Three plots representing two stages of old-field succession and the climax were selected in each of three vegetation types in Oklahoma: oak-pine forest, post oak-blackjack oak forest, and tall grass prairie. Soil samples from the 0-15 and 45-60 cm levels were analyzed every other month for 1 yr for exchangeable ammonium nitrogen and for nitrate. On alternate months numbers of Nitrosomonas and Nitrobacter were determined in the 0-15 cm level. The amount of ammonium nitrogen was lowest in the first-successional stage, intermediate in the second successional stage, and highest in the climax stand. This trend was remarkably consistent hroughout all sampling periods, all vegetation types, and both sampling levels in the soil. The amount of nitrate was highest in the first successional stage, intermediate in the second successional stage, and lowest in the climax stand in both sampling levels, all vegetation types, and virtually all sampling peri6ds. The numbers of nitrifiers were high in the first successional stage, generally, and decreased to a very low level in the climax. In fact, there was often no Nitrobacter in the climax stands. These results indicate that the nitrifiers are inhibited in the climax so that ammonium nitrogen is not oxidized to nitrate as readily in the climax as in the successional stages. Evidence from other geographic areas and vegetation types strongly supports this conclusion. This would certainly appear to be a logical trend in the evolution of ecosystems because of the increased conservation of nitrogen and energy. The ammonium ion is positively charged and is adsorbed on the negatively charged colloidal micelles, thus preventing leaching below the depth of rooting. On the other hand, nitrate ions are negatively charged, are repelled by the colloidal micelles in the soil, and thus readily leach below the depth of rooting or are washed away in surface drainage. There is growing evidence also that many plant species can use ammonium nitrogen as effectively or more so than nitrate nitrogen. If ammonium nitrogen is used directly, this eliminates four chemical steps because nitrogen which is oxidized to nitrite and then to nitrate must be reduced back to nitrite and then to ammonium nitrogen before it can react with keto-acids in the formation of amino acids. The two reduction reactions require considerable xpenditure of energy.

Inhibition of nitrification by climax ecosystems

The same nine plots were used in this study as in our previous study on inhibition of nitrification (Rice and Pancholy, 1972). These consisted of three stands representing two stages of old field succession and the climax in each of three vegetation types in Oklahoma: tall grass prairie, post oak-blackjack oak forest, and oak-pine forest. Soil samples were analyzed three times during the growing season of 1972 for exchangeable ammonium nitrogen, nitrate, and numbers of Nitrosomonas and Nitrobacter. Results were similar to those obtained during the entire year of 1971. The amount of ammonium nitrogen was lowest in the first successional stage, intermediate in the intermediate successional stage, and highest in the climax. The amount of nitrate was highest in the first successional stage, intermediate in the intermediate successional stage, and lowest in the climax. The numbers of nitrifiers were highest in the first successional stage usually and decreased to a very low number in the climax. These data furnish additional evidence that the nitrifiers are inhibited in the climax so that ammonium nitrogen is not oxidized to nitrate as readily in the climax as in the successional stages. This would aid in the conservation of nitrogen and energy in the climax ecosystem. Some inhibition of nitrification occurred in the intermediate stage of succession also. Previous studies of tannins indicated that these are inhibitory to nitrification, so all important plant species in the intermediate successional stage and the climax were analyzed for total tannin content. A method for extracting and quantifying condensed tannins from soils was developed and the amounts of tannins were determined in each 15-cm level down to 60 cm in the same two plots in each vegetation type. Gallic and ellagic acids, which result from the digestion of hydrolyzable tannins in oak species, were also extracted and quantified in the climax oak-pine forest. All the important herbaceous species, including the grasses, were found to have considerable amounts of condensed tannins. The highest amounts of tannins occurred in the oaks and pine, however. Condensed tannins, hydrolyzable tannins, ellagic acid, gallic acid, digallic acid, and commercial tannic acid (hydrolyzable tannin), in very small concentrations, were all found to completely inhibit nitrification by Nitrosomonas in soil suspensions for 3 weeks, the duration of the tests. Slightly larger concentrations were required to inhibit nitrification by Nitrobacter under similar conditions. The concentrations of tannins, gallic acid, and ellagic acid found in the soil of the research plots were several times higher than the minimum concentrations necessary to completely inhibit nitrification. The inhibition of nitrification was always greater in the climax stand than in the intermediate successional stage in each vegetation type, and the concentration of tannins in the top 15 cm of soil was always higher in the climax stand than in the intermediate successional stage. Moreover, the amounts of tannins calculated to be added to each plot each year are much less than the amounts found in the soil, indicating that the tannins accumulate over a period of time. Thus, it appears that the tannins and tannin derivatives may play a continuous and rather prominent role in the inhibition of nitrification by vegetation.

Inhibition of nitrification by climax ecosystems. ii. additional evidence and possible role of tannins

Amylase, cellulase, invertase, dehydrogenase, and urease activities in soil were determined every other month for a year in two old-field successional stages and a climax stand in each of three vegetation types; tall grass prairie, post oak-blackjack oak forest and oak-pine forest. The activities of amylase, cellulase, and invertase were highest generally in the first successional stage, intermediate in the second stage, and lowest in the climax stand. On the other hand, dehydrogenase and urease activities were generally lowest in the first successional stage, intermediate in the second stage, and highest in the climax. These trends were observed in all three vegetation types throughout the year. No correlation was found between soil enzymatic activity and amount of organic matter or soil pH. The type of vegetation and thus the type of organic matter added to the soil during succession seem to be the chief determiners of the activity gradients of the enzymes under study. Additional Index Words: carbohydrases, hydrolases, plant succession. View complete article To view this complete article, insert Disc 3 then click button8

Elroy L. Rice

Papers

Allelopathy—An update

Inhibition of nitrification by climax ecosystems

Inhibition of nitrification by climax ecosystems. ii. additional evidence and possible role of tannins

Soil Enzymes in Relation to Old Field Succession: Amylase, Cellulase, Invertase, Dehydrogenase, and Urease1

Allelopathy as Expressed by Helianthus annuus and Its Role in Old-Field Succession