On the dynamics of vegetation: Succession in model communities

Summary Successional change is thought to be at least partially driven by forees originating from within the community, namely by reaction and competition. Both processes operate through changes in the environment, but from the literature on the subject it is not clear how they differ.To clarify these issues successiens of model communities are studied. This leads us to conclude that competition represent an instantaneous interaction, whereas reaction has historical aspects since it relies on cumulative changes in the environment. The three models considered-one relying on reaction to cause vegetational change, one relying on competition and differential growth rates, and a hybrid third one-yield very similar predictions: roughly bell-shaped curves displaced along the time axis. This shows that the mere fit of a certain model to successional data may easily be spurious (recently some workers have empirically fitted models identical to one derived here from first principles). The three models do behave radically different under perturbation, however: any model relying completely or partially on historical interactions cannot account for the well known possibility of artificially arresting succession. Even if the importance of historical interactions in succession (i.e. the Markovian character of succession) cannot easily be ascertained, one can nevertheless ask whether historical interactions are at all necessary for the explanation of successional change. It is argued here that succession can be entirely understood in terms of instantaneous interactions, notably competition. The argument rests upon the well known relationship between colonizing and competitive ability, and on the fact, proven here, that stress, defined as expressing itself in severe random fluctuations in the growth parameters, is negatively correlated with competition intensity.The author gratefully acknowledges the critical support be received from Professor László Orlóci. This paper forms part of a wider research program supported by a NCR grant to Proc. Orlóci.     

Successional change in species composition alters climate sensitivity of grassland productivity

Succession theory predicts altered sensitivity of ecosystem functions to disturbance (i.e., climate change) due to the temporal shift in plant community composition. However, empirical evidence in global change experiments is lacking to support this prediction. Here, we present findings from an 8‐year long‐term global change experiment with warming and altered precipitation manipulation (double and halved amount). First, we observed a temporal shift in species composition over 8 years, resulting in a transition from an annual C₃‐dominant plant community to a perennial C₄‐dominant plant community. This successional transition was independent of any experimental treatments. During the successional transition, the response of aboveground net primary productivity (ANPP) to precipitation addition magnified from neutral to +45.3%, while the response to halved precipitation attenuated substantially from ?17.6% to neutral. However, warming did not affect ANPP in either state. The findings further reveal that the time‐dependent climate sensitivity may be regulated by successional change in species composition, highlighting the importance of vegetation dynamics in regulating the response of ecosystem productivity to precipitation change.     

Vegetation development on the outer islands of the Bothnian Bay

This paper summarizes succession studies on the outer islands of the Bothnian Bay (Finland). The study area is characterized by consistent and relatively rapid uplift (vertical rise on average 75 cm per century). The spatial and temporal succession has been deduced from the sequence of the littoral and epilittoral vegetation as well as from the historical records of the island development.Four types of successional series are distinguished. The relationships between substrate type (boulder, gravel, sand and clay-silt shores) and 25 typical plant communities are outlined and treated as successional sequences.     

Spatial Heterogeneity in Soil Microbes Alters Outcomes of Plant Competition

Plant species vary greatly in their responsiveness to nutritional soil mutualists, such as mycorrhizal fungi and rhizobia, and this responsiveness is associated with a trade-off in allocation to root structures for resource uptake. As a result, the outcome of plant competition can change with the density of mutualists, with microbe-responsive plant species having high competitive ability when mutualists are abundant and non-responsive plants having high competitive ability with low densities of mutualists. When responsive plant species also allow mutualists to grow to greater densities, changes in mutualist density can generate a positive feedback, reinforcing an initial advantage to either plant type. We study a model of mutualist-mediated competition to understand outcomes of plant-plant interactions within a patchy environment. We find that a microbe-responsive plant can exclude a non-responsive plant from some initial conditions, but it must do so across the landscape including in the microbe-free areas where it is a poorer competitor. Otherwise, the non-responsive plant will persist in both mutualist-free and mutualist-rich regions. We apply our general findings to two different biological scenarios: invasion of a non-responsive plant into an established microbe-responsive native population, and successional replacement of non-responders by microbe-responsive species. We find that resistance to invasion is greatest when seed dispersal by the native plant is modest and dispersal by the invader is greater. Nonetheless, a native plant that relies on microbial mutualists for competitive dominance may be particularly vulnerable to invasion because any disturbance that temporarily reduces its density or that of the mutualist creates a window for a non-responsive invader to establish dominance. We further find that the positive feedbacks from associations with beneficial soil microbes create resistance to successional turnover. Our theoretical results constitute an important first step toward developing a general understanding of the interplay between mutualism and competition in patchy landscapes, and generate qualitative predictions that may be tested in future empirical studies.     

Marine invertebrates in an algal succession. I. Variations in abundance and diversity with succession

This is a study of the effects of successional algal changes on the community of small invertebrate animals living in the algae at a marine intertidal boulder field. At each of 14 sampling times over a 2-yr period, replicate samples were taken from all stages of succession. By sampling all successional stages simultaneously at each date over an extended period, we could analyze spatial and temporal variations within and among stages. Species richness, diversity, and abundance of the whole invertebrate community increased from early to middle successional stages, while remaining similar from middle to late stages. Evenness remained about constant throughout succession. The increases occur primarily through the addition of species to the community. Some taxonomic groups in this study showed different patterns. Unlike patterns seen in birds, insects, and spiders associated with terrestrial plant succession, species replacements are rare, and there are few early and middle successional specialists. Trophic structure changed markedly during succession. Spatial variation did not change with succession but temporal variation declined. Many of these results are different from the predictions of equilibrium-based theory. Succession in this marine animal community is very different from that in terrestrial animal groups, probably because of the higher rate of exchange of matter in this open coast system.     

Temporal patterns in rates of community change during succession

While ecological dogma holds that rates of community change decrease over the course of succession, this idea has yet to be tested systematically across a wide variety of successional sequences. Here, I review and define several measures of community change rates for species presence-absence data and test for temporal patterns therein using data acquired from 16 studies comprising 62 successional sequences. Community types include plant secondary and primary succession as well as succession of arthropods on defaunated mangrove islands and carcasses. Rates of species gain generally decline through time, whereas rates of species loss display no systematic temporal trends. As a result, percent community turnover generally declines while species richness increases--both in a decelerating manner. Although communities with relatively minor abiotic and dispersal limitations (e.g., plant secondary successional communities) exhibit rapidly declining rates of change, limitations arising from harsh abiotic conditions or spatial isolation of the community appear to substantially alter temporal patterns in rates of successional change.     

Explanations in evolutionary theory1

The theory of biological evolution is defined in many ways, leading to considerable confusion in its application and testing against objective empirical observations. Evolutionary change is usually defined as genetic which would exclude both cultural and template evolution; hence the qualifying adjective genetic should not be included in the definition of biological evolution. Darwin's theory, always described by him in the singular, is actually a bundle of five independent theories about evolution as advocated by Mayr. Furthermore only one of these theories, that of common descent, is historical, and the other four – evolution as such, gradualism, processes of phyletic evolution and of speciation, and causes of evolution – are nomological. Hence not all evolutionary theory is historical. Biological comparisons can be divided into horizontal and vertical ones and valid conclusions from one type of comparisons cannot be automatically extrapolated to the other. All phyletic evolutionary change, no matter how extensive it may be, never crosses species taxa boundaries; hence it is not possible to distinguish ‘trans‐specific evolution’ (= evolution beyond or above the level of the species) from evolution within the species level. Macroevolution does not differ from microevolution except in the scale of the overall change; no special causes or processes of macroevolution exist.     

Successional theories

Succession is a fundamental concept in ecology because it indicates how species populations, communities, and ecosystems change over time on new substrate or after a disturbance. A mechanistic understanding of succession is needed to predict how ecosystems will respond to land-use change and to design effective ecosystem restoration strategies. Yet, despite a century of conceptual advances a comprehensive successional theory is lacking. Here we provide an overview of 19 successional theories (‘models’) and their key points, group them based on conceptual similarity, explain conceptual development in successional ideas and provide suggestions how to move forward. Four groups of models can be recognised. The first group (patch & plants) focuses on plants at the patch level and consists of three subgroups that originated in the early 20th century. One subgroup focuses on the processes (dispersal, establishment, and performance) that operate sequentially during succession. Another subgroup emphasises individualistic species responses during succession, and how this is driven by species traits. A last subgroup focuses on how vegetation structure and underlying demographic processes change during succession. A second group of models (ecosystems) provides a more holistic view of succession by considering the ecosystem, its biota, interactions, diversity, and ecosystem structure and processes. The third group (landscape) considers a larger spatial scale and includes the effect of the surrounding landscape matrix on succession as the distance to neighbouring vegetation patches determines the potential for seed dispersal, and the quality of the neighbouring patches determines the abundance and composition of seed sources and biotic dispersal vectors. A fourth group (socio-ecological systems) includes the human component by focusing on socio-ecological systems where management practices have long-lasting legacies on successional pathways and where regrowing vegetations deliver a range of ecosystem services to local and global stakeholders. The four groups of models differ in spatial scale (patch, landscape) or organisational level (plant species, ecosystem, socio-ecological system), increase in scale and scope, and reflect the increasingly broader perspective on succession over time. They coincide approximately with four periods that reflect the prevailing view of succession of that time, although all views still coexist. The four successional views are: succession of plants (from 1910 onwards) where succession was seen through the lens of species replacement; succession of communities and ecosystems (from 1965 onwards) when there was a more holistic view of succession; succession in landscapes (from 2000 onwards) when it was realised that the structure and composition of landscapes strongly impact successional pathways, and increased remote-sensing technology allowed for a better quantification of the landscape context; and succession with people (from 2015 onwards) when it was realised that people and societal drivers have strong effects on successional pathways, that ecosystem processes and services are important for human well-being, and that restoration is most successful when it is done by and for local people. Our review suggests that the hierarchical successional framework of Pickett is the best starting point to move forward as this framework already includes several factors, and because it is flexible, enabling application to different systems. The framework focuses mainly on species replacement and could be improved by focusing on succession occurring at different hierarchical scales (population, community, ecosystem, socio-ecological system), and by integrating it with more recent developments and other successional models: by considering different spatial scales (landscape, region), temporal scales (ecosystem processes occurring over centuries, and evolution), and by taking the effects of the surrounding landscape (landscape integrity and composition, the disperser community) and societal factors (previous and current land-use intensity) into account. Such a new, comprehensive framework could be tested using a combination of empirical research, experiments, process-based modelling and novel tools. Applying the framework to seres across broadscale environmental and disturbance gradients allows a better insight into what successional processes matter and under what conditions.     

Shade tolerance,canopy gaps and mechanisms of coexistence of forest trees

The belief that canopy gaps are important for the maintenance of tree species diversity appears to be widespread, but there have been no formal theoretical models to assess under what conditions gap phase processes allow coexistence. Much of the empirical research on niche differentiation in response to gaps has focused on evidence for an interspecific tradeoff between low light survival and high light growth. The objectives of this study are first to distinguish the possible mechanisms allowing coexistence based on this tradeoff, and second, to explore their limitations. We present a theory of forest dynamics driven by small‐scale disturbances as a special case of the theory of coexistence in variable environments. We demonstrate that temporal and spatial heterogeneity in light conditions that results from canopy gaps can allow stable coexistence as a result of three previously documented general mechanisms: ‘relative non‐linearity’, ‘the successional niche’ and the ‘storage effect’. We find that temporal fluctuations in light availability alone allow the stable coexistence of only two species. Spatial variation in disturbance synchronicity and intensity allows three species to coexist in a narrow parameter space. The rate of extinction is, however, extremely slow and there is transient coexistence of a larger number of species for a long period of time. We conclude that while the low light survival/high light growth tradeoff may be ubiquitous in forest tree species, it is unlikely to function as an important mechanism for the stable coexistence of several tree species.     

Nonequilibrium thermodynamics and different axioms of evolution

Proponents of two axioms of biological evolutionary theory have attempted to find justification by reference to nonequilibrium thermodynamics. One states that biological systems and their evolutionary diversification are physically improbable states and transitions, resulting from a selective process; the other asserts that there is an historically constrained inherent directionality in evolutionary dynamics, independent of natural selection, which exerts a self-organizing influence. The first, the Axiom of Improbability, is shown to be nonhistorical and thus, for a theory of change through time, acausal. Its perception of the improbability of living states is at least partially an artifact of closed system thinking. The second, the Axiom of Historically Determined Inherent Directionality, is supported evidentially and has an explicit historical component. Historically constrained dynamic populations are inherently nonequilibrium systems. It is argued that living, evolving systems, when considered to be historically constrained nonequilibrium systems, do not appear improbable at all. Thus, the two axioms are not compatible. Instead, the Axiom of Improbability is considered to result from an unjustified attempt to extend the contingent proximal actions of natural selection into the area of historical, causal explanations. It is thus denied axiomatic status, and the effects of natural selection are subsumed as an additional level of constraint in an evolutionary theory derived from the Axiom of Historically Determined Inherent Directionality.     

The role of land-use history in driving successional pathways and its implications for the restoration of tropical forests

Secondary forests are increasingly important components of human-modified landscapes in the tropics. Successional pathways, however, can vary enormously across and within landscapes, with divergent regrowth rates, vegetation structure and species composition. While climatic and edaphic conditions drive variations across regions, land-use history plays a central role in driving alternative successional pathways within human-modified landscapes. How land use affects succession depends on its intensity, spatial extent, frequency, duration and management practices, and is mediated by a complex combination of mechanisms acting on different ecosystem components and at different spatial and temporal scales. We review the literature aiming to provide a comprehensive understanding of the mechanisms underlying the long-lasting effects of land use on tropical forest succession and to discuss its implications for forest restoration. We organize it following a framework based on the hierarchical model of succession and ecological filtering theory. This review shows that our knowledge is mostly derived from studies in Neotropical forests regenerating after abandonment of shifting cultivation or pasture systems. Vegetation is the ecological component assessed most often. Little is known regarding how the recovery of belowground processes and microbiota communities is affected by previous land-use history. In published studies, land-use history has been mostly characterized by type, without discrimination of intensity, extent, duration or frequency. We compile and discuss the metrics used to describe land-use history, aiming to facilitate future studies. The literature shows that (i) species availability to succession is affected by transformations in the landscape that affect dispersal, and by management practices and seed predation, which affect the composition and diversity of propagules on site. Once a species successfully reaches an abandoned field, its establishment and performance are dependent on resistance to management practices, tolerance to (modified) soil conditions, herbivory, competition with weeds and invasive species, and facilitation by remnant trees. (ii) Structural and compositional divergences at early stages of succession remain for decades, suggesting that early communities play an important role in governing further ecosystem functioning and processes during succession. Management interventions at early stages could help enhance recovery rates and manipulate successional pathways. (iii) The combination of local and landscape conditions defines the limitations to succession and therefore the potential for natural regeneration to restore ecosystem properties effectively. The knowledge summarized here could enable the identification of conditions in which natural regeneration could efficiently promote forest restoration, and where specific management practices are required to foster succession. Finally, characterization of the landscape context and previous land-use history is essential to understand the limitations to succession and therefore to define cost-effective restoration strategies. Advancing knowledge on these two aspects is key for finding generalizable relations that will increase the predictability of succession and the efficiency of forest restoration under different landscape contexts.     

Some nutritional and excretional interactions and the growth of an organ or colony

Following the general form for the differential equation of organism and colonial growth, there is derived a rational formulation for the growth of a bounded cell community (e.g., an organ) equipped with a food supply and a waste removal mechanism. It is shown how, from the integral form and an empirical curve, the vital coefficients of the equation can be derived. Changes to be expected in these coefficients are discussed, and the analytic methods for assessing them are set forth. It is hoped that these equations and similar ones will make it possible to relate empirical curves to the mathematico-biophysical theory of the cell. The opinions or assertions contained herein are the private ones of the writers, and are not to be construed as official or reflecting the views of the Navy Department or the Naval Service at large.     

The relative importance of population versus community processes in microbial primary succession

Interpretations of successional patterns in ecological communities have traditionally adhered to the dichotomy between the Clementsian view that emphasizes community level processes and the Gleasonian view that stresses individual population responses. The present study evaluates the relative importance of each type of process during protistan primary succession in initially barren aquatic isolates (200-1 plastic pools) over a 170-d period. Species availability to these systems was manipulated by erecting exclosures around individual mesocosms to successively eliminate access to different dispersal vectors responsible for passive protistan dispersal. Increased exclosure significantly reduced access of autotrophs to the pools, but had little effect on heterotroph species availability. The species replacement process was directional through time and occurred at similar rates in all treatments. Both lower and upper temporal boundaries of heterotrophic and autotrophic species were contagious through time, as predicted by the Clementsian hypothesis, although the independence of these two boundary types suggested an individualistic model. Dominant and subdominant species were correlated into four temporal groups: pioneer, early successional, mid-successional, late successional. The dominance of several mid- and late successional species was reduced with increased exclosure. The loss of these species from successional pathways in more exclosed pools had no significant effect on the distribution of other species within the same temporal group. However, the establishment of these other mid- and late successional species may be dependent on initial colonization by pioneer and early successional species. Increased abundances of mid- and late successional species in less exclosed pools coincided with significant attenuations in the distribution of many early successional species. Interactions between successional groups may be related to the supply of inorganic resources as well as allelopathic effects. Patterns of protist succession are the result of both population and community processes; while species-specific characteristics (i.e., dispersal ability) may dominate the process in more isolated systems, increased species availability increases the relative importance of interspecific interactions.     

Monitoring succession from space: A case study from the North Carolina Piedmont

Question: Is the successional transition from pine to hardwood, which has been inferred from chronosequence plots in previous studies, validated through a time line of satellite imagery? Location: Durham, North Carolina, USA. Methods: We examined successional trends in a time‐series of winter‐summer pairs of Thematic Mapper imagery from 1986 to 2000. We calculated the normalized difference of vegetation index (NDVI) for winter and summer, as well as the difference between summer and winter NDVI (i.e., summer increment NDVI). A set of approximately 50 forest stands of known age and phenology were used to interpret patterns in winter and summer increment NDVI over successional time, and a continuum was found to exist between pine‐dominance and hardwood‐dominance. We fitted a series of linear regressions that modeled the change in winter and summer increment NDVI as a function of initial winter and summer increment NDVI, and additional explanatory variables. Results: All regressions were highly significant (P < 0.0001, R²= ca. 0.3). Predicted dynamics are in accord with successional theory, with pixels moving from evergreen dominance to deciduous dominance along a line of fairly constant summer NDVI. A large disturbance event that occurred over the course of this study, Hurricane Fran, appeared to slow rates of succession in the short term (1–3 years), but increase the rate of conversion to hardwoods over longer time spans. Conclusions: We conclude that temporal sequences of remote sensing images provide an excellent opportunity for broad‐scale monitoring of successional processes, and that continuous metrics of that change are essential to accurate monitoring.     

Vegetation as a component of a non-nested hierarchy: a conceptual model

Abstract. A general conceptual model of vegetation based on hierarchy theory is presented. The model emphasizes that prediction of vegetation requires consideration of both mechanisms of vegetation change and the constraints within which it occurs. The mechanisms of vegetation change are the responses to and effects upon their surroundings of individual plants. The most general constraints upon vegetation are aspects of the environment not affected by vegetation over successional time, and the pool of species within dispersal range. Examples of such environmental factors include macroclimate and soil parent material. In some cases, vegetation may alter important labile environmental factors such as soil nutrient and water availability. Some vegetation compositions appear to be resistant to changes in the general constraints. Due to both sources, there are multiple possible vegetation compositions given the same general constraints. Disturbance is defined as an abrupt change in the constraints on the vegetation resulting in a change in the vegetation's state or dynamics. Both the recognition of disturbance and the distinction between independent and labile environmental factors depend on the spatial and temporal scale of observation. For example, a particular wildfire at a given stand may be a disturbance, whereas at a larger scale of observation the same event may contribute to the wildfire regime, part of the constraints at that scale. Similarly, levels of soil organic matter may constrain vegetation over short time scales, due to influencing availability of water and nutrients. Over long time scales, the vegetation itself is a primary determinant of soil organic matter content. This model contains elements of both the initial, holistic theory of vegetation and recent, reductionistic approaches. It reiterates the need to considerboth mechanisms and constraints, stressed by contemporary and earlier workers. Hierarchy theory provides new insights concerning sufficient conditions for prediction, possible limits on predictability, and appropriate research strategy.     

Empirical and theoretical challenges in aboveground–belowground ecology

A growing body of evidence shows that aboveground and belowground communities and processes are intrinsically linked, and that feedbacks between these subsystems have important implications for community structure and ecosystem functioning. Almost all studies on this topic have been carried out from an empirical perspective and in specific ecological settings or contexts. Belowground interactions operate at different spatial and temporal scales. Due to the relatively low mobility and high survival of organisms in the soil, plants have longer lasting legacy effects belowground than aboveground. Our current challenge is to understand how aboveground–belowground biotic interactions operate across spatial and temporal scales, and how they depend on, as well as influence, the abiotic environment. Because empirical capacities are too limited to explore all possible combinations of interactions and environmental settings, we explore where and how they can be supported by theoretical approaches to develop testable predictions and to generalise empirical results. We review four key areas where a combined aboveground–belowground approach offers perspectives for enhancing ecological understanding, namely succession, agro-ecosystems, biological invasions and global change impacts on ecosystems. In plant succession, differences in scales between aboveground and belowground biota, as well as between species interactions and ecosystem processes, have important implications for the rate and direction of community change. Aboveground as well as belowground interactions either enhance or reduce rates of plant species replacement. Moreover, the outcomes of the interactions depend on abiotic conditions and plant life history characteristics, which may vary with successional position. We exemplify where translation of the current conceptual succession models into more predictive models can help targeting empirical studies and generalising their results. Then, we discuss how understanding succession may help to enhance managing arable crops, grasslands and invasive plants, as well as provide insights into the effects of global change on community re-organisation and ecosystem processes.     

Structure and stand development of secondary forests in Eastern Prealps (Italy)

The composition and temporal variations in species recruitment were examined by means of annual dendrochronological data, to determine the historical development and successional history of ash and sycamore mixed stands in five secondary forests on the Eastern Prealps. The first step of the investigation was to describe species composition and age structure. In accordance with data derived from land register and time series of orthophoto images, differences in time of colonization among the five stands were detected. The second step was to describe spatial colonization patterns: only in one case a frontal pattern was found while in others the colonization started from some old alder or ash trees preserved by farmers in the meadows.     

植物群落演替过程中植物生理生态学特性及其主要环境因子的变化

对20世纪80年代以来有关植物群落演替的生理生态学机制研究的主要结果进行了综述.演替早期和演替后期各种植物所处环境常有很大差别.演替早期的生境具有开放性和光照充足等特点,各环境因子富于变化;演替后期的生境由于植被的缓冲作用,一般较为封闭和稳定,各环境因子在空间尺度上的异质性较强.演替早期和演替后期群落不仅物种组成不同,而且在演替不同阶段中出现的物种的生理生态特性以及对环境的适应性也有很大差别,这些物种的生理生态差异使得物种更替现象经常发生, 也使得演替能够顺利进行.在全球气候变化影响下,生态系统将会出现更多的次生演替和长时间停留于演替早期阶段的情况.     

Maximizing in Jajmaniland: A Model of Caste Relations

Insofar as it may be regarded as a purely logicomathematical system, lacking empirical content, economic theory may, by appropriate empirical interpretation, be applied to domains quite apart from the market. Here such use yields a model of caste or jajmani relations. The chief assumptions of the model regarding the stability of a caste system are the following: (1) it requires a high concentration of political power; (2) it requires "prices" that do not change "freely" with supply and demands; (3) it requires consonance of political power, wealth, and ritual rank. These assumptions and various related deductions are tested statistically by data derived from village studies in India, Pakistan, and Ceylon. The evidence indicates that the model is generally plausible and may be applicable to hereditary hierarchies everywhere.     

The selective advantage of reaction norms for environmental tolerance

A tolerance curve defines the dependence of a genotype's fitness on the state of an environmental gradient. It can be characterized by a mode (the genotype's optimal environment) and a width (the breadth of adaptation). It seems possible that one or both of these characters can be modified in an adaptive manner, at least partially, during development. Thus, we extend the theory of environmental tolerance to include reaction norms for the mode and the width of the tolerance curve. We demonstrate that the selective value of such reaction norms increases with increasing spatial heterogeneity and between-generation temporal variation in the environment and with decreasing within-generation temporal variation. Assuming that the maintenance of a high breadth of adaptation is costly, reaction, norms are shown to induce correlated selection for a reduction in this character. Nevertheless, regardless of the magnitude of the reaction norm, there is a nearly one to one relationship between the optimal breadth of adaptation and the within-generation temporal variation perceived by the organism. This suggests that empirical estimates of the breadth of adaptation may provide a useful index of this type of environmental variation from the organism's point of view.