Species dispersion profiles of anthropogenic grasslands in the Italian Eastern Pre-Alps

A method is proposed to analyse the dispersion profiles of species in classes of environmental variables, based on the decomposition of the expected frequencies in contingency tables with many interacting species. The method has been applied to data of dominant or very frequent graminoid species in grasslands of the Natisone Valley (Friuli, Italy). It allowed to make predictions by removing the random component of variation.Nomenclature follows: P. Zangheri (1976). Flora Italica, CEDAM, Padua.The authors were recipients of an Italian CNR grant (E. Feoli) and a Canadian NSERC grant (L. Orlóci) during tenure of this project. The results are in partial fulfillment of a commitment to the Centro Regionale per la Sperimentazione Agraria per il Friuli-Venezia Giulia.     

Leaf and canopy adaptations in a high-elevation desert on Tenerife,Canary Islands

Quantitative data on the morphology of leaves and canopy in a high elevation desert stand on Tenerife have been analyzed by multivariate methods. The results show a trend in morphological variation that is related to plant height. An interpretation of this trend in terms of the adaptive values of different character combinations suggests that this could be related to a vertical microclimatic gradient. Three main groups of species have been identified, which have evolved different adaptive strategies to different niches within a single stand. The functional values of the character combinations of each species group are discussed.Nomenclature follows Bramwell & Bramwell (1983).This research is part of a broader co-operative project Convergent evolution of plant communities in distant areas, codirected by D. Lausi (University of Trieste, Italy) and L. Orlóci (University of W-Ontario, London, Canada), and funded by the Italian C.N.R. and the Canadian N.S.E.R.C.     

Releve ranking based on a sum of squares criterion

Summary When a very large number of phytosociological types have to be compared, a reduction of the number of relevés is desirable. In this paper a method of relevé selection from given phytosociological tables is suggested. The method is based on a sum of squares criterion. The advantage, in comparison with other selection procedures, is that this method provides a means on the basis of which the efficiency of a relevé selection can be objectively measured.Contribution from the Working Group for Data-Processing in Phytosociology, International Society for Vegetation Science.The work was completed at the Department of Plant Sciences of the University of Western Ontario, London, Canada. We wish to thank Prof. L. Orlóci for the hospitality and the helpful discussions. The work was supported by Italian C.N.R., within the project Promozione qualità dell'ambiente subproject Metodologie matematiche e basi di dati.     

On the dynamics of vegetation: Patterns of environmental and vegetational change

Summary Ecological succession theory deals with temporal change in biological communities. It consists largely of generalizations based on temporal sequences inferred from spatial ones. The predictive content of the theory is low, since predictions are derived from unconditional trends rather than conditional laws. There exist several conflicting theories purporting to explain successional change, but their empirical vacuousness prevents an assessment on empirical terms. It is argued here that one can nevertheless advocate a theory which accounts for the ubiquity of successional change and explains the most conspicuous characteristics of the successional process, even though it cannot predict the detailed dynamics. Such a theory is derived here from an analysis of adaptive strategies.It is also pointed out that a persistant confusion exists in the ecological literature between what are considered to be the driving forces of successional change, competition and reaction. The former is taken to be an instantaneous type of interaction, whereas the latter has historical (cumulative) aspects. It is not at all obvious whether interactions of the historical type play an important role in driving vegetational change, although it is usually suggested that they do.     

Towards the analysis of vegetation succession

Attention is drawn to canonical analysis as a plausible model for analyzing vegetation succession. An assessment of the opportunities afforded by canonical analysis for this purpose is then made by reference to two applications of the method. The applications deal with characteristics of hydroseral processes and with the dynamic status of an area of lowland tropical rain forest. On the basis of these and other studies the conclusion is drawn that canonical analysis could contribute usefully in efforts to place the study of dynamic ecosystem processes on a more analytic footing.This work was supported by a Natural Sciences and Engineering Research Council award to L. Orlóci. It is a pleasure to acknowledge my indebtedness both to Dr. Orlóci for his support and encouragement and to the Department of Plant Sciences at the University of Western Ontario for hospitality and help. Appreciation is expressed also to Dr. John Ogden, Research School of Pacific Studies, The Australian National University, Canberra for kindly placing his rain forest data at my disposal and for his invaluable help in interpreting the results of analyses of these data.     

Species selection by interactive ranking

Ranking methods represent a means of giving weight to species according to their measured or perceived importance. Stepwise methods, such as Orlóci's (1973) yield a small list of species which can effectively describe groups and gradients in the data. These lists often include species which are difficult to use and the method does not allow incorporation of internal information in the analysis. The present method overcomes these problems. It uses Orlóci's (1973) ranking as a basis, but allows the user to reject species if undesired on ecological or practical grounds. The method is illustrated on a set of 34 relevés from peat bogs, and interactive ranking proves only slightly less efficient than un iased ranking.Nomenclature follows Hess et al. (1967–1972) for vascular plants.Acknowledgement. The author expresses his thanks to Dr L. Orlóci for advice and comments.     

Species ordering on a variance criterion

Summary A technique for ordering, to identify diagnostic species, based on variance ratios has been described. The method was illustrated with river terrace data.The author would like to thank Dr. László Orlóci for permission to use the vegetation data employed in the illustration of the algorithm. This work was supported by a grant from the National Research Council of Canada.     

Analysis of concentration and detection of underlying factors in structured tables

Summary If the rows and columns of a data table are rearranged in such a way that the non-zero values are concentrated in distinct blocks, the table is said to be structured. In such tables, two specific properties are often subject to interpretations: the sharpness of the block structure and the compositional variation among the blocks. A simple chi square analysis is satisfactory to test the overall sharpness of the block structure. The interpretation of compositional variation is more difficult, since the dimensionality of such variation will almost certainly exceed the dimensionality of the table. A solution can nevertheless be obtained based on a two-step analysis. In the first step, the canonical variates of compositional variation are identified. These are characteristics of the vegetation. In the second step, the environmental variables are identified which are highly correlated with the canonical variates.The example in the paper uses data from fossil flood plains. The data table is structured into 18 blocks of 6 species groups and 3 relevé groups. The chi square test indicates a sharp block structure. The structuring generated two canonical variates. The dominant of these two signifies the environmental influence of changing elevation on the flood plain. The second seems to be a response of the vegetation to disturbance.The algorithm which performs analysis of concentration is AOC. A program is available to enquirers free of charge. The programming language is BASIC.This work is part of a research project supported by a National Research Council of Canada Grant (Orlóci) and an Italian C.N.R. grant: project Promozione qualità dell'ambiente, Subproject Metodologie matematiche e basi di dati (Feoli).     

Secondary plant succession: how is it modified by insect herbivory?

The effects of foliar- and root-feeding insects on the dynamics of an early successional plant community, representing the first four years of colonisation, were examined. Subterranean insect herbivores were found to increase in density with increasing successional age of the plant community. In early succession, chewing insects mainly Coleoptera (Scarabaeidae) and Diptera (Tipulidae) were dominant. This was in direct contrast to the foliar-feeding insects, which were dominated by sap-feeders (mainly Auchenorrhynchan Hemiptera).Reduction of both foliar- and root-feeding insects with appropriate insecticides had different, but dramatic, consequences for the plant community. Reducing foliar herbivory resulted in large increases in perennial grass growth, with plant species richness being reduced as the grasses outcompeted the forbs. Reducing subterranean herbivory prolonged the persistence of annual forbs, greatly increased perennial forb colonisation and, as a consequence, plant species richness. Foliar-feeding insects thus act to delay succession by slowing grass colonisation. In contrast, root-feeding insects accelerate succession by reducing forb persistence and colonisation. The structure of early successional plant communities is therefore modified by the two modes of herbivory.This paper was presented at the Vth International Congress of Ecology (INTECOL), Japan, 1990, entitled Successional Communities of Plants and Insects.     

Cursorial spiders and succession: age or habitat structure?

Summary The structure of cursorial spider assemblages was examined along a gradient of four temperature successional communities. Species diversity (H), richness (S), and evenness (J) exhibited a dichotomy between herbaceous and woody communities rather than a progressive change with community age: all three parameters were higher in the two younger fields than in the two older woodlands, which is contrary to conventional successional theory. Species importance curves were steeper in the two woody communities. The breadth of the distribution of adult body lengths was greater in the two herbaceous communities. Indices of community similarity revealed neither a successional trend nor the vegetative dichotomy. We suggest the hypothesis that habitat structure is a more important determinant of cursorial spider diversity than successional age per se, and that the switch in dominance from herbaceous to woody vegetation is the critical change. We further suggest that competition for prey is more important to cursorial spiders in early successional (herbaceous) communities, because of a switch in the limiting resource from prey in these communities to the amount of accumulating litter (a spatial resource) in older woody stands. This may explain the greater variation in adult body size of these generalist predators in the two younger communities.     

Secondary succession and summer herbivory in a subarctic grassland: community structure and diversity

A field experiment was established in a subarctic grassland in the Finnish Lapland to study the role of summer herbivory in plant community succession Perennial vegetation and moss cover were removed in an area of 324 m² The site was divided into four blocks, of which two were fenced to prevent herbivory by large mammals (reindeer, hare)
Early successional changes in the vegetation were assessed Mean species richness per 3 × 3 m plot was consistently higher in the fenced area, indicating that herbivory can suppress small-scale diversity Herbivory affected the height of several plant species However, there was no correlation between frequency and height of individual species There was a weak indication that taller species were more successful m early succession when grazed Light competition is apparently not a key process determining successional change Thus, in early stage of succession, summer herbivory has little effect on diversity by limiting light competition, and most species are equally successful in grazed and ungrazed plots There was some indirect evidence about competitive interactions in the developing community However, unlike temperate grasslands, large mammal herbivory and competition for light seem not to be important determinants of community change in this subarctic grassland (at least what concernes early successional stages) This may be explained by the harshness of local climate, and abundance of light due to the polar day     

Relationships between successional situation and production of miscanthus sinensis communities at Kirigamine Heights,Central Japan

Summary In the Miscanthus sinensis type grasslands of the Kirigamine Heights situated in a high altitude area (1,600 m) of Central Japan, the biomass in various successional phases was measured and the degree of succession (DS) was calculated. The maximum biomass during the growing season was used to compare the productivity of different grassland communities.The range and maximum point of DS in the DS-biomass curve of Miscanthus grasslands at Kirigamine was small compared with that of other Miscanthus grasslands in Japan.Between successional phases and production in the Miscanthus type grassland we found, that the total biomass increases, as succession proceeds to the maximum point. The of DS, and decreases after the maximum point. The biomass of Miscanthus sinensis changes as a normal curve against the axis of DS.Accordingly, in this Miscanthus type grassland, the production of the community decreases temporarily when the dominants of seral stages change. These results confirm Numata's (1966) model.Nomenclature followsA Handbook for Plant Collectors by H. Okuyama (1974).     

Scales of disturbance and their role in plankton ecology

The role of hydraulic and hydrographic disturbances in delaying, arresting or diverting successional sequences from achieving stable, climactic equilibria is discussed by reference to case studies. The critical time scale is expressed in terms of planktonic reproductive generation times. Environmental constancy persisting over some 12–16 generations may permit a climactic condition to be achieved. An Intermediate Disturbance, if sustained, can establish a new successional sequence or, if not, can lead to a reversion to a sequence similar to the predisturbance succession. At intervals of 1 generation time, species are selected according to their ability to accommodate disturbances at the physiological level. Highly disturbed environments are considered to be likely to maintain plagioclimactic associations.     

A heuristic test for homogeneity in species populations

Summary This paper elaborates on the notion of homogeneity in plant populations and its quantification. It describes a method for testing homogeneity without the need to make assumptions about generalized distributions. The implementation of the method is illustrated through actual numerical examples.This paper presents results from a study of plant populations for which a National Research Council of Canade grant has been received (L. Orlóci).     

Consequences of behavioral dynamics for the population dynamics of predator-prey systems with switching

This article explores how different mechanisms governing the rate of change of the predators preference alter the dynamics of predator-prey systems in which the predator exhibits positive frequency-dependent predation. The models assume that individuals of the predator species adaptively adjust a trait that determines their relative capture rates of each of two prey species. The resulting switching behavior does not instantaneously attain the optimum for current prey densities, but instead lags behind it. Several mechanisms producing such lags are discussed and modeled. In all cases examined, our question is whether a realistic behavioral lag can significantly change the dynamics of the system relative to an analogous case in which the predators switching is effectively instantaneous. We also explore whether increasing the rate parameters of dynamic models of behavior results in convergence to the population dynamics of analogous models with instantaneous switching, and whether different behavioral models produce similar population dynamics. The analysis concentrates on systems that undergo endogenously generated predator-prey cycles in the absence of switching behavior. The average densities and the nature of indirect interactions are often sensitive to the rate of behavioral change, and are often qualitatively different for different classes of behavioral models. Dynamics and average densities can be very sensitive to small changes in parameters of either the prey growth or predator switching functions. These differences suggest that an understanding of switching in natural systems will require research into the behavioral mechanisms that govern lags in the response of predator preference to changes in prey density.     

The effect of a seed source on primary succession in a forest ecosystem

An analysis of the Relative Importance Value Index (RIVI) of later successional tree species and the Shannon diversity indexH of all tree and shrub species was undertaken on previously mined sites within a sparsely forested area in central Florida, USA. A comparative analysis of the distance to a seed source and the age of a site suggested that the distance to a seed source was the best predictor (R ²=0.85) of the regeneration of the later successional species and a good predictor of species diversity. Both theRIVI of the later successional species and the diversity index decreased with the distance from the seed source. The lack of a seed source containing climax species resulted in arrested succession at some sites. It is suggested that biological information is the causative means of succession and the dispersal of this information entails spatial dependency while its introduction and development are time dependent processes.Research was part of a project of the Center for Wetlands, University of Florida, Gainesville under contract with the Florida Institute of Phosphate Research. Interactions of Wetlands with Phosphate Mining, H. T. Odum and G. R. Best, principal investigators. B. T. Rushton, J. Butner, G. R. Best, R. S. Schnoes and S. R. Humphrey are thanked for the use of their data.     

Vegetation responses to past climatic variation

Vegetation responses to climatic change can be studied retrospectively by utilizing the Quaternary fossil record. There has been controversy over the extent to which major changes in vegetation patterns at the continental scale lag behind the climatic changes that drive them, and to what extent vegetation can ever be said to be in equilibrium with climate. The equilibrium question has no single answer. The predominant mode of vegetation response to climatic change depends on the space and time frame and resolution of the data set in which the response is observed.Vegetation (as observed on particular space and time scales) can be in dynamic equilibrium with climate if its response time is sufficiently fast in relation to the rate of climatic change to which it is observed to be responding. Several processes can be involved in the response: successional, migrational, edaphic, and evolutionary. Successional response times can be deduced from forest succession models. The other processes are less well understood and different ideas exist concerning their rates. According to one hypothesis, migrational lags caused delays of thousands of years in the postglacial dynamics of forest composition. The alternative hypothesis explains these changes as dynamic equilibrium responses to changes in climatic seasonality and climatic anomaly patterns. Neither hypothesis need be universally true; gradient analysis and forest succession models are among the techniques that can be used in inferential tests of these hypotheses for particular space-time regions.Dynamic equilibrium may often be a reasonable approximation for the responses of the broadest continental-scale forest patterns to orbitally induced climatic changes. But as spatial and temporal frames of observation are diminished and resolution increased, biotic processes must eventually come to dominate. At sufficiently fine scales the main observable phenomena are successional responses to natural disturbance events. The late-Quaternary record of vegetation change allows a choice of observation scales and thus provides a continuum of possibilities for study, ranging from long-term dynamic bioclimatology to more conventional vegetation dynamics.I thank Margaret Davis, Honor Prentice, Jim Ritchie, Al Solomon, Geoff Spaulding and Tom Webb for their reviews of earlier drafts. Research supported by a US Department of Energy, Carbon Dioxide Research Division, grant to Brown University and a Swedish Natural Science Research Council grant to the project SlsSimulation of Natural Forest Dynamics.I thank Margaret Davis, Honor Prentice, Jim Ritchie, Al Solomon, Geoff Spaulding and Tom Webb for their reviews of earlier drafts. Research supported by a US Department of Energy, Carbon Dioxide Research Division, grant to Brown University and a Swedish Natural Science Research Council grant to the project SlsSimulation of Natural Forest Dynamics.     

A resemblance function based on probability: Applications to field and simulated data

A resemblance function (COCHIS) based on the probability of species association and correlation is proposed. It is a development of the underlying idea of association analysis and can serve to produce similarity matrices for polythetic classifications and ordinations. Examples are given utilizing field and simulated data.The work has been supported by the Italian Ministry of Education. We would like to thank the Profs. D W. Goodall, E. van der Maarel and I. Orlóci for comments.     

Developing tests of successional hypotheses with size-structured populations,and an assessment using long-term data from a Ugandan rain forest

In 1947, W. J. Eggeling published an account of forest succession at Budongo, Uganda. This interpretation was based on a large-scale comparative plot study, performed in the 1930s and 1940s. This account, with its implication that species richness declines in late succession, endures as a controversial corner-stone in theories and disputes about community diversity. Data have now been collected over six decades from five of Eggeling's original plots.This paper evaluates Eggeling's successional interpretation of the Budongo vegetation. The first set of analyses assesses the consistency of the original data with the predictions of compositional progression and convergence implicit in Eggeling's model. The second analyses do the same for the time-series observations. A logical approach shows how temporal information may be derived from both between plot, and within plot, evaluations using size-structured data. A Detrended-Correspondence-Analysis (DCA) of canopy-tree composition, from the original data, ranks the plots in perfect correspondence to Eggeling's successional sequence. A development-scoring procedure is developed using passive-ordination against this sequence; this is then applied to composition by plot and stem-size class.Eggeling's original data are consistent with each prediction assessed. The analyses show compositional progression and apparent convergence across the plot series, and also progression and convergence within each plot. A monodominant-Cynometra forest is the natural end-point of this progression. The time-series results, though in apparent agreement for one early successional plot, do not generally accord with Eggeling's ideas. The analyses illustrate a general means for evaluating explicit and implicit compositional trends in communities with structured populations.     

Pattern analysis in successional communities – An approach for studying shifts in ecological interactions

Abstract. Many ecological studies have addressed issues of vegetation spatial patterns in attempts to understand the processes generating them. We investigated changes in ecological processes during succession via the analysis of shrubs’ spatial patterns in a system of linear sand dunes, an arid ecosystem located in the Negev Desert in Israel during three consecutive years. We hypothesized that spatial patterns change from clustered to regular as succession progresses due to changes in the relative importance of facilitation and competition in this environment. In this ecosystem communities of early successional stages are frequently disturbed by high rates of sand movement, whereas in later successional stages sand stability is high. We mapped in the field individual shrubs on high‐resolution aerial photographs, and converted the digital images to a GIS data set. Using Ripley's K‐function we analysed spatial patterns at three levels: the single‐species level, among species and at the individual level, in three communities characterizing different successional stages. In the early successional communities we found clustered spatial patterns, in comparison with stable habitats where spatial patterns tended to be regular. We argue that these shifts in spatial patterns are indicative of the assumption that in this sand‐dune system ecological interactions change from facilitation to competition as succession progresses. Further, we argue that these interactions operate in different spatial scales at the different successional stages, and that the study of these processes should be conducted at the spatial scales specific to each community.