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The addition of two urban sites, based in Baltimore, Maryland, and Phoenix, Arizona, to the Long Term Ecological Research (LTER) program in 1997, posed challenges for creating a truly integrated social-ecological framework. Proposals to include social “core areas” to sites with social science agendas were developed in concert with national environmental priorities being set within the scientific community. Although the National Science Foundation rejected these proposals for LTER as a whole, researchers at the urban sites pursued their goals of developing more sophisticated multi-disciplinary frameworks for the study of urban patterns and processes. The Baltimore Ecosystem Study (BES) illustrates how the conceptual basis of urban ecology evolved over 20 years, developing new strategies to manage cross-disciplinary interactions and relationships. Researchers drew especially on the concept of “boundary objects” as articulated by S. L. Star and J. Griesemer, and on General Stanley A. McChrystal’s conception of the “team of teams” as a way to engage with complex problems. Looking forward, BES is positioned now to expand to a long-term, transdisciplinary science platform which we call the Baltimore Ecosystem Alliance. Its goals include developing an authentic urban ecology on local and regional levels, while also serving as a national and international leader in urban ecology.

Urbanization dramatically affects hydrology, water quality and aquatic ecosystem composition. Here we characterized changes in diatom assemblages along an urban-to-rural gradient to assess impacts of urbanization on stream conditions in Beijing, China. Diatoms, water chemistry, and physical variables were measured at 22 urban (6 in upstream and 16 in downstream) and 7 rural reference stream sites during July and August of 2013. One-way ANOVA showed that water physical and chemical variables were significantly different ( p p > 0.05). Similarly, structural metrics, including species richness (S), Shannon diversity (H′), species evenness (J′) and Simpson diversity (D′), were significantly different ( p p > 0.05) between reference and urban upstream sites. However, diatom assemblages were very different among all sites. Achnanthidium minutissima was a consistent dominant species in reference sites; Staurosira construens var. venter and Pseudostaurosira brevistriata were the dominant species in urban upstream sites; and Nitzschia palea was the dominant species in urban downstream sites. Clustering analyses based on the relative abundance of diatom species, showed all the samples fit into three groups: reference sites, urban upstream sites, and urban downstream sites. Canonical correspondence analysis (CCA) and Monte Carlo permutation tests showed that concentration of K + , EC, TN, Cl − and pH were positively correlated with relative abundance of dominant diatom species in urban downstream samples; WT and F − were correlated with reference and urban stream diatom composition. Our results demonstrate that the composition of diatom species was more sensitive to urbanization than the water physical and chemical parameters, and that diatom assemblage structure metrics more accurately assessed water quality. Some species, such as Amphora pediculus and Cocconeis placentula were among the dominant species in low nutrients stream sites; however, they were considered to be high nutrient indicators in some streams in USA. We suggest using caution in applying indicator indices based on species composition from other regions. It is necessary to build a complete set of diatom species data and their co-ordinate environment data for specific regions.

The ability to estimate and model future vegetation dynamics is a central focus of contemporary ecology and is essential for understanding future ecological trajectories. It is therefore critical to understand when the influence of initial post-disturbance regeneration versus stochastic processes dominates long-term post-disturbance ecological processes. Often, conclusions about post-disturbance dynamics are based upon initial regeneration in the years immediately after disturbances. However, the degree to which initial post-disturbance regeneration indicates longer-term trends is likely to be contingent on the types, intensities, and combinations of disturbances, as well as pre-disturbance ecosystem structure and composition. Our relatively limited understanding of why initial post-disturbance regeneration is sometimes a poor predictor of future ecosystem trajectories represents a critical gap in post-disturbance ecological forecasting. We studied the composition and density of regeneration of tree species following wind blowdown in 1997, wildfire in 2002, and compounded disturbances by blowdown and wildfire in subalpine forests of Colorado. We examined regeneration of Picea engelmannii, Abies lasiocarpa, Pinus contorta, and Populus tremuloides in 180 permanent plots across 12 sites (classified by pre-disturbance age and composition) in 2003, 2010, and 2015. At sites that were blown down but not burned, regeneration was dense and dominated by Picea and Abies. At these sites, regeneration observed from 2003 to 2005 (hereafter initial regeneration) was also highly predictive of regeneration 5–10 yr later. In contrast, at sites that were burned and sites that were blown down and burned, regeneration was less dense and dominated by a mix of species. At these sites, initial regeneration was a poor predictor of longer-term trends as species dominance and overall density fluctuated over the 13-yr period. These findings call into question our ability to confidently predict ecosystem trajectories based upon observations made in the years immediately after large, severe disturbances such as wildfires and compounded disturbances. As compounded disturbances become more common under climatically driven changes in disturbance regimes, post-disturbance ecosystem trajectories may become increasingly stochastic and unpredictable.

The ecological concept of disturbance has scarcely been applied in urban systems except in the erroneous but commonplace assumption that urbanization itself is a disturbance and cities are therefore perennially disturbed systems. We evaluate the usefulness of the concept in urban ecology by exploring how a recent conceptual framework for disturbance (Peters et al. 2011 , Ecosphere , 2, art 81) applies to these social–ecological–technological systems (). Case studies, especially from the Long‐Term Ecological Research sites of Baltimore and Phoenix, are presented to show the applicability of the framework for disturbances to different elements of these systems at different scales. We find that the framework is easily adapted to urban and that incorporating social and technological drivers and responders can contribute additional insights to disturbance research beyond urban systems.

Numerous mechanisms driving alien plant invasions have been described in a rapidly growing body of literature. However these are frequently case specific, making generalizations across species and systems difficult. A number of conceptual approaches have been proposed to help synthesize the literature, stimulating healthy debate among scientists. We build on these syntheses, presenting an expanded framework that incorporates the processes contributing to invasions, and the context within which they must interact. We also provide a model template into which the framework we develop is incorporated, illustrating both with examples. Our general framework includes three contributing processes: these are (1) the characteristics of the introduced species, (2) system context, within which the invasion takes place, and (3) the features of the receiving habitat. System context refers to the influences arising outside of the receiving environment, both spatially and temporally. Each contributing process is comprised of specific mechanisms, drawn from literature on invasion ecology and other related fields. The framework invokes relevant mechanisms for a specific species or situation. Although, a number of frameworks already consider the characteristics of the invading species or those of the receiving habitat, they seldom include all possible characteristics of both. We propose that these approaches alone are inadequate to provide a comprehensive understanding of the invasion process, without explicitly examining the context within which the invasion takes place. The model template we present relates the contributing processes described for a particular invasion, to the change in habitat from one state to another. Each of the contributing processes defined in the framework modulates the degree to which the habitat is changed. We suggest that these additional tools and the explicit inclusion of all three contributing processes, provide for further synthesis and improved understanding of invasions by alien plants.

Motivation: The Buell-Small Succession Study (BSS) is the longest running study of post agricultural succession in North America. To honor this program, a symposium at the Ecological Society of America meetings was organized to explore the state of succession theory and its contribution to the field of ecology and its application to restoration. The BSS was originally motivated by two controversies in the literature during the 1950's. The first was between a community versus and individual basis of secondary succession. The second was the validity of the Initial Floristic Composition hypothesis. Location: Hutcheson Memorial Forest, Somerset, New Jersey, USA Methods: Vegetation composition and cover has been continuously quantified in permanent plots established in 10 old fields. Continued Research Motivation: The rich data set has documented population and community dynamics and the spatio-temporal controls and historical contingencies that influence those dynamics. The regulation of community dynamics continues to be a line of inquiry as does the application of results to restoration and understanding the dynamics of non-native species. Conclusions: Long term vegetation studies are uncommon in ecology yet they are uniquely valuable for understanding system dynamics – particularly if the studies capture periodic events or system shifts such as droughts and invasions by non-native species. Resilient long term studies, of which the BSS is an example, maintain methods and data structure while allowing motivating questions to evolve along side advancements in the theoretical and conceptual realms of the field. Succession continues to serve as a basic tenet of ecology which is demonstrated by the papers making up this special issue.

While exotic plant invasions are thought to lead to declines in native species, the long-term impacts of such invasions on community structure are poorly known. Furthermore, it is unknown how exotic plant invasions compare to invasions by native species. We present data from 40 yr of continuous vegetation sampling of 10 fields released from agriculture to examine the effects of invasions on species richness. The effects of both exotic and native species invasions on species richness were largely driven by variations among fields with most species not significantly affecting species richness. However, invasion and dominance by the exotics Agropyron repens, Lonicera japonica, Rosa multiflora, Trifolium pratense and the native Solidago canadensis were associated with declines in richness. Invasions by exotic and native species during old field succession have similar effects on species richness with dominance by species of either group being associated with loss of species richness. Exotic species invasions tended to have stronger effects on richness than native invasions. No evidence was found of residual effects of invasions because the impacts of the invasion disappeared with the decline of the invading population. When pooled across species, heavy invasion by exotic species resulted in greater loss of species richness than invasion by native species. Studies of invasion that utilize multiple sites must account for variability among sites. In our study, had we not included field as a factor we would have incorrectly concluded that invasion consistently resulted in changes in species richness.

We investigated the branching structure of shrubs to quantify architectural changes along a successional gradient and to compare these findings with the behaviour of trees. Although shrubs did not differ in bifurcation ratio, it was discovered that branch length, interbranch distance and branching angle are all important display characters. Later successional shrubs had longer whole branches or interbranch lengths than early successional shrubs. In addition, there is a tradeoff between interbranch length versus whole branch length and angle size as part of the overall strategy for minimizing leaf overlap in the later successional shrubs. Early successional shrubs allocate less to long branches as a component of multilayer display of numerous, small leaves. We conclude the existence of great flexibility in the shrub form which utilizes horizontal areas rather cheaply and exploits different successional positions by manipulating the same small-scale branching features that differ within single tree crowns.

As defined by Ascher, biocomplexity results from a “multiplicity of interconnected relationships and levels.” However, no integrative framework yet exists to facilitate the application of this concept to coupled human–natural systems. Indeed, the term “biocomplexity” is still used primarily as a creative and provocative metaphor. To help advance its utility, we present a framework that focuses on linkages among different disciplines that are often used in studies of coupled human–natural systems, including the ecological, physical, and socioeconomic sciences. The framework consists of three dimensions of complexity: spatial, organizational, and temporal. Spatial complexity increases as the focus changes from the type and number of the elements of spatial heterogeneity to an explicit configuration of the elements. Similarly, organizational complexity increases as the focus shifts from unconnected units to connectivity among functional units. Finally, temporal complexity increases as the current state of a system comes to rely more and more on past states, and therefore to reflect echoes, legacies, and evolving indirect effects of those states. This three-dimensional, conceptual volume of biocomplexity enables connections between models that derive from different disciplines to be drawn at an appropriate level of complexity for integration.

Specialization can become detrimental to a discipline if it fosters intellectual isolation. A bibliographic analysis of several research areas in plant ecology (invasion biology, succession ecology, gap/patch dynamics, and global change effects on plants) revealed that plant ecologists do not regularly make use of the findings and insights of very similar studies being conducted in other research subdisciplines, nor do they try to make their findings and insights easily accessible to researchers in other areas. Invasion papers were least likely to be cross-linked (6%) with other fields, whereas gap/patch dynamics papers were most likely to be cross-linked (15%). This tendency toward intellectual isolation may be impeding efforts to achieve more powerful generalizations in ecology by reducing the number of potentially productive exchanges among researchers. In this paper, we illustrate this problem using the example of several speciality areas that study vegetation change. We argue that, rather than characterizing studies of vegetation change on the basis of what distinguishes them from one another, plant ecologists would benefit from concentrating on what such studies have in common. As an example, we propose that several speciality areas of plant ecology could be reunified under the term ecology of vegetation change. Individual researchers, journals, and ecological societies all can take specific steps to increase the useful exchange of ideas and information among research areas. Promoting rapid and more effective communication among diverse researchers may reduce the proliferation of narrow theories, concepts, and terminologies associated with particular research areas. In this way, we can expedite our understanding of the ecological mechanisms and consequences associated with plant communities. (c) 2004 Elsevier GmbH. All rights reserved.

Recent experimental and simulation results, and competition-based ecological theory, predict a simple relationship between species richness and the invasibility of communities at small spatial scales – likelihood of invasion decreases with increasing richness. Here we show data from 42 continuous years of sampling old field succession that reveal quite different dynamics of plant invasion. Contrary to experimental studies, when richness was important in explaining invasion probability, it was typically positively associated with species invasion. Invasion of several species had a unimodal response to resident species richness, which appeared to be a mixture of compositional influences and a richness effect. Interestingly, invasions by native and exotic species did not fundamentally differ. Control of species invasion in this system is individualistic, caused by a variety of community-level mechanisms rather than a single prevailing richness effect.

We compared soil N and P availability in similar oak forest stands located in either an urban or a rural area. To compare N and P availability in the urban and rural soils, we measured: (1) net N-mineralization potential and extractable inorganic P; (2) plant demand for NH4+, NO3− and PO43− using a root nutrient uptake bioassay; and (3) N and P acquisition by red oak (Quercus rubra L.) seedlings grown in the contrasting soils. Although net N-mineralization potential did not differ between the urban and rural soils, we found a 5-fold increase in net nitrification in urban compared to rural soils. Despite an apparent trend toward lower labile inorganic P in the urban versus the rural soils, the treatment means did not differ statistically. Root nutrient uptake bioassays indicated that demand for NH4+ was higher (i.e. availability lower) in the urban than in the rural soils in 1995, but did not differ in 1996. A trend toward greater root demand for NO3− in the rural than the urban soils was observed but the means did not differ significantly. We also found that demand for PO43− was significantly higher in oak roots collected from the urban compared to the rural soils in both 1995 and 1996. Total seedling N content (mg N in tissue) and leaf N concentration were significantly lower in Q. rubra seedlings grown in urban compared to rural soils, with decreased N content associated with lower leaf, stem and tap root nitrogen. Leaf and stem P contents were also significantly lower in urban-grown Q. rubra seedlings compared to rural-grown seedlings. We found no differences in tissue P concentration between urban and rural seedlings. Together our results suggest that soil P and, to a lesser degree, N availability is lower in the urban than the rural stands and that these differences are associated with anthropogenic impacts. We discuss the potential for differences in litter quality, exotic earthworms and N deposition between the urban and rural soils to lower soil N and P availability in the urban forest stands.

While exotic plant species often come to dominate disturbed communities, long-term patterns of invasion are poorly known. Here we present data from 40 yr of continuous vegetation sampling, documenting the temporal distribution of exotic plant species in old field succession. The relative cover of exotic species decreased with time since abandonment, with significant declines occurring 20 yr post-abandonment. The number of exotic species per plot also declined with time since abandonment while field-scale richness of exotics did not change. This suggests displacement occurring at small spatial scales. Life history types changed from short-lived herbaceous species to long-lived woody species for both native and exotic plant species. However, shrubs and lianas dominated woody cover of exotic plants while trees dominated native woody cover. The species richness of exotic and native species was positively correlated at most times. In abandoned hay fields, however, the proportion of exotic plant cover per plot was inversely related to total species richness. This relationship suggests that it is not the presence, but the abundance of exotic species that may cause a reduction in community diversity. While the development of closed-canopy forest appears to limit most introduced plant species, several shade-adapted exotic species are increasing within the fields. These invasions may cause a reversal of the patterns seen in the first 40 yr of succession and may result in further impacts on community structure.

We investigated the influence of differing patch types on the germination, growth, and survival of two species of tree seedlings in a catastrophic, old-growth forest windthrow. We documented environmental conditions, and experimentally introduced seeds, transplanted seedlings, and manipulated litter and herbivore access to determine the effects of patch types during initial forest regrowth. Among patch types defined by substrate condition and exposure, we found significant differential influence of patch type on germination of seeds of a common colonizer, Betula alleghaniensis. Patch types that provided exposed mineral soil allowed greater germination than other patch types, regardless of light availability. Germination of Acer rubrum varied among patch types in a fashion similar to that of B. alleghaniensis, although differences among patch types were not significant. Litter can also be the basis of patch classification. Both type and amount of litter significantly influenced emergence of seedlings, with broad-leaf litter (in amounts typical of intact forest) producing greater inhibition of emergence than amounts of conifer needles typical of intact forest. Survival and performance of fenced and unfenced Acer rubrum seedlings transplanted into different patch types reflected influences of both herbivory and competition with existing vegetation. Survival was lowest where the Acer seedlings were most apparent to mammalian herbivores. Growth of Acer seedlings was greater when they were protected from herbivory and free of competition from surrounding vegetation. These findings demonstrate that patch types offer different suitability as regeneration microsites during early tree life history stages. Because these patch types are common and easy to recognize, the differences in species' responses may be sufficiently general to apply in other disturbances of this type. If general species responses to patch types can be combined with knowledge of propagule abundance and patch abundance, it could provide a basis for improved understanding of post-disturbance regeneration, via patch influences.

Summary 1 Forest edges, which are prominent features in the north-eastern United States landscape, may control the flux of organisms between forest and non-forest habitats. Previous studies have described edge structure rather than function, as determined by interaction with such fluxes. 2 The function of the forest edge may be linked to the structure of its vegetation. We tested this hypothesis by experimentally altering the structure of the vegetation at two deciduous forest edges in Millbrook, New York, USA. Intact and thinned plots were established at each edge and we determined whether the structure of the edges influenced the flux of herbivores, as measured by herbivore damage to transplanted tree seedlings. 3 Herbivore damage to seedlings at site 1 was affected by edge vegetation structure and by distance from the edge. The edge structure effect was due to herbivory by voles, which was significantly greater in the intact than in the thinned treatment. Regardless of treatment, voles damaged seedlings only on the edge and 30–40 m from the edge and did no damage in the forest interior (90–100 m), whereas deer damaged significantly more seedlings in the forest interior than on the edge. At site 2, where vole damage was concentrated on the edge, damage to seedlings was affected only by distance from the edge, not edge structure. 4 The two dominant herbivores, white-tailed deer and meadow voles, preferentially damaged different seedling species. In addition, tree seedlings browsed by deer resprouted more frequently than those clipped by voles. Our results suggest that both edge structure and distance from the edge influence herbivore activity and, as a result, influence the spatial arrangement, density and composition of populations of tree seedlings during regeneration in forest fragments.

As the density and species composition of insects may change in relation to distance from the forest edge, the role of herbivory in tree establishment may also change across edges. To determine the importance of insect herbivory in tree establishment, insect densities were experimentally altered at different distances from the forest edge. Plots were established at three distances from the edge, with plots located in forest, edge, and field habitats. In half of each plot, insect densities were reduced by insecticide application. Seeds of two tree species, Acer rubrum and Fraxinus americana, were planted into each plot in 1995. The experiment was repeated in 1996 with the addition of Quercus palustris and Quercus rubra. Distance from the forest edge was the most important factor in determining seedling emergence and mortality. Overall seedling performance increased from field to edge to woods, although responses varied among species. In 1995, a drought year, insect removal increased emergence and decreased mortality of tree seedlings. In 1996, a year with normal precipitation, insect removal had much less effect on A. rubrum and F. americana. For the two Quercus species, mortality was reduced by insect removal. The tree species differed in their susceptibility to insect herbivory, with Acer rubrum the most susceptible and Fraxinus americana the least. Herbivory by insects was shown to have the potential to affect both the composition and spatial pattern of tree invasions. Herbivore importance differed greatly between the two years of the study, making the interaction between insects and tree seedlings variable both in space and time.

We studied the patterns and mechanisms of regeneration of a 400-ha wind- throw in an old-growth beech-hemlock forest caused by a tornado on 31 May 1985. Starting in 1986, and over a period of six growing seasons, we recorded percent cover and density of woody stems, and monitored seedling demography of nearly 5000 seedlings in the windthrow and adjacent forest. Plant community response to the disturbance was dramatic: by August of 1986, species richness, tree seedling density and total percent cover were significantly greater in the windthrow than in the adjacent forest. Shade-intolerant herbs (e.g., Erechtites hieracifolia) and shrubs (e.g., Rubus allegheniensis) established and rapidly increased in abundance during the first 3 yr, but began declining by the 5th yr of the study. Tree seedlings established in decreasing amounts through the 6 yr of the study, and the young tree canopy was dominated in 1991 by seedlings and sprouts that established prior to 1987. Fagus grandifolia, a shade-tolerant species that established via advanced regeneration, was dominant the first 3 yr, but was surpassed in the 5th yr by Betula alleghaniensis, a species of intermediate tolerance that established from seed germination just before or shortly after the disturbance. Tsuga canadensis seedling densities were initially high, but deer browsing prevented substantial growth and a drought in 1988 caused heavy mortality of browsed seedlings. Regeneration thus differed from the predictions of the gap and Hubbard Brook models of forest regeneration (which predict dominance by shade-intolerant species), and the severity model (which predicts dominance by shade-tolerant species). The differences point out important influences of availability of propagules and the impact of herbivory; and the need for more attention to models that incorporate multiple contingencies.

We explored local variation in rate of plant succession by computing three contrasting measures of change for 10 New Jersey old fields through the first 18 yr after abandonment. We found that all three measures, euclidean distance in ordination space, turnover analysis, and regression analysis of species loss from the initial cohort, showed the same trend of decrease in the rate of plant succession with time. However, we did not find a monotonic decrease, and the specific years of transient increase differed with the analytical measure used. Both turnover and regression analysis showed that those old fields that had previously been planted in orchard grass (Dactylis glomerata) had a much lower initial change than fields previously in soybeans, or soybeans and sorghum, but this difference dissolved after 10 yr of succession. The study fields compared favorably to other mesic successions and agreed with the general pattern of more rapid rate decrease in mesic compared to xeric successions. Our results echo ...

We surveyed the type of treefall (snapped or uprooted), and for snapped trees the height of break and sprouting tendency, in a catastrophic windthrow in an old-growth hemlock-hardwoods forest. Two-thirds of the 630 trees sampled were uprooted and one-third snapped. Trunk size (diameter at breast height) was more important than species in determining whether trees snapped or uprooted. Tree characteristics interacted with storm meteorology to determine the overall pattern of treefalls. We found that only 25% of the snapped trees sprouted in the four growing seasons after the tornado, and that the tendency to sprout varied significantly among species. Of those snapped trees that sprouted, only 68% were alive at the end of the fourth growing season. In contrast to small gaps in the tropics, where sprouted trees contribute a substantial portion of the post-disturbance canopy, sprouting will be of little importance in the re-establishment of forest in this catastrophic windthrow.

We studied the effects of litter ofthe annual grass Setariafaberii, the perennial herb Solidago spp. (mostly S. canadensis), and leaves of the hardwood tree Quercus alba on a successional plant community. We also assessed light interception by these litter types in the laboratory. Light extinction followed the Beer-Lambert exponential law. Solidago litter had the highest transmittance constant and Quercus litter the lowest. The three types of litter produced different light mosaics at the microsite (0.8 cm diameter) scale. In the field, all three litter types affected community structure, but the effect of Quercus was the strongest. Litter reduced the density of the two dominant grasses, Setaria faberii and Panicum dichotomiflorum. Quercus and Setaria litter resulted in biomass compensation (i.e., fewer but larger individuals) by S. faberii but not by P. dichotomiflorum, which probably was always outcompeted by S. faberii. Solanum carolinense, the main dicot in the community, was unaffected by litter addition. Setaria and Solidago litter enhanced the establishment of Erigeron annuus, but Quercus litter reduced it. Litter reduced the number of flowering individuals of S. faberii and the number of seeds per plot; Quercus litter increased the production of seeds per individual. We conclude that species-dependent effects of litter on plant populations may significantly alter interspecific interactions and change plant community structure through direct and indirect effects.

We discuss the dynamics of plant litter, the effects of litter on the chemical and physical environment, the direct and indirect effects of plant litter on plant populations and communities, and different adaptative traits that may be related to litter accumulation. The production of litter depends primarily on the site productivity, but other properties of the environment, as well as chance, may introduce important variation. The existence of time lags between the production of plant organs and their transformation into litter appears as a relevant character of litter dynamics seldom included in models. Herbivory, and other processes that destroy biomass or reduce productivity, may reduce the amount of litter produced. The destruction of litter encompasses a complex of interactions. The main processes, including physical and chemical degradation, consumption by invertebrates and decomposition, are differentially affected by the environment and by the physical and chemical characteristics of the litter itself. The relative importance of those processes varies among systems.

This quote captures the spirit of the new urban emphasis in the US Long-Term Ecological Research (LTER) network. We know now that Earth abounds with both subtle and pronounced evidence of the influence of people on natural ecosystems (Russell 1993, Turner and Meyer 1993). Arguably, cities are the most human dominated of all ecosystems. Recent calls for studies on “human-dominated ecosystems” (Vitousek et al. 1997) finally have been heeded, over 60 years after Tansley penned his warning, with the addition of two metropolises (Phoenix and Baltimore) to the LTER network. In this article, we describe an emerging approach to understanding the ecology of urban areas by contrasting these two metropolises, and we present a call to action for ecologists to integrate their science with that of social scientists to achieve a more realistic and useful understanding of the natural world in general and its ecology in particular (Pickett and McDonnell 1993, Ehrlich 1997). We begin by framing a conceptual basis for the study of urban ecological systems: the rationale, contrasting approaches, and special considerations for including human interactions at different scales and in a spatial context. We then discuss the application of our conceptual approach by comparing site conditions and initial research results in Baltimore and Phoenix. We conclude with a summary and synthesis of implications for the integration of social and ecological sciences.

Ecologists and paleoecologists have become increasingly aware that the temporal and spatial scales of the two disciplines overlap considerably and provide complementary information. Pollen and macrofossil evidence from thousands of radiocarbon-dated sites worldwide indicate that species respond to environmental change independently, that communities are relatively open assemblages, and that instability and change characterize Quaternary environments and biotas. The extended temporal view provided by paleoecology also enables detection of the occurrence, intensity and changing frequency of periodic and unique events such as disturbances and environmental fluctuations. As these insights contribute to our understanding of a dynamic environment and biota, they may help to increase our ability to anticipate future changes in communities.

The roles of disturbance and resource availability in influencing species richness and plant abundance were examined by conducting a factorial experiment for 2 yr in an old field in New Jersey, USA, dominated by goldenrods (Solidago spp.). Replicate plots were treated by (1) adding macronutrients, (2) adding water, (3) tying back tall herbs to increase light to ground layer species, and (4) creating gaps in the vegetation by disturbing the soil with a hand trowel early in the growing season. Nutrient additions increased the cover of the herbaceous canopy and enhanced the dominance of Solidago spp. Water additions also increased canopy cover during the second growing season, a year with low seasonal rainfall. Light enrichment as a consequence of canopy manipulations resulted in the competitive release of subcanopy species, primarily Fragaria virginiana. Other subcanopy dominants were limited by different resources; Hier- acium pretense responded most strongly to water and Rumex acetosella to nutrients. These differences in limiting resources may be responsible for the continued coexistence of these three subcanopy species. Additionally, we observed marked seasonal and yearly fluctuations in the severity of resource limitation which might serve to favor different species in this community at different times. Species richness was decreased by light enrichment alone, whereas light with irrigation interacted to increase species richness. Nutrient additions had little direct effect on species richness. A single small disturbance at the beginning of the growing season had little impact on species richness or abundance. Relative to the influence of resources, small-scale dis- turbance plays only a minor role in this community. Our results suggest that nonequilibrium organization of old-field communities can occur via a shifting resource base in the absence of natural disturbances.

An emerging generalization from theoretical and empirical studies on conservation biology is that high levels of environmental stochasticity increase the likelihood of population extinction. However, coexistence theory has illustrated that there are circumstances under which environmental stochasticity can increase the chance of population persistence. These theoretical studies have shown that the sign of the effect of environmental stochasticity on population persistence is determined by interactions between life history and environmental stochasticity. These interactions mean that the stochastic and deterministic rates of population growth might differ fundamentally. Although difficult to demonstrate in real systems, observed life histories and variance in the vital rates of populations suggest that this phenomenon is likely to be common, and is therefore of much relevance to conservation biologists.

Conservation biology has traditionally focused on the fine scale and the species level of biological organization (Soule and Wilcox 1980), and biotic conservation is only one of the various goals that has directed the preservation and management of natural areas and resources. Resource management goals have largely been utilitarian or commercial, such as the maintenance of large areas for watersheds; preservation of marshes, bogs, and seasonal wetlands for flood water storage, nursery areas, and flow continuity in river systems, and the sustained yield of forests, fisheries, and wildlife (Temple et al. 1988; Aplet, Laven, and Fiedler in press). There have, however, been both practically and theoretically motivated calls for widening the focus to include scales beyond that of the individual site and levels of organization above that of the species (Soule 1989; Western 1989) to approach new goals. This essay will explore the relationship of this new frontier for conservation biology to advances in ecology. Much of the new ecological focus of conservation biology is driven by the shift in the overarching paradigm of ecology itself. We begin by defining the concept of paradigm and characterizing the classical paradigm of ecology. The classical paradigm had definite implications for conservation, and these are shown to be problematical. Therefore, we outline the contemporary paradigm in ecology and show how the science of ecology has been affected by the shift in paradigm.

Closed canopy vegetation often prevents the coloni- zation of plant species. Therefore the majority of plant species are expected to appear at the initial phase of post-agricultural succession in mesic forest environment with moderate levels of resources. This hypothesis was tested with data from the Buell- Small Successional Study, NJ, USA, one of the longest continu- ous fine-scale studies of old-field succession. The study started in 1958, including old fields with different agricultural histories, landscape contexts, and times of abandonment. In each year of the study, the cover values of plant species were recorded in 48 permanent plots of 1 m 2 in each field. We analysed the temporal patterns of colonization at plot scale and related these to precipi- tation data and other community characteristics. The number of colonizing species decreased significantly after ca. 5 yr, coinciding with the development of a continuous canopy of perennial species. However, species turnover remained high throughout the whole successional sequence. The most re- markable phenomenon is the high inter-annual variation of all studied characteristics. We found considerable temporal collapses of vegetation cover that were synchronized among fields despite their different developmental stages and dis- tinctive species compositions. Declines of total cover were correlated with drought events. These events were associated with peaks of local species extinctions and were followed by increased colonization rates. The transitions of major suc- cessional stages were often connected to these events. We suggest that plant colonization windows opened by extreme weather events during succession offer optimum periods for intervention in restoration practice.

CADENASSO, M.L., S.TA. PIcKETr (Institute of Ecosystem Studies, Box AB, Millbrook, NY 12545) AND PJ. MoRiN (Department of Ecology, Evolution, and Natural Resources, 14 College Farm Road, Cook College, Rutgers University, New Brunswick, NJ 08901). Experimental test of the role of mammalian herbivores on old field succession: community structure and seedling survival. J. Torrey Bot. Soc. 129:228-237. 2002.-When elucidating plant community dynamics, investigators have focused on plant-plant and plant-environment interactions and on the characteristics of individual plants. The role of animals has been relatively neglected. Herbivory may affect vegetation dynamics by altering community composition and structure and by influencing the performance of species. Therefore, herbivory can be an important factor in succession. We experimentally tested the role of mammalian herbivores in two fields at the Hutcheson Memorial Forest Center in central New Jersey. Both fields were released from active cultivation, one at the beginning of the experiment and the other 17 years earlier. Large exclosures (5 X 5 m) were used to exclude mammalian herbivores, and open units of the same size were used as controls. The effect of herbivore exclusion on the structure and composition of the plant community was analyzed. In addition, tree seedlings of three species-Acer rubrum, Cornus florida, and Juniperus virginiana-were planted in the experimental units in both fields. These species typically invade earlyto mid-successional old fields. The survival of these individuals was calculated and, using height as an indicator, their performance assessed. Herbivore exclusion primarily influenced the structure of the plant community rather than its composition. Percent cover of species and species richness of all plants were not affected by herbivory, but in one field the relative cover of exotics was greater in the exclosures. The height profile of the plant community in the exclosures was significantly taller than the community in the open plots. The survival and growth of planted A. rubrum and C. florida seedlings was significantly greater when protected from herbivores. In contrast, the survival and growth of J. virginiana seedlings was not affected by herbivores. Growth and survival of J. virginiana did differ by the field they were planted in, suggesting that physiological constraints may be more important than herbivory for this species. These results reenforce that herbivorous mammals play a key role in old field succession, particularly in the pivotal shift of dominance from herbaceous to woody cover.

Efford 1xEnvironmental stochasticity cannot save declining populations. Efford, M. Trends Ecol. Evol. 2001; 16: 177Abstract | Full Text | Full Text PDFSee all References1 raised valid concerns regarding the conclusions of our paper 2xPredicting extinction risks for plants: environmental stochasticity can save declining populations. Higgins, S.I. et al. Trends Ecol. Evol. 2000; 15: 516–520Abstract | Full Text | Full Text PDF | PubMed | Scopus (62)See all References2. Here, however, we explain why these concerns do not change our original conclusion. In our paper, we use a log-normal distribution to describe the recruitment rate of a plant population. Such a distribution seems appropriate for plants because recruitment rates must be ≥0 and are typically right skewed. In our simulations, we held the mean of the logged recruitment constant and explored the effect of increases in variance on population growth. Using this approach, we concluded that increasing levels of variance in recruitment rates can promote population growth for populations that can store reproductive potential between generations. Efford questions our conclusion by pointing out that the mean (μ) of a log-normal random variable X is not the same as, μL, the mean of the transform Y=log(X). In particular, because μ = exp(μL+0.5σL2), where σL2 is the variance of the transformed variable, the difference between μ and μL increases as σL2 increases. Efford argues that this relationship between μ and μL means that effects we attributed to σL2 while holding μL constant are, in fact, attributable to increases in μ and not increases in σ2. It has been demonstrated 3xThe demography of chance extinction. Goodman, D. : 11–34CrossrefSee all References, 4xModelling extinction of small populations. Wissel, C. et al. : 67–103See all References, however, that the effect of environmental variance on the mean time to extinction is influenced by not just μ, but also by the ratio (α) of 2μ to σ2 such that,(1)where C is a constant that, depending on the application, could describe patch size, carrying capacity or resource availability. We can use the relationships μ=exp(μL+0.5σL2) and σ2=exp(2μL+σL2)(exp{σL2}−1) to convert from the transform parameters (μL,σL2) into the original scale parameters (μ,σ2) and thereby check whether our conclusion remains contrary to this theory:(2)Substituting α from Eq. (2)Eq. (2) into Eq. (1)Eq. (1), it can be seen that, if we increase σL2 and hold μL constant, α decreases and the time to extinction shortens. This means that, even though μ increases as σL2 increases, existing theory 3xThe demography of chance extinction. Goodman, D. : 11–34CrossrefSee all References, 4xModelling extinction of small populations. Wissel, C. et al. : 67–103See all References would still predict that the increase in μ is insufficient to compensate for the corresponding increase in σ2. Consequently, our result cannot be explained by the relationship between μ and μL.We should also consider the implications of Efford's point for the application of population viability analyses. From a practical perspective, we should aim to simulate environmental variance so as to mimic nature closely; this means selecting the parameters that are most consistent with the empirical data. Although data limitations force us to speculate, we would anticipate that for empirical data on recruitment μ will increase as σ2 increases and, hence, that the approach we adopted is more consistent with empirical data. In summary, from both a theoretical and empirical perspective, our conclusion remains that for plants with life histories that allow the storage of reproductive potential between generations, increasing variance in recruitment can increase the population growth rate because these populations can persist long enough to benefit from occasional bumper recruitment years. Conversely, for plants that cannot store reproductive potential between generations, increasing variance in recruitment must reduce the population growth rate because the effect of a zero, or a series of low recruitment generations would be terminal for such plant populations.

One of the most important ways the discipline of ecology can advance is to facilitate synthesis and integration. In turn synthesis can be stimulated by attending to the cultural factors that promote and constrain the process. Culture refers to special training and development or to the totality of socially transmitted behavior patterns, arts, beliefs, institutions, and all other products of human work and thought. Certain aspects of the general concept of culture can be applied to scientific practice, help explain the role synthesis currently plays in ecology, and identify ways to facilitate synthesis in the discipline. Potential cultural barriers to synthesis include a dominance by narrow, reductive methods without a well-developed counterpoise of integrative and creative habits of mind. The literature on the creative process provides ecologists with a model of the long-term commitment required, the use of analogy and radical juxtaposition, and a vanquishing of the eureka myth. Vertical integration, linkage across a diverse field, cross-disciplinary fertilization, and increased use of the diversity among the community of ecologists can be stimulated by a culture of synthesis.

Ascertaining whether an ecosystem, a community or any other ecological unit has completely changed. whether it has been destroyed or whether it remains the same is a common task for ecologists. At the heart of this task is the question of what constitutes the identity of a unit through time. which we call self-identity. The determination of self-identity requires that we know what the essence of an ecological unit is. This is highly dependent on the definition and specification of the ecological unit of interest, which in turn, depends upon the question(s) being asked. We develop a framework to provide unambiguous definitions of ecological units. The components of this framework are different criteria describing the spatial or process-based boundaries of a unit, the degree of required internal relationship and the level of abstraction t an ecological unit is specified. These criteria can be represented in a three-dimensional graphical model which locates the definitions of ecological units within a conceptual volume. We describe the application of the model for the assessment of the self-identity of ecological units through time. The specification of ecological units and the possibility of assessing the self-identity of ecological objects are also dependent upon the specific ecological theories applied (and upon issues that reside outside the realm of pure ecological science). The universe of meaningful questions and of ecological units is constrained by both societal and scientific interests. We argue that a precise specification of ecological units is necessary for advancing both ecological understanding and its application.

Because the fate of seed rain is critical to understanding the invasion of old fields by trees, we investigated spatial and temporal variation and difference among six tree species in seed predation. Very few Juniperus virginiana seeds were lost to predation but predation for all other test species was reduced by additions of Quercus litter and solidago litter. Carya tomentosa seed predation was also reduced by increasing distance from the forest edge. In addition to litter and distance effects, differing starting densities of Acer rubrum seeds significantly affected predation rates but Fraxinus americana rates were not affected (...)

The pattern of significant associations between plants was examined in ten old fields during 31 years of succession by calculating rank correlations for species pairs in each old field during each sample year. Three hypotheses were tested concerning the dynamics of species interactions through succession, and correspondence was explored between the pattern of association and published results from field and glasshouse experiments. The proportion, number and level of significance of associations between plants all declined with time. Annuals and biennials had a higher portion of significant associations and more positive associations than perennial species (...)

MYSTER, R. W. (Terrestrial Ecology Division, University of Puerto Rico, San Juan, PR 00936), AND S. T. A. PicyErr (Institute of Ecosystem Studies, Mary Flagler Cary Arboretum, Millbrook, NY 12545). Effects of palatability and dispersal mode on spatial patterns of trees in oldfields. Bull. Torrey Bot. Club 119: 145-151. 1992.-The spatial patterns of tree stems in 480 plots were examined on a series of ten experimental oldfields adjacent to an old growth forest over a 31 -year period. Tree stem density decreased exponentially with distance from the forest edge early in succession. Later in succession the predictive power of the negative exponential model decreased, suggesting that processes such as animal seed dispersal within a field become important. Species palatable to herbivores were denser than unpalatable species close to the forest edge which implies that consumer pressure originates in the field rather than in the forest. Stems of all species were clumped on the scale of the 1 m2 plots. However, stems of bird-dispersed species showed the highest degree of clumping as expected if perches serve as recruitment foci. Results suggest that spatial patterns of tree seedlings in oldfields are significantly influenced by successional age, herbivory, dispersal vector and proximity to forest edge.

We examined the PCA ordination scores of 10 old fields through 22 yr of succession. Clonal species Dactylis glomerata and Hieracium pratense had the largest correlation with the first principal component in the 1st 18 yr of succession. After 18 yr, Rosa multiflora and Toxicodendron radicans had the largest correlation with the first principal component. Successional pathways of the 10 old fields show great variation in early succession but variation of pathway decreases later. Comparison with a null model reveals that the trend of increasing similarity of old fields through time is real and convergence is occurring. Because mechanisms causing convergence might include the effect of initial abandonment conditions, we divided the 10 old fields into groups defined by the last crop, season of abandonment, year of abandonment, and mode of abandonment (plowed or not). Analysis of the minimum spanning tree needed to connect the old fields of the same age in ordination space showed that the greatest difference between groups was in the 1st 8 yr between old fields that had Dactylis glomerata (orchard grass) as a last crop and those that did not. Of common abandonment conditions, last crop greatly influences successional pathways and convergence in these old fields.

Island biogeographic theory has been applied to the design of nature reserves. However, immigration, which is important in maintaining species equilibrium on true islands, will not contribute significantly to the maintenance of equilibrium on reserves in the future because of the disappearance of recolonisation sources. Consequently, extinction becomes the dominant population process, and the internal disturbance dynamics become the critical design feature of reserves. The design of reserves should be based on ‘minimum dynamic area’, the smallest area with a natural disturbance regime which maintains internal recolonisation sources and hence minimises extinctions. Determination of minimum dynamic area must be based on knowledge of disturbance-generated patch size, frequency, and longevity, and the mobilities of the preserved species. These features have not all been explicitly considered in the previous island biogeographic design recommendations.

Single- (33–37 m2) and multi-tree (51–151 m2) gaps were created in an Allegheny Plateau northern hard-woods forest to investigate environmental and herb layer response to canopy opening. After gap creation, noon light on clear summer days was brightest north of opening center. At other times of the day, and when skies were overcast, there was no difference in the light quantity beneath opened and closed canopy. Nor was the distribution of soil moisture or of soil or air temperature greatly affected by gap creation. Species establishment tended to be higher near opening centers; otherwise, there was no pronounced effect of canopy opening on plant cover or species richness during the first four years after gap creation. Biotic responses were not significantly correlated with any environmental factor.