Decomposing biodiversity data using the Latent Dirichlet Allocation model,a probabilistic multivariate statistical method

We propose a novel multivariate method to analyse biodiversity data based on the Latent Dirichlet Allocation (LDA) model. LDA, a probabilistic model, reduces assemblages to sets of distinct component communities. It produces easily interpretable results, can represent abrupt and gradual changes in composition, accommodates missing data and allows for coherent estimates of uncertainty. We illustrate our method using tree data for the eastern United States and from a tropical successional chronosequence. The model is able to detect pervasive declines in the oak community in Minnesota and Indiana, potentially due to fire suppression, increased growing season precipitation and herbivory. The chronosequence analysis is able to delineate clear successional trends in species composition, while also revealing that site‐specific factors significantly impact these successional trajectories. The proposed method provides a means to decompose and track the dynamics of species assemblages along temporal and spatial gradients, including effects of global change and forest disturbances.     

Assessing long-term changes in tropical forest dynamics: a first test using tree-ring analysis

There is growing evidence that tree turnover in tropical forests has increased over the last decades in permanent sample plots. This phenomenon is generally attributed to the increase in atmospheric CO₂, but other causes cannot be ruled out. A proper evaluation of historical shifts in tree turnover requires data over longer periods than used so far. Here, we propose two methods to use tree-ring data for detecting long-term changes in tree turnover. We apply these methods to two non-pioneer tree species in a Bolivian moist forest. First, we checked for temporal changes in the frequency of growth releases to determine whether this frequency has increased over time. Second, we calculated the degree of temporal autocorrelation—a measure that indicates temporal changes in growth rates that are likely related to canopy dynamics—and checked for changes in this parameter over time. In addition, we performed analyses that corrected for ontogenetic increases in the measures used by analyzing residuals from size–growth relations. No evidence for the occurrence of a large-scale disturbance was found as we did not observe synchronization in the occurrence of releases in time. For both species, we did not detect changes in autocorrelation or release frequency over the last 200–300 years. Only in one size category, we found increased release frequency over time, probably as a result of a remaining ontogenetic effect. In all, our analyses do not provide evidence for long-term changes in tree turnover in the study area. We discuss the suitability of the proposed methods.     

Temporal trends of genetic diversity in European barley cultivars (<Emphasis Type="Italic">Hordeum vulgare</Emphasis> L.)

The changes of genetic diversity over time were monitored in 504 European barley cultivars released during the 20th century by genotyping with 35 genomic microsatellites. For analysis, the following four temporal groups were distinguished: 1900–1929 (TG1 with 19 cultivars), 1930–1949 (TG2 with 40 cultivars), 1950–1979 (237 cultivars as TG3), and 1980–2000 (TG4 consisting of 208 cultivars). After rarefaction of allelic diversity data to the comparable sample size of 18 varieties, of the 159 alleles found in the first group (TG1) 134 were retained in the last group (TG4) resulting in a loss of only 15.7% of alleles. On the other hand 51 novel alleles were discovered in the group representing the last investigated time period (TG4) in comparison with the TG1. Novel alleles appeared evenly distributed over the genome, almost at all investigated genomic loci, with up to five such novel alleles per locus. Alleles specific for a temporal group were discovered for all investigated time periods, however analysis of molecular variance (AMOVA) did not reveal any significant population structure attributable to temporal decadal grouping. Only 2.77% of the total observed variance was due to differences between the four temporal groups and 1.42% between individual decades of the same temporal group, while 95.81% of the variance was due to variation within temporal groups. The distinction between two-rowed and six-rowed genetic types accounted for 19.5% of the total observed variance by AMOVA, whereas the comparison between ‘winter’ and ‘spring’ types accounted for 17% of the total observed variation. The analysis of linkage disequilibrium did not reveal statistically significant differences between the temporal groups. The results indicated that the impact of breeding effort and variety delivery systems did not result in any significant quantitative losses of genetic diversity in the representative set of barley cultivars over the four time periods.     

The Value of Long-Term Stream Invertebrate Data Collected by Citizen Scientists

The purpose of this investigation was to systematically examine the variability associated with temporally-oriented invertebrate data collected by citizen scientists and consider the value of such data for use in stream management. Variability in invertebrate data was estimated for three sources of variation: sampling, within-reach spatial and long-term temporal. Long-term temporal data were also evaluated using ordinations and an Index of Biotic Integrity (IBI). Through two separate investigations over an 11-year study period, participants collected more than 400 within-reach samples during 44 sampling events at three streams in the western United States. Within-reach invertebrate abundance coefficient of variation (CV) ranged from 0.44–0.50 with approximately 62% of the observed variation strictly due to sampling. Long-term temporal CV ranged from 0.31–0.36 with 27–30% of the observed variation in invertebrate abundance related to climate conditions (El Niño strength) and sampling year. Ordinations showed that citizen-generated assemblage data could reliably detect differences between study streams and seasons. IBI scores were significantly different between streams but not seasons. The findings of this study suggest that citizen data would likely detect a change in mean invertebrate density greater than 50% and would also be useful for monitoring changes in assemblage. The information presented here will help stream managers interpret and evaluate changes to the stream invertebrate community detected by citizen-based programs.     

Stopover of migrating birds: simultaneous analysis of different marking methods enhances the power of capture–recapture analyses

Analyses of stopover parameters of migrating birds with Cormack–Jolly–Seber (CJS) capture–recapture models often suffer from low precision due to sparse data sets. Low recapture rates result in low power to detect violations of the underlying assumptions and factors influencing stopover behaviour. We studied stopover behaviour of Palearctic migrant passerines in an oasis in Mauritania, West Africa. Using capture–recapture data and systematic observations of colour-ringed birds, we analysed the effect of increased sample size on probability of stay and recapture probability and the influence of a possible trap response on these parameters. We analysed capture–recapture data with the conventional CJS model and compared the results with those from a multistate model using in addition resighting data. The analyses including resighting data resulted in a higher precision of the estimates of the probabilities of stay compared to analyses using only capture–recapture data of the same individuals. Moreover, the power to detect transients was substantially enhanced. Capture had no effect on the estimates of probability to stay and recapture probability; birds did not leave the stopover site or avoid nets as a reaction to capture. The estimates of probability of stay were up to 15.7% higher when resighting data were included, probably due to the higher power to detect transients and the elimination of the bias induced by non-random temporary emigration when both data types are considered. As a consequence, stopover duration would have been underestimated when only the capture–recapture data were available. We conclude that additional resightings of colour ringed birds can produce more accurate results needed for enhancing our understanding of stopover ecology of migrants. Electronic supplementary material Supplementary material is available in the online version of this article at and is accessible for authorized users. From the Swiss Ornithological Institute project on bird migration across the Sahara.     

Effects of Watershed History on Dissolved Organic Matter Characteristics in Headwater Streams

Dissolved organic matter (DOM) is recognized as a major component in the global carbon cycle and is an important driver in aquatic ecosystem function. Climate, land use, and forest cover changes all impact stream DOM and alter biogeochemical cycles in terrestrial environments. We determined the temporal variation in DOM quantity and quality in headwater streams at a reference watershed (REF), a watershed clear-cut 30 years ago (CC), and a watershed converted to a white pine plantation 50 years ago (WP) at the US Forest Service, Coweeta Hydrologic Laboratory, in the Nantahala Mountains of western North Carolina, USA. Average stream dissolved organic carbon (DOC) concentrations in CC or WP were 60 and 80% of those in REF, respectively. Stream DOM composition showed that the difference was mainly due to changes in humic-like components in chromophoric DOM. In addition, excitation–emission matrix fluorescence data with parallel factor analysis indicate that although the concentration of protein-like components did not differ significantly among watersheds, their relative abundance showed an enrichment in CC and WP compared to REF. The ratio of humic acid-type to fulvic acid-type components was highest and lowest at REF and WP, respectively. Our data suggest that forest ecosystem disturbance history affects the DOM quantity and quality in headwater streams over decades as a result of changes in watershed soil organic matter characteristics due to differences in organic matter inputs.     

基于动态当量的乐山市生态系统服务价值时空演变及驱动力研究

生态系统服务价值评估作为土地资源合理配置、生态环境保护和生态补偿标准制定的重要前提,是当前的研究热点之一。以乐山市为研究区域,基于2000年、2005年、2010年、2015年4期土地利用数据,运用生态系统服务价值动态当量法、地理探测器模型等方法,定量分析乐山市2000—2015年间生态系统服务价值时空变化规律及驱动机制。结果表明:(1)2000—2015年乐山市生态系统服务价值整体呈下降趋势,仅在2005—2010年有所回升,总量共减少了23.22亿元,主要与林地大量减少和建设用地大量增加有关;(2)空间上呈现西南高、东北低的分布态势,且各区域变化不同,西南方向山地以下降为主,东部平原丘陵以上升为主,河流两侧上升明显;(3)影响生态系统服务价值时空分布的主导因素为土地利用、NPP、降水量和坡度,除此之外气温、高程、土壤类型也有较大决定力;(4)自然因子决定力大于社会经济因子,但社会经济因子的影响越来越突出。研究揭示了乐山市生态系统服务价值的时空变化规律及驱动因子,为该地区土地利用规划与生态环境保护机制的建立提供了科学参考。     

“Less is better” and “only above threshold”: Two incompatible paradigms for human toxicity in life cycle assessment?

The absence of spatial and temporal information in the data from a typical Life Cycle Inventory puts constraints on the possibilities of subsequent Life Cycle Impact Assessment to predict actual impact. Usual methods for Life Cycle Impact Assessment (often referred to as “less is better” methods) make only limited use of spatial and temporal information, because they predict concentration increases rather than full concentrations. As a consequence it does not seem possible to evaluate whether a threshold value is surpassed. The resulting poor accordance between the predicted impact and the expected occurrence of actual impact is a major problem. This problem is particularly relevant for human toxicity assessment, since the probability of surpassing thresholds here traditionally is the main point of attention. A considerable group of practitioners suggests to follow an “only above threshold” principle by introduction of assessment tools from risk assessment and environmental impact assessment in LCA. Intensive debate is going on about possibilities and limitations of “less is better” and “only above threshold”. The debate is obscured by two underlying discussions (about no-effect-levels and about data-availability) that are partly, but not fully intertwined. Both principles tend to be given fixed positions in these discussions, and are therefore often put forward as fundamentally different and incompatible with each other. This article entwines the discussions, shows parallels between both principles, and uses these parallels to present a new method for Life Cycle Impact Assessment of human toxicity from air emissions that — with limited data requirement from Life Cycle Inventory — can take as well threshold evaluation and spatial source-differentiation into account.     

Power, precaution, Type II error and sampling design in assessment of environmental impacts

Increasingly, environmental managers attempt to incorporate precautionary principles into decision making. In any quantitative analysis of impacts, precaution is closely related to the power of the analysis to detect an impact. Designs of sampling to detect impacts are, however, complex because of natural spatial and temporal variability and the intrinsic nature of the statistical interactions which define impacts. Here, pulse and press responses and impacts that affect time courses (temporal variance) were modelled to determine the influences of increasing temporal replication—sampling more times in each of several longer periods before and again after an impact.Increasing the number of control or reference locations and number of replicate sample units at each time and place of sampling investigated the influence of spatial replication on power. From numerous scenarios of impacts, with or without natural spatial and temporal interactions (i.e. not caused by an impact), general recommendations are possible. Detecting press impacts requires maximal numbers of control locations. Shorter-term pulse impacts are best detected when the number of periods sampled is maximized. Impacts causing changes in temporal variance are most likely to be detected by sampling with the greatest possible number of periods or times within periods.To allow precautionary decision making, the type of predicted impact should be specified with its magnitude and duration. Only then can sampling be designed to be powerful, thereby allowing precautionary concepts to be invoked.     

Mammalian Faunal Dynamics During the Last 1.8 Million Years of the Cretaceous in Garfield County, Montana

This study provides an analysis of biotic change in successive mammalian communities during the last 1.8 million years of the Cretaceous (67.3–65.58 Ma) from the Hell Creek Formation in Garfield County, Montana. Results show changes in relative abundances of species, mean individual body size, and to some extent taxonomic composition through the Hell Creek Formation. These results are interpreted as “normal” mammalian responses to fluctuating temperatures during the latest Cretaceous. By contrast, the extinction of 22–27 mammalian species at or near the Cretaceous-Tertiary (K-T) boundary cannot be explained by the coincident cooling interval alone. At the scale of temporal resolution available, these fossil data are inconsistent with an extended gradual pattern of extinction (linear-response) and are most consistent with either a non-linear response pattern for the K-T extinction, resulting from the accumulated stress of multiple long- and short-term environmental perturbations (e.g., climate change, sea-level regression, volcanism, an extraterrestrial impact), or a single, short-term cause (an extraterrestrial impact).     

Theoretical and experimental study of growth and remodeling in the developing heart

A theoretical model is presented for growth and remodeling in the developing embryonic heart. The model is a thick-walled tube composed of two layers of orthotropic pseudoelastic material. The analysis includes stress and strain dependent volumetric growth, with changes in material properties specified to reflect the evolving structure of the heart wall. For use in model validation, experimental measurements of ventricular opening angles are reported for 3–4-day old chick embryos under control and pressure overload conditions. Owing to changes in residual stress in the overloaded heart, the opening angle decreased from 31 ± 10° to −8 ± 12° (mean ± SD) within 12 h and then increased slightly. The opening angle at 12 h was significantly less than the control value. With an appropriate choice of parameters, the model yields reasonable agreement with these and other published opening angle data, as well as with temporal changes in lumen radius, wall thickness, epicardial strains, and pressure–volume curves during development before and after birth. Received: 26 November 2001 / Accepted: 21 December 2001     

Use of Life Cycle assessment in the procedure for the establishment of environmental criteria in the catalan ECO-label of leather

Life Cycle Assessment (LCA) has been used to detect the environmental ‘hot spots’ in the chrome-tanned bovine leather industry. We have studied those stages in the life cycle of leather, which occur ‘from cradle to gate’. The production chain studied starts with the agricultural products (fertiliser and pesticide production is also included) needed for cattle raising, it is followed by the slaughterhouse, and ends at the tanning industry gate. Main chemicals and waste flows in and out of this chain have also been included in the analysis. One of the main conclusions is that the tannery is an important stage in most of the impact categories, mainly due to the landfilling of the tannery wastes. Agriculture and — to a lesser extent — cattle raising also play a very important role in most of the impact categories; the former, due to the related energy consumption and use of fertilisers, and the latter due to the emissions associated with animal care. The Autonomous Government of Catalonia is using the results of this study to establish the environmental criteria that a leather product must fulfil in order to attain the Catalan eco-label.     

BaySTDetect: detecting unusual temporal patterns in small area data via Bayesian model choice

Space-time modeling of small area data is often used in epidemiology for mapping chronic disease rates and by government statistical agencies for producing local estimates of, for example, unemployment or crime rates. Although there is typically a general temporal trend, which affects all areas similarly, abrupt changes may occur in a particular area, e.g. due to emergence of localized predictors/risk factor(s) or impact of a new policy. Detection of areas with "unusual" temporal patterns is therefore important as a screening tool for further investigations. In this paper, we propose BaySTDetect, a novel detection method for short-time series of small area data using Bayesian model choice between two competing space-time models. The first model is a multiplicative decomposition of the area effect and the temporal effect, assuming one common temporal pattern across the whole study region. The second model estimates the time trends independently for each area. For each area, the posterior probability of belonging to the common trend model is calculated, which is then used to classify the local time trend as unusual or not. Crucial to any detection method, we provide a Bayesian estimate of the false discovery rate (FDR). A comprehensive simulation study has demonstrated the consistent good performance of BaySTDetect in detecting various realistic departure patterns in addition to estimating well the FDR. The proposed method is applied retrospectively to mortality data on chronic obstructive pulmonary disease (COPD) in England and Wales between 1990 and 1997 (a) to test a hypothesis that a government policy increased the diagnosis of COPD and (b) to perform surveillance. While results showed no evidence supporting the hypothesis regarding the policy, an identified unusual district (Tower Hamlets in inner London) was later recognized to have higher than national rates of hospital readmission and mortality due to COPD by the National Health Service, which initiated various local enhanced services to tackle the problem. Our method would have led to an early detection of this local health issue.     

Hybrid modeling as a QbD/PAT tool in process development: an industrial <Emphasis Type="Italic">E. coli</Emphasis> case study

Process understanding is emphasized in the process analytical technology initiative and the quality by design paradigm to be essential for manufacturing of biopharmaceutical products with consistent high quality. A typical approach to developing a process understanding is applying a combination of design of experiments with statistical data analysis. Hybrid semi-parametric modeling is investigated as an alternative method to pure statistical data analysis. The hybrid model framework provides flexibility to select model complexity based on available data and knowledge. Here, a parametric dynamic bioreactor model is integrated with a nonparametric artificial neural network that describes biomass and product formation rates as function of varied fed-batch fermentation conditions for high cell density heterologous protein production with E. coli. Our model can accurately describe biomass growth and product formation across variations in induction temperature, pH and feed rates. The model indicates that while product expression rate is a function of early induction phase conditions, it is negatively impacted as productivity increases. This could correspond with physiological changes due to cytoplasmic product accumulation. Due to the dynamic nature of the model, rational process timing decisions can be made and the impact of temporal variations in process parameters on product formation and process performance can be assessed, which is central for process understanding.     

Metabolomic analysis of the potato tuber life cycle

An analysis of the potato (Solanum tuberosum L.) tuber life cycle has been completed using a range of mass-spectrometry (MS) based approaches. Six stages have been examined which included developing and mature tubers, sprouting mature tubers and mature tubers stored at 5 or 10°C. The impact of excising developing tubers from the mother plant (source-sink manipulation) was also determined. Data was subjected to Principal Components Analysis, Analysis of Variance and Hierarchical Cluster Analysis to assess the potential for separating the life cycle stages, to define the major profiles for metabolite changes during the life cycle stages examined, and to inform on which metabolites underpinned these profiles. We have shown that it is possible to separate all of the stages using combined analytical approaches and that five major profiles can be used to describe the changes in metabolite levels. Data also indicate that, within a relatively short timeframe, manipulation of source-sink relations has a significant impact on metabolite pools beyond what is currently known for sugar–starch metabolism. We have also demonstrated that the metabolomics data can be mined to provide answers to specific questions––in this case to identify temporal changes in metabolites related to acrylamide-forming potential.     

Ontogenetic and temporal trajectories of chemical defence in a cyanogenic eucalypt

Many studies have shown that similarly aged plants within a species or population can vary markedly in the concentration of defence compounds they deploy to protect themselves from herbivores. Some studies have also shown that the concentration of these compounds can change with development, but no empirical research has mapped such an ontogenetic trajectory in detail. To do this, we grew cyanogenic Eucalyptus yarraensis seedlings from three half-sibling families under constant glasshouse conditions, and followed their foliar cyanogenic glycoside (prunasin) concentration over time for 338 days after sowing (DAS). Plants in all families followed a similar temporal pattern. Plants increased in foliar prunasin concentration from a very low level (10 μg cyanide (CN) equivalents g⁻¹) in their first leaves, to a maximum of, on average, 1.2 mg CN g⁻¹ at about 240 DAS. From 240 to 338 DAS, prunasin concentration gradually decreased to around 0.7 mg CN g⁻¹. Significant differences between families in maximum prunasin concentration were detected, but none were detected in the time at which this maximum occurred. In parallel with these changes in prunasin concentration, we detected an approximately linear increase in leaf mass per unit leaf area (LMA) with time, which reflected a change from juvenile to adult-like leaf anatomy. When ontogenetic trajectories of prunasin against LMA were constructed, we failed to detect a significant difference between families in the LMA at which maximum prunasin concentration occurred. This remarkable similarity in the temporal and ontogenetic trajectories between individuals, even from geographically remote families, is discussed in relation to a theoretical model for ontogenetic changes in plant defence. Our results show that ontogeny can constrain the expression of plant chemical defense and that chemical defense changes in a nonlinear fashion with ontogeny.     

耕地占补面积时空变化对碳储量的影响测度——以湖北省为例

基于InVEST模型定量评估湖北省2000-2020年耕地占补面积时空变化对陆地生态系统碳储量的影响,在县级行政单元尺度探究由耕地占补导致的碳储量变化量在空间上的集聚程度,并运用PLUS模型模拟区域未来四种发展情景下的土地利用格局及碳储量变化趋势。结果表明：（1）2000年至2020年,湖北省耕地面积净变化率为-3.89%,耕地面积略微下降,基本实现耕地占补数量平衡。（2）2000-2005年、2005-2010年、2010-2015年、2015-2020年耕地占用和补偿导致的碳储量变化值占该时段碳储量变化值的比例分别为68.45%、59.45%、57.86%、55.46%,二十年整体占比为61.38%。耕地碳储量的变化对陆地生态系统碳储量的影响巨大。碳固持的地块面积为1.29×10⁵ hm²,碳损失的地块面积为3.88×10⁵ hm²。（3）2000-2020年,湖北省耕地占补导致的碳储量变化值具有明显的空间集聚性,"高高聚集"区主要分布在西部和西南部山区,"低低聚集"区分布在中南部的江汉平原。（4）2020-2030年,湖北省陆地生态系统碳储量在自然发展情景下减少5.50×10⁶ t,在耕地保护情景下减少1.22×10⁶ t,在城镇开发情景下减少8.89×10⁶ t,在生态保护情景下增加2.43×10⁶ t。与其他三种情景相比,生态保护情景是未来发展的最优情景。研究通过定量评估耕地占补平衡背景下湖北省耕地面积时空变化对区域碳储量的影响为未来的国土空间规划以及增汇政策的制定提供决策依据和科学参考,对于实现土地资源的可持续利用和生态环境保护具有重要意义。     

Energy model for contrast detection: spatiotemporal characteristics of threshold vision

A model for contrast detection of spatiotemporal stimuli is proposed which consists of a spatiotemporal linear filter, an energy device and a threshold device. Assuming the existence of independent intrinsic noise, the probability of stimulus detection was approximated by a Weibull function of the response energy. With this assumption, the stimulus energy is a constant at fixed detection probability. This energy model for contrast detection satisfactorily accounted for the elliptical threshold contours of line pairs at stimulus separations within the range 2–30 min and at stimulus onset asynchronies within the range 20–140 ms. The threshold contour at a large stimulus onset asynchrony (300 ms) was in the form of a rounded square. This finding was explained by assuming that the probability of seeing the line pair was determined by the joint probability that at least one stimulus had been detected. With the energy model, the temporal and spatial autocorrelation functions of the response to a flashed line were evaluated. The autocorrelation functions thus determined were used to predict the temporal contrast sensitivity function to a flickering line stimulus and the spatial contrast sensitivity function to flashed gratings, which were in agreement with the experimental data. The data obtained were fitted adequately by an impulse response approximated by a spatiotemporal Gabor-like function. Received: 08 December 1997 / Accepted in revised form: 26 January 1999     

区域气候模式(PRECIS)对黄土高原降水模拟能力的评估

黄土高原水资源短缺,严重制约其社会经济发展;全球变暖背景下,需要对该区水资源状况进行详细的影响评估。区域气候模式可提供气候变化情景下的数据,但模式的模拟精度直接影响评估结果。为此利用ERA40再分析数据作为边界条件驱动PRECIS,从降水频率、降水量和极端事件3个方面,评估了PRECIS对黄土高原1960—2000年降水的模拟能力。结果表明,PRECIS能够模拟出各要素东南-西北方向变化的空间分布特征,还可模拟出整体的时间变化趋势,其中对非汛期的模拟较好,而汛期降水日数和降水量等被严重高估;并且涉及干旱的指标普遍偏低;还发现对于极端降水事件模式对强度指标的模拟能力优于频率指标。因此,还需要进一步探讨订正方法,才能更好的应用于气候变化水文效应评估。     

Temporal dynamics of bird community composition: an analysis of baseline conditions from long-term data

Numerous anthropogenic activities threaten the biodiversity found on earth. Because all ecological communities constantly experience temporal turnover due to natural processes, it is important to distinguish between change due to anthropogenic impact and the underlying natural rate of change. In this study, we used data sets on breeding bird communities that covered at least 20 consecutive years, from a variety of terrestrial ecosystems, to address two main questions. (1) How fast does the composition of bird communities change over time, and can we identify a baseline of natural change that distinguishes primeval systems from systems experiencing varying degrees of human impact? (2) How do patterns of temporal variation in composition vary among bird communities in ecosystems with different anthropogenic impacts? Time lag analysis (TLA) showed a pattern of increasing rate of temporal compositional change from large-scale primeval systems to disturbed and protected systems to distinctly successional systems. TLA slopes of <0.04 were typical for breeding bird communities with natural turnover, while communities subjected to anthropogenic impact were characterised by TLA slopes of >0.04. Most of the temporal variability of breeding bird communities was explained by slow changes occurring over decades, regardless of the intensity of human impact. In most of the time series, medium- and short-wave periodicity was not detected, with the exception of breeding bird communities subjected to periodic pulses (e.g. caterpillar outbreaks causing food resource peaks).