The influence of spatial heterogeneity on the behavior and growth of two herbivorous stream insects

Environmental heterogeneity can affect the behavior of organisms, but the consequences of patchiness for organismal energetics (e.g., growth, fitness) are not well understood. This study demonstrates that spatial heterogeneity can affect the growth of aquatic stream insects in laboratory streams, and reveals the behavioral mechanisms for these effects. In a 2×2 factorial design, I experimentally manipulated resource distribution (homogeneous vs. patchy, with the same overall resource levels) and current velocity (fast vs. slow) to investigate the direct and interactive effects of these factors on the drift behavior and growth of two mobile stream grazers, the mayflies Baetis bicaudatis and Epeorus deceptivus. B. bicaudatis nymphs grew larger in environments with homogeneously distributed resources than in patchy environments, and both species grew larger in fast than slow current environments. Patterns of drift behavior over the course of the study corresponded to observed differences in growth. Both species grew to larger body size in treatments where they drifted more successfully among substrates (fast-current treatments) and where they entered the drift less frequently (fast current for both species, and homogeneous treatments for B. bicaudatis). Overall, these results demonstrate that patchiness can significantly influence both the behavior of aquatic insects and the size to which these insects grow. In the light of previously published relationships between nymphal mayfly body mass and fecundity, these results suggest that patchiness in streams may have important consequences for mayfly populations.     

Soil nutrient patchiness and plant genotypes interact on the production potential and decomposition of root and shoot litter: evidence from short-term laboratory experiments with Triticum aestivum

Aims

The purpose of this study was to test the hypotheses that soil nutrient patchiness can differentially benefit the decomposition of root and shoot litters and that this facilitation depends on plant genotypes.

Methods

We grew 15 cultivars (i.e. genotypes) of winter wheat (Triticum aestivum L.) under uniform and patchy soil nutrients, and contrasted their biomass and the subsequent mass, carbon (C) and nitrogen (N) dynamics of their root and shoot litters.

Results

Under equal amounts of nutrients, patchy distribution increased root biomass and had no effects on shoot biomass and C:N ratios of roots and shoots. Roots and shoots decomposed more rapidly in patchy nutrients than in uniform nutrients, and reductions in root and shoot C:N ratios with decomposition were greater in patchy nutrients than uniform nutrients. Soil nutrient patchiness facilitated shoot decomposition more than root decomposition. The changes in C:N ratios with decomposition were correlated with initial C:N ratios of litter, regardless of roots or shoots. Litter potential yield, quality and decomposition were also affected by T. aestivum cultivars and their interactions with nutrient patchiness.

Conclusions

Soil nutrient patchiness can enhance C and N cycling and this effect depends strongly on genotypes of T. aestivum. Soil nutrient heterogeneity in plant communities also can enhance diversity in litter decomposition and associated biochemical and biological dynamics in the soil.     

Spatial covariance in resources affects photosynthetic rate and water potential,but not the growth of Glechoma longituba fragments

Glechom longituba is a widespread clonal plant and usually experiences diverse patchiness of its growing sites. The hypothesis was tested that spatial covariance in resources differentially affects the growth and physiology of G. longituba. Plants were exposed to two patchy habitats where above- and below-ground resources were either positively or negatively associated, and to four control habitats. When equal amounts of resources were given, G. longituba fragments from two patchy habitats had similar biomass, root weight ratio, leaf weight ratio, fluorescence yield, specific petiole length, and specific root length; fragments under reciprocal patchiness significantly increased photosynthetic rate and decreased leaf water potential. Biomass of clones grown in patchy habitats was equal or less than that of counterparts from the control habitats, not supporting the notion that clonal growth is more advantageous in patchy habitats than in uniform habitats. In addition, no evidence was detected for spatial division of labour because biomass allocation to roots and leaves was similar in patchy habitats compared with the control habitats.     

Stochastic eco-epidemiological model of dengue disease transmission by Aedes aegypti mosquito

We present a stochastic dynamical model for the transmission of dengue that takes into account seasonal and spatial dynamics of the vector Aedes aegypti. It describes disease dynamics triggered by the arrival of infected people in a city. We show that the probability of an epidemic outbreak depends on seasonal variation in temperature and on the availability of breeding sites. We also show that the arrival date of an infected human in a susceptible population dramatically affects the distribution of the final size of epidemics and that early outbreaks have a low probability. However, early outbreaks are likely to produce large epidemics because they have a longer time to evolve before the winter extinction of vectors. Our model could be used to estimate the risk and final size of epidemic outbreaks in regions with seasonal climatic variations.     

The influence of habitat heterogeneity on host-pathogen population dynamics

The influence of spatial heterogeneity on the population dynamics of a naturally occurring invertebrate host-pathogen system was experimentally investigated. At ten week intervals over a two year period, I quantified the spatial distribution of natural populations of the terrestrial isopod crustacean Porcellio scaber infected with the isopod iridescent virus (IIV). During the seasonally dry periods of summer and early fall in central California, isopod populations were highly aggregated and the degree of patchiness and distance between inhabited patches was greatest. Coincident with increased patchiness and patch spacing was an increase in isopod density within patches. During the wet seasons of winter and spring, isopod population patchiness, inter-patch spacing, and within-patch density was low. Seasonal changes in virus prevalence were negatively correlated with within-patch density, patchiness, and inter-patch spacing. The influence of the spatial distribution of isopods on virus prevalence was also tested in field experiments. The virus was introduced into arrays of artificial habitat patches colonized by isopods in which interpatch distance was varied. The prevalence of resulting infections was monitored at weekly intervals. In addition, dispersal rates between artificial patches and natural patches were quantified and compared. The results showed that isopods in treatments with the smallest inter-patch spacing had the highest virus prevalence, with generally lower prevalence among isopods in more widely spaced patches. The spacing of experimental patches significantly affected virus prevalence, although the experiments did not resolve a clear relationship between patch spacing and virus prevalence. Rates of dispersal between patches decreased with increased patch spacing, and these rates did not differ significantly from dispersal between natural patches. The results suggest that rates of dispersal between isopod subpopulations may be an important component of the infection dynamics in this system. I discuss the consequences of these findings for host-pathogen dynamics in fragmented habitats, and for other ecological interactions in spatially heterogeneous habitats.     

Heterogeneous distribution of soil nutrients increase intra-specific competition in the clonal plant Glechoma hederacea

Small-scale heterogeneity strongly affects plant fitness and many ecological processes, and it can significantly influence the growth of individual plants, populations and communities. Generally, clonal species achieve significantly more growth when essential resources are patchily distributed than when resources are uniformly distributed. In this study, we aim to determine the effect of spatial heterogeneity in soil resources on intraspecific competition in the clonal plant Glechoma hederacea. We report the outcomes of a greenhouse experiment where high and low densities of plants were exposed to patchy and uniform distribution of nutrients. Our results showed that patchy distribution of resources exacerbated intra-specific competition between clonal systems. We found a reduction of total mass of clonal systems growing at high-density, especially under patchy conditions. Patchy distribution of resources conduct to high concentration of resources located in small areas, and as consequence increase the competition interaction between plants. This study demonstrates that full understanding of plant–plant competitive interactions requires consideration of spatial heterogeneity in nutrient supply.     

缀块性和缀块动态:Ⅰ.概念与机制

缀块性(patchiness)是自然界中最为普遍的现象之一,它存在于各种生态学系统的每一时空尺度上。森林、农田、草地、湖泊等生态系统,通常构成景观缀块(landscape patches),每一景观缀块内部又具有大小、持续时间以及内容都不同的各种缀块。在不同时、空     

Scaling and spatial dynamics in plant-pathogen systems: from individuals to populations

Components of transmission for primary infection from soil-borne inoculum and secondary (plant to plant) infection are estimated from experiments involving single plants. The results from these individual-based experiments are used in a probabilistic spatial contact process (cellular automaton) to predict the progress of an epidemic. The model accounts for spatial correlations between infected and susceptible plants due to inhomogeneous mixing caused by restricted movement of the pathogen in soil. It also integrates nonlinearities in infection, including small stochastic differences in primary infection that become amplified by secondary infection. The model predicts both the mean and the variance of the infection dynamics of R. solani when compared with replicated epidemics in populations of plants grown in microcosms. The broader consequences of the combination of experimental and modelling approaches for scaling-up from individual to population behaviour are discussed. <br>     

Foundations of spatial ecology: the reification of patches through quantitative description of patterns and pattern repetition

Insect populations tend to be patchy, and the nature of the patches is a critical component of ecology. Predator-prey interactions, coexistence of competing species, survival of rare species as habitat is destroyed, and damage to crops are just a few examples of spatially-dependent ecological processes. For want of tractable quantitative approaches, understanding of spatial ecology has lagged far behind recognition of its importance. We assert that a quantitative foundation of a spatial ecology involves the reification of patches as objects of study. We introduce two new measures of patch dynamics: total covariance for comparing degrees of patchiness between populations, and quantile variance for quantifying the constancy of dispersion patterns through time. These new measures, in combination with the long-established spatial covariance from geostatistics, comprise a rudimentary toolbox for reification of patches and empirical field studies in spatial ecology.     

Influence of soil fauna and habitat patchiness on plant (Betula pendula) growth and carbon dynamics in a microcosm experiment

We tested (1) how the presence of a diverse soil faunal community affects ecosystem carbon balance and (2) whether habitat patchiness modifies the influence of soil fauna on plant growth and carbon dynamics. We constructed cylindrical microcosms that contained coniferous forest humus and different litter materials either mixed or in separate patches, and in the presence or absence of diverse soil mesofauna. A birch seedling was planted in the centre of each microcosm. The experiment continued for two growing periods during which net carbon assimilation was measured continuously. At the end of the experiment, the microcosms were destructively sampled for plant biomass, soil fauna, and soil physical and chemical properties. All systems, independently of treatment, were net CO₂ producers in the beginning. In the presence of a diverse fauna, the plant growth was drastically increased, and the mixed-litter systems respired more than the patchy ones. During the second season, the patch effect disappeared, while the birch seedlings and mosses continued to grow better in the microcosms with diverse fauna. In the long term, patchiness did not modify the effect of fauna on plant growth or carbon balance. By the end of the experiment, the carbon balance approached zero in the refaunated microcosms, while it remained negative in the "simple" systems. The weak impact of patchiness in comparison to the faunal effect may be due to a homogenising role of plant roots and progressive decay of the substrates.     

Spatial heterogeneity of daphniid parasitism within lakes

Spatially explicit models show that local interactions of hosts and parasites can strongly influence invasion and persistence of parasites and can create lasting spatial patchiness of parasite distributions. These predictions have been supported by experiments conducted in two-dimensional landscapes. Yet, three-dimensional systems, such as lakes, ponds, and oceans, have received comparatively little attention from epidemiologists. Freshwater zooplankton hosts often aggregate horizontally and vertically in lakes, potentially leading to local host–parasite interactions in one-, two-, or three-dimensions. To evaluate the potential spatial component of daphniid parasitism driven by these local interactions (patchiness), we surveyed vertical and horizontal heterogeneity of pelagic Daphnia infected with multiple microparasites in several north temperate lakes. These surveys uncovered little evidence for persistent vertical patchiness of parasitism, since the prevalence of two parasites showed little consistent trend with depth in four lakes (but more heterogeneity during day than at night). On a horizontal scale of tens of meters, we found little systematic evidence of strong aggregation and spatial patterning of daphniid hosts and parasites. Yet, we observed broad-scale, basin-wide patterns of parasite prevalence. These patterns suggest that nearshore offshore gradients, rather than local-scale interactions, could play a role in governing epidemiology of this open water host–parasite system. Electronic Supplementary Material Supplementary material is available for this article at     

Long-term primary succession: a comparison of non-spatial and spatially explicit inferential techniques

Trajectories of plant primary succession are commonly inferred from temporal changes in non-spatially explicit metrics that characterise the whole sampling area with a single statistic (e.g. community diversity). However, the derivation of these metrics is affected by the presence of spatial structure (patchiness) in vegetation. The emergence of spatial patchiness during succession is therefore likely to have an impact on attempts to infer the rate and direction of vegetation development. This study examines the impact of patchiness on inferred developmental trajectories by comparing a non-spatial analysis of long-term primary succession with a spatially explicit analysis of the same data. The data used in the analysis were collected from an 850-year-old chronosequence of 7 lava flows in southern Iceland. The non-spatial analysis captured broad developmental trends, including an overall increase in community diversity with time, and a split between early pioneer communities (sites <150-year-old) dominated by cryptogams and later assemblages (sites older than ≈150 years) where vascular plants were more important. However, the non-spatial analysis missed key community processes apparent in the spatially explicit analysis, including divergence in vegetation development related to metre-scale topographic differences. The results of this study emphasise the need for spatially explicit, multi-scale studies of vegetation development, both in the inference of past vegetation dynamics, and in modelling the response of spatially patchy vegetation to future environmental change.     

Temporal and spatial spread of Lettuce mosaic virus in lettuce crops in central Spain: factors involved in Lettuce mosaic virus epidemics

Lettuce mosaic virus (LMV) is transmitted by aphid vectors in a nonpersistent manner as well as by seeds. The virus causes severe disease outbreaks in commercial lettuce crops in several regions of Spain. The temporal and spatial patterns of spread of LMV were studied in autumn 2002 in the central region of Spain. Symptomatic lettuce (var. Cazorla) plant samples were collected weekly, first at the seedling stage from the greenhouse nursery and later outdoors after transplantation. The exact position of symptomatic plants sampled in the field was recorded and then material was tested by enzyme‐linked immunosorbent assay to assess virus infection. Cumulative spatial data for infected plants at different growth stages were analysed using spatial analysis by distance indices. For temporal analysis, the monomolecular, Gompertz, logistic and exponential models were evaluated for goodness of fit to the entire set of disease progress data obtained. The results indicated that the disease progress curve of LMV epidemics in the selected area is best described by a Gompertz model and that the epidemic follows a polycyclic disease progression. Our data suggest that secondary cycle of spread occurs when noncolonising aphid species land on the primary infected plants (probably coming from infected seed) and move to adjacent plants before leaving the crop. The role of weeds growing close to lettuce fields as potential inoculum sources of virus and the aphid species most likely involved in the transmission of LMV were also identified.     

Percolation on heterogeneous networks as a model for epidemics

We consider a spatial model related to bond percolation for the spread of a disease that includes variation in the susceptibility to infection. We work on a lattice with random bond strengths and show that with strong heterogeneity, i.e. a wide range of variation of susceptibility, patchiness in the spread of the epidemic is very likely, and the criterion for epidemic outbreak depends strongly on the heterogeneity. These results are qualitatively different from those of standard models in epidemiology, but correspond to real effects. We suggest that heterogeneity in the epidemic will affect the phylogenetic distance distribution of the disease-causing organisms. We also investigate small world lattices, and show that the effects mentioned above are even stronger.     

Coupled Contagion Dynamics of Fear and Disease: Mathematical and Computational Explorations

Background

In classical mathematical epidemiology, individuals do not adapt their contact behavior during epidemics. They do not endogenously engage, for example, in social distancing based on fear. Yet, adaptive behavior is well-documented in true epidemics. We explore the effect of including such behavior in models of epidemic dynamics.

Methodology/Principal Findings

Using both nonlinear dynamical systems and agent-based computation, we model two interacting contagion processes: one of disease and one of fear of the disease. Individuals can “contract” fear through contact with individuals who are infected with the disease (the sick), infected with fear only (the scared), and infected with both fear and disease (the sick and scared). Scared individuals–whether sick or not–may remove themselves from circulation with some probability, which affects the contact dynamic, and thus the disease epidemic proper. If we allow individuals to recover from fear and return to circulation, the coupled dynamics become quite rich, and can include multiple waves of infection. We also study flight as a behavioral response.

Conclusions/Significance

In a spatially extended setting, even relatively small levels of fear-inspired flight can have a dramatic impact on spatio-temporal epidemic dynamics. Self-isolation and spatial flight are only two of many possible actions that fear-infected individuals may take. Our main point is that behavioral adaptation of some sort must be considered.     

Dynamics of annual influenza A epidemics with immuno-selection

 The persistence of Influenza A in the human population relies on continual changes in the viral surface antigens allowing the virus to reinfect the same hosts every few years. The epidemiology of such a drifting virus is modeled by a discrete season-to-season map. During the epidemic season only one strain is present and its transmission dynamics follows a standard epidemic model. After the season, cross-immunity to next year's virus is determined from the proportion of hosts that were infected during the season. A partial analysis of this map shows the existence of oscillations where epidemics occur at regular or irregular intervals. Received: 16 February 2001 / Revised version: 11 June 2002 / Published online: 28 February 2003 Key words or phrases: Infectious disease – Influenza drift – Cross-immunity – Seasonal epidemics – Iterated map     

Stomatal patchiness

Different behaviour of small groups of stomata on a single leaf blade (stomatal patchiness) is reviewed. The occurrence of stomatal patchiness depends on plant species, age, leaf position, environmental conditions,etc. The possibility of errors in conventional evaluation of stomatal and non-stomatal (biochemical) limitations of photosynthesis resulting from patchy stomatal closure is analysed. The consequences of stomatal patchiness for leaf and plant photosynthesis and water economy are discussed. A brief survey of the techniques currently used for detection and quantification of stomatal patchiness is presented.     

巴丹吉林沙漠东缘天然梭梭种群空间分布异质性

梭梭(Haloxylon ammodendron(C.A.Mey.)Bunge)是生长于沙漠地区的一种特有灌木,在维持荒漠生态系统平衡和地区经济发展中发挥着不可替代的作用。为掌握天然梭梭种群空间分布规律,以巴丹吉林沙漠东缘塔木素地区的"野生肉苁蓉及梭梭产籽基地"为试验区,采用样线法测定梭梭株高、冠幅直径,统计梭梭分布密度,分析梭梭种群数量特征空间分布特征及其与生境之间的关系。结果表明:梭梭林密度、株高、冠幅直径均符合正态分布,表现为强度变异;株高、密度、冠幅直径的半方差函数理论模型均为高斯理论模型,其相应的变程为1249、909、1035 m;空间自相关比例均超过70%,受随机因素影响较小,保持着较好的天然分布和生长状态。梭梭种群数量特征的分形维数均大于1.5,空间依赖性强,空间结构性好。长期风蚀作用,小地形海拔高度体现为不同沙层厚度,梭梭株高、冠幅直径和密度在空间上的分布呈西高东低,北高南低的趋势,与海拔高度存在极显著相关;梭梭群落的空间异质性表现出一定的适度沙埋效应。     

Space and patchiness affects diversity–function relationships in fungal decay communities

The space in which organisms live determines health and physicality, shaping the way in which they interact with their peers. Space, therefore, is critically important for species diversity and the function performed by individuals within mixed communities. The biotic and abiotic factors defined by the space that organisms occupy are ecologically significant and the difficulty in quantifying space-defined parameters within complex systems limits the study of ecological processes. Here, we overcome this problem using a tractable system whereby spatial heterogeneity in interacting fungal wood decay communities demonstrates that scale and patchiness of territory directly influence coexistence dynamics. Spatial arrangement in 2- and 3-dimensions resulted in measurable metabolic differences that provide evidence of a clear biological response to changing landscape architecture. This is of vital importance to microbial systems in all ecosystems globally, as our results demonstrate that community function is driven by the effects of spatial dynamics.Subject terms: Fungal biology, Microbial ecology, Metabolomics, Community ecology, Biodiversity