Degree Of Patchiness Research Articles

Radio surface fluctuations in radio relics

Recent observations have revealed detailed structures of radio relics across a wide range of frequencies. In this work, we performed three-dimensional magnetohydrodynamical (3D MHD) simulations of merger shocks propagating through a turbulent magnetized intracluster medium. We then employed on-the-fly Lagrangian particles to explore the physical processes behind the origination of radio substructures and their appearance in high and low-frequency observations. We employed two cosmic-ray (CR) electron acceleration models, with a fresh injection of electrons from the thermal pool and the re-acceleration of mildly relativistic electrons. We used the relative surface brightness fluctuations, δSν, to define a “degree of patchiness.” First, we found that patchiness is produced if the shock’s surface has a distribution of Mach numbers, rather than a single Mach number. Second, radio relics appear patchier if the Mach number distribution consists of a large percentage of low Mach numbers (ℳ ≲ 2.5). Furthermore, as the frequency increases, the patchiness also becomes larger. Nevertheless, if radio relics are patchy at high frequencies (e.g., 18.6 GHz), they necessarily will also be patchy at low frequencies (e.g., 150 MHz). Then, to produce noticeable differences in the patchiness at low and high frequencies, the shock front should have a Mach number spread of σℳ ≳ 0.3 − 0.4. Finally, the extent of the patchiness depends on the Mach number distribution as well as the CR acceleration model. We propose δSν as a potential tool for extracting merger shock properties and information on particle acceleration processes at shocks in radio observations.

Seasonal and Spatial Variability in Patchiness of Microphytobenthos on Intertidal Flats From Sentinel-2 Satellite Imagery

Understanding the spatial structure of microphytobenthos (MPB) on intertidal flats is necessary to gain insight in the benthic community structure and ecosystem processes. The increasing availability of high resolution satellite sensors provides the opportunity to better understand spatial patterns of MPB on various (meter to km) scales. We tested how MPB patch size (indicated by the range derived from a semi-variogram) and degree of patchiness (indicated by the sill) vary as function of seasons, salinity, tidal flat type (muddy fringing versus sandy mid-channel tidal flats) or ecotopes (defined by hydrodynamics, silt content and elevation), in the Westerschelde estuary, the Netherlands. We used Sentinel-2 imagery (2016-2019) with 10 m spatial resolution to derive (omnidirectional) semi-variogram parameters from the NDVI (used as indicator for MPB biomass) and evaluated (seasonality in) patchiness of MPB in the different categories. We demonstrated that MPB patch size (the range) remains constant from winter to summer, while the sill increased from winter to summer. The location of patches on tidal flats was variable throughout the year and shows a remarkable similarity with seasonality in the spatial heterogeneity of the silt content on tidal flats. The patch size and degree of patchiness is higher on relatively sandy mid-channel tidal flats than on relatively silt rich fringing tidal flats. This implies that spatial patterning of MPB biomass on the meso scale is likely closely linked to abiotic conditions and that spreading processes or grazing activity play a minor role. We observed visually that some areas with a relatively high MPB biomass (‘patches’) remain visible throughout the year, while other patches were only present during a particular season.

Abundance/occupancy/patchiness relations in estuarine seagrass macrobenthos

Using >1800 macrobenthic samples from 25 representative intertidal seagrass sites in the Knysna estuarine bay (warm-temperate South Africa), abundance/occupancy relations were examined in the light of Foggo et al.'s (2003) earlier work concerning British estuarine faunas. In addition, the relationship of degree of variation in macrobenthic abundance across unit area, 'patchiness', to this macroecological picture was investigated. Across the group of 30 dominant faunal components as a whole, the most widely and abundantly distributed species had (a) the highest mean abundances and (b) the least spatially-patchy densities in each of Knysna's diverse regions and at all spatial scales from landscape to local; however no significant relationship was observed between mean abundance and degree of patchiness. Individual dominant species displayed considerable variation; some showing positive occupancy/patchiness correlations, and several displaying significant positive and others negative abundance/patchiness ones. As expected along the estuarine gradient, positive and negative correlations of both abundance and occupancy with distance upstream were found in different species (both metrics in a given species always responding in the same manner), although significant upstream relationships with patchiness were found in only two. Local patchiness clearly contributes macroecological information additional to that provided by abundance or occupancy.

Insights for selecting the most suitable nonparametric species-richness estimators for subtropical Brazilian Atlantic Forests

In large-scale biodiversity surveys, the observed species-richness inevitably underestimates the real species richness of a certain geographic region. Nonparametric estimators may be feasible tools for predicting the potential tree/shrub species richness using data from large-scale forest surveys that gather reliable information. In light of the many nonparametric estimators available in the literature, we evaluated their statistical performance (bias and precision) in estimating the γ-diversity of different forest types by comparing their estimates with theoretical total richness values. In addition, we investigated their behaviors on simulated data regarding the spatial distribution of species among sample plots (patchiness). To accomplish these tasks, we used data from a wide-ranging systematic forest survey conducted at 418 sample plots on three subtropical forest types located in a Brazilian hotspot. We established theoretical real tree/shrub species-richness values for each forest type from a vast and carefully revised database for the neotropics. Due to the considerable amount of species with restricted geographic distribution, the incidence-based estimators were shown to be more suitable than the abundance-based estimators for estimating γ-diversity, presenting the smaller overall combination of performance metrics. The abundance-based estimators (Chao1 and ACE) were notedly sensitive to different degrees of patchiness, especially to patchy species distributions. On the other hand, Jackknife1 and the sample-based extrapolation were less influenced by patchiness. Regardless of the differences in species composition and structures among forest types, the Jackknife2 estimator presented the best overall performance and quite accurate estimates for all the cases.

Volumetric bounds on subsurface fluid substitution using 4D seismic time shifts with an application at Sleipner, North Sea

A method is presented for the volumetric estimation of subsurface fluid substitution based on the analysis of 4D seismic time shifts. Since time shifts cannot resolve for fluid saturation and layer thickness simultaneously without additional constraints, mass estimates are derived from the complete set of possible fluid saturations and layer thicknesses. The method considers velocity-saturation relationships that range from uniform fluid mixing to patchy fluid mixing. Based on a generalized velocity-saturation relationship that is parameterized by the degree of patchiness, explicit upper and lower fluid mass bounds are provided. We show that the inherent ambiguity between fluid saturation, fluid mixing, and layer thickness has a severe impact on the convergence of these mass bounds. That is, fluid substitution scenarios with patchy fluid mixing and roughly linear velocity-saturation relationships allow for more accurate fluid quantification than scenarios associated with uniform mixing. Application of the method to two 4D seismic monitor data sets from Sleipner results in [Formula: see text] mass bounds that are consistent with the true injected masses of [Formula: see text]. Moreover, a linear relationship between progressively developing 4D time shifts and known injected [Formula: see text] mass is observed, suggesting that the evolving patterns of fluid saturation and fluid mixing in the [Formula: see text] plume have remained roughly constant with time.

NO MORE ACTIVE GALACTIC NUCLEI IN CLUMPY DISKS THAN IN SMOOTH GALAXIES ATz∼ 2 IN CANDELS/3D-HST

We use CANDELS imaging, 3D-HST spectroscopy, and Chandra X-ray data to investigate if active galactic nuclei (AGNs) are preferentially fueled by violent disk instabilities funneling gas into galaxy centers at 1.3 < z < 2.4. We select galaxies undergoing gravitational instabilities using the number of clumps and degree of patchiness as proxies. The CANDELS visual classification system is used to identify 44 clumpy disk galaxies, along with mass-matched comparison samples of smooth and intermediate morphology galaxies. We note that despite being mass-matched and having similar star formation rates, the smoother galaxies tend to be smaller disks with more prominent bulges compared to the clumpy galaxies. The lack of smooth extended disks is probably a general feature of the z ∼ 2 galaxy population, and means we cannot directly compare with the clumpy and smooth extended disks observed at lower redshift. We find that z ∼ 2 clumpy galaxies have slightly enhanced AGN fractions selected by integrated line ratios (in the mass-excitation method), but the spatially resolved line ratios indicate this is likely due to extended phenomena rather than nuclear AGNs. Meanwhile, the X-ray data show that clumpy, smooth, and intermediate galaxies have nearly indistinguishable AGN fractions derived from both individual detections and stacked non-detections. The data demonstrate that AGN fueling modes at z ∼ 1.85—whether violent disk instabilities or secular processes—are as efficient in smooth galaxies as they are in clumpy galaxies.

Interactions between seagrass complexity, hydrodynamic flow and biomixing alter food availability for associated filter-feeding organisms.

Seagrass shoots interact with hydrodynamic forces and thereby a positively or negatively influence the survival of associated species. The modification of these forces indirectly alters the physical transport and flux of edible particles within seagrass meadows, which will influence the growth and survivorship of associated filter-feeding organisms. The present work contributes to gaining insight into the mechanisms controlling the availability of resources for filter feeders inhabiting seagrass canopies, both from physical (influenced by seagrass density and patchiness) and biological (regulated by filter feeder density) perspectives. A factorial experiment was conducted in a large racetrack flume, which combined changes in hydrodynamic conditions, chlorophyll a concentration in the water and food intake rate (FIR) in a model active filter-feeding organism (the cockle). Results showed that seagrass density and patchiness modified both hydrodynamic forces and availability of resources for filter feeders. Chlorophyll a water content decreased to 50% of the initial value when densities of both seagrass shoots and cockles were high. Also, filter feeder density controlled resource availability within seagrass patches, depending on its spatial position within the racetrack flume. Under high density of filter-feeding organisms, chlorophyll a levels were lower between patches. This suggests that the pumping activity of cockles (i.e. biomixing) is an emergent key factor affecting both resource availability and FIR for filter feeders in dense canopies. Applying our results to natural conditions, we suggest the existence of a direct correlation between habitat complexity (i.e. shoot density and degree of patchiness) and filter feeders density. Fragmented and low-density patches seem to offer both greater protection from hydrodynamic forces and higher resource availability. In denser patches, however, resources are allocated mostly within the canopy, which would benefit filter feeders if they occurred at low densities, but would be limiting when filter feeder were at high densities.

Unexpected Feature of the Lepidoptera Assemblages on the Small Shelf Islands in the Peter the Great Gulf (Sea of Japan, South of Russian Far East)

The article describes and discusses unexpected feature of agrological structure of the moths' species assemblages on four small shelf islands in the Peter the Great Gulf (Sea of Japan, south of Russian Far East). Before insect fauna of these islands was almost unknown. The study was conducted according to standard entomological procedures of insect collecting and chorological analysis of local faunas. Statistical analysis was performed using the detrended correspondence analysis. Typification of areas was made following the principles by Gorodkov. As a result, it was found that geometrid fauna demonstrates high degree of patchiness between the islands and species richness at times lesser than in neighboring continental localities. Nevertheless, a high similarity of agrological structure of the moths inhabited the islands and the continental localities was explored. This is unexpected because of low species richness of geometrids on the islands and their mosaic distribution between them, being original current climatic conditions on the islands and their separation from mainland at beginning of Holocene in different climatic epoch. Starting from the theory of island biogeography, and taking into account geography of the gulf, this fact could be explained by assuming that the moths on the islands represented mainly by resident populations, which occasionally, but regularly over long time intervals, are replenished by random immigrants from mainland. This random immigration could alignment of areological structure of moths between islands and mainland. This model could explain both a high degree of patchiness of the species composition of the moths on the islands, as well as a high degree of compliance of their arealogical structure to neighboring continental localities. However, this hypothesis assumes an equivalence of distribution of migratory ability of moths in large arealogical groups, which requires special research.

INTERTIDAL ZONATION AND KEY-SPECIES ASSOCIATIONS OF THE FLAT ROCKY SHORES OF SINAI, USED FOR SCALING ENVIRONMENTAL VARIABLES AFFECTING CERITHIID GASTROPODS

ABSTRACT Horizontal and near-horizontal rocky platforms at about mean sea level dominate a significant portion of the Sinai Red Sea rocky coasts. Four key-environmental variables govern the distribution of cerithiid gastropods (probably many other species as well) on the platforms' surfaces: wave exposure, fluctuations in moisture, temperature and salinity and presence of sediment and its depth and sediment movement. These variables are interdependent and difficult to assess directly, but, being associated with other easily observable environmental features, they can be indirectly determined. In spite of differences in species richness and diversity and the degree of patchiness between the Gulf of Suez and the Gulf of Elat, the universal scheme of intertidal zonation could be commonly applied to both gulfs. Within this scheme, key-species associations were consistently associated each with a specific segment of the gradients formed by the four environmental variables. As a result, a system for scaling the...

Patchiness and Co-Existence of Indigenous and Invasive Mussels at Small Spatial Scales: The Interaction of Facilitation and Competition

Ecological theory predicts that two species with similar requirements will fail to show long-term co-existence in situations where shared resources are limiting, especially at spatial scales that are small relative to the size of the organisms. Two species of intertidal mussels, the indigenous Perna perna and the invasive Mytilus galloprovincialis, form mixed beds on the south coast of South Africa in a situation that has been stable for several generations of these species, even though these populations are often limited by the availability of space. We examined the spatial structure of these species where they co-exist at small spatial scales in the absence of apparent environmental heterogeneity at two sites, testing: whether conspecific aggregation of mussels can occur (using spatial Monte-Carlo tests); the degree of patchiness (using Korcak B patchiness exponent), and whether there was a relationship between percent cover and patchiness. We found that under certain circumstances there is non-random conspecific aggregation, but that in other circumstances there may be random distribution (i.e. the two species are mixed), so that spatial patterns are context-dependent. The relative cover of the species differed between sites, and within each site, the species with higher cover showed low Korcak B values (indicating low patchiness, i.e. the existence of fewer, larger patches), while the less abundant species showed the reverse, i.e. high patchiness. This relationship did not hold for either species within sites. We conclude that co-existence between these mussels is possible, even at small spatial scales because each species is an ecological engineer and, while they have been shown to compete for space, this is preceded by initial facilitation. We suggest that a patchy pattern of co-existence is possible because of a balance between direct (competitive) and indirect (facilitative) interactions.

Fractal measures of habitat structure: maximum densities of juvenile cod occur at intermediate eelgrass complexity

Habitat patchiness is known to alter the relation of a population to both its predators and its prey. We developed a biologically interpretable measure of habitat complexity to test whether juvenile fish density depends on degree of patchiness. In Newfoundland coastal waters, juvenile fish species, including Atlantic cod Gadus morhua, are associated with eelgrass Zostera marina. How- ever, the association of density with eelgrass cover is dependent upon site and the scale of analysis. We tested for non-monotonic relationships (with an intermediate optimum) between 5 spatial charac- teristics of eelgrass and density of 3 juvenile (Age-0) fish species: Atlantic cod, Greenland cod G. ogac, and white hake Urophycis tenuis. We used aerial photography to determine eelgrass perimeter and area measurements at multiple scales, fractal dimensions of perimeter (DP) and area (DA), and a measure that combines perimeter and area complexity at these scales (βP/A). Fish densities were esti- mated at each site using a seine net. We found parabolic relationships between βP/A and density for all 3 species, indicating highest fish densities at sites of intermediate patchiness and edge regularity. Furthermore, we determined that βP/A provided a less ambiguous estimate of spatial configuration than other measures. This intermediate maximum may reflect a trade-off, whereby eelgrass sites of intermediate spatial complexity provide juvenile fish with both optimal protective cover and oppor- tunity to feed. This pattern may apply to any species requiring open areas in which to forage as well as shelter to offset predation risk.

Widespread gene flow and high genetic variability in populations of water volesArvicola terrestrisin patchy habitats

Theory predicts that the impact of gene flow on the genetic structure of populations in patchy habitats depends on its scale and the demographic attributes of demes (e.g. local colony sizes and timing of reproduction), but empirical evidence is scarce. We inferred the impact of gene flow on genetic structure among populations of water voles Arvicola terrestris that differed in average colony sizes, population turnover and degree of patchiness. Colonies typically consisted of few reproducing adults and several juveniles. Twelve polymorphic microsatellite DNA loci were examined. Levels of individual genetic variability in all areas were high (H(O) = 0.69-0.78). Assignments of juveniles to parents revealed frequent dispersal over long distances. The populations showed negative F(IS) values among juveniles, F(IS) values around zero among adults, high F(ST) values among colonies for juveniles, and moderate, often insignificant, F(ST) values for parents. We inferred that excess heterozygosity within colonies reflected the few individuals dispersing from a large area to form discrete breeding colonies. Thus pre-breeding dispersal followed by rapid reproduction results in a seasonal increase in differentiation due to local family groups. Genetic variation was as high in low-density populations in patchy habitats as in populations in continuous habitats used for comparison. In contrast to most theoretical predictions, we found that populations living in patchy habitats can maintain high levels of genetic variability when only a few adults contribute to breeding in each colony, when the variance of reproductive success among colonies is likely to be low, and when dispersal between colonies exceeds nearest-neighbour distances.

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.

Evaluation of the dry weight rank method for botanical analysis of grassland by means of simulation

With the Dry Weight Rank (DWR) method of 't Mannetje and Haydock [see Journal of British Grassland Society (1963) 18, 268-275] for botanical analysis in pastures, the dry weight proportions of species are estimated from their first, second and third ranks in dry weight in single quadrats. The yield correction of Haydock and Shaw [see Australian Journal of Experimental Agriculture and Animal Husbandry (1975) 15, 663-670] is used additionally to solve the problem of the respective under- and overestimates of the dry weight proportions of high and low yielding species when these grow in patches. In this paper the DWR method is evaluated by means of computer simulation. Main element of the simulation model is a computer sampling program with which a fictitious vegetation can be sampled with a circular quadrat. The output shows that the DWR method works well using relatively small sampling quadrats with, on average, only a few plants per quadrat, irrespective of the horizontal vegetation structure. In vegetations where species grow patchwise, satisfactory results are also obtained using large quadrats with much more plants (i.e. tens) per quadrat. The reason is that in these cases also minor species can compete successfully for first, second and third ranks. However, it appeared that only a certain degree of patchiness is necessary, and with the usually applied quadrat sizes up to 25 dmsuperscript 2, probably in most vegetations this condition is fulfilled. Care should be taken in applying the DWR method for estimating species composition in recently sown grasslands where species usually occur more or less at random. In those cases, in principle a very small sampling quadrat (smaller than 1 dmsuperscript 2) could be used. However, this has practical limitations since the quadrat size should not be too small for realistic yield estimations, needed for the Haydock &amp; Shaw yield correction. The simulations revealed that one condition (i.e., that the sampling quadrat should be at least as large that it usually contains three or more species) is not necessary because of the almost always perfect functioning of the correction for missing ranks. Generally speaking, a sampling quadrat should be chosen not larger than is strictly necessary from the viewpoint of horizontal vegetation structure and from the viewpoint of realistic yield estimations. Multipliers calculated from simulation data could satisfactorily mimic the original multipliers of DWR given by 't Mannetje &amp; Haydock. It is postulated that the DWR method is well suited for studying vegetation changes in old, floristically diverse grasslands with dominant species often in moderate dry weight proportions and species usually growing in patches.

Neighbourhood effects in forests: implications for within‐stand patch structure

1 MOSAIC, a spatially referenced Markov model was used to show how interactions among trees in a neighbourhood may influence the patch structure of forests. A series of two‐species simulations were conducted with neighbourhood strength ranging from neutral (chances of species replacing each other independent of neighbourhood composition) to strong (chance of replacement for each species proportional to neighbourhood composition), and with neighbourhood sizes including 1–50 neighbours. 2 Neighbourhood strength was positively correlated with the degree of patchiness. Very high neighbourhood strength is necessary to form mono‐specific patches composed of hundreds of individual trees. Intermediate neighbourhood sizes (5–12 neighbours) led to the most distinct patches where individuals were arranged so that contact between species was minimized. 3 Neighbourhood effects alone are unlikely to lead to large areas (several ha) dominated by one species. However, simulations showed that neighbourhood effects can augment small differences in the environment, resulting in large mono‐specific patches. 4 Simulations with 4 and 5 species indicated that groups of species can interact to form spatially distinct communities, starting from a random mixture on a uniform environment. This implies that neighbourhood effects may be responsible for some unexplained variability in studies that attempt to relate environmental parameters to forest composition. 5 Patch structures that develop due to neighbourhood effects are usually not recognized by current vegetation classification schemes or by forest managers, and this lack of recognition could lead to the loss of certain natural spatial structures on forested landscapes.

Spatial and temporal distributions of smallmouth bass (Micropterus dolomieu) nests in Lake Opeongo, Ontario

Smallmouth bass (Micropterus dolomieu) nests were patchily distributed within the littoral zone of Lake Opeongo at two spatial scales (1 km and 100 m shoreline segments). Nest locations were recorded by snorkelling along 155 and 6.3 km of littoral zone over 4 and 11 years, respectively. The degree of patchiness was greater and occurred more consistently at the 1-km than at the 100-m spatial scale. However, the degree of patchiness was not significantly affected by 200% differences in spawning population size, implying that competitive interactions did not strongly influence nest locations over the study period. High-density nesting areas remained stationary between years at the 1-km and 100-m scales. This suggests that habitat variables having stationary spatial characteristics, influence nest site choice. Since the locations of nest patches are less stationary and less consistent among 100-m than among 1-km scale sites, influential habitat variables at the 100-m scale are either less important to the locations of nests or less stationary from year-to-year in their effects on nest distributions. If stationary nest patches are typical of spawning smallmouth bass in lakes, permanent protection of known patch locations could enhance their reproductive success.

Prey patchiness and larval fish growth and survival: inferences from an individual-based model

We used an individual-based simulation model to evaluate how prey patchiness and fish swimming behavior affect larval fish survival and mortality source (predation or starvation). Simulations revealed that cohort survival increased linearly with greater average patch residence times and that patch residence times for individual fish with different fates (survived, starved, eaten) diverged substantially during the first few days of feeding. Further, by examining the interaction of patch spatial distribution (uniform random versus clumped) with three possible swimming behaviors, we found that swimming behavior, via its effect on prey encounter and feeding rates, affected both cohort survival rates and whether fish died from predation or starvation, but that the spatial distribution of patches (fine-scale (100s m)) only influenced whether fish died from predation or starvation. Within a particular patch spatial arrangement, however, patch intensity (division of food between patches and non-patches) had a major effect on survival. Except at high food levels, fish did not survive when there were no patches, suggesting that average, well-mixed prey densities will not support sufficiently rapid growth for survival. As patch intensity increased, survival increased to a maximum and then declined, with peak survival occurring at higher patch intensity as average food concentration declined. Finally, the degree of patchiness also determined the intensity of selection on growth rates. In patchier environments, there was stronger selection for fast growth rates leading to eight-fold differences in average cohort growth rates after only three days of growth. In general, survival was not directly related to fast cohort growth the best survival occurred with high average prey densities and weak patchiness. Prey patchiness, by influencing the average as well as the variance in individual growth rates, can have a substantial impact on survival rates of larval fish cohorts.

Large- and small-scale effects of habitat structure on rates of predation: how percent coverage of seagrass affects rates of predation and siphon nipping on an infaunal bivalve.

Landscape ecology, predominantly a terrestrial discipline, considers the effect of large-scale (tens of meters to kilometers) spatial patterns of habitats on ecological processes such as competition, predation, and flow of energy. In this study, a landscape-ecology approach was applied to a marine soft-sediment environment to examine rates of predation and transfer of secondary production in and around vegetated habitats. Seagrass beds naturally occur in a variety of spatial configurations from patches 1-10s of meters across with interspersed unvegetated sediments (i.e., patchy coverage) to more continuous coverage with little or no bare sediment. I designed experiments to address how percent coverage of seagrass in a 100-m2 area of seafloor, and the spatial arrangement (degree of patchiness or fragmentation) of an equal area (100 m2) of vegetation affected predation (lethal) and siphon nipping (sublethal) intensity on an infaunal bivalve, Mercenaria mercenaria (hard clam). Measures of seagrass density and biomass with different percent coverage of seagrass were also made. When clams were placed in both the vegetated and unvegetated portions of the seafloor nearly twice as many clams were recovered live with 99% seagrass cover than with 23% seagrass cover, while survivorship was intermediate with 70% cover. Cropping of clam siphons from both the vegetated and unvegetated sediments was also affected by the amount of seagrass cover in a 100-m2 area of seafloor: mean adjusted siphon weights were approximately 76% heavier from the 99% seagrass cover treatment than from the 70% or 23% cover treatments. Survivorship of clams placed within an equal area of seagrass in very patchy, patchy, and continuous spatial configurations was 40% higher in the continuous seagrass treatment than in either of the two patchy treatments. This study demonstrates that transfer of secondary production in the form of predation and cropping on an infaunal organism is altered as the percent cover of seagrass changes. While large-scale changes in the amount and spatial patterning of vegetation may affect habitat utilization patterns and foraging HGLoopbehavior, increased seagrass density and biomass with increased percent coverage of seagrass limit any conclusions concerning predator foraging behavior and feeding success in response to patch shapes and sizes. Instead, local changes in seagrass characteristics provide the most compelling explanation for the observed results.

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.

PATCHY: simulating and visualizing the effects of stomatal patchiness on photosynthetic CO2 exchange studies

Abstract. In this study, computer modelling has been used to simulate and analyse the effects which non‐homogeneous stomatal conductance (patchiness) might have on the appearance of the macroscopic relationship between photosynthetic CO2 assimilation (A) and intercellular CO2 (Q) in leaves. The problem was formalized using the assumptions that (1) the biochemical model of Farquhar, von Caemmerer &amp; Berry applies [Planta, 149, 78–90 (1980)]; (2) that the parameters for the model are fixed for the period required to determine the relationship; (3) that the distribution of conductances in any leaf area is normal (but restricted to positive values); and (4) that the leaf is perfectly heterobaric. The model is interactive, allowing the user to explore–well beyond the conditions for which data are presented here–the effects of carboxylation capacity, photosynthetic electron transport rate and photorespiration over a range of possible conductances and degrees of patchiness. Regardless of the parameters used in the model, the results fail to predict the change in appearance of the A‐C1 curve which has been attributed to patchiness in other reports: even when conductance varies with a standard deviation of twice its mean value, the effects on the curve are minor. The need for reconsideration of the methods currently used to interpret gas exchange studies is indicated.