Increasing Patch Size Research Articles

Clonal structure of Elytrigia atherica along different successional stages of a salt marsh

Elytrigia atherica is a tall clonal grass species typical of higher salt marshes, but is gradually invading to the lower marshes. At young successional stages of a salt marsh, E. atherica is found sparsely dispersed in small groups of ramets. These patches increase in size and ramet density over time, eventually forming extensive swards as succession proceeds. This study investigates the change in the clonal diversity of E. atherica stands during colonization as a result of its reproductive strategy. Clonal diversities of differently sized patches of E. atherica were investigated on two lower salt-marsh sites of different age, 25 years and 35 years, respectively. Microsatellite fingerprint patterns were used to determine genet identities and to estimate relatedness and genetic differentiation between the sites, between patches within sites and within patches. The majority of the patches on both sites contained more than one genet. On the older site, the clonal diversity was higher than on the younger site. However, the clonal diversity tended to decrease with increasing patch size. Low genetic differentiation was found between the two sites, indicating habitat differentiation, whereas differentiation between patches within sites was high. It is reasoned that different environmental conditions could have resulted in different clonal structures: On an older marsh, the increase of successful seedling recruitment, due to more suitable environmental conditions, leads to an increase in clonal diversity. Over time, with increasing ramet density, intraspecific competition is likely to increase, resulting in a decrease of clonal diversity.

Landscape context outweighs local habitat quality in its effects on herbivore dispersal and distribution

Past studies with spatially structured herbivore populations have emphasized the primacy of intrinsic factors (e.g., patch quality), patch geometry (e.g., patch size and isolation), and more recently landscape context (e.g., matrix composition) in affecting local population abundance and dispersal rate. However, few studies have examined the relative importance of each factor, or how they might interact to affect herbivore abundance or dispersal. Here, we performed a factorial field experiment to examine the independent and interactive effects of patch quality (plant biomass, leaf protein, leaf phenolics) and matrix composition [mudflat or non-host grass (Bromus inermis)] on planthopper (Prokelisia crocea) emigration from host-plant patches (prairie cordgrass, Spartina pectinata). In addition, a field survey was conducted to examine the relative importance of patch quality, geography, and matrix composition on planthopper occupancy and density. In the experiment, we found that rates of emigration from low and intermediate quality patches were, on average, 21% percent higher for patches embedded in brome than mudflat. In contrast, the emigration rate was unaffected by matrix composition in nutrient-rich patches. Within matrix types, plant quality had little effect on emigration. In the survey, planthopper density and the patch occupancy rate of planthoppers increased nonadditively with increasing patch size and the percentage of the surrounding matrix composed of mudflat. This study suggests that landscape-level factors, such as the matrix, may be more important than factors intrinsic to the patches.

Patch-distribution effect on diffusion-limited process in dilute suspension of partially active spheres

The normalized overall rate constant, kp/kf for diffusion-limited processes in a dilute suspension of spheres, partially covered with active patches of varying distribution states, is studied with sped-up Brownian dynamic simulations. A dimensionless separation index Is is defined to quantify the characteristics of patch distribution on the sphere surfaces, with values of 0 and 1 corresponding to the states of the most compact and loosest patch distributions, respectively. Remarkably, the normalized overall rate constant is found to strongly correlate with the dimensionless separation index at fixed patch coverage fcover exhibiting a positive, linear relationship. In addition, the slope of the kp/kf vs Is line, a measure of sensitivity of kp/kf to variation in the separation state of the distributed patches, is found to depend on patch coverage and patch size. This sensitivity exhibits a maximum value with respect to an increase in patch coverage, and the magnitude of the maximum sensitivity decreases with increasing patch size. The patch coverage, at which the maximum sensitivity occurs, increases with increasing patch size.

Effects of fragmentation and trampling on carabid beetle assemblages in urban woodlands in Helsinki, Finland

We studied the effects of fragmentation (edge effects and patch size) and trampling (path cover) on carabid beetle assemblages in urban woodland patches in Helsinki, Finland. We expected that (1) open habitat and generalist species would benefit and forest species would suffer from increased woodland fragmentation, and (2) most carabid species would respond negatively to increased levels of trampling. A total of 2088 carabid individuals representing 37 species were collected. A cluster analysis distinguished sites in the interior of large woodland patches, with low or moderate path cover, from the other sites. The other sites did not cluster meaningfully, suggesting increased variation in the carabid fauna with increasing human impact. All species and ecological species-groups decreased with increasing distance from the edge toward the woodland interior and with increasing patch size. This pattern is in accordance with our expectation for open habitat and generalist species but opposite to what we expected for forest species. The reason for these surprising results may be that (1) the species we collected are not true forest interior species, (2) urban woodland edges are optimal habitats for many forest carabids, or (3) edges are actually sub-optimal, and high catches simply reflect increased activity of beetles moving away from the edge. Trampling did not have an overall negative effect on carabids as hypothesized. Species associated with moist forest habitat responded as predicted: they decreased in abundance with increasing path cover. Furthermore, open habitat species decreased with increasing path cover but more straightforward than we had predicted. Model elaboration, by dropping the highly trampled sites from the analyses, suggested that our data of high trampling may be too scarce: the results without these sites were more in accordance with our predictions than with the full dataset.

Testing the importance of patch scale on forest birds

The relationship between population density and habitat area is of central importance to conservation biology, particularly for species dependent on declining habitat. A recent study by Lee et al. in Oikos in 2002 found that the densities of three forest interior bird species (ovenbirds, wood thrushes, and red‐eyed vireos) decline with increasing patch size, contradicting many other studies that demonstrate a positive correlation between area and density for these species. The authors’ argument is based on a misapplied criticism of the prevailing approach based on density‐and on an inappropriate statistical methodology, caused by incorrect specification of the null hypothesis after log–transformation. We use three different methods of testing area‐dependence: a corrected log–abundance log–area regression; a similar abundance–area regression; and the prevailing density‐area approach. For each species, all three methods agree broadly, suggesting the robustness of each approach. For ovenbirds and wood thrushes, we do not find that population density is negatively correlated to patch size. While our analyses of the red‐eyed vireo data find a negative correlation between the two factors, the strength of the correlation is far weaker than that of Lee et al. and may derive from landscape factors unconsidered in the original data. Nevertheless, we do not find positive density–area relationships for any of these forest‐interior species, further underscoring the site‐specificity of the underlying mechanisms of area‐ sensitivity.

Estimating relevant source area of pollen for small Danish lakes around AD 1800

The aims of this study were to obtain estimates of the relevant source area of pollen (RSAP) that can be used to interpret fossil pollen diagrams from medium sized lakes, and to elucidate factors affecting RSAP for such lakes. A data set of sediment pollen assemblages dating from AD 1800 from 25 Danish lakes (3-27 ha) and land cover around the same lakes from historical maps was used. The plant abundance data were distance weighted using a species-specific model of pollen dispersal/deposition and other distance weighting functions (1, Ild and lId2). Extended R-value models were applied to evaluate the relationship between pollen and plant abundance and to estimate RSAP. The choice of distance weighting function influenced the RSAP estimates. When using the species-specific model, wind speed and species-specific properties of pollen dispersal had little effect on the RSAP estimates, which were approximately 1700 m in radius, when all lakes were analysed together. When the pollen types were classified into fewer groups, the RSAP estimate increased, probably because this procedure affected the spatial pattern of the plant abundance data by, in effect, increasing patch size. When the lakes were classified into two distinct groups (western Jutland and remaining sites) using a multivariate statistical technique (TWINSPAN), we obtained different RSAP estimates for each of the two groups. This result could be explained by a difference in average patch size of the land cover types between regions. A series of simulations supported our interpretation that the patch size was the main factor determining the size of the RSAP for the Danish lakes at AD 1800.

Iron in the Ross Sea: 2. Impact of discrete iron addition strategies

Presented are results of a regional‐scale numerical investigation into the effectiveness of Fe fertilization as a means to increase the efficiency of the biological pump in Fe‐limited waters of the Ross Sea, Antarctica. This investigation was conducted using a modified version of the Coupled Ice And Ocean (CIAO) ecosystem model of the Ross Sea sector of the Southern Ocean. Four sets of experiments were performed, investigating the impacts of differences in (1) timing of fertilization, (2) duration of fertilization, (3) amount of Fe added, and (4) size of the fertilized patch. Results show that the stimulation of air‐sea CO2 exchange (FCO2) depends primarily on the timing of fertilization, regardless of the amount of Fe added. When Fe was added at the optimal time of year, FCO2 from the atmosphere into the Ross Sea was increased by 3–22%, depending on fertilization strategy. Increasing patch size produced the largest response, and increasing initial Fe concentration produced the smallest. In all cases, as the intensity of Fe fertilization increased, the fertilization efficiency (increase in CO2 uptake per unit added Fe) dropped. Strategies that maximized the fertilization efficiency resulted in relatively little additional CO2 being drawn out of the atmosphere. To markedly increase oceanic uptake of atmospheric CO2 would require the addition of large amounts of Fe due to the low fertilization efficiencies associated with maximum air‐sea CO2 exchange. Our results also show that differences in the fertilization strategy should be kept in mind when comparing the results of different Fe fertilization experiments.

Differential establishment and survival of species in deciduous and evergreen shrub patches and on bare ground, Mt. Koma, Hokkaido, Japan

The effects of patches formed by a deciduous shrub (Salix reinii) and an evergreen shrub (Gaultheria miqueliana) on cohabitants were examined on an active volcano Mt. Koma, northern Japan, using bareground as a comparative baseline (control). Species richness increased with increasing the areas of three habitats (Salix patches, Gaultheria patches and bareground), but significantly slowed in Gaultheria patches where the shoot densities of cohabitants also decreased. Calamagrostis hakonensis and Spiranthes sinensis were 2–3 times more likely to establish in Salix patches than in bareground, Anaphalis margaritacea var. angustior and Campanula lasiocarpa were 4–5 times more likely to establish on bareground than in Salix patches. Cohabitants was fewer with Gaultheria. Light intensities of the two patch types were ca 1/3 of that on bareground. Light intensity on Gaultheria was ca 88% of that on Salix patch and decreased with increasing patch size, because the differences in stem and leaf architectures and phenology between the two patch-forming species. Moisture and total nitrogen in volcanic deposits were 2–3 times higher in the patches than in bareground. In the patches, low light intensity restricted seed germination, in particular, for the plants characteristic of bareground, e.g., Anaphalis, while high soil moisture increased the survival rates of Calamagrostis. In Gaultheria patches, despite high moisture, the reduced light intensity influenced seed germination and inhibited the establishment of most species. Seedbanks estimated by a lab-experiment indicated that Salix patches trapped more seeds. As a consequence, Salix patches act as facilitator, in particular, for Calamagrostis, by improving moisture and nutrients status of volcanic deposits, moderating light intensity and seed trapping. Deciduous shrub patches, such as Salix reinii, have an improving role in accelerating rates of succession in harsh environments, while evergreen shrub patches slow plant establishment.

Nonlinear processes in seagrass colonisation explained by simple clonal growth rules

The development of single clones of the seagrass Cymodocea nodosa was analysed using a growth model based on the formation of structures limited by diffusive aggregation. The model implemented the measured clonal growth rules (i.e. rhizome elongation and branching rates, branching angle, and spacer length between consecutive shoots) and shoot mortality rate for C. nodosa at Alfacs Bay (Spain). The simulated patches increased their size nonlinearly with time displaying two different domains of growth. Young patches showed a rapid increase with time of the length of rhizome network and the number of living shoots, which depended on rhizome branching rate, and increased the radial patch size (Rg) algebraically with the number of living shoots as Rg ∝ Ns1/Df, being Df the fractal dimension of the patch structure. Patches older than 4 years increased the production of rhizome network and the number of living shoots much more slowly, while their radial patch size behaved as Rg ∝ Ns0.5 resulting from an internal patch compactation. Moreover, the linear growth rate of the simulated patches changed up to 30 fold during patch development, increasing with increasing patch size until patches reached an intermediate size. The modelled patch development was found to closely reproduce the observed patch structure for the species at the Alfacs Bay (Spain). Hence, the growth of C. nodosa patches initially proceeds with a growth mode controlled by the branching pattern (branching frequency and angle) of the species, producing sparse and elongated patches. Once patches exceed 4–5 years of age and contained >500 shoots, becoming dense and circular, they shifts to a growth model typical of compact structures. These results explain previously unaccounted evidence of the emergence of nonlinear patch growth from simple clonal growth rules, and highlight the importance of branching frequency and angles as critical determinants of the space occupation rate of seagrasses and probably other clonal plants.

Intra‐ and interspecific aggregation of north temperate dung beetles on standardised and natural dung pads: the influence of spatial scale

Abstract. 1. The spatial distribution of north temperate dung beetles (Aphodius, Sphaeridium, and Margarinotus) was investigated at three spatial scales (scale a between all pads – large spatial scale, small patch size; scale b between block – large spatial scale, large patch size; scale c within block – small spatial scale, small patch size) in the field using standardised and naturally dropped dung pads that varied in size and volume.2. Results indicated that all the major colonising species of dung beetle expressed strong intraspecific aggregation, while interspecific aggregation, though present, was of a lesser magnitude.3. In both controlled and natural dung treatments, intra‐ and interspecific aggregation decreased with increasing patch size (from pad to block) and also decreased with decreasing spatial scale (from whole plot to within‐block scale).4. The data also suggested that intraspecific aggregation was more important than interspecific aggregation in the distribution of adult dung beetles in natural and standardised dung pads and hence has the potential to facilitate coexistence.5. Intra‐ and interspecific aggregation was greater in the natural dung compared with that of the standardised dung, and the effect of pad size partly explains this phenomenon.6. The within‐patch spatial distribution of dung beetles observed in the natural and standardised dung was possibly mediated through within‐patch differences between the two treatments. Six confounding factors could explain this difference and these factors are discussed in relation to resource utilisation by north temperate dung beetles.

SPIDER EFFECTS ON PLANTHOPPER MORTALITY, DISPERSAL, AND SPATIAL POPULATION DYNAMICS

Nonlethal (trait-mediated) effects of predators on prey populations, particularly with regard to prey dispersal, scarcely have been considered in spatial ecological studies. In this study, we report on the effects of spider predators on the mortality, dispersal, and spatial population dynamics of Prokelisia crocea planthoppers (Hemiptera: Delphacidae) in a prairie landscape. Based on a three-generation survey of host-plant patches (Spartina pectinata; Poaceae), the density of cursorial and web-building spiders declined significantly with increasing patch size (a pattern the opposite of that for the planthopper). Independent of patch size effects, an increase in the density of web-building and cursorial spiders had a negative effect on planthopper density in one of three generations each. Finally, the likelihood of extinction of local (patch) populations of planthoppers increased significantly with an increase in the density of web-building spiders. Planthoppers in small host-plant patches with high densit...

Initiation and maintenance of vegetation mosaics in an Arctic salt marsh

Summary 1 Vegetation mosaics of grassland and exposed soil are often the outcome of intense herbivory and represent alternate stable states, although the vegetational and soil processes that both initiate and maintain the states in response to herbivory are poorly understood. 2 The vegetation mosaic resulting from grubbing by geese was examined on the intertidal flats at La Perouse Bay, Manitoba. Soil properties of the alternate states were measured in conjunction with the ability of transplanted tillers of Puccinellia phryganodes to grow in exposed soil and intact swards. Simulated grubbed patches (2.5–40 cm diameter) were prepared at three sites and the soil properties and inward clonal growth of the grass in each patch were examined during two growing seasons. 3 Where an organic layer was still present on exposed areas, soil properties were similar to those in the rooting zone beneath intact swards, except that soils were hyper-rather than hyposaline. Older exposed mineral soils, where the organic layer had been eroded, were similarly hypersaline, but were also low in soil moisture and total nitrogen, and compacted with a low infiltration rate. Transplanted tillers survived in intact swards, but few survived in any hypersaline soils. 4 Changes in soil properties over two seasons depended on size of experimentally created patches. Again, high salinities developed in soils of the larger patches, and there was a loss of nitrogen and soil moisture and an increase in bulk density with increasing patch size. Although inward clonal growth of P. phryganodes took place from plot perimeters, this is likely to be curtailed by high salinities in plots over 20 cm diameter. 5 As in tropical Africa, intense herbivory can lead to an alternate stable state in this Arctic salt marsh. Small patches of exposed soil are characterized by deteriorating soil conditions and the inability of vegetation to re-establish. As patches coalesce with continued herbivory, an ecological sere (the intertidal salt marsh) is lost.

The effect of patch size and competitor number on aggression among foraging house sparrows

We examined the effect of patch size and competitor number on aggression among house sparrows, Passer domesticus, foraging at patches of seven different sizes in a doubling series (0.014, 0.029, 0.058, 0.116, 0.230, 0.462, and 0.922 m 2 ). Contrary to our expectations, the birds did not defend an entire patch, even when it was small as 0.014 m 2 . The frequency of aggression among the birds decreased gradually with increasing patch size, in contrast to the step decline predicted by resource defense theory. Moreover, the birds fought more frequently and more intensely as competitor density increased. Both results are consistent with the predictions of a modified hawk-dove model for shared patches. Females were more aggressive and fed at a higher rate than did males. The proportion of females increased as patch size decreased, and aggression became more frequent and intense. Even when patches are shared, patch size has an important effect on the frequency and intensity of foraging competition and the size and composition of foraging groups. Key words: aggression, competitor density, hawk-dove, interference, patch size, Passer domesticus, resource defense, truncated phenotypic distribution. [Behav Ecol 15:412–418 (2004)]

Trends in fire patterns in a southern African savanna under alternative land use practices

Climate, topography, vegetation and land use interact to influence fire regimes. Variable fire regimes may promote landscape heterogeneity, diversification in vegetation pattern and biotic diversity. The objective was to compare effects of alternative land use practices on landscape heterogeneity. Patch characteristics of fire scars were measured from 21 annual burn maps produced from 1972 to 2001 Landsat imagery. Trends in fire patterns under alternative land use practices were compared across a 250,000 ha savanna in southern Africa partitioned into three land use zones. Zone 1, Madikwe Game Reserve (MGR), has had mostly prescribed fires since 1993. Zone 2, cattle farms near MGR in South Africa (SAF), has experienced occasional fires. Zone 3, communal grazing lands in neighboring Botswana (BOT), has had the fewest fires. Cattle ranching was the predominant land use throughout the study area until 1992, when land use switched to conservation and eco-tourism in MGR. Sixteen landscape metrics were applied to this data set to uncover trends in the patch characteristics of the fire scars. A principal components analysis (PCA) reduced the dimensionality of the results so trends in the 10 most important size, shape, and proximity metrics could be better interpreted. The PCA results showed that more burning over time in MGR, and to a lesser extent in SAF, increased patch size, size variability, shape complexity and proximity, while fire exclusion in BOT produced no change or decreasing trends. We tested for significant differences in these metrics between the three land use zones and between two periods, 1972–1992 and 1993–2001. Most patch characteristics in MGR and SAF differed significantly from those in BOT, especially during the latter period, while between MGR and SAF they did not. Patch area, shape complexity and core area increased significantly between periods in MGR, while patch size, size variability and core area increased significantly between periods in SAF. In BOT, no patch characteristics changed significantly between periods. Within the time span analyzed for the study area, we conclude that increased fire occurrence promoted landscape heterogeneity while fire exclusion did not.

Patch use by the greater glider (Petauroides volans) in a fragmented forest ecosystem. I. Home range size and movements

This paper examines home-range attributes of 40 greater gliders (Petauroides volans) in five patches of remnant eucalypt forest surrounded by stands of radiata pine (Pinus radiata) near Tumut in south-eastern Australia. Fixed-kernel smoothing methods were used to estimate home-range size for P. volans. For males, home-range size varied from 1.38–4.10 ha (mean = 2.6 ± 0.8 ha, n = 12) and was significantly larger (P < 0.05) than for females (1.26–2.97 ha, mean = 2.0 ± 0.6 ha, n = 11). Home-range size increased significantly with increasing patch size and reduced patch population density. Thus, small patches had more animals per unit area with smaller home ranges and greater home-range overlap. Our findings illustrate flexibility in the use of space by P. volans. Such results have not previously been reported for P. volans or any other species of arboreal marsupial. Considerable home-range overlap (at 95th percentile isopleth level) was observed between male and female P. volans. Pairs of females also exhibited home-range overlap. Males tended to maintain home ranges exclusive of other males, although some shared common areas. Contrary to the large variations observed in home-range area, core areas (50th isopleth) remained relatively constant, regardless of patch size, population density or sex. This may indicate that core areas are an essential requirement for individuals and resources they contain cannot be shared with congeners.

Modeling the density–area relationship in a dynamic landscape: an examination for the beetle Tetraopes tetraophthalmus and a generalized model

The relationship between local population density and habitat area is an important factor in spatial population ecology. I examined how density dependence in the growth of local insect populations and their host plant patches, combined with patch birth and death, and insect dispersal affect the density–area relationship. I constructed a simulation model to examine the relationship for an insect herbivore, Tetraopes tetraophthalmus, inhabiting patches of its host plant, Asclepias syriaca. Given the observed growth of insect populations, patch growth, dispersal of insects, and change in the number of patches within the landscape, the model predicts that T. tetraophthalmus density should decrease with increasing A. syriaca patch size. The model also predicts moderate amounts of temporal variation in the relationship. A more general insect herbivore-host plant model was also developed to extend the results. The general model shows that density dependence in patch and insect population growth rates have large effects on the density–area relationship. The density–area relationship was strongly affected by density dependence in insect growth. Increasing density dependence in insect growth caused insect density to decrease with increasing patch size. Temporal variation in the relationship was most strongly affected by density dependence in patch growth. Variation in the density–area relationship increased as density dependence in patch growth increased. The results of this study show that density–area relationships can be variable and are not necessarily a species-specific trait. Application of density–area relationships, especially in dynamic landscapes, need to be aware of and account for factors that affect the size and number of patches as well as growth and dispersal of the target populations.

Seed predators are undeterred by nectar-feeding ants on Chamaecrista nictitans (Caesalpineaceae)

There are many examples of mutualistic interactions between ants and plants bearing extrafloral nectaries (EFN). The annual legume Chamaecrista nictitans (Caesalpineaceae) secretes nectar from EFN, specialized structures that attract ants, spiders, and other arthropods. The effects of manipulated C. nictitans patch size and location on plant-ant interactions were tested. Defense from herbivores was not detected; plants with ants did not set significantly more fruit or seed than plants with ants excluded. On the contrary, in one year, plants without ants set more fruit and seed than C. nictitans with ants. The cause of this was not determined. Furthermore, insect herbivore damage was low during three years of observations. Sennius cruentatus (Bruchidae), a specialist seed predator beetle, escaped ant defense despite the presence of numerous ants. Beetle progeny are protected during development by living inside maturing C. nictitans fruit and preventing fruits from dehiscing before emerging as adults. Although ants reduced percent of infestation in 1995, the total number of S. cruentatus per plant was not affected by ants in years of infestation. Overall, larger experimental C. nictitans patches attracted more ants, parasitoid wasps, and percent infestation by S. cruentatus while insect herbivores declined with increasing patch size. Location of patches within fields, however, did not affect numbers of arthropod visitors. Similar to manipulated populations, very little insect herbivory occurred in four reference populations. In contrast to the experimental populations, no S. cruentatus were recovered in reference populations of C. nictitans. Herbivory by insects may not always depress seed set by C. nictitans or may not exceed a threshold level. Thus, herbivory-reduction by ants may not have been detectable in these results. Seed predation may be more influential on C. nictitans reproduction.

Testing the resource concentration hypothesis with tarnished plant bug on strawberry: density of hosts and patch size influence the interaction between abundance of nymphs and incidence of damage

Abstract. 1. Attributes of patches with strawberry plants were manipulated experimentally to assess the impact of patch size and host density on the abundance of tarnished plant bug (Hemiptera: Miridae) and incidence of fruit damage.2. The density of nymphs per inflorescence increased with patch size and host density for some but not all generations of plant bug, providing partial support for the resource concentration hypothesis, while emphasising the importance of replicating experiments across several generations of herbivore.3. Increasing density of nymphs with increasing patch size and host density did not translate into a high incidence of damage, which may be due to the relatively low feeding impact of nymphs in patches with numerous fruits and to the relatively high perimeter‐to‐area ratio in small patches resulting in a relatively high proportion of damaged fruits.4. The decreasing proportion of damaged fruits with increasing density of fruits per patch suggests that nymphs exhibit a saturating functional response to density of strawberry fruits. Functional responses of herbivores may be a critical yet overlooked component that influences interactions between insects and their host plant. In particular, concentrated plant resources may result in increased numbers of herbivores yet sill have a negligible influence on plant damage and/or fitness.

Blobs strengthen repetition but weaken symmetry

The human visual system is more sensitive to symmetry than to repetition. According to the so-called holographic approach [J. Math. Psychol. 35 (1991) 151; Psychol. Rev. 103 (1996) 429; Psychol. Rev. 106 (1999) 622], however, this perceptual difference between symmetry and repetition depends strongly on spatial scaling. This was tested in three experiments, using symmetry and repetition stimuli that consisted of black and white patches, with patch size as the critical variable. In Experiment 1, patch size was increased in the entire pattern, yielding fewer but larger patches (or blobs). This is known to have hardly any effect on symmetry but, as found now, it does have a strengthening effect on repetition. In the second experiment, we increased patch size in subpatterns only, yielding salient blob areas. This again strengthens repetition but, as double-checked in experiment 3, it can weaken symmetry. These results agree with the holographic approach, and enable an integration of computational, algorithmic, and implementational aspects of vision.

Shrub encroachment in Argentinean savannas

Abstract. In the Wet Chaco region of Argentina, increasing shrub encroachment in savannas over the last few decades has led to important changes in the structure and functioning of the landscape. Some sectors of this territory are characterized by the appearance of circular clusters of woody patches, dispersed throughout the grassland matrix. The increasing size of these patches leads to a gradual change from grassland to dense shrubland. We studied these circular woody patches in the eastern region of the Argentine province of Formosa and characterized the variation in terms of floristic composition, diversity and predominant seed dispersal mode in different size patches. We observed an increase in species richness, diversity and compositional heterogeneity among patches with increasing patch size. Seed dispersal by animals, especially birds, is an important factor in the expansion of these woody vegetation patches within the grassland matrix.