Eye movements and the enhancement of edges

A mathematical examination of retinal photochemistry leads to a hypothesis for Mach band phenomena based on eye movements. This retinal model suggests why minimally distinct borders fade under eye fixation and agrees qualitatively with subjective measures of border contrast as a function of overall field luminance.Deceased     

Habitat choice of some field-inhabiting carabid beetles (Coleoptera: Carabidae) studied by recapture of marked individuals

ABSTRACT.

• 1 Habitat choice of some field-inhabiting carabid beetles was studied in the field, and included the adult-overwintering Bembidion lampros Herbst, Pterostichus cupreus L. and Agonum dorsale Pontoppidan, and the larval-overwintering Trechus secalis Paykull, Pterostichus melanarius Illiger, P.niger Schaller and Harpalus rufipes De Geer. These were compared to the forest-inhabiting and adult-overwintering species, Carabus nemoralis Müller.
• 2 Marked beetles were released in the centre of a circular enclosure which was placed on the edge between a cereal field and a wood. The direction of movement in this circle was compared with that in a control circle placed entirely in the cereal field.
• 3 In the field/wood circle, adult-overwinterers and the larval-overwin-terer H.rufipes choose to move into the cereal field. The majority of T.secalis. P.melanarius and C.nemoralis, however, moved into the wood, while P.niger exhibited no preference.
• 4 In the field circle, adult-overwinterers moved towards the more sun-exposed parts of the circle, whereas the movements of larval-overwin-terers were random.
• 5 Some species-specific characteristics considered important for habitat choice by these carabid beetles, and the relevance of these characteristics for their dispersal behaviour are discussed.

     

Foraging site use and interspecific competition between bluegills and golden shiners

Synopsis Laboratory experiments examined the foraging performances of a dietary generalist, bluegill,Lepomis macrochirus, and a dietary specialist, golden shiner,Notemigonus crysoleucas, as they fed from devices simulating four foraging sites (bottom substrate, water column, submerged macrophytes, and water surface). Fishes foraged in monospecific and mixed-species groups of two and four individuals. For monospecific groups, foraging rates of bluegills did not differ among the four sites, but golden shiners had significantly higher rates on bottom and midwater sites than on plant and surface sites. The size of monospecific groups did not affect foraging rates of either species. In mixed-species trials, bluegills removed more food items than golden shiners from plant and surface sites in two- and four-fish groups and from bottom sites in two-fish groups. Bluegills' foraging performances improved with experience, golden shiners' performances did not. Experimental results are discussed with respect to interactions between bluegills and golden shiners in natural assemblages.     

Enhancement of habitat heterogeneity and species richness on rocky shores inundated by sand

Summary Many rocky shores are subject to periodic inundation by sand, which is often thought to reduce species richness by eliminating organisms intolerant of sand scour or sand smothering. However, regular disturbance (e.g. inundation) should promote richness by preventing the development of low diversity climax communities. A study of faunal richness on 10 regularly inundated shores showed that inundation does promote richness, but by increasing habitat heterogeneity. Some species are excluded from parts of the shore by sand, but because of the patchiness of sand deposits they are rarely excluded from the entire shore. Other species are found only on rocks associated with sand, while typically sandy shore animals occur in the sand deposits themselves. Total richness (281 species) was greater than for local noninundated shores and sandy beaches combined.     

The first born,their dispersal,and vole cycles

Summary On the basis of some empirical data and the existing theory of vole cycles, a new hypothesis is proposed. It explains cyclicity as an effect of obligatory dispersal of the first seasonal cohort of young (Y1) from their natal (optimal) habitats into vacant (suboptimal) habitats. This behaviour could evolve, because it increases contribution of genetic lineages with dispersing Y1, to subsequent generation. It is assumed that the dispersal of Y1 is common among vole species, it does not change during consecutive phases of the cycle, and it does not vary between cyclic and non-cyclic populations. It results in multiannual cycles only under a certain set of spatial and climatic conditions, which are discussed in a paper. Otherwise it results in annual dynamics.     

Small-scale environmental heterogeneity and the analysis of species distributions along gradients

Abstract. The observed distribution of a species along an environmental gradient is strongly affected by environmental variability within a quadrat. Because a quadrat does not represent a point along an environmental gradient, but rather a range of conditions, it is likely to contain species not typically associated with the mean conditions in the quadrat. Systematic relationships exist between a species' true distribution, the observed distribution as a function of mean quadrat environment, and the frequency distribution of the environment within that quadrat. The observed species habitat breadth increases and the observed maximum abundance decreases as within-quadrat environmental heterogeneity increases. If species distributions or beta diversities are to be compared among species or coenoclines, they should be correctedforintra-quadratheterogeneity.Wederive simple corrections for environmental heterogeneity. The distributions of hardwood forest understory species along a soil acidity gradient in the North Carolina piedmont are presented as an example.     

Controlled experiments of habitat fragmentation: a simple computer simulation and a test using small mammals

Habitat fragmentation involves a reduction in the effective area available to a population and the imposition of hard patch edges. Studies seeking to measure effects of habitat fragmentation have compared populations in fragments of different size to estimate and area effect but few have examined the effect of converting open populations to closed ones (an effect of edges). To do so requires a shift in spatial scope-from comparison of individual fragments to that of fragmented versus unfragmented landscapes. Here we note that large-scale, controlled studies of habitat fragmentation have rarely been performed and are needed. In making our case we develop a simple computer simulation model based on how individual animals with home ranges are affected by the imposition of habitat edges, and use it to predict population-level responses to habitat fragmentation. We then compare predictions of the model with results from a field experiment on Peromyscus and Microtus. Our model treats the case where home ranges/territories fall entirely within or partially overlap with that of sample areas in continuous landscapes, but are restricted to areas within habitat fragments in impacted landscapes. Results of the simulations demonstrate that the imposition of hard edges can produce different population abundances for similar-sized areas in continuous and fragmented landscapes. This edge effect is disproportionately greater in small than large fragments and for species with larger than smaller home ranges. These predictions were generally supported by our field experiment. We argue that large-scale studies of habitat fragmentation are sorely needed, and that control-experiment contrasts of fragmented and unfragmented microlandscapes provide a logical starting point.     

Stochastic determinants of assemblage patterns in coral reef fishes: a quantification by means of two models

Synopsis One perspective emphasizing the importance of stochastic processes in determining coral reef fish assemblages implies that there is little organization in species richness, abundance structure, and spatial distribution. We examine the degree to which this perspective is correct by analyzing distribution of fishes on a collection of patch reefs (Discovery Bay, Jamaica). We ask the question whether these patches accumulate species and individuals in a manner consistent with stochastic expectations. To address this question we use two conceptual models, each permitting a different insight. One assumes that fish are distributed stochastically on patches while the other assumes presence of restrictions on fish distribution due to habitat structure. For each conceptual model we use two types of benchmark: we compare observed patterns to those predicted by theoretical models, and we also compare observed patterns to those obtained from a random reallocation of fish individuals to patches. We found that the conceptual model assuming stochastic processes appeared to provide weaker explanation of patterns than the conceptual model that includes restrictions due to habitat structure. Further, and more importantly, we found that (i) the community is shaped by a mixture of stochastic and non-stochastic mechanisms, and (ii) the stochastic assembly processes decrease in importance for species restricted to fewer microhabitat types and sites. Our study therefore indicates that patches do accumulate individuals and species in a manner consistent with stochastic expectations, however, this applies primarily to the habitat generalists (unrestricted species). By the same token, increased habitat specialization by some species imposes constraints on the stochastic model such that it eventually fails.     

Coexistence of Banksia species in southwestern Australia: the role of regional and local processes

Abstract. 60 of the 75 Banksia species are confined to southwestern Australia where five or six species often coexist. We explored the role of regional species richness, niche differentiation, and habitat specialization in structuring banksia assemblages. The diversity of growth forms and categories of seed production and response to fire were assessed in actual assemblages at 40 sites throughout southwestern Australia. Diversity indices at each site were compared with those from null communities assembled on the basis of the abundance and sociability of taxa in regional species pools. The relationship between local and regional species richness suggests that processes at the scale of 100-m² quadrats limit local richness and therefore coexistence. However, there was no consistent evidence that taxa are differentiated by growth form or regeneration strategy. No particular biological profile makes a banksia adept at coexisting with a wide range of other taxa. Habitat specialization is an important factor contributing to lower local richness than would be predicted from niche differentiation of taxa in regional pools. There is recent empirical evidence of several mechanisms whereby the number of coexisting banksias is increased beyond the limits suggested by simple niche theories. Variability in the fire regime also provides a mechanism for maintaining local species richness because different fires favour recruitment of different taxa.     

Fine-scale temporal variation in recruitment of a temperate demersal fish: the importance of settlement versus post-settlement loss

In order to understand variability in recruitment to populations of benthic and demersal marine species, it is critical to distinguish between the contributions due to variations in larval settlement versus those caused by post-settlement mortality. In this study, fine-scale (1–2 days) temporal changes in recruit abundance were followed through an entire settlement season in a temperate demersal fish in order to determine 1) how dynamic the process of recruitment is on a daily scale, 2) whether settlement and post-settlement mortality are influenced by habitat structure and conspecific density, and 3) how the relationship between settlement and recruitment changes over time. Settlement is considered to be the arrival of new individuals from the pelagic habitat, and recruitment is defined as the number of individuals surviving arbitrary periods of time after settlement. Replicate standardized habitat units were placed in 2 spatial configurations (clumped and randomly dispersed) and monitored visually for cunner (Tautogolabrus adspersus) settlement and recruitment every 1–2 days throughout the settlement season. The process of recruitment in T. adspersus was highly variable at a fine temporal scale. Changes in the numbers of recruits present on habitat units were due to both settlement of new individuals and mortality of animals previously recruited. The relative importance of these two processes appeared to change from day to day. The magnitude of the change in recruit number did not differ between the clumped and random habitats. However, post-settlement loss was significantly greater on randomly dispersed than clumped habitats. During several sampling dates, the extent of the change in recruit abundance was correlated with the density of resident conspecifics; however, on other dates no such relationship appeared to exist. Despite the presence of significant relationships between the change in recruit number and density, there was no evidence of either density-dependent mortality or settlement. Initially, there was a strong relationship between settlement and recruitment; however, this relationship weakened over time. Within 2 months after the cessation of settlement, post-settlement loss was greater than 99%, and no correlation remained between recruitment and the initial pattern of settlement. The results of this study demonstrate that the spatial arrangement of the habitat affects the rate and intensity of post-settlement loss. Counter to much current thinking, this study suggests that in order to understand the population ecology of reef fishes, knowledge of what habitats new recruits use and how mortality varies with structural aspects of the habitats is essential.