Edge effects in patchy seagrass landscapes: The role of predation in determining fish distribution

Predation is often described as an underlying mechanism to explain edge effects. We assessed the importance of predation in determining edge effects in seagrass using two approaches: a video survey to sample predators at small scales across seagrass edges, and a tethering experiment to determine if predation was an underlying mechanism causing edge effects. Underwater videos were placed at four positions: middle of seagrass patches; edge of seagrass; sand immediately adjacent to seagrass and sand distant from seagrass. Fish abundances and the time fish spent in view were measured. The main predatory fish (Australian salmon, Arripis spp.) spent more time over adjacent sand than other positions, while potential prey species (King George whiting, Sillaginodes punctata (Cuvier), recruits) were more common in the middle of seagrass patches. Other species, including the smooth toadfish, Tetractenos glaber (Freminville), and King George whiting adults, spent more time over sand adjacent to seagrass than distant sand, which may be related to feeding opportunities. King George whiting recruits and pipefish (Stigmatopora spp.) were tethered at each of the four positions. More whiting recruits were preyed upon at outer than inner seagrass patches, and survival time was greater in the middle of shallow seagrass patches than other positions. Relatively few pipefish were preyed upon, but of those that were, survival time was lower over sand adjacent to seagrass than at the seagrass edge or middle. Video footage revealed that salmon were the dominant predators of both tethered King George whiting recruits and pipefish. The distribution of predators and associated rate of predation can explain edge effects for some species (King George whiting) but other mechanisms, or combinations of mechanisms, are determining edge effects for other species (pipefish).     

Dugong habitat use in relation to seagrass nutrients,tides, and diel cycles

The dugong is the only herbivorous mammal that is strictly marine and a seagrass community specialist. The pasture available to the dugong varies with the tides because seagrass occurs in both intertidal and subtidal areas. We GPS-tracked seven dugongs within a 24 km², intensively used seagrass habitat in subtropical Australia in winter. We modeled resource selection within the habitat by comparing the dugongs’ use of space with the distribution of seagrass in an area defined using the combined space-use of the tracked animals. Selection by dugongs for seagrass quantity (biomass) and quality (nutrients) was analyzed within six time/tide combinations to examine the influences of tidal periodicity and the diel cycle on resource selection. Dugong habitat use was consistently centered over seagrass patches with high nitrogen concentrations, except during the day at low tides when the animals had fewer habitat choices and their space use was centered over high seagrass biomass. The association of dugongs with seagrass high in starch was positive during both day and night high tides when the animals could access the intertidal areas where seagrass biomass was generally low. Associations between dugongs and seagrass species were less definite, reflecting the potential for dugongs to exploit several species. Our model of dugong resource selection suggests that nitrogen is the primary limiting nutrient for dugong populations and also confirms the preference of dugongs for high-energy foods.     

Fine-scale spatial and temporal variations in diets of the pipefish Stigmatopora nigra within seagrass patches

Diets of the pipefish Stigmatopora nigra were analysed to determine if food availability was causing S. nigra to distribute according to habitat edge effects. Gut analysis found little difference in the diets of S. nigra at the edge and interior of seagrass patches, regardless of time of day or season. Fish diets did, however, vary with seagrass density: S. nigra in denser seagrass consumed more harpacticoid copepods and fewer planktonic copepods. The lack of difference in prey eaten by S. nigra at the edge and interior of patches suggests either that food was not determining S. nigra distribution patterns within patches or that differences in fish densities across patches meant that relative fish-prey densities were similar at edge and interior positions. Alternatively, any edge effects in diet might be masked by gradients in seagrass structure.     

Emergent properties of patch shapes affect edge permeability to animals

Animal travel between habitat patches affects populations, communities and ecosystems. There are three levels of organization of edge properties, and each of these can affect animals. At the lowest level are the different habitats on each side of an edge, then there is the edge itself, and finally, at the highest level of organization, is the geometry or structure of the edge. This study used computer simulations to (1) find out whether effects of edge shapes on animal behavior can arise as emergent properties solely due to reactions to edges in general, without the animals reacting to the shapes of the edges, and to (2) generate predictions to allow field and experimental studies to test mechanisms of edge shape response. Individual animals were modeled traveling inside a habitat patch that had different kinds of edge shapes (convex, concave and straight). When animals responded edges of patches, this created an emergent property of responding to the shape of the edge. The response was mostly to absolute width of the shapes, and not the narrowness of them. When animals were attracted to edges, then they tended to collect in convexities and disperse from concavities, and the opposite happened when animals avoided edges. Most of the responses occurred within a distance of 40% of the perceptual range from the tip of the shapes. Predictions were produced for directionality at various locations and combinations of treatments, to be used for testing edge behavior mechanisms. These results suggest that edge shapes tend to either concentrate or disperse animals, simply because the animals are either attracted to or avoid edges, with an effect as great as 3 times the normal density. Thus edge shape could affect processes like pollination, seed predation and dispersal and predator abundance.     

Response of fauna in seagrass to habitat edges,patch attributes and hydrodynamics

This study has investigated the taxon‐specific responses of fauna to patch edges, and how these relate to patch attributes (patch size, seagrass biomass and water depth), and hydrodynamics in the seagrass habitat. Faunal abundances were sampled at the edge, 2 m in from the edge, and in the middle of 10 seagrass patches of variable size in Port Phillip Bay, Australia. Five of nine taxa showed edge effects. There were higher abundances at the edge compared with the middle for porcellid harpacticoids, and an increase in abundance from the edge to the middle of the patches for tanaids and isopods. For caprellid and gammarid amphipods, the edge effect varied across patches. Changes in current within the patch and patch size were related to the variability in the edge effect pattern of caprellids. None of the measured environmental variables (seagrass biomass, current and water depth) or patch size had a role in the variable edge effect pattern of gammarid amphipods. At the patch level, the distribution of six of nine taxa in this study, namely isopods, polychaetes, ‘other harpacticoids’, porcellid harpacticoids, cumaceans and gammarid amphipods, was related to differences in average water depth, average seagrass biomass and patch size. Our study indicates that the faunal response to edges cannot be generalized across seagrass habitat, and the implications of habitat area loss will vary depending on the taxon under consideration.     

Effects of seagrass habitat fragmentation on juvenile blue crab survival and abundance

Seagrasses form temporally dynamic, fragmented subtidal landscapes in which both large- and small-scale habitat structure may influence faunal survival and abundance. We compared the relative influences of seagrass (Zostera marina L.) habitat fragmentation (patch size and isolation) and structural complexity (shoot density) on juvenile blue crab (Callinectes sapidus Rathbun) survival and density in a Chesapeake Bay seagrass meadow. We tethered crabs to measure relative survival, suction sampled for crabs to measure density, and took seagrass cores to measure shoot density in patches spanning six orders of magnitude (ca. 0.25-30,000 m²) both before (June) and after (September) seasonally predictable decreases in seagrass structural complexity and increases in seagrass fragmentation. We also determined if juvenile blue crab density and seagrass shoot density varied between the edge and the interior of patches. In June, juvenile blue crab survival was not linearly related to seagrass patch size or to shoot density, but was significantly lower in patches separated by large expanses of unvegetated sediment (isolated patches) than in patches separated by <1 m of unvegetated sediment (connected patches). In September, crab survival was inversely correlated with seagrass shoot density. This inverse correlation was likely due to density-dependent predation by juvenile conspecifics (i.e. cannibalism); juvenile blue crab density increased with seagrass shoot density, was inversely correlated with crab survival, and was greater in September than in June. Shoot density effects on predator behavior and on conspecific density also likely caused crab survival to be lower in isolated patches than in connected patches in June. Isolated patches were either large (patch area >3000 m²) or very small (<1 m²). Large isolated patches had the lowest shoot densities, which may have allowed predators to easily find tethered crabs. Very small isolated patches had the highest shoot densities and consequently a high abundance of predators (=juvenile conspecifics). Though shoot density did not differ between the edge and the interior of patches, crabs were more abundant in the interior of patches than at the edge. These results indicate that seagrass fragmentation does not have an overriding influence on juvenile blue crab survival and density, and that crab cannibalism and seasonal changes in landscape structure may influence relationships between crab survival and seagrass habitat structure. Habitat fragmentation, structural complexity, faunal density, and time all must be incorporated into future studies on faunal survival in seagrass landscapes.     

Relationships between spatial configuration of tropical forest patches and woody plant diversity in northeastern Puerto Rico

The destruction and fragmentation of tropical forests are major sources of global biodiversity loss. A better understanding of anthropogenically altered landscapes and their relationships with species diversity and composition is needed in order to protect biodiversity in these environments. The spatial patterns of a landscape may control the ecological processes that shape species diversity and composition. However, there is little information about how plant diversity varies with the spatial configuration of forest patches especially in fragmented tropical habitats. The northeastern part of Puerto Rico provides the opportunity to study the relationships between species richness and composition of woody plants (shrubs and trees) and spatial variables [i.e., patch area and shape, patch isolation, connectivity, and distance to the Luquillo Experimental Forest (LEF)] in tropical forest patches that have regenerated from pasturelands. The spatial data were obtained from aerial color photographs from year 2000. Each photo interpretation was digitized into a GIS package, and 12 forest patches (24–34 years old) were selected within a study area of 28 km². The woody plant species composition of the patches was determined by a systematic floristic survey. The species diversity (Shannon index) and species richness of woody plants correlated positively with the area and the shape of the forest patch. Larger patches, and patches with more habitat edge or convolution, provided conditions for a higher diversity of woody plants. Moreover, the distance of the forest patches to the LEF, which is a source of propagules, correlated negatively with species richness. Plant species composition was also related to patch size and shape and distance to the LEF. These results indicate that there is a link between landscape structure and species diversity and composition and that patches that have similar area, shape, and distance to the LEF provide similar conditions for the existence of a particular plant community. In addition, forest patches that were closer together had more similarity in woody plant species composition than patches that were farther apart, suggesting that seed dispersal for some species is limited at the scale of 10 km.     

Seagrass patch size affects fish responses to edges

1.  Patch area and proximity of patch edge can influence ecological processes across patchy landscapes and may interact with each other. Different patch sizes have different amounts of core habitat, potentially affecting animal abundances at the edge and middle of patches. In this study, we tested if edge effects varied with patch size.
2.  Fish were sampled in 10 various-sized seagrass patches (114–5934 m²) using a small (0·5 m²) push net in three positions within each patch: the seagrass edge, 2 m into a patch and in the middle of a patch.
3.  The two most common species showed an interaction between patch size and the edge–interior difference in abundance. In the smallest patches, pipefish ( Stigmatopora nigra ) were at similar densities at the edge and interior, but with increasing patch size, the density at the edge habitat increased. For gobies ( Nesogobius maccullochi ), the pattern was exactly the opposite.
4.  This is the first example from a marine system of how patch size can influence the magnitude and pattern of edge effects.
5.  Both patch area and edge effects need to be considered in the development of conservation and management strategies for seagrass habitats.     

Post-larval French grunts (Haemulon flavolineatum) distinguish between seagrass, mangrove and coral reef water: Implications for recognition of potential nursery habitats

Environmental cues like sound, magnetic field, oceanic currents, water chemistry or habitat structure are believed to play an important role in the orientation of reef fish towards their settlement habitat. Some species of coral reef fish are known to use seagrass beds and mangroves as juvenile habitats. Once oceanic larvae of these fish have located a coral reef from the open ocean, they still have to find embayments or lagoons harbouring these juvenile habitats. The sensory mechanisms that are used for this are still unknown. In the present study, experiments were conducted to investigate if recruits of the French grunt (Haemulon flavolineatum) respond to habitat differences in water type, as mangrove/seagrass water may differ in biotic and abiotic compounds from coral reef water. Our results show that post-larvae of a reef fish that is highly associated with mangroves and seagrass beds during its juvenile life stage, choose significantly more often for water from mangroves and seagrass beds than for water from the coral reef. These results provide a more detailed insight in the mechanisms that play a role in the detection of these juvenile habitats.     

Seagrass growth and patch dynamics: cross-scale morphological plasticity

By examining the spatial distribution of rhizome morphological characteristics of the seagrass Halodule wrightii, in relation to a seasonal pattern of seagrass patch dynamics, we attempted to derive a mechanistic explanation for the variety of changes exhibited by seagrass patch shapes. Rhizome morphological characteristics (mean internodal distance, branching frequency and biomass) were measured at three spatially-recognized regions (Flood edge, Center, Ebb edge) of 5 seagrass patches, reflecting position relative to hydrodynamic flow. In addition, maps (1 resolution) of the seagrass patches were used to quantify changes in seagrass patch margins across the growing season. Rhizome morphological characteristics varied with spatial position: longer internodal distances were recorded on both edges of the patch relative to patch center, and rhizomes from Flood edges exhibited longer internodes than plants on the Ebb edge of patches. Branching frequency showed no spatially-explicit distribution across the seagrass patches. Patch change analysis indicated a pattern of increase in patch area on the Flood edges of seagrass patches and recession (or no change) on the Ebb edges. Patch margin change was significantly correlated with internodal distances: the more positive the increase in patch seagrass coverage on an edge, the greater the internodal distances.Sediment nutrients were explored as a potential mechanism for the distinct spatial distribution of morphologies found; experimental addition of phosphorus, but not nitrogen, significantly altered the rhizome morphology and biomass, but measurement of ambient sediment nutrient concentrations produced no significant correlations with the in situ distribution of rhizome morphologies. These results suggest that larger-scale landscape characteristics of patch dynamics may be determined by predictable behaviors of small-scale components, but the results do not conclusively describe a mechanism for this system.     

Effects of habitat edges on American lobster abundance and survival

Habitat edges frequently possess distinct ecological conditions that affect interactions such as competition and predation. Within a species' preferred habitat, the structural complexity and resource availability of adjacent habitats may influence the effect of edges on ecological processes. In nearshore waters of New England, American lobsters (Homarus americanus) inhabit fragmented cobble reefs that often are bordered by unvegetated sediment and occasionally by seagrass. We determined whether proximity to cobble patch edges, microhabitat characteristics within cobble habitat, and the type of habitat adjacent to cobble patches (seagrass or unvegetated sediment) influence the density and survival of juvenile and adult American lobsters in Narragansett Bay, Rhode Island, USA. We surveyed naturally occurring cobble patches and artificial cobble reefs to determine how the odds of finding lobsters varied with distance from the edge and habitat type. Additionally, we tethered lobsters at different distances from the edge inside and outside of cobble patches to determine how lobster relative survival varied with edge proximity and habitat type. In cobble habitat, the odds of finding large lobsters (adolescents and adults > 40 mm carapace length (CL)) were highest near patch edges regardless of adjacent habitat type, whereas smaller lobsters (e.g. emergent juveniles 15-25 mm CL) were more abundant in patch interiors when seagrass bordered cobble patches. The odds of finding lobsters also increased with the relative amount of cobble cover within patches. In predation experiments, lobster relative survival after 6 h was lowest outside of cobble and increased toward cobble patch interiors, but after 24 h this trend disappeared or reversed. Seagrass appeared to offer greater refuge for lobsters than did unvegetated sediment. Our results suggest that proximity to patch edges influences lobster distribution and survival, and that edge effects on lobsters vary with life history phase and with the type of habitat adjacent to cobble patches.     

Inverted sand dollars actively orient themselves in flow to increase likelihood of righting

The fact that sand dollars are often dislodged and inverted is an inescapable consequence of living at or slightly below the sediment–water interface. Once inverted, however, how do sand dollars effectively right themselves, given their small spines and stiff internal skeletons? Here, we examined the possibility that individuals of Mellita quinquiesperforata and Dendraster excentricus may take advantage of the interaction of their morphology and flow to increase the likelihood of righting. Based on flow tank observations, the critical velocity required to flip an inverted sand dollar varies with orientation and increases with test size. For both species, the critical velocity was lower when inverted sand dollars were oriented with the posterior margin facing directly downstream, compared with when the posterior margin was positioned in an upstream orientation. To test whether inverted sand dollars would actively rotate into a more advantageous position for flipping, we exposed inverted animals in three starting orientations – with their posterior edge directed upstream (the least favored position for flipping), perpendicular, and downstream to flow – to the minimum flow expected to induce flipping and compared their responses. Time‐lapse photography showed that regardless of initial orientation, within one hour, a majority of individuals of both species rotated into positions that were not statistically different from the downstream orientation (the most favored position for flipping). These results for D. excentricus were further confirmed in a field experiment. Taken together, these data suggest that inverted sand dollars are able to recognize flow direction and respond by modifying their orientation to maximize lift and drag for righting.     

Microscale imaging sheds light on species-specific strategies for photo-regulation and photo-acclimation of microphytobenthic diatoms

Intertidal microphytobenthic (MPB) biofilms are key sites for coastal primary production, predominantly by pennate diatoms exhibiting photo-regulation via non-photochemical quenching (NPQ) and vertical migration. Movement is the main photo-regulation mechanism of motile (epipelic) diatoms and because they can move from light, they show low-light acclimation features such as low NPQ levels, as compared to non-motile (epipsammic) forms. However, most comparisons of MPB species-specific photo-regulation have used low light acclimated monocultures, not mimicking environmental conditions. Here we used variable chlorophyll fluorescence imaging, fluorescent labelling in sediment cores and scanning electron microscopy to compare the movement and NPQ responses to light of four epipelic diatom species from a natural MPB biofilm. The diatoms exhibited different species-specific photo-regulation features and a large NPQ range, exceeding that reported for epipsammic diatoms. This could allow epipelic species to coexist in compacted light niches of MPB communities. We show that diatom cell orientation within MPB can be modulated by light, where diatoms oriented themselves more perpendicular to the sediment surface under high light vs. more parallel under low light, demonstrating behavioural, photo-regulatory response by varying their light absorption cross-section. This highlights the importance of considering species-specific responses and understanding cell orientation and photo-behaviour in MPB research.     

Selection of food patches by sympatric herbivores in response to concealment and distance from a refuge

Small herbivores face risks of predation while foraging and are often forced to trade off food quality for safety. Life history, behaviour, and habitat of predator and prey can influence these trade‐offs. We compared how two sympatric rabbits (pygmy rabbit, Brachylagus idahoensis; mountain cottontail, Sylvilagus nuttallii) that differ in size, use of burrows, and habitat specialization in the sagebrush‐steppe of western North America respond to amount and orientation of concealment cover and proximity to burrow refuges when selecting food patches. We predicted that both rabbit species would prefer food patches that offered greater concealment and food patches that were closer to burrow refuges. However, because pygmy rabbits are small, obligate burrowers that are restricted to sagebrush habitats, we predicted that they would show stronger preferences for greater cover, orientation of concealment, and patches closer to burrow refuges. We offered two food patches to individuals of each species during three experiments that either varied in the amount of concealment cover, orientation of concealment cover, or distance from a burrow refuge. Both species preferred food patches that offered greater concealment, but pygmy rabbits generally preferred terrestrial and mountain cottontails preferred aerial concealment. Only pygmy rabbits preferred food patches closer to their burrow refuge. Different responses to concealment and proximity to burrow refuges by the two species likely reflect differences in perceived predation risks. Because terrestrial predators are able to dig for prey in burrows, animals like pygmy rabbits that rely on burrow refuges might select food patches based more on terrestrial concealment. In contrast, larger habitat generalists that do not rely on burrow refuges, like mountain cottontails, might trade off terrestrial concealment for visibility to detect approaching terrestrial predators. This study suggests that body size and evolutionary adaptations for using habitat, even in closely related species, might influence anti‐predator behaviors in prey species.     

Modification of animal habitat by large plants: mechanisms by which seagrasses influence clam growth

Summary Field experiments withMercenaria mercenaria in a relatively high-energy environment demonstrated that clams on unvegetated sand flats failed to grow during autumn while those within seagrass beds grew substantially. Clam growth rates at the seagrass margin that first receives the faster-flowing, flood-tidal currents were about 25% less than at the opposite edge. In a second experiment, pruning, which reduced average blade length by 50–75%, was shown to enhance near-bottom current velocities and to reduce shell growth ofMercenaria during summer by about 50%. As in the first experiment, clams in the unvegetated sand flats exhibited no net growth. Clam mortality, caused mostly by predatory crabs and whelks, was much higher on sand flats than in seagrass beds and intermediate in clipped seagrass. Although consistent with some previous reports, these growth results are still surprising given that they contradict the generalization that suspension feeders grow faster under more rapid current regimes.Three types of indirect interactions might explain the observed effect of seagrass on growth of buried clams: (1) altering food supply; (2) changing the intensity of biological disturbance on feeding clams; and/or (3) affecting the physical stability of the sediments. Previous research on this question has focused almost exclusively on processes that alter food supply rates. In this study, food concentrations, as indicated by suspended chla, were 30% higher inside than outside one seagrass bed, whereas chla concentrations in two other beds were not different from those on adjacent sand flats. This result is sufficient to show that more intense food depletion was not induced by the reduction in flow velocities under the seagrass canopy. Nevertheless, the possible small difference in food concentrations between vegetated and unvegetated bottom seems insufficient to explain the absence of growth of sand-flat clams, especially given the virtual lack of food limitation among suspension feeders in this system. Two data sets demonstrated that the effects of biological disturbance agents cannot be ignored. An outdoor laboratory experiment showed that even in the absence of physical contact between predator and prey the presence of a whelk reduces the amount of time spent feeding byMercenaria. This result suggests that sand flats, where predation rates are higher, may be sites of lower clam growth than seagrass beds because of greater consumer interference with clam feeding. Furthermore, clam siphons are proportionately larger inside seagrass than on sand flats, implying that siphon nipping may not be as intense inside seagrass. This process, too, would reduce net growth of sand-flat clams. Finally, no explicit test was conducted of the hypothesis that enhanced sediment transport in the absence of flow baffling and root binding by seagrass inhibits net growth of clams on high-energy sand flats. Nevertheless, this is a reasonable explanation for the pattern of enhanced growth of seagrass clams, and could serve to explain the otherwise unexplained pattern of lower clam growth at the edge of the seagrass bed that experiences the faster flood-tidal current velocities. Each broad process, changing fluid dynamics, altering consumer access, and varying sediment stability, represents a mechanism whereby habitat structure, provided by the dominant plant, has an important indirect influence on the functional value of the habitat for resident animals.     

Effects of current on photosynthesis and distribution of seagrasses

Studies of the effect of current on seagrass physiology and distribution are few, and the influence of current may be severely underestimated. Current flow may enhance nutrient uptake at the leaf surface of seagrasses by reducing the diffusion boundary layer, modifying the scale of turbulence within the canopy and presenting more nutrients to the leaf. Preliminary laboratory studies suggest that currents between 2 and 50 cm s⁻¹ affect leaf production of Zostera marina L. under light-saturated conditions. Canopy modification of flow structure and light capture efficiency by the deflected canopy should be examined as strategies that could help explain species distribution under different current regimes.Current velocity (together with wave action) creates hydraulic regimes that influence seagrass and seedling distribution. Currents and waves have been correlated with meadow configuration, relief and blowout formation and migration, as well as the distribution of seagrass detritus.     

Ocular dominance stripe formation by regenerated isogenic double temporal retina in Xenopus laevis

In lower vertebrates such as frogs and fish, long ocular dominance stripes with anterior-posterior (A-P) orientation can be produced by causing both eyes to innervate one optic tectum during the course of development. Similar experiments on adult animals usually produce patches rather than stripes. During development, new retinal fibers from the nasal retina segregate into appropriate stripes at the growing edge of the posterior (P) tectum while new temporal fibers segregate at the non-growing anterior (A) tectal edge. Fiber segregation into long A-P oriented stripes might depend upon a template produced by new nasal fibers initiating stripe orientation in the vicinity of new tectal cells; new nasal fibers would orient to the nascent (posterior) edge of the template while temporal fibers would orient to the anterior (non-growing) end of the template. To test the dependence of stripe formation on the matching of nascent retinal cells with nascent tectal cells, we compared stripe orientation in animals with isogenic double nasal innervation and isogenic double temporal innervation of the tectum. In double nasal innervation, the oldest retinal cells innervate the anterior tectum; new fibers from the entire retinal periphery always innervate the newest tectal cells at the posterior tectum. Stripes are oriented A-P, consistent with a maturation front model. In contrast, the oldest retinal cells innervate the newest (posterior) tectal cells in double temporal innervation of the tectum; the growing retinal periphery innervates the non-growing anterior tectum. Stripes are also oriented A-P, indicating that the production of long stripes does not depend upon maturation front matching of nascent retinal fibers and nascent tectal cells.     

Habitat corridors function as both drift fences and movement conduits for dispersing flies

Corridors connect otherwise isolated habitat patches and can direct movement of animals among such patches. In eight experimental landscapes, we tested two hypotheses of how corridors might affect dispersal behavior. The Traditional Corridor hypothesis posits that animals preferentially leave patches via corridors, following them into adjacent patches. The Drift Fence hypothesis posits that animals dispersing through matrix habitat are diverted into patches with corridors because they follow corridors when encountered. House flies (Musca domestica L.), a species that prefers the habitat of our patches and corridors, were released in a central patch (100×100 m) and recaptured in peripheral patches that were or were not connected by a corridor. Flies were captured more frequently in connected than unconnected patches, thereby supporting the Traditional Corridor hypothesis. The Drift Fence hypothesis was also supported, as flies were captured more frequently in unconnected patches with blind (dead end) corridors than in unconnected patches of equal area without blind corridors. A second experiment tested whether these results might be dependent on the type of patch-matrix boundary encountered by dispersing flies and whether edge-following behavior might be the mechanism underlying the observed corridor effect in the first experiment. We recorded dispersal patterns of flies released along forest edges with dense undergrowth in the forest (“closed” edges) and along edges with little forest understory (“open” edges). Flies were less likely to cross and more likely to follow closed edges than open edges, indicating that when patch and corridor edges are pronounced, edge-following behavior of flies may direct them along corridors into connected patches. Because edges in the first experiment were open, these results also suggest that corridor effects for flies in that experiment would have been even stronger if the edges around the source patches and corridors had been more closed. Taken together, our results suggest that corridors can affect dispersal of organisms in unappreciated ways (i.e., as drift fences) and that edge type can alter dispersal behavior.     

Which species will succesfully track climate change? The influence of intraspecific competition and density dependent dispersal on range shifting dynamics

Understanding the ability of species to shift their geographic range is of considerable importance given the current period of rapid climate change. Furthermore, a greater understanding of the spatial population dynamics underlying range shifting is required to complement the advances made in climate niche modelling. A simulation model is developed which incorporates three key features that have been largely overlooked in studies of range shifting dynamics: the form of intraspecific competition, density dependent dispersal and the transient dynamics of habitat patches. The results show that the exact shape of the response depends critically on both local and patch dynamics. Species whose intraspecific competition is contest based are more vulnerable than those whose competition is scramble based. Contesters are especially sensitive when combined with density dependent dispersal. Species living in patches whose carrying capacity grows slowly are also susceptible to rapid shifts of environmental conditions. A complementary analytic approach further highlights the importance of intraspecific competition.     

Single-channel and whole-cell currents evoked by acetylcholine in dissociated sympathetic neurons of the rat

Single acetylcholine-activated channels have been recorded from neurons dissociated from the sympathetic chain of 17-21 day old rats. The mean single channel conductance is 35 pS in normal medium containing 1 mM calcium, and 51 pS in the absence of calcium. The measured current amplitudes are about five times more variable than at the frog endplate, at least in part because the current, while the channel is open, is much noisier than when it is shut. Single activations of the receptor by acetylcholine (ACh) produce a burst of openings; the distribution of the burst length has two components, the longer of which is of primary importance in synaptic transmission. Whole-cell currents, in response to ACh (up to 30 microM), show strong inward rectification with no outward current being detectable. This phenomenon is similar whether the intracellular ion is sodium or cesium, whether or not divalent cations are present, and whether or not atropine is present. Nevertheless, outward single-channel currents (of normal conductance) are detectable in isolated outside-out patches.