Enhancement of sensitivity in photoreceptors of the honey bee drone by light and by Ca2+

Summary Deeply dark adapted (1 h) photoreceptor cells of the honey bee drone show a light-induced enhancement of sensitivity (facilitation) as an aftereffect of illumination or in the presence of dim backgrounds.The Ca²⁺-dependency of this effect was studied: Reduction of extracellular Ca²⁺ to 0.1 mM decreases the sensitivity of a dark adapted cell, and the light-induced increase in sensitivity due to repetitive, dim, 20 ms test flashes is slower than in normal saline. After a sensitizing conditioning light, the sensitivity drops faster in low-calcium saline. The light-induced enhancement of sensitivity is mimicked by pressure injections of low amounts of Ca²⁺ (Ca²⁺/EGTA-buffers; 0.15 M free Ca²⁺) into a dark adapted cell. Injection of EGTA alone decreases the sensitivity. Injection of a solution containing ca1 mM free Ca²⁺ sequentially decreases and later increases the sensitivity transiently.These results suggest a model in which a progressive increase in intracellular Ca²⁺ concentration by light first increases (facilitates), and, at higher concentrations, decreases (light adapts) the sensitivity of the cells. One possible site of action for this positive and negative feedback control of cell sensitivity by Ca²⁺ is the endoplasmic reticulum.     

Facilitative interactions between two lepidopteran herbivores of Asimina

Summary Insect herbivores that require young foliage for successful larval development are often restricted to a single generation during a year by the scarcity of suitable food over most of the growing season. The major specialist herbivore attacking shrubs in the genus Asimina in Florida, Eurytides marcellus, requires young foliage for successful larval development. Field manipulations were used to investigate the role of the young foliage produced by Asimina in response to defoliation by the late-season feeder Omphalocera munroei, a second specialist herbivore of Asimina in Florida, in maintaining Eurytides populations during the summer months when young foliage is otherwise scarce. Defoliation by Omphalocera proved to be the major inducer of young growth during the summer because Omphalocera defoliated Asimina shrubs so frequently and severely. When compared to young leaves produced in the absence of damage, the teaves produced by Asimina in response to defoliation were equally as suitable as food for Eurytides larvae and as acceptable as oviposition sites by Eurytides females. The availability of young foliage in an Asimina population was correlated with the size of the associated Eurytides population. The combination of regular, severe defoliation by Omphalocera and lack of a defensive response to damage by Asimina lead to a positive affect of Omphalocera on Eurytides population size, and may be central to other facilitative interactions between herbivores as well.     

Early primary succession on the volcano Mount St. Helens

Abstract. Primary succession on Mount St. Helens, Washington State, USA, was studied using long-term observational and experimental methods. Distance from potential colonists is a major factor that impedes early primary succession. Sites near undisturbed vegetation remain low in plant cover, but species richness is comparable to intact vegetation. Sites over 500 m from sources of potential colonists have as many species, but mean species richness is much lower than in undisturbed plots. Cover is barely measurable after 11 growing seasons. Highly vagrant species of Asteraceae and Epilobium dominate isolated sites. Sites contiguous to undisturbed communities are dominated by large-seeded species. For a new surface to offer suitable conditions to invading plants, weathering, erosion and nutrient inputs must first occur. The earliest colonists are usually confined to specific microsites that offer some physical protection and enhanced resources. Primary succession on Mount St. Helens has been very slow because most habitats are isolated and physically stressful. Well-dispersed species lack the ability to establish until physical processes ameliorate the site. Species capable of establishment lack suitable dispersal abilities. Subsequently, facilitation may occur, for example through symbiotic nitrogen fixation, but these effects are thus far of only local importance. Lupinus lepidus usually facilitates colonization of other species only after it dies, leaving behind enriched soil lacking any competitors. Experiments and fine-scale observations suggest that successional sequences on Mount St. Helens are not mechanistically necessary. Rather, they result from local circumstances, landscape effects and chance.     

Effect of fish predation on intertidal benthic fauna is modified by crab bioturbation

The burrowing crab Chasmagnathus granulatus is an important bioturbator in SW Atlantic estuaries where they generate dense and extended intertidal beds. Its bioturbation leads to profound changes in the structure, quality and dynamics of sediments with concomitant impacts on the entire benthic community. In this study, we evaluate whether the presence of this crab affects the predator-prey interaction between juvenile fishes and their benthic prey. Gut content and benthic prey selection by juvenile fishes inside and outside crab beds were evaluated, and predation effect was experimentally contrasted between areas using fish exclosures. The results show that in crab beds the percentage of fish with empty guts was lower and the number of polychaetes consumed by fish higher than outside crab beds. The silverside Odontesthes argentinensis and the catfish Pimelodella laticeps fed on larger polychaetes outside than inside crab bed areas, while the white mouth croaker Micropogonias furnieri preyed upon larger polychaetes inside crab beds. In addition, field experiments shows that fish predation decreases polychaete abundances only in crab beds. These results suggest that crab bioturbation facilitate fish predation on benthic prey.     

Negative Plant–Soil Feedback and Positive Species Interaction in a Herbaceous Plant Community

Increasing evidence shows that facilitative interaction and negative plant–soil feedback are driving factors of plant population dynamics and community processes. We studied the intensity and the relative impact of negative feedback on clonal growth and seed germination of Scirpus holoschoenus, a ‘ring’ forming sedge dominant in grazed grassland, and the consequences for species coexistence. The structure of aboveground tussocks was described. A Lithium tracer assessed belowground distribution of functional roots. Seed rain and seedling emergence were compared for different positions in relation to Scirpus tussocks. Soil bioassays were used to compare growth on soil taken from inside and outside Scirpus tussocks of four coexisting species (Mentha acquatica, Pulicaria dysenterica, Scirpus holoschoenus and Dittrichia viscosa). We also compared plant performance of dominant plant species inside and outside Scirpus tussocks in the field. The ‘ring’ shaped tussocks of S. holoschoenus were generated by centrifugal rhizome development. Roots were functional and abundant under the tillers and extending outside the tussocks. The large roots mats that were present in the inner tussock zone were almost all dead. Seedling emergence and growth both showed a strong negative feedback of Scirpus in the inner tussock zone. Scirpus clonal development strongly reduced grass biomass. In the degenerated tussock zone, Pulicaria and Mentha mortality was lower, and biomass of individual plants and seed production were higher. This positive indirect interaction could be related to species-specific affinity to soil conditions generated by Scirpus, and interspecific competitive release in the degenerated tussock zone. We conclude that Scirpus negative feedback affects its seedling emergence and growth contributing to the development of the degenerated inner tussock zone. Moreover, this enhances species coexistence through facilitative interaction because the colonization of the inner tussock zone is highly species-specific.     

Can isotropy vs. anisotropy in the spatial association of plant species reveal physical vs. biotic facilitation?

In dryland ecosystems and other harsh environments, a large part of the vegetation is often clustered, appearing as ‘islands’. If ‘independent’ species, usually colonizers, can be distinguished from species which are ‘dependent’ on the presence of the colonizing species for successful establishment and/or persistence, the type of spatial pattern of the association ‐ isotropic (spatially symmetric) or anisotropic (spatially asymmetric) ‐ can give information on the underlying environmental factors driving the process of association. Modified spatial pattern analysis based on Ripley's K‐function can be applied to bivariate clustered patterns by cardinal direction in order to detect possible anisotropy in the pattern of association. The method was applied to mapped distribution patterns of two types of semi‐arid shrubland in southeastern Spain. In shrubland of Retama sphaerocarpa, low shrubs of Artemisia barrelieri were significantly clustered under the canopy of the Retama shrubs in all four cardinal directions, suggesting an isotropic facilitation effect. In low shrubland dominated by Anthyllis cytisoides and Artemisia barrelieri, Anthyllis shrubs occurred more frequently than expected on the eastern side (and downslope) of an Artemisia shrub. The possible environmental factors driving the two association patterns are discussed and recommendations for further applications of the analytical method are given.     

Mycorrhizal networks and distance from mature trees alter patterns of competition and facilitation in dry Douglas-fir forests

The distribution of dry Douglas-fir forests in western North America is expected to shift northward with climate change and disappear from the grassland interface in the southern interior of British Columbia. This shift may be accentuated by clearcutting, a common harvesting practice that aims to reduce the competitive effects of residual mature trees on new regeneration, but in so doing, ignores their facilitative effects. In this study, we investigated the net effects of competition from and facilitation by mature trees retained on harvested sites on seedling establishment in the dry interface Douglas-fir forests. We demonstrate that access to a mycorrhizal network (MN) and proximity to trees have important influences on seedling performance. On six sites, we established trenched plots around 24 mature Pseudotsuga menziesii var. glauca (Douglas-fir) trees, then planted Douglas-fir seedlings into four mesh treatments that served to restrict MN access (i.e., planted into mesh bags with 0.5-, 35-, or 250-μm pores, or without mesh) or into impermeable bags (grown in isolation) at four distances (0.5, 1.0, 2.5, or 5.0 m). Seedling survival tended to be greater and water stress lower where seedlings had full access to the MN. Seedling height, shoot biomass, needle biomass, and nutrient uptake peaked at 2.5–5.0 m from mature trees. Seedlings 0.5 m from mature trees had lower CO₂ assimilation rates and wood δ¹³C compared to seedlings 5.0 m away. Competition for soil resources was highest near mature trees but facilitation was relatively greater at further distances, resulting in a zone of net benefit for seedlings. These results show that intraspecific tree-seedling interactions are both competitive and facilitative in dry Douglas-fir forests, and that they are spatially dependent. After disturbance, maintaining residual mature trees may be important for their beneficial regeneration zones. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Spatial variation in plant interactions across a severity gradient in the sub-Antarctic

The stress–gradient hypothesis predicts that the intensity of interspecific positive interactions increases along environmental severity (i.e. stress and disturbance) gradients faster than the intensity of negative interactions. This study is the first to test if the stress–gradient hypothesis is supported for a location in the climatically extreme and species-poor sub-Antarctic. To do so, we investigate the fine-scale spatial distribution of plant species across altitude- and aspect-related abiotic severity gradients on a scoria cone on Marion Island. A clear altitudinal severity gradient was observed across the scoria cone, with lower temperatures, stronger winds and greater soil movement at higher altitudes. The altitudinal severity gradient was matched by stronger interspecific spatial association between the four dominant species at higher altitudes and in areas of lower vegetation cover. This suggests that, relative to the intensity of competition, the intensity of facilitation is greater under more severe conditions, supporting the stress–gradient hypothesis at the community level (i.e. for multiple pairs of species) and corroborating its usefulness for predicting variation in plant interactions at high latitudes and altitudes. Furthermore, the directional intraspecific aggregation and interspecific association plant cover patterns found within the gradient suggest that protection from the prevailing wind and from burial by loose substrate are the dominant facilitative mechanisms. Thus, plants benefit from the presence of neighbours when they provide shelter and substrate stability, and the relative intensity of this positive interaction is greatest at higher altitudes, and varies between species pairs. This study, therefore, not only provides support for the stress–gradient hypothesis in the sub-Antarctic, but also demonstrates fine-scale directional spatial patterns between multiple species nested within the severity gradient. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Implications of self/non-self discrimination for spatial patterning of clonal plants

Many clonal plants are characterised by tussock growth forms, but the mechanisms that account for their formation and maintenance are still vague. Here we examine the possible effects of the recently identified phenomenon of self/non-self discrimination on the spatial distribution and patterning of ramets, tussocks and clones in stands of clonal plants. Spatially explicit ramet-based simulation modeling of growth and competition have shown that compact tussocks can be generated as a transient phenomenon that typically disappears at equilibrium. We introduced self/non-self discrimination into a spatial model by decreasing competition between neighbouring ramets on the same clonal fragment. The results demonstrate that self/non-self discrimination can have significant effects on clonal growth and architecture with a clear tendency to generate long-lasting and self-sustaining clumps. Interestingly, this effect was qualitatively independent of other architectural and growth attributes of the plants, making it a candidate mechanism of stable clumped growth forms observed in many clonal plants and communities. Furthermore, the introduction of self/non-self discrimination shifted competition from the level of ramets to that of clonal fragments, which in turns strongly increased genet extinction rates. Our results stress the need for greater attention to the rather neglected scaling up of physiological and morphogenetical controls to the population and community levels.     

Patch-size dependent habitat modification and facilitation on New England cobble beaches by<Emphasis Type="Italic"> Spartina alterniflora</Emphasis>

Most marine habitats are generated by the presence of habitat-modifying species. However, little is know about many aspects of this process, such as how individual- and population-level traits of habitat modifiers affect their ability to reduce environmental stress and thus facilitate other species. An important habitat modifier in New England is the intertidal grass Spartina alterniflora which facilitates the establishment and persistence of cobble beach plant communities by reducing wave-related disturbance. The objectives of this study were to (1) quantify the modification of cobble beach habitats by S. alterniflora, (2) determine how this process is related to S. alterniflora bed traits, and (3) determine why small patches of S. alterniflora generally remain unoccupied by cobble beach plants. Our results demonstrate that S. alterniflora substantially reduces flow-related physical disturbance on cobble beaches. Behind S. alterniflora, mean flow velocity was reduced by 40–60% and substrate stability was dramatically increased compared to portions of the shoreline not bordered by this species. These comparative results were supported by a S. alterniflora shoot removal experiment, which resulted in a 33% increase in average flow velocity and an 85% increase in substrate instability relative to control areas. There was a strong inverse logarithmic relationship between bed length and both average flow velocity and substrate instability behind S. alterniflora. Most S. alterniflora beds were small and bed length was significantly related to the presence of one or more cobble beach plant species. Only 13% of beds <25 m and 40% of beds 30–40 m in length were occupied, in contrast to an occupancy rate of 87% for beds >40 m long. Seeds of two annual cobble beach species (Suaeda linearis and Salicornia europaea) were added to plots behind large (>100 m in length) and small S. alterniflora (<25 m) beds with and without a substrate stabilization manipulation. Seedlings of both species only emerged and established behind small beds when the substrate was stabilized. These results indicate that smaller S. alterniflora patches are usually unoccupied because they do not stabilize the substrate to a degree that meets the establishment requirements of seedlings. Thus, both habitat modification and facilitation by S. alterniflora are patch-size dependent. The conditionality of this facilitation appears to generate a pattern of patchy yet predictable population and community distribution at a landscape spatial scale. Received: 2 November 1998 / Accepted: 15 July 1999