Different physiological functions of free D- and L-alanine in three body parts of the intertidal sipunculid Phascolosoma arcuatum

Free D- and L-alanine contents were comparable in the body wall and introvert cum retractor muscles of Phascolosoma arcuatum. In contrast, the content of free D-alanine in the internal organs was twice that of free L-alanine. Since alanine aminotrans-ferase from P. arcuatum was L-alanine specific, D-alanine appeared to be synthesized from L-alanine through the action of alanine racemase. Alanine racemase activity was higher in the D-alanine-forming direction in the three body parts of P. arcuatum. In addition, the ratio of DL/LD racemase activity in the internal organs was the lowest among the body parts studied. These results indicate that free D-alanine might be of lesser importance than the free D-isomer to the internal organs as compared to the body wall and introvert cum retractor muscles. Indeed, L-alanine inhibited pyruvate kinase from the body wall and introvert cum retractor muscles but had no effect on the pyruvate kinase from the internal organs. Furthermore, the activity of alanopine dehydrogenase present in the internal organs was significantly lower than those of the body wall and introvert cum retractor muscles. L-Alanine was an essential substrate for alanopine formation in the body wall and introvert cum retractor muscles during hypoxia since alanopine dehydrogenases from these body parts were L-alanine specific. When P. arcuatum was confronted with hypo-osmotic stress, the free D-alanine/total free alanine ratio in the internal organs increased approximately from 0.6 to 0.8 as the total free alanine content decreased. In comparison, those ratios in the body wall and introvert cum retractor muscles remained relatively constant. It was concluded that D- and D-alanine had different physiological functions in the three body parts of P. arcuatum.Abbreviations ADP adenosine-5-diphosphate - ADH alanopine dehydrogenase - ALT alanine aminotransferase - AOD amino acid oxidase - BW body wall - EDTA ethylenediaminetetra-acetic acid - EGT A ethylene glyco-bis (-aminoethyl ether) - N,N,N,N tetra-acetic acid - ICRM introvert cum retractor muscles - IO internal organs - I ₅₀ inhibitor concentration producing 50% inhibition of enzyme activity - -KG -ketoglutarate - LDH lactate dehydrogenase - NAD nicotinamide adenine dinucleotide - NADH nicotinamide adenine dinucleotide (reduced form) - PEP phosphoenolpy-ruvate - PEPCK phosphoenolpyruvate carboxykinase - PK pyruvate kinase - PMSF phenylmethylsulphonyl fluoride - SE standard error - SW sea water - TCA trichloroacetic acid     

Habitat linkages: the effect of intertidal saltmarshes and adjacent subtidal habitats on abundance, movement, and growth of an estuarine fish

In this study we used pinfish (Lagodon rhomboides) in field experiments to examine linkages between intertidal saltmarsh and adjacent subtidal habitats. Pinfish are more than twice as abundant in intertidal marshes adjacent to seagrass beds than in those adjacent to the unvegetated subtidal bottom. Movement of pinfish between the marsh edge and the adjacent subtidal habitat was greater for fish captured in areas with both intertidal and subtidal vegetation than in those with intertidal vegetation and adjacent unvegetated mudflats. This movement provides an important link between habitats, allowing transfer of marsh-derived secondary production to subtidal seagrass beds and vice versa. Pinfish held in enclosures with both intertidal and subtidal vegetation were, on average, approximately 90% heavier than fish held in enclosures with intertidal vegetation and unvegetated subtidal bottom. Because saltmarshes and seagrass beds contribute to the production of living marine resources, active measures are being taken to preserve and restore these habitats. The results from this study have direct application to decisions concerning site selection and optimal spatial proximity of saltmarsh and seagrass habitats in the planning of restoration and mitigation projects. To maximize secondary production and utilization of intertidal marshes, managers may opt to restore and/or preserve marshes adjacent to subtidal seagrass beds. Received: 31 May 1996 / Accepted: 23 October 1996     

N-acetyl-L-cysteine as a source of sulfane sulfur in astrocytoma and astrocyte cultures: correlations with cell proliferation

Summary. N-acetyl-L-cysteine (NAC), a precursor of L-cysteine, not only elevates the level of glutathione in both astrocytoma and astrocyte cultures, but also affects the cellular level of sulfane sulfur. Astrocytoma cells were investigated using the stable U373 human cell line. In the U373 cells, N-acetyl-L-cysteine, depending on the concentration in the culture medium and culture duration, either elevated or diminished the level of sulfane sulfur, and this was respectively accompanied by decreased or increased cellular proliferation. In murine astrocytes, in turn, NAC was capable of lowering the level of sulfane sulfur and in this way decreased cellular proliferation. It seems that normal (astrocyte) and transformed (astrocytoma) cells differed in their reaction to NAC in the culture medium. The effect of N-acetyl-L-cysteine on astrocytoma cells was advantageous in that it inhibited their proliferation through the elevation of the level of sulfane sulfur. Authors’ address: Maria Wróbel, Chair of Medical Biochemistry, Jagiellonian University Medical College, Kopernika 7 St., 31-034 Kraków, Poland     

Winter production of CO2 and N2O from alpine tundra: environmental controls and relationship to inter-system C and N fluxes

Fluxes of CO₂ and N₂O were measured from both natural and experimentally augmented snowpacks during the winters of 1993 and 1994 on Niwot Ridge in the Colorado Front Range. Consistent snow cover insulated the soil surface from extreme air temperatures and allowed heterotrophic activity to continue through much of the winter. In contrast, soil remained frozen at sites with inconsistent snow cover and production did not begin until snowmelt. Fluxes were measured when soil temperatures under the snow ranged from –5°C to 0°C, but there was no significant relationship between flux for either gas and temperature within this range. While early developing snowpacks resulted in warmer minimum soil temperatures allowing production to continue for most of the winter, the highest CO₂ fluxes were recorded at sites which experienced a hard freeze before a consistent snowpack developed. Consequently, the seasonal flux of CO₂ –C from snow covered soils was related both to the severity of freeze and the duration of snow cover. Over-winter CO₂ –C loss ranged from 0.3 g C m⁻² season⁻¹ at sites characterized by inconsistent snow cover to 25.7 g C m⁻² season⁻¹ at sites that experienced a hard freeze followed by an extended period of snow cover. In contrast to the pattern observed with C loss, a hard freeze early in the winter did not result in greater N₂O–N loss. Both mean daily N₂O fluxes and the total over-winter N₂O–N loss were related to the length of time soils were covered by a consistent snowpack. Over-winter N₂O–N loss ranged from less 0.23 mg N m⁻² from the latest developing, short duration snowpacks to 16.90 mg N m⁻² from sites with early snow cover. These data suggest that over-winter heterotrophic activity in snow-covered soil has the potential to mineralize from less than 1% to greater than 25% of the carbon fixed in ANPP, while over-winter N₂O fluxes range from less than half to an order of magnitude higher than growing season fluxes. The variability in these fluxes suggests that small changes in climate which affect the timing of seasonal snow cover may have a large effect on C and N cycling in these environments. Received: 5 April 1996 / Accepted: 25 November 1996     

Blood flow in the carotid artery during breath-holding in relation to diving bradycardia

The present study investigated the mechanism of diving bradycardia. A group of 14 healthy untrained male subjects were examined during breath-holding either out of the water (30–33°C), in head-out immersion, or in whole-body submersion (27–29°C) in a diving pool. Blood velocity, blood volume flow in the carotid artery, diastolic blood pressure and electrocardiogram were measured and recorded during the experiments. The peak blood velocity increased by 13.6% (P < 0.01) and R-wave amplitude increased by 57.1% (P < 0.005) when the subjects entered water from air. End-diastolic blood velocity in the carotid artery increased significantly during breath-holding, e.g. increased from 0.20 (SD 0.02) m · s⁻¹ at rest to 0.33 (SD 0.04) m · s⁻¹ (P < 0.001) at 50.0 s in breath-hold submersion to a 2.0-m depth. Blood volume flow in the carotid artery increased by 26.6% (P < 0.05) at 30 s and 36.6% (P < 0.001) at 40 s in breath-hold submersion to a 2.0-m depth. Diastolic blood pressure increased by 15.4% (P < 0.01) at 60 s during breath-holding in head-out immersion. Blood volume flow, and diastolic blood pressure increased significantly more and faster during breath-holding in submersion than out of the water. There was a good negative correlation with the heart rate: the root mean square correlation coefficient r was 0.73 (P < 0.001). It was concluded that an increased accumulation of blood in the aorta and arteries at end-diastole and decreased venous return, caused by an increase in systemic peripheral resistance during breath-holding, underlies diving bradycardia. Accepted: 22 November 1996     

Changes in selected aspects of immune function in the leopard frog, Rana pipiens, associated with exposure to cold

The effect of exposure to low temperatures (5 °C) on lymphocyte proliferation, leukocyte populations, and serum complement levels was examined in the northern leopard frog, Rana pipiens. Proliferation of T lymphocytes in response to phytohemagglutinin stimulation was significantly decreased in frogs kept for 2, 3, and 5 months at 5 °C compared to that of animals kept at 22 °C. A significant increase in the average percentage of neutrophils and a decrease in the mean percentage of eosinophils was observed in the blood of frogs held for 5 months in the cold compared to animals held at 22 °C for the same length of time. Mean serum complement activity after 1 month at 5 °C was significantly reduced in comparison to animals held at 22 °C and was not detectable after 5 months in the cold. Recovery of complement levels at room temperature (22 °C) was also examined after cold exposure. Complement levels were significantly higher than controls (at 22 °C) in frogs returned to 22 °C for 7 and 14 days after 5 months in the cold. After frogs were held at 5 °C for 1 month, serum complement levels increased significantly within 2 days after returning to 22 °C and continued to rise 5 and 9 days after warming. Injections with Aeromonas hydrophila following a 5-week exposure to 5 °C failed to cause death or observable symptoms of disease in frogs that were returned to 22 °C. Accepted: 20 November 1996     

Application of a 15N tracer to simulate and track the fate of atmospherically deposited N in the coastal forests of the Waquoit Bay Watershed, Cape Cod, Massachusetts

The central grassland region of the United States encompasses major gradients in temperature and precipitation that determine the distribution of plant life forms, which in turn may influence key ecosystem processes such as nutrient cycling and soil organic matter dynamics. One such gradient is the threefold increase in precipitation from the eastern Colorado shortgrass-steppe, in the rain shadow of the Rocky Mountains, to the tallgrass prairie in eastern Kansas. We investigated the relative roles of plant species and plant cover in influencing soil C and N cycling in three sites along this gradient. Plant cover (i.e., the presence or absence of an individual plant) was relatively more important than plant species in explaining variability in soil properties at the dry site, the Central Plains Experimental Range in␣northeastern Colorado. However, plant species explained relatively more of the variability in soil properties than did plant cover at the two wetter sites, Hays and Konza, in central and eastern Kansas. The wetter sites had more continuous plant cover, resulting in less plant-cover-induced variation in soil C and N, than did the dry site, which had distinct patches of bare ground. Plant species at the wetter sites had higher and more variable levels of tissue C:N than plant species at the dry site, due to both within species changes and changes in species composition. Aboveground tissue C:N was better correlated with net nitrogen mineralization rates at the wet sites than the dry site. Thus, tissue chemistry appears to exert more control on nitrogen dynamics at the wet than the dry sites. The results suggest that plant species traits that are relevant to nutrient cycling (e.g., tissue C:N ratios, spatial patterns, productivity) reflect environmental limitations as well as species' physiological potentials. Furthermore, a dominant environmental driver such as precipitation may ameliorate or exaggerate the importance of individual species traits for nutrient cycling. Received: 11 July 1996 / Accepted: 5 December 1996     

Responses to object approach by a wide field visual neurone, the LGMD2 of the locust: Characterization and image cues

The LGMD2 belongs to a group of giant movement-detecting neurones which have fan-shaped arbors in the lobula of the locust optic lobe and respond to movements of objects. One of these neurones, the LGMD1, has been shown to respond directionally to movements of objects in depth, generating vigorous, maintained spike discharges during object approach. Here we compare the responses of the LGMD2 neurone with those of the LGMD1 to simulated movements of objects in depth and examine different image cues which could allow the LGMD2 to distinguish approaching from receding objects. In the absence of stimulation, the LGMD2 has a resting discharge of 10–40 spikes s⁻¹ compared with <1 spike s⁻¹ for the LGMD1. The most powerful excitatory stimulus for the LGMD2 is a dark object approaching the eye. Responses to approaching objects are suppressed by wide field movements of the background. Unlike the LGMD1, the LGMD2 is not excited by the approach of light objects; it specifically responds to movement of edges in the light to dark direction. Both neurones rely on the same monocular image cues to distinguish approaching from receding objects: an increase in the velocity with which edges of images travel over the eye; and an increase in the extent of edges in the image during approach. Accepted: 23 October 1996     

Oxygen consumption in the foraging honeybee depends on the reward rate at the food source

Oxygen consumption of the honeybee Apis mellifera ligustica was measured as a function of the flow rate supply of sucrose solution at an automatic feeder located inside a respirometric chamber. Trained bees freely entered the respirometric chamber and collected the sucrose solution supplied. The mean value of the O₂ consumption rate per visit increased with the sucrose flow rate, and for a given flow rate, with increasing locomotor activity. However, when no locomotor activity was displayed, O₂ consumption also increased with increasing nectar flow rate. Crop load attained at the end of the visit showed a positive relationship with the nectar flow rate; however, for a given flow rate, O₂ consumption showed either no correlation or a negative one with the final crop load attained. It is concluded that the energy expenditure of the foraging bee is controlled by a motivational drive whose intensity depends on the reward rate at the food source. Accepted: 30 July 1996     

Young chickens learn to localize the centre of a spatial environment

Young chickens were trained to find food by ground-scratching in the centre of a closed uniform arena and were then tested in arenas of similar areas but of different shapes. Chickens showed localized searching behaviour in the square-shaped arena, and maintained this behaviour when placed in a circular or triangular (both equilateral and isosceles) arena. With a rectangular-shaped arena, obtained by doubling the original square-shaped one, chickens showed more dispersed searching along the major axis, but searching tended to be concentrated around the centres of the composing squares and around the centre of the rectangle itself. When trained in a square- or triangle-shaped arena and then tested in an arena of the same shape but a larger area, chickens displayed searching behaviour at two different distances from the wall of the arena, one corresponding to the correct distance (i.e. centre) in the smaller (training) arena, the other to the actual centre of the test arena. On the other hand, in a circular arena, chickens searched mainly at a distance midway between the radius of the small (training) and of the large (testing) circular arena. These results suggest that, during training, chickens encoded information on both the absolute and the relative distance of the food from the walls of the arena, the latter information being more accurate when the arena displayed identifiable features such as corners. Accepted: 28 December 1996     

Neural correlates of habituation of the proleg withdrawal reflex in larvae of the hawk moth, Manduca sexta

The larval proleg withdrawal reflex of the hawk moth, Manduca sexta, exhibits robust habituation. This reflex is evoked by deflecting one or more mechanosensory planta hairs on a proleg tip. We examined neural correlates of habituation in an isolated proleg preparation consisting of one proleg and its segmental ganglion. Repeated deflection of a single planta hair caused a significant decrease in the number of action potentials evoked in the proleg motor nerve (which carries the axons of proleg retractor motor neurons). Significant response decrement was seen for interstimulus intervals of 10 s, 60 s and 5 min. Response decrement failed to occur in the absence of repetitive stimulation, the decremented response recovered spontaneously following a rest, and electrical stimulation of a body wall nerve facilitated the decremented response (a neural correlate of dishabituation). Adaptation of sensory neuron responses occurred during repeated hair deflections. However, when adaptation was eliminated by direct electrical stimulation of sensory neurons, the response in the proleg motor nerve still decreased significantly. Muscle recordings indicated that the response of an identified proleg retractor motor neuron decreased significantly during habituation training. Thus, habituation of the proleg withdrawal reflex includes a central component that is apparent at the level of a single motor neuron. Accepted: 20 December 1996     

Directionality of auditory nerve fiber responses to pure tone stimuli in the grassfrog, Rana temporaria. II. Spike timing

We studied the directionality of spike timing in the responses of single auditory nerve fibers of the grass frog, Rana temporaria, to tone burst stimulation. Both the latency of the first spike after stimulus onset and the preferred firing phase during the stimulus were studied. In addition, the directionality of the phase of eardrum vibrations was measured. The response latency showed systematic and statistically significant changes with sound direction at both low and high frequencies. The latency changes were correlated with response strength (spike rate) changes and were probably the result of directional changes in effective stimulus intensity. Systematic changes in the preferred firing phase were seen in all fibers that showed phaselocking (i.e., at frequencies below 500–700 Hz). The mean phase lead for stimulation from the contralateral side was approximately 140° at 200 Hz and decreased to approximately 100° at 700 Hz. These phaseshifts correspond to differences in spike timing of approximately 2 ms and 0.4 ms respectively. The phaseshifts were nearly independent of stimulus intensity. The phase directionality of eardrum vibrations was smaller than that of the nerve fibers. Hence, the strong directional phaseshifts shown by the nerve fibers probably reflect the directional characteristics of extratympanic pathways. Accepted: 23 November 1996     

An Aspergillus nidulans nuclear protein, AnCP, involved in enhancement of Taka-amylase A gene expression, binds to the CCAAT-containing taaG2 , amdS , and gatA promoters

Using AnCP (Aspergillus nidulans CCAAT-binding protein) as a CCAAT-specific binding factor model, the possibility that one factor is able to recognize CCAAT sequences in several different genes in A.␣nidulans was examined. DNase I protection analysis showed that AnCP specifically bound to CCAAT sequence-containing regions comprising 21 to 36 bp of the taa, amdS and gatA genes. Furthermore, replacement of the CCAAT sequence with CGTAA was found to abolish the binding of AnCP and to have an inhibitory effect on taa promoter activity. This clearly demonstrates a positive function of the CCAAT element. However, amylase was induced by starch and repressed by glucose in a CCAAT-box disruptant, as in wild-type cells. Received: 28 June 1996 / Accepted: 7 October 1996     

Multisegmental motor activity in the segmentally restricted gin trap behavior in Manduca sexta pupae

Stimulation of sensory neurons innervating hairs in the gin traps on the abdomen of Manduca sexta pupae evokes a rapid bending of the abdomen that is restricted to one or more of the three articulating posterior segments. However, electrical stimulation of the gin trap sensory nerve in an isolated abdominal nerve cord evokes characteristic motor neuron activity in every abdominal segment. To determine if the segmentally distributed motor activity also occurred in intact animals and how it contributed to the segmentally restricted reflex movement, mechanical stimulation of the sensory hairs in intact animals was used to evoke reflex responses that were recorded as electromyograms synchronized with video recordings of the behavior. Motor activity was monitored during movements to determine if there was activity in many segments when the movement was restricted to one segment. Coordinated muscle activity was evoked throughout the abdomen in response to stimulation of any of the three gin traps, even when movement was restricted to one segment. Differences in the timing of ipsilateral and contralateral motor activity among segments allowed the closing of gin traps to be segmentally restricted. These findings suggest that the neural circuit underlying the gin trap reflex is distributed throughout the abdominal nerve cord. This network generates a complex, yet coordinated, motor pattern with muscular activity in many abdominal segments that produces a localized bending reflex. Accepted: 10 January 1997     

Growth, herbivory and disease in relation to gender in Salix viminalis L.

Many species of dioecious plants show sex-related differences in growth rate and rates of attack by various herbivores and diseases. The common pattern is for males to grow faster than females and to be less well defended against herbivores. In willows (Salix spp.), the predominance of female-biased sex ratios has been ascribed in part to differential feeding by herbivores. In this study of Salix viminalis, seven families grown on agricultural land showed no gender-related variation in shoot biomass or rates of herbivory by insects (lepidopterans and cecidomyiids). However, Melampsora rust disease was found to be more severe on females than on males when the plants were in a non-reproductive stage. After flowering and seed-set females tended to be more affected in some families but less affected in others. Although, on average, there was a female bias in the sex ratio of S. viminalis, sex ratios differed significantly between families. These ratios were not related to any of the recorded biotic agents, but rather to relationships between families. These results are interpreted in terms of resource allocation between reproduction, growth and defence, and causes for divergence from the expected patterns are discussed. The results may have implications for S. viminalis breeding strategies where the aim is to produce biofuel. For instance, these findings suggest that gender can be ignored when selecting for a high growth rate and resistance to Melampsora and certain insect pests. Received: 2 November 1996 / Accepted: 8 February 1997     

Adaptive reconfiguration of a reflex circuit during different motor programmes in the locust

The cuticle strain which develops in the hindleg tibiae when a locust prepares to kick, or when the tibia thrusts against an obstacle, is detected by two campaniform sensilla, which reflexly excite the fast extensor tibiae motoneuron, some of the flexor tibiae motoneurons and nonspiking interneurons. The reflex excitation is adaptive for the extensor motoneuron during both co-activation and thrusting, but is only adaptive for the flexor motoneurons during co-activation, and is maladaptive during thrusting. We show that the femoral chordotonal organ, which monitors tibial position, controls the efficacy of the strain feedback. The campaniform sensilla-induced depolarization in the extensor motoneuron is about twice as large when the tendon is in mid position (reflecting a tibial-femoral angle of 90°) than when fully stretched (reflecting tibial flexion), while in the flexors the reverse is true. The amplitudes of excitatory postsynaptic potentials evoked by single campaniform sensilla spikes, are, however, not affected. Our data suggests that the chordotonal organ modulates the gain of the strain feedback onto the motoneurons by exciting interneuronal circuits whose output sums with the former. Thrusting typically occurs with the tibia partially extended, therefore the actions of the chordotonal organ support the production of a maximal thrusting force. Accepted: 27 December 1996     

HU protein binding to the replication origin of the rolling-circle plasmid pKYM enhances DNA replication

The RepK protein, which is encoded by the rolling-circle plasmid pKYM, binds to the PR I site in the pKYM DNA replication origin. We have identified HU as a protein that binds to the PR II and PR III sites in the replication-enhancing region which is downstream of PR I. DNA footprinting assays show that HU binds to these two sites only when RepK is bound to PR I, and that HU also enhances the binding of RepK to PR I. In vivo, pKYM was unable to transform an HU null strain. Two mutant RepK proteins, RepKW179Y, which contains a Trp-to-Tyr exchange at position 179, and RepKD277L, which contains an Asp-to-Leu mutation at residue 277, initiate DNA replication in vivo in the absence of HU. In vitro, these mutant RepK proteins form more stable complexes with the pKYM origin region than does the wild-type RepK protein. These results indicate that HU plays a role in the formation of a stable RepK-origin complex, which is required for the initiation of pKYM DNA replication. Received: 24 July 1996 / Accepted: 30 December 1996     

c-fos expression in the putative avian suprachiasmatic nucleus

c-fos induction was investigated as a potential component in the avian photic entrainment pathway and as a possible means of locating the central pacemaker in birds. In both quail (Coturnix coturnix japonica) and starlings (Sturnus vulgaris) exposure to 1 h of light induced Fos-lir in the visual suprachiasmatic nucleus but not in the medial suprachiasmatic nucleus. However, the degree of c-fos induction in the visual suprachiasmatic nucleus was similar at different circadian times despite the fact that the light pulses caused differential phase shifts in the locomotor rhythm. For golden hamsters the same experiment resulted in significantly different levels of Fos-lir in the suprachiasmatic nucleus, as well as different phase shifts. Starlings and hamsters were also entrained to T-cycles that caused a large daily phase shift (T = 21.5 h in starlings, T = 22.67 hours in hamsters), or no daily phase shift (T = free running period). No difference in the induced levels of Fos-lir in the visual suprachiasmatic nucleus region was observed between the two groups of starlings, but in hamsters there were significantly different levels of Fos-lir in the suprachiasmatic nucleus between the two groups. Accepted: 15 November 1996     

Life on the edge: conspecific attraction and recruitment of populations to disturbed habitats

Variation in the recruitment of benthic marine invertebrates is often attributed to the interaction of the supply of new individuals to a habitat and the availability of space for colonisation when they arrive. Also important in determining variation in recruitment is the response of the larvae to the characteristics of the habitat. Larvae of many benthic marine invertebrates have shown great specificity of requirements in setting their limits of distribution at the time of selection of a habitat. The tubeworm Galeolaria caespitosa shows great variation in recruitment from place to place on rocky intertidal seashores and is a gregarious animal with larvae showing directed responses to conspecific adults on the substratum. I hypothesised that, if variation in recruitment of G. caespitosa were independent of conditions on the substratum, the magnitude of recruitment in patches of the same shape but different sizes cleared within continuous mats of conspecific adults would be directly related to the area available for colonisation in the patch. Alternatively, if variation in recruitment were due to the response of larvae to conspecific adults on the substratum, the magnitude of recruitment would be a function of the perimeter of the patch, which, given patches of the same shape, is a measure of the influence of conspecific adults in that patch. To distinguish between these alternatives, small (area = 25 cm²; perimeter = 20 cm) and large (area = 225 cm²; perimeter = 60 cm) square patches were cleared within continuous mats of conspecific adults at four sites and recruitment of G. caespitosa was monitored over two seasons of recruitment. The density of recruits per unit area was, on average, almost three times greater in small than in large patches, indicating that recruitment of G. caespitosa is not directly related to the area of the patch. In contrast, the density of recruits per unit perimeter was not significantly different between small and large patches, indicating that recruitment of G. caespitosa is related to the proximity of conspecific adults in the patch. Therefore, at a given site, the perimeter of patches within mats of G. caespitosa is a better predictor of the relative magnitude of recruitment among patches than that provided by their areas. These results are contrary to many models of invertebrate recruitment that assume close linkage between available space on the substratum and settlement. Moreover, they highlight the importance of behavioural responses of animals at the time of selection of habitat in accounting for variation in recruitment. For populations of organisms that display gregarious behaviour at settlement, or conspecific attraction, this direct relationship between the perimeter of patches and recruitment could be used as a tool in restoring populations to disturbed habitats. The added benefit of such facilitative interactions in restoring populations is that they provide increasing returns to the population for a given supply of potential colonists to a habitat. Received: 1 November 1996 / Accepted: 20 January 1997     

Post-dispersal seed predation and the establishment of vertebrate dispersed plants in Mediterranean scrublands

The post-dispersal fate of seeds and fruit (diaspores) of three vertebrate-dispersed trees, Crataegus monogyna, Prunus mahaleb and Taxus baccata, was studied in the Andalusian highlands, south-eastern Spain. Exclosures were used to quantify separately the impact of vertebrates and invertebrates on seed removal in relation to diaspore density and microhabitat. The three plant species showed marked differences in the percentage of diaspores removed, ranging from only 5% for C. monogyna to 87% for T. baccata. Although chaffinches (Fringilla coelebs) fed on diaspores, rodents (Apodemus sylvaticus) were the main vertebrate removers of seed and fruit. Two species of ant (Cataglyphis velox and Aphaenogaster iberica) were the only invertebrates observed to remove diaspores. However, the impact of ants was strongly seasonal and they only removed P. mahaleb fruit to any significant extent. While removal of seed by rodents was equivalent to predation, ants were responsible for secondary dispersal. However, their role was limited to infrequent, small-scale redistribution of fruit in the vicinity of parent trees. Rodents and ants differed in their use of different microhabitats. Rodents foraged mostly beneath trees and low shrubs and avoided open areas while the reverse was true of ants. Thus, patterns of post-dispersal seed removal will be contigent on the relative abundance and distribution of ants and rodents. Studies which neglect to quantify separately the impacts of these two guilds of seed removers may fail to elucidate the mechanisms underlying patterns of post-dispersal seed removal. The coincidence of both increased seed deposition by the main avian dispersers (Turdus spp.) and increased seed predation with increasing vegetation height suggested that selection pressures other than post-dispersal seed predation shape the spatial pattern of seed dispersal. Rather than providing a means of escaping post-dispersal seed predators, dispersal appears to direct seeds to microhabitats most suitable for seedling survival. Nevertheless, the reliance of most vertebrate-dispersed trees on regeneration by seed and the absence of persistent soil seed banks imply that post-dispersal seed predators may exert a strong influence on the demography of the plants whose seeds they consume. Even where microsites are limited, the coincidence of the most suitable microhabitats for seedling establishment with those where seed predation is highest provide a means by which selective seed predators can influence community composition. Received: 19 August 1996 / Accepted: 25 January 1997