Effect of seed predation on seed bank size and seedling recruitment of bush lupine (Lupinus arboreus)

Whether seed consumers affect plant establishment is an important unresolved question in plant population biology. Seed consumption is ubiquitous; at issue is whether seedling recruitment is limited by safe-sites or seeds. If most seeds inhabit sites unsuitable for germination, post-dispersal seed consumption primarily removes seeds that would otherwise never contribute to the population and granivory has minimal impacts on plant abundance. Alternatively, if most seeds ultimately germinate before they lose viability, there is greater potential for seed consumption to affect plant recruitment. Of the many studies on seed consumption, few ask how seed loss affects seedling recruitment for species with long-lived seed banks. We examined post-dispersal seed predation and seedling emergence in bush lupine (Lupinus arboreus), a woody leguminous shrub of coastal grasslands and dunes in California. We followed the fate of seeds in paired experimental seed plots that were either protected or exposed to rodent granivores in grassland and dune habitats. Significantly more seeds were removed by rodents in dunes than grasslands. In dunes, where rodent granivory was greatest (65% and 86% of seeds removed from plots by rodents in two successive years), there is a sparse seed bank (6.6 seeds m⁻²), and granivory significantly reduced seedling emergence (in the same two years, 18% and 19.4% fewer seedlings emerged from exposed versus protected plots), suggesting seed rather than safe-site limited seedling recruitment. In contrast, rodents removed an average of 6% and 56% of seeds from grassland plots during the same two years, and the grassland seed bank is 43-fold that of the dunes (288 seeds m⁻²). Even high seed consumption in the second year of the study only marginally influenced recruitment because seeds that escaped predation remained dormant. Burial of seeds in both habitats significantly reduced the percentage of seeds removed by rodents. Results suggest that granivores exert strong but habitat-dependent effects on lupine seed survival and seedling emergence. Received: 24 October 1996 / Accepted: 4 February 1997     

Density-dependent predation by skunks using olfactory search images

The formation of search images can create density-dependent predation. Predators have been shown to form search images when searching for many small prey items in one feeding session. This paper reports experiments that test whether striped skunks can form olfactory search images in other situations: when prey are found over several days, when prey are large, and when prey are found in certain habitats. Striped skunks were raised in captivity, and their reaction distance to food was measured outside in a natural grassy area. In experiment 1 skunks were offered many small food items for several days in a row. From one day to the next, skunks initially detected food from further away, they increased detection distance faster and their maximum detection distance increased – i.e., they formed olfactory search images faster and stronger from one day to the next. In experiment 2 skunks formed search images over several days when finding only one large food item per day. In experiment 3 skunks lost olfactory search images when they entered habitats in which they had previously searched for another type of food. These long-term search images magnify the effects of short-term search images, extend the effects of short- term search image to longer time spans, and affect different species from short-term search images. Received: 26 July 1996 / Accepted: 13 December 1996     

Interspecific competition and insect herbivory reduce bush lupine (Lupinus arboreus ) seedling survival

Seedlings suffer high mortality in most plant populations, with both competition and herbivory proposed as being important mechanisms causing seedling death. The relative strength of these factors, however, is often unknown. Here I ask how interspecific competition for light and insect herbivory jointly affect seedling survival of bush lupine (Lupinus arboreus), a native shrub common to coastal California. Bush lupine seedlings germinate in grasslands during winter, and throughout spring potentially compete for light with surrounding fast-growing annual grasses. By early summer, after grasses have died, seedlings can be defoliated by a locally abundant caterpillar, the western tussock moth (Orgyia vetusta). I examined the relative importance of competition and insect herbivory on seedling survival in two separate experiments. First, I compared seedling mortality in plots either exposed to or protected from tussock moth larvae. Plants were protected from herbivory by the judicious use of insecticide; control plants were sprayed with water. Tussock moth herbivory resulted in significantly greater (31%) seedling mortality. To determine the effects of interspecific competition for light on seedling survival, I manipulated the density of grass surrounding lupine seedlings. I removed all vegetation surrounding some individuals, and left intact vegetation surrounding others. Reducing competition resulted in a 32% increase in seedling survival from February to June, as well as changes in seedling growth. To determine whether there are interactive effects of competition and herbivory on seedling survival, I enclosed tussock moth larvae on half of all surviving seedlings within each of the two prior competition treatments, comparing growth and survival of defoliated and undefoliated seedlings. Defoliation in June led to an additional 50% mortality for individuals that had grown with competitors through spring, and a 53% additional mortality for seedlings that grew without competitors through spring. Thus, although competition and herbivory both caused substantial seedling mortality, there was no statistical interaction between these factors. Competition-free plants were not less vulnerable to herbivory than plants that previously grew with competitors. Taken together, these experiments indicate that competition and herbivory are both important sources of mortality for bush lupine seedlings. Received: 4 April 1996 / Accepted: 5 November 1996     

High vapour pressure deficit exacerbates xylem cavitation and photoinhibition in shade-grown Piper auritum H.B. & K. during prolonged sunflecks

Water relations dynamics during simulated sunflecks at high (36°C) and medium (27°C) temperatures and high and low vapour pressure deficits beween leaf and air (VPD) were studied on shade-grown Piper auritum H.B. & K. plants, a pioneer tree, common in gaps and clearings of tropical rain forests. The leaves of P. auritum wilt rapidly when exposed to high light. Exposure to high VPD and high light caused substantial and rapid dehydration of leaves. Dehydration could be prevented under high humidity irrespective of temperature. Water stored in leaf cells served as initial source for transpiration upon high light exposure. This effect increased with increasing VPD and temperature. The pronounced decrease in leaf water content over time in high light caused a rapid decrease in leaf water potential (Ψ_l) and a concomitant increase in water potential gradient (ΔΨ/Δx) between trunk and leaf, yet the high leaf elasticity (small bulk elastic modulus, ε) allowed turgor maintenance under most conditions. Under high VPD and high temperature, stomata remained open and ΔΨ/Δx frequently exceeded 0.95 MPa · m⁻¹, the cavitation-inducing threshold (ΔΨ/Δx _cav) causing high rates of acoustic emissions from stems and leaf petioles and leading to concomitant losses in hydraulic conductance per leaf area (k _l). At medium temperature (high VPD), stomatal closure contained xylem embolism by keeping ΔΨ/Δx at or below this threshold. We argue that wilting substantially contributes to creating a sufficient driving force for water uptake from the soil, and reducing the VPD (through a decrease in radiation load and thus leaf temperature) to avoid excessive dehydration. Received: 3 March 1996 / Accepted: 10 November 1996     

Two uvs genes of Aspergillus nidulans with different functions in error-prone repair: uvsI , active in mutation-specific reversion, and uvsC , a recA homolog, required for all UV mutagenesis

Two genes of Aspergillus nidulans are known to function in UV mutagenesis, but have been assigned to different epistasis groups: uvsC, which is also required for meiosis and mitotic recombination, and uvsI, which may have no other function. To clarify their role in error-prone repair and to investigate their interaction, uvsI and uvsC single and uvsI;uvsC double mutant strains were further tested for mutagen sensitivities and characterized for effects on mutation. Spontaneous and induced frequencies were compared in forward and reverse mutation assays. All results confirmed that uvsI and uvsC are members of different epistasis groups, and demonstrated that these uvs mutants have very different defects in UV mutagenesis. The uvsI strains showed wild-type frequencies in all forward mutation tests, but greatly reduced spontaneous and UV-induced reversion of some, but not other, point mutations. In contrast, uvsC had similar effects in all assay systems: namely pronounced mutator effects and greatly reduced UV mutagenesis. Interestingly, the uvsI;uvsC double mutant strains differed from both single mutants; they clearly showed synergism for all types of reversion tested: none were ever obtained spontaneously, nor after induction by UV or EMS (ethylmethane sulfonate). Based on these results, we conclude that uvsI is active in a mutation-specific, specialized error-prone repair process in Aspergillus. In contrast, uvsC, which is now known to show sequence homology to recA, has a basic function in mutagenic UV repair in addition to recombinational repair, similar to recA of Escherichia coli. Received: 23 September 1996 / Accepted: 2 December 1996     

Temperature regulation and metabolism of an Australian bat, Chalinolobus gouldii (Chiroptera: Vespertilionidae) when euthermic and torpid

The thermal and metabolic physiology of Chalinolobus gouldii, an Australian vespertilionid bat, was studied in the laboratory using flow-through respirometry. Chalinolobus gouldii exhibits a clear pattern of euthermic thermoregulation, typical of endotherms with respect to body temperature and rate of oxygen consumption. The basal metabolic rate of euthermic Chalinolobus gouldii is approximately 86% of that predicted for a 17.5-g mammal and falls into the range of mass-specific basal metabolic rates ascribed to vespertilionid bats. However, like most vespertilionid bats, Chalinolobus gouldii displays extreme thermolability. It is able to enter into torpor and spontaneously arouse at ambient temperatures as low as 5 °C. Torpid bats thermoconform at moderate ambient temperature, with body temperature ≈ ambient temperature, and have a low rate of oxygen consumption determined primarily by Q ₁₀ effects. At low ambient temperature (< 10 °C), torpid C. gouldii begin to regulate their body temperature by increased metabolic heat production; they tend to maintain a higher body temperature at low ambient temperature than do many northern hemisphere hibernating bats. Use of torpor leads to significant energy savings. The evaporative water loss of euthermic bats is relatively high, which seems unusual for a bat whose range includes extremely arid areas of Australia, and is reduced during torpor. The thermal conductance of euthermic C. gouldii is less than that predicted for a mammal of its size. The thermal conductance is considerably lower for torpid bats at intermediate body temperature and ambient temperature, but increases to euthermic values for torpid bats when thermoregulating at low ambient temperature. Accepted: 22 August 1996     

How symbiotic bacteria influence plant utilisation by the polyphagous aphid, Aphis fabae

To explore the effect of rearing-plant species on the contribution of the symbiotic bacterium, Buchnera, to aphid performance, larvae of Aphis fabae that contained the bacteria (symbiotic aphids) and larvae experimentally deprived of the bacteria (aposymbiotic aphids) were reared on 16 plant species. Mortality of aphids was low on most plant species. The relative growth rate (RGR) of the larvae varied with plant species, and was generally depressed by elimination of the bacteria; the mean values of RGR varied between 0 and 0.29 μg μg⁻¹ day⁻¹ for symbiotic aphids and 0 and 0.17 μg μg⁻¹ day⁻¹ for aposymbiotic aphids. The extent to which RGR was depressed by aposymbiosis varied significantly between plant species, suggesting that aphid host plant may influence the contribution of the bacteria to plant utilisation. It is proposed that the bacteria may be particularly important on plants with phloem sap of high amino acid content of low quality, i.e. low concentrations of essential amino acids. Received: 18 August 1996 / Accepted: 13 January 1997     

Fishing spiders, green sunfish, and a stream-dwelling water strider: male-female conflict and prey responses to single versus multiple predator environments

Many studies have experimentally addressed the effects of a particular predator species on prey behavior. In nature, however, prey frequently face multiple species of predators that often vary in their predatory mode and in their level of predation risk. Relatively few studies have considered prey responses under these complex conditions. In Kentucky, the stream-dwelling water strider (Aquariusremigis) coexists with many potentially dangerous predators, two of which are the green sunfish (Lepomiscyanellus) and the fishing spider (Dolomedesvittatus). Green sunfish occupy stream pools and attack water striders from below. In contrast, fishing spiders hunt along stream shorelines where they perch on overhanging vegetation or rocks and attack water striders near shore. We compared how A. remigis individuals respond to these two very different predators in pools with one or both predators. The presence of sunfish in pools had strong effects on male water strider behavior, including increased use of three types of refuge from sunfish (riffles, climbing out of the water, sitting on the water but at the edges of pools), decreased activity and a decreased number of aggressive males on the water. Spiders also influenced water strider behavior; male water striders avoided spiders by shifting away from the edges of pools. Comparisons of the effects of the two predator species showed that in general, antipredator responses by male water striders were stronger in pools with fish alone than in those with spiders alone. In the presence of both predators, male water strider behavior (microhabitat use and activity) was generally similar to behavior in the presence of fish alone. In contrast, female water striders showed no significant response to the presence of sunfish, and little response to the presence of spiders. This lack of response could be because females spent much of their time in refuges even in the absence of predators (apparently hiding from harassment by males). Both spiders and fish caused decreases in water strider mating activity. The presence of fish reduced both the number of matings per pool (mating frequency), and mean mating durations. Spiders induced a decrease in mean mating duration, but not in mating frequency. The largest reductions in mating activity occurred in pools with both predators present. Pools with either spiders or fish alone suffered 15–20% water strider mortality during our experiment (versus no mortality in predator-free pools). Extant theory suggests that when prey face conflicting microhabitat responses to two predators (as in this study), the predators should have facilitative effects on predation rates (i.e., prey that avoid one predator are often killed by the other and vice versa). Mortality rates in pools with both predators present, however, were not significantly different from that predicted by a null model of multiple predator effects. The lack of predator facilitation can be explained by the compensatory reductions in water strider activity and mating activity in the presence of both predators. Received: 26 August 1996 / Accepted: 12 June 1998     

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     

Cold hardiness in Entomobrya nivalis (Collembola, Entomobryidae): annual cycle of polyols and antifreeze proteins, and antifreeze triggering by temperature and photoperiod

In the Swiss Prealps Entomobrya nivalis hibernates in an inactive state, hidden under bark flakes on spruce. For freeze avoidance it relies on thermal hysteresis proteins (THPs) and polyols (mainly ribitol, with small amounts arabitol and threitol). Polyols are present only during the inactive state, THPs additionally protect during the transition phase in spring and autumn, when animals are still active but frosts may occur. Peak values were recorded in February/March for THPs (3.5 °C hysteresis between melting and freezing point) and for polyols (26 μg mg⁻¹ FW; hemolymph osmolality 680 mosmol l⁻¹). E. nivalis is able to control its hemolymph osmolality independently of body water content. Mean osmolality in summer was 350– 440 mosmol l⁻¹, in winter it was elevated to 650 mosmol l⁻¹, due to a synthesis mainly of ribitol. Body water content varied between 1.8 and 3.3 mg H₂O mg⁻¹ DW, depending on humidity conditions. Experiments on triggering of antifreeze synthesis showed the action of temperature and photoperiod as cues, but there was also evidence for an endogenous rhythm. No clear correlation between antifreeze concentration and supercooling ability could be established, suggesting that gut content or other parameters also play an inportant role. Accepted: 18 November 1995     

Non-consumptive effects of larval Salamandra on crustacean prey: can eggs detect predators?

Predators affect prey populations not only by prey consumption but also in nonconsumptive ways including modifying prey behavior. I tested the effects of fire salamander larvae (Salamandra infraimmaculata) on populations of co-occurring crustacean species in artificial outdoor pools. I also tested whether these effects were due entirely to prey consumption by Salamandra larvae or alternatively to some nonconsumptive effect. The soil (containing crustacean eggs) added to the artificial pools was collected from a dried-out temporary pool that is inhabited by Salamandra during the early part of the hydroperiod. I randomly assigned the pools to one of three treatments: control, free Salamandra, or caged Salamandra. Free salamander larvae could roam the entire pool and prey upon crustaceans. Caged salamander larvae were placed within a cage with having 250-μm mesh windows. They could not prey upon the crustaceans but could, for example, influence them by chemical cues. Densities of the three dominant crustacean species (Arctodiaptomus similis, Ceriodaphnia quadrangula and Cyzicus sp.) were drastically reduced in both salamander treatments compared to the control. Crustacean densities, however, were not significantly different between the two salamander treatments. One plausible explanation is that crustacean eggs can detect the presence of this predator via chemical cues and delay hatching. Received: 4 March 1996 / Accepted, 23 October 1996     

Influence of soil hydric parameters on the winter cold hardiness of a burrowing beetle, Leptinotarsa decemlineata (Say)

This investigation examined the influence of soil moisture and associated parameters on the cold hardiness of the Colorado potato beetle (Leptinotarsa decemlineata Say), a temperate-zone species that overwinters in terrestrial burrows. The body mass and water content of adult beetles kept in sand at 4 °C varied over a 16-week period of diapause according to the substratum's moisture content. Changes in body water content, in turn, influenced the crystallization temperature (range −3.3 to −18.4 °C; n = 417), indicating that environmental moisture indirectly determined supercooling capacity, a measure of physiological cold hardiness. Beetles held in dry sand readily tolerated a 24-h exposure to temperatures ranging from 0° to −5 °C, but those chilled in sand containing as little as 1.7% water (dry mass) had elevated mortality. Thus, burrowing in dry soils not only promotes supercooling via its effect on water balance, but may also inhibit inoculative freezing. Mortality of beetles exposed to −5 °C for 24 h was lower in substrates composed of sand, clay and/or peat (36–52%) than in pure silica sand (78%) having an identical water content (17.0% dry mass). In addition to moisture, the texture, structure, water potential, and other physico-chemical attributes of soil may strongly influence the cold hardiness and overwintering survival of burrowing insects. Accepted: 10 September 1996     

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     

Acorn selection by Mexican jays: a test of a tri-trophic symbiotic relationship hypothesis

By caching acorns, jays serve as important dispersal agents for oak (Quercus) species. Yet little is known about which acorn characteristics affect selection by jays. In the traditional model of jay/oak symbiosis, large, brown, ripe acorns free of invertebrate parasites (e.g., Curculio acorn weevils) are selected by jays. Recently, it has been suggested that a tri-trophic relationship between oaks, jays, and weevils may have evolved to counter the negative dietary effects of acorn tannins. Under the tri-trophic model, jays would preferentially select acorns containing weevil larvae. We tested the assumptions that (1) acorns containing curculionid larvae exist in sufficient quantities to support jay populations and (2) jays can detect, and preferentially select, acorns containing weevil larvae, and investigated the cues by which jays select acorns. Captive Mexican jays (Aphelocomaultramarina) were presented Emory oak (Quercusemoryi) acorns in aviary feeding trials. Large, dense, viable acorns free of curculionid larvae were preferentially selected. Contrary to results of previous research, color did not affect selection. Acorn viability increased and curculionid larval occupancy decreased in adjacent savannas and isolated stands relative to existing oak woodland, perhaps favoring oak recruitment into adjacent lower-elevation grasslands. Our results compel us to reject the tri-trophic model for this system, and are consistent with the traditional jay/oak symbiosis model. Relatively long-distance dispersal of viable acorns favors Emory oak replacement, and spatial patterns of acorn viability and curculionid parasitism suggest expansion of Emory oak into adjacent low-elevation semi-arid grasslands. Received: 29 February 1996 / Accepted: 26 September 1996     

Seasonal changes in fatty acid composition associated with cold-hardening in third instar larvae of Eurosta solidaginis

Third-instar larvae of the goldenrod gall fly Eurosta solidaginis (Diptera: Tephritidae) survive extended periods in winter during which tissue water is frozen. Both low temperature and reduced water activity during freezing present challenges for the structural integrity of cellular lipids. Fatty acids of both phospholipids and triacylglycerols from fat body cells of E. solidaginis were analyzed throughout fall and early winter, a period that encompasses the acquisition of freeze-tolerance, to determine if adaptations to freezing include changes in fatty acid unsaturation. The five most abundant fatty acids from both fractions were (in decreasing order) oleic (40–65%), palmitoleic (18–20%), palmitic (12–17%), linoleic (5–10%), and stearic acids (4 –7%). This represents a typical complement of Dipteran fatty acids, although oleic acid levels were higher in E. solidaginis than those reported from other Dipterans (˜28%; Downer 1985). From September to November, monounsaturates increased from 59 to 70% in phospholipids at the expense of saturated fatty acids (25% –20%) suggesting activation of a Δ⁹-desaturase enzyme. These changes resulted in an increase in the ratio of unsaturated to saturated fatty acids (U/S) from 3.0 to 4.2, although there was no change in the average number of double bonds per fatty acid (unsaturation index, UI ≈ 1.2 in phospholipids and 0.9 in triacylglycerols throughout the season). These changes were temporally correlated to decreasing ambient temperatures and increasing larval and fat body cell freeze-tolerance. Accepted: 31 October 1996     

Invasive capacity of Tamarix ramosissima in a Mojave Desert floodplain: the role of drought

Tamarix ramosissima (Tamaricaceae) is a woody phreatophyte that has invaded thousands of hectares of floodplain habitat in the southwestern U.S. In this study, we examined the response of gas exchange and stem sap flow of Tamarix and three co-occurring native phreatophytes (Pluchea sericea (Asteraceae), Prosopis pubescens (Fabaceae) and Salix exigua (Salicaceae)) to drought conditions in an early successional floodplain community in the Mojave Desert of southern Nevada. In an analysis of a size/age series of each species across the whole floodplain (both mature and successional stands), stem growth rate was lowest for Tamarix. However, along the same successional chronosequence, Tamarix came to dominate the 50+ year old stands with dense thickets of high stem density. Xylem sap flow, when expressed on a sapwood area basis, was highest in Tamarix under early drought conditions, but comparable between the four species toward the end of the summer dry season. Multivariate analysis of the gas exchange data indicated that the four species differentiated based on water use under early drought conditions and separated based on plant water potential and leaf temperature (indices of drought effects) at the end of the summer dry season. This analysis suggests that the invasive Tamarix is the most drought tolerant of the four species, whereas Salix transpires the most water per unit leaf surface area and is the least tolerant of seasonal water stress. Therefore, Salix appears to be well adapted to early successional communities. However, as floodplains in this arid region become more desiccated with age, Tamarix assumes greater dominance due to its superior drought tolerance relative to native phreatophytes and its ability to produce high density stands and high leaf area. Received: 8 August 1996 / Accepted: 29 January 1997     

Effect of masking the parietal eye on the diurnal activity and body temperature of two sympatric species of monitor lizards, Varanus s. salvator and Varanus b. nebulosus

 Five water monitor lizards, Varanus salvator salvator, and four clouded monitor lizards, Varanus bengalensis nebulosus, were caught on Tioman island in Malaysia. A radio-thermistor transmitter was implanted into the buccal cavity of each animal, and they were released into an enclosure measuring 5.5 × 6.5 metres. The lizards were observed for 9 and 8 days, respectively, before and after the parietal eye was covered with aluminium foil. With uncovered parietal eye, both species showed a clear diurnal rhythm, being active only during day time. After covering the parietal eye, the mean locomotor activity of five V. s. salvator decreased from 791 to 107 min · day^–1 but remained unchanged around 850 min · day^–1 for V. b. nebulosus. The mean duration of locomotor activity decreased in V. s. salvator and V. b. nebulosus after the parietal eye was covered, but V. b. nebulosus maintained its locomotor activity by increasing the number of locomotor bouts. The water monitor spent very little time on thermoregulation. Its body temperature ranged between 26.3 and 28.4 °C, which decreased after the parietal eye was covered. The clouded monitor thermoregulated around 28.8–36.0 °C, which remained unchanged after the parietal eye was covered. In both species, there was a strong correlation between body temperature and ambient temperature. Behavioural abnormalities were recorded among V. s. salvator with covered parietal eye. They were often observed to be active by night and often slept outside a burrow. The circadian rhythm of V. b. nebulosus appeared unaffected by shielding of its parietal eye. Captivity combined with shielded parietal eye induced agonistic behaviour in both species. Accepted: 11 September 1996     

Effects of plant hybridization on herbivore-parasitoid interactions

We studied the effects of host plant hybridization on the survival and mortality of the leaf-mining moth Phyllonorycter salicifoliella on hybrid and parental willow plants in the field and in a common garden experiment. P. salicifoliella survival differed significantly among three willow taxa in the field in 1994 but not in the field in 1995 or in the common garden. Parasitism by eulophid wasps differed significantly among taxa in 1994 and appeared to account for the variation in their survival. In the field in 1995, host feeding predation varied significant among taxa. The theory of tritrophic interactions predicts that plant genotype can affect natural enemy impact, and this study supports this prediction. Significant variation in survival and eulophid parasitism was also found among genotypes within taxa in the field in both years and in the common garden experiment. The common garden results show that genetic differences in plants affect the herbivore-parasitoid interaction. Variation among years in the patterns of survival and causes of mortality among field plants suggest that genotype by environment interactions may be important. Received: 1 March 1996 / Accepted: 4 November 1996     

Rhythmic components of photoperiodic time measurement in the pitcher-plant mosquito, Wyeomyia smithii

Photoperiodic time measurement regulating larval diapause in the pitcher-plant mosquito, Wyeomyia smithii, varies in a close relationship with latitude. The critical photoperiod mediating the maintenance and termination of diapause is positively correlated with latitude (r ² = 0.977) among six populations from southern (30–31° N), intermediate (40° N), and northern (46–49° N) latitudes in North America. The developmental response to unnaturally short and to unnaturally long photoperiods declines with increasing latitude, so that longer critical photoperiods are associated with a downward rather than a lateral shift in the photoperiodic response curve. Exotic light and dark cycles of varying period (T) with a short (10 h) photophase and a scotophase ranging from 14 (T = 24) to 62 (T = 72) h, reveal two geographic patterns: a decline in perturbability of the photoperiodic clock with increasing latitude, and no change with latitude in the 21-h period of rising and falling development with increasing T. These results show (1) that there is a rhythmic component to photoperiodic time measurement in W. smithii, (2) that the period of this rhythm is about 21 h in all populations, and (3) that more northern populations show decreasing responsiveness to photoperiod and increasing stability against perturbation by exotic period lengths (T > 24). Previous studies on W.␣smithii indicate that this single temperate species of a tropical and subtropical genus has evolved from south to north. We therefore conclude that the evolution of increasing critical photoperiod in W. smithii during its adaptive radiation into North America has more likely involved the amplitude and not the period of the underlying circadian pacemaker. Received: 22 July 1996 / Accepted: 30 September 1996