Competition intensity as a function of resource availability in a semiarid ecosystem

Two field experiments were conducted using three dominant perennial species of the Chihuahuan Desert: Hilaria mutica (a tussock grass), Larrea tridentata (a microphyllous shrub) and Opuntia rastrera (a flat-stemmed succulent cactus). Two hypotheses concerning competition in arid plant communities were tested. (1) Marked resource partitioning with no interspecific competition could be expected since the three species belong to different life-forms, and that plant growth in deserts is basically limited by harsh environmental conditions. (2) Alternatively, resource scarcity (particularly water) will result in strong plant competition. In a 1-year removal experiment, water status and plant growth of the three species were monitored in twelve 10 m × 10 m plots randomized in three blocks and assigned to the following treatments: (a) removal of all species, except H. mutica; (b) removal of all species, except L. tridentata; (c) removal of all species, except O. rastrera, and (d) control without any manipulation. In a watering experiment, under two neighbourhood conditions (growing isolated or in associations of plants of the three species in plots of 20 m²), the water status of the three species and the growth of H.mutica and L.tridentata were studied for 32 days after an irrigation equivalent to 30 mm of rain, similar to a strong storm event at the site. In the removal experiment, where plants were free to capture water, no evidence of competition was observed. However, during the watering experiment, in which water was forced into the soil, competitive effects were observed. Associated individuals of L. tridentata had lower xylem water potentials and osmotic potentials (OPs) and produced shorter twigs and less leaves and nodes. Although less pronounced, neighbours also had a negative effect on the OP in O. rastrera. According to these results, the intensity of the interspecific competition for water seems to depend on the level of resource availability in the soil. Thus, the validity of the two hypotheses tested in this study also depends on the level of resources. Competition could be absent or very low in years of low precipitation, as in the year of this study (173 mm against a 25-year average of 264 mm). However, when soil water availability is high, e.g. following heavy rain, the negative interactions between species could be more intense. Received: 3 October 1997 / Accepted: 23 March 1998     

Interspecific Aggression between Lizard Competitors,Anolis cooki and Anolis cristatellus

Aggressive encounters were staged between two species of Puerto Rican lizards (Anolis cooki and Anolis cristatellus) which share the same microhabitat (syntopic). The intensity of these interspecific matches was as great as their respective conspecific matches. When the two species interacted with either “look alike” congeners (A. cooki with A. monensis and A. cristatellus with A. gundlachi) or with the quite different appearing A. evermanni, aggressive intensity was only 1/6 that of the cooki-cristatellus encounters. It seems that A. cooki and A. cristatellus recognize each other as competitors; it was improbable that this interspecific aggression was caused by the two species misidentifying each other as conspecifics. Because A. cristatellus eventually dominated A. cooki during staged encounters, it is predicted A. cristatellus should ecologically displace A. cooki in their syntopic zones of contact.     

Heterospecific attraction and food resources in migrants' breeding patch selection in northern boreal forest

We studied experimentally how heterospecific attraction may affect habitat selection of migrant passerine birds in Finnish Lapland. We manipulated the densities of resident tit species (Parus spp.). In four study plots residents were removed before the arrival of the migrants in the first study year, and in four other plots their densities were increased by releasing caught individuals. In the second year the treatments of the areas were reversed, allowing paired comparisons within each plot. We also investigated the relative abundance of arthropods in the study plots by the sweep-net method. This allowed us to estimate the effect of food resources on the abundance of birds. The heterospecific attraction hypothesis predicts that densities of migrant species (especially habitat generalists) would be higher during increased resident density. Results supported this prediction. Densities and number of the most abundant migrant species were significantly higher when resident density was increased than when they were removed. On the species level the redwing (Turdus iliacus) showed the strongest positive response to the increased abundance of tits. Migrant bird abundances seemed not to vary in parallel with relative arthropod abundance, with the exception of the pied flycatcher (Ficedula hypoleuca) which showed a strongly positive correlation with many arthropod groups. The results of the experiment indicate that migrants can use resident tit species as a cue to a profitable breeding patch. The relationship between the abundance of the birds and arthropods suggests that annual changes in food resources during the breeding season probably do not have a very important effect on bird populations in these areas. The results stress the importance of positive interspecific interactions in structuring northern breeding bird communities. Received: 1 September 1997 / Accepted: 22 January 1998     

Spatial discrimination of pheromones and behavioural antagonists by the tortricid moths Cydia pomonella and Adoxophyes orana

Male moths responding to their species-specific sex pheromone, may cease their upwind flight when pheromone components of sympatric species are added to the mixture. The interspecific interaction between the pheromone response of the tortricid moths Cydia pomonella and Adoxophyes orana was investigated in field-trapping and wind-tunnel studies. Addition of the A. orana pheromone [(Z9)-tetradecenylacetate and (Z11)-tetradecenylacetate] to a source containing the C. pomonella pheromone [(E8, E10)-dodecadienol] resulted in a significant inhibition of attraction by male C. pomonella. It is demonstrated that this behavioural antagonist for C. pomonella must be emitted from the same point source to induce this inhibitory effect. A spatial separation of the two interspecific pheromones (at 14 cm, 5 cm and 0.5 cm crosswind) restored the attraction of the conspecific pheromone for male C. pomonella. In contrast to C. pomonella, male A. orana were not inhibited by point sources releasing both the C. pomonella and A. orana pheromone. We suggest that the discrepancy in the interspecific pheromone interaction between these two tortricids can be explained if we consider the evolutionary ecology of interspecific pheromone communication in C. pomonella and A. orana. Accepted: 18 July 1999     

An experimental investigation of the competitive displacement of a native gecko by an invading gecko: no role for parasites

On islands across the Pacific the invasion of the gecko Hemidactylusfrenatus has caused a decline in the abundance of a resident gecko, Lepidodactyluslugubris. In a previous study we demonstrated that the prevalence of the cestode Cylindrotaenia sp. is higher in the resident gecko on islands where it is sympatric with the invader than on islands where it occurs alone. In the present study we experimentally test whether the presence of the invading gecko causes an increase in parasites, particularly Cylindrotaenia sp., in the resident. In addition, we test whether the effect of the invader on parasite prevalence in the resident is mediated through an increase in corticosterone in the resident. Corticosterone is the primary glucocorticoid, or “stress” hormone in lizards, and chronic elevation in corticosterone may suppress some types of immune responses. After experimental manipulations of interspecific interactions (single vs. mixed species treatments) and intraspecific densities (high vs. low), we detected no difference in parasite prevalence or circulating corticosterone among the experimental treatments in either L. lugubris or H. frenatus. Circulating levels of corticosterone were higher in geckos␣sampled at night than geckos sampled during the day, indicating a circadian cycle in corticosterone levels in these nocturnal animals. Circulating levels of corticosterone were higher in experimental geckos than in geckos that had not been used in the experiment, and, in some groups, higher in geckos infected with cestodes than in uninfected geckos. Circulating levels of corticosterone did not differ between non-experimental H. frenatus and L. lugubris, but when geckos used in the experiment were compared, circulating levels of corticosterone were significantly higher in H.␣frenatus than in L. lugubris. Received: 17 March 1997 / Accepted: 5 December 1997     

Use of cover habitat by bull trout, <Emphasis Type="Italic">Salvelinus confluentus</Emphasis>, and lake trout, <Emphasis Type="Italic">Salvelinus namaycush</Emphasis>, in a laboratory environment

Lacustrine-adfluvial bull trout, Salvelinus confluentus, migrate from spawning and rearing streams to lacustrine environments as early as age 0. Within lacustrine environments, cover habitat provides refuge from potential predators and is a resource that is competed for if limiting. Competitive interactions between bull trout and other species could result in bull trout being displaced from cover habitat, and bull trout may lack evolutionary adaptations to compete with introduced species, such as lake trout, Salvelinus namaycush. A laboratory experiment was performed to examine habitat use and interactions for cover by juvenile (i.e., <80 mm total length) bull trout and lake trout. Differences were observed between bull trout and lake trout in the proportion of time using cover (F _1,22.6 = 20.08, P < 0.001) and bottom (F _1,23.7 = 37.01, P < 0.001) habitat, with bull trout using cover and bottom habitats more than lake trout. Habitat selection ratios indicated that bull trout avoided water column habitat in the presence of lake trout and that lake trout avoided bottom habitat. Intraspecific and interspecific agonistic interactions were infrequent, but approximately 10 times greater for intraspecific interactions between lake trout. Results from this study provide little evidence that juvenile bull trout and lake trout compete for cover, and that species-specific differences in habitat use and selection likely result in habitat partitioning between these species.     

Competition and facilitation among plants for pollination: can pollinator abundance shift the plant–plant interactions?

We hypothesize interactions among plants for pollination may depend on pollinator abundance, which always varies among years and habitats and has different effects on plant reproductive success. Honeybee-pollinated plants, Lotus corniculatus, and its commonly coflowering neighbor, Potentilla reptans var. sericophylla, were used in a two-year project. We designed six types of plant combinations with different conspecific and interspecific flower densities in 2011 and repeated this in the same site in 2012. Meanwhile, we artificially increased pollinator abundance by hiring beehives only in 2011. Pollinator abundance as well as flower density significantly affected pollination of L. corniculatus plants from both the conspecific and interspecific plots. Total number of bees visiting a plot was enhanced by an increase in both the conspecific and interspecific flower densities regardless of high or low pollinator abundance. However, changes in visitation rates and fruit sets in the focal plants when flower densities were increased depended on pollinator abundance. Under high pollinator abundance, an increase in both the conspecific and interspecific flower densities significantly enhanced pollinator visits to L. corniculatus. However, under low pollinator abundance, the pollinator visitation rate remained unchanged as conspecific flower density increased, but decreased when there was an increase in interspecific flower density. Coflowering plants enhanced fruit sets of L. corniculatus only when the pollinator abundance was high. The findings suggest that the interactions among plants for pollination are influenced not only by a plant density threshold, but also by a pollinator abundance threshold.     

Modulation of gene expression by (CA)n microsatellites in the filamentous ascomycete Podospora anserina

A microsatellite consisting of the alternating pyrimidine-purine sequence (CA)_n.(TG)_n is found to occur in very conserved form in the genome of various races of the filamentous ascomycete Podospora anserina. Screening of a cDNA library revealed that this sequence is frequently transcribed. In this study, we focused our attention on a short (CA)₅ microsatellite located in the 5′ untranslated sequence of the glyceraldehyde-3-phosphate dehydrogenase (gpd) gene of P. anserina. Specifically, we investigated whether or not the number of repeat units present in the microsatellite affects the expression of the β-d-glucuronidase (gusA) reporter gene introduced on an autonomously replicating plasmid into fungal protoplasts. The results show that an increase in the number of microsatellite repeat units positively affects reporter gene expression. Received: 27 November 1998 / Received revision: 12 February 1999 / Accepted: 20 March 1999     

Top-down effects of insect herbivores during early succession: influence on biomass and plant dominance

We tested the hypothesis that phytophagous insects would have a strong top-down effect on early successional plant communities and would thus alter the course of succession. To test this hypothesis, we suppressed above-ground insects at regular intervals with a broad-spectrum insecticide through the first 3 years of old-field succession at three widely scattered locations in central New York State. Insect herbivory substantially reduced total plant biomass to a similar degree at all three sites by reducing the abundance of meadow goldenrod, Solidago altissima. As a result, Euthamia graminifolia dominated control plots whereas S. altissima dominated insecticide-treated plots by the third year of succession. S. altissima is the dominant old-field herbaceous species in this region but typically requires at least 5 years to become dominant. Past explanations for this delay have implicated colonization limitation whereas our data demonstrate that insect herbivory is a likely alternative explanation. A widespread, highly polyphagous insect, the xylem-tapping spittlebug, Philaenus spumarius, appeared to be the herbivore responsible for the reduction in standing crop biomass at all three sites. Insect herbivory typically caused little direct leaf tissue loss for the ten plant species we examined, including S. altissima. Consequently, the amount of leaf area removed was not a reliable indicator of the influence of insect herbivory on standing crop biomass or on early succession. Overall, we found a strong top-down effect of insect herbivores on biomass at several sites, so our results may be broadly applicable. These findings run counter to generalizations that top-down effects of herbivores, particularly insects, are weak in terrestrial systems. These generalizations may not apply to insects, such as spittlebugs, that can potentially mount an effective defense (i.e., spittle) against predators and subsequently reach relatively high abundance on common plant species. Our results suggest that insect herbivory may play an important but often overlooked role during early old-field succession. Received: 26 December 1998 / Accepted: 3 April 1999     

Aspects of the reproductive biology of two archer fishes <Emphasis Type="Italic">Toxotes chatareus</Emphasis>, (Hamilton 1822) and <Emphasis Type="Italic">Toxotes jaculatrix</Emphasis> (Pallas 1767)

Various aspects of the reproductive biology of two archer fishes Toxotes chatareus and Toxotes jaculatrix were studied to describe gonad development, spawning season, sex ratio, and fecundity. Spawning season was assessed using monthly changes in gonadosomatic index (GSI) and histological inspection of the gonads. Both species exhibit two modes of oocytes; a mode of small primary growth oocytes and a single mode that increases with size as oocytes sequester vitellogenin and undergo maturation, showing the synchronous oocyte development typical of total spawners. Based on GSI values and advanced stages of oocyte maturity, T. chatareus and T. jaculatrix, females appear to spawn from November to December. The average fecundity of female T. chatareus was 55 000 ± 5538 eggs, and T. jaculatrix was 50 000 ± 3440 eggs; fecundity ranged from 20 000 to 150 000 eggs for both species, with relative fecundities of 600 to 1100 eggs/g body weight, and a mean value of 800 ± 32 for T. chatareus; relative fecundity ranged from 500 to 1100 with a mean value of 700 ± 23 for T. jaculatrix. Sex ratio, defined as the proportion of males to females, was 2.2 and 2.5 in T. chatareus and T. jaculatrix, respectively. The apparent abundance of males in samples could be due to females being positioned lower in the water column and therefore being sampled less frequently. Our results indicate that in both species, spawning occurs between the months of November and December during the monsoon season, which provides the mangrove coastal waters inhabited by these species with an abundance of food resources and additional floodplain nursery habitat for larvae and juveniles.     

The interactions of plant clone and abiotic factors on a gall-making midge

Within and around Tampa Bay, Florida, monoclonal populations of the sea daisy, Borrichia frutescens, can be found on small, isolated islands growing within the intertidal zone. Stem tips of Borrichia are attacked by the gall-making cecidomyiid, Asphondyliaborrichiae. We used reciprocal transplants of Borrichia clones between islands to assess the importance of plant genotype and local environmental conditions (shade and host-plant nitrogen) on gall abundance. In another experiment, we controlled for host genotype effects by inducing differences in local environmental conditions through the addition of NH₄NO₃ fertilizer and/or shade to field plots at the only monoclonal site with a large enough population of Borrichia to facilitate the experiment. We also examined the effect of these variables on attack levels of Asphondylia by parasitoids. In the reciprocal transplant, while some Borrichia clones always supported more galls than others, regardless of environmental conditions, all four clones developed more galls when they were placed in the shade, compared to those in the sun, at all four sites. In addition, some islands always supported more galls than others and we found a significant clone × site interaction. In the single-clone experiment, Borrichia in fertilized- and shaded-only plots developed more Asphondylia galls than those from nonmanipulated control plots, and plants that received both shading and fertilizer developed the most galls. Although shade and fertilization produced an additive increase in plant nitrogen content, their effects resulted in a synergistic decrease in C:N ratio. Neither shading nor host plant nitrogen content had a significant effect on levels of parasitism between experimental and control plants. Our results suggest that genetic differences in Borrichia's susceptibility to Asphondylia attack are important in shaping the distribution of galls, but environmental factors such as soil nitrogen and degree of shading are at least as important as genetic differences between host plants. Received: 12 June 1997 / Accepted: 6 April 1998     

Influence of plant resistance at the third trophic level: interactions between parasitoids and entomopathogenic fungi of cereal aphids

Host-plant resistance can affect herbivorous insects and their natural enemies such as parasitoids and entomopathogenic fungi. This tritrophic effect acts on interspecific interactions between the two groups of natural enemies distantly related in phylogenetic terms. The intra- and extra-host aspects of the interaction between the cereal aphid parasitoid Aphidius rhopalosiphi and the entomopathogenic fungus Erynia neoaphidis developing on the grain aphid, Sitobion avenae, on resistant and susceptible wheat (Triticum aestivum) cultivars, were studied. The competitive outcome of the intra-host interaction depended on the timing of parasitoid oviposition and fungal infection and was affected by wheat resistance. In particular, survival of the parasitoid was lower on the resistant wheat cultivar than the susceptible wheat cultivar, when the competitive outcome of the interaction was favourable for either parasitoid or fungal development. Before and after this period the influence of plant resistance was not significant. Furthermore, the extra-host interaction was not affected by the wheat cultivar, although an increase in fungal infection of S. avenae was observed when parasitoids foraged in the experimental arena with sporulating aphid cadavers compared with foraging in the absence of sporulating cadavers. Our results showed that the host plant may affect interspecific interactions between parasitoids and fungi and that these interactions depended on the timing of parasitoid oviposition and fungal infection. Received: 16 March 1998 / Accepted: 24 August 1998     

Quantification of extraradical soil mycelium and ectomycorrhizas of Boletus edulis in a Scots pine forest with variable sporocarp productivity

The availability of most edible ectomycorrhizal mushrooms depends on their natural fructification. Sporocarp formation of these fungi is linked to habitat characteristics and climate conditions, but these data alone do not explain all the trends of fungal fruiting and dynamics. It could be hypothesized that the amount of soil mycelia could also be related to the production of carpophores. Soil samples (five cylinders of 250 cm³ per plot) were taken monthly, from September to November, in five fenced permanent plots (5 × 5 m) in Pinar Grande (Soria, Spain), a Pinus sylvestris stand situated in the north of the Sistema Ibérico mountain range. Plots were chosen to establish a gradient of Boletus edulis productivity from 0 to 38.5 kg/ha year, according to the mean fresh weight of sporocarps collected during the last 10 years. B. edulis ectomycorrhizal root tips were identified in each soil sample according to its morphology and counted. DNA extractions were performed with the PowerSoil^TM DNA Isolation Kit and quantification of extraradical soil mycelium by real-time polymerase chain reaction using specific primers and a TaqMan? probe. The concentration of soil mycelium of B. edulis (mg mycelium/g soil) did not differ significantly between plots (p = 0.1397), and sampling time (p = 0.7643) within the fructification period. The number of mycorrhizal short roots per soil volume showed significant differences between the plots (p = 0.0050) and the three sampling times (p < 0.0001). No significant correlation between the number of mycorrhizas and the productivity of the plot (kg of B. edulis/ha year) was detected (p = 0.615). A statistically significant positive correlation (p = 0.0481) was detected between the concentration of mycelia of B. edulis in the soil samples and the presence of short roots mycorrhizal with B. edulis in these samples. The productivity of the plots, in terms of sporocarps produced during the last 10 years, was not correlated either with the concentration of soil mycelium or with the presence or abundance of ectomycorrhizas.     

Intertidal zone of Svalbard 3. Littoral of a subarctic, oceanic island: Bjornoya

Twenty-two stations in the intertidal and shallow sublittoral of Bjornoya (74 °N, 19 °E) were studied in August 1994 revealing a large and diverse standing crop of macro-algae (16 species) and littoral macrofauna (at least 17 species). In most places the biomass of littoral macroorganisms exceeded 100 g ww/m². In the shallow sublittoral, between 2 and 20 m, 45 animal taxa and 23 algae species were collected. Littoral coarse sand meiofauna was dominated by Turbellaria, while, on algae, Halacaridae and Harpacticoida predominated. Meiofauna densities ranged from 0 to 169 ind./10 cm² and biomass from 0 to 0.4 g dw/m². The abundance of littoral species and their zoogeographic origin resemble that of Spitsbergen more than that of the northern Scandinavian coast, although both are of equal distance from Bjornoya. The first record of the boreal bivalve Mytilus edulis from the island is presented. Another striking feature was the presence of the arctic amphipod Gammarus setosus and the absence of its boreal sibling species G.oceanicus. Received: 19 March 1996 / Accepted: 1 December 1996     

Varying responses of herbivorous and invertebrate-feeding fishes to macroalgal reduction on a coral reef

 The consequences of macroalgal overgrowth on reef fishes and means to reverse this condition have been little explored. An experimental reduction of macroalgae was conducted at a site in the Watamu Marine National Park in Kenya, where a documented increase in macroalgal cover has occurred over the last nine years. In four experimental 10 m by 10 m plots, macroalgae were greatly reduced (fleshy algal cover reduced by 84%) by scrubbing and shearing, while four similar plots acted as controls. The numerical abundance in all fish groups except wrasses and macroalgal-feeding parrotfishes (species in the genera Calotomus and Leptoscarus) increased in experimental algal reduction plots. Algal (Sargassum) and seagrass (Thalassia) assays, susceptible to scraping and excavating parrotfishes, were bitten more frequently in the algal reduction plots one month after the manipulation. Further, surgeonfish (Acanthurus leucosternon and A. nigrofuscus) foraging intensity increased in these algal reduction plots. The abundance of triggerfishes increased significantly in experimental plots relative to control plots, but densities remained low, and an index of sea urchin predation using tethered juvenile and adult Echinometra mathaei showed no differences between treatments following macroalgal reduction. Dominance of reefs by macrofleshy algae appears to reduce the abundance of fishes, mostly herbivores and their rates of herbivory, but also other groups such as predators of invertebrates (triggerfishes, butterflyfishes and angelfishes). Accepted: 2 February 1999     

Interspecific variation in the escape responses of aphids: effect on risk of predation from foliar-foraging and ground-foraging predators

A series of laboratory experiments was conducted to determine the effect of interspecific differences on prey defensive behavior on the susceptibility of two aphid species (Acyrthosiphon pisum and A. kondoi) to a ground-foraging predator, Harpalus pennsylvanicus, and a foliar-foraging predator, Coccinella septempunctata. These organisms are representative of a biologically and economically important predator/prey system in alfalfa. The primary defensive behavior of both aphid species toward C. septempunctata was to “drop” from the plant. Both aphid species were significantly more likely to drop from the plant in the presence of C. septempunctata. However, when C. septempunctata was present, a significantly lower proportion of A. kondoi individuals dropped (0.42 ± 0.07) compared to A. pisum (0.73 ± 0.08). As a result of their lower propensity to drop from the plant A. kondoi individuals are significantly more likely to be consumed by C. septempunctata. Conversely, the higher propensity of A. pisum individuals to drop increased their susceptibility to ground-foraging predators. When A. pisum was the prey species, ground-foraging predators made a significant contribution to overall aphid suppression and there was a significant synergistic interaction between ground and foliar-foraging predators. When A. kondoi was the prey there was no interaction between the predator species. As either a cause or consequence of its higher propensity to drop, A. pisum seems to be more adapted for survival and dispersal off the plant. In comparison to A. kondoi individuals, A. pisum individuals relocate plants more quickly (63 ± 41 s vs. 164 ± 39 s), disperse farther (18 ± 1.7 cm vs. 13 ± 0.66 cm), and survive longer (37 ± 2.0 h vs. 25 ± 2.0) off the plant. This study demonstrates the importance of prey defensive behavior in determining the susceptibility of a prey species to a multiple-predator complex. Received: 24 February 1997 / Accepted:17 December 1997     

Annual and interannual variability in phytoplankton at a permanent station off Kerguelen Islands, Southern Ocean

From November 1992 to February 1995 a quantitative and qualitative phytoplankton study was conducted at a permanent station (Kerfix) southwest off the Kerguelen Islands, in the vicinity of the Polar Front (50°40′S–68°25′E). Phytoplankton populations are low in this area both during summers and winters. They consist, in order of decreasing cell abundance, of pico- and nanoflagellates (1.5–20 μm), coccolithophorids (<10 μm), diatoms (5–80 μm) and dinoflagellates (6–60 μm). Flagellates form the dominant group throughout the year and attain the highest summer average of 3.0 × 10⁵ cells l⁻¹. Next in abundance year-round are coccolithophorids with the dominant Emiliania huxleyi (highest summer 1992 average 1.9 × 10⁵ cells l⁻¹), diatoms (summer 1992 average 1.0 × 10⁵ cells l⁻¹) and dinoflagellates (average 3.8 × 10⁴ cells l⁻¹). Winter mean numbers of flagellates and picoplankton do not exceed 8.4 × 10⁴ cells l⁻¹; those of the three remaining algal groups together attain 2 × 10⁴ cells l⁻¹. Summer peaks of diatoms and dinoflagellates are mainly due to the larger size species (>20 μm). The latter group contributes most to the total cell carbon biomass throughout the year. Dominant diatoms during summer seasons include: Fragilariopsis kerguelensis, Thalassionema nitzschioides, Chaetoceros dichaeta, C. atlanticus, Pseudonitzschia heimii, and P. barkleyi/lineola. This diatom dominance structure changes from summer to summer with only F. kerguelensis and T. nitzschioides retaining their first and second positions. Any one of the co-dominant species might be absent during some summer period. The variable diatom community structure may be due to southward meandering of the Polar Front bringing “warmer” species from the north, and to the mixing of the water masses in this area. The entire community structure characterized both during summer and winters by the dominance of flagellates can be related to deep mixing (ca. 40–200 m) of the water column as the probable controlling factor. Received: 13 November 1997 / Accepted: 11 May 1998     

Enzyme function and polymorphism: A test in two Anolis lizard species

Electrophoretic surveys of two lizard species were used to test hypotheses that relate levels of enzyme variability to enzyme function (single-substrate versus multiple-substrate enzymes, regulatory versus nonregulatory enzymes). Anolis roquet behaviorally regulates its body temperature, but its congener A. gundlachi is passive to variations in the thermal environment. As a result, populations of A. gundlachi probably experience the thermal environment as temporally coarse-grained, whereas populations of A. roquet do not. We therefore predicted that A. gundlachi would exhibit greater enzyme heterozygosity than A. roquet and that different enzyme classes would contribute disproportionately to this interspecific difference. The data show (1) that A. gundlachi does have a higher heterozygosity and (2) that this difference appears to result from high levels of heterozygosity at loci coding for multiple-substrate enzymes. The dichotomy between regulatory and nonregulatory enzymes offers no explanation for the variability in heterozygosity among enzyme loci in these species.E.Z. was supported by a grant from the Natural Sciences and Engineering Council of Canada. The study was accomplished while P.E.H. held a postdoctoral fellowship from the Killam Trust of Dalhousie University and a grant from the Research Development Fund of Dalhousie University. The collection of material was made possible by grants (to P.E.H.) from National Science Foundation (DEB 75-16334), the Explorers Club of New York, Sigma Xi, and the Richmond and Anderson Funds of Harvard University. We thank Dr. D. W. Foltz for his help with the calculations.     

Modeling growth for predicting the contamination level of guava nectar by Candida pelliculosa under different conditions of pH and storage temperature

The combined effects of temperature (2–46°C) and pH (1.55–6.25) on the growth of Candida pelliculosa isolated from guava nectar produced in Cameroon were studied using a turbidity method, ie measurement of optical density at 630 nm. A quadratic polynomial model was constructed to predict the effects and interactions of these two environmental conditions on the maximal optical density obtained (r ² = 0.97). The relation between optical density and population density of C. pelliculosa (CFU ml⁻¹) was also established using an exponential regression (r ² = 0.99). According to the model, maximal growth conditions were 37°C and pH 6.25 for obtaining the maximal optical density of 1.25 corresponding to about 60 × 10⁶ CFU ml⁻¹. A good agreement of the model was found between the predicted values and the observed values of maximal optical density. The model was validated by the experimental values of maximal optical density obtained in the growth of C. pelliculosa in commercial guava nectar (pH 3.15). Received 01 December 1995/ Accepted in revised form 30 August 1996     

Invasion by a Perennial Herb Increases Decomposition Rate and Alters Nutrient Availability in Warm Temperate Lowland Forest Remnants

We determined the impact of the invasive herb, Tradescantia fluminensis Vell., on litter decomposition and nutrient availability in a remnant of New Zealand lowland podocarp–broadleaf forest. Using litter bags, we found that litter beneath mats of Tradescantia decomposed at almost twice the rate of litter placed outside the mat. Values of k (decomposition quotient) were 9.44±0.42 yrs for litter placed beneath Tradescantia and 5.42±0.42 yrs for litter placed in native, non-Tradescantia plots. The impact of Tradescantia on decomposition was evident through the smaller forest floor mass in Tradescantia plots (2.65±1.05 t ha⁻¹) compared with non-Tradescantia plots (5.05±1.05 t ha⁻¹), despite similar quantities of annual leaf litterfall into Tradescantia plots (6.85±0.85 t ha⁻¹ yr⁻¹) and non-Tradescantia plots (7.45±1.05 t ha⁻¹ yr⁻¹). Moreover, there was increased plant nitrate available, as captured on resin bags, in Tradescantia plots (25.77 ± 8.32 cmol(−)/kg resin) compared with non-Tradescantia plots (9.55±3.72 cmol(−)/kg resin). Finally, the annual nutrient uptake by Tradescantia represented a large proportion of nutrients in litterfall (41% N, 61% P, 23% Ca, 46% Mg and 83% K), exceeded the nutrient content of the forest floor (except Ca), but was a small proportion of the topsoil nutrient pools. Taken together, our results show that Tradescantia increases litter decomposition and alters nutrient availability, effects that could influence the long-term viability of the majority of podocarp–broadleaf forest remnants affected with Tradescantia in New Zealand. These impacts are likely mostly due to Tradescantia's vegetation structure (i.e., tall, dense mats) and associated microclimate, compared with native ground covers. This revised version was published online in July 2006 with corrections to the Cover Date.