摘要:

Water—filled treeholes provide an experimental setting for examining processes within an ecosystem, and influences of external factors on those processes. Using a limnological, experimental approach involving both natural tree holes and laboratory microcosms of the tree hole ecosystem, we identified and studied interacting, biotic processes, including dynamics of bacterial populations and variation in concentration of inorganic nutrients in tree hole water, and density—dependent competition for food among larvae of the mosquito Aedes triseriatus. We characterized the influence of external factors (inputs of leaf detritus and stemflow) on those processes. Analyses of water samples over time showed that tree hole water was rich and dynamic in nutrients (nitrite, nitrate, ammonium, phosphate, and sulfate); ammonium was the dominant form of inorganic nitrogen. Variation in nutrient concentrations in microcosms depended upon exogenous inputs (leaf detritus and stemflow water), dilution of nutrients by stemflow, nutrient cycling processes (nitrification, dentrification, and sulfate reduction), and ammonium excretion by mosquito larvae. The densities of bacteria in tree hole water, obtained using direct counts of DAPI—fluorochrome stained samples and epifluorescence microscopy, ranged from 2.0 ° 106to 6.0 ° 107cells/mL, and in microcosms from 4.6 ° 105to 2.6 ° 108cells/mL. Experimentation involving microcosms revealed that bacterial abundance was reduced by mosquito feeding and stemflow flushing. Further experiments showed that stemflow flushing increased mosquito productivity from microcosms several—fold and released mosquitoes from density—dependent competition. This effect was likely related to nutrient input and the simultaneous removal of toxic metabolites owing to inputs of stemflow water. We conclude that disturbance by a physical factor, stemflow, has a major influence on the interactions of nutrient dynamics, bacterial populations, and mosquito productivity in temperate tree—hole ecosystems.

ISSN:0012-9658    

DOI:10.2307/1940953

出版商:Ecological Society of America    

年代:1991

数据来源: WILEY

摘要:

Methods are available in the literature for the measurement of instantaneous growth rates in field samples of photosynthetic microbes and procaryotic saprotrophic microbes, but there has been no such method for eucaryotic saprotrophic microbes (members of the kingdom Fungi). We have devised a technique for estimating instantaneous growth rates for ergosterol—containing fungi in field material, and preliminarily applied the technique to estimation of fungal productivities and throughputs in two types of standing—dead grass (one saltwater, one freshwater), obtaining plausible values (e.g., conversion efficiencies, total fungal production ° leaf mass loss, were calculated to be 36—64%). This method is based on measuring rates of radiolabeled acetate incorporation into ergosterol. We examined several potential problem areas for use of radiolabeled precursors in measurement of microbial molecular syntheses, and its extrapolation to microbial productivity. Key findings were: (a)5 mmol/L added acetate (required for maximization of detection of change in rates) had positive or neutral impact on 48—h fungal growth, and no shift—up (sudden upward change in rate) or lag in incorporation of acetate into ergosterol occurred between 0.25 and 1.25 h of incubation of naturally decaying leaf (Spartina alteriflora) samples with 5 mmol/L acetate; (b) bacterial assemblages on leaves did not contribute radioactivity to the ergosterol fraction after incubation with radioacetate (as opposed to>30 Bq per centimetre length of leaf containing fungal mycelium); (c) the average empirical factor for conversion from nanomoles14C—acetate incorporated to micrograms organic fungal mass produced was 8.2 mg/nmol (coefficient of variation, 27%). We tentatively conclude that the acetate—to—ergosterol method is tenable for measurement of natural fungal productivity.

ISSN:0012-9658    

DOI:10.2307/1940954

出版商:Ecological Society of America    

年代:1991

数据来源: WILEY

摘要:

We asked what factors keep the seed beetles Stator limbatus and S. pruininus (Bruchidae) restricted to different species of Acacia (Leguminosae) in sympatry. Factors assessed were plant microenvironment, interspecific competition, suitably of seeds for food, and enemy impact. Females selectively oviposited on seeds of their normal host when given a choice of acacia seeds in their normal host microenvironment. This result occurred whether seeds of the other acacia were offered with or without potential competitors in them. Host suitability was determined by comparing full—siblings on both acacias. Three performance variables were measured: (1) emergence per egg laid, (2) time to emergence, and (3) size of females. Stator limbatus performed best on its normal host in all three variables measured, while S. pruininus performed best on its normal host in emergence time but showed no difference in percent emergence or female size. We also assessed the impact of the egg parasite Uscana semifumipennis (Trichogrammatidae) on the two species of bruchids when the bruchids occurred in their normal hosts. Uscana attacked both bruchids at the same level (20% of eggs parasitized). Thus, the proximate determinant of locality restricted use of hosts is oviposition behavior, which we suggest relates best to differences in the suitability of hosts for larvae.

ISSN:0012-9658    

DOI:10.2307/1940955

出版商:Ecological Society of America    

年代:1991

数据来源: WILEY

摘要:

Experiments were conducted to determine if three plant defenses affected the parasitoid Cotesia congregata in a fashion that would influence herbivore—parasitoid interactions. We evaluated the developmental rate, size, and survival of the parasitoid Cotesia congregata, when reared from Manduca sexta fed on diets containing either nicotine, rutin, or hordenine. These results were compared to the effects of the same three chemicals on unparasitized M. sexta. The influences of the three plant defenses differed. In general, however, the effects of these allelochemicals on the parasitoid paralleled those on the unparasitized hornworn. The one major exception was that concentrations of nicotine that had little or no effect on the hornworm caused significant mortality of parasitoids. Neither rutin nor hordenine had large or consistent negative effects, although rutin had a pronounced effect on both hornworm and parasitoid developmental times.

ISSN:0012-9658    

DOI:10.2307/1940956

出版商:Ecological Society of America    

年代:1991

数据来源: WILEY

摘要:

We determined sex ratio, initial egg complement, and size of adult Aphytis melinus, an ectoparasitoid of California red scale, Aonidiella aurantii. Host scales were reared on leaves of four citrus cultivars previously shown to differ in suitability for scale growth and survival. Wasps from scales reared on leaves of lemon (Citrus limon) produced nearly twice the proportion of female progeny as did wasps from scales reared on leaves of grapefruit (Citrus paradisi), orange (Citrus sinensis), or mandarin (Citrus unshiu). Moreover, female wasps from lemon—reared scales had highest initial egg complement, followed by wasps reared from grapefruit, mandarin, and orange. The quality of scales for the production of fecund, female wasp progeny also varied seasonally over all cultivars pooled. Scales reached largest size when reared in the cooler autumn temperatures and were more than twice as suitable for the production of fecund, female A. melinus as scales reared in early summer or midsummer. Results suggest that the mechanisms allowing persistence of the interaction between California red scale and A. melinus may vary among different citrus cultivars.

ISSN:0012-9658    

DOI:10.2307/1940957

出版商:Ecological Society of America    

年代:1991

数据来源: WILEY

摘要:

We tested the commonly made assumption that reduction in leaf quality reduces the survivorship of herbivores indirectly by increasing their exposure to natural enemies. Caterpillars of the butterfly Pieris rapae were placed on collard plants that had been experimentally manipulated to have leaves either rich or poor in nitrogen. On low—nitrogen plants caterpillars both developed more slowly and devoted more of their time to feeding than on high—nitrogen plants. When exposed to large (>2 mm) predators and parasites in a natural setting, the survivorship of caterpillars on low—nitrogen collards was reduced greatly compared to caterpillars protected from predators and parasites. Survivorship of caterpillars on high—nitrogen plants was reduced significantly less dramatically when exposed to predators and parasites. Small (<1—2 mm) pupal parasites that were able to reach the caterpillars through the mesh of the predator exclusion cages emerged more frequently from individuals reared on high—nitrogen plants.

ISSN:0012-9658    

DOI:10.2307/1940958

出版商:Ecological Society of America    

年代:1991

数据来源: WILEY

摘要:

I conducted a field test of the hypothesis that the dynamics of a predator—prey interaction should be affected by the number of interacting subpopulations within an assemblage. I used a "successful" biological control system, the phytophagous European red mite (Panonychus ulmi) and its predator, the phytoseiid mite Typhlodromus pyri. An orchard was established where apple trees were arranged in groups of 1, 4, or 16 and the dynamics of the mite populations on the trees followed over a season. The pests reached highest densities and were most persistent on trees in the largest groups. These results are explained in terms of an interaction between prey immigration/emigration and predation.

ISSN:0012-9658    

DOI:10.2307/1940959

出版商:Ecological Society of America    

年代:1991

数据来源: WILEY

摘要:

The aspen blotch miner, Phyllonorycter salicifoliella, has occurred at extremely high, "epidemic" densities on its quaking aspen host trees for at least 10 yr in Itasca Park, Minnesota, USA. During the 1986 through 1988 growing seasons, I censused density, survival, and sources of mortality for eggs, larvae, and pupae on branches at two heights on four host trees. I constructed cohort life tables to determine: (1) dominant sources of mortality, and (2) which sources of mortality were spatially density dependent. The impact of parasitism on leaf miner success was also measured by excluding parasitoids with cages from four trees in 1986. Most mortality occurred during larval development and was caused by interference, parasitism, and unknown causes. Interference means that one larva kills another when mines coalesce during the first three instars. Egg mortality, predation, and death from premature leaf abscission were generally low, except in 1988 when a severe drought caused significant early leaf fall. I tested if dominant sources of mortality were density dependent at four spatial scales: per leaf, twig, branch and tree. Larval interference was the only consistently density—dependent cause of death. No between—trophic—level interactions imposed density—dependent mortality. Although interference was spatially density dependent, it appears incapable of regulating P. salicifoliella population size. Instead, P. salicifoliella densities approached population "ceilings" delimited each year by availability of young leaves during oviposition. Cessation of the outbreak is unlikely until stochastic meteorological events increase overwintering mortality and/or severely disrupt synchrony between ovisposition flights and availability of young foliage.

ISSN:0012-9658    

DOI:10.2307/1940960

出版商:Ecological Society of America    

年代:1991

数据来源: WILEY

摘要:

The effects of different light and soil nutrient conditions on foliar chemistry and acceptability of Inga oerstediana seedlings to leaf—cutter ants were investigated. I determined differences among (1) leaves that were initiated and matured under identical conditions but subsequently were subjected to different light and fertilization treatments and (2) leaves that were initiated and matured under different soil, light, and fertilization treatments. I also assessed the relationship between environmentally induced differences in plant growth and the production of carbon—based secondary chemicals (specifically, tannins). Finally, I determined whether increases in tannins corresponded to decreases in the acceptability of leaves to leaf—cutter ants. Once a leaf matured the concentrations of tannins did not change as a function of light and soil nutrient conditions despite very large differences in growth rates among the treatments. Leaves that expanded to maturity under different soil, light, and fertilization treatments did differ in tannin chemistry, and treatment differences in plant growth rates appeared to dictate tannin chemistry. When growth was light limited, tannin concentrations were low, but when growth was nutrient limited, tannin concentrations were high. Only leaves that expanded to maturity under different environmental conditions differed in their acceptability to leaf—cutter ants. Decreases in tannins did not result in parallel increases in acceptability. Despite higher concentrations of tannins, leaf—cutter ants preferred the leaves of seedlings that were grown at 20% light over leaves of seedlings grown at 2% light. However, fertilization increased the acceptability of leaves when seedlings were grown at 20% light, a result consistent with the avoidance of very—high—tanning leaves. From these results I suggest that spatial variation in resource availability in lowland tropical rainforests would result in differences in tannin chemistry, and in susceptibility to leaf—cutter ants. However, temporal changes in resource availability would only be important to tannin chemistry and susceptibility if the duration of the change were long enough to allow new leaves to be produced.

ISSN:0012-9658    

DOI:10.2307/1940961

出版商:Ecological Society of America    

年代:1991

数据来源: WILEY

摘要:

Pines growing in unproductive habitats experience more damage by moose than those growing in more productive habitats. This suggests that trees in unproductive habitats are qualitatively different, e.g., more palatable to moose, and/or less able to recover from browsing. Moose browsing was compared on four phenotypes of Scots pine taken from a gradient of forest productivity. The trees were exposed, side by side, at test stations during winter. Consumption of twig biomass was highest on pines from the most productive habitat and lowest on those from the moderately productive habitat with an overstory of mature pines. Consumption was intermediate on pines from the least productive habitat and from the moderately productive habitat without an overstory. Pines from the highly productive habitat had greater nitrogen, potassium, and phosphorus content and more readily degradable dry matter (when incubated in the rumen of a cow) than did pines from the less productive habitats. Even though they provided a lower quality food for moose, pines in the less productive habitats generally suffered most from moose browsing. The slender twigs and the slow growth rate of these pines allow the moose to browse the main part of the needle—bearing twigs, in contrast to pines from more productive habitats. We also recorded the mortality response to simulated winter browsing in living pines on this same forest productivity gradient. Pine mortality was greater in the less productive habitats than in the more productive ones. These findings largely explain why pines growing in habitats of low and medium productivity suffer more than pines in highly productive habitats, where the possibility for growth is better. Severe damage to stands growing on poor soils most probably arises where moose density is high and where moose have already harvested most of the available twigs from pines growing in the more productive habitats. The moose are then forced to utilize the suboptimal food resources available in less productive habitats.

ISSN:0012-9658    

DOI:10.2307/1940962

出版商:Ecological Society of America    

年代:1991

数据来源: WILEY