摘要:

This field experiment examined whether early—season damage to a wild, native shrub affected the survival and cumulative numbers over several generations of its most abundant herbivore, Bucculatrix thurberiella. The effects of reduced plant density on these herbivores and possible interactions between density and induced resistance were also considered. Half of the plants were damaged experimentally at the start of the season by feeding caterpillars; the other half served as controls. Half of the plants experienced experimentally reduced densities of neighbors, while the other half were surrounded by natural densities. Early—season damage did not influence the initiation of new mines, although plants experiencing reduced density received significantly more mines than controls. Miners were less likely to survive on plants that had been damaged in 1989. This was also true in 1990 but only for those plants at reduced density. Effects on one stage in the life cycle of an insect need not translate into similar effects on the size of its population on a host plant. In this study, I was able to measure effects of resistance induction on the cumulative numbers of B. thurberiella because it completes many generations in each season and because a record of each individual is preserved at the end of the season. Damaged plants and those that had natural densities of neighbors had fewer cumulative numbers of mines than plants that had not been previously damaged and those with reduced densities of neighbors. Plant density had a greater effect than induced resistance on the cumulative number of mines produced over an entire season.

ISSN:0012-9658    

DOI:10.2307/1939495

出版商:Ecological Society of America    

年代:1993

数据来源: WILEY

摘要:

Many plants employ induced defenses that are activated after attack rather than constitutive defenses that are always present. Current theory predicts that one benefit of induced defenses is to optimize the plant's allocation to defense, growth, and reproduction. According to this reasoning induced defenses are expected under the following conditions: (1) Attack is unpredictable and not fatal. (2) Defense is costly; when herbivores are not present, the plant can save resources by not being defended and use them for growth and reproduction. (3) When herbivores are present, activation of the defense increases plant growth and reproduction. This study tested the allocation hypothesis by examining the effects of early—season damage to a desert shrub, Gossypium thurberi, in environments that either included or did not include its most abundant herbivore, Bucculatrix thurberiella. A companion paper demonstrated that induction occurred. Plants damaged early in the season supported fewer miners than controls that were not previously induced. If there was a benefit of induced resistance, I expected that survival, growth, and seed production would be greater for plants that had been induced experimentally by caterpillars early in the season compared to those that had not. I found no evidence that plants benefited by responding to damage early in the season, despite reductions in the populations of herbivores on damaged plants. If there were costs of induced resistance, I predicted that survival, growth, and seed production would be lower for induced plants than for controls when herbivores were excluded from both treatments. I found no evidence that induced resistance cost the plant anything in terms of survival, growth, or seed production. Finally, to determine if plant crowding was costly and if crowding interacted with induced resistance, survival, growth, and seed production were compared for plants that had no neighbors within 50 cm vs. those experiencing endemic densities of neighbors. Plants that experienced reduced density grew more and produced twice as many seeds as controls that experienced endemic densities of neighbors. No interactions between damage and plant density were observed. There are many different ways to interpret these negative results. I measured only a subset of the plant's lifetime fitness and the statistical power of my tests may have been insufficient. My experiment to test the benefits of induced resistance was much weaker than my tests of the costs of damage and of crowding. Herbivory may be unimportant in this system, although indirect evidence suggests otherwise. This and other empirical results suggest that the cost—benefit model may not be appropriate to explain the advantages of induced resistance over constitutive resistance. I discuss three alternative hypotheses to explain the existence of induced resistance.

ISSN:0012-9658    

DOI:10.2307/1939496

出版商:Ecological Society of America    

年代:1993

数据来源: WILEY

摘要:

A series of experiments indicate that induced changes occur in the foliage of the annual morning glory Ipomopea purpurea when subjected to prior damage. These changes reduce the growth rate, consumption rate, growth efficiency of larvae of the generalist lepidopteran Spodoptera eridania. They also alter the pattern of damage caused by specialist flea beetles, Chaetocnema confinis, and by generalist insect herbivores in the field. No effects of induction were detected on specialist tortoise beetles, Deloyala guttata and Metriona bicolor. Although total damage by both flea beetles and generalist insects did not differ between induced and noninduced plants, this result does not necessarily imply that the effects on these insects were nonadaptive. The observed effects of induced response are consistent with both "diffuse" and "pairwise" views of the selection imposed by herbivores on plant resistance. Finally, the existence of induced responses suggests that previous conclusions by Simms and Rausher that resistance to flea beetles and generalist insects is not costly in I. purpurea may be premature.

ISSN:0012-9658    

DOI:10.2307/1939497

出版商:Ecological Society of America    

年代:1993

数据来源: WILEY

摘要:

Seedling representing four families of European white birch (Betula pendula) were grown through their first growing season in two fertilization and three shading treatments. The growth responses of seedlings were measured, and after complete winter hardening the basal parts of seedlings were offered to voles (Microtus agrestis) and the top parts of the same seedlings were fed to hares (Lepus timidus). The growth response of 1—yr—old birch seedlings to fertilization and shading was family specific. Nitrogen fertilization generally increased the biomass accretion and the palatability of the seedlings to voles. Fertilization significantly decreased resistance of seedlings to hares in only one out of the four birch families. Shading decreased the biomass accretion of seedlings and increased their palatability to hares but tended to decrease the palatability to voles. The differing preferences of the two mammalian herbivores for seedlings grown under different environmental conditions may be due to vertical differences in the defense of birch seedlings to herbivore feeding. No trade—off was found between seedling defense and growth.

ISSN:0012-9658    

DOI:10.2307/1939498

出版商:Ecological Society of America    

年代:1993

数据来源: WILEY

摘要:

Experiments were conducted over several years to assess the effects of the three most abundant herbivores (a thrips, a spittlebug, and a plume moth) on the growth and reproduction of their long—lived, iteroparous host plant. In a greenhouse experiment, thrips reduced root biomass of seedlings, but not shoot biomass. No correlation was found between density of thirps and change in aboveground area of plants after monitoring for 3 yr in the field. Thirps fed on the phyllaries and corollas of flower heads, as well as on leaves, and were particularly damaging to ray petals. Placed in artificial arrays, flower heads with rays damaged by thirps were less likely to be visited by pollinators than were undamaged flower heads. Plants in a common garden were subjected to three seasons of feeding by either spittlebugs, plume moth caterpillars, both, or neither. Survival, growth, and flower production were measured for those 3 yr and for an additional 3 yr following the herbivory treatments. Plant survival was unaffected by these insect herbivores, although gophers (Thomomys bottae) were a major source of mortality. Plants with caterpillars produced more rosettes than those without moths. By consuming apical buds the caterpillars released axillary buds, thereby causing plants to produce more side rosettes. This difference in architecture did not result in an increase in flower production. Plants that had experienced 3 yr of spittlebug or caterpillar attack produced fewer flower heads than controls during their 3 yr. One year later, and after the herbivory treatments were discontinued, plants that had been damaged by spittlebugs (but not plume moth caterpillars) still produced fewer flowers than those in other treatments. Thereafter, no long—terms effects of herbivory could be detected. Al three herbivores were found to adversely affect some components of plant growth and reproduction. Because they are consistently abundant at Bodega Bay, these herbivores may reduce the fitness of their host plant.

ISSN:0012-9658    

DOI:10.2307/1939499

出版商:Ecological Society of America    

年代:1993

数据来源: WILEY

摘要:

The role of plant chemistry in determining the suitability of herbivore—free individuals within a plant population has rarely been investigated. In this study, we examined the importance of furanocoumarin chemistry in determining distribution patterns of the parsnip webworm (Depressaria pastinacella) in a population of its host plant, wild parsnip (Pastinaca sativa). Parsnip webworms feed almost exclusively on wild parsnip, a species that produces furanocoumarins, compounds that are toxic or deterrent to many insects. Samples of plants hosting actively feeding parsnip webworms (attacked) and plants exhibiting no damage (unattacked) were collected in the field and brought to the laboratory, where they were examined for content of three furanocoumarins, xanthotoxin, bergapten, and sphondin. In addition, they were bioassayed with ultimate—instar parsnip webworms; growth, consumption rate, and rate of in vitro metabolism of furanocoumarins were compared between larvae fed undamaged tissue from attacked plants and larvae fed tissue from unattacked plants. Attacked plants contained less than half the amount of xanthotoxin and sphondin, two of the three furanocoumarins assayed, than did unattacked plants. Growth and consumption rates of larvae fed tissue of unattacked plants also were reduced significantly (40 and 24%, respectively) compared to larvae fed attacked plants. Moreover, in vitro metabolism of xanthotoxin and bergapten was reduced by 50% in larvae fed tissue from unattacked plants compared to larvae fed tissues of attacked plants. Cluster analysis placed plants into three groups based on furanocoumarin chemistry. One group, characterized by high concentrations of all three furanocoumarins, contained only unattacked individuals. A second group consisting of plants intermediate in furanocoumarin concentration, contained predominantly unattacked individuals, and last group, characterized by low concentrations of all three furanocoumarins, was dominated by attacked individuals. When large larvae are fed plant tissues with below—average total furanocoumarin concentrations, consumption rate is the most important larval characteristic determining growth rate. However, when larvae are fed plant tissues with above—average concentrations of furanocoumarins, the capacity to metabolize furanocoumarins is also important. These results indicate that plant chemistry, relative to larval ability to tolerate plant chemicals, can to a large extent explain the distribution of this insect within a population of its host plant.

ISSN:0012-9658    

DOI:10.2307/1939500

出版商:Ecological Society of America    

年代:1993

数据来源: WILEY

摘要:

We examined the seasonal changes in plant chemical defenses of key tree species (Robinia pseudoacacia, Liriodendron tulipifera, Acer rubrum, and Cornus florida) regenerating during early succession in different patch sizes within Southern Appalachian forests. Trees of each species were sampled from five sizes of forest openings (0.016, 0.08, 0.4, 2.0, and 10 ha; two replicates each) and from the understory of surrounding forests. Leaves of rapidly growing, pioneer species (Robinia and Liriodendron) were slightly less tough and contained lower levels of hydrolyzable tannins and total phenolics, but higher condensed tannin concentrations than the slower growing, more shade—tolerant species (Acer and Cornus). These results generally support earlier findings indicating the evolutionary development of greater levels of constitutive phenolics in slow—growing than fast—growing species. The differential use of condensed and hydrolyzable tannins by slow— and fast—growing species may reflect adaptive trade—offs in the allocation of different types of phenolics. Regenerating trees often had tougher leaves and contained higher levels of phenolics in large than small openings by midsummer. The differences in phenolics were more pronounced for condensed tannins and when comparing the smallest canopy openings with larger patch sizes. Plant phenolics were also generally lower in understory trees than in conspecific sprouts in openings. These findings may reflect phenotypic carbon/nutrient adjustments as light availability changes across the patch—size gradient. The greater sunlight in large than small openings apparently promotes excess carbon production and a metabolic buildup of phenolic compounds. Independent measures of phenolic compounds peaked at different times during the growing season. Hydrolyzable tannins reached peak levels quite early whereas condensed tannins increased later in the growing season. Total phenolics peaked in midsummer. Herbivore damage was consistently high (5—9%) on Robinia sprouts in all openings. Insect herbivory on the remaining three species (1—5%) was significantly lower in large than small patch sizes. Tree species regenerating in the high sunlight of larger openings in Southern Appalachian forests thus tend to have tougher leaves, contain greater phenolics, and experience less herbivore damage than conspecifics in small openings.

ISSN:0012-9658    

DOI:10.2307/1939501

出版商:Ecological Society of America    

年代:1993

数据来源: WILEY

摘要:

Four California annual grassland species were used to examine the hypothesis that different plant species have equivalent competitive effects. We investigated the effects of the annuals on soil water availability and the growth responses of blue oak (Quercus douglasii Hook&Arn.) to neighbor—induced water depletion. Neighborhoods of annuals were composed of species from California annual grassland with differing phenology and morphology that were hypothesized to show non—equivalent competitive effects on both a per—individual— and a per—unit—tissue basis. Three introduced species of winter annuals were sown at each of three densities (10, 30, or 100 seeds/dm2) around a single oak acorn. The grass Bromus diandrus Roth. and the forb Erodium botrys Cav. flower in early spring, while the grass B. mollis L. flowers slightly later. A native summer annual forb, Hemizonia luzulaefolia DC., was sown at only the intermediate density. Plants were grown outdoors in 15 cm diameter X 1 m deep tubes filled with soil from an oak woodland site. Identity and density of annual species had independent and interactive effects on the water resource level. Soil water potential was negatively density—dependent, remaining highest when oaks were grown without competitors. At the intermediate density soil water potentials decreased in the following order: B. mollis (slowest), B. diandrus, E. botrys, and H. luzulaefolia (most rapid). This order followed the pattern of root length developed in the neighborhoods, which was species— and density—dependent. The relationship between sowing density and neighborhood dry mass also differed among the annual species. Correlations between root length and soil water potential were stronger for the forbs than for the grasses. A given root length of the summer annual, H. Luzulaefolia, depleted soil water to a greater extent than did the same root length of any species. Oak seedling growth, gas exchange rates, and survivorship were negatively related to the rate and extent of soil water depletion. Additionally, the relationship between annual root length and oak seedling height or root length differed by neighborhood species. Thus, neighborhood species were non—equivalent in competitive effect on both a per—individual and a per—unit—root—length basis. Phenologically different species may represent different functional groups of competitors.

ISSN:0012-9658    

DOI:10.2307/1939502

出版商:Ecological Society of America    

年代:1993

数据来源: WILEY

摘要:

The South African succulent Carpobrotus edulis has been invading native and non—native plant assemblages in California since its introduction in the early 1900s. This study was designed to determine mechanisms controlling invasion by this species into three different communities: coastal grassland, backdune, and coastal scrub. The communities differed in several factors that influenced invasion by Carpobrotus: soil disturbance, herbivory, and identity of competitors. Disturbances of the soil was required for establishment of Carpobrotus at the grassland site. Soil disturbance did not promote Carpobrotus establishment in the coastal scrub or backdune. Establishment in these sites was restricted by herbivory. The influence of neighboring vegetation on Carpobrotus establishment was site dependent. At the grassland site, Carpobrotus seedling survival was reduced by grasses, and the only surviving seedlings were on gopher mounds. Growth of larger Carpobrotus plants, however, was unaffected by grasses. Proximity to shrubs, the dominant plant type at the backdune and coastal scrub sites, did not affect Carpobrotus seedling survival. It did, however, decrease both root and shoot biomass of seedlings and cuttings. Invasion is expected to occur most rapidly at the grassland site. Although germination and early survival there depend on the occurrence of rodent disturbances, these occur frequently, and seedling recruitment in the grassland was observed during this study. Once established, clonal growth is great and is not affected by herbivory or competition. In contrast, at the coastal growth, intense herbivory prevented successful establishment during this study. Cage experiments suggested that herbivory counteracted the high growth potential of Carpobrotus at this site. At the dune site, even caged plants showed very poor growth. Thus, although seedling recruitment does not occur there, invasion is expected to be slow due to harsh physical conditions that limit both survival and growth. These findings demonstrate strong community differences in the factors governing invasability by Carpobrotus edulis.

ISSN:0012-9658    

DOI:10.2307/1939503

出版商:Ecological Society of America    

年代:1993

数据来源: WILEY

摘要:

Increasing rates of relative sea—level rise (RSL) have been linked to coastal wetland losses along the Gulf of Mexico and elsewhere. While such losses have yet to be reported for New England tidal marshes, rapidly rising RSL may still be affecting these systems. Studies of the Wequetequock—Pawcatuck tidal marshes over four decades have documented dramatic changes in vegetation that appear to be related primarily to differential rates of marsh accretion and sea—level rise. Other environmental factors such as sediment supply and anthropogenic modifications of the system may be involved as well. When initially studied in 1947—1948 the high marsh supported a Juncus gerardi—Spartina patens belting pattern typical of many New England salt marshes. On the most of the marsh complex the former Juncus belt has been replaced by forbs, primarily Triglochin maritima, while the former S. patens high marsh is now a complex of vegetation types–stunted Spartina alterniflora, Distichlis Spicata, forbs, and relic stands of S. patens. These changes are documented by vegetation sampling that closely followed the 1947—1948 methods and by peat core analysis. Marsh elevations were determined by leveling, and the mean surface elevation of areas where the vegetation has changed is significantly lower than that of areas still supporting the earlier pattern (4.6 vs. 13.9 cm above mean tide level). The differences in surface elevation reflect differences in accretion of marsh peat. Calculations based on sandy overwash layers deposited during historically recorded storms as well as on experimentally placed marker horizons of known age indicate that stable areas have been accreting at the rate of local sea—level rise, 2.0—2.5 mm/yr at least since 1938; changed areas have accreted at about one half that rate. Lower surface elevations result in greater frequency and duration of tidal flooding, and thus in increased peat saturation, salinity, and sulfide concentrations, and in decreased redox potential, as directly measured over the growing season at both changed and stable sites. It is proposed that these edaphic changes have combined to favor establishment of a wetter, more open vegetation type dominated by to distinctive communities–Stunted S. alterniflora and forbs. Changes documented on the Wequetequock—Pawcatuck system have been observed on the other Long Island Sound marshes and may serve as a model for the potential effects of seal—level rise on New England tidal salt marshes.

ISSN:0012-9658    

DOI:10.2307/1939504

出版商:Ecological Society of America    

年代:1993

数据来源: WILEY