摘要:

In many plants, extrafloral nectars attract ants that protect the plant from herbivory. Previous work indicates that both sugars and amino acids contribute to ant attraction and that herbivory may affect nectar production or composition. This study was designed to measure the effect of defoliation on sugar and amino acid concentrations in the extrafloral nectar of Impatiens sultani. Twenty—nine fall—raised plants and 45 spring—raised plants were subjected to varying degrees of defoliation in which 25, 50 or 75% of each leaf was removed. Extrafloral nectar was collected immediately before and 24 h after defoliation for the fall—raised plants; additionally, nectar was collected 24h before and 48 and 72 hr after defoliation in the spring—raised plants. Nectars were analyzed for sugar and amino acid content by means of high—pressure liquid chromatography (HPLC). Twenty four hours after defoliation, amino acid content increased dramatically in plants subjected to all three levels of defoliation' amino acid concentrations of nectars 72 h after defoliation were indistinguishable from predefoliation levels. No change in sugar concentrations or nectar volumes was detected. Additional work is needed to determine if the increase in amino acid is adaptive and serves to attract more plant protectors.

ISSN:0012-9658    

DOI:10.2307/1940251

出版商:Ecological Society of America    

年代:1990

数据来源: WILEY

摘要:

Several aspects of nectarivore foraging behavior have been interpreted as responses to spatial reward patchiness of the kind documented for Delphinium nelsonii floral nectar by Pleasants and Zimmerman (1979). Working with this same species over 3 yr, however, we were unable to detect substantial pattern in nectar standing volumes, either through contingency analyses or spatial autocorrelation. Although spatial outocorrelations between rewards of neighboring plants were positive in 5 or 6 samples examined, only one value was statistically significant. Spatial autocorrelations over longer distances were erratic. We used computer simulations of nectarivores foraging in a large plant population to explore factors that promote reward patchiness. Simulations suggest that moderate patchiness will develop at all but extremely low or high flower visitation rates. Rates were intermediate at our sites and those of Pleasants and Zimmerman, however, so visitation intensity does not seem to explain the discrepancy between our results and theirs. On the other hand, reward patchiness in simulations declined substantially as nectarivores exhibited less area—restricted foraging. Hummingbirds were important visitors at our sites, and fly farther between plants than the bumble bees that predominated at Pleasant's and Zimmerman's sites. Finally, simulations suggest that spatial patchiness is lower when interplant coefficient of variation in nectar production rate is large, as in our populations.

ISSN:0012-9658    

DOI:10.2307/1940252

出版商:Ecological Society of America    

年代:1990

数据来源: WILEY

摘要:

Phratora vitellinae and Galerucella lineola are two leaf beetles that feed on willows (Salix) in central Sweden. When disturbed, larvae of P. vitellinae exude droplets of a defensive secretion from dorsal glands. The secretion contains salicyl aldehyde, the precursors for which are plant—derived salicylates like salicin and other chemically similar phenolic glycosides. In contrast, larvae of G. lineola lack dorsal glands, cannot convert salicylates to salicyl aldehyde, and no not produce such a defensive secretion. We evaluate the adult oviposition pattern, larval performances, and suceptibility to predators of these two beetles on three species of willows, two rich in salicylates (Salix fragilis and S. dasyclados) and one poor in salicylates (S. viminalis). Females of G. lineola oviposited preferentially on S. viminalis and S. fragilis where larvae developed rapidly and survival was high, and avoided S. dasyclados where larval performance was poor. Variation in leaf pubescence, toughness, and nitrogen content across willow species not explain observed patterns of larval performance. However, larval performance was inversely related to the concentration of salicylates in the three willows. By contrast, the oviposition preferences of P. vitellinae did not correspond well to patterns of larval performance. Salix fragilis was by far preferred choice of oviposition, and females rarely placed eggs on either S. viminalis or S. dasyclados. The performance of P. vitellinae larvae differed markedly on the two salicylate—rich willows; larvae survived best and developed most rapidly on S. fragilis, but mortality was high and development slow on S. dasyclados. Poor larval performance on S. dasyclados was associated with high concentrations of simple phenolic compounds other than salicylates. Despite differences in performance, larvae fed these two salicylate—rich willows produced a defensive secretion which effectively repelled coccinellid predators. Larvae fed the salicylate—poor S. viminalis survived and developed well in the absence of enemies, but lacked the salicylate—based defensive secretion and were readily eaten by coccinellids. Females of P. vitellinae apparently avoided S. viminalis as an oviposition host, not because larvae performed poorly, but because larvae were defenseless against some predators when raised on this salicylate—poor willow. We argued that herbivores such as P. vitellinae obtain enemy—free space on hosts from which they sequesters plant—derived allelochemicals for defense. The result appears to be dietary specialization on host—plant species that provide these precursors for defense. Other related herbivores like G. lineola that do not employ plant—derived defensive secretions, are not so constrained and feed on a wider range of host willows.

ISSN:0012-9658    

DOI:10.2307/1940253

出版商:Ecological Society of America    

年代:1990

数据来源: WILEY

摘要:

To learn what scope there may be for a mother plant to modify whether her seeds are removed by ants, and how quickly, we investigated (1) how initial seed removal by ants may vary in times (years, season, time of day) and space (within and between sites, and (2) how large these sources of variation are. We focused on seed of two common ant—dispersed species in a field experiment on 20 x 20 m plots at each o three sites in Kuringai Chase National Park in southeast Australia. The most important influence on removal rate over the first 12 h of exposure was the precise location of seeds on the ground (54% of total variation). This variation did not persist over longer periods of time and, except during winter, all seeds had a high probability of being removed within 2—3 d. Time of year also had a strong influence had a strong influence on removal rate (21% of total variation) with the highest rates occurring in summer and the lowest in winter. These changes were mainly attributable to changes in ambient temperature. Differences in removal rates between the two seed species used was the third most important source of variation (4%). There was significant variation in removal rate between the same months in different years, in two of the three between—year comparisons. The effect of year, and of interactions between year and other factors, accounted for between 7 and 18% of total variation. Complementary experiments showed that ants were the only important removers of seed at the study sites and that seed age up to 1 yr did not influence removal rate. Experiments comparing removal rates of clumped seeds to those of single seeds showed no consistence difference. The high probability of removal of elaiosome—bearing seeds within 2—3 d of seedfall means it is unlikely that ant—dispersed plants in this vegetation ever face the problem of their seeds not being encountered and transported by an ant. Instead, the problem may be how to be removed by those ant species whose behavior will give the seed the best chance of survival and establishment.

ISSN:0012-9658    

DOI:10.2307/1940254

出版商:Ecological Society of America    

年代:1990

数据来源: WILEY

摘要:

Stands of two species of African Acacia were monitored for 1 yr in a natural savanna ecosystem, to compare shoot regrowth and leaf chemistry in lightly browsed and heavily browsed trees. Where ungulates concentrated at a seasonal waterhole, A. nigrescens was more severely browsed than the thornier and finer leafed A. tortilis. Shoot regrowth in heavily browsed A. nigrescens more than compensated for herbivory, as net annual shoot extension was not significantly different from that in light browsed trees. Foliage of heavily browsed A. nigrescens was higher in nutrients and lower in condensed tannins than foliage of light browsed trees. We propose that severe pruning by browsing ungulates reduces intershoot competition for nutrients, promoting rapid shoot regrowth. Carbohydrate demands of rapid regrowth reduce carbon—based secondary metabolite synthesis. This results in patches of highly palatable browse that attract further browsing, generating a browsing regrowth feedback loop. Such patches may be considered analogous to grazing lawns.

ISSN:0012-9658    

DOI:10.2307/1940255

出版商:Ecological Society of America    

年代:1990

数据来源: WILEY

摘要:

Balsam fir (Abies balsamea) population parameters were analyzed in Isle Royale National Park, Michigan, to asses moose (Alces alces andersoni) herbivory in relation to the population density of both fir and moose. Fir population parameter were determined at nine study sites, each representing a different combination of relative fir and moose density (high, medium, and low levels, respectively). In general, moose suppression of fir height growth and recruitment to the canopy increased with increasing moose density and decreasing fir density. Where fir densities were low, sapling heights were restricted to<1 m regardless of moose density. In such sites, sapling growth suppression has been continuous since a peak in the moose population in the 1920s. Canopy fir trees at most sites established prior to arrival of moose on Isle Royale around 1910. At high fir density release from growth inhibition was common following a period of low moose numbers from the mid—1970s to early 1980s. Here this release should facilitate recruitment of fir and effect replacement of canopy losses due to senescene. In other areas, however, continuing canopy looses and lack of recruitment due to intensive herbivory by moose will greatly reduce the fir component of the canopy, effectively preventing any return to the fir—dominated forests of the past.

ISSN:0012-9658    

DOI:10.2307/1940256

出版商:Ecological Society of America    

年代:1990

数据来源: WILEY

摘要:

To investigate the effects of differences in light and nutrient availability on growth, we planted seven species of shrubs in two genera, Miconia (Melastomataceae) and Piper (Piperaceae), into the centers, edges, and adjacent forest understory of four natural treefall gaps (275—3355 m2) in the tropical premontane rain forest of Costa Rica. We used rooted cutting of species typical of forest understory environments on the one hand and large clearings or disturbed areas on the other. We also compared growth rates of three Miconia species grown in shade houses under 2, 20, and 40% full sunlight. Both light and nutrient availability in newly formed gaps of these sizes were strongly buffered by the canopy and root systems of the surrounding forest. Total incident radiation was higher in gap centers (9—23% full sunlight) than in gap—forest edges (3—11%) or under intact forest canopy (.04—2%), but varied among similar microhabitats from different sites. Relative stem growth rates (RGRs) of all field—grown plants were significantly greater in gap centers than at edges or beneath forest understories. Fertilization did not significantly affect growth rate in any light environment. Light appears to be the most critical resource limiting growth at these gap sizes. In general, shade—tolerant species were less plastic than light—demanding species, but at these gap sizes grew as fast or faster in the gap centers. In shade—houses, the shade—tolerant species grew faster at 20% full sunlight and light—demanding species grew faster at 40% full sunlight. We found no evident of a trade—off between growth and foliar phenolic concentration in these species.

ISSN:0012-9658    

DOI:10.2307/1940257

出版商:Ecological Society of America    

年代:1990

数据来源: WILEY

摘要:

Field experiments were performed over two growing seasons to investigate the response of Schizachyrium scoparium (C4photosynthetic pathway) and Poa partensis (C3) to natural and simulated bison urine deposition in a northern, mixed prairie in South Dakota. We also assessed potential feedbacks of urine deposition on herbivore grazing by monitoring grass response to defoliation and herbivore grazing preference for vegetation occupying urine patches. Total aboveground biomass and root mass were higher and root: shoot ratios were lower on urine patches than in the surrounding plant community. Higher total aboveground biomass on urine patches resulted primarily from increased aboveground P. pratensis production. Urine deposition in May had little effect on aboveground production of S. scoparium except during July when S. scoparium was most active. Urine disposition date and plant penology appear important in determining changes in species composition. Following urine deposition, aboveground N concentrations of P. pratensis and S. scoparium were higher on patches relative to conspecifics off patches. This increase in N concentration following urine deposition was greater in P. pratensis. We suggest the largest increase in P. pratensis biomass following urine deposition is related to its relatively large response to increased soil N availability and its rhizomatous habit. Root N concentrations were higher on urine patches. Poa pratensis on urine patches initiated growth earlier in the season and postponed senescene relative to plants off patches. Aboveground production following clipping was greater on urine patches and N concentrations in regrowth of both species were higher than concentrations in plants not previously clipped. Above ground herbivore utilization was greater on urine patches than on adjacent vegetation. Although urine patches covered only 2% of the study site, they provided 7% of the biomass and 14% of the N consumed by aboveground herbivores from June through August. Urine patches probably provided an even greater source of forage and N for herbivores earlier and later in the growing season when surrounding vegetation was most quiescent.

ISSN:0012-9658    

DOI:10.2307/1940258

出版商:Ecological Society of America    

年代:1990

数据来源: WILEY

摘要:

A successional process model has been adapted for use with species from ponderosa pine/Douglas—fir (Pinus ponderosa var. ponderosa)/(Pseudotsuga menziesii var. glauca) forests of the inland Northwest. Its design allows modification for application to other forest types. This model, FIRSUM, simulates tree establishment, growth, and mortality, along with live and dead fuel accumulation, fire behavior, and fuel reduction on a 400—m2plot. The modeling contains algorithms for influences on tree establishment and growth including temperature, water stress, light tolerance, and site quality. The model was used to predict 200 yr of forest succession for five different disturbance regimes. This allowed comparison of patterns of basal area by species, of duff and fuel accumulation, and on fire intensities among the following scenarios: (1) no fires (fire suppression), (2) consistent fire intervals of 10, 20, and 50 yr, and (3) a natural fire regime of variable intervals reconstructed from fire scarred trees. Frequent fires (10— and 20—yr intervals) were simulated to be of low intensity, resulting in scorch heights of 0.5—3.0 m. These fires prevented Douglas—fir saplings from surviving and becoming part of the overstory. Simulation results for the 10— and 20—yr fire intervals were similar to those for the natural fire regime, based on fire occurrence between AD 1600 and 1900. However, the occasional long fire intervals within the natural fire regime allowed greater regeneration success for ponderosa pine. Fires at regular intervals of 50 yr were more severe and resulted in a decrease of western larch (Larix occidentalis) after 150 yr, with a corresponding increase in ponderosa pine. Douglas—fir slowly increased in basal area and became established in the overstory after 200simulations years. The no—fire scenario allowed Douglas—fir to achieve dominance in the understory, and eventually in the overstory, thereby limiting survival of ponderosa pine and western larch regeneration. A test of the model showed predictions to be within 19% of field observations, and a sensitive analysis of FIRESUM showed parameters associated with the growth algorithm to be most critical for predicting successional trends.

ISSN:0012-9658    

DOI:10.2307/1940259

出版商:Ecological Society of America    

年代:1990

数据来源: WILEY

摘要:

Four sizes of forest opening (0.016, 0.08, 0.4, 0.4, and 2.0 ha; two replicates each) were established in a Southern Appalachian forest to examine the effects of disturbance size on earl successional community structure and function. Solar radiation, soil temperature, and air temperature were all higher in large openings than small openings and increased from edge to center of disturbance patches. Aboveground net primary productivity (NPP) was 3—4 times as highe in larger (2.0 ha) as small (0.016 ha) openings, presumably in response to greater light availability in large patches. Stump and root sprouts of tree species accounted for the largest fraction of NPP in all patch sizes. Herbs, vines, shrubs, advance regeneration trees, and tree seedlings had progressively smaller NPP, respectively. Vegetation biomass reached 0.7—2.6% of undistributed forest levels and aboveground NPP reached 17—58% of forest levels by the 2nd yr after cutting. Plant species richness was generally higher in large than small patches. Tree species composition shifted considerably followed disturbance. Liriodendron tulipifera was important before and after logging. Large canopy dominants such as oaks and hickories were relatively unimportant sources of sprouts during early revegetation. Instead, minor canopy and understory species such as Robinia pseudoacacia, Halesia carolina, Acer rubrum, Cornus florida, and Magnolia fraseri were the major sprouters in all patch sizes. The N—fixing black locust (Robinia) was much more important in large than small openings. Disturbance size within the Southern Appalachians thus affects microenvironment, species composition, and NPP during early revegetation.

ISSN:0012-9658    

DOI:10.2307/1940260

出版商:Ecological Society of America    

年代:1990

数据来源: WILEY