摘要:

To examine nonclimatic factors controlling small—peatland development we expanded the classical model of peatland development by hypothesizing the existence of three horizontal zones. We then tested five predictions derived from the expanded model. Prediction 1: Peat Strata. There are two vertical strata of peat, an upper layer consisting of peat formed in the floating or grounded mat (mat peat) and a lower layer of peat formed in the floating mat, but dropped from the side or bottom of the mat (debris peat). Underlying these strata are lake sediments originating in the water column. Prediction 2: Strata Boundary. Chamaedaphne calyculata provides the framework for mat growth, and therefore stems and roots of this plant form the boundary between the two peat strata. The lower boundary of mat peat is continuous from the lake edge to the upland. Prediction 3: Strata Thickness. Thickness of mat peat increases as a function of the depth of the original basin and the length of time peat has been accumulating. Prediction 4: Peat Density. The density of peat increases with distance from the lake edge up to a zone in which peat has reached a maximum density and mean density is constant. Prediction 5: Vertical Accumulation Rate. The long—term rate of vertical accumulation (in grams per square metre per year) decreases with distance from the lake edge. To test these predictions we used data collected on peat stratigraphy, lake—edge vegetation, and peat age and density in Fallison Bog, a 5.5—ha peatland—lake system in northern Wisconsin, and to a lesser extent from 13 other peatlands in the region. Each prediction was confirmed, except that thickness of mat peat appears to be independent of peat accumulation time. We estimated the vertical growth rate of the mat by radiocarbon dating of C. calyculata stems. By dating twigs at different locations in the mat we estimated that the rate of vertical peat accumulation ranges from 34—75 g°m—2°yr—1, depending on location within the peatland. The relationships among peat densities, vertical accumulation rates, and distance from the lake edge suggest that three horizontal zones occur in Fallison Bog: (1) a zone of thickening near the lake edge where vertical accumulation of organic matter thickness the floating mat, (2) a zone of compaction farther from the lake edge where vertical accumulation compacts underlying peat, and (3) a zone of equilibrium farthest from the lake edge where peat has reached a maximum density (100—110 kg/m3in Fallison Bog) and, in the absence of a perched water table, no peat accumulates. Our results underscore the importance of spatial dynamics in peatland development.

ISSN:0012-9658    

DOI:10.2307/1938507

出版商:Ecological Society of America    

年代:1986

数据来源: WILEY

摘要:

Benefits of host specificity for Uroleucon nigrotibium (Homoptera: Aphididae) include not only improved physiological efficiency but also enhanced success of dispersal and defense tactics. I tested the physiological efficiency hypothesis, which predicts a higher reproductive rate for this specialist that for the generalist U. gravicorne when each is reared on their shared host, Solidago nemoralis. Between—species comparisons of developmental times and fecundities of individuals reared on naturally occurring and potted screenhouse host plants supported this prediction for the spring but not in summer, when most reproduction occurs. Greater physiological efficiency of the specialist is also supported by its greater size, which may aid in dispersing and in escaping enemies by walking. Field colonies of the specialist were found to produce few winged forms and to disperse frequently via apterae walking between hosts; consequently, U. nigrotibium infestations are larger and longer lived and are attacked more by enemies than those of its generalist congener. In experiments using potted host plants, the specialist was better able than the generalists to escape predation by walking to nearby hosts. This strategy of persistence depends on specificity to a long—lived, clumped, bushy host such as S. nemoralis.

ISSN:0012-9658    

DOI:10.2307/1938508

出版商:Ecological Society of America    

年代:1986

数据来源: WILEY

摘要:

We investigated the effects of crowding and host plant nutrition on the development time and body length (a correlate of fecundity) of Prokelisia marginata, a wing—dimorphic planthopper that feeds on the phloem sap of the intertidal marsh grass Spartina alterniflora. Crowding delayed development time for males and females of both volant macropters and flightless brachypters; however, host plant nutrition did not affect development time. The body length of both sexes increased when they were raised on nutritionally superior hosts, but the nutritional effect on body size was greater for females than males. Body length exhibited a significant crowding x plant nutrition interaction; crowding had a greater reducing effect on body length if planthoppers were raised on marginal—quality hosts than on nutritionally superior hosts. These findings are discussed in the context of the performance, population dynamics, and migration behavior of P. marginata in its various habitats.

ISSN:0012-9658    

DOI:10.2307/1938509

出版商:Ecological Society of America    

年代:1986

数据来源: WILEY

摘要:

The resource concentration hypothesis states that specialized herbivores are more likely to find remain in pure, large, and/or dense stands of their hosts plants. This study examined whether in fact herbivores tended to remain longer in large vs. small patches of host plants and attempts to explain in the results by modelling herbivore movement. Both movement within a patch and emigration of marked Mexican bean beetles (MBBs) were studied by releasing beetles into small (19 plants) and large (61 plants) patches. Beetles left small patches at a faster rate than large patches, but the difference was not statistically significant. A stochastic model that was developed translates individual patterns of MBB movement into population patterns of beetle distribution within a patch, and predicts how this distribution changes with time. Estimating and testing a model on two separate sets of data provides a more rigorous test than applying the model to the data set on which it was developed. The parameters of the model, therefore, were estimated from data collected in the large patches, and the model was tested by comparing the predicted with the observed spatial distribution of beetles in small patches (i.e., a novel situation). Predicted distribution were very similar to the observed distributions; the predicted proportion of insects remaining in a patch at each census time differed from the observed by @<10% on the average. The model predicts that MBB emigration rate should decrease with patch size, confirming the observed trend.

ISSN:0012-9658    

DOI:10.2307/1938510

出版商:Ecological Society of America    

年代:1986

数据来源: WILEY

摘要:

I tested for an association between intraspecific size differences and differences in resource utilization in worker bumble bees (Bombus ternarius and B. pennsylvanicus). Bees were observed foraging in two fields in Minnesota where flower species with short corollas and species with long corollas occurred in both single—species and mixed—species stands. In mixed—species stands, foragers on the species with short corollas were found to have shorter proboscides than conspecific foragers on the species with long corollas. However, proboscis lengths of foragers on the species with a long corolla in single—species stands did not differ from those on the species with a short corolla in single—species stands. Thus, where a choice exists, bumble bee foragers select the species having a corolla most compatible to their proboscis length. I then compared conspecific foragers on a single flower species in one—species versus mixed—species stands. For foragers on flowering species with a short corolla, proboscis length was significantly shorter in the mixed—species than in the single—species stand, but this difference was not found for the species with a long corolla. Thus, presence of other flowering species can influenced the pollinator population of a species, relative to a single—species stand. Resource choice by foragers was analyzed using a probability function, calculated as the ratio of foragers with a given proboscis length on the species with a long corolla to the number with that proboscis length collected on both species. The shape of the probability function was fitted to a sigmoidal curve using probit analysis Regression of the linearized probabilities was significant. This procedure is a better measure of intra— or interspecies association than are measures such as niche overlap, because it quantifies a continuous distribution and accounts for variance in the species distribution.

ISSN:0012-9658    

DOI:10.2307/1938511

出版商:Ecological Society of America    

年代:1986

数据来源: WILEY

摘要:

Colonizing stem mothers of the gall—forming aphid Pemphigus betae defend galls sites on narrowleaf cottonwood, Populus angustifolia. Kicking—shoving contests are used to establish microterritories 3—5 mm in length. The largest stem mothers usually win in competitive bouts. Small differences in body size (i.e., a 3.4% difference in prothorax widths) are more importantly associated with territorial interactions than with fecundity. Stem mothers that win the superior basal position of a leaf produce an average of 56% more progeny than stem mothers displaced to inferior distal positions. Differences in reproductive performance within a leaf result from microhabitat variation in leaf quality. A change of only a few millimetres in the position of the gall on the leaf blade affects aphid reproduction. Experimental removal of either member of a competing pair allows the remaining stem mother to cross the former territorial boundary and enlarge her own territory. The impact of competitive release on reproductive success is asymmetrical. The distal stem mother of a competing pair achieves an average 48.5% increase in number of progeny, whereas the basal stem mother achieves an 18.5% increase in number of progeny. Because both members of a competing pair suffer reduced success from competitive interactions, selection favors stem mothers that occupy leaves singly. The largest stem mothers are solitary; their despotic behavior prevents smaller competitors from settling.

ISSN:0012-9658    

DOI:10.2307/1938512

出版商:Ecological Society of America    

年代:1986

数据来源: WILEY

摘要:

The herbicide atrazine was applied to 0.045—ha experimental ponds in two replicate concentrations of 0, 20, 100, and 500 mg/L each. Physical, chemical, and biological variables were measured and aquatic insect community structure was monitored in the eight ponds using partially submerged funnel emergence traps. Atrazine had no effect on water temperature or oxygen concentration. Mean turbidity did increase significantly with atrazine concentration, but turbidity could not be correlated with abundance or species richness of emerging insects. Macrophyte production decreased with increasing herbicide concentration; however, the macroalga Chara sp. showed resistance up to 100 mg/L. The benthic insect community was also negatively affected by atrazine. Abundance of emerging individuals of the chironomid Labrundinia pilosella was significantly reduced at atrazine concentrations as low as 20 mg/L, while other, less abundant species showed similar declines. Benthic insect species richness (S), species equitability (J'), and total emergence all declined significantly with atrazine addition. In general, abundances of nonpredatory insects were greatly reduced with the addition of atrazine (20 mg/L), while predatory insects showed no response to the herbicide. In addition, emergence periods of several herbivorous insect species shifted to an earlier time in atrazine—treated ponds; the shift was statistically significant for Oxyethira pallida. The lowest concentration at which atrazine affected aquatic insects in the pond system (20 mg/L) is one order of magnitude lower than the lowest concentration previously shown to have a direct toxic effect on the midge Chironomus tentans in single—species laboratory experiments. Furthermore, because of the differential response of predatory and nonpredatory insect species to atrazine in this study, it is suggested that the effect of the herbicide on the insect community in the experimental ponds may be primarily indirect, presumably through reduction of the food of nonpredators (periphyton, macrophytes) and, to some extent, their habitat (macrophytes).

ISSN:0012-9658    

DOI:10.2307/1938513

出版商:Ecological Society of America    

年代:1986

数据来源: WILEY

摘要:

First—instar mantid nymphs (Tenodera sinensis) of a single cohort exhibited a Holling's Type II functional response to prey density in the laboratory. However, functional response was a poor indicator of success. Predation rate increased well beyond maximum gain in survival, development time, and size at ecdysis, all of which are directly related to fitness in this species. Food limitation is common for this species in nature, particularly during the first instar, and apparently selection has not been adequate to optimize predation rate. Predation rate varied over time as well as with density. Daily feeding rate among nymphs fed ad libitum reached a peak midway through the instar, and then dropped sharply just prior to ecdysis. These and other sources of variability will influence and impact of these generalist predators on arthropod assemblages in the field.

ISSN:0012-9658    

DOI:10.2307/1938514

出版商:Ecological Society of America    

年代:1986

数据来源: WILEY

摘要:

Both tannin and fat concentration in acorns have been postulated to affect squirrel preferences and foraging strategy. Our study directly tests the effects of tannin and fat on the food preferences of free—ranging squirrels (Sciurus carolinensis). Using food items processed from white oak (Quercus alba) acorns with various amounts of tannin and fat added, we find that added tannin significantly reduces the probability that a food item will be eaten and the length of time spent eating it. The addition of fat is shown to attenuate the effects of tannin, but the overall effect is that squirrels do not forage in a manner that maximizes daily energy intake. These results are contrary to the findings of previous studies. A hypothesis to presented to account for the apparent contradiction: The squirrels may maximize energy intake over the entire fall/winter season rather than over a shorter period, such as each foraging bout or day.

ISSN:0012-9658    

DOI:10.2307/1938515

出版商:Ecological Society of America    

年代:1986

数据来源: WILEY

摘要:

Four sympatric species: Desmognathus quadramaculatus, D. monticola, D. fucus, and D. ochrophaeus, showed different substrate preferences in experimental trials. In addition, the choices of substrates and of cover objects were influenced by the presence of other salamanders. Significant differences in substrate choice were found for each pair of species. The ratio of rocky to woody substrate chosen was lower for the more terrestrial species. The ecologically intermediate species, D. monticola, exhibited the broadest choice of substrate and shifted its choice when confined with congeners. Adults of D. monticola became more active and avoided the preferred substrate of D. quadramaculatus when confined with that species. Juvenile of D. monticola became less active but also avoided preferred substrates of the larger D. quadramacultatus. In 1—m2arenas with four cover objects, small individuals of the three more terrestrial species avoided cover occupied of large individuals of these species. No aggressive behavior patterns were observed in 100 encounters of different individuals. These results suggest that this assemblage of salamanders is structured by species—specific microenvironmental preferences that are affected by interspecific interactions. Avoidance of other salamanders is the common response that segregates individuals and species. This is likely an adaptation to the severe risk of predation faced by small salamanders.

ISSN:0012-9658    

DOI:10.2307/1938516

出版商:Ecological Society of America    

年代:1986

数据来源: WILEY