摘要:

In 1982, experimental nitrogen gradients were established on both existing and disturbed (disked) vegetation in three fields (abandoned 14, 25, and 48 yr) and on existing vegetation in native oak savannah. Each of these seven gradients contained five or six replicates of each of nine treatments that differed in the annual rate of nitrogen addition. In none of the fields did plant biomass, height, species richness, or light penetration respond to addition of P, K, Ca, Mg, S, and trace metals. In contrast, plant biomass and height increased significantly, and light penetration and species richness decreased significantly, with added nitrogen along all seven gradients. On average,>60% of the species had been displaced from high—nitrogen treatments by 1985. Nitrogen addition led to a period of transient dominance by certain species. Species that reached peak relative abundance in high—nitrogen treatments in 1982 tended to be rare in all but the low—nitrogen treatments by 1985. In contrast, the relative abundances of most species that dominated the high—nitrogen treatments in 1985 did not increase along the nitrogen gradients in 1982. The relative or absolute abundances of most common species changed significantly along the experimental gradients in at least 1 yr. By 1985, many common species were differentiated in their distributions along the seven gradients. In general, early successional annuals and short—lived perennials and plants of short stature at maturity reached their peak abundance in low—nitrogen plots, whereas plots, receiving high rates of nitrogen addition were dominated by long—lived herbaceous and woody species that are taller at maturity. A survey of 22 old fields at Cedar Creek, Minnesota, showed that total and available soil nitrogen increased during succession and that major species had individualistic, fairly Gaussian distributions along this temporal nitrogen gradient. The distributions along the experimental gradients of most of the common species were consistent with the pattern observed in the old—field survey, demonstrating that nitrogen influences the pattern of secondary succession at Cedar Creek. The major exception was Agropyron repens, and early successional grass that dominated high—nitrogen treatments on six of the seven gradients. Comparisons of species responses on the disturbed plots with those on plots of existing (undisturbed) vegetation showed that, by 1985, most species responded similarly to the nitrogen gradients despite great differences in their initial abundances. For instance, Agropyron repens, an initial dominant of Field A but rare in Fields B and C, was dominant in the high—nitrogen treatments in both the disturbed and undisturbed plots of these three fields. It invaded into and rapidly increased in abundance in numerous high—nitrogen plots from which it was initially absent. Schizachyrium scoparium declined along the nitrogen gradients both in undisked plots in which it was initially dominant and in disked plots in which it was initially rare. Such similarities suggest that the outcome of interspecific interactions among old—field plants is highly dependent on nitrogen supply rates, but fairly independent of initial plant abundances.

ISSN:0012-9615    

DOI:10.2307/2937080

出版商:Ecological Society of America    

年代:1987

数据来源: WILEY

摘要:

Life history attributes for the western chuckwalla, Sauromalus obesus, and environmental variables, e.g., rainfall and vegetational growth, were recorded simultaneously during a 7—yr investigation in the Colorado desert of southeastern California. The timing of rainfall was most critical to the growth of annual plants, and the standing crop of winter annuals was greatest when substantial rainfall occurred early (October—January). Chuckwallas preferred to eat winter annuals during the spring. However, broad and opportunistic feeding habits were observed, and the persistence of relatively mild winters and frequent summer rainfall enabled individuals to feed throughout most of the year. Males typically achieved reproductive maturity at 125 mm snout—vent length (SVL) and 2 yr of age. Females achieved reproductive maturity at 125 mm SVL and 2—3 yr only when optimal environmental conditions prevailed. Clutch size was highly correlated with body size, and the mean for the study period was 6.9 eggs. Mean values for other reproductive attributes were: egg mass, 8.4 g; relative clutch mass (RCM), 0.343; and expenditure per progeny (EPP), 0.053. Clutch size, egg mass, RCM, and EPP did not differ significantly among years for a given body size. Mean annual frequency of reproduction was 52%, but ranged from 0 to 95% during the study period. Mean 1st—yr survivorship was 38% but showed considerable year—to—year variability. Egg mortality had the greatest impact on 1st—yr survivorship. Mean survivorship for chuck—wallas older than 1 yr approached 75% and values for males and females were not significantly different. Adults of both sexes appeared to be considerably more susceptible to predation after achieving 165 mm SVL. Consequently, large adult individuals were never common. Annual recruitment was 20% and was largely the result of reproduction. Population densities showed nearly a twofold annual variation, ranging from 15 to 30 individuals/ha. Life table analysis showed that younger females (3—6 yr) contributed to 50% of the replacement rate; mean generation time was 8.2 yr, and the life expectancy was ≈15 yr. Year—to—year consistency of various reproductive attributes (clutch size, egg mass, and RCM), indicative of "boom or bust" iteroparity, is possibly a typical adaption for extremely long—lived lizard species. Relatively mild winters and occurrence of summer rainfall in the Colorado desert greatly promoted early maturity and frequent reproductions. In addition, summer rainfall improved the survivorship of eggs, probably by providing adequate moisture for incubation. Such conditions were responsible for relatively high densities and predation rates. However, under drier conditions maturity was delayed, reproduction less frequent, and egg survivorship much lower; such conditions are typical in the Mojave desert. The relative stability of clutch size, egg mass, and RCM, but apparent variability of age at reproductive maturity and reproductive frequency in this study, may indicate that the life history of chuckwallas evolved under a variable but predictable environmental setting.

ISSN:0012-9615    

DOI:10.2307/2937081

出版商:Ecological Society of America    

年代:1987

数据来源: WILEY

摘要:

The structure of animal communities and the energy flux through them may be characterized by biomass ratios, ecological efficiencies, and production efficiencies of the component organisms. Here, we interpret these ratios in terms of the elementary processes of food intake, specific production rate, and gross growth efficiency that underlie them. Recent information confirms that the magnitude of all these processes is related to the average body mass of the organisms involved. However, our analysis shows that this well—known dependence reflects the influence of two different basic biological properties. One of these is the metabolism—body—size relation of individual that is familiar from physiology. The other less well—recognized property appears as an ecological population factor reflected in the distribution of particle sizes within animal groups in the community and is probably related to the relative sizes and distributions of predators and their prey. It appears that both the physiological and ecological size relationships have to be recognized as scaling factors in order to transform measures of biological production of various parts of communities into common terms for comparison. Current data on the generality and stability of community structure and production suggest that by using this twofold size scaling, trophic energy flow within the community can be determined from the distribution of body sizes without the necessity of specifying trophic levels of the organisms involved. The ecological size scaling can be seen as an index of the system nature of ecosystems.

ISSN:0012-9615    

DOI:10.2307/2937082

出版商:Ecological Society of America    

年代:1987

数据来源: WILEY

摘要:

Observations over a two and a half year period in the Northeast United States and adjacent Canada indicate that White—winged Crossbills (Loxia leucoptera) and Red Crossbills (L. curvirostra) shift their diets among the seeds of various conifer species in a seasonal pattern. Both crossbill species forage on white spruce (Picea glauca) in late summer. White—winged Crossbills continue foraging on white spruce or tamarack (Larix laricina) until late autumn or winter, when they switch to black spruce (P. mariana). Red Crossbills switch to white pine (Pinus strobus) in early autumn and to red pine (P. resinosa) or other pines in winter or spring. This pattern of diet shifts is consistent with the hypothesis that crossbills forage to maximize food intake rate. Profitability (milligrams of kernel ingested per second) was measured for crossbills foraging on each of these conifers by observing rates of seed ingestion in the field, and subsequently measuring dry seed kernel masses. Profitability for a given conifer increases as cones and seeds mature, then declines as seeds are shed from the cones. Because conifers differ in the timing of cone ripening, crossbills experience sequential peaks in profitability. Both crossbill species usually foraged predominately on the most profitable conifer species, with switches in conifer use coinciding with shifts in relative profitabilities. However, crossbills often forage on more than one conifer species at a time, even though intake rates might be maximized by foraging on only the most profitable conifer. Predation does not appear to influence diet selection and large—scale patterns of conifer use. Crossbill movements and patterns of abundance, both on local and continent—wide scales, are correlated with patterns of profitability. Dietary overlap between crossbill species is greatest, often approaching 100%, in late summer when seed is most abundant. Overlap then declines rapidly when white pine cones open, and usually remains negligible most of the remainder of the year. Patterns of dietary overlap are a result of differences in profitability for each crossbill species that are largely inherent in the ripening phenology of conifer cones and seed accessibility, not seed depletion by crossbills. Interspecific competition is most likely between White—winged Crossbills and either the smallest form of the Red Crossbill or redpolls (Carduelis sp.) during the late winter of "invasion" years, which occur every 3—4 yr. Intraspecific competition is also likely to be most intense at these times.

ISSN:0012-9615    

DOI:10.2307/2937083

出版商:Ecological Society of America    

年代:1987

数据来源: WILEY