摘要:

While ecologists involved in management or policy often are advised to learn to deal with uncertainty, there are a number of components of global environmental change of which we are certain–certain that they are going on, and certain that they are human—caused. Some of these are largely ecological changes, and all have important ecological consequences. Three of the well—documented global changes are: increasing concentrations of carbon dioxide in the atmosphere; alterations in the biogeochemistry of the global nitrogen cycle; and ongoing land use/land cover change. Human activity–now primarily fossil fuel combustion– has increased carbon dioxide concentrations from °280 to 355 mL/L since 1800; the increase is unique, at least in the past 160 000 yr, and several lines of evidence demonstrate unequivocally that it is human—caused. This increase is likely to have climatic consequences–and certainly it has direct effects on biota in all Earth's terrestrial ecosystems. The global nitrogen cycle has been altered by human activity to such an extent that more nitrogen is fixed annually by humanity (primarily for nitrogen fertilizer, also by legume crops and as a by product of fossil fuel combustion) than by all natural pathways combined. This added nitrogen alters the chemistry of the atmosphere and of aquatic ecosystems, contributes to eutrophiction of the biosphere, and has substantial regional effects on biological diversity in the most affected areas. Finally, human land use/land cover change has transformed one—their to one—half of Earth's ice—free surface. This in and of itself probably represents the most important component of global change now and will for some decades to come; it has profound effects on biological diversity on land and on ecosystems downwind and downstream of affected areas. Overall, any clear dichotomy between pristine ecosystems and human—altered areas that may have existed in the past has vanished, and ecological research should account for this reality. These three and other equally certain components of global environmental change are the primary causes of anticipated changes in climate, and of ongoing losses of biological diversity. They are caused in turn by the extraordinary growth in size and resource use of the human population. On a broad scale, there is little uncertainty about any of these components of change or their causes. However, much of the public believes the causes–even the existence–of global change to be uncertain and contentious topics. By speaking out effectively, we can help to shift the focus of public discussion towards what can and should be done about global environmental change.

ISSN:0012-9658    

DOI:10.2307/1941591

出版商:Ecological Society of America    

年代:1994

数据来源: WILEY

摘要:

We determined if drought—induced nitrogen retranslocation occurs in perrenial grasses of tallgrass prairie, as suggested from studies of annual changes in plant N content. To test this, we analyzed six C4grasses representing a wide range of drought tolerance for shoot, rhizome, and root N before and after controlled drought. Shoot N concentration decreased in all species during drought (31—41%), including in recently expanded leaves (23—38%). No consistent pattern with respect to drought tolerance was apparent in these decreases or in observed changes in distribution of whole—plant N, although there was some suggestion of a mesic—to—xeric gradient in the magnitude of retranslocation. For example, the proportion of total plant N allocated to shoots decreased during drought 20—29% in the most mesic species over three experiments, 2—12% in the three intermediate species, and 4—6% in the two most xeric species, for pre— vs. post—drought comparisons. However, when drought—stressed plants were compared to well—watered age controls, the respective values were 20—21%, 12—20%, and 0.19%, the apparent result of size—related changes in N allocation in control plants in one experiment. In most cases, shoot N was moved primarily into rhizomes, though in one species with intermediate drought tolerance, evidence suggested that much of the retranslocated shoot N was apparently lost through fine—root turnover. Retranslocation of shoot N to rhizomes and roots, confirmed by monitoring movement of35S—methionine, was in response to drought stress rather than phenology and involved the entire shoot (e.g., blades, culms, recently expanded leaves). Post—drought photosynthesis and leaf N concentration remained well below predrought levels 6 d following rewatering. Thus decreases in leaf N status during drought as a consequence of retranslocation likely result in lower photosynthetic capacity and decreased whole—plant carbon gain following relief of water stress after rain. Drought—induced retranslocation may serve to protect plant N from loss of herbivory, fire, and volatilization during periods when soil N uptake and carbon assimilation are limited by water availability.

ISSN:0012-9658    

DOI:10.2307/1941592

出版商:Ecological Society of America    

年代:1994

数据来源: WILEY

摘要:

Stable carbon isotope composition was determined on leaves of woody plants sampled along an 800—km transect on the western flank of the Rocky Mountains at altitudes ranging from 610 to 2650 m above mean sea level. Discrimination decreased by 1.20 ± 0.11% (mean ± 1 se) per km of altitude (n = 15, F1,13= 127.8, P<0.0001). The change in discrimination was just sufficient to maintain a constant CO2partial pressure gradient from ambient air to the intercellular spaces within the leaf for both deciduous (P = 0.60) and evergreen (P = 0.90) species. However, the CO2gradient so maintained was significantly steeper among evergreen (11.31 ± 0.14 Pa) than among deciduous (9.64 ± 0.14 Pa) species (t = 8.4, 27 df, P<00001). As a consequence, the evergreens had lower discrimination than the deciduous species at any given altitude. After the data were corrected for altitude, further analysis revealed significant differences in discrimination and in CO2partial pressure gradient among species. Thuja plicata (western red—cedar), a scale—leaved evergreen, had lowest mean discrimination (16.67 ± 0.50%, n = 4) and the steepest CO2gradient from ambient to intercellular spaces (14.5 ± 0.5 Pa). Larix occidentalis (western larch), a deciduous conifer, had the highest discrimination (20.95 ± 0.34%, n = 9) and the flattest CO2gradient (8.3 ± 0.4 Pa). A simple model of water—use efficiency predicted that evergreen species would average 18 ± 2% higher in water—use efficiency at any given altitude and that mean water—use efficiency would triple across a 2000—m altitude gradient. The difference between evergreen and deciduous species is attributable to variation in the CO2partial pressure gradient, but the tripling with altitude was almost exclusively a consequence of reduced evaporative demand.

ISSN:0012-9658    

DOI:10.2307/1941593

出版商:Ecological Society of America    

年代:1994

数据来源: WILEY

摘要:

Proleptic flowering is common among plant species from tropical seasonal forests. In these species flower initiation and anthesis are separated by a prolonged rest period (the unfavorable season), and the two processes are controlled separately. Proleptic species are appropriate for assessing the within— and between—season) effects of herbivory on flowering and reproductive success. We investigated the reproductive consequences of artificial defoliation of the proleptic shrub Erythroxylum havanense, in a seasonal dry forest in western Mexico. Twenty individuals in each of three sites were randomly assigned to one of four treatments: control (no manipulation), and three levels of leaf area removed: 0, 25, and 100%. Plants in the latter three treatments were sprayed with insecticide to prevent additional foliage consumption by natural herbivores. Within a season undamaged plants (control and 0% defoliation) produced from two to four times more seeds than damaged ones (12 and 100% defoliation). In addition, completely defoliated plants exhibited a significant increase in both fruit maturation time (8 d) and average seed mass (9.5%), when compared to the plants in the other treatments. There was no evidence of a trade—off between seed number and seed mass, or between ripening time and seed mass. One year after defoliation, vegetative growth (basal area increment) was reduced only by 100% defoliation. Likewise, flower and fruit production were significantly reduced only in this treatment. Although defoliation reduced seed production in both years, only the first year's reduction was a direct consequence of defoliation. Seed production decline in year 2 was an indirect effect of the reduction in flower number, suggesting that fruit and seed maturation do not depend on resources stored, but on annual production of photosynthates. Plant survival was not affected up to 5 yr after defoliation. Since most of the effects were evident only in the 100% defoliated plants, our results reflect those instances when insect outbreaks occur and E. havanese plants are completely defoliated.

ISSN:0012-9658    

DOI:10.2307/1941594

出版商:Ecological Society of America    

年代:1994

数据来源: WILEY

摘要:

Seasonal and spatial patterns of seed dissemination of a canopy tree Cornus controversa were studied over two fruiting seasons in a temperate deciduous forest in Japan. Seedfall was monitored every 2 wk in seed traps (N = 221) distributed over a 1—ha study plot that include 12 mature Cornus. Estimated annual seed production was 1.57 x 106seeds/ha. In both years, the peak of fruit ripening had already ended before migratory frugivorous birds entered the area and only 17% of the seeds were disseminated by birds. Directly fallen seeds were deposited close to the parent trees (usually within 10 m, infrequently as far as 35 m from parent trees). In contrast, bird—disseminated seeds showed a markedly scattered distribution; the birds deposited more than half of the seeds they ate>5 m from the crown edge, and density of bird—disseminated seeds exceeded that of directly fallen seeds at distances>15 m, with some found>40 m from the nearest mature Cornus. In addition, birds seemed to selectively eat fruits that contained viable seeds. The density of bird—disseminated seed decreased significantly with increasing distance from fruiting trees, both conspecific and heterospecific. Distance to the nearest edge of canopy gaps that were several years old had little effect on the spatial pattern of bird—disseminated seeds. This means that seeds of C. controversa were not directionally dispersed into canopy gaps by birds.

ISSN:0012-9658    

DOI:10.2307/1941595

出版商:Ecological Society of America    

年代:1994

数据来源: WILEY

摘要:

Seed fate pathways were investigated for antelope bitterbrush (Purshia tridentata), a common and economically important shrub, from all 1988 to fall 1992 at the Whittell Forest and Wildlife Area in Little Valley, Washoe County, on the east slope of the Sierra Nevada in western Nevada. Rodents harvested bitterbrush seeds from late July to September, taking them either directly off the fruiting twigs or picking up fallen fruits from the ground surface and carrying from 63 to 80% of the seeds away intact from shrubs. Radioactively labelled bitterbrush seeds placed under four source shrubs were removed and scatter hoarded primarily by yellow pine chipmunks (Tamias ameonus). Caches contained a mean of 4—11 seeds and were located within 25 m from the source shrubs. Cache depth and microhabitat selection were generally favorable for bitterbrush seedling establishment. Chipmunks and other rodents found many of the seed caches and either ate the seeds (53%) or reached them elsewhere. Chipmunks probably also carried some intact seeds to larders in hibernacula to use as a winter food source. In spring,>99% of emerging seedlings occurred in clumps containing from 2 to 104 individuals. An estimated 5.3 and 8.5% of those seeds cached by rodents and 1.1 and 2.4% of the seeds produced by shrubs in 1990 and 1991 produced emergent seedlings the following spring. Seedling mortality rates during their 1st yr ranged from 78 to 92%. Grazing by rodents was the most important source of mortality during early spring, and most seedling deaths were caused by desiccation during summer. Despite considerable attrition of individuals within clumps, most mature "shrubs" consisted of two or more individuals. The seed fate pathway that resulted in the highest probability of seedling establishment at this study site was harvest, dispersal, and scatter hoarding by chipmunks, and subsequent neglect of those caches. Retrieval of cached seeds by chipmunks followed by redistribution of those seeds to neglected secondary caches also resulted in many seedlings. Modes of seed dispersal based solely on abiotic agents appear to be insufficient to account for significant recruitment of bitterbrush seedlings.

ISSN:0012-9658    

DOI:10.2307/1941596

出版商:Ecological Society of America    

年代:1994

数据来源: WILEY

摘要:

Seagrasses rely on both vegetative (rhizome elongation) and sexual (seeds) propagation for maintenance of existing beds and colonization of new areas. Yet mechanisms of seed dispersal and survival of seeds in new areas remain poorly described. We conducted seed dispersal experiments in the field and laboratory to better describe seed dispersal characteristics in one species, Zostera marina L. (eelgrass), the dominant seagrass species in the temperate zone of the United States, Japan, and Europe. Seeds were broadcast by hand into unvegetated 5 m diameter plots at three locations over 3 yr (1989—1991) in the York River, Virginia (Chesapeake Bay). These sites had been previously vegetated but were devoid of any vegetation prior to (since 1972) and during the course of the experiments. Resultant seedling distributions closely matched broadcast patterns, with 80% of all seedlings found within the 5 m diameter plots, despite the fact that geophysical processes would appear sufficient to transport seeds greater distances. Wind records for the 2—mo period between seed broadcasting and germination revealed time—averaged wind speeds in excess of 40 km/h on>12 d in each of the 3 yr and galeforce winds (17 km/h) in 2 of 3 yr. A three—dimensional hydrographic computer simulation model of the York River provided instantaneous current velocity estimates from which maximum bottom shear velocities (u*) in the study area were approximated (flood tide: 1.26 cm/s, ebb tide: 1.20 cm/s). These estimates exceeded the critical erosion threshold (U*crit= 0.7 cm/s) for Z. marina seeds determined from laboratory flume experiments. We postulate that small—scale topographic features on the bottom (burrows, pits, mounds, ripples) shield the seeds from the flow. Our results suggest that seeds settle rapidly, dispersing only up to a few metres under the influence of currents and become rapidly incorporated into the sediment. The limited dispersal capabilities of seeds underscore the need to address restoration goals and questions of seagrass ecology in the context of landscape—scale distributional patterns and metapopulation analyses.

ISSN:0012-9658    

DOI:10.2307/1941597

出版商:Ecological Society of America    

年代:1994

数据来源: WILEY

摘要:

Segregation of species occurs when a species tends to be found near conspecifics. This is frequently investigated using a contingency table, classifying each point by its species and the species of its nearest neighbor. Pielou proposed using a 1—df chisquare test of independence as a test of segregation. This test is inappropriate if all locations within a study area are mapped. For completely mapped data, I derive the expectations and variances of the cell counts in the nearest—neighbor contingency table under the null hypothesis that species labels are randomly assigned to points. The properties of the cell counts suggest a new 2—df chi—square test of spatial segregation, a pair of species—specific tests, and a pair of species—specific measures of segregation. In small samples, the proposed tests have the appropriate size, unlike the Pielou test. The new test is illustrated with three examples: Pielou's Douglas—fir/ponderosa pine data, a realization of a mother—daughter process, and the locations of male and female water tupelo trees.

ISSN:0012-9658    

DOI:10.2307/1941598

出版商:Ecological Society of America    

年代:1994

数据来源: WILEY

摘要:

The influence of biotic and abiotic factors on seed fates are exceptionally difficult to analyze in natural populations because it is not usually possible to identify an emerging seedling with its history as a seed. Large seed size and the unusual feature that the seed body remains attached to the new seedling in the ant—dispersed herb Calathea ovandensis provided an opportunity for field experiments that examine the influence of (1) seed dispersers, (2) treefall gaps, and (3) post—dispersal predators on seed survival, seed dormancy, seedling emergence, and seedling distributions. In a seed depot experiment, seed—dispersing ants significantly affected the seedling emergence patterns in three ways. Seeds with rewards for ants: (a) had improved seedling emergence, by 1.6—fold (17.4 vs. 10.6%); (b) gave rise to seedlings with a mean dispersal distance that was threefold farther (34 vs. 114 cm); and (c) generated a non—leptokurtic seedling shadow (kurtosis: —0.60, not significant vs. 10.84, P<0.001) in contrast to seeds without ant rewards. There was significant spatial variation in the effect of ant rewards on seedling emergence and also temporal variation in seedling emergence, the greatest number of seedlings emerging in an el Nino year. In two germination box experiments: (1) treefall gaps enhanced seedling emergence 180—fold (72.4 vs 0.4%) compared to the deep shade, and ungerminated seeds in the deep shade were mostly dormant, not dead; (2) post—dispersal seed predation was very low (overall mean of 6.2%), with considerable microspatial variability, and pathogens and small invertebrates accounted for only one—fourth as much seed mortality as larger (vertebrate and large insect) post—dispersal predators. Seedling emergence in this ant—dispersed herb with a relatively long—lived seed bank depends upon a complex of factors, suggesting an interaction of the effects of biotic dispersal to local safe microsites with larger scale effects of spatial and temporal variation in the abiotic environment.

ISSN:0012-9658    

DOI:10.2307/1941599

出版商:Ecological Society of America    

年代:1994

数据来源: WILEY

摘要:

The difficulty of distinguishing allelopathy from resource competition among plants has hindered investigations of the role of phytotoxic allelochemicals in plant communities. The effects of allelopathic substances on competitive outcome when two species differ in their sensitivity to an inhibitor were modelled by applying atrazine, a commonly used herbicide for broadleaf weeds, to corn—soybean mixtures. A target—neighbor design was used, in which differing densities of a neighbor species are planted around one individual of the target species. This design is particularly appropriate to investigations of allelopathy, due to the density—dependent nature of phytotoxic effects. Neighbor density greatly influenced the response to the toxin. At a corn densities of 0, 3, 6, 9, and 12 plants per pot and atrazine treatment of 3.0 mg/kg, the dry mass of the soybean (target) plant increased from 0.2 g with no neighbors to 0.5 g with 9—12 neighboring corn plants. The increased growth of soybean at higher corn densities is contrary to the predicted effects of resource competition and is due to uptake of atrazine by the corn plants, which decreased the amount available to the soybean target. Detoxification of soil by neighbors may explain in part the conflicting assessments of some putatively allelopathic species, such as black walnut (Juglans nigra).

ISSN:0012-9658    

DOI:10.2307/1941600

出版商:Ecological Society of America    

年代:1994

数据来源: WILEY