摘要:

This paper investigates whether body size—related constraints on home—range resource harvesting could lead to coexistence between interspecific competitors of different body size under conditions of complete niche overlap. With this objective, I analyzed the influence of body size on induced and sustainable resource limitation and its implications on the interaction between an individual and a fixed biomass of larger competitors in a four—dimensional space, consisting of two spatial dimensions describing competitor home range, resource availability, and time. It was shown that body size—related spatiotemporal constraints on home—range resource harvesting, trophic optimization, and relativity of resource availability determine absolute and relative amounts of unused home—range resources, restricting the influence of resource limitation induced by an individual to a definite size distance around its size. It was therefore concluded that: (1) when competition occurs asymmetrically with a superiority of large animals, size differences alone could allow coexistence, independently of any kind of resource partitioning; and (2) superiority of large animals should result from the resource density control that a larger competitor imposes on the smaller one whenever size differences for stable coexistence occur. Existing evidence of an inefficiency in home—range resource exploitation seems large enough to suggest a view of guilds based on a hierarchy of inclusive home ranges.

ISSN:0012-9658    

DOI:10.2307/1940913

出版商:Ecological Society of America    

年代:1995

数据来源: WILEY

摘要:

The ecological basis for breeding—system evolution was explored in Ecballium elaterium (L.) A. Rich. (Cucurbitaceae). Two subspecies are parapatric on the Iberian Peninsula: dioicum (dioecious) is found in southern, more arid regions and elaterium (monoecious) is found in northern, wetter regions. Six experimental gardens were set up in three sites that span the climatic gradient of the species' natural range in Spain in order to address two issue: (1) gender specialization in reproductive characters and life history, and (2) ecological specialization of the two subspecies across environments. Overall, the dioecious subspecies outperformed the monoecious subspecies. Monoecious plants, on average, were shorter—lived in all environments. Females of the dioecious subspecies produced similar numbers of female flowers, but more fruits and seeds, than did monoecious plants (cosexuals). The fruits of females contained more, but smaller, seeds than the fruits of cosexuals. Males always produced more male flowers than did cosexuals, disproportionately so with increased plant size (canopy volume); no such disproportionate increase in seed production was found in females. A geographic analysis of gender (proportion of female flowers) in the monoecious plants revealed a striking pattern: dimorphism with increased gender differentiation in drier sites, in contrast to monomorphism with respect to gender in wetter sites. Differences between the subspecies and between habitat types can be related directly to breeding—system differences. The higher overall performance of dioecious plants may be due to enhanced heterozygosity resulting from outcrossing enforced by dioecy. On the other hand, the gender differentiation of the monoecious plants in the drier sites may reflect gender allocation "decisions" enforced by limited resource availability.

ISSN:0012-9658    

DOI:10.2307/1940914

出版商:Ecological Society of America    

年代:1995

数据来源: WILEY

摘要:

Tristylous plant populations should exhibit equal frequencies of the three style morphs at equilibrium. In contrast, New England and central Ontario populations of Decodon verticillatus (Lythraceae) show a marked deficiency of the mid—styled (M) morph. This pattern was also evident in an independent sample of 30 populations from eastern Ontario: all nine cases of morph loss involved the M morph, and this morph also occurred at low frequencies, especially in large trimorphic populations. The hypothesis that a fitness disadvantage to the M morph accounts for its low frequency was not supported by data from a series of field and glasshouse experiments. Measures of reproductive success from two populations showed no disadvantage to the M morph, but revealed substantially higher seed set in the mid— (M) and short—styled (S) morphs than in the long—styled (L) morph. A pollen addition experiment showed that low seed set in the L morph can only partly be explained by reduced pollen receipt. Comparison of morph frequencies between adult plants and their open—pollinated progeny in three populations failed to reveal any morph—specific fitness differences. Analysis of growth and flowering of open—pollinated progeny from three populations in a 2—yr glasshouse experiment revealed differences among morphs for some parameters in some populations, but no consistent patterns were evident. The deficiency of the M morph may have resulted from an historical accident during post—glacial colonization of parts of the northern range. Computer calculations examining the return of morph frequencies to equilibrium after major perturbations indicated that populations of long—lived clonal species like D. verticillatus may preserve skewed morph ratios for>10 000 yr. Moreover, the distribution of fossil seeds indicates that this species has occurred in its post—glacial range in southern Ontario for only °5000 yr. It is important to distinguish between ecological and evolutionary time scales when testing selective interpretations of microevolutionary patterns involving clonal taxa in glaciated regions.

ISSN:0012-9658    

DOI:10.2307/1940915

出版商:Ecological Society of America    

年代:1995

数据来源: WILEY

摘要:

Variations in growth, flowering, and phenology of two forbs, Acomastylis rossii and Bistorta bistortoides, were compared among six years (1983—1988) and five plant communities (fellfield, dry meadow, moist meadow, were meadow, snowbed) at an alpine site in the Front Range of Colorado. The purpose was to determine the extent to which the phenolgical patterns of these species varied among plant communities and how interannual climate variability affects phenology and growth. There were significant differences in growth among communities for both species. In B. bistortoides, there were also significant differences among years, due primarily to the influence of a single year (1983) when leaf length increased by °10% and the average number of B. bistortoides leaves nearly doubled. Key phenological events of both species (initiation of growth, date of maximum leaf length, leaf number, and flower number) were related to snowmelt patterns, resulting in differences among communities. There were also significant differences among years, again primarily related to the single year 1983, the year of a major El Nino Southern Oscillation (ENSO) event that produced high snowfall amounts at the site. The increased leaf length in a high snow year is counterintuitive to the commonly accepted notion that alpine species may be limited by season length. We hypothesize that changes in phenology related to changes in snowfall of snowmelt will cause detectable changes in growth, but that these will not be predicted simply from phenology alone. Rather, the timing of snowmelt in relation to nutrient availability, soil moisture, and air temperature will be critical in determining how individual species respond.

ISSN:0012-9658    

DOI:10.2307/1940916

出版商:Ecological Society of America    

年代:1995

数据来源: WILEY

摘要:

We conducted three experiments in which we applied additional hand pollination, flower removal, and leaf removal treatments in various combinations to Primula veris, a perennial spring—flowering rosette species. The purpose of the study was to determine whether the seed set of Primula veris was limited by pollen availability or by other resources, and whether there were measurable cost of reproduction. Hand pollination in the beginning of experiments significantly increased current seed set in only one of the three experiments. It also increased the next—year fruiting probability in that first experiment. In the second experiment, hand pollination did not significantly affect current seed set, but we nevertheless observed enhanced leaf growth in the treatment year and the two following years, and increased flowering frequency, fruiting frequency, and survival in the two following years. In the third experiment, after hand pollination we observed a higher net photosynthetic capacity of the leaves and, again, increased leaf growth in the treatment year and a higher flowering probability in the following year. The positive effect of hand pollination was even clearer when the leaves of the plants were removed at the beginning of the experiment. However, the treatment in which all the flowers were removed had effects on subsequent performance similar to those of the hand—pollination treatment, suggesting trade—offs. Thus, we did not observed any costs associated with reproduction after supplemental hand pollination; on the contrary, hand pollination resulted in increased survival, size, and reproduction of the plants in the subsequent years.

ISSN:0012-9658    

DOI:10.2307/1940917

出版商:Ecological Society of America    

年代:1995

数据来源: WILEY

摘要:

In order to clarify regeneration traits and their fitness, we compared seed fall, seedling emergence, and first—year seedling of four co—occurring species. Carpinus tschonoskii, C. japonica, C. laxiflora, and C. cordata, in an old—growth temperate deciduous forest in Ogawa Forest Reserve, central Japan. Seedling emergence ratio of C. cordata (5.5%), whose seeds mostly germinated 1.5 yr after seed fall, was lower than the other species (8.5—11.2%). First—year seedlings of Carpinus tschonoskii and C. japonica had much lower survival rates (2.7%, both species) than C, laxiflora and C. cordata (7.5—8.5%) under closed canopy, suggesting that the former two are more disturbance dependent than the later two. They are characterized by their seed and seedling performances as seed rain type fundamentally; however, C. cordata and C. laxiflora are able to establish a seedling bank, and C. cordata has a seed bank too. These regenerative strategies partly explain the abundance and population structure of the four species in the old—growth forest. Multiple regression analyses showed that topography was important for survival of all species but C. cordata. This result was consistent with the adult distribution of these species in this forest. Wind dispersal of Carpinus is effective, both to escape from density— or distance—dependent high mortality and to increase the chance of arriving at canopy gaps, where seedling survival rate is high. However, escaping high mortality seems relatively more important for C. laxiflora and C. cordata (seedling bank strategy), and arriving at a canopy gap is relatively more important for C. tschonoskii and C. japonica (seed rain strategy).

ISSN:0012-9658    

DOI:10.2307/1940918

出版商:Ecological Society of America    

年代:1995

数据来源: WILEY

摘要:

The analysis of records of species occurrences on islands in an attempt to detect interactions between species has been an area of controversy in recent years in terms of the validity of some of the statistical methods used. In this note I make two contributions to the continuing debate. First, I advocate a generalized Monte Carlo testing procedure because this is easy to implement, is computationally efficient, and has guaranteed properties when the null hypothesis of no species interactions is correct. Second, I propose a test statistic that can be decomposed into a component for each individual species, and I demonstrate how this makes it possible to isolate species with unusual patterns of co—occurrence with other species, even after an allowance for multiple testing is made.

ISSN:0012-9658    

DOI:10.2307/1940919

出版商:Ecological Society of America    

年代:1995

数据来源: WILEY

摘要:

The effect of plant community structure on nutrient cycling is fundamental to our understanding of ecosystem function. We examined the importance of plant species and plant cover (i.e., plant covered microsites vs. bare soil) on nutrient cycling in shortgrass steppe of northeastern Colorado. We tested the effects of both plant species and cover on soils in an area of undisturbed shortgrass steppe and an area that had undergone nitrogen and water additions from 1971 to 1974, resulting in significant shifts in plant species composition. Soils under plants had consistently higher C and N mineralization rates and, in some cases, higher total and microbial C and N levels than soils without plant cover. Four native grasses, one sedge, and one shrub differed from one another in the quantity and quality of above— and belowground biomass but differences among the six species in soil nutrient cycling under their canopies were slight. However, soils under bunchgrasses tended to have higher C mineralization and microbial biomass C than soil under the rhizomatous grass, Agropyron smithii. Also, the one introduced annual in the study, Kochia scoparia, had soils with less plant—induced heterogeneity and higher rates of C and N mineralization as well as higher levels of microbial biomass C than soils associated with the other species. This species was abundant only on plots that had received water and nitrogen for a 4—yr period that ended 20 yr ago, where it has persisted in the absence of resource additions for 20 yr, suggesting a positive feedback between plant persistence and soil nutrient status. Plant cover patterns had larger effects on ecosystem scale estimates of soil properties than the attributes of a particular plant species. This result may be due to the semiarid nature of this grassland in which plant cover is discontinuous and decomposition and nutrient availability are primarily limited by water, not by plant species—mediated characteristics such as litter quality. That local plant—induced patterns in soil properties significantly affected ecosystem scale estimates of these properties indicates that consideration of structural attributes, particularly plant cover patterns, is critical to estimates of ecosystem function in shortgrass steppe.

ISSN:0012-9658    

DOI:10.2307/1940920

出版商:Ecological Society of America    

年代:1995

数据来源: WILEY

摘要:

In forest gaps, the geometry of canopy openings above a plant determines the spatial distribution of diffuse and direct radiation, which may be received from different sectors of the sky. We examined crown orientation and light interception by seedlings of four species of pioneer trees, Cecropia obtusifolia, Heliocarpus appendiculatus, Piper auritum, and Trema micrantha, transplanted into natural forest gaps at the Los Tuxtlas Tropical Biology Station, Veracruz, Mexico. The four species differed in the number and mean size of leaves, but total leaf area was not significantly different among equal age plants. The mean azimuth and zenith angle of diffuse and direct radiation were determined from hemispherical photographs taken above each plant. Mean diffuse radiation vectors were distributed around the zenith, with a significant clustering in an easterly direction, while mean direct radiation vectors were distributed along the solar track in the southern portion of the sky. The mean orientation of the crown of each seedling was determined from detailed, three—dimensional reconstruction of leaf display using a canopy architecture model. A directional correlation test indicated that seedling crowns were orientation of individual leaves. The mechanisms of this differential response to diffuse vs. direct radiation are not known. For diffuse radiation, the total light capture capacity integrated over all sky directions increased with seedling leaf area, while efficiency of capture per unit area decreased. Based on simulations, the light interception efficiency of each plant in its own microsite was 5—25% greater than would be achieved in the sites of conspecifics of in symmetric canopy openings centered on the zenith. These analyses provide a quantitative estimate of the value of nonrandom leaf and crown orientation for whole—plant light interception in natural environments.

ISSN:0012-9658    

DOI:10.2307/1940921

出版商:Ecological Society of America    

年代:1995

数据来源: WILEY

摘要:

Through plasticity in traits controlling clonal morphology, clonal plants possess the potential to selectively place ramets within a heterogeneous environment or "forage." Although many studies document plant responses that are consistent with foraging, few studies test directly whether plants can preferentially locate "good" patches or avoid "bad " patches when grown in a heterogeneous environment. We conducted such a test for Hydrocotyle bonariensis, a clonal dune species that inhabits soils known to be extremely patchy on both time and space. We subjected Hydrocotyle to a temporally constant but spatially variable soil resource environment created by clipped patches of grass. Clonal morphology on the patch treatment was compared to that in two homogeneous treatments: no grass and full grass. In order to predict the effect of our treatments on the long—term expansion of clones, we calibrated diffusion models of clonal growth with data on the morphological response of Hydrocotyle to our treatments. Clones of Hydrocotyle bonariensis were able to respond to the presence of patches by selectively placing ramets outside of grass patches, thus providing direct evidence of an effective foraging response. For each of the three traits identified as potentially contributing to an overall foraging response (branching, internode distance, and direction of rhizome growth), the response in and out of grass patches was substantially different from the degree of plasticity manifested by the two homogeneous treatments. For example, no difference in main rhizome internode lengths was found between the two homogeneous treatments. In contrast, ramets in the heterogeneous (patchy grass) treatment responded to their local environment by increasing internode lengths when in the unfavorable (grass patch) portion of the environment. Empirically calculated diffusion models indicate that habitat complexity has considerable impact on the long—term expansion of clones: only in the heterogeneous treatment did clones expand less rapidly in favorable habitat than they did in unfavorable habitat. In the heterogeneous treatment, Hydrocotyle rhizomes exhibited a previously undocumented behavior: they appeared to veer away from patches of grass. Finally, we discuss how the foraging response of Hydrocotyle ramets may be enhanced by its previously documented capacity for resource integration.

ISSN:0012-9658    

DOI:10.2307/1940922

出版商:Ecological Society of America    

年代:1995

数据来源: WILEY