摘要:

Abstract:Successional patterns in seed rain, seed predation, seed bank, and the density and survival of seedlings were studied for fleshy-fruited plants in a patchy vegetation of dry grassland with interspersed scrub in central Europe. These hay meadows were abandoned 20-30 years ago. Three successional scrub stages (phases) and the adjacent mown and abandoned meadows were used for the investigation. Seeds of 29 fleshy-fruited species were collected in seed traps over two years. The spatial pattern of seed dispersal strongly depended on the successional stage: seed rain density was highest beneath the mature scrub phase and lowest in the grassland. This was also true for seed predation by rodents, and frequency of fleshy-fruited plants in the seed bank. The density of seedlings showed a different pattern; it was highest in the intermediate phase of scrub development. In this phase the seed rain was less abundant than under the mature phase, but the destructive influence of seed predation and the mortality of seedlings (particularly due to shading) were not as strong as beneath the mature phase. Therefore, it can be assumed that this intermediate phase is a regeneration window for fleshy-fruited scrub species.

ISSN:1195-6860    

DOI:10.1080/11956860.1995.11682286

出版商:Taylor&Francis    

年代:1995

数据来源: Taylor

摘要:

Abstract:In this paper we propose that an evaluation of the ecological and evolutionary consequences of seed dispersal requires the recognition of patches where seeds have different probabilities to reach the seedling stage as well as the spatial and temporal dynamics of such patches. To analyze the relative importance of seed sources on the regeneration of trees growing in patchy environments, we use metapopulation models that combine matrix demographic models with Markovian models that simulate patch dynamics. The models are applied to the cases of two tree species with contrasting life-histories at Los Tuxtlas tropical rain forest, Mexico. For pioneer tree species, we show that the relative contribution of propagules from the seed rain and seed bank depends on the temporal regime of natural disturbance and the mortality rates of soil seeds. For shade-tolerant trees, results suggest that dispersal may increase seedling recruitment when seed movement occurs from patches where mother plants have higher fecundity and/or seeds have higher pre-dispersal survival to patches where soil-seeds and/or seedlings have higher survival. The metapopulation models also show that inferences about critical stages in the evolution of species life-histories contrast between a static and dynamic patchy scenario.

ISSN:1195-6860    

DOI:10.1080/11956860.1995.11682287

出版商:Taylor&Francis    

年代:1995

数据来源: Taylor

摘要:

Abstract:Typically seed rain density decreases as a function of distance from the parent plant. Most models of recruitment assume a seed (and a seedling) mortality function monotonically decreasing with distance from the parent (hence, positively density-dependent). This results in either higher recruitment at a distance from the parent (Janzen-Connell model) or near it (Hubbell model), or else a spatially uniform recruitment (exact compensation model), depending upon the specific shape of the mortality function in relation to distance from the parent. However, mortality may increase with distance from the parent (negatively density-dependent) or be spatially uniform, both of those scenarios resulting in a recruitment similar to that predicted by the Hubbell model. Recruitment may also be unpredictable on the sole basis of the parent plant position if mortality is spatially variable and density-independent. For several tree species in a homogeneous cold temperate forest (Houle, 1992a), recruitment could not be predicted from seed rain spatial patterns. Seeds and seedlings were contagiously distributed and mortality, at both stages, was spatially variable and density-independent. In a similar forest type (present study), in which two species were segregated along a topographical gradient, recruitment of each species was higher where respective seed rain abundance was higher. At this study site, also mortality, for both seeds and first-year seedlings was not density-dependent, with the result that clumping at the seedling level and positive relationship between seed rain and recruitment were maintained through time. A chain of events from seed dispersal through seedling establishment and subsequent survival controls recruitment. Simple models tend to mask the complexity of the processes involved between dispersal and recruitment.

ISSN:1195-6860    

DOI:10.1080/11956860.1995.11682289

出版商:Taylor&Francis    

年代:1995

数据来源: Taylor

摘要:

Abstract:It is essential to investigate both the spatio-temporal pattern of seed dispersal and the subsequent fate of offspring to evaluate the effect of dispersal on plant fitness. This paper examines the effectiveness of seed dispersal in terms of three selective advantages (i.e. avoidance of high mortality around the parent plant, finding‘safe sites’, and directional dispersal) by synthesizing the results of studies on seed dispersal, post-dispersal mortality of seeds, and seedling survivorship of several tree species in an old-growth temperate forest community, the Ogawa Forest Reserve (OFR), Japan. Some species in the OFR apparently avoid high offspring mortality around parents in much the same way as other species have been found to do in other forests. Finding‘safe sites’and directional dispersal could also be important elements of seed dispersal; however, the importance of‘safe sites’in this context and their spatio-temporal distribution have not been sufficiently characterized to discuss their relative significance for recruitment. A trade-off among seed size, dormancy and dispersal has been suggested, but should be tested by long-term investigations on spatio-temporal patterns of both seed dispersal and‘safe site’distributions.

ISSN:1195-6860    

DOI:10.1080/11956860.1995.11682290

出版商:Taylor&Francis    

年代:1995

数据来源: Taylor

摘要:

Abstract:Changes in plant population size are usually attributed to exogenous factors such as drought or herbivore outbreaks. However, in theory, populations may oscillate from endogenous density-dependent causes alone. Under some conditions these oscillations are very irregular and are called chaos. There are very few examples of endogenous oscillations in plants and these are controversial. Large fluctuations in population size occur after fires in many species of Proteaceae in South African and Australian shrublands. These non-sprouting species form single-aged cohorts with recruitment dating from the last fire. Variation in cohort size has usually been attributed to exogenous factors, especially the length of the fire interval, fire season, and post-burn weather conditions. We studied the importance of endogenous density-dependence in generating population oscillations in these proteoid shrubs. Using a variety of approaches, we show that density-dependent reduction in fecundity may lead to endogenous oscillations in some species. We explore the“seeds of chaos”by defining attributes of species and populations most likely to generate endogenous oscillations. Finally, we note the implications of the phenomenon for research in, and management of, these systems.

ISSN:1195-6860    

DOI:10.1080/11956860.1995.11682291

出版商:Taylor&Francis    

年代:1995

数据来源: Taylor

摘要:

Abstract:Fates of antelope bitterbrush (Purshia tridentata) seeds were monitored from the time the seeds were cached by yellow pine chipmunks (Tamias amoenus) until they germinated nine months later. One thousand seeds were numbered and then radioactively labelled with scandium-46 so that the histories of individual seeds could be followed. The labelled seeds were placed under a bitterbrush shrub in the morning and by that evening chipmunks had gathered most of the seeds and made 110 primary caches within 16 m of the source shrub. During the fall, chipmunks and other rodents visited many of these caches and removed some of the seeds. Thirty-one caches completely disappeared, 28 other caches had some but not all of the seeds removed, and 51 caches remained intact until the time of seed germination. Chipmunks recached about 30% of the seeds they took from primary caches during the late summer and fall at 12 secondary and two tertiary cache sites. The histories of seeds were often complicated. Seeds survived to the time of germination at 86 of the 124 cache sites (79 primary caches, six secondary caches, and one tertiary cache). The population of caches from a particular source plant is dynamic. Seed-hoarding animals frequently move seeds from one storage site to another, and this secondary dispersal has important consequences for the seeds being moved.

ISSN:1195-6860    

DOI:10.1080/11956860.1995.11682292

出版商:Taylor&Francis    

年代:1995

数据来源: Taylor

摘要:

Abstract:Seed dispersal determines the conditions that seeds, seedlings, and saplings confront, and thus potentially provides a unifying theme for plant population ecology. This potential is seldom realized, however, because dispersal is infrequently linked effectively with its consequences. We advocate the importance of explicitly designing studies quantifying spatial patterns of dispersal at a variety of scales, causes of those spatial patterns, and consequences of alternative patterns of dispersal for all stages from seed to new adult. It is critical to link this chain of events with thorough investigations of individual dispersal systems rather than continuing to collect small pieces of the story from a multitude of systems. We first present a brief overview of spatial patterns, causes, and consequences of dispersal. We then develop the argument that a correspondence, or lack of correspondence, between patterns of seed arrival and adult recruitment tells little about causal relationships between the two. Finally, we suggest that a combination of observational and experimental approaches taking into account the complexities of recruitment processes will better link dispersal with its consequences. Experimental manipulations of patterns of seed arrival and causal modelling are potentially powerful approaches, but may be limited in application to easily manageable species.

ISSN:1195-6860    

DOI:10.1080/11956860.1995.11682293

出版商:Taylor&Francis    

年代:1995

数据来源: Taylor

摘要:

Abstract:Echolocating insectivorous bats under natural conditions, do not make the fine-detailed discriminations based on target shape and texture that laboratory data suggest they are capable of. The purpose of this study was to test the discrimination ability of a visually-orienting predator, the common nighthawk (Chordeiles minor), to allow for a comparison with ecologically similar bats. We hypothesized that vision is better for discrimination than echolocation and predicted that nighthawks would be better than bats at discriminating between targets. We tested the ability of nighthawks by presenting edible and non-edible targets to free-flying birds in a cafeteria-style experiment. Like bats, birds responded to all targets presented, apparently not discriminating, at least initially, between targets based on size or shape. However, unlike bats, nighthawks rejected the targets in the final phase leading to attack and rarely made contact. We conclude that visually orienting nighthawks are capable of more fine-grained target discrimination than echolocating bats. However, because the birds almost never attacked targets, we cannot say what cues such discrimination is based on.

ISSN:1195-6860    

DOI:10.1080/11956860.1995.11682294

出版商:Taylor&Francis    

年代:1995

数据来源: Taylor

摘要:

Abstract:We studied the effects of food quantity (prey biomass), nutritional quality (prey-biomass energy equivalent), and food availability (prey behaviour) on the daily energy ingestion and instar duration ofRanatra montezuma(Heteroptera: Nepidae).Ranatrawas reared in the laboratory on two prey types (amphipods and damselfly nymphs) that differed in biomass energy, maximum body size and behaviour. Damselfly nymphs had greater biomass energy equivalent(= 21.4 Joules/mg dry weight) and larger maximum size (14 mm) than amphipods(= 14.4 J/mg dry weight; maximum size 6 mm), while amphipods were considerably more active. Total instar duration forR. montezumareared on amphipods was significantly less than total instar duration for individuals reared on damselfly nymphs, even though the total quantity of energy ingested by instars reared on amphipods was one-half (481 J) that of damselfly nymphs (972 J). We propose thatRanatraconserved energy in the sit-and-wait foraging strategy employed for swimming amphipod prey, but consumed considerable energy searching for the more sedentary damselfly nymphs. Our results demonstrate that prey behaviour is an important factor influencing ingestion rates and energy flow in aquatic foodwebs.

ISSN:1195-6860    

DOI:10.1080/11956860.1995.11682295

出版商:Taylor&Francis    

年代:1995

数据来源: Taylor

摘要:

Abstract:Sestonic particles composed of phytoplankton, bacteria, and detritus are a potential food source for most zooplankton. In the present study, the effect of zooplankton on the stoichiometry of seston was examined in a small eutrophic pond from April to August, because the relative elemental contents of seston relates with phytoplankton growth rate and food quality. In the pond, most of the sestonic particles were edible size (<30µm), and sestonic C:P and N:P ratios were high at low zooplankton biomass but low at high zooplankton biomass, suggesting that the zooplankton activity is one of the important factors inducing changes in seston stoichiometry. This possibility was substantiated by grazer gradient experiments that showed a significant increase in the relative P content of the seston with increasing zooplankton biomass. However, slopes of the regression line of the elemental ratios against the zooplankton biomass varied between seasons, and were greater at lower nutrient concentration and higher seston abundance. The relationship implies that the magnitude of the mass-specific effect of zooplankton depends on the relative nutrient concentration available for phytoplankton. In parallel with the grazer gradient experiments, zooplankton grazing was simulated by artificially eliminating some fraction of seston in order to separate the direct (eliminating particles) and indirect (nutrient release) effects on the seston stoichiometry. The response of sestonic elemental ratios to simulated grazing revealed that the zooplankton can affect seston stoichiometry by increasing theper capitaavailability of a deficient nutrient for phytoplankton through grazing as well as nutrient release. These results suggest that the effect of zooplankton on seston particles is feedback to their growth rate by changing not only food abundance but also food quality.

ISSN:1195-6860    

DOI:10.1080/11956860.1995.11682296

出版商:Taylor&Francis    

年代:1995

数据来源: Taylor