摘要:

This paper reveals that key factor analysis, a popular method of life table analysis, cannot address the central issue of population dynamics: the pattern of population fluctuation and its causal mechanism. The basic idea of key factor analysis is to identify factors that contribute significantly to the variation in population. Several statistical methods to identify a key factor have subsequently been proposed in the literature. These are, in terms of the familiar relationship, K = k1+ k2+... + kn, to correlate K with a k, regress K on a k, or regress a k on K, the last method being currently most frequently used. These methods, in effect, look for those k's whose variances and covariances constitute a significant part of the variance of K. The fundamental problem here is that the variation in population is inappropriately interpreted as the variance. I show that, in general, different factors play different roles in generating the pattern and that none of these aspects can be indicated merely by the variance of K and its components, the variances and covariances of the k's. I contend that, for judging which factor is major, criteria are multiple and subtle beyond the simplistic idea of key factors.

ISSN:0012-9658    

DOI:10.2307/2265658

出版商:Ecological Society of America    

年代:1996

数据来源: WILEY

摘要:

The effect of landscape heterogeneity on the dispersal of organisms between habitat islands is poorly understood. Preferred pathways for dispersal (i.e., corridors), as well as dispersal barriers, are difficult to identify when the landscape matrix is composed of a complex mixture of land cover types. We developed an individual—based dispersal model to measure immigration and emigration rates between habitat islands within heterogeneous landscapes. Dispersing individuals of a model organism were simulated as self—avoiding random walkers (SAW) traversing a digital land cover map, with each habitat type assigned a priori a probability that the SAW would enter that habitat type. Each individual began the dispersal process on a random site at the edge of a deciduous forest patch and was allowed to move until it reached a different deciduous forest patch. Visualization of the movement patterns across the landscape was achieved by tabulating the frequency of visitation of successful dispersers to each grid cell on the map. The model was used to estimate the probabilities of disperser transfer between patches by varying the a priori probabilities of movement into each habitat type in order to: (1) estimate the effect of changing landscape heterogeneity on the transfer probabilities, and (2) visualize dispersal corridors and barriers as perceived by model organisms operating by specific movement rules and at specific scales. The results show that 89% of the variability in dispersal success can be accounted for by differences in the size and isolation of forest patches, with closer and larger patches having significantly greater exchange of dispersing organisms. However, changes in the heterogeneity of the landscape matrix could significantly enhance or decrease emigration success from an individual patch, depending on the landscape. Changes in emigration success from an individual patch resulting from changes in matrix heterogeneity were not predictable, and transfer rates between patches were not symmetrical due to differences in the proximity of neighboring patches, and differences in the funneling attributes of certain landscape patterns. Visualizations showed that corridors are often diffuse and difficult to identify from structural features of the landscape. A wide range of organisms with differing movement capabilities can be simulated using the approach presented to increase our understanding of how landscape structure affects organism dispersal.

ISSN:0012-9658    

DOI:10.2307/2265659

出版商:Ecological Society of America    

年代:1996

数据来源: WILEY

摘要:

Recent history plays an important role in the physiology, behavior, and ecology of individuals, and in the dynamics of populations and assemblages of species. In this paper, we examine the impact of history on the species composition of intertidal reef corals, by comparing simulation models that incorporate four different levels of knowledge about the recent past (over a time scale of 1—27 yr). The models are Markov or semi—Markov transition probability matrix models, based on rates of colonization, persistence, and species replacement measured from a long—term study spanning three decades at Heron Island, Great Barrier Reef. Rates of colonization (transitions from free space) varied 20—fold for different species groups, while mortality (transitions to free space) ranged fivefold, reflecting a wide range of life histories among the coral assemblage. Virtually all species groups could undergo reciprocal transitions (e.g., from A to B, and B to A) in a single time interval, indicating the lack of a single competitive dominant that was capable of outcompeting all or most other species. Transition probabilities changed markedly as a function of history. For most species groups, the probability of persisting (i.e., transitions from A to A) increased with time. Thus, a colony that had occupied space for some time was generally more likely to continue to do so than a new arrival. This result is consistent with an escape in size for older colonies from mortality agents such as competition and predation. However, three species groups showed the opposite pattern. Algae, Pocilloporid corals, and fragile tabular Acropora showed marked increases in transitions to free space after 3—5 yr, reflecting a more ephemeral suite of life history traits. Similarly, free space that had recently been generated had a higher rate of colonization than substratum that had been unoccupied for some time. These results falsify a major assumption of standard first—order models, i.e., that transition probabilities are constant, and that history is irrelevant. Although the changes in transition probabilities as a function of history were often striking, the four different models we employed show only minor variation in community composition in both transitory and climax (equilibrium) phases. Thus, while recent history was important in determining transition probabilities, it had little effect on community dynamics and structure in this system. This discrepancy is due to the rapid turnover of corals and algae on shallow reef crests, where only a small proportion of colonies survive long enough to display effects of history. All models agreed that the length of time required for this system to reach an equilibrium community structure is far longer than the observed interval between recurrent disturbances from tropical cyclones.

ISSN:0012-9658    

DOI:10.2307/2265660

出版商:Ecological Society of America    

年代:1996

数据来源: WILEY

摘要:

A native asexual gecko, Lepidodactylus lugubris, declines numerically when the sexual gecko Hemidactylus frenatus invades urban/suburban habitats throughout the Pacific. Previous studies showed that the competitive displacement occurs rapidly and is facilitated by clumped insect resources. Five lines of evidence suggest that the mechanism of displacement is primarily due to differences in the ability of each species to exploit insect resources. (1) These species show nearly complete diet overlap. (2) Insects are a limiting resource for both geckos as evidenced by positive demographic effects with increased insect abundance. (3) Hemidactylus frenatus depletes insect resources to lower levels than L. lugubris, which results in reduced rates of resource acquisition in L. lugubris. (4) This reduced resource acquisition translates into significant reductions in the body condition, fecundity, and survivorship of L. lugubris individuals. (5) Evidence for interference (and other) mechanisms does not account for these negative demographic effects on L. lugubris. Interspecific competition is stronger than intraspecific competition for L. lugubris, with increasing L. lugubris density having negligible effect on H. frenatus, mirroring the asymmetry of the large—scale displacement. The superior harvesting ability of H. frenatus is most pronounced when insects are clumped spatially and temporally, and is attributable to a variety of species—specific traits such as their larger body size, faster running speed, and reduced intraspecific interference while foraging. We conclude that clumped resources can increase interspecific exploitation competition, and this mechanism may contribute to species turnover when human environmental alterations redistribute resources.

ISSN:0012-9658    

DOI:10.2307/2265661

出版商:Ecological Society of America    

年代:1996

数据来源: WILEY

摘要:

The role of biotic interactions including vertebrate predation and interspecific competition was studied among members of a semiarid small mammal assemblage in a long—term field manipulation in north central Chile. Fences and netting selectively excluded mammalian and avian predators, and a putative rodent competitor, the degu (Octodon degus), from large, replicated grids. Small mammal populations were censused monthly over>4 yr, and effects of predator and degu exclusions on numbers and survival of small mammals were assessed with repeated measures ANOVAs. Marginally significant predator exclusion effects were detected on O. degus, and highly significant year X predation interactions in both wet and dry seasons; degu survival (= persistence) was also significantly longer on exclusion grids. Predation had significant effects on numbers of Phyllotis darwini, and a significant month X predator interaction existed in the wet season; survival was also significantly higher on predator exclusion grids. There was a marginally significant year X competitor interaction in the dry season for this species. Akodon olivaceus had significant month X predator and month X competitor interactions in the wet season only; survival was significantly longer on degu exclusion grids. An irruptive species, Oligoryzomys longicaudatus, showed no significant predator or competitor effects in 1991—1992; however, survival was significantly longer on predator + competitor exclusion (vs. control) grids. An uncommon species, Abrothrix longipilis, had marginally nonsignificant time X competitor interactions but no treatment differences in survival. Finally, a second uncommon rodent, Abrocoma bennetti (not analyzed numerically) had significantly longer survival on predator exclusion grids with degus. Degus and P. darwini were significantly overrepresented (relative to availability) in diets of culpeo foxes (Pseudalopex culpaeus) and three nocturnal owls, respectively, whereas A. bennetti was significantly overrepresented in predator diets generally. Vertebrate predation has pervasive effects on some species that, however, may be related to their intrinsic demographic characteristics. Competition may be mediated by subtle behavioral interactions among assemblage members. Long—term studies are necessary to elucidate the role of biotic and abiotic factors in semiarid environments

ISSN:0012-9658    

DOI:10.2307/2265662

出版商:Ecological Society of America    

年代:1996

数据来源: WILEY

摘要:

Although seasonal migrations of large predatory mammals that follow migrating prey are well documented, no equivalent phenomenon has been described previously in terrestrial reptiles. We surveyed and radio—tracked water pythons (Liasis fuscus) in Fogg Dam and its adjacent floodplain in the wet—dry tropics of northern Australia to document patterns of movement, with particular emphasis on the ways in which the snakes exploit their major prey species, the dusky rat (Rattus colletti). The distribution and abundance of these rodents vary seasonally. During the dry season the rats live in soil crevices in the floodplain, but wet—season flooding forces them to higher ground, primarily to natural levee banks. Python and rat abundances on the floodplain adjacent to Fogg Dam were significantly correlated through time: both reached a maximum during the dry season, and fell dramatically during the wet season. Activity of pythons was centered around Fogg Dam during the dry season, but all of the radio—tracked snakes moved away from this area during the wet season. Most pythons migrated to the vicinity of levee banks on the floodplain up to 12 km away from their dry—season range. By migrating seasonally, water pythons can efficiently utilize a migratory prey species that would otherwise be unavailable for much of the year.

ISSN:0012-9658    

DOI:10.2307/2265663

出版商:Ecological Society of America    

年代:1996

数据来源: WILEY

摘要:

This study examines the non—lethal effects of an odonate predator (Anax junius) on the competitive interactions among several size classes of anuran larvae. In an outdoor experiment using cattle watering tanks, we estimated the effects of both large and small bullfrogs (Rana catesbeiana) on themselves, on each other, and on small green frogs (R. clamitans) in the absence and non—lethal (caged) presence of Anax. The presence of Anax depressed both growth and survivorship of small bullfrogs and green frogs. In contrast, the presence of Anax had positive effects on growth rates and size at metamorphosis of the large bullfrogs. Increasing density of competitors also decreased survivorship of small classes, and growth rates of all classes. The per—unit—biomass competitive effects of the small bullfrogs on target classes were much greater than those of large bullfrogs. The presence of Anax significantly altered the per—unit—biomass competitive effects of small bullfrogs but not large bullfrogs, presumably because individuals in the small class reduced their activity rates in the presence of Anax. Overall production of new tadpole biomass was quite similar across experimental units, with decreases in production of small size classes in the presence of Anax compensated for by increases in production of the large size class. Thus the non—lethal presence of Anax had substantial effects on the nature of competitive interactions in this system, and we discuss the implications of such behavioral indirect effects in the study of ecological communities. Our results also illustrate the futility of attempting to partition the effects of competitors and predators, as both competitors and the non—lethal presence of predators significantly affected growth rates and death by starvation of small larvae. Finally, our results illustrate how individual behavioral responses may be translated to community and ecosystem properties.

ISSN:0012-9658    

DOI:10.2307/2265664

出版商:Ecological Society of America    

年代:1996

数据来源: WILEY

摘要:

Most empirical studies of specialists and generalists have focused on comparisons among different species. Interspecific comparisons have recently been criticized because phenotypic trade—offs can evolve after ecological specialization. As a result, it has been suggested that intraspecific variation is the appropriate level to demonstrate functional trade—offs. We demonstrate functional trade—offs for two components of fitness in pumpkinseed sunfish (Lepomis gibbosus) from a single trophically polymorphic population: condition factor and growth rate. Condition factor was estimated using somatic lipid content, while growth rate was determined from scale measurements. Fish with extreme benthic and limnetic morphologies are on average superior to fish with intermediate morphologies, especially in the pelagic habitat. Our study is a particularly stringent test of functional trade—offs at the intraspecific level because the morphological variation is very subtle in this population.

ISSN:0012-9658    

DOI:10.2307/2265665

出版商:Ecological Society of America    

年代:1996

数据来源: WILEY

ISSN:0012-9658    

DOI:10.2307/2265666

出版商:Ecological Society of America    

年代:1996

数据来源: WILEY

摘要:

The ontogeny of many size—structured organisms is marked by a series of niche shifts that divides a population into ecologically distinct stages. Consequently, overall population dynamics are affected both by processes operating within individual stages and by factors that influence transitions between stages. In largemouth bass (Micropterus salmoides), there is a diet shift from littoral invertebrates to fish that typically occurs sometime in their 1st yr. I examined factors that influenced this shift by tracking diet and growth trajectories of young—of—year (YOY) bass cohorts in several Michigan lakes over 2 yr. Cohorts differed in consumption rates of invertebrate prey, which led to an early divergence in bass growth among lakes. This variation in early growth had dramatic consequences for the timing of the shift to piscivory. In lakes with rapid growth in the invertebrate feeding stage, bass were able to gain a size advantage over YOY bluegill (Lepomis macrochirus), their primary fish prey, which facilitated a shift to piscivory as soon as these bluegill became available as food. In contrast, slow growth during the invertebrate feeding stage reduced the size advantage of bass over bluegill and delayed or even prevented the shift to piscivory in the 1st yr. Niche shifts in bass were also sensitive to environmental conditions early in the growing season. In 1992, low temperatures caused a reduction in growth rates that prevented the establishment of a size advantage over bluegill in all but one lake. Bass grew much faster after switching to piscivory; thus the shift from invertebrates to fish strongly influenced sizes attained by YOY bass by autumn. Because survivorship of YOY bass is strongly size dependent (particularly through the first winter), the timing of the shift to piscivory may be important in determining survival and recruitment rates. Therefore, processes that operate on bass in the invertebrate feeding stage can influence the shift to piscivory, and this can potentially have important effects on the dynamics of the population as a whole.

ISSN:0012-9658    

DOI:10.2307/2265667

出版商:Ecological Society of America    

年代:1996

数据来源: WILEY