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Results for “Oikos”

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70 records · Page 4Linked to original sources
USGS1991

Density cycles in an island population of deer mice, Peromyscus maniculatus

We report on eight years of data for a population of deer mice ( Peromyscus maniculatus ) on Santa Barbara Island, California which reaches exceptionally high densities and fluctuates markedly in an apparent three- to four-year cycle. The cyclic increase follows winters with high rainfall, and the decline may be similarly associated with low rainfall winters. The peak and early decline is marked by nearly complete cessation of breeding, along with heavy predation by barn owls ( Tyto alba ), whose numbers track those of the deer mice. This island population of Peromyscus differs from the well-established pattern for the species on the mainland, where populations occur in low to moderate numbers and are relatively stable from year to year. The pattern seen on the island is instead similar to that of cyclic microtines. We compare Microtus and the Santa Barbara Island deer mice and discuss parallels in the possible causes of the respective cycles.

California↗
USGS1993

The marine snail, Cerithidea californica, matures at smaller sizes where parasitism is high

I investigated life-history and parasitism in the salt marsh snail, Cerithidea californica. Latitude and growing conditions were important factors determining maturation size. After accounting for environmental variation, there was a negative association between the maturation size of snails and the prevalence of parasitic castration by larval trematodes. As predicted by life-history theory, this may represent an adaptation against parasitism that is similar to previous observations of life-history adaptations in species subject to predation or disturbance. However, it was unclear whether this adaptation was due to phenotypic plasticity or genetic differences among populations resulting from natural selection so I conducted a reciprocal transplant between sites with high and low prevalence and found source population differences in maturation size. It appears, therefore, that the life-history differences between these populations are at least partially genetic or may represent an adaptive developmental switch that was initiated prior to the transplant.

Oikos↗
USGS1993

The examination of a competition matrix for transitivity and intransitive loops

Recent examinations of competition matrices for transitivity (species A > species B > species C) have used techniques that can be subject to certain biases. First, recent theoretical and empirical analyses have shown that traditional measures of competitive performance are biased in favor of the larger species. It is argued that this size bias has the potential to bias analyses of transitivity. Second, analytical techniques used to test matrices for transitivity can be shown to be insensitive to the presence of intransitive loops. Techniques are presented for exploring both these types of erros and these techniques are illustrated using the results from a six-species study of marsh plants. In addition, two published studies are partially reanalyzed using a technique designed to detect intransitivities. Results for both the new data set as well as for the published data sets fail to reveal intransitivities. For the marsh plant study, the size bias associated with traditional measures of competitive success did not bias in favor of transitivity. We conclude (1) that the studies examined do not possess intransitive loops and (2) care must be taken in order to avoid biased analyses if intransitive loops are to be detected.

Oikos↗
USGS1990

Multiple scales of patchiness and patch structure: a hierarchical framework for the study of heterogeneity

We develop a hierarchical model of heterogeneity that provides a framework for classifying patch structure across a range of scales. Patches at lower levels in the hierarchy are more simplistic and correspond to the traditional view of patches. At levels approaching the upper bounds of the hierarchy the internal structure becomes more heterogeneous and boundaries more ambiguous. At each level in the hierarchy, patch structure will be influenced by both contrast among patches as well as the degree of aggregation of patches at lower levels in the hierarchy. We apply this model to foraging theory, but it has wider applications as in the study of habitat selection, population dynamics, and habitat fragmentation. It may also be useful in expanding the realm of landscape ecology beyond the current focus on anthropocentric scales.

Oikos↗
USGS1994

The relationship between species richness and community biomass: The importance of environmental variables

Several studies have used plant community biomass to predict species richness with varying success. In this study we examined the relationship between species richness and biomass for 36 marsh communities from two different watersheds. In addition, we measured several environmental variables and estimated the potential richness (the total number of species known to be able to occur in a community type) for each community. Above ground living and dead biomass combined was found to be weakly correlated with species richness (R 2 =0.02). Instead, a multiple regression model based on elevation (R 2 =0.47), salinity (R 2 =0.30), soil organic matter (R 2 =0.18), and biomass was able to explain 82% of the variance in species richness. It was found that environmental conditions could explain 89% of the variation in potential richness. Biomass had no relation to potential richness. When used as a predictor variable, potential richness was found to explain 72% of the variation in realized (observed) richness and biomass explained an addition 9% of the variance in realized richness. This finding suggests that realized richness in our system was controlled primarily by environmental regulation of potential richness and secondarily by biomass (as an indicator of competition). Further examination of the data revealed that when sites exposed to extreme environmental conditons were eliminated from the analysis, biomass became the primary predictor of realized richness and potential richness was of secondary importance. We conclude that community biomass has a limited capacity to predict species richness across a broad range of habitat conditions. Of particular importance is the inability of biomass to indicate the effect of environmental factors and evolutionary history on the potential species richness at a site.

Louisiana↗
USGS1996

Seed dispersal by specialist versus generalist foragers: The plant's perspective

I examined the seed dispersal ecology of the stem parasitic plant, desert mistletoe (Phoradendron californicum, Viscaceae), with the objectives of (1) determining the relative effectiveness of specialist and generalist foragers for seed dispersal, (2) determining the extent to which desert mistletoe fruiting characteristics correspond to those predicted for plants attracting specialist versus generalist foragers, and (3) examining the potential consequences of the observed dispersal strategy for mistletoe reproduction. Three species of birds, phainopepla, Gila woodpecker, and northern mockingbird, fed on desert mistletoe at my study site. The specialist, phainopepla, was the most abundant and the most likely to perch in host species, where defecated seeds had a greater probability of lodging in a site suitable for establishment. Gila woodpeckers, although abundant, spent little time in host plants, thus dooming most of the seeds they consumed. Mockingbirds may disperse a small number of seeds, but were abundant enough to consume only a small portion of the available fruits. As expected for plants attracting specialist frugivores, mistletoes produced fruits throughout the 6-month season in which phainopeplas reside in the Sonoran desert. Contrary to expectation, numbers of fruits produced far exceeded the amount that could be consumed by the frugivores at my study site. Fruit crop size was positively related to absolute fruit removal, but not to proportional removal at the scale of the entire study site. However, crop size was positively related to proportional removal within the neighborhood of mistletoes occupying an individual host tree. Frugivores were attracted to infected hosts, host attractiveness increased, although proportional removal of fruit declined, with number of female mistletoes. The observed dispersal ecology of desert mistletoe suggests the likelihood of increasingly clumped distributions of mistletoe plants, as more and more seeds are deposited on previously infected hosts, and increased density of mistletoes attract ever more visits by birds. I observed no decline in vigor, in terms of fruit production, within the levels of infestation at my study site. The seed dispersal strategy of desert mistletoe thus includes aspects of that expected both for plants dependent on specialists and those dependent on generalists. Fruits are available through the entire season to maintain the specialist. Production far exceeds that expected, but serves to attract the specialist within a neighborhood of vigorously fruiting conspecifics.

Oikos↗
USGS1994

On the importance of sampling variance to investigations of temporal variation in animal population size

Our purpose here is to emphasize the need to properly deal with sampling variance when studying population variability and to present a means of doing so. We present an estimator for temporal variance of population size for the general case in which there are both sampling variances and covariances associated with estimates of population size. We illustrate the estimation approach with a series of population size estimates for black-capped chickadees (Parus atricapillus) wintering in a Connecticut study area and with a series of population size estimates for breeding populations of ducks in southwestern Manitoba.

Oikos↗
USGS1997

Developmental instability analysis of BKD-infected spring Chinook salmon (Onchorhynchus tshawytscha) prior to seawater exposure

Stress in organisms results in energy dissipation, making developmental pathways less stable. Effects of chronic stress, manifested as small random departures from phenotypic symmetry, reflect developmental instability, are considered to be epigenetic and an effect produced by compromised fitness. Instability is detectable and effectively interpreted among sites or populations if samples are collected randomly, the stressor is present throughout character development, characters are identified accurately and excessive mortality does not erase the existence of developmental instability. Bacterial kidney disease (BKD) is a chronic systemic disease in salmonids that, after vertical transmission from parent to egg, persists and spreads throughout ontogeny, potentially affecting developmental processes. Because levels of progeny infection reflect parental infection levels, groups of offspring from parents with high and low levels of BKD infection can be compared to assess the effects of disease-mediated developmental stress. Analyses of fluctuating asymmetry in five bilateral characters were inconclusive, but significant reductions in the proportion of unusable scales, in the number of circulus errors, and in the directional asymmetry of branchiostegal rays were observed in fish from the high-BKD group. This group also contained individuals of significantly larger size. These results are opposite to those expected from traditional developmental instability theory in suggesting that surviving high-BKD fish have greater developmental stability. This reversal appears to be produced by selective mortality having a greater effect than sublethal stress in altering developmental instability patterns. These results are discussed with respect to size selectivity, heterosis and the assumptions supporting developmental instability as a tool for detecting chronic sublethal stress.

Oikos↗
USGS1999

The relationship between species density and community biomass in grazed and ungrazed coastal meadows

Previous studies have indicated that the relationship between community biomass and species density can be represented by a multivariate model in which abiotic variables influence species density both through effects on biomass and through effects on the species pool. In this paper, we use data from grazed and ungrazed coastal meadows in Finland to evaluate and extend this general conceptual model of the factors controlling species density. Structural equation analysis was used to evaluate a model for all meadows and then to perform a multigroup analysis to determine how grazed and ungrazed meadows differ. By itself, biomass could explain only 12% of the variation in species density while the multivariate model was able to explain 47% using five types of predictor variables: site, soil, flooding, grazing, and biomass. Analyses found that flooding explained the greatest amount of variability in species density, primarily through negative effects on the species pool. Grazing was also found to have a strong effect on species density and results suggest that its negative influence may be largely through reductions in the species pool in grazed meadows. The most important difference found between grazed and ungrazed meadows was that species density had a strong negative relationship to biomass in the ungrazed meadows but no significant relationship in the grazed ones. Thus, it appears that the influence of competition on species density was much greater in ungrazed meadows compared to grazed ones.

Oikos↗
USGS1998

Thermally induced chronic developmental stress in coho salmon: Integrating measures of mortality, early growth and fluctuating asymmetry

Developmental stability, or homeostasis, facilitates the production of consistent phenotypes by buffering against stress. Fluctuating asymmetry is produced by developmental instability and is manifested as small random departures from bilateral symmetry. Increased fluctuating asymmetry is thought to parallel compromised fitness, in part, because stress promotes energy dissipation. Compensatory energy expenditures within the organism are required to complete development, thus promoting instability through reductions in homeostasis. Increased heterozygosity may enhance developmental stability by reducing energy dissipation from stress through increased metabolic efficiency, possibly by providing greater flexibility in metabolic pathways. Traditionally, fluctuating asymmetry has been used as a bioindicator of chronic stress, provided that selective mortality of less fit individuals did not reduce stress-mediated increases in fluctuating asymmetry to background levels produced by natural developmental error, or create data inconsistencies such as higher asymmetry in groups exposed to lower stress. Unfortunately, absence of selective mortality and its effects, while often assumed, can be difficult to substantiate. We integrated measures of early growth, mortality, fluctuating asymmetry (mandibular pores, pectoral finrays, pelvic finrays, and gillrakers on the upper and lower arms of the first branchial arch) and directional asymmetry (branchiostegal rays) to assess chronic thermal stress (fluctuating temperatures as opposed to ambient temperatures) in developing eggs from two different coho salmon (Oncorhynchus kisutch) stocks and their reciprocal hybrids. Hybridization provided insight on the capacity of heterozygosity to reduce stress during development. Although egg losses were consistently higher in crosses exposed to fluctuating temperatures, egg mortality was predominantly a function of maternal stock of origin. Post-hatch losses were higher in crosses exposed to ambient temperatures than in crosses exposed to fluctuating temperatures during embryogenesis. Observed patterns of early growth revealed no heterosis, but instead reflected maternal effects, with some crosses slowing growth and yolk utilization when exposed to fluctuating temperatures. Analyses of fluctuating asymmetry also showed no effects from heterosis. While analyses of composite asymmetry scores and branchiostegal rays were inconclusive, analyses of individual characters showed significantly higher fluctuating asymmetry in pelvic finray counts and a marginal change in the numbers of fish asymmetric for this character in crosses exposed to chronic thermal stress. In contrast, the fluctuating asymmetry in lower gillraker counts was significantly higher in crosses exposed to ambient temperatures and there were significantly more fish asymmetric for this character. Data on mortalities and fluctuating asymmetry indicate pelvic finray development was thermally stressed, while the heightened fluctuating asymmetry in lower gillraker counts under ambient temperatures was due to a greater frequency of less fit fish that had not been culled by thermal stress. Changes in early growth patterns in response to developmental stress yielded no parallel responses in meristic characters. We conclude that chronic thermal stress produced both selectively lethal and sublethal effects that directly shaped fluctuating asymmetry and fitness profiles in these crosses. Implicit in this conclusion is that developmental instability analyses can detect more than just chronic sublethal stress, thus providing substantial credence for using instability studies as proactive bioassessment methodologies.

Oikos↗
USGS1998

Estimating species richness: The Michaelis-Menten model revisited

The Michaelis-Menten model has been widely used to estimate the richness (S) of species pools, but is largely untested. We tested whether (1) species accumulation curves follow the form predicted by the model, (2) the model gives unbiased estimates (Ŝ and B̂, respectively) of S and of the sample size, B, needed to detect S/2 species, and (3) performance is robust to community structure. Performance varied with community structure. For model communities with species-abundance distributions based on MacArthur's broken-stick model with 100 or 1000 species, deviations from predicted accumulation curves were slight, and Ŝ and B̂ were unbiased (P ≥ 0.18). For broken-stick communities with 10 species, Ŝ and B̂ overestimated S and B by an average of 17% and 63%, respectively (P < 0.001). For model communities with species-abundance distributions based on Tokeshi's (1990) random-fraction model with 10, 100, or 1000 species, deviations from predicted accumulation curves were large; on average, Ŝ underestimated S by 7-37% (P < 0.001), and (for S = 100 or 1000) B̂ underestimated B by 67-80% (P < 0.001). Vascular plant inventories (S = 42 to 99 species) also showed large deviations from predicted curves; on average, Ŝ underestimated S by 35% (P < 0.001) and B̂ underestimated B by 72% (P < 0.001). Because most natural communities are better described by the random-fraction than the broken-stick model, we suggest the Michaelis-Menten model will typically yield poor estimates of S. Moreover, we argue that accepted criteria for evaluating estimators of S are inadequate.

Oikos↗
USGS1989

Model-based estimates of annual survival rate are preferable to observed maximum lifespan statistics for use in comparative life-history studies

Estimates of longevity are available for many animals, and are commonly used in comparative life-history analyses. We suggest that annual survival rate is a more appropriate life history parameter for most comparative life history analyses. Observed maximum lifespans estimate complicated functions of survival and sampling probabilities. Annual survival rate estimates derived from modern band-recovery statistical procedures are becoming available for a variety of organisms. We compiled annual survival rate estimates and observed maximum longevities derived from band recovery data for North American waterfowl. Observed maximum longevities were not correlated with the annual survival rate estimates and appear to be unstable over time. We recommend that observed maximum lifespans not be used in life history analyses.

Oikos↗
USGS1990

Temporal and spatial variation in survival rates of the tropical lizard Anolis limifrons

We evaluated survival of the lizard, Anolis limifrons at two sites, AVA and Lutz, from 1976-1979 and during two periods at Lutz site, 1971-1976 vs 1976-1979, at Barro Colorado Island, Panama. Survival of adult females and males did not differ nor did survival of juveniles and adults. In contrast, survival was significantly higher at Lutz site during 1971-1976 than during 1976-1979 and survival was significantly higher at AVA than at Lutz site during 1976-1979. On an annual basis, mean survival rates were 0.042,0.013,0.055 for Lutz 1971-1976, Lutz 1976-1979, and AVA 1976-1979, respectively. These rates are in accord with reports of annual population turnover for this and other small mainland Anolis. Temporal and spatial variation in survival was not associated with habitat, season, year of observation, or numbers and abundances of avian predators. Survival patterns of mainland Anolis are contrasted with those of West Indian species in terms of life history evolution.

Barro Colorado Island↗
USGS2012

Inferring local competition intensity from patch size distributions: a test using biological soil crusts

Dryland vegetation is inherently patchy. This patchiness goes on to impact ecology, hydrology, and biogeochemistry. Recently, researchers have proposed that dryland vegetation patch sizes follow a power law which is due to local plant facilitation. It is unknown what patch size distribution prevails when competition predominates over facilitation, or if such a pattern could be used to detect competition. We investigated this question in an alternative vegetation type, mosses and lichens of biological soil crusts, which exhibit a smaller scale patch-interpatch configuration. This micro-vegetation is characterized by competition for space. We proposed that multiplicative effects of genetics, environment and competition should result in a log-normal patch size distribution. When testing the prevalence of log-normal versus power law patch size distributions, we found that the log-normal was the better distribution in 53% of cases and a reasonable fit in 83%. In contrast, the power law was better in 39% of cases, and in 8% of instances both distributions fit equally well. We further hypothesized that the log-normal distribution parameters would be predictably influenced by competition strength. There was qualitative agreement between one of the distribution's parameters (&mu;) and a novel intransitive (lacking a 'best' competitor) competition index, suggesting that as intransitivity increases, patch sizes decrease. The correlation of &mu; with other competition indicators based on spatial segregation of species (the C-score) depended on aridity. In less arid sites, &mu; was negatively correlated with the C-score (suggesting smaller patches under stronger competition), while positive correlations (suggesting larger patches under stronger competition) were observed at more arid sites. We propose that this is due to an increasing prevalence of competition transitivity as aridity increases. These findings broaden the emerging theory surrounding dryland patch size distributions and, with refinement, may help us infer cryptic ecological processes from easily observed spatial patterns in the field.

Oikos↗