Search

International audience

We use the method of surrogates to test the structure of variability in nine paleoclimate reconstructions and to compare temperature trends with that of the modern temperature record in the Northern hemisphere. Three different algorithms are used to generate surrogate time series: the iterated amplitude adjusted Fourier transform (IAAFT), the statically transformed autoregressive process (STAP) and a modification of STAP, which generates surrogates of arbitrary length (STAPL). We assessed through formal statistical tests that the surrogates preserve the LTP structure of the reconstructed time series of global temperature, using different measures (Hurst exponent, DFA exponent and R/S analysis). Then using the same surrogates we tested for the presence of a linear trend at least as great as the trend of the modern time series against the null hypothesis that the observed trend is only due to LTP. The null hypothesis could be rejected at the lowest possible significance level for all but two of the reconstructions. The result from the non-parametric test adds further statistical evidence to that of earlier parametric studies that the observed global warming trend in the modern time series cannot be adequately explained by natural agents of variability. Climatic Change

The worldwide loss of natural habitats leads not only to the loss of habitat-endemic species but also to further and protracted extinctions in the reduced areas that remain. How rapid is this process? We use the neutral theory of biodiversity to answer this question, and we compare the results taken with observed rates of avifaunal extinctions. In the neutral model, we derive an exact solution for the rate of species loss in a closed community. The simple, closed-form solution exhibits hyperbolic decay of species richness with time, which implies a potentially rapid initial decline followed by much slower rates long term. Our empirical estimates of extinction times are based on published studies for avifaunal extinctions either on oceanic islands or in forest fragments, which span a total of six orders of magnitude in area. These estimates show that the time to extinction strongly depends on the area. The neutral-theory predictions agree well with observed rates over three orders of magnitude of area (between 100 and 100,000 ha) both for islands and forest fragments. Regarding the species abundance distribution, extinction times based on a broken-stick model led to better agreement with observation than if a log-series model was used. The predictions breakdown for very small or very large areas. Thus, neutrality may be an affordable assumption for some applications in ecology and conservation, particularly for areas of intermediate size. Proc Natl Acad Sci U S A

The long-term variability of marine turtle populations remains poorly understood, limiting science and management. Here we use basin-scale climate indices and regional surface temperatures to estimate loggerhead sea turtle (Caretta caretta) nesting at a variety of spatial and temporal scales. Borrowing from fisheries research, our models investigate how oceanographic processes influence juvenile recruitment and regulate population dynamics. This novel approach finds local populations in the North Pacific and Northwest Atlantic are regionally synchronized and strongly correlated to ocean conditions-such that climate models alone explain up to 88% of the observed changes over the past several decades. In addition to its performance, climate-based modeling also provides mechanistic forecasts of historical and future population changes. Hindcasts in both regions indicate climatic conditions may have been a factor in recent declines, but future forecasts are mixed. Available climatic data suggests the Pacific population will be significantly reduced by 2040, but indicates the Atlantic population may increase substantially. These results do not exonerate anthropogenic impacts, but highlight the significance of bottom-up oceanographic processes to marine organisms. Future studies should consider environmental baselines in assessments of marine turtle population variability and persistence. PLoS One

Translocations are increasingly important tools for endangered species conservation, but their success is often uncertain. We analyzed 125 time series of grazing mammal translocations in South African protected areas. Some 94% of translocations succeeded (66% unambiguously) even though most populations began with

Statistical processes with long-term persistence (LTP) offer a promising approach to modeling the natural dynamics of Earth's temperature. One Such process, the family of 1/f-noises, is used here to assess the plausibility of a natural origin for recent global warming. Following earlier studies, a model of natural variability with UP is parameterized via paleoclimate reconstructions. The method developed here resolves a number of limitations in existing studies, primarily the problem of inaccuracies in estimating the spectral exponent of LTP. The output of the model is compared with the observed rate of temperature rise (0.61 degrees C/century between 1850 and 2007 for Northern hemispheric land air temperatures). We find that rates comparable with the observed global warming are very rarely generated by the model of natural variability (the probability is less than 2.3 x 10(-4)). Thus, natural agencies are not a Plausible explanation for the observed global warming unless all the paleoclimate reconstructions through Which the model is parameterized are underestimating natural variance by a factor of at least four. (C) 2009 Elsevier B.V. All rights reserved. Physica a-Statistical Mechanics and Its Applications

Aim To examine the way in which 'area' and 'habitat diversity' interact in shaping species richness and to find a simple and valid way to express this interaction. Location The Natura 2000 network of terrestrial protected areas in Greece, covering approximately 16% of the national territory. Methods We used the Natura 2000 framework, which provides a classification scheme for natural habitat types, to quantify habitat heterogeneity. We analysed data for the plant species composition in 16,143 quadrats in which 5044 species and subspecies of higher plants were recorded. We built a simple mathematical model that incorporates the effect of habitat diversity on the species-area relationship (SAR). Results Our analysis showed that habitat diversity was correlated with area. However, keeping habitat diversity constant, species richness was related to area; while keeping area constant, species richness was related to habitat diversity. Comparing the SAR of the 237 sites we found that the slope of the species-area curve was related to habitat diversity. Conclusions Discussion of the causes of the SAR has often focused on the primacy of area per se versus habitat heterogeneity, even though the two mechanisms are not mutually exclusive and should be considered jointly. We find that increasing habitat diversity affects the SAR in different ways, but the dominant effect is to increase the slope of the SAR. While a full model fit typically includes a variety of terms involving both area and habitat richness, we find that the effect of habitat diversity can be reduced to a linear perturbation of the slope of the species accumulation curve. Global Ecology and Biogeography

Question: How does the spatial pattern of habitat disturbance and availability affect the selection of dispersal distance? Modelling approach: We use ail individual-based simulation model. By looking at the outcome of competition between individuals using different dispersal distances, the model simulates the selection of dispersal distance. Both habitat and disturbance are spatially structured and can range from a purely random pattern, through a fractal to complete aggregation (a solid block). Conclusions: The disturbance regime affects the selection of dispersal distance more strongly than landscape pattern does. Spatial aggregation of disturbance favours the selection of longer dispersal distances. Lower habitat availability favours shorter dispersal distances. When disturbance is not highly autocorrelated, aggregation of suitable habitat favours shorter dispersal distances. Evolutionary Ecology Research

Marine Ecology-Progress Series

In this paper, we measured the variance of fisheries landings using 344 time series from the Food and Agriculture Organization (FAO) of the United Nations: the series chosen had neither blank records nor zeroes over the period 1950-1996. We compared the results with 431 terrestrial populations from the Global Population Dynamics Database. The logarithm of landings (for fish) and the logarithm of population abundance (for terrestrial organisms) were employed. For both original and detrended time series, the Hurst exponent (a time series autocorrelation index that detects non-random changes) was used to measure the rate of increase of variance as a function of time span. For most FAO time series, variability increased with series length. A large proportion of fish landings series had very large Hurst exponents, suggesting a greater contribution to the total variance by longer-period oscillations for marine data relative to terrestrial ones but this difference between the marine and terrestrial data virtually disappeared when the series were detrended. In fish landings and terrestrial series there was no indication of variance stabilizing at a finite value. This growth in variance may extend to longer time scales, as indicated by analysis of two of the longest time series for marine fish. Our findings have major ecological and management implications as continuously increasing variability implies that there can be no long-term equilibrium yield. This is incompatible with many models currently used for fisheries management that assume bounded equilibrium yields. Models or management strategies looking to the long-term must account for this phenomenon. Fish and Fisheries

The features of 1/f-noise processes offer important new insights into the field of population biology, greatly helping our quest for understanding and for prediction of ecological processes. 1/f-noises account quite satisfactorily for the observed nature of ecological fluctuations. This article reviews the application of 1/f-noise processes to ecology. After a discussion of the basic problems of population ecology that makes such an innovation necessary, we review the features of 1/f-noises concentrating especially on those aspects that make these processes attractive as a solution. We also present a discussion of the analysis of real ecological data, which confirms that there are good empirical as well as good theoretical reasons to establish a leading role for pink 1/f noise. We then discuss the consequences of such a model for our understanding of ecology. The article finishes with a number of observations about some aspects of ecological data and applications that are likely to drive research in a different direction from that associated with engineering and the physical sciences. Fluctuation and Noise Letters

Fractals have found widespread application in a range of scientific fields, including ecology. This rapid growth has produced substantial new insights, but has also spawned confusion and a host of methodological problems. In this paper, we review the value of fractal methods, in particular for applications to spatial ecology, and outline potential pitfalls. Methods for measuring fractals in nature and generating fractal patterns for use in modelling are surveyed. We stress the limitations and the strengths of fractal models. Strictly speaking, no ecological pattern can be truly fractal, but fractal methods may nonetheless provide the most efficient tool available for describing and predicting ecological patterns at multiple scales. Ecol Lett

1. The temporal variability of many animal populations increases with the length of census period. This pattern is associated with the reddened spectral colour, quantified by the spectral exponent, which is typically around + 1. 2. We used simulation models to explore the effects of various population-level processes on the spectral colour of the simulated time series of abundances. 3. Our results showed that the observed spectral exponents could be explained as the effect of a combination of measurement error and natural variability (in the form of white environmental noise). 4. Thus, it may not be necessary to invoke complex varieties of environmental noise to explain the observed spectral exponents. Journal of Animal Ecology

Estimates of the reliability of population viability analysis (PVA), accounting for uncertainties in model parameters, often arrive at confidence intervals for extinction probability so wide as to be almost meaningless. This lack of precision is a consequence of extreme sensitivity to average linear growth rate, when predicting to a distant time horizon. Longer-term trends or drift in parameter values (a form of 'reddened' environmental variability) will also affect the accuracy of such forecasts. This letter reports how, contrary to intuition, introducing such a component of variability may improve the precision of extinction forecasts. The paradoxical result arises because the dependence of extinction probability on growth rate is weakened, and shifted onto other parameters (e.g. diffusion strength) where dependence is less sensitive. This offers hope that, with reasonable knowledge of environmental stochasticity, it may still be meaningful to carry out longer range PVAs. Ecol Lett

We compared the potential allelopathic activity of 47 monoterpenoids of different chemical groups, by estimating their effect on seed germination and subsequent growth of Lactuca sativa seedlings. Apart from individual compounds, eleven pairs at different proportions were also tested. As a group, the hydrocarbons, except for (+)-3-carene, were the least inhibitory. Of the oxygenated compounds, the least inhibitory were the acetates; whenever the free hydroxyl group of an alcohol turned into a carboxyl group, the activity of the resulting ester was markedly lower ( against both germination and seedling growth). Twenty-four compounds were extremely active against seedling growth ( inhibiting it by more than 85%), but only five against seed germination. The compounds that were most active against both processes belonged to the groups of ketones and alcohols; they were terpinen-4-ol, dihydrocarvone, and two carvone stereoisomers. We used a model to investigate whether compounds acted independently when applied in pairs. The combined effect varied. In half of the cases, it followed the pattern expected under the assumption of independence; in the rest, either synergistic or antagonistic interactions were found in both germination and elongation. However, even in cases of synergistic interactions, the level of inhibition was not comparable to that of a single extremely active compound, unless such a compound already participated in the combination. The specific structural factors that operate and determine the activity of monoterpenoids still remain rather obscure. The same holds true for the combined effect; its character cannot in general be predicted on the basis of individual compounds acting alone. J Chem Ecol

We carry out a simulation study of the estimation of fractal dimension in a grid-based setting typical of ecological species distributions, using null landscape models. We calculate the box-counting dimension for samples taken in various types of sampling geometry. Sampler geometries include simple blocks, Cantor grids and line transects. This method may be used to measure fractal dimension of a species distribution, but the accuracy depends on a number of criteria. The most important is sampling effort: any estimate will be inaccurate if the sampling effort is low. We also find the geometry of the sampler to be important. For a given sampling effort, schemes based on the Cantor grids performed better than either line transects or simple blocks. Sampling effort can be improved either by using a bigger sampler over a larger area or by repeated sampling of a smaller area: optimum performance is often a trade-off between these two mechanisms. However, performance is also highly sensitive to the type of fractal object being sampled, with certain types of object requiring a much greater effort for an accurate estimate of fractal dimension. These results raise the possibilities of using novel sampling techniques to estimate fractal dimension, when confronted with limited resources and time, but underline also the need for an understanding of the "type" of fractality expected in ecological situations. Landscape Ecology

We studied the flowering phenology of the genus Campanula (represented by nine species) along the elevation gradient of Mt Olympos (Greece) in order to assess whether there are elevation patterns at the genus level and whether these relate to patterns previously observed along such gradients at the community level. The traits examined were time and duration of the flowering period, flower life span, and duration of male and female flower phases. Floral attributes, such as number of flowers per plant, flower biomass, flower size, were also studied in order to examine whether they change with elevation or influence flower phenology. Flowering of Campanula species started in mid-May and ended in late September. The average duration of flowering of the genus was ca 27 d and the average floral longevity 4 d. In all but one species, the female phase lasted longer than the male. Campanula versicolor differed remarkably from all others in flower phenology and other floral traits. Nearly all Campanula populations studied had right or positively skewed flowering distributions indicating that flowering begins more abruptly than it ends. At the genus level, the time of flowering increased with elevation by 2-3 d for every 100 m. Floral longevity also increased with elevation, by 0.2 d for every 100 m. Neither duration of flowering nor duration of the flower phases showed any consistent change with elevation. The same is true of the non-phenological floral traits examined. No trade-off between duration of flowering and flower life span or between structure and maintenance of flowers was apparent. The pattern of increasing floral longevity along the elevation gradient suggests a mechanism of compensation for reduced pollinator availability. Ecography

In order to predict extinction risk in the presence of reddened, or correlated, environmental variability, fluctuating parameters may be represented by the family of 1/f noises, a series of stochastic models with different levels of variation acting on different timescales. We compare the process of parameter estimation for three 1/f models (white, pink and brown noise) with each other, and with autoregressive noise models (which are not 1/f noises), using data from a model time-series (length, T) of population. We then calculate the expected increase in variance and the expected extinction risk for each model, and we use these to explore the implication of assuming an incorrect noise model. When parameterising these models, it is necessary to do so in terms of the measured ("sample") parameters rather than fundamental ("population") parameters. This is because these models are non-stationary: their parameters need not stabilize on measurement over long periods of time and are uniquely defined only over a specified "window" of timescales defined by a measurement process. We find that extinction forecasts can differ greatly between models, depending on the length, T, and the coefficient of variability, CV, of the time series used to parameterise the models, and on the length of time into the future which is to be projected. For the simplest possible models, ones with population itself the 1/f noise process, it is possible to predict the extinction risk based on CV of the observed time series. Our predictions, based on explicit formulae and on simulations, indicate that (a) for very short projection times relative to T, brown and pink noise models are usually optimistic relative to equivalent white noise model; (b) for projection timescales equal to and substantially greater than T, an equivalent brown or pink noise model usually predicts a greater extinction risk, unless CV is very large; and (c) except for very small values of CV, for timescales very much greater than T, the brown and pink models present a more optimistic picture than the white noise model. In most cases, a pink noise is intermediate between white and brown models. Thus, while reddening of environmental noise may increase the long-term extinction probability for stationary processes, this is not generally true for non-stationary processes, such as pink or brown noises. (C) 1999 Academic Press. Theoretical Population Biology

We examined the asymptotic rate of population extinction beta when the population experiences density-dependent population regulation, demographic stochasticity, and environmental stochasticity. We assume discrete-generation population dynamics, in which some parameters fluctuate between years. The fluctuation of parameters can be of any magnitude, including both fluctuation traditionally treated as diffusion processes and fluctuation from catastrophes within a single scheme. We develop a new approximate method of calculating the asymptotic rate of population extinction per year, beta = integral(0)(infinity) exp(-x) u(x) dx, where u(x) is the stationary distribution of adult population size from the continuous-population model including environmental stochasticity and population-regulation but neglecting demographic stochasticity, The method can be regarded as a perturbation expansion of the transition operator for population size. For several sets of population growth functions and probability distributions of environmental fluctuation, the stationary distributions can be calculated explicitly. Using these, we compare the predictions of this approximate method with that using a full transition operator and with the results of a direct Monte Carlo simulation, The approximate formula is accurate when the intrinsic rate of population increase is relatively large, though the magnitude of environmental fluctuation is also large, This approximation is complementary to the diffusion approximation. (C) 1998 Academic Press. Theoretical Population Biology

J. Appl. Ecol.

1. We have studied the density-dependent persistence of four tephritid flies on patches of their host vegetation using a semi-analytic model. The model takes into account density dependence, environmental stochasticity and immigration. Basic statistical methods were used to parameterize the model for data collected in our field study. We have assumed that the extinction process is due mainly to a fluctuation of the amount of resources. 2. The model was used to predict local extinction probability. For all species, these predictions were compared to, and found to be in reasonable agreement with, the observed levels of local extinction. 3. The model can also be used to estimate the effects of increasing fragmentation on a given fly species. The occupancy of larger patches is strongly dependent on initial conditions. Initially-occupied large patches can last a long time, irrespective of isolation, but unoccupied ones must be colonized first, requiring that the patch be reasonably close to the source of dispersers. Small patches, irrespective of initial conditions, will, on average, only be occupied if it lies close to a large 'mainland' patch or group of patches; that is, its isolation is less than the 'dispersal distance' for that species. 4. On the scale of the field system studied, immigration is the dominant factor determining persistence. The rate of immigration appears to be proportional to the area of a patch, but the isolation of almost all patches (distance to the next patch) is significantly less than the measured dispersal distance of all the species studied. Further studies will be needed to confirm what our model has to say about the relative importance of migration in more highly fragmented habitats. Journal of Applied Ecology

This paper describes the first cellular automaton model of a multispecies fungal community and presents some of the results of this model. The model assumes that competition occurs between two pairs of species which break down litter in two stages of succession. The litter can be introduced into the arena either continuously or seasonally. The model predicts numerous observed features of fungal dynamics: the formation of ring structures; the displacement of one mycelium species by another in simple competitive situations and the coexistence of several species in a variable environment. The predicted response of species richness to resource level is dome-shaped. The model also predicts the formation of strikingly heterogeneous spatial patterns when the resource input is regular. Oikos

We present a population dynamics model for annual plants subject to density dependent competition and a decline in mean individual fitness with inbreeding. An analysis of this model provides three distinct sets of parameter values that define the relative influence of inbreeding depression and density on population growth. First, a population with a relatively high finite rate of increase and a relatively small environmental carrying capacity can persist in spite of low levels of inbreeding depression. These types of population may occur during a bottleneck event that is caused by pure predation (or collecting) pressure rather than loss of habitat. Second, there can exist a minimum viable population size when the finite rate of increase is relatively low and the population is also affected by density: the growth or decline of the population will depend on the initial population size. Third, when the population is small enough to be simultaneously effected by density and by inbreeding depression, there can be no viable population. Evolutionary Ecology

Ieee Journal of Quantum Electronics

Journal of Applied Physics

Many have asserted that Sacred Natural Sites (SNS) play an important role in nature protection but few have assessed their conservation effectiveness for different taxa. We studied sacred groves in Epirus, NW Greece, where a large number of such SNS have been identified. Based on historical, ethnographic and ecological criteria, we selected eight of these groves and matching control sites and in them we studied fungi, lichens, herbaceous plants, woody plants, nematodes, insects, bats and passerine birds. Our results reveal that the contribution of SNS to species conservation is nuanced by taxon, vegetation type and management history. We found that the sacred groves have a small conservation advantage over the corresponding control sites. More specifically, there are more distinct sets of organisms amongst sacred groves than amongst control sites, and overall biodiversity, diversity per taxonomic group, and numbers of species from the European SCI list (Species of Community Interest) are all marginally higher in them. Conservationists regard the often small size of SNS as a factor limiting their conservation value. The sizes of SNS around the globe vary greatly, from a few square meters to millions of hectares. Given that those surveyed by us (ranging from 5 to 116 ha) are at the lower end of this spectrum, the small conservation advantage that we testified becomes important. Our results provide clear evidence that even small-size SNS have considerable conservation relevance; they would contribute most to species conservation if incorporated in networks.