Showing papers in "The Open Ecology Journal in 2009"
TL;DR: A new, free software is introduced calculating a large spectrum of network indices, visualizing bipartite networks and generating null models, and enables ecologists to readily contrast their findings with null model expectations for many different questions, thus separating statistical inevitability from ecological process.
Abstract: Many analyses of ecological networks in recent years have introduced new indices to describe network properties. As a consequence, tens of indices are available to address similar questions, differing in specific detail, sensitivity in detecting the property in question, and robustness with respect to network size and sampling intensity. Furthermore, some indices merely reflect the number of species participating in a network, but not their interrelationship, requiring a null model approach. Here we introduce a new, free software calculating a large spectrum of network indices, visualizing bipartite networks and generating null models. We use this tool to explore the sensitivity of 26 network indices to network dimensions, sampling intensity and singleton observations. Based on observed data, we investigate the interrelationship of these indices, and show that they are highly correlated, and heavily influenced by network dimensions and connectance. Finally, we re-evaluate five common hypotheses about network properties, comparing 19 pollination networks with three differently complex null models: 1. The number of links per species ("degree") follow (truncated) power law distributions. 2. Generalist pollinators interact with specialist plants, and vice versa (dependence asymmetry). 3. Ecological networks are nested. 4. Pollinators display complementarity, owing to specialization within the network. 5. Plant-pollinator networks are more robust to extinction than random networks. Our results indicate that while some hypotheses hold up against our null models, others are to a large extent understandable on the basis of network size, rather than ecological interrelationships. In particular, null model pattern of dependence asymmetry and robustness to extinction are opposite to what current network paradigms suggest. Our analysis, and the tools we provide, enables ecologists to readily contrast their findings with null model expectations for many different questions, thus separating statistical inevitability from ecological process.
1,222 citations
TL;DR: This paper discusses the appropriate valuation methods for pollination at different scales, and presents an analysis of the value of the pollination service based on a literature review, and discusses the need to adjust the valuation approach to the scale of the analysis.
Abstract: Pollination is an ecosystem service that is essential to support the production of a wide range of crops. The service is increasingly under threat, as a consequence of among others habitat loss of pollinators and increasing use of pesticides. In order to support maintaining the pollination service in agriculture, there is a need to better understand the economic value generated by the pollination service. This paper discusses the appropriate valuation methods for pollination at different scales, and presents an analysis of the value of the pollination service based on a literature review. At the local scale, the value of the service is highly variable depending on the crop and the market conditions. At the national scale, value estimates of the pollination service range from 1% to 16% of the market value of agricultural production. At the global scale, there is currently no reliable estimate of the value of this service. When the economic value of the pollination service is analysed, it is critical to adjust the valuation approach to the scale of the analysis.
81 citations
TL;DR: This article examined citation rates for 5883 articles in relation to number of authors, first author's primary language, and gender, and found that citation rates were unrelated to primary language and gender but increased with author number.
Abstract: Citation metrics are widely used as a surrogate measure of scientific merit; however, these indices may be sensitive to factors and influences unrelated to merit. We examined citation rates for 5883 articles in relation to number of authors, first author's primary language, and gender. Citation rates were unrelated to primary language and gender but increased with author number. These findings add to a growing body of indirect evidence for potential attitudinal bias in the perceived merit of publications within ecology.
75 citations
TL;DR: Data on coyote Canis latrans abundance and diet composition from two habitats in the Chihuahuan Desert of Mexico supported both the above cited predictions and the hypothesis that prey vulnerability can influence habitat use by coyotes.
Abstract: Under predation risk, prey species are more abundant in areas of low predation risk even at the expense of forage quality. As a result two predictions are possible, 1) predators should choose to hunt in areas with fewer but easier to catch prey than areas where they are more abundant but harder to catch; and 2) the frequency of prey species in the diet of predators using low risk areas should be greater than, or at least equal to, the diet of predators using high risk areas. To test these two predictions, we used data on coyote Canis latrans abundance and diet composition from two habitats in the Chihuahuan Desert of Mexico that have different abundances of jackrabbits (Lepus californicus) and rodents. We used the number of coyote scats found in transects in the two areas to assess coyote abundance and analyzed the contents of these scats to determine diet composition. We found significantly more coyote scats/yr (22.6 ± 4.7 (SE) vs. 12.2 ± 2.4 scats/yr, d.f. = 7, paired t = 3.80, P = 0.007) in the habitat with less jackrabbits and more rodents. However, the percent occurrence of jackrabbits (54.3 ± 6.7% vs. 60.1 ± 7.7%) and rodents (32.6 ± 6.5% vs. 30.1 ± 6.0%) in coyote scats did not differ between the two habitats. These results supported both the above cited predictions and the hypothesis that prey vulnerability can influence habitat use by coyotes.
32 citations
TL;DR: Investigations into the spatial and temporal distributions of the entomopathogenic fungus L. lecanii suggest that it may very well be responsible for the density-dependent control thought necessary for spatial pattern formation of ant nests in this system.
Abstract: Recent studies have shown that the spatial pattern of nests of an arboreal ant, Azteca instabilis (Hymenoptera: Formicidae), in a tropical coffee agroecosystem may emerge through self-organization. The proposed self-organization process involves both local expansion and density-dependent mortality of the ant colonies. We explored a possible mechanism for the density-dependent mortality involving the entomopathogenic fungus Lecanicillium lecanii. L. lecanii attacks a scale insect, Coccus viridis (Coccidae, Hemiptera), which is tended by A. instabilis in a mutualistic association. By attacking C. viridis, L. lecanii may have an indirect, negative effect on ant colony survival. To explore this hypothesis, we conducted investigations into the spatial and temporal distributions of L. lecanii. We measured incidence and severity at 4 spatial scales: (1) throughout a 45 hectare study plot; (2) in two 40 X 50 meter plots; (3) on coffee bushes within 4 m of two ant nests; and (3) on individual branches in a single coffee bush. The plot-level censuses did not reveal a clear spatial pattern, but the finer scale surveys show distinct patterns in the spread of infection over time. We also developed a simple cellular automata model of the coupled ant nest-L. lecanii system which is able to produce spatial patterns qualitatively and quantitatively similar to that found in the field. The accumulated evidence suggests that L. lecanii may very well be responsible for the density-dependent control thought necessary for spatial pattern formation of ant nests in this system.
26 citations
TL;DR: A spatially explicit simulation model of a host and its specialized parasitoid and simulated their spatiotemporal population dynamics in virtual landscapes found that the total habitat amount in the landscape modulates the impact of fragmentation on parasitoids persistence.
Abstract: In the context of agricultural landscapes, conservation biocontrol practitioners attempt to secure and enhance the presence and effectiveness of natural enemies of insect pest species, for example parasitoids. Conservation biocontrol aims at maximizing both parasitoid persistence and parasitation rate. It is, however, still poorly understood how the amount, fragmentation and isolation of non-crop habitat of the host and its parasitoid affect persistence and parasitation rate. We developed a spatially explicit simulation model of a host and its specialized parasitoid and simulated their spatiotemporal population dynamics in virtual landscapes. We found that the total habitat amount in the landscape modulates the impact of fragmentation on parasitoid persistence. If habitat is abundant, parasitoid persistence decreases with fragmentation, whereas if habitat is scarce, persistence is highest at intermediate levels of fragmentation. In any case, persistence is best for intermediate levels of isolation. Parasitation rate, on the other hand, is negatively influenced by fragmentation and isolation regardless of the habitat amount. Our results suggest that in landscapes with abundant habitat, both parasitation rates and parasitoid persistence can be increased by arranging habitat to be as clumped as possible. However, if habitat is scarce, landscape management can optimize either parasitation rates or parasitoid persistence but not both simultaneously.
17 citations
TL;DR: It seems likely that having species within a community whose spatial variability responds in different ways to stress may be crucial to smoothing out tensions between species and increasing resilience.
Abstract: The relationship between stress and population variability is essential for predicting whether communities will exhibit stability and resilience when faced with stress. Stress is generally considered to increase biological variability, even before mean responses exhibit change. However, generalities related to spatial variability of populations have not emerged, as large-scale perturbations tend to reduce variability in affected areas (i.e., a homogenising effect), and a posi- tive relationship between mean and variance is expected at all scales. To investigate whether stress does increase the spa- tial variability of macrobenthic species abundances, we analysed the response of survey and experimental data, collected over a variety of space (50 m to 5 km) and time scales (15 d to 15 yr), to two different stressors. We observed no consis- tent increase in variability as a response to stress, even within studies. Moreover, a complex relationship was observed be- tween spatial variance and mean abundances that was not represented by a simple power law. However, one consistent re- sponse was observed across stressors and study type; the number of common species exhibiting changes (either increases or decreases) to their spatial variability, beyond natural levels, increased with stress. It seems likely that having species within a community whose spatial variability responds in different ways to stress (rather than spatial variability of all spe- cies increasing) may be crucial to smoothing out tensions between species and increasing resilience.
14 citations
TL;DR: It is found that bet hedging effects and key reproductive traits are often sufficient to produce an optimal size-number balance and can account for the frequent tendency of both brood size and body mass to increase with brood resources (host size).
Abstract: The trade-off between the number of offspring in a brood and the sizes of those offspring has been documented in a diverse array of species. Here we consider the factors that might account for the way that the trade-off achieves a particular size-number balance for female offspring. In particular, we determine whether bet hedging, along with traits influencing the expected short-term reproductive success of the brood, could select for a brood size and body mass that maximize long-term fitness. We also evaluate whether the optima based on these characteristics agree qualitatively with documented responses to brood resource levels. To develop a model incorporating these phenomena and keep it as simple and general as possible, we consider an organism that produces clonal broods, eliminating the complexities associated with parent-offspring and sibling conflict. We use the polyembryonic parasitoid wasp Copidosoma bakeri as the focal example. We find that bet hedging effects and key reproductive traits (the number of searching females, their host-finding efficiency, and survival prospects) are often sufficient to produce an optimal size-number balance and can account for the frequent tendency of both brood size and body mass to increase with brood resources (host size). Under some conditions, however, balancing the trade-off requires an additional minimum body-size constraint. Future empirical work and spatially explicit models must better establish the parameter magnitudes and functional relationships so that a deeper understanding and more precise predictions are obtained.
11 citations
TL;DR: The architectural properties of rice food webs from West Bengal are examined, based on replicated plots of folk variety (organic) and modern (chemicalised) rice systems, to understand the functional significance of species richness and ecosystem complexity of rice farms.
Abstract: Modern rice farms are characterized by the use of synthetic agrochemicals, which eliminate a large segment of biodiversity on-farm. In contrast, organic rice farms tend to preserve much of natural biodiversity. While biodiversity- productivity relationship in organic vs. chemicalised rice farms is contested, the relationship of on-farm biodiversity with food web structural properties and ecosystem services remains to be explored. To understand the functional significance of species richness and ecosystem complexity of rice farms, I examine here the architectural properties of rice food webs from West Bengal, based on replicated plots of folk variety (organic) and modern (chemicalised) rice systems. All rice food webs, constructed from observational data collected over three years, show prominent scale dependence of dietary links, link density, web height, diversity of natural enemies to pests, predator-pest ratio, and the numbers of omnivores and omnivory levels. Organic folk rice webs tend to have greater mean species richness, predator diversity, predator-pest ratio and chain length than modern rice farm webs, yet both systems show homogeneity of distribution of the web properties. Analyses of 16,400 computerized analog webs, following non-random rules of species association drawn on real-life, seasonally distinct rice food webs, validate the robustness of conclusions.
10 citations
TL;DR: It is suggested that sites with high N concentrations in stomach content and with higher NC-ratio imply better habitats that support higher population densities of voles and the diet quality could be related to this ratio.
Abstract: We found that concentrations of nitrogen (N) in the stomach of voles varied between forest sites. Concentrations of nitrogen as well as the ratio of nitrogen to carbon (C) in stomach content were not different between the sexes or with season. Body mass varied between sites and between seasons but were unrelated to N concentration in spite that the body mass of voles in the spring season were on average 4.8 g heavier than in the autumn A positive relationship was found between N and C concentrations in the stomach content implying the diet quality could be related to this ratio. N concentrations in the stomach per unit body mass were positively and significantly related to catch per unit effort of voles. We suggest that sites with high N concentrations in stomach content and with higher NC-ratio imply better habitats that support higher population densities of voles.
8 citations
TL;DR: It is believed that facilitation is not a true species interaction and should be kept within the framework of succession.
Abstract: Current ecological literature considers facilitation to be a positive interaction alongside symbiosis or mutualism. But unlike these interactions, the fitness of one of the species involved in facilitation remains unaffected which precludes coevolution. Therefore, we believe that facilitation is not a true species interaction and should be kept within the framework of succession.
TL;DR: Though T. rutilus is an aggressive predator, it does not cause an increase in prey species diversity as a keystone predator would, and the relationship between the predator and diversity in treeholes appears to be unrelated to predation on the dominant competitors and is instead caused by some other habitat characteristic.
Abstract: The effects of predation on water-filled treehole communities in North Carolina were examined using mesocosm experiments and observations in natural treeholes. The presence of the predator Toxorhynchites rutilus and leaf litter abundance were manipulated in mesocosms to examine interactions between resources and predation. Long-term examination of interactions in unmanipulated treeholes provided data on natural variation in water volume and predator density. Toxorhynchites rutilus preys upon two common treehole insects, Aedes triseriatus and Culicoides guttipennis. We predicted that T. rutilus would act as a keystone predator and reduce the density of these dominant species. This would allow other species to coexist and lead to an increase in diversity. We also predicted that effects of predation would be reduced in habitats with high levels of resources, due to either increased refugia or decreased competition. The results did not entirely support the predictions. In both mesocosms and treeholes T. rutilus depressed densities of the most abundant prey type, A. triseriatus. In treeholes, the presence of T. rutilus depressed densities of the midge C. guttipennis, and predator densities were positively associated with insect diversity in treeholes. Strikingly, higher diversity was also associated with high densities of the dominant prey in treeholes. In addition, mesocosms showed no relationship between T. rutilus presence and species diversity. The relationship between the predator and diversity in treeholes appears to be unrelated to predation on the dominant competitors and is instead caused by some other habitat characteristic. While there were effects of resources on densities and diversity in both mesocosms and treeholes, neither of the resources analyzed, leaf litter or water, appear to be the sole characteristic that mutually allows for large populations of T. rutilus and high species diversity. We conclude that though T. rutilus is an aggressive predator, it does not cause an increase in prey species diversity as a keystone predator would. Further research is needed to determine the conditions that favor the presence of predator and high prey diversity.
TL;DR: An exploratory analysis of four broad-scale hypotheses for changes in species richness over Central American mayfly species, and compared their outcomes at different taxonomic levels found only area availability was an effective predictor of mayfly genus and family richness.
Abstract: I performed an exploratory analysis of four broad-scale hypotheses (area availability, energy availability, habi- tat heterogeneity and geometric constraints) for changes in species richness over Central American mayfly species, and compared their outcomes at different taxonomic levels. I employed an eigenvector-based spatial filtering to control spatial autocorrelation effects and conducted multiple spatial eigenvector regressions to identify the strongest predictors of spe- cies, genus and family richness. The usefulness of higher-taxa as surrogates for species richness was assessed by Pearson correlations. Mayfly species richness is characterized by a patchy pattern. None of the variables accounted for observed patterns. Only area availability was an effective predictor of mayfly genus and family richness, although both habitat het- erogeneity and energy presented marginal effects on genus richness. I did not observe any relationship between species and higher-taxa richness. Broad-scale hypotheses did not explain species richness patterns of mayflies, which instead can be explained by interactions among studied variables and spatial structure.
TL;DR: In this article, the authors captured birds in a central Guatemalan cloud forest (Sierra Yalijux) in two adjacent habitats, with different habitat structure to assess the influence of land use on bird populations.
Abstract: I captured birds in a central Guatemalan cloud forest (Sierra Yalijux) in two adjacent habitats, with different habitat structure to assess the influence of land use on bird populations. The regional population of Common Bush- Tanager (Chlorospingus ophthalmicus) showed a pattern of spatial separation of 1 st -year individuals vs. adults, a pattern not shown so far for any bird species. During the early post-breeding season, I captured similar numbers of adult individuals in both natural and young secondary forest (38 vs. 43), while 1 st -year individuals were captured mainly in young secondary vegetation (3 vs. 40). The findings indicate that inexperienced, young or less competitive individuals may use other habitats than adults, at least temporarily.