Revisiting matrix games: the concept of neighborhood invader strategies

We extend the concept of neighborhood invader strategy (NIS) to finite-dimensional matrix games and compare this concept to the evolutionarily stable strategy (ESS) concept. We show that these two concepts are not equivalent in general. Just as ESS's may not be unique, NIS's may also not be unique. However, if there is an ESS and a NIS then these strategies must be the same. We show that an ESNIS (an ESS and NIS) for any matrix game is unique and that a mixed ESS with full support is a NIS. Thus a mixed ESS with full support is not invadable by any pure or mixed strategy and it can invade any pure or mixed strategy. An ESS which is an ESNIS, therefore, has better chance of being established evolutionarily through dynamic selection.     

Determination of zooplankton dietary shift following a zebra mussel invasion, as indicated by stable isotope analysis

1. Freshwaters with established zebra mussel populations typically exhibit reduced chlorophyll a concentrations, but the subsequent impacts upon zooplankton are varied. We hypothesised that in an invaded system with less phytoplankton but available allochthonous subsidy, zooplankton may utilise greater proportions of allochthonous matter and that this could be traced by analysis of stable carbon and nitrogen isotopes. 2. We used archived zooplankton samples which had been consistently preserved and which spanned the invasion period of an Irish lake, Lough Erne. Increasing reliance upon allochthonous resources would be reflected in an increase in zooplankton δ¹³C away from phytoplankton which is relatively ¹³C‐depleted in humic‐stained L. Erne. 3. Analysis of a series of monthly samples (1992–96, 1999–2003) revealed significant ¹³C‐enrichment of mixed zooplankton, Eudiaptomus gracilis and Mysis relicta post‐zebra mussel invasion; δ¹³C values approached ?27‰ typical of terrestrial organic matter during spring and autumn. Changes in zooplankton elemental composition also suggested a switch to a lower quality diet. However, analysis of zooplankton δ¹³C from an annual, single‐point (June) time series spanning 28 years (1977–2004) suggested that when phytoplankton was sufficiently abundant, zooplankton used this resource and their δ¹³C remained relatively constant around ?32‰. Post‐invasion enrichment of mysid δ¹⁵N may reflect a shift towards carnivory, but planktonic prey abundance was reduced and a subsequent loss of body condition could result in the same isotopic changes. 4. Our results indicate that in L. Erne, when phytoplankton was reduced by zebra mussel filtering, zooplankton assimilated more from allochthonous matter, and potentially sustained a higher population than would otherwise be possible. Thus, zebra mussel impact on foodweb structure and function is likely to be different in lakes subject to varying subsidy levels.     

Trophic Niche Segregation in the Nilotic Ichthyofauna of Lake Albert (Uganda, Africa)

Synopsis Nile perch, Lates niloticus, and Nile tilapia, Oreochromis niloticus, were originally transplanted from Lake Albert in western Uganda to the African Great Lakes, Lake Victoria and Lake Kyoga, where they are partially implicated in reduction of the fish species diversity. Lake Albert is facing multiple environmental changes, including declining fish species diversity, hyper-eutrophication, hypoxia, and reduced fish catches. To examine the role of Nile perch and Nile tilapia in the food web in their native Lake Albert, we estimated their diets using stable nitrogen and carbon isotopes. In Lake Albert, the tilapiine congeners (closely related species), Tilapia zillii, Oreochromis leucostictus, and Sarethorodon galilaeus, and the centropomid Nile perch congener, Lates macrophthalmus, have narrower diet breath in the presence of the native O. niloticus and L. niloticus. A computerized parameter search of dietary items for five commercially important fish species (Hydrocynus forskahlii, Bagrus bayad, L. niloticus, Alestes baremose and Brycinus nurse) was completed using a static isotopic mixing model. The outcome of the simulation for most fish species compared favorably to previously published stomach contents data for the Lake Albert fishes dating back to 1928, demonstrating agreement between stable isotope values and analyses of stomach contents. While there were some indications of changes in the diets of L. niloticus and A. baremose diets over the past 20 years in parallel with other changes in the lake, for the most part, food web structure in this lake remained stable since 1928. The Lake Albert fish assemblage provides insight into the invasion success of L. niloticus and O. niloticus.     

If the antibody fails--a mass western approach

SUMMARY: Sucrose-phosphate synthase (SPS) has attracted the interest of plant scientists for decades. It is the key enzyme in sucrose metabolism and is under investigation in various plant species, e.g. spinach, tobacco, poplar, resurrection plants, maize, rice, kiwi and Arabidopsis thaliana. In A. thaliana, there are four distinct SPS isoforms. Their expression is thought to depend on environmental conditions and plant tissue. However, these data were derived from mRNA expression levels only. No data on SPS protein identification from crude extracts have been available until now. An antibody approach failed to distinguish the four isoforms. Therefore, we developed a method for SPS quantification and isoform-specific identification in A. thaliana complex protein samples. Samples were separated on SDS-PAGE, digested and directly applied to liquid chromatography/triple-stage quadrupole mass spectrometry (LC/TSQ-MS). In this approach, known as mass Western, samples were analysed in multi-reaction monitoring (MRM) mode, so that all four SPS isoforms could be measured in one experiment. In addition to the relative quantification, stable isotope-labelled internal peptide standards allowed absolute quantification of SPS proteins. Protein extracts from various plant tissues, samples harvested during the day or the night, and cold-stressed plants were analysed. The stress-specific SPS5a isoform showed increased concentrations in cold-stressed leaf material.     

Soil carbon sequestration in a pine forest after 9 years of atmospheric CO2 enrichment

The impact of anthropogenic CO₂ emissions on climate change may be mitigated in part by C sequestration in terrestrial ecosystems as rising atmospheric CO₂ concentrations stimulate primary productivity and ecosystem C storage. Carbon will be sequestered in forest soils if organic matter inputs to soil profiles increase without a matching increase in decomposition or leaching losses from the soil profile, or if the rate of decomposition decreases because of increased production of resistant humic substances or greater physical protection of organic matter in soil aggregates. To examine the response of a forest ecosystem to elevated atmospheric CO₂ concentrations, the Duke Forest Free‐Air CO₂ Enrichment (FACE) experiment in North Carolina, USA, has maintained atmospheric CO₂ concentrations 200 μL L^?1 above ambient in an aggrading loblolly pine (Pinus taeda) plantation over a 9‐year period (1996–2005). During the first 6 years of the experiment, forest‐floor C and N pools increased linearly under both elevated and ambient CO₂ conditions, with significantly greater accumulations under the elevated CO₂ treatment. Between the sixth and ninth year, forest‐floor organic matter accumulation stabilized and C and N pools appeared to reach their respective steady states. An additional C sink of ～30 g C m^?2 yr^?1 was sequestered in the forest floor of the elevated CO₂ treatment plots relative to the control plots maintained at ambient CO₂ owing to increased litterfall and root turnover during the first 9 years of the study. Because we did not detect any significant elevated CO₂ effects on the rate of decomposition or on the chemical composition of forest‐floor organic matter, this additional C sink was likely related to enhanced litterfall C inputs. We also failed to detect any statistically significant treatment effects on the C and N pools of surface and deep mineral soil horizons. However, a significant widening of the C : N ratio of soil organic matter (SOM) in the upper mineral soil under both elevated and ambient CO₂ suggests that N is being transferred from soil to plants in this aggrading forest. A significant treatment × time interaction indicates that N is being transferred at a higher rate under elevated CO₂ (P=0.037), suggesting that enhanced rates of SOM decomposition are increasing mineralization and uptake to provide the extra N required to support the observed increase in primary productivity under elevated CO₂.     

Changing skewness: an early warning signal of regime shifts in ecosystems

Empirical evidence for large-scale abrupt changes in ecosystems such as lakes and vegetation of semi-arid regions is growing. Such changes, called regime shifts, can lead to degradation of ecological services. We study simple ecological models that show a catastrophic transition as a control parameter is varied and propose a novel early warning signal that exploits two ubiquitous features of ecological systems: nonlinearity and large external fluctuations. Either reduced resilience or increased external fluctuations can tip ecosystems to an alternative stable state. It is shown that changes in asymmetry in the distribution of time series data, quantified by changing skewness, is a model-independent and reliable early warning signal for both routes to regime shifts. Furthermore, using model simulations that mimic field measurements and a simple analysis of real data from abrupt climate change in the Sahara, we study the feasibility of skewness calculations using data available from routine monitoring.     

Coupling of canopy and understory food webs by ground‐dwelling predators

Understanding food‐web dynamics requires knowing whether species assemblages are compartmentalized into distinct energy channels, and, if so, how these channels are structured in space. We used isotopic analyses to reconstruct the food web of a Kenyan wooded grassland. Insect prey were relatively specialized consumers of either C₃ (trees and shrubs) or C₄ (grasses) plants. Arboreal predators (arthropods and geckos) were also specialized, deriving c. 90% of their diet from C₃‐feeding prey. In contrast, ground‐dwelling predators preyed considerably upon both C₃‐ and C₄‐feeding prey. This asymmetry suggests a gravity‐driven subsidy of the terrestrial predator community, whereby tree‐dwelling prey fall and are consumed by ground‐dwelling predators. Thus, predators in general couple the C₃ and C₄ components of this food web, but ground‐dwelling predators perform this ecosystem function more effectively than tree‐dwelling ones. Although prey subsidies in vertically structured terrestrial habitats have received little attention, they are likely to be common and important to food‐web organization.     

A novel fitness proxy in structured locally finite metapopulations with diploid genetics, with an application to dispersal evolution

Many studies of evolutionarily stable strategies (ESS) for technical reasons make the simplification that reproduction is clonal. A post-hoc justification is that in the simplest eco-evolutionary models more realistic genetic assumptions, such as haploid sexual or diploid sexual cases, yield results compatible with the clonal ones. For metapopulations the technical reasons were even more poignant thanks to the lack of accessible fitness proxies for the diploid case. However, metapopulations are also precisely the sort of ecological backdrop for which one expect discrepancies between the evolutionary outcomes derived from clonal reproduction and diploid genetics, because substantially many mutant homozygotes appear locally even though the mutant is rare globally. In this paper we devise a fitness proxy applicable to the haploid sexual and diploid sexual case, in the style of Metz and Gyllenberg [Metz, J.A.J., Gyllenberg, M., 2001. How should we define fitness in structured metapopulation models? Including an application to the calculation of ES dispersal strategies. Proc. R. Soc. Lond. B 268, 499-508], that can cope with local population fluctuations due to environmental and demographic stochasticity. With the use of this fitness proxy we find that in dispersal evolution the studied clonal model is equivalent with the haploid sexual model, and that there are indeed many differences between clonal and diploid ESS dispersal rates. In a homogenous landscape the discrepancy is but minor (less than 2%), but the situation is different in a heterogeneous landscape: Not only is the quantitative discrepancy between the two types of ESSs appreciable (around 10%-20%), but more importantly, at the same parameter values, evolutionarily stability properties may differ. It is possible, that the singular strategy is evolutionarily stable in the clonal case but not in the diploid case, and vice versa.     

The ideal free distribution: a review and synthesis of the game-theoretic perspective

The Ideal Free Distribution (IFD), introduced by Fretwell and Lucas in [Fretwell, D.S., Lucas, H.L., 1970. On territorial behavior and other factors influencing habitat distribution in birds. Acta Biotheoretica 19, 16-32] to predict how a single species will distribute itself among several patches, is often cited as an example of an evolutionarily stable strategy (ESS). By defining the strategies and payoffs for habitat selection, this article puts the IFD concept in a more general game-theoretic setting of the “habitat selection game”. Within this game-theoretic framework, the article focuses on recent progress in the following directions: (1) studying evolutionarily stable dispersal rates and corresponding dispersal dynamics; (2) extending the concept when population numbers are not fixed but undergo population dynamics; (3) generalizing the IFD to multiple species.For a single species, the article briefly reviews existing results. It also develops a new perspective for Parker’s matching principle, showing that this can be viewed as the IFD of the habitat selection game that models consumer behavior in several resource patches and analyzing complications involved when the model includes resource dynamics as well. For two species, the article first demonstrates that the connection between IFD and ESS is now more delicate by pointing out pitfalls that arise when applying several existing game-theoretic approaches to these habitat selection games. However, by providing a new detailed analysis of dispersal dynamics for predator-prey or competitive interactions in two habitats, it also pinpoints one approach that shows much promise in this general setting, the so-called “two-species ESS”. The consequences of this concept are shown to be related to recent studies of population dynamics combined with individual dispersal and are explored for more species or more patches.     

Food resources of shredders and other benthic macroinvertebrates in relation to shading conditions in tropical Hong Kong streams

1. Gut content analyses (GCA) of benthic macroinvertebrates, supplemented by carbon and nitrogen stable isotope analyses (SIA), were used to determine the relative contribution of leaf litter and autochthonous food sources to consumer biomass in five shaded and five unshaded streams in tropical Hong Kong. 2. Only four obligate shredders and two facultative shredders were identified out of 58 morphospecies dissected. Non‐shredder taxa consumed little (<23% food eaten) coarse particulate organic matter (CPOM) in spite of its abundance in streams, and GCA revealed that fine particulate organic matter was the major food (25–99%) of most primary consumers. 3. Stable isotope analysis results were in general agreement with the findings of GCA, and confirmed that three of the four obligate shredders had a high dependence (55–78% of assimilated carbon) on CPOM. 4. Autochthonous energy sources were important in all streams: non‐shredding primary consumers examined, which accounted for 72% of total macroinvertebrate abundance in shaded streams, derived (on average) 61% of their biomass from autochthonous foods; the equivalent values for unshaded streams were 72% (abundance) and 71% (biomass).