Darwinian evolution does not rule out the gaia hypothesis

This study explores so-called Darwinian Daisyworlds mathematically rigorously in detail. The original Daisyworld was introduced by Watson & Lovelock (1983) to demonstrate how two species of daisies regulate the global temperature of their planet through competition among these species against the rising solar luminosity, i.e. the Gaia hypothesis. Its variants are Darwinian Daisyworlds in which daisies can adapt themselves to the local temperature. Robertson & Robinson (1998) insist their Darwinian daisies lose the ability for temperature regulation on the basis of their spreadsheet simulations. Lenton & Lovelock (2000) point out that the constraints on adaptation recovers Darwinian daisies' ability of temperature regulation on the basis of their Euler-code simulations. The present study shows there exist the exact and closed-form solutions to these two Daisyworlds. The results contradict the former studies: Robertson and Robinson's daisies do regulate the global temperature even longer than non-adaptive daisies; Lenton and Lovelock's daisies are less adaptive than Robertson and Robinson's daisies because of the constraints on adaptation; the introduction of weak adaptability drives species into a dead end of evolution. Thus, the present results confirm that the Gaia hypothesis and Darwinian evolution can coexist.     

The consequence of maximum thermodynamic efficiency in Daisyworld

The imaginary planet of Daisyworld is the simplest model used to illustrate the implications of the Gaia hypothesis. The dynamics of daisies and their radiative interaction with the environment are described by fundamental equations of population ecology theory and physics. The parameterization of the turbulent energy flux between areas of different biological cover is similar to the diffusive-type approximation used in simple climate models. Here I show that the small variation of the planetary diffusivity adopted in the classical version of Daisyworld limits the range of values for the solar insolation for which biota may grow in the planet.Recent studies suggest that heat transport in a turbulent medium is constrained to maximize its efficiency. This condition is almost equivalent to maximizing the rate of entropy production due to non-radiative sources. Here, I apply the maximum entropy principle (MEP) to Daisyworld. I conclude that the MEP sets the maximum range of values for the solar insolation with a non-zero amount of daisies. Outside this range, daisies cannot grow in the planet for any physically realistic climate distribution. Inside this range, I assume a distribution of daisies in agreement with the MEP. The results substantially enlarge the range of climate stability, due to the biota, in comparison to the classical version of Daisyworld. A very stable temperature is found when two different species grow in the planet.     

Coexistence introducing regulation of environmental conditions

Interactions between environmental conditions and environment-affecting species have not been investigated extensively. In this study, the population dynamics of species yielding regulative feedback between temperature (a representative of environmental condition) and species with a temperature-altering trait was examined. We considered a simple closed model that described the population of two species (at least one of them had a temperature-altering trait) competing for one resource. The long-term outcomes of the competition and changes of temperature were explored against increasing background temperature. As a result of simulations, the regulation of temperature was accompanied by the coexistence of two species, which was contrary to the 'Gause's exclusion principle'. The steady-state analysis showed that (i) the temperature-altering trait allowed species to coexist and (ii) the coexistence of species with the trait could introduce the regulation of temperature. A 'trade-off' in their ability to utilize a resource plays a key role in this coexistence and homeostasis of temperature. This may imply that actual environmental conditions can be automatically stabilized by resource competition among species in natural ecosystems.     

One-dimensional daisyworld: spatial interactions and pattern formation

The zero-dimensional daisyworld model of Watson and Lovelock (1983) demonstrates that life can unconsciously regulate a global environment. Here that model is extended to one dimension, incorporating a distribution of incoming solar radiation and diffusion of heat consistent with a spherical planet. Global regulatory properties of the original model are retained. The daisy populations are initially restricted to hospitable regions of the surface but exert both global and local feedback to increase this habitable area, eventually colonizing the whole surface. The introduction of heat diffusion destabilizes the coexistence equilibrium of the two daisy types. In response, a striped pattern consisting of blocks of all black or all white daisies emerges. There are two mechanisms behind this pattern formation. Both are connected to the stability of the system and an overview of the mathematics involved is presented. Numerical experiments show that this pattern is globally determined. Perturbations in one region have an impact over the whole surface but the regulatory properties of the system are not compromised by transient perturbations. The relevance of these results to the Earth and the wider climate modelling field is discussed.     

Similarities and differences in the peering-jump behavior of three grasshopper species (Orthoptera: Caelifera)

The peering-jump behavior was studied for the common field grasshopper Chorthippus brunneus , the meadow grasshopper C. parallelus and the alpine grasshopper Miramella alpina (Orthoptera, Caelifera). It was found that immediately before jumping M. alpina executes primarily unilateral object-related peering movements, with approximately twice the amplitude and velocity of the predominantly bilateral object-related peering movements of the other two species. Whereas M. alpina almost always jumped toward the black stripes in the experimental arena, the other species jumped toward both the black stripes and the white spaces between them. All three species preferred the same pattern of black stripes, which permitted them to view one black stripe frontally, with an additional black stripe to the left and right, in the lateral visual field. The similarities and differences in the peering-jump behavior of the three grasshopper species is discussed with regard to visual perception (parallax cues) and environmental adaptation.     

Coexistence and emergent neutrality generate synchrony among competitors in fluctuating environments

Many competitive communities exhibit a puzzling amount of species diversity. In this study, we model a community of symmetric competitors in a fluctuating environment. We use biologically realistic temperature-dependent growth curves with a widely hypothesized trade-off between maximum growth and nice breadth to control the shapes of the curves of different species. We perform three analyses of the community dynamics to investigate the role of environmental fluctuations in community composition and species diversity. We initiate communities with equal abundances of all species and randomize the temperature fluctuations so that there is no correlation between species responses, only noise. We initiate single populations and allow other species to randomly invade the community. We also knock out extant species one by one from an established community and allow them to reinvade after the remaining species have adjusted. We find that competitors with sufficiently different temperature niches coexist via temporal niche differentiation. We also find long-term persistence of species that are very similar to a dominant competitor. This creates communities with species clumped along a temperature niche axis, with stable coexistence between groups and near neutrality within groups. The near neutrality results in interspecific synchrony within the groups, providing an explanation for the maintenance of high diversity in competitive communities where synchrony is commonly observed.     

Choice of hunting site as a consequence of experience in late-instar crab spiders

Earlier experiences may play an important role in the choice of hunting sites, but their effects on the foraging repertoire of most animals remain poorly understood. I tested the role of previous flower choices (hunting sites) by penultimate-instar female crab spiders Misumena vatia in making subsequent patch-choice decisions. M. vatia is a sit-and-wait predator, and the two flower species used, ox-eye daisy Chrysanthemum leucanthemum and common buttercup Ranunculus acris, are important hunting sites. Spiders with different immediate experience showed similar short-term (<1 day) giving-up times on the two flower species, independent of their previous substrate. However, four-fifths of the individuals that remained a day or longer tended to leave buttercups sooner than daisies, especially if they had previously occupied daisies. Thus they may directly assess the quality of a potential hunting site, perhaps in response to prey abundance, but previous experience may play a minor role as well. Of spiders that made several consecutive choices of hunting sites, those on daisies often confined these runs to daisies (one of two years); those on buttercups did not exhibit comparable fidelity. Spiders molting into the adult stage almost always subsequently chose the same flower species (either daisy or buttercup) as the one on which they molted. Thus, juvenile experiences may influence adults, the critical stage when virtually all of the spiders' reproductive resources are gathered, even if this resulted from imprinting on their molt sites rather than carrying information over the molt. Received: 26 December 1998 / Accepted: 21 April 1999     

Frequency distribution and environmental correlates of plumage polymorphism in the grey fantail Rhipidura fuliginosa

Abstract

The grey fantail (Rhipidura fuliginosa) in New Zealand displays a striking plumage polymorphism. Some individuals are coloured almost entirely black (the “black morph"), while other individuals sport a contrasting brown and white plumage (the “pied morph"). The adaptive significance of plumage polymorphism in this species is unknown. We mapped the relative distribution and frequency of each morph across the entire South Island range of the fantail, and correlated the frequency of the morphs with a variety of ecological variables. The black morph comprised <5% of individuals across the South Island and, contrary to previous observations, was least frequent at the southern extremes of its range. From historical records, the frequency of the black morph also appears to have declined, although we cannot rule out a bias in reporting rates of the black morph in the literature. The relative frequency of the two morphs was not related to vegetation type, annual rainfall, altitude, or mean annual temperature. Although we could not identify an environmental variable that might explain the distribution of the two morphs over the South Island, changes in the relative abundance of each morph suggest a dynamic process that warrants further long‐term study.     

Bayesian calibration of the Unified budburst model in six temperate tree species

Numerous phenology models developed to predict the budburst date of trees have been merged into one Unified model (Chuine, 2000, J. Theor. Biol. 207, 337–347). In this study, we tested a simplified version of the Unified model (Unichill model) on six woody species. Budburst and temperature data were available for five sites across Belgium from 1957 to 1995. We calibrated the Unichill model using a Bayesian calibration procedure, which reduced the uncertainty of the parameter coefficients and quantified the prediction uncertainty. The model performance differed among species. For two species (chestnut and black locust), the model showed good performance when tested against independent data not used for calibration. For the four other species (beech, oak, birch, ash), the model performed poorly. Model performance improved substantially for most species when using site-specific parameter coefficients instead of across-site parameter coefficients. This suggested that budburst is influenced by local environment and/or genetic differences among populations. Chestnut, black locust and birch were found to be temperature-driven species, and we therefore analyzed the sensitivity of budburst date to forcing temperature in those three species. Model results showed that budburst advanced with increasing temperature for 1–3 days °C⁻¹, which agreed with the observed trends. In synthesis, our results suggest that the Unichill model can be successfully applied to chestnut and black locust (with both across-site and site-specific calibration) and to birch (with site-specific calibration). For other species, temperature is not the only determinant of budburst and additional influencing factors will need to be included in the model.     

Re-examination of the roles of environmental factors in the control of body-color polyphenism in solitarious nymphs of the desert locust Schistocerca gregaria with special reference to substrate color and humidity

This study re-examines the effects of environmental factors including substrate color, humidity, food quality, light intensity and temperature on the green-brown polyphenism, black patterning and background body color of solitarious (isolated-reared) nymphs of Schistocerca gregaria. All individuals reared in yellow-green or yellow containers became green morphs, whereas those reared in white, ivory-colored, blue, grey, brown, zinc-colored and black containers produced brown morphs in similar proportions. The intensity of black patterns was negatively correlated with the brightness of the substrate color of the containers. Humidity, which previous studies claimed controls green-brown polyphenism in this species, exerted no significant influence on either the green-brown polyphenism or the black patterning. Food quality also had little effect on body color. High temperature tended to inhibit darkening. The background body color on the thorax was greatly influenced by the substrate color of rearing containers and a close correlation was found between these two variables, indicating that, in contrast to what has been suggested by others, this species exhibits homochromy to match the body color to the substrate color of its habitat. Similar responses were observed in another strain, although some quantitative differences occurred between the two strains examined. Based on these results, a new model explaining the control of body-color polyphenism in this locust is proposed and the ecological significance of black patterns in solitarious nymphs is discussed.     

Competition between unit-restricted fungi: a metapopulation model

We aimed to provide a theoretical framework for dynamic studies of competition between fungi living on divided and ephemeral resources. We previously adapted the seminal Skellam's patch-occupancy model (Skellam, 1951) to describe the population dynamics of one species of unit-restricted fungus whose mycelial growth occurs within resource units and which colonizes new resource units by spore dispersal (Gourbiere et al., 1999). In this study, we extended this model to describe the competition between a pair of unit-restricted fungal species that interact with each other inside units by decreasing their spore production. Accordingly, we designed a discrete-time metapopulation model where all patches go extinct at each generation and species interact by lowering their propagule production in jointly occupied patches. We showed that the two species easily coexist although there is no trade-off between their competitive and colonization abilities. Furthermore, the outcome of the competition process can depend on a founder effect. Founder effect determines either which species is excluded or the relative densities of each species when they coexist. We investigated the implications of these results on the distribution and abundance of fungal species along environmental gradients. This work bridges the gap between the mycological theory of "Resource Units" and the metapopulation theory, showing the specificity of fungal exploitation competition. We suggest that unit-restricted fungal species are appropriate biological models to test the theoretical results of the metapopulation theory, such as the appearance of alternative stable equilibria.     

Symbiotic physiology promotes homeostasis in Daisyworld

A connection is hypothesized between the physiological consequences of mutualistic symbiosis and life's average long-term impact on certain highly biologically conserved environmental variables. This hypothesis is developed analytically and with a variant of the Daisyworld model. Biological homeostasis is frequently effective due to co-ordination between opposing physiological “rein” functions, which buffer an organism in response to an external (often environmental) perturbation. It is proposed that during evolutionary history the pooling of different species' physiological functions in mutualistic symbioses increased the range of suboptimal environmental conditions that could be buffered against—a mutual tolerance benefit sometimes sufficient to outweigh the cost of cooperation. A related argument is that for a small number of biologically-crucial physical variables (i) the difference between organism interiors and the life-environment interface is relatively low, and (ii) the biologically optimum level of that variable is relatively highly conserved across different species. For such variables, symbiosis tends to cause (at a cost) an increase in the number of environmental buffering functions per unit of selection, which in turn biases the overall impact of the biota on the state of the variable towards the biological optimum. When a costly but more temperature-tolerant and physiologically versatile symbiosis between one black (warming) and one white (cooling) “daisy” is added to the (otherwise unaltered) Daisyworld parable, four new results emerge: (1) The extension of habitability to a wider luminosity range, (2) resistance to the impact of “cheater” white daisies with cold optima, that derive short-term benefit from environmental destabilisation, (3) the capacity to maintain residual, oscillatory regulation in response to forcings that change more rapidly than allele frequencies and (crucially) (4) “succession”-type dynamics in which the tolerant symbiosis colonises and to an extent makes habitable an otherwise lifeless environment, but is later displaced by free-living genotypes that have higher local fitness once conditions improve. The final result is arguably analogous to lichen colonisation of the Neoproterozoic land surface, followed by the Phanerozoic rise of vascular plants. Caution is necessary in extrapolating from the Daisyworld parable to real ecology/geochemistry, but sufficiently conserved variables may be water potential, macronutrient stoichiometry and (to a lesser extent) the temperature window for metabolic activity.     

Predicting coexistence of plants subject to a tolerance-competition trade-off

Ecological trade-offs between species are often invoked to explain species coexistence in ecological communities. However, few mathematical models have been proposed for which coexistence conditions can be characterized explicitly in terms of a trade-off. Here we present a model of a plant community which allows such a characterization. In the model plant species compete for sites where each site has a fixed stress condition. Species differ both in stress tolerance and competitive ability. Stress tolerance is quantified as the fraction of sites with stress conditions low enough to allow establishment. Competitive ability is quantified as the propensity to win the competition for empty sites. We derive the deterministic, discrete-time dynamical system for the species abundances. We prove the conditions under which plant species can coexist in a stable equilibrium. We show that the coexistence conditions can be characterized graphically, clearly illustrating the trade-off between stress tolerance and competitive ability. We compare our model with a recently proposed, continuous-time dynamical system for a tolerance-fecundity trade-off in plant communities, and we show that this model is a special case of the continuous-time version of our model.     

Efficiency of a New Covering System for the Environmental Control of Biopiles Used for the Treatment of Contaminated Soils

The objectives of the study were to first characterize a new covering system that allows the temperature inside the biopiles to be maintained at a level where biodeg-radation can take place despite unfavorable climatic conditions, and then second to develop a mathematical simulation of the biopile temperature profile knowing local meteorological conditions and the covering system used. A field study was undertaken with four 60?m³ biopiles of contaminated soil. The performance of conventional semipermeable black geotextile was compared with that of two sealed double polyethylene membrane systems (a white/white and a black/translucid polyethylene membrane). Heat transfer was favored or restricted by choosing the color of the polyethylene membranes and by the presence of an insulating air layer between the two polyethylene membranes. Results showed that the air layer allowed to increase soil temperature up to a range that could enhance biodegradation. For example, the biopile temperature was maintained above 10°C during fall conditions using the double membrane system, while it remained between of 5 to 10°C when using the conventional black geotextile. The white/white polyethylene membrane was considered to be the covering system offering the best performance because it allowed not only the temperature level to increase during fall conditions but also to reduce the temperature gradient within the biopile. A mathematical model describing the temperature profile within a biopile was developed, taking into account soil thermal properties, covering material properties, and local meteorological conditions. Close agreements between simulation results and actual measurements were found with maximum deviation within 2°C. This validated model can now be used to predict thermal profiles within biopiles without costly tests on site.     

Dispersal and the evolution of specialisation in a two-habitat type metapopulation

Metapopulation theory for the evolution of specialisation is virtually absent. In this article, therefore, we study a metapopulation model for consumers with a fitness trade-off between two habitats. We focus on effects of habitat abundance, dispersal rate and trade-off strength on the evolution of specialisation under two types of trade-off. Adaptation affects either the intrinsic growth rates r or the carrying capacities K. Depending on dispersal rate and trade-off strength, evolution can result in one generalist, one specialist or two specialist types. Higher dispersal rate and a weaker trade-off favour the evolution of a generalist, for both trade-off structures. However, we also find differences between the two trade-off structures. Our results are qualitatively similar to analyses of two-patch models, suggesting that insights from such simpler models can be extrapolated to metapopulation models. Additional effects, however, occur because in classical metapopulations patch lifetime depends on extinction rate. Counterintuitively, this favours the evolution of specialisation when the trade-off affects r.     

Whole-plant nitrogen- and water-relations traits, and their associated trade-offs, in adjacent muskeg and upland boreal spruce species

 Black and white spruce (Picea mariana and P. glauca) exhibit a striking micro-geographic distribution pattern at the southern edge of the boreal forest. Black spruce grows in flooded nutrient-poor muskegs, while white spruce is found primarily on drier upland sites, and the two rarely form mixed stands. In an attempt to characterize the physiological and, hence, mechanistic basis of this pattern, we sampled five adjacent populations of black and white spruce from northern British Columbia and measured a suite of physiological and allocative characteristics, and associated trade-offs, that may be important to survival in habitats limited in nutrient or water availability. Two laboratory experiments were conducted: a greenhouse dry-down experiment to assess relative degree of drought tolerance; and a 2×2 nested factorial experiment in which seedlings were subjected to varying water and nitrogen regimes for approximately 16 weeks. White spruce was more drought-tolerant (i.e., maintained positive net photosynthesis at lower shoot water potential) and more efficient in water-use (as indicated by carbon isotopic composition) than black spruce. Black spruce was found to be significantly less sensitive to nitrogen stress, exhibited greater plasticity in nitrogen-use efficiency (measured as the carbon-to-nitrogen ratio in total plant tissue), and had a greater specific N absorption rate under high-N conditions than white spruce. Trade-offs hypothesized to be associated with these nitrogen and water relations traits were examined, but few were confirmed. Water-use efficiency and nitrogen-use efficiency did not trade-off between species, but did trade-off plastically (i.e., across treatments) within species. When exposed to simultaneous limitations of N and water both species were forced to utilize each resource with suboptimal efficiency. The change in isotopic composition per unit change in C/N ratio was not the same in the two species. This difference may reflect optimization of the trade-off, whereby each species maximizes the use efficiency of the most limiting resource (respective to its habitat), while minimizing the concomitant reduction in the use efficiency of the other resource. Received: 10 June 1996 / Accepted: 8 October 1996     

Host‐parasitoid dynamics in periodic boreal moths

We analyse the population and spatial structures of coastal annual-plant communities, across ten dunes and three years, to explore the role of seed mass in structuring these communities. One suggestion is that annual-plant communities are structured by competition-colonization trade-offs driven by difference among species in seed-allocation strategies, while another perspective is that seed mass influences the ways in which species respond to environmental variation. In support of the competition-colonization trade-off, the two largest-seeded species found on the dunes ( Erodium cicutarium and Geranium molle ) were negatively associated with the other guild members at the 10-mm scale in 1995, suggesting they locally excluded smaller-seeded species in that year (when population densities were high). In support of the environmental response hypothesis, populations of annual plants declined between 1995 and 1996 on eight of the ten dunes, underscoring the importance of year-to-year environmental fluctuations in determining population sizes. The species that became relatively uncommon also became more aggregated in space, and this effect was most pronounced among the small-seeded species. Thus, small-seeded species may be forced to retreat into refuges when conditions are unfavourable, where reduced frequencies of interspecific contacts may increase their chances of persistence. We also show that small-seeded species sometimes reach much higher population densities than larger-seeded species, consistent with earlier findings, but reason that this abundance/seed mass relationship could have resulted from either a competition-colonization trade-off or from different responses of small- and large-seeded species to environmental variation. We conclude that dune-annual species with contrasting seed masses respond differently to environmental variation, while the competition-colonization trade-off plays a lesser role in community dynamics than previously considered.     

Mutation of albedo and growth response produces oscillations in a spatial Daisyworld

We extended a two-dimensional cellular automaton (CA) Daisyworld to include mutation of optimal growth temperature as well as mutation of albedo. Thus, the organisms (daisies) can adapt to prevailing environmental conditions or evolve to alter their environment. We find the resulting system oscillates with a period of hundreds of daisy generations. Weaker and less regular oscillations exist in previous daisyworld models, but they become much stronger and more regular here with mutation in the growth response. Despite the existence of a particular combination of mean albedo and optimum individual growth temperature which maximises growth, we find that this global state is unstable with respect to mutations which lower absolute growth rate, but increase marginal growth rate. The resulting system oscillates with a period that is found to decrease with increasing death rate, and to increase with increasing heat diffusion and heat capacity. We speculate that the origin of this oscillation is a Hopf bifurcation, previously predicted in a zero-dimensional system.     

Synergistic effects of direct and indirect defences on herbivore egg survival in a wild crucifer

Evolutionary theory of plant defences against herbivores predicts a trade-off between direct (anti-herbivore traits) and indirect defences (attraction of carnivores) when carnivore fitness is reduced. Such a trade-off is expected in plant species that kill herbivore eggs by exhibiting a hypersensitive response (HR)-like necrosis, which should then negatively affect carnivores. We used the black mustard (Brassica nigra) to investigate how this potentially lethal direct trait affects preferences and/or performances of specialist cabbage white butterflies (Pieris spp.), and their natural enemies, tiny egg parasitoid wasps (Trichogramma spp.). Both within and between black mustard populations, we observed variation in the expression of Pieris egg-induced HR. Butterfly eggs on plants with HR-like necrosis suffered lower hatching rates and higher parasitism than eggs that did not induce the trait. In addition, Trichogramma wasps were attracted to volatiles of egg-induced plants that also expressed HR, and this attraction depended on the Trichogramma strain used. Consequently, HR did not have a negative effect on egg parasitoid survival. We conclude that even within a system where plants deploy lethal direct defences, such defences may still act with indirect defences in a synergistic manner to reduce herbivore pressure.     

Morphological and genetic differentiation of the acorn barnacle Tetraclita squamosa (Crustacea, Cirripedia) in East Asia and description of a new species of Tetraclita

The common intertidal barnacle Tetraclita squamosa occurs in two morphologically and genetically distinct forms in East Asia. The north-western Pacific form (Japan, Okinawa and Taiwan) has green parietes and the tergo-scutal flaps are black without any patterns. The south China form (Xiamen, Hong Kong) also has green parietes but the tergo-scutal flaps are black with two white spots on the tergal and scutal margin. Compared to the NW Pacific form, the south China form has a beaked tergum, a sharper tergal spur and cirrus I lacks serrulate type setae that have four rows of setules. The two forms differ by 15–16% in COI divergence, which is comparable to values for other congeneric barnacle species. The 12S rRNA and ITS1 sequences are also distinct between the two forms. Our results support the conclusion that the two forms are genetically differentiated species. We describe the NW Pacific form as a new species, Tetraclita pacifica . We are treating the other species as Tetraclita squamosa based on the fact that Pilsbry, in 1916, redescribed T. squamosa squamosa using samples collected from south China and the Philippines. Further studies are needed to confirm the identity and geographical distribution of the 'widely distributed' T. squamosa in the Indo-West Pacific.