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.     

A fitness based analysis of Daisyworld

The Gaia hypothesis [Lovelock, J., Margulis, L., 1974. Atmospheric homeostasis: the Gaia hypothesis. Tellus 26, 1], that the earth functions as a self-regulating system, has never sat particularly comfortably with ideas in mainstream biology [Anon, 2002. In pursuit of arrogant simplicities. Nature 416, 247]. A lack of any clear role for evolution in the model has led to claims of teleology-that self-regulation emerges because it is pre-ordained to do so [Doolittle, W.F., 1981. Is nature really motherly? CoEvol. Q. 58-63; Dawkins, R., 1979. The Extended Phenotype. Oxford University Press, Oxford]. The Daisyworld parable [Watson, A.J., Lovelock, J.E., 1983. Biological homeostasis of the global environment--the parable of Daisyworld. Tellus B 35, 284], a simple mathematical illustration of Gaia, went some way to addressing these critiques but, despite recent success in incorporating natural selection [Stocker, S.,1995. Regarding mutations in Daisyworld models. J. Theor. Biol. 175, 495; Lenton, T.M., 1998. Gaia and natural selection. Nature 394, 439; Lenton, T.M., Lovelock, J.E., 2001. Daisyworld revisited: quantifying biological effects on planetary self-regulation. Tellus B 53, 288; Wood, A.J., Ackland, G.J., Lenton, T.M., 2006. Mutation of albedo and growth response leads to oscillations in a spatial Daisyworld. J. Theor. Biol. 242, 188], it remains a widely held view that the ideas are inconsistent with biological principles. We show that standard methodology from quantitative genetics can be used to predict the stationary states and dynamic behaviour of Daisyworlds. The system regulates its temperature due to the low-level evolutionary dynamics of competition between the thermally coupled daisies, no higher level principle is invoked. A reconciliation of Gaia with evolutionary theory may allow further development of evolutionary arguments for the existence of global self-regulatory systems.     

Maximization principles and daisyworld

We investigate whether the equilibrium time-averaged state of a self-organizing system with many internal degrees of freedom, 2D-daisyworld, can be described by optimizing a single quantity. Unlike physical systems where a principle of maximum energy production has been observed, daisyworld follows evolutionary dynamics rather than Hamiltonian dynamics. We find that this is sufficient to invalidate the maximum entropy production principle, finding instead a different principle, that the system self-organizes to a state which maximizes the amount of life.     

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.     

Environmental regulation can arise under minimal assumptions

Models that demonstrate environmental regulation as a consequence of organism and environment coupling all require a number of core assumptions. Many previous models, such as Daisyworld, require that certain environment-altering traits have a selective advantage when those traits also contribute towards global regulation. We present a model that results in the regulation of a global environmental resource through niche construction without employing this and other common assumptions. There is no predetermined environmental optimum towards which regulation should proceed assumed or coded into the model. Nevertheless, polymorphic stable states that resist perturbation emerge from the simulated co-evolution of organisms and environment. In any single simulation a series of different stable states are realised, punctuated by rapid transitions. Regulation is achieved through two main subpopulations that are adapted to slightly different resource values, which force the environmental resource in opposing directions. This maintains the resource within a comparatively narrow band over a wide range of external perturbations. Population driven oscillations in the resource appear to be instrumental in protecting the regulation against mutations that would otherwise destroy it. Sensitivity analysis shows that the regulation is robust to mutation and to a wide range of parameter settings. Given the minimal assumptions employed, the results could reveal a mechanism capable of environmental regulation through the by-products of organisms.     

基于系统动力学的雏菊世界模型气候控制敏感性分析

Lovelock于20世纪70年代提出了著名的盖娅假说,认为地球是一个由生物和非生物环境组成的“超级有机体”,有能力调节自身的气候和化学组成,使之适合于生物的生存.盖娅假说遭到了生物学家,尤其是新达尔文主义者的强烈批评,认为它是目的论;随后,Lovelock通过雏菊世界模型论证了该假说.模型中的黑雏菊和白雏菊通过反射较少或较多的太阳光来调节地球的温度.基于雏菊世界模型,通过假设不同的初始条件——不同初始面积分配和不同的太阳光反射率组合,运用系统动力学对雏菊世界的温度调节能力进行了敏感性分析.结果证明,既使初始条件有差异,雏菊世界仍能够通过自我调节机制调节系统稳定时的星球温度,达到最适合生物生存的温度.雏菊世界模型对于初始面积分配、光照反射率组合条件不敏感.最后,雏菊世界模型的优点在于系统分析思想和以及对地球系统的简化,但是随着人类活动对地球系统影响的日益显著,建议模型应包含更多的环境、生物变量以及反馈关系和人类活动因素,将盖娅系统融入到自然-人类耦合的大系统中.     

Daisyworld inhabited with daisies incorporating a seed size/number trade-off: the mechanism of negative feedback on selection from a standpoint of the competition theory

We reexamined a Daisyworld model from the traditional view of competition theory. Unlike the original model, white and black daisies in our model incorporate a seeding/germination trade-off against bare ground area without assuming the local temperature reward. As a result, the planetary temperature is automatically regulated by two species if the following conditions are met: (i) the species react equally to an environmental condition, but one can alter the environmental condition in the opposite direction to the other. (ii) that one of the two cannot have both a higher maximal growth rate (mu(max)) and lower half-saturation constant (K) than those of the other. In other words, a pair of phenotypes incorporates a trade-off between quality and number of seeds. We found that the homeostatic regulation can also be reconciled with the adaptive evolution of optimal temperature. The results of simulation imply that biotic environmental feedback can also be maintained when the emergence of polymorphisms (black and white daisies) is closely linked to such a trade-off.     

Scale dependence in desert plant diversity

The relationship between the number of species and the area sampled is one of the oldest and best-documented patterns in community ecology. An equation of the form S = cA ^z describing more precisely the species–area relation for plant species in smaller area is proposed as a result of intensive examination of species presence. Several study and field data from a wild range of plant and animal taxa suggest that the slope, z, of a graph of the logarithm of species richness against the logarithm of area is not a constant to the grassland or woodland community. We collected replicated and randomized plant data at 6 spatial scales from 1 m² to 1 km² in the desert region of northwest China to identify the scale dependence in desert plant biodiversity. The results showed that the slope of the log–log plot varied systematically with spatial scale. The value of z was high (0.37) at small scales from 1 to 10 m² and it decreased with increased spatial scale subsequently. When spatial scales varied from 1 m² to 1 km², the value of z varied from 0.37 to 0.035, suggesting that desert plant diversity has strongly scale-dependence at the small scales (less than 100 m²). The result is different from the research of grassland and woodland communities.     

The effect of <Emphasis Type="Italic">Cercidium praecox</Emphasis> and <Emphasis Type="Italic">Prosopis laevigata</Emphasis> on vertical distribution of soil free-living nematode communities in the Tehuacán Desert,Mexico

Vegetation cover is known to act as an abiotic mediator influencing the structure of soil fauna communities in arid and semi-arid ecosystems. The aim of the current research was to determine the spatial dispersion of the soil free-living nematode community under the canopy of Cercidium praecox and Prosopis laevigata during the rainy season. These shrubs are the dominant plant associations in the western part of the Tehuacán-Cuicatlán Valley in Mexico. Soil samples were taken from each 10-cm depth between 0 and 50 cm in August 2004. Our results demonstrated that the abundance and structure of the soil free-living nematode communities in the study area were strongly dependent on plant effects, specified by limited factors such as soil moisture and organic matter availability. The greatest degree of abundance of soil-free-living nematodes (88%) was found in the upper (0–10 cm) soil layer. Plant parasites were the most abundant trophic group under the two plants (58 and 36% under Parkinsonia (Cercidium) praecox and Prosopis laevigata, respectively), whereas omnivore-predators were the most dominant (96%) in inter-plant spaces. The fungivore/bacterivore (F/B) ratio was found to be the most useful tool of the ecological indices tested in the present study, reflecting the vertical distribution of the free-living nematode communities beneath different plant species in the different soil layers. The soil free-living nematode communities and their vertical distribution were found to be affected by plant ecophysiological adaptation, soil moisture, and the interaction between them.     

Leaf-twig carbon isotope ratio differences in photosynthetic-twig desert shrubs

Summary Carbon isotope ratios were determined for bulk tissues of both leaves and current season twigs of 29 species of Mohave Desert shrubs. Leaf and twig tissues were found to differ in their carbon isotope ratios, only in those species which had photosynthetic twigs. These data suggest that the twigs of these species operate at lower intercellular CO₂ values than leaves, an interpretation which is consistent with available gas-exchange data. An effect of microhabitat was also evident between the mean isotope ratios of leaves from wash versus slope habitats.This research has been supported by NSF Grant BSR-8410943 and by DOE Grant DE-FGO2-86ER60399.     

Clump spacing in a desert dwarf shrub community

A Monte Carlo method based on Ripley's K function-a cumulative function related to the number of plants encountered at different distances from other plants-is used to test the null hypothesis of random distribution of shrub clumps in a desert dwarf shrub community in Namaqualand, South Africa, where Psilocaulon arenosum is the dominant shrub. The method takes into account the apparent regularity of pattern caused by the finite size (up to 2 m diameter) of the clumps. It is shown that the clump centres are significantly aggregated (compared to random expectation) at distances on the order of 1 m. Such aggregation is expected, as a simple result of regeneration near to seed sources, if the time between catastrophic droughts is short in relation to the time required for development of a non-aggregated or regular pattern determined by moisture competition. No significant regulatiry was detected at distances of 3 m or less. One subplot showed regularity above 3 m, but this pattern was not shown by the other subplot and may not be a competition effect. These results support a hypothesis of aggregation caused by regeneration pattern decaying slowly toward randomness as larger individuals compete.We thank C. J. F. ter Braak for suggesting the method of spatial analysis; 1. Noy-Meir, C. J. F. ter Braak and two unnamed reviewers for helpful comments; the Netherlands Science Research Council (ZWO) for financial support to I. C. Prentice (grant B84-218); and Prof. D. von Willert for material support to M. J. A. Werger.     

Recovery of lichen-dominated soil crusts in a hyper-arid desert

Soil crust lichens can be the dominant vegetation in desert regions that are unsuitable for higher plants, and are vital to soil stabilization and primary production. Biological soil crusts are vulnerable to disturbance and there is little evidence of the lichen components achieving full recovery following human disturbances in semi-arid to arid environments, and no records of recovery in hyper-arid deserts. Eight sites with varying anthropogenic, mechanical disturbance regimes were assessed in the hyper-arid Namib Desert for levels of recovery and successional convergence, based on a comparative analysis of overall lichen cover and community composition in disturbed and control locations. Recovery time estimations ranged from 5 to 530 years, with no detected linear relationship to impact gradient (low to high impact). Variables that were found to most strongly influence recovery rates were the overall cover of lichen growth and total number of lichen species in the bordering undisturbed areas, followed by the extent of soil compaction in the disturbed area, altered soil surface microrelief and vitality of subsurface soil crust components. An assessment of pioneering species demonstrated a link between increased soil depressions, i.e. track ruts, and the occurrence of fragmenting, wind-dispersing species. Track ruts in hype-arid deserts are not as vulnerable to the water erosion found in less arid deserts, and may be advancing recovery by trapping fragments. However, the lichen community structure was significantly different between all of the disturbed and control areas, regardless of the recovery phase, suggesting that while the lichen community composition may not. The ecological consequences of such disturbances may be far reaching in hyper-arid deserts where lichens are primary heterotrophs soil stabilizers. Given the economic development occurring within coastal hyper-acid deserts of the world, these impacts undoubtedly call for conservation attention.     

毛乌素沙地的生态背景及其草地建设的原则与优化模式

 本文全面分析了毛乌素沙地生态过渡带的自然环境、植被的现状和历史变迁过程,以定性和定量相结合的方法阐述了环境和植被之间的动态关系,在此基础上提出了该地区草地建设的原则及优化生态模式,其中包括：水分平衡原则,半固定沙丘持续发展原则,网带状种植原则,滩地草、农、林复合系统模式、软梁半人工草地复合系统模式和硬梁天然草地放牧系统模式。     

Differential utilization of summer rains by desert plants

Summary Seasonal changes in the hydrogen isotope ratios of xylem waters were measured to determine water sources used for growth in desert plants of southern Utah. While all species used winter-spring recharge precipitation for spring growth, utilization of summer rains was life-form dependent. Annuals and succulent perennials exhibited a complete dependence on summer precipitation. Herbaceous and woody perennial species simultaneously utilized both summer precipitation and remaining winter-spring precipitation, with herbaceous species much more reliant on the summer precipitation component. Several of the woody perennials exhibited no response to summer precipitation. Currently, precipitation in southern Utah is evenly partitioned between winter and summer time periods; however, global circulation models predict that summer precipitation will increase in response to anticipated climate change. Our data indicate that components within the community will differentially responde to the change in precipitation patterns. These results are discussed in relation to impact on competition and possible changes in community structure.     

Decomposition rate of ocotillo (Fouquieria splendens) wood in the desert of southern California and its use in estimating adult survival by life-cycle graph analysis

Dead and downed ocotillos (Fouquieria splendens) are common in the Colorado Desert of southern California and pose a question concerning the life-span of adult plants that have been reported to be long-lived: if ocotillo is long-lived, decay rates must be very slow. Our study focused on decomposition and disintegration of above-ground ocotillo wood over 14 years to obtain a rate of change in dead mass. Disintegration rate was used in a new application of life-cycle graph analysis to calculate persistence time of carcasses, which in turn was used to estimate adult survival rates. We conclude that previous estimates of life spans may be too conservative and half-life of adult shrubs is over 200 years. Our approach to estimating adult survival from decay rates provides an independent test of survival transitions based on live plants for some species and so is an additional tool for developing life cycle models.     

Molecular diversity and allergenic profiles of Alternaria spp. from desert environments in Arizona

This study examined the genetic diversity of small-spored Alternaria species in the southwest desert of the USA by sampling 552 isolates from different habitats (soil and plant debris) in different locations (urban and an undisturbed desert). To estimate the genetic diversity, Amplified Fragment Length Polymorphism (AFLP) fingerprinting analysis was performed for all isolates. Strains representative of the sampled genotypic diversity (n = 125) were further characterized according their sporulation pattern and the capability to produce allergens. Morphological characterization assigned the majority of the strains to the Alternaria alternata and Alternaria tenuissima morpho-groups with only two isolates assigned to the Alternaria arborescens morpho-group. AFLP fingerprinting differentiated the A. arborescens morpho-groups, but could not distinguish between the A. alternata and A. tenuissima morpho-groups. Western blot analysis showed that a large number of allergenic proteins were produced by strains. These proteins were not specific for any morpho-group nor source of isolation. A hierarchical analysis of molecular variance was performed on the AFLP data to quantify molecular variation and partition this variation among sampled locations and habitat. No statistically significant differentiation among locations and habitat was detected indicating a lack of population structure across environments.     

Solitary male chacma baboons in a desert canyon

Solitary and paired adult (nine) and subadult (one) male chacma baboons, Papio ursinus, were observed over a period of years living in part of a wooded desert canyon not used by adjacent troops. These extratroop males were silent when alone and gave only one alarm vocalization, the “wa-hoo” call, when paired. The space occupied by them is unsuitable for use by troops according to criteria for adequate sleeping sites and access to water. But the foods available to them, especially figs, but also other fruits and fresh acacia seeds, were abundant. These foods are more highly preferred by baboons than those foods available to troop members. Troop members deplete these resources and shift to less preferred foods with lower water content and longer processing times. All of the adult members of the troop adjacent to these isolated males were infected with a skin disease. Isolated males were not so afflicted and so cannot have originated from, or ever been a part of, this troop. They probably moved to the space where they were observed from other inland troops, traveling to their current home range along the narrow canyon river course.     

小毛足鼠繁殖活动的季节变化

有关小毛足鼠 (Phodopusroborovskii)冬季活动和季节性繁殖模式的资料很少。九年来 ,我们在自然温度和光照条件下维持了一个实验室种群。这些自然条件说明其繁殖活动与野生条件下的很一致。甚至在零下 4 0度的冬季 ,罗氏仓鼠都不休眠。第一个繁殖高峰 (每 10对鼠繁殖 2 4窝 ,每窝平均 3 5只幼鼠 )在 4月份 ,第二个繁殖高峰 (每 10对鼠繁殖 2 9- 2 6窝 ,每窝平均 3 9- 3 6只幼鼠 )在 6 - 7月份。在秋季和初冬 ,繁殖强度降低到每 10对鼠产 0 1- 0 4窝 ,但繁殖并不完全终止。全年内 ,所有成年雄鼠可见下降的睾丸 ,雌鼠可见开放的阴道口。雄鼠的血液睾酮浓度在夏季最高 ,秋季和初冬降低。但是 ,独居鼠在最短的光周期和寒冷季节保持高的睾酮浓度。春季 ,睾酮平均水平显著升高 ,但未达到夏季的水平     

Preliminary assessment of plant community structure and arbuscular mycorrhizas in rangeland habitats of Cholistan desert, Pakistan

Plant species dominance and arbuscular mycorrhizas (AMs) were evaluated in four different habitats of Cholistan desert, Pakistan. The sites were selected on the basis of variations in topography, floristic composition and biotic interference. The phytosociological data revealed obvious differences in the floristic composition, plant species diversity, AM colonization and arbuscular mycorrhizal fungi (AMF) spore densities in the rhizospheres. The vegetation pattern and composition was governed by topography and edaphic characteristics of a particular site. Grasses were dominant and main components of vegetation at all the study sites and were invariably mycorrhizal. Some plant species were mycorrhizal at one site but non-mycorrhizal at the other. The AMF survey reported here offers an important starting point from which to analyse AMF community structure in different phytosociological habitats and land uses of Cholistan desert.