Scaling up population dynamics: integrating theory and data

How to scale up from local-scale interactions to regional-scale dynamics is a critical issue in field ecology. We show how to implement a systematic approach to the problem of scaling up, using scale transition theory. Scale transition theory shows that dynamics on larger spatial scales differ from predictions based on the local dynamics alone because of an interaction between local-scale nonlinear dynamics and spatial variation in density or the environment. Based on this theory, a systematic approach to scaling up has four steps: (1) derive a model to translate the effects of local dynamics to the regional scale, and to identify key interactions between nonlinearity and spatial variation, (2) measure local-scale model parameters to determine nonlinearities at local scales, (3) measure spatial variation, and (4) combine nonlinearity and variation measures to obtain the scale transition. We illustrate the approach, with an example from benthic stream ecology of caddisflies living in riffles. By sampling from a simulated system, we show how collecting the appropriate data at local (riffle) scales to measure nonlinearities, combined with measures of spatial variation, leads to the correct inference for dynamics at the larger scale of the stream. The approach provides a way to investigate the mechanisms and consequences of changes in population dynamics with spatial scale using a relatively small amount of field data.     

Costs of phenotypic plasticity

Phenotypically plastic organisms display alternative phenotypes in different environments. It is widely appreciated that possessing alternative phenotypes can affect fitness. However, some investigators have suggested that simply carrying the ability to be plastic could also affect fitness. Evolutionary models suggest that high costs of plasticity could constrain the evolution of optimal phenotypes. However, costs (and limits) of plasticity are primarily hypothetical. Little empirical evidence exists to show that increased plasticity leads to reduced growth and development, leads to increased developmental instability, or limits the ability of organisms to produce more extreme phenotypes. I used half-sib families of larval wood frogs (Rana sylvatica) reared in outdoor mesocosms to examine how tadpoles altered behavioral, morphological, and life-historical traits in response to larval dragonfly predators (Anax longipes). The predators induced lower activity and the development of relatively large tails and small bodies in wood frogs. As a result, wood frogs experienced reduced growth and development. I then examined whether tadpole sibships with higher plasticity experienced fitness costs (above and beyond the costs of expressing a particular phenotype) and whether they were limited in producing extreme phenotypes. Fitness effects of plasticity were widespread. Depending on the trait examined and the environment experienced, increased plasticity had either positive effects, negative effects, or no effects on tadpole mass, development, and survivorship. I found no relationship between increased plasticity and greater developmental instability. There was also no evidence that sibships with increased plasticity produced less extreme phenotypes; the most extreme trait states were always produced by the most plastic genotypes. This work suggests that costs of plasticity may be pervasive in nature and may substantially impact the evolution of optimal phenotypes in organisms that live in heterogeneous environments.     

Role of phenotypic plasticity and population differentiation in adaptation to novel environmental conditions

Species can adapt to new environmental conditions either through individual phenotypic plasticity, intraspecific genetic differentiation in adaptive traits, or both. Wild emmer wheat, Triticum dicoccoides, an annual grass with major distribution in Eastern Mediterranean region, is predicted to experience in the near future, as a result of global climate change, conditions more arid than in any part of the current species distribution. To understand the role of the above two means of adaptation, and the effect of population range position, we analyzed reaction norms, extent of plasticity, and phenotypic selection across two experimental environments of high and low water availability in two core and two peripheral populations of this species. We studied 12 quantitative traits, but focused primarily on the onset of reproduction and maternal investment, which are traits that are closely related to fitness and presumably involved in local adaptation in the studied species. We hypothesized that the population showing superior performance under novel environmental conditions will either be genetically differentiated in quantitative traits or exhibit higher phenotypic plasticity than the less successful populations. We found the core population K to be the most plastic in all three trait categories (phenology, reproductive traits, and fitness) and most successful among populations studied, in both experimental environments; at the same time, the core K population was clearly genetically differentiated from the two edge populations. Our results suggest that (1) two means of successful adaptation to new environmental conditions, phenotypic plasticity and adaptive genetic differentiation, are not mutually exclusive ways of achieving high adaptive ability; and (2) colonists from some core populations can be more successful in establishing beyond the current species range than colonists from the range extreme periphery with conditions seemingly closest to those in the new environment.     

Ecological consequences of phenotypic plasticity

Phenotypic plasticity is widespread in nature, and often involves ecologically relevant behavioral, physiological, morphological and life-historical traits. As a result, plasticity alters numerous interactions between organisms and their abiotic and biotic environments. Although much work on plasticity has focused on its patterns of expression and evolution, researchers are increasingly interested in understanding how plasticity can affect ecological patterns and processes at various levels. Here, we highlight an expanding body of work that examines how plasticity can affect all levels of ecological organization through effects on demographic parameters, direct and indirect species interactions, such as competition, predation, and coexistence, and ultimately carbon and nutrient cycles.     

Evolutionary role of phenotypic plasticity

Phenotypic plasticity, i.e., the ability of a genotype to produce various phenotypes in response to changes in the environment, plays an important, although poorly understood and often underestimated, role in evolution. Both adaptive and nonadaptive phenotypic plasticity modulate the strength and direction of selection acting on a population and can, depending on conditions, either accelerate or inhibit adaptation, divergence, and speciation. Phenotypic plasticity also affects the direction of evolutionary change, which can either coincide with the direction of plastic changes (genetic assimilation) or be the opposite (genetic compensation). A special case of phenotypic plasticity is phenotypic change of the host caused by changes in its symbiotic microbiota. In the current review, we discuss the main forms of phenotypic plasticity and the current data on their impact on the rate and direction of evolutionary change. Special attention is paid to the results of recent experimental work, including the long-term evolutionary experiment on Drosophila melanogaster, which is being held at the Department of Evolutionary Biology, School of Biology, Moscow State University.     

Developmental causes of allometry: new models and implications for phenotypic plasticity and evolution

Shapes change during development because tissues, organs, and various anatomical features differ in onset, rate, and duration of growth. Allometry is the study of the consequences of differences in the growth of body parts on morphology, although the field of allometry has been surprisingly little concerned with understanding the causes of differential growth. The power-law equation y?=?ax(b), commonly used to describe allometries, is fundamentally an empirical equation whose biological foundation has been little studied. Huxley showed that the power-law equation can be derived if one assumes that body parts grow with exponential kinetics, for exactly the same amount of time. In life, however, the growth of body parts is almost always sigmoidal, and few, if any, grow for exactly the same amount of time during ontogeny. Here, we explore the shapes of allometries that result from real growth patterns and analyze them with new allometric equations derived from sigmoidal growth kinetics. We use an extensive ontogenetic dataset of the growth of internal organs in the rat from birth to adulthood, and show that they grow with Gompertz sigmoid kinetics. Gompertz growth parameters of body and internal organs accurately predict the shapes of their allometries, and that nonlinear regression on allometric data can accurately estimate the underlying kinetics of growth. We also use these data to discuss the developmental relationship between static and ontogenetic allometries. We show that small changes in growth kinetics can produce large and apparently qualitatively different allometries. Large evolutionary changes in allometry can be produced by small and simple changes in growth kinetics, and we show how understanding the development of traits can greatly simplify the interpretation of how they evolved.     

Costs and limits of phenotypic plasticity

The costs and limits of phenotypic plasticity are thought to have important ecological and evolutionary consequences, yet they are not as well understood as the benefits of plasticity. At least nine ideas exist regarding how plasticity may be costly or limited, but these have rarely been discussed together. The most commonly discussed cost is that of maintaining the sensory and regulatory machinery needed for plasticity, which may require energy and material expenses. A frequently considered limit to the benefit of plasticity is that the environmental cues guiding plastic development can be unreliable. Such costs and limits have recently been included in theoretical models and, perhaps more importantly, relevant empirical studies now have emerged. Despite the current interest in costs and limits of plasticity, several lines of reasoning suggest that they might be difficult to demonstrate.     

Promising directions in plant phenotypic plasticity

A research agenda for the next phase of plasticity studies calls for contributions from a diverse group of biologists, working both independently and collaboratively, to pursue four promising directions: examining dynamic, anatomical/architectural, and cross-generational plasticity along with simpler growth traits; carefully assessing the adaptive significance of those plasticity patterns; investigating the intricate transduction pathways that lead from environmental signal to phenotypic response; and considering the rich environmental context of natural systems. Progress in these areas will allow us to address broad and timely questions regarding the ecological and evolutionary significance of plasticity and the nature of phenotypic determination.     

Endocrine regulation of predator-induced phenotypic plasticity

Elucidating the developmental and genetic control of phenotypic plasticity remains a central agenda in evolutionary ecology. Here, we investigate the physiological regulation of phenotypic plasticity induced by another organism, specifically predator-induced phenotypic plasticity in the model ecological and evolutionary organism Daphnia pulex. Our research centres on using molecular tools to test among alternative mechanisms of developmental control tied to hormone titres, receptors and their timing in the life cycle. First, we synthesize detail about predator-induced defenses and the physiological regulation of arthropod somatic growth and morphology, leading to a clear prediction that morphological defences are regulated by juvenile hormone and life-history plasticity by ecdysone and juvenile hormone. We then show how a small network of genes can differentiate phenotype expression between the two primary developmental control pathways in arthropods: juvenoid and ecdysteroid hormone signalling. Then, by applying an experimental gradient of predation risk, we show dose-dependent gene expression linking predator-induced plasticity to the juvenoid hormone pathway. Our data support three conclusions: (1) the juvenoid signalling pathway regulates predator-induced phenotypic plasticity; (2) the hormone titre (ligand), rather than receptor, regulates predator-induced developmental plasticity; (3) evolution has favoured the harnessing of a major, highly conserved endocrine pathway in arthropod development to regulate the response to cues about changing environments (risk) from another organism (predator).     

Origins of differentiation via phenotypic plasticity

How cell types of multicellular organisms came to be differentiated is still an open issue. Here I offer a model that posits that the origins of some cell differentiation patterns were originally passive outcomes of environmental effects. As cells' contact with the external environment was diminished, their patterns of gene expression were altered, due to changes in concentrations of externally supplied substances. Later, as multicellular growth continued, the relationships of cell layers to each other shifted, producing concentration gradients of signaling molecules. These gradients emanated both from the external cell layer toward the inside and from internal cell layers to adjacent layers. In this scenario then, differentiation arose initially as a by-product of the changing patterns of gene expression and of the complex mixtures and changing concentrations of substances passing among layers. Subsequent selection would operate to stabilize the expression patterns in those cell layers whose phenotypes provide a fitness advantage to the organism.     

Ecological limits to plant phenotypic plasticity

Phenotypic plasticity is considered the major means by which plants cope with environmental heterogeneity. Although ubiquitous in nature, actual phenotypic plasticity is far from being maximal. This has been explained by the existence of internal limits to its expression. However, phenotypic plasticity takes place within an ecological context and plants are generally exposed to multifactor environments and to simultaneous interactions with many species. These external, ecological factors may limit phenotypic plasticity or curtail its adaptive value, but seldom have they been considered because limits to plasticity have typically addressed factors internal to the plant. We show that plastic responses to abiotic factors are reduced under situations of conservative resource use in stressful and unpredictable habitats, and that extreme levels in a given abiotic factor can negatively influence plastic responses to another factor. We illustrate how herbivory may limit plant phenotypic plasticity because damaged plants can only rarely attain the optimal phenotype in the challenging environment. Finally, it is examined how phenotypic changes involved in trait-mediated interactions can entail costs for the plant in further interactions with other species in the community. Ecological limits to plasticity must be included in any realistic approach to understand the evolution of plasticity in complex environments and to predict plant responses to global change.     

The ecological effect of phenotypic plasticity — Analyzing complex interaction networks (COIN) with agent-based models

Analyzing complex dynamics of ecological systems is complicated by two important facts: First, phenotypic plasticity allows individual organisms to adapt their reaction norms in terms of morphology, anatomy, physiology and behavior to changing local environmental conditions and trophic relationships. Secondly, individual reactions and ecological dynamics are often determined by indirect interactions through reaction chains and networks involving feedback processes.

We present an agent-based modeling framework which allows to represent and analyze ecological systems that include phenotypic changes in individual performances and indirect interactions within heterogeneous and temporal changing environments. We denote this structure of interacting components as COmplex Interaction Network (COIN).

Three examples illustrate the potential of the system to analyze complex ecological processes that incorporate changing phenotypes on the individual level:

• A model on fish population dynamics of roach (Rutilus rutilus) leads to a differentiation in fish length resulting in a conspicuous distribution that influences reproduction capability and thus indirectly the fitness.

• Modeling the reproduction phase of the passerine bird Erithacus rubecula (European Robin) illustrates variation in the behavior of higher organisms in dependence of environmental factors. Changes in reproduction success and in the proportion of different activities are the results.

• The morphological reaction of plants to changes in fundamental environmental parameters is illustrated by the black alder (Alnus glutinosa) model. Specification of physiological processes and the interaction structure on the level of modules allow to represent the reaction to changes in irradiance and temperature accurately.

Applying the COIN-approach, individual plasticity emerges as a structural and functional implication in a self-organized manner. The examples illustrate the potential to integrate existing approaches to represent detailed and complex traits for higher order organisms and to combine ecological and evolutionary aspects.

Scaling up of renewable chemicals

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