Jane Molofsky

Empirical and theoretical challenges in aboveground–belowground ecology

A growing body of evidence shows that aboveground and belowground communities and processes are intrinsically linked, and that feedbacks between these subsystems have important implications for community structure and ecosystem functioning. Almost all studies on this topic have been carried out from an empirical perspective and in specific ecological settings or contexts. Belowground interactions operate at different spatial and temporal scales. Due to the relatively low mobility and high survival of organisms in the soil, plants have longer lasting legacy effects belowground than aboveground. Our current challenge is to understand how aboveground–belowground biotic interactions operate across spatial and temporal scales, and how they depend on, as well as influence, the abiotic environment. Because empirical capacities are too limited to explore all possible combinations of interactions and environmental settings, we explore where and how they can be supported by theoretical approaches to develop testable predictions and to generalise empirical results. We review four key areas where a combined aboveground–belowground approach offers perspectives for enhancing ecological understanding, namely succession, agro-ecosystems, biological invasions and global change impacts on ecosystems. In plant succession, differences in scales between aboveground and belowground biota, as well as between species interactions and ecosystem processes, have important implications for the rate and direction of community change. Aboveground as well as belowground interactions either enhance or reduce rates of plant species replacement. Moreover, the outcomes of the interactions depend on abiotic conditions and plant life history characteristics, which may vary with successional position. We exemplify where translation of the current conceptual succession models into more predictive models can help targeting empirical studies and generalising their results. Then, we discuss how understanding succession may help to enhance managing arable crops, grasslands and invasive plants, as well as provide insights into the effects of global change on community re-organisation and ecosystem processes.

Population Dynamics and Pattern Formation in Theoretical Populations

Simple individual-based models of population dynamics and spatial patterns are developed using cellular automata theory. These models assume that individuals within a population are distributed in space and that important interactions among individuals take place over some predefined local scale. Local density dependence in the model produces dynamics that depend on the scale of the dispersal distance. When dispersal occurs over long distances, the population enters a stable limit cycle. However, when colonization is limited to the same local neighborhood where competition is taking place, the spatial and temporal patterns become chaotic. Habitat size also affects the dynamics. Chaotic dynamics become cyclical as habitat size decreased. These results illustrate how a wide array of complex dynamics can arise with simple spatially distributed models and that the resulting dynamics can depend on the habitat size.

Coexistence under positive frequency dependence.

Negative frequency dependence resulting from interspecific interactions is considered a driving force in allowing the coexistence of competitors. While interactions between species and genotypes can also result in positive frequency dependence, positive frequency dependence has usually been credited with hastening the extinction of rare types and is not thought to contribute to coexistence. In the present paper, we develop a stochastic cellular automata model that allows us to vary the scale of frequency dependence and the scale of dispersal. The results of this model indicate that positive frequency dependence will allow the coexistence of two species at a greater rate than would be expected from chance. This coexistence arises from the generation of banding patterns that will be stable over long time–periods. As a result, we found that positive frequency–dependent interactions over local spatial scales promote coexistence over neutral interactions. This result was robust to variation in boundary conditions within the simulation and to variation in levels of disturbance. Under all conditions, coexistence is enhanced as the strength of positive frequency–dependent interactions is increased.

A novel theory to explain species diversity in landscapes: positive frequency dependence and habitat suitability

Theories to explain the diversity of species have required that individual species occupy unique niches and/or vary in their response to environmental factors. Positive interactions within a species, although common in communities, have not been thought to maintain species diversity because in non–spatial models the more abundant species always outcompetes the rarer species. Here, we show, using a stochastic spatial model, that positive intraspecific interactions such as those caused by positive frequency dependence and/or priority effects, can maintain species diversity if interactions between individuals are primarily local and the habitat contains areas that cannot be colonized by any species, such as boulders or other physical obstructions. When intraspecific interactions are primarily neutral, species diversity will eventually erode to a single species. When the landscape is homogeneous (i.e. does not contain areas that cannot be colonized by any species), the presence of strong intraspecific interactions will not maintain diversity.

NEGATIVE FREQUENCY DEPENDENCE AND THE IMPORTANCE OF SPATIAL SCALE

Issues of spatial scale are inherent in many ecological systems. This study uses a spatially explicit cellular automaton model to explore how the scale of dispersal interacts with the scale and strength of negative frequency dependence to determine patterns of species distribution. Counter to expectation, strong local frequency-dependent interactions result in random spatial patterns. When dispersal scale and interaction scale are decoupled, the resulting patterns are not necessarily random. For strong negative frequency dependence, stable bands result when the scale of interaction exceeds the scale of dispersal, and bands with two-point cycles result when the scale of dispersal exceeds the scale of interaction. However, for weaker interactions occurring over intermediate scales, only random patterns result. Thus, our results call into question the utility of inferring any ecological interaction from only the spatial distributions of the putatively interacting species. Furthermore, our results call for new experimental studies that explicitly manipulate the strength and the scale of the processes being studied.

A New Kind of Ecology

Abstract Mathematical models have a long history of use for understanding ecological systems. In a recent book, A New Kind of Science, Steve Wolfram calls into question traditional modeling approaches and calls for the increased application of cellular automata (CA) models in all areas of scientific inquiry. With reference to Wolframs book, we review the past uses of CA models in ecology and discuss the relative utility of using traditional models versus CA models to understand ecological communities.

Control Strategies for the Invasive Reed Canarygrass (Phalaris arundinacea L.) in North American Wetlands: the Need for an Integrated Management Plan

Abstract Phalaris arundinacea L. (reed canarygrass) is one of the most noxious invasive species in North American wetlands, rivers, and lakes. As is true for many invasive species, detailed research may give insights into the ecological and evolutionary factors that promote reed canarygrass invasion. However, important insights into control strategies of reed canarygrass may be gleaned from a synthesis of all the relevant ecological and management studies. We assessed the control strategies previously applied to contain reed canarygrass invasions, the potential for new promising strategies, and the research that is still needed to improve its control in North America. We showed that no one method is sufficient, and that the most successful strategies require both physical and chemical methods, coupled with hydrological management. Moreover, subsequent restoration of the community structure and composition is needed to limit new infestations of reed canarygrass or other invaders. Biological control has not been developed yet for reed canarygrass. Finally, the current knowledge of ecological factors that enhance reed canarygrass invasion suggests that any attempt to eradicate it and limit its spread will be jeopardized if an integrated pest management strategy is not undertaken. Given the high sensitivity of wetlands to plant invasion, management of invasive species must switch from isolated efforts of stand eradication to a landscape approach, emphasizing infestation prevention and accounting for surrounding human activities and the socio-economic context.

Genetic variation in photosynthetic characteristics among invasive and native populations of reed canarygrass (Phalaris arundinacea)

With the extensive spread of invasive species throughout North America and Europe there is an urgent need to better understand the morphological and physiological characteristics of successful invasive plants and the evolutionary mechanisms that allow introduced species to become invasive. Most ecological studies have focused on morphological differences and changes in community dynamics, and physiological studies have typically explored the differences between native and invasive species. In this study, 15 different genotypes of Phalaris arundinacea from both its native (European) and invasive (North American) range were grown in a common garden experiment to monitor the physiological differences between native and invasive genotypes. Here we present data that suggests high variability exists in the physiological traits among genotypes of P. arundinacea, yet genotypes from the native range are not necessarily physiologically inferior to the hybridized invasive genotypes. Previous work has shown that multiple introductions of P. arundinacea from various European locations to the United States resulted in numerous hybridization events, yielding more genetic variability and phenotypic plasticity in the invasive range. Of the genotypes studied, both morphological and physiological traits of genotypes with French origin were significantly different from the plants from the Czech Republic, North Carolina, and Vermont. The lack of clear differences between native and invasive genotypes indicates that physiological traits may be highly conserved in P. arundinacea and enhanced photosynthetic rates are not indicative of successful invasive genotypes. Instead, morphological traits and defensive secondary compound metabolism may play a more important role in the success of P. arundinacea within its invasive range, and patterns of genetic variation in physiological traits between invasive and native range may be more important than the mean traits of each region when explaining reed canarygrass’ invasive potential in North America.

The ubiquity of phenotypic plasticity in plants: a synthesis.

Adaptation to heterogeneous environments can occur via phenotypic plasticity, but how often this occurs is unknown. Reciprocal transplant studies provide a rich dataset to address this issue in plant populations because they allow for a determination of the prevalence of plastic versus canalized responses. From 31 reciprocal transplant studies, we quantified the frequency of five possible evolutionary patterns: (1) canalized response–no differentiation: no plasticity, the mean phenotypes of the populations are not different; (2) canalized response–population differentiation: no plasticity, the mean phenotypes of the populations are different; (3) perfect adaptive plasticity: plastic responses with similar reaction norms between populations; (4) adaptive plasticity: plastic responses with parallel, but not congruent reaction norms between populations; and (5) nonadaptive plasticity: plastic responses with differences in the slope of the reaction norms. The analysis included 362 records: 50.8% life-history traits, 43.6% morphological traits, and 5.5% physiological traits. Across all traits, 52% of the trait records were not plastic, and either showed no difference in means across sites (17%) or differed among sites (83%). Among the 48% of trait records that showed some sort of plasticity, 49.4% showed perfect adaptive plasticity, 19.5% adaptive plasticity, and 31% nonadaptive plasticity. These results suggest that canalized responses are more common than adaptive plasticity as an evolutionary response to environmental heterogeneity.

Comparing the genetic architecture and potential response to selection of invasive and native populations of reed canary grass

Evolutionary processes such as migration, genetic drift, and natural selection are thought to play a prominent role in species invasions into novel environments. However, few empirical studies have explored the mechanistic basis of invasion in an evolutionary framework. One promising tool for inferring evolutionarily important changes in introduced populations is the genetic variance–covariance matrix (G matrix). G matrix comparisons allow for the inference of changes in the genetic architecture of introduced populations relative to their native counterparts that may facilitate invasion. Here, we compare the G matrices of reed canary grass (Phalaris arundinacea L.) populations across native and invasive ranges, and between populations along a latitudinal gradient within each range. We find that the major differences in genetic architecture occur between populations at the Northern and Southern margins within each range, not between native and invasive populations. Previous studies have found that multiple introductions in introduced populations caused an increase in genetic variance on which selection could act. In addition, we find that differences in the evolutionary potential of Phalaris populations are driven by differences in latitude, suggesting that selection also shapes the evolutionary trajectory of invasive populations.