Matrix projection models are among the most widely used tools in plant ecology. However, the way in which plant ecologists use and interpret these models differs from the way in which they are presented in the broader academic literature. In contrast to calls from earlier reviews, most studies of plant populations are based on < 5 matrices and present simple metrics such as deterministic population growth rates. However, plant ecologists also cautioned against literal interpretation of model predictions. Although academic studies have emphasized testing quantitative model predictions, such forecasts are not the way in which plant ecologists find matrix models to be most useful. Improving forecasting ability would necessitate increased model complexity and longer studies. Therefore, in addition to longer term studies with better links to environmental drivers, priorities for research include critically evaluating relative ⁄ comparative uses of matrix models and asking how we can use many short-term studies to understand long-term population dynamics.

Abstract:Predicting the speed of biological invasions and native species migrations requires understanding the ecological and evolutionary dynamics of spreading populations. Theory predicts that evolution can accelerate species' spread velocity, but how landscape patchiness, an important control over traits under selection, influences this process is unknown. We manipulated the response to selection in populations of a model plant species spreading through replicated experimental landscapes of varying patchiness. After six generations of change, evolving populations spread 11% further than non-evolving populations in continuously favorable landscapes, and 200% further in the most fragmented landscapes. The greater effect of evolution on spread in patchier landscapes was consistent with the evolution of dispersal and competitive ability. Accounting for evolutionary change may be critical when predicting the velocity of range expansions.One Sentence Summary: Evolution on ecological timescales increases the velocity of experimental plant populations spreading through patchy habitats. Main text:In an era of global environmental change, biological invasions and the movement of species ranges with climate change present two of the greatest threats to natural and managed ecosystems (1, 2). At the core of each dynamic is the spread of populations across landscapes fragmented by natural and anthropogenic barriers to movement. That habitat fragmentation slows the velocity of spread has long been appreciated (3, 4), but its influence on the potential for evolution to increase population expansion is unknown (5). Theory shows that natural selection at the low-density front of populations expanding through continuously favorable landscapes coupled with the spatial sorting of offspring favors traits contributing to fecundity and dispersal, both of which accelerate the invasion velocity (6-10). Whether this eco-evolutionary process operates similarly in systems fragmented by unsuitable habitat is uncertain because spread in these systems depends on the build-up of high density populations capable of dispersing over gaps (5, 11). Any factor that alters selection on an expanding population can influence spread, but whether evolution through selection or genetic drift predictably affects spread velocity on the rapid time scale of ecological dynamics remains an open question. Answering such questions has Page 2 of 10! important implications for predicting the future spread of biological invasions and climate change migrants.Empirical progress toward understanding evolution in populations spreading through fragmented landscapes is limited, largely because the process occurs over many generations and at geographic spatial scales. Due to these constraints, nearly all empirical evidence for evolution affecting spread comes from a few retrospective, observational analyses (12-16). The spread velocity of cane toads, for example, increased 6-fold after introduction to Australia, consistent with evolved changes in dispersal (14,17,18). No...

No abstract

Understanding the movement of species' ranges is a classic ecological problem that takes on urgency in this era of global change. Historically treated as a purely ecological process, range expansion is now understood to involve eco-evolutionary feedbacks due to spatial genetic structure that emerges as populations spread. We synthesize empirical and theoretical work on the eco-evolutionary dynamics of range expansion, with emphasis on bridging directional, deterministic processes that favor evolved increases in dispersal and demographic traits with stochastic processes that lead to the random fixation of alleles and traits. We develop a framework for understanding the joint influence of these processes in changing the mean and variance of expansion speed and its underlying traits. Our synthesis of recent laboratory experiments supports the consistent role of evolution in accelerating expansion speed on average, and highlights unexpected diversity in how evolution can influence variability in speed: results not well predicted by current theory. We discuss and evaluate support for three classes of modifiers of eco-evolutionary range dynamics (landscape context, trait genetics, and biotic interactions), identify emerging themes, and suggest new directions for future work in a field that stands to increase in relevance as populations move in response to global change.

Uncertainty associated with ecological forecasts has long been recognized, but forecast accuracy is rarely quantified. We evaluated how well data on 82 populations of 20 species of plants spanning 3 continents explained and predicted plant population dynamics. We parameterized stage-based matrix models with demographic data from individually marked plants and determined how well these models forecast population sizes observed at least 5 years into the future. Simple demographic models forecasted population dynamics poorly; only 40% of observed population sizes fell within our forecasts' 95% confidence limits. However, these models explained population dynamics during the years in which data were collected; observed changes in population size during the data-collection period were strongly positively correlated with population growth rate. Thus, these models are at least a sound way to quantify population status. Poor forecasts were not associated with the number of individual plants or years of data. We tested whether vital rates were density dependent and found both positive and negative density dependence. However, density dependence was not associated with forecast error. Forecast error was significantly associated with environmental differences between the data collection and forecast periods. To forecast population fates, more detailed models, such as those that project how environments are likely to change and how these changes will affect population dynamics, may be needed. Such detailed models are not always feasible. Thus, it may be wiser to make risk-averse decisions than to expect precise forecasts from models.

Invasive plants may respond through adaptive evolution and/or phenotypic plasticity to new environmental conditions where they are introduced. Although many studies have focused on evolution of invaders particularly in the context of testing the evolution of increased competitive ability (EICA) hypothesis, few consistent patterns have emerged. Many tests of the EICA hypothesis have been performed in only one environment; such assessments may be misleading if plants that perform one way at a particular site respond differently across sites. Single common garden tests ignore the potential for important contributions of both genetic and environmental factors to affect plant phenotype. Using a widespread invader in North America, Cynoglossum officinale, we established reciprocal common gardens in the native range (Europe) and introduced range (North America) to assess genetically based differences in size, fecundity, flowering phenology and threshold flowering size between native and introduced genotypes as well as the magnitude of plasticity in these traits. In addition, we grew plants at three nutrient levels in a pot experiment in one garden to test for plasticity across a different set of conditions. We did not find significant genetically based differences between native and introduced populations in the traits we measured; in our experiments, introduced populations of C. officinale were larger and more fecund, but only in common garden experiments in the native range. We found substantial population-level plasticity for size, fecundity and date of first flowering, with plants performing better in a garden in Germany than in Montana. Differentiation of native populations in the magnitude of plasticity was much stronger than that of introduced populations, suggesting an important role for founder effects. We did not detect evidence of an evolutionary change in threshold flowering size. Our study demonstrates that detecting genetically based differences in traits may require measuring plant responses to more than one environment.

No abstract

A central question in ecology concerns how some exotic plants that occur at low densities in their native range are able to attain much higher densities where they are introduced. This question has remained unresolved in part due to a lack of experiments that assess factors that affect the population growth or abundance of plants in both ranges. We tested two hypotheses for exotic plant success: escape from specialist insect herbivores and a greater response to disturbance in the introduced range. Within three introduced populations in Montana, USA, and three native populations in Germany, we experimentally manipulated insect herbivore pressure and created small-scale disturbances to determine how these factors affect the performance of houndstongue (Cynoglossum officinale), a widespread exotic in western North America. Herbivores reduced plant size and fecundity in the native range but had little effect on plant performance in the introduced range. Small-scale experimental disturbances enhanced seedling recruitment in both ranges, but subsequent seedling survival was more positively affected by disturbance in the introduced range. We combined these experimental results with demographic data from each population to parameterize integral projection population models to assess how enemy escape and disturbance might differentially influence C. officinale in each range. Model results suggest that escape from specialist insects would lead to only slight increases in the growth rate (lambda) of introduced populations. In contrast, the larger response to disturbance in the introduced vs. native range had much greater positive effects on lambda. These results together suggest that, at least in the regions where the experiments were performed, the differences in response to small disturbances by C. officinale contribute more to higher abundance in the introduced range compared to at home. Despite the challenges of conducting experiments on a wide biogeographic scale and the logistical constraints of adequately sampling populations within a range, this approach is a critical step forward to understanding the success of exotic plants.