Mark Rees

Models suggesting field experiments to test two hypotheses explaining successional diversity.

A simple mathematical model of competition is developed that includes two alternative mechanisms promoting successional diversity. The first underpins the competition‐colonization hypothesis in which early successional species are able to persist because they colonize disturbed habitats before the arrival of late successional dominant competitors. The second underpins the niche hypothesis, in which early successional species are able to persist, even with unlimited colonization by late successional dominants, because they specialize on the resource‐rich conditions typical of recently disturbed sites. We modify the widely studied competition‐colonization model so that it also includes the mechanism behind the niche hypothesis. Analysis of this model suggests simple experiments that determine whether the successional diversity of a field system is maintained primarily by the competition‐colonization mechanism, primarily by the niche mechanism, by neither, or by both. We develop quantitative metrics of the relative importance of the two mechanisms. We also discuss the implications for the management of biodiversity in communities structured by the two mechanisms.

QUANTIFYING THE IMPACT OF COMPETITION AND SPATIAL HETEROGENEITY ON THE STRUCTURE AND DYNAMICS OF A FOUR-SPECIES GUILD OF WINTER ANNUALS

We develop statistical methods appropriate for the analysis of spatially structured population data. The methods are used to study the structure and dynamics of a four-species annual plant guild recorded in 1,000 permanent squares over a 10-yr period. We parameterize models that predict population density from one year to the next. In agreement with theoretical expectation all the models have locally stable equilibria, and overcompensation is rare. We demonstrate that interspecific interactions are extremely weak, relative to intraspecific ones, and that the spatial arrangement of species and individuals within them is critical to the observed dynamics. The impact of spatial density-dependent population growth on observed densities was calculated. In 52% of the cases population size would have been increased by at least a factor of 1.5 had there been no interactions between individuals, and in 9% of these it would have increased by a factor of four or more. This effect is shown to be largely a result of intraspecific interactions. We discuss possible explanations for the weakness of interspecific interactions.

Biological control of Scotch broom: modelling the determinants of abundance and the potential impact of introduced insect herbivores.

Simulation and analytical models are developed for the European shrub Scotch broom Cytisus scoparius Link (Fabaceae). The simulation model is spatially explicit and allows us to explore not only changes in population size but also the proportion of ground covered by the weed. The simulation model incorporates spatially local density-dependent competition, asymmetric competition between seedlings and established plants, a seed bank, local seed dispersal and an age-structured established plant population. This model is designed to incorporate much of the known population biology of broom. The analytical models are simple approximations of the simulation. The basic model contains nine parameters: the probability a site is disturbed, p dist ; the probability a seed becomes a seedling, g; the probability a seedling survives the first year, s; the probability a seed is lost from the seed bank, d; the minimum age for reproduction, A min ; maximum plant age, A max ; seed production per site, F; the probability a seed is retained in the parental site, fh; and the probability a site becomes suitable for colonization after broom senesces, p so . We review published data on the demography of broom from studies around the world, and also present some previously unpublished data. These data suggest that broom in some exotic habitats can achieve higher fecundities and live longer than in its native range. Analytical approximations provide a good description of the simulation results over a wide range of biologically reasonable parameter values. Specifically, the analytical modekls work well when plants are long-lived or highly fecund. Analysis of the models indicates that when broom colonizes all suitable sites with probability one, that the fraction of sites occupied by broom is determined by only three parameters: the probability of disturbance, p dist the probability a site becomes suitable for colonization following plant senescence, p so ; and maximum longevity, A max . In exotic habitats, where individual broom plants can produce several thousand seeds, differences in these parameters are the most likely reason why broom populations are more weedy than in the native range. The impact of insect herbivores, which reduce plant fecundity, on broom abundance is explored for several environmental scenarios. This analysis suggests that potential biological control agents are most likely to have a substantial impact if the disturbance rate is high, plant fecundity is low, and seedling survival is low. Even herbivores that reduce seed production by only 75% can have a dramatic impact on broom abundance, in contrast to several published predictions. Extensions to the models to allow for arbitrary patterns of age-dependent senescence, and site-specific probabilities of disturbance are presented.

Evolutionary bet-hedging in the real world: empirical evidence and challenges revealed by plants

Understanding the adaptations that allow species to live in temporally variable environments is essential for predicting how they may respond to future environmental change. Variation at the intergenerational scale can allow the evolution of bet-hedging strategies: a novel genotype may be favoured over an alternative with higher arithmetic mean fitness if the new genotype experiences a sufficiently large reduction in temporal fitness variation; the successful genotype is said to have traded off its mean and variance in fitness in order to ‘hedge its evolutionary bets’. We review the evidence for bet-hedging in a range of simple plant systems that have proved particularly tractable for studying bet-hedging under natural conditions. We begin by outlining the essential theory, reiterating the important distinction between conservative and diversified bet-hedging strategies. We then examine the theory and empirical evidence for the canonical example of bet-hedging: diversification via dormant seeds in annual plants. We discuss the complications that arise when moving beyond this simple case to consider more complex life-history traits, such as flowering size in semelparous perennial plants. Finally, we outline a framework for accommodating these complications, emphasizing the central role that model-based approaches can play.

Coexistence and Relative Abundance in Annual Plant Assemblages: The Roles of Competition and Colonization

Although an interspecific trade‐off between competitive and colonizing ability can permit multispecies coexistence, whether this mechanism controls the structure of natural systems remains unresolved. We used models to evaluate the hypothesized importance of this trade‐off for explaining coexistence and relative abundance patterns in annual plant assemblages. In a nonspatial model, empirically derived competition‐colonization trade‐offs related to seed mass were insufficient to generate coexistence. This was unchanged by spatial structure or interspecific variation in the fraction of seeds dispersing globally. These results differ from those of the more generalized competition‐colonization models because the latter assume completely asymmetric competition, an assumption that appears unrealistic considering existing data for annual systems. When, for heuristic purposes, completely asymmetric competition was incorporated into our models, unlimited coexistence was possible. However, in the resulting abundance patterns, the best competitors/poorest colonizers were the most abundant, the opposite of that observed in natural systems. By contrast, these natural patterns were produced by competition‐colonization models where environmental heterogeneity permitted species coexistence. Thus, despite the failure of the simple competition‐colonization trade‐off to explain coexistence in annual plant systems, this trade‐off may be essential to explaining relative abundance patterns when other processes permit coexistence.

Delayed Germination of Seeds: A Look at the Effects of Adult Longevity, the Timing of Reproduction, and Population Age/Stage Structure

The effects of adult longevity, the timing of reproduction, and population age/stage structure on the evolution of seed dormancy are explored in both constant and variable environment models. In the constant environment models complete germination is the evolutionarily stable strategy (ESS) regardless of adult longevity. Incorporating a cost of reproduction on subsequent survival does not alter this result. In contrast, in a variable environment changes in adult longevity can exert a strong selection pressure against seed dormancy. Incorporating a cost of reproduction for iteroparous species reduces adult longevity, which selects for more seed dormancy. The magnitude of the change in ESS germination probability depends on several factors, including which life-history stage is variable (e.g., fecundity, seedling survival), whether seeds can detect favorable sites for establishment, and the age/stage structure of the population. In general, increases in adult longevity select against seed dormancy, but exceptions to this pattern are discussed. The idea that established plant traits are uncoupled from those of the regenerative phase, as assumed by J. P. Grimes competition-stress-ruderal model, is considered critically.

Game-theoretical evolution of seed mass in multi-species ecological models

Within plant communities seed mass often varies over 3 to 5 orders of magnitude, yet simple evolutionary models predict a single optimum seed mass. Here we explore a class of models where seed mass determines 1) the number of seeds produced via a size-number trade-off and 2) competitive ability - plants arising from large seeds are assumed to have a competitive advantage over those derived from small seeds. In this setting the existence of a single-species global ESS seed mass requires the competitive advantage of large seeds over small ones to be unbounded. If there is a limit on the competitive advantage that large seeds obtain then it is always possible to find a smaller seed mass that will successfully invade. In such circumstances there might be a multi-species coevolutionarily stable coalition of several species each with a different seed mass. In this way a wide range of seed masses could be promoted by evolution. In general the adaptive landscape generated by these models is extremely flat leading to slow evolutionary dynamics. The implications of these results for the interpretation of observational, comparative and experimental studies are discussed.

Evolutionary demography of monocarpic perennials

Monocarpic plants, which flower once then die, are ideal systems for testing evolutionary ideas because the cost of reproduction is easily quantified and the timing of flowering is a key determinant of darwinian fitness. Monocarps should flower at the size that maximizes fitness, enabling models of life-history evolution to be tested. These models are becoming increasingly sophisticated and accurate, making a review of the techniques and underlying theory timely. Here, we review the long-term demography of monocarpic species, focusing on how demographic rates vary with size and age. We then examine the broad array of evolutionary models, and question what aspects of the demography are crucial for the successful prediction of the size and age at flowering, shedding light on both the important aspects of monocarp demography and current advances in life-history modelling.

Snow Tussocks, Chaos, and the Evolution of Mast Seeding

One hitherto intractable problem in studying mast seeding (synchronous intermittent heavy flowering by a population of perennial plants) is determining the relative roles of weather, plant reserves, and evolutionary selective pressures such as predator satiation. We parameterize a mechanistic resource‐based model for mast seeding in Chionochloa pallens (Poaceae) using a long‐term individually structured data set. Each plant’s energy reserves were reconstructed using annual inputs (growing degree days), outputs (flowering), and a novel regression technique. This allowed the estimation of the parameters that control internal plant resource dynamics, and thereby allowed different models for masting to be tested against each other. Models based only on plant size, season degree days, and/or climatic cues (warm January temperatures) fail to reproduce the pattern of autocovariation in individual flowering and the high levels of flowering synchrony seen in the field. This shows that resource‐matching or simple cue‐based models cannot account for this example of mast seeding. In contrast, the resource‐based model pulsed by a simple climate cue accurately describes both individual‐level and population‐level aspects of the data. The fitted resource‐based model, in the absence of environmental forcing, has chaotic (but often statistically periodic) dynamics. Environmental forcing synchronizes individual reproduction, and the models predict highly variable seed production in close agreement with the data. An evolutionary model shows that the chaotic internal resource dynamics, as predicted by the fitted model, is selectively advantageous provided that adult mortality is low and seeds survive for more than 1 yr, both of which are true for C. pallens. Highly variable masting and chaotic dynamics appear to be advantageous in this case because they reduce seed losses to specialist seed predators, while balancing the costs of missed reproductive events.

Effects of Temporal Variability on Rare Plant Persistence in Annual Systems

Traditional conservation biology regards environmental fluctuations as detrimental to persistence, reducing long‐term average growth rates and increasing the probability of extinction. By contrast, coexistence models from community ecology suggest that for species with dormancy, environmental fluctuations may be essential for persistence in competitive communities. We used models based on California grasslands to examine the influence of interannual fluctuations in the environment on the persistence of rare forbs competing with exotic grasses. Despite grasses and forbs independently possessing high fecundity in the same types of years, interspecific differences in germination biology and dormancy caused the rare forb to benefit from variation in the environment. Owing to the buildup of grass competitors, consecutive favorable years proved highly detrimental to forb persistence. Consequently, negative temporal autocorrelation, a low probability of a favorable year, and high variation in year quality all benefited the forb. In addition, the litter produced by grasses in a previously favorable year benefited forb persistence by inhibiting its germination into highly competitive grass environments. We conclude that contrary to conventional predictions of conservation and population biology, yearly fluctuations in climate may be essential for the persistence of rare species in invaded habitats.