Eric S. Menges

Population viability analyses in plants: challenges and opportunities

This review of 95 plant population viability analyses (PVAs) reveals that most studies consider one species, only a few populations and are based on data collected for less than five years. Only five studies referred to themselves as PVAs. Plants offer numerous challenges, such as seed banks and periodic recruitment, but these can be answered with suitable data collection and modeling. New approaches, such as metapopulation models, inclusion of disturbance cycles, and integration of genetics and demography, are producing more realistic PVAs. Although exact solutions are fraught with limitations, plant PVAs can be useful in comparing management regimes, populations and microhabitats, and in using these results to guide conservation and management.

Diversity of ageing across the tree of life

Evolution drives, and is driven by, demography. A genotype moulds its phenotype’s age patterns of mortality and fertility in an environment; these two patterns in turn determine the genotype’s fitness in that environment. Hence, to understand the evolution of ageing, age patterns of mortality and reproduction need to be compared for species across the tree of life. However, few studies have done so and only for a limited range of taxa. Here we contrast standardized patterns over age for 11 mammals, 12 other vertebrates, 10 invertebrates, 12 vascular plants and a green alga. Although it has been predicted that evolution should inevitably lead to increasing mortality and declining fertility with age after maturity, there is great variation among these species, including increasing, constant, decreasing, humped and bowed trajectories for both long- and short-lived species. This diversity challenges theoreticians to develop broader perspectives on the evolution of ageing and empiricists to study the demography of more species.

LONGEVITY CAN BUFFER PLANT AND ANIMAL POPULATIONS AGAINST CHANGING CLIMATIC VARIABILITY

Both means and year-to-year variances of climate variables such as temperature and precipitation are predicted to change. However, the potential impact of changing climatic variability on the fate of populations has been largely unexamined. We analyzed multiyear demographic data for 36 plant and animal species with a broad range of life histories and types of environment to ask how sensitive their long-term stochastic population growth rates are likely to be to changes in the means and standard deviations of vital rates (survival, reproduction, growth) in response to changing climate. We quantified responsiveness using elasticities of the long-term population growth rate predicted by stochastic projection matrix models. Short-lived species (insects and annual plants and algae) are predicted to be more strongly (and negatively) affected by increasing vital rate variability relative to longer-lived species (perennial plants, birds, ungulates). Taxonomic affiliation has little power to explain sensitivity to increasing variability once longevity has been taken into account. Our results highlight the potential vulnerability of short-lived species to an increasingly variable climate, but also suggest that problems associated with short-lived undesirable species (agricultural pests, disease vectors, invasive weedy plants) may be exacerbated in regions where climate variability decreases.

Plant strategies in relation to elevation and light in floodplain herbs

We examined the distribution of herbaceous species in southern Wisconsin floodplain forests in relation to two major environmental factors: elevation and light. Most species had definite elevation optima and could be categorized as either light specialists for high or low light, or light generalists. To test Grimes ideas regarding the existence of three primary plant strategies (competitive, stress tolerant, and ruderal), we used simple autecological indices to classify species into guilds. Higher elevation areas were dominated by tall competitive perennial forbs. As flooding frequency increased, smaller perennial forbs (stress-tolerant competitors) and tall annuals (competitive ruderals) increased in importance. In areas of greatest flooding frequency, the dichotomy in plant strategies was most extreme: slowgrowing, flood-tolerant sedges and grasses (stress tolerators) shared the ground with small, fast-maturing, annual forbs (ruderals). Tolerance to flooding in sedges and grasses may be related to the position of their shoot apex, which affords protection from mechanical damage caused by turbulent flooding. Light was important in differentiating guilds (high light specialists, light generalists, small spring-flowering forbs) only when flooding was infrequent. Grimes triangle of plant strategies maps onto axes of disturbance frequency and physiological tolerance, emphasizing the interaction of environment and adaptation in determining strategy. One environmental event, such as flood, fire, or drought, may be seen as a disturbance by short-lived ruderal species and as a stress by tolerant, long-lived perennials. Combinations of more than one important stress or disturbance further complicate Grimes model.

INTERACTIVE EFFECTS OF FIRE AND MICROHABITAT ON PLANTS OF FLORIDA SCRUB

Fire, microhabitat, and their interactions affect Florida scrub ecosystems and their plant species. Concepts of vegetation change in the Florida upland landscape have followed successional theory, with recent models emphasizing the resilience of Florida scrub to fire and the interactive effects of the vegetation and fire regime. We extend these models by incorporating greater complexity in vegetation types and emphasizing that departures from modal fire frequencies may alter vegetation. In particular, fire exclusion leads to structural and compositional changes that, in turn, alter vegetation changes following the reintroduction of fire. Individual species responses to fire can be categorized by the demographic mechanisms of the response (e.g., resprouting, clonal growth, seedling recruitment) and by typical patterns of abundance during fire-free intervals. Various types of scrub differ in these life-history traits. For example, xeric rosemary scrub supports more herbs, more endemics, more specialized spe...

How do plant ecologists use matrix population models

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.

Restoration demography and genetics of plants: when is a translocation successful?

Restorations are complex, often involving restoring ecological processes, vegetation structure, and species’ populations. One component of restorations is translocation of key species. Translocations (introductions, reintroductions, augmentations) are often necessary to recover species diversity and install key species. In this review, I consider the ways translocations have been evaluated at various stages during the process of restoration. Vital rates (survival, growth, fecundity) of propagules (seeds, transplants) are commonly used to evaluate initial success. Transplants usually provide greater initial success than do sown seeds. Beyond initial rates, completion of the life cycle through flowering, fruiting, dispersal and subsequent seedling recruitment is a key benchmark. Modelling population viability of translocated populations is a logical next step and can bring in many powerful inferential tools. Of factors affecting the success of translocations, genetic issues are paramount, as restorationists need to consider inbreeding depression, reproductive viability, local adaptation, and evolutionary potential of translocated populations. The success of translocations is also clearly context-dependent, with herbivory, disturbance, competition and other ecological factors important. Future translocations need to make better use of comparisons with reference populations, a long-term perspective on success and an experimental framework that can provide both practical and basic knowledge. Demographic data collection and analysis in restorations has great potential to elucidate causes of translocation failure and improve the prognosis of future restorations.

POPULATION VIABILITY WITH FIRE IN ERYNGIUM CUNEIFOLIUM: DECIPHERING A DECADE OF DEMOGRAPHIC DATA

We analyzed and modeled the demography of Eryngium cuneifolium ,a n herbaceous species endemic to the fire-prone Florida scrub, using 10 annual censuses (1990- 1999) of 11 populations at Archbold Biological Station. Nearly every aspect of the de- mography of this plant is affected by time since fire. Year, time since fire, life history stage, and plant age affected survival, growth, and fecundity of E. cuneifolium, but time since fire and life history stage had the most consistent effects. Survival, flowering stem pro- duction, and early seedling survival were highest in recently burned sites. Long-term sur- vival, growth, and fecundity were highest for yearling cohorts recruiting recently after fire, with the largest contrast between plants recruiting two years postfire and those recruiting more than a decade postfire. Prior (historical) stage also affected individual plant fates. For example, plants with prior stasis or regression in stage subsequently died in greater numbers than plants with prior advancement in stage. Historical analyses did not suggest any cost associated with the initiation of flowering. We used a matrix selection approach to explicitly model Eryngium cuneifolium popu- lation viability in relation to fire. This simulation strategy included preserving observed data and variances within projection matrices formed for individual combinations of pop- ulation and year. We built 54 of these matrices, each with six stages (seed bank, yearlings, vegetative plants, and three reproductive stages). Each of these matrices also represented a specific time since fire. In building matrices, we minimized the use of pooled data while retaining specific matrices whenever possible. In this way, we preserved both the correlation structure within individual matrices (populations, years) and protected patterns among ma- trices across the time-since-fire gradient. To deal with less-detailed data on recruitment processes, we evaluated 13 fertility and seed bank scenarios that bracketed a range of outcomes. All scenarios were similar in showing the positive effects of fire on the demography of E. cuneifolium. The scenario with high seed bank survival (0.5) and low germination rates (0-0.005) was the best predictor of observed postfire years of peak aboveground population size (;8 yr) and aboveground population disappearance (30-34 yr), and also did a good job of reproducing observed population trajectories. Finite rates of increase (l) were .1 only during the first decade postfire but then declined beyond a decade postfire. Although prior (historical) stage affected most individual de- mographic parameters, it did not significantly influence finite rates of increase. Elasticities were highest for stasis and germination from the seed bank. Elasticities for survival in- creased with time since fire, while growth and fertility elasticities decreased. In historical models (those with information on stage from the second-to-last year), the elasticities for stasis were higher and the elasticities for growth lower, compared to models without this history. Bootstrapping suggested small standard errors for several types of model output. Most matrix elements were positively correlated, suggesting that favorable conditions affect many life history stages similarly, and that simulations using element selection would provide a less conservative risk assessment than the matrix selection technique used. We used a stochastic simulation program to simulate changing demography with time since fire, with various fire-return intervals, and for various initial population sizes. We obtained estimates of extinction risk and probability of population decline. Even populations as large as thousands of individuals will become extinct in the absence of fire. Fire-return intervals of 15 yr or less are necessary for E. cuneifolium persistence at individual sites. Fires at intervals longer than 20 yr create substantial extinction risks, and intervals longer than 12 yr produce declining populations. Cycles of widely divergent, alternating short and long fire-return intervals caused slightly higher chances of extinction compared to regular fire-return intervals. Although shrub regrowth is implicated in the decreased viability of E. cuneifolium populations under regimes of infrequent fire, aboveground fuel increases are often too slow

Integrating demography and fire management: an example from Florida scrub

In this work, I have used life-history and demographic data to define fire return intervals for several types of Florida scrub, a xeric shrubland where fire is the dominant ecological disturbance but where fire suppression is a major issue. The datasets combine chronosequence and longitudinal approaches at community and population levels. Resprouting shrubs, which dominate most types of Florida scrub, recover rapidly after fires (although their limits under frequent fires are not well known) and also increasingly dominate long-unburned areas. These dominant shrubs can prosper over a range of fire return intervals. Obligate-seeding scrub plants, which often have persistent seed banks, can be eliminated by frequent fire but often decline with infrequent fire. Population viability analyses of habitat specialists offer more precision in suggesting ranges of appropriate fire return intervals. For two types of Florida scrub (rosemary scrub and oak-hickory scrub), plant-population viability analyses narrow the interval and suggest more frequent fires than do previous recommendations, at intervals of 15-30 and 5-12 years, respectively. Variation in fire regimes in time and space (pyrodiversity) is recommended as a bet-hedging fire-management strategy and to allow co-existence of species with disparate life histories.

Twenty years of vegetation change in five long-unburned Florida plant communities

Rates and directions of change over a 20-yr interval in five long-unburned (> 60 yr) plant communities were stud- ied using multivariate analyses and compositional vectors. The study sites were located in fire and summer-drought adapted, xerophytic vegetation with many endemics on acidic, nutrient-poor, sandy soils in south-central peninsular Florida. Sizes of individual stems from 72 sets of nested permanent quadrats were measured in 1969, 1979, and 1989. Patterns of vegetation change differed by community. Flatwood and bayhead quadrats showed rapid increases in densities and basal areas of Persea borbonia (red bay). In the southern ridge sandhill community, evergreen clonal Quercus species (oaks) and Pinus clausa (sand pine) increased in dominance and grasses declined. Oaks (especially Q. geminata) also increased in importance in scrubby flatwoods. Sand pine scrub was relatively stable in composition, but experienced marked structural changes due to substantial sand pine mortal- ity (18 % during 1969-1979, 39 % during 1979-1989). Com- positional changes in the absence of fire were greatest whereas structural changes were least in southern ridge sandhill and scrubby flatwoods, both communities which normally receive frequent, recurrent fire. Compositional changes were lowest in sand pine scrub, which is normally infrequently burned. Classic successional patterns such as species replacement, decreases in density, and increases in basal area were gener- ally lacking. Tree densities increased in two of four communi- ty types (southern ridge sandhill, scrubby flatwoods); while basal area declined in the flatwoods/bayhead and sand pine scrub sites. Directions of compositional vectors included di- vergent, opposing, and complex patterns, suggesting vegeta- tion change in the absence of fire has a strong stochastic component.