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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 ...

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.

1. Relatively few studies of the host-finding ability of specialised, phytophagous insects involve direct observations of individual insects moving among intact hosts and non-hosts. Information from such studies can inform the design of restoration programmes for species of conservation concern. 2. The movement of caterpillars of the threatened Oregon silverspot butterfly, Speyeria zerene hippolyta (Edwards) (Lepidoptera: Nymphalidae) was studied in the field in cleared arenas with 10 cm radii. Caterpillars were placed in the centre, surrounded by three individuals of their host, Viola adunca, and three different non-host individuals, separated by bare ground. In a second experiment, second instars were placed between a host and a non-host, 3–6 cm away. Caterpillars were observed to determine if they walked to their host more often than expected by chance. 3. Caterpillars walked to vegetation significantly more often than expected by chance. They did not, however, reach their hosts more often than expected, based on plant availability. 4. It is concluded that S. z. hippolyta caterpillars can distinguish vegetation from bare ground from 10 cm away. There is no evidence that they can distinguish their host plant from other herbaceous species at distances of 3 cm.

Motivated by data demonstrating fluctuating relative and absolute fitnesses for white- versus blue-flowered morphs of the desert annual Linanthus parryae, we present conditions under which temporally fluctuating selection and fluctuating contributions to a persistent seed bank will maintain a stable single-locus polymorphism. In L. parryae, blue flower color is determined by a single dominant allele. To disentangle the underlying diversity-maintaining mechanism from the mathematical complications associated with departures from Hardy-Weinberg genotype frequencies and dominance, we successively analyze a haploid model, a diploid model with three distinguishable genotypes, and a diploid model with complete dominance. For each model, we present conditions for the maintenance of a stable polymorphism, then use a diffusion approximation to describe the long-term fluctuations associated with these polymorphisms. Our protected polymorphism analyses show that a genotype whose arithmetic and geometric mean relative fitnesses are both less than one can persist if its relative fitness exceeds one in years that produce the most offspring. This condition is met by data from a population of L. parryae whose white morph has higher fitness (seed set) only in years of relatively heavy rain fall. The data suggest that the observed polymorphism may be explained by fluctuating selection. However, the yearly variation in flower color frequencies cannot be fully explained by our simple models, which ignore age structure and possible selection in the seed bank. We address two additional questions--one mathematical, the other biological--concerning the applicability of diffusion approximations to intense selection and the applicability of long-term predictions to datasets spanning decades for populations with long-lived seed banks.

Analyses of population projection models are increasingly being used by conservation biologists and land managers to assess the health of sensitive species and to evaluate the likely effects of management strategies, harvesting, grazing, or other manipulations. Here I describe some of the limitations of this approach and illustrate how these limitations may affect its usefulness. I do this by comparing the results of such an analysis, performed in 1979 on two populations of a perennial plant, Arisaema triphyllum, with new information about the size and structure of these same populations gathered in 1994, 15 years later. While one population changed as the model projected it would, the other behaved quite differently from the projection. Instead of increasing in size, this population decreased between 1979 and 1994. Possible shortcomings in the data and in the model include: too few plants to provide accurate transition probabilities; too few years to capture accurately the complete range of year-to-year ...

Demographic transition matrices are among the most commonly applied population models for both basic and applied ecological research. The relatively simple framework of these models and simple, easily interpretable summary statistics they produce have prompted the wide use of these models across an exceptionally broad range of taxa. Here, we provide annual transition matrices and observed stage structures/population sizes for 20 perennial plant species which have been the focal species for long-term demographic monitoring. These data were assembled as part of the “Testing Matrix Models” working group through the National Center for Ecological Analysis and Synthesis (NCEAS). The data represent 82 populations with >460 total population-years of observations. It is our hope that making these data available will help promote and improve our ability to monitor and understand plant population dynamics. The complete data sets corresponding to abstracts published in the Data Papers section of the journal are publ...

A size-classified transition matrix model of the demography of Arisaema triphyllum, a perennial herb of temperate deciduous forest, was constructed from 3 yr of field data from two different sites. Arisaema is quite unusual in that individuals change sex in a pattern determined by their sizes: small individuals do not flower, larger individuals reproduce as males, and the largest as females. As plants increase or decrease in size, they also may change sex. Though demographic parameters varied with the site and with the year, some consistent patterns were seen. Mortality rates ranged from 20 to 40%/yr for small, nonflowering plants, but decreased to much lower levels as plants reached reproductive size. For these large plants, mortality rates did not differ between sexes or among size-classes. Because rates of sexual reproduction were severely pollinator limited, levels of seed production were low (averaging 7 seeds female-1 yr-1) and were not correlated with plant size. All plants engaged in clonal reproduction, regardless of their size. Transition matrices based on these data were compared statistically and were found to vary in time, as a result of year-to-year environmental variability. No population had achieved a stable size distribution. The classical method of calculating rate of increase, X, was therefore rejected, and an alternative method incorporating temporal stochasticity substituted. A sensitivity analysis was per- formed to evaluate the effect on population growth of changing various life history parameters. Differences in rates of individual growth and of clonal reproduction were found to be largely respon- sible for the relative success of the two populations, one of which was increasing, the other declining. Arisaema's status as a climax-forest herb suggests that temporally varying transition matrices might be common even in environments traditionally regarded as "stable." The prevalence of tem- poral variability has consequences both for the future direction of demographic theory and for the design and analysis of field studies.

Summary Information is reviewed on the life histories and demographic patterns of shade-tolerant herbs of temperate deciduous forest. Most of the species that have been studied are deciduous perennials. These species generally have a juvenile (non-flowering) period of several years followed by an extended period of flower and seed production. About half the species reviewed are self-incompatible, the others are at least partially self-compatible. Although vegetative reproduction is common, replacement does not always occur by vegetative spread; many species appear to be able to replace themselves by seed. Sexual reproductive effort ranges from 1–5 to 50 % among the species reviewed; seed weight averages 2–6 mg. Short dormancy periods (one season) are most common, with germination rates generally below 50%. Mortality rates of small plants are high, but decline as plant size increases. ‘Ageing’ does not appear to occur. No information exists on population growth rates of temperate deciduous forest herbs, or on the temporal stability of their population sizes or structures. The transition matrix model is suggested as a promising approach to future studies in plant demography.

Plant-pollinator relationships have been recognized as excellent systems in which to test optimal foraging theory (Pyke et al., 1977). In particular, Pyke (1978a, 1978b, 1978c, 1979) has considered the movements of hummingbirds and bumblebees within and among inflorescences to determine the extent to which the behavior of these organisms can be considered "optimal," i.e., that the net rate of energy gain is maximized. Pyke's (1978c, 1979) predictions of how an optimally foraging bee should behave have been based on general theoretical arguments rather than on precise quantitative considerations of the energetic "costs" and "rewards" of foraging. In addition, recent work by Waddington and Heinrich (1979) has revealed new information about the flexibility of response of foraging bees to changes in the presentation of rewards. In this paper we examine the relationship between Digitalis purpurea (Scrophulariaceae) and its most frequent visitor, Bombus flavifrons dimidiatus. We construct a quantitative model for the foraging behavior of energy-maximizing bees on Digitalis, predicting where on the inflorescence bees will begin foraging, the direction of their movement, and at what flower position they will leave. We then seek to answer the following questions, raised by the model: 1) Is the spatial arrangement of nectar rewards within the inflorescence exploited by foraging bees in the manner predicted by our model? 2) Does the plant's pattern of nectar rewards