Teresa Valverde

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.

CANOPY CLOSURE RATE AND FOREST STRUCTURE

Natural disturbances produce openings in forest canopies. Gap opening and closure create a mosaic of regenerating phases in the forest, which are a source of environmental heterogeneity, contributing to the maintenance of community diversity. In this study we measured the rate of canopy closure using hemispherical photographs taken in the same points in August 1992, 1993, and 1994 at three temperate forests in Britain. We analyzed the change in percentage canopy openness in time and found that canopy closure occurred exponentially. These results were used to build a matrix model that incorporated canopy dynamics as a linear Markov-chain process of forest succession by classifying forest patches into types (according to percentage canopy openness) and calculating the probability of each patch type becoming the following patch type in one year’s time. Standard matrix analysis allowed us to project forest structure (i.e., the proportion of patches of each type) at equilibrium, and to explore the effect of di...

The COMPADRE Plant Matrix Database: an open online repository for plant demography.

Summary 1. Schedules of survival, growth and reproduction are key life-history traits. Data on how these traits vary among species and populations are fundamental to our understanding of the ecological conditions that have shaped plant evolution. Because these demographic schedules determine population

A METAPOPULATION MODEL FOR PRIMULA VULGARIS, A TEMPERATE FOREST UNDERSTOREY HERB

1 We suggest that Primula vulgaris in woodland habitats may form metapopulations: colonization is associated with the opening of canopy gaps and local populations become extinct as the canopy closes. The metapopulation is seen as a permanently changing system in which colonizations and extinctions are constantly occurring, coupled with forest canopy dynamics. 2 To describe metapopulation dynamics we use a Markovian model that incorporates the different stages through which local populations pass; we calculate the transition probabilities among those stages and estimate colonization and extinction rates. We combine empirical information on the demography of local populations and the dynamics of the forest mosaic in the model and use it to explore the effect of seed dispersal, fd (i.e. the fraction of seeds dispersing out of populations) and forest disturbance rate, k, on metapopulation growth rate, XM (i.e. the rate at which the number of populations in the metapopulation increases or decreases). 3 Increasing either fd or k has a positive effect on the projected metapopulation growth rate: higher disturbance rates increase the frequency of newly opened gaps in the environment and higher seed dispersal levels increase the probability of gap colonization. 4 Metapopulation growth rate is also affected by a number of other variables. It declines when time to local population extinction decreases and when fecundity is lower. The latter emphasizes that sufficient seeds must be available for dispersal to ensure gap colonization. 5 Stable patch/population-type distributions given different parameter values were obtained as the right eigen-vectors of metapopulation matrices. The proportion of forest patches occupied by P. vulgaris populations at equilibrium would be expected to be negligible under low k andfd values. 6 Metapopulation structure may break down under high disturbance rates because P. vulgaris local populations may never go extinct and thus seed dispersal would not play a major role in local population re-establishment.

demographic analysis of a rare columnar cactus (neobuxbaumia macrocephala) in the tehuacan valley, mexico

In this study we used population projection matrices to evaluate the conservation status of Neobuxbaumia macrocephala, a columnar cactus endemic to a small region in the Tehuacan Valley, in central Mexico. Demographic data included 2-year observations on growth, fecundity and survival of individuals classified by size. Our results indicate that the population is comprised of 70% juveniles. Population growth rate was 0.979 and 0.994 for the 1997/1998 and the 1998/1999 periods, respectively. The slight increase in λ in 1998/1999 was a result of increased fecundity and seedling survival. The highest elasticity values correspond to the survival of large/old individuals. Numerical simulations were performed by changing the value of particular matrix entries and directly evaluating their effect on λ. Population growth rate reached values above unity only when either fecundity or seedling survival probability were increased 10-fold. Given these limitations for population growth, along with its limited distribution range and low population densities, we propose N. macrocephala to be classified as a rare species and to promote its conservation by favoring management practices aimed to increase germination and seedling establishment success.

spatial variation in the seed ecology of a woodland herb (primula vulgaris) in relation to light environment

Patches of Primula vulgaris occur across a range of light conditions within forests. To understand how light affects the stages of the life cycle involved in reproduction and recruitment, several fecundity-related variables (flower and capsule production, seed set, seed germination and seedling establishment) were studied in eight populations across a range of light conditions. The mean number of seeds per plant varied greatly, from seven to 136, and the percentage of seedling establishment varied from 0.4 to 17.2%. These variables, as well as the number of flowers per plant and fecundity, were positively correlated with the light conditions. Primula vulgaris populations establish in newly opened gaps. How this species colonizes new clearings was investigated by looking at seed survival in the soil and seed removal by potential dispersers, in particular rodents and ants. After 14 months in the greenhouse, only three P. vulgaris seedlings were observed germinating from 200 soil samples taken from two populations. Germination of retrieved seeds from buried samples was generally lower than 10%. This suggests that only a small number of seeds remain viable in the soil for long periods of time. Seed removal was high in the populations studied (more than 70% in 10 days). Rodents harvested a greater proportion of seeds than ants. Seed dispersal by ants is known to occur within short distances but rodents may be long-distance dispersers, which may be important for the long-term persistence of P. vulgaris in a site because it allows seeds to reach newly opened gaps.

Population dynamics of Mammillaria magnimamma Haworth. (Cactaceae) in a lava-field in central Mexico

One of the habitats occupied by Mammillaria magnimamma is a 2000-year old lava-field, in Mexico City. The great ecological interest on this lava-field and the little knowledge there is regarding cacti population ecology have compelled us to analyse the demography of this species to evaluate its present conservation status at this site. We studied two populations of this species within the lava-field: one in a disturbed site (i.e., recently burned) and another one in a well preserved site. For each population we built two size-based population projection matrices (1996/97 and 1997/98). Demographic data were gathered directly from observations of plant fates from one year to the next. Additionally, seed germination and seedling establishment experiments were carried out in the field to estimate fecundity values and seedling survival probabilities. The four matrices built were used to perform numerical analyses simulating yearly stochastic demographic variation to project the overall populations long-term behaviour under these changing conditions. Three of the four matrices showed λ values slightly below unity. In these cases elasticity values were highest for matrix entries corresponding to plants remaining in their same category. The matrix that showed a λ value above unity (well preserved site, 1997/98) had higher elasticity values for entries referring to seedling survival and growth. The numerical simulations of demographic stochasticity showed that the population appears to be growing at a slow rate. According to the simulation results, the variation in overall population size over time may be accounted for by yearly variation in seed germination and seedling survival. Population persistence probability might decrease significantly if fire frequency increases.

An integrated model of demography, patch dynamics and seed dispersal in a woodland herb, Primula vulgaris

Primula vulgaris is a woodland herb often associated with canopy gaps. The canopy regeneration cycle results in a permanently changing environment which confers a dynamic character to the overall system of P. vulgaris populations inhabiting a forest. In order to describe the dynamics of this spatially structured population, we built a matrix model that incorporates both the spatio-temporal variation in its population dynamics and the dynamics of the forest habitat given by the opening and closure of canopy gaps. To build the model, we combined empirical data on 1) the demography of populations in various forest patches and 2) canopy closure rate. We then used the model to test the effect of disturbance rate and long-distance (i.e., off-patch) seed dispersal on overall population dynamics. Simulation results suggest that overall population growth rate of P. vulgaris would be enhanced by high gap opening rates, as this would increase the relative frequency of clearings, where population growth rate is maximal. Our analysis also predicts a negative effect of long-distance seed dispersal on overall population dynamics, since seeds dispersing out of occupied patches would have a high probability of landing in closed canopy patches. These results are an example of the costs of long-distance seed dispersal on overall population dynamics. Finally, the potential role of long-distance seed dispersal in the colonization of newly opened gaps is discussed.

Comparative demographic analysis of three Neobuxbaumia species (Cactaceae) with differing degree of rarity

In this study we use a demographic approach to analyse the differing abundance of three congeneric columnar cacti: Neobuxbaumia macrocephala (the rarest), Neobuxbaumia tetetzo (intermediate), and Neobuxbaumia mezcalaensis (the most common). Populations of these species were studied in the Tehuacan Valley (Central Mexico) over a 3-year period. We employed traditional models and life table response experiments (LTRE) to explore the association between particular demographic traits and the degree of rarity of each species. Most matrices showed population growth rate (λ) values close to unity; the only exception was N. mezcalaensis in 2001–2002 (λ=1.091±0.088). In the three species the highest elasticity values corresponded to entries referring to the stasis of pre-reproductive plant and small adults. However, the LTRE indicated that most important differences in the λ values between years and species were associated with variation in fruit production and seedling recruitment. N. mezcalaensis yielded the highest mean λ due to its high fecundity in 2001–2002. This result suggests that the variation in demographic behaviour observed between species is sufficient to explain the higher abundance of N. mezcalaensis compared to the rare N. macrocephala. The high fecundity values and recruitment success observed in the former, even if they occur only seldom, may result in higher average λ values accounting for its high population densities.

Variation in the population dynamics of the palm Sabal yapa in a landscape shaped by shifting cultivation in the Yucatan Peninsula, Mexico

To understand the population dynamics of a species, it is necessary to document the way in which its demographic behaviour varies through space and time. Anthropogenic disturbance, such as shifting cultivation, is an important factor causing demographic variation in many tropical non-timber forest products. The leaves of the palm Sabal yapa are an important non-timber forest product used for thatching by Mayan peoples. The demography of Sabal yapa was studied in three habitats (mature forest, successional forest and crop fields), representing successional phases along the slash-and-burn agricultural cycle in the Yucatan Peninsula. Matrix population models, along with elasticity analyses and life-table-response experiments were employed. Population growth rate differed between patches (MF: λ = 1.043; SF: λ = 1.027; CF: λ = 0.959). Only the λ value of the mature forest was significantly higher than unity. Fecundity and seedling survival were lowest in the crop fields and highest in the mature forest. The elasticity analyses and life-table-response experiments showed that entries with a high positive contribution to λ also showed high elasticity values, while those with a negative contribution to λ showed low elasticity. Thus, both analyses are crucial to understand the demography of a species and to aid in conservation and management practices.