Glenda M. Wardle

Causes and consequences of variation in plant population growth rate: a synthesis of matrix population models in a phylogenetic context

Explaining variation in population growth rates is fundamental to predicting population dynamics and population responses to environmental change. In this study, we used matrix population models, which link birth, growth and survival to population growth rate, to examine how and why population growth rates vary within and among 50 terrestrial plant species. Population growth rates were more similar within species than among species; with phylogeny having a minimal influence on among-species variation. Most population growth rates decreased over the observation period and were negatively autocorrelated between years; that is, higher than average population growth rates tended to be followed by lower than average population growth rates. Population growth rates varied more through time than space; this temporal variation was due mostly to variation in post-seedling survival and for a subset of species was partly explained by response to environmental factors, such as fire and herbivory. Stochastic population growth rates departed from mean matrix population growth rate for temporally autocorrelated environments. Our findings indicate that demographic data and models of closely related plant species cannot necessarily be used to make recommendations for conservation or control, and that post-seedling survival and the sequence of environmental conditions are critical for determining plant population growth rate.

Prescribed burning: how can it work to conserve the things we value?

Prescribed burning is a commonly applied management tool, and there has been considerable debate over the efficacy of its application. We review data relating to the effectiveness of prescribed burning in Australia. Specifically, we address two questions: (1) to what extent can fuel reduction burning reduce the risk of loss of human life and economic assets posed from wildfires? (2) To what extent can prescribed burning be used to reduce the risk of biodiversity loss? Data suggest that prescribed burning can achieve a reduction in the extent of wildfires; however, at such levels, the result is an overall increase in the total area of the landscape burnt. Simulation modelling indicates that fuel reduction has less influence than weather on the extent of unplanned fire. The need to incorporate ecological values into prescribed burning programmes is becoming increasingly important. Insufficient data are available to determine if existing programs have been successful. There are numerous factors that prevent the implementation of better prescribed burning practices; most relate to a lack of clearly defined, measurable objectives. An adaptive risk management framework combined with enhanced partnerships between scientists and fire-management agencies is necessary to ensure that ecological and fuel reduction objectives are achieved.

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

Social organization and movements of desert rodents during population ''booms'' and ''busts'' in central Australia

Abstract We monitored populations of 2 species of desert rodents, the sandy inland mouse (Pseudomys hermannsburgensis) and spinifex hopping-mouse (Notomys alexis), over 18 years in the Simpson Desert, central Australia. Populations fluctuated synchronously from very low numbers, or “busts,” during prolonged dry periods to high numbers, or “booms,” after heavy rainfall 3 times over the study period. On the basis of observations that food resources expand after rainfall, we predicted that rodents would show increased rates of recapture, fidelity to burrows, and burrow sharing during population increase (boom) phases compared with decline or bust phases, and also reduce their movements and foraging activity in open habitats during population booms. The behavior of both species was similar but not as we had anticipated. Burrow fidelity and numbers of animals per burrow were roughly 2-fold higher during both the population increase and decrease phases as compared with the population-low phase, whereas rates of movement were reduced by about half. As revealed by giving-up density trials, animals foraged less at experimental food patches during population increase and decrease phases than during busts, and also foraged less in open than in covered habitats. Recaptures of N. alexis were similar across all population phases, whereas P. hermannsburgensis was recaptured more often when populations were decreasing than at other times. The results suggest that both species are dispersed and highly mobile during bust periods but sedentary and more social during population increases and collapses. These changes in movements and social organization appear to be unusual in desert rodents, and we propose that future studies seek to identify the roles of food and other factors in driving them.

The fire history of an arid grassland: the influence of antecedent rainfall and ENSO

Implementing appropriate fire regimes has become an increasingly important objective for biodiversity conservation programs. Here, we used Landsat imagery from 1972 to 2003 to describe the recent fire history and current wildfire regime of the north-eastern Simpson Desert, Australia, within each of the region’s seven main vegetation classes. We then explored the relationship between antecedent rainfall and El Nino–Southern Oscillation with wildfire area. Wildfires were recorded in 11 years between 1972 and 2003, each differing in size. In 1975, the largest wildfire was recorded, burning 55% (4561 km2) of the study region. Smaller fires in the intervening years burnt areas that had mostly escaped the 1975 fire, until 2002, when 31% (2544 km2) of the study region burnt again. Wildfires burnt disproportionally more spinifex (Triodia basedowii) than any other vegetation class. A total of 49% of the study area has burnt once since 1972 and 20% has burnt twice. Less than 1% has burnt three times and 36% has remained unaffected by wildfire since 1972. The mean minimum fire return interval was 26 years. Two years of cumulative rainfall before a fire event, rainfall during the year of a fire event, and the mean Southern Oscillation Index from June to November in the year before a fire event could together be used to successfully predict wildfire area. We use these findings to describe the current fire regime.

Spatial dynamics of small mammals in central Australian desert habitats: the role of drought refugia

Abstract Populations of small mammals often fluctuate dramatically in arid environments, persisting at very low density during drought and erupting briefly but dramatically after rain. We selected 3 species that exhibit such boom and bust dynamics in spinifex grassland in the Simpson Desert, central Australia, and asked how they survive prolonged dry periods when they are scarce or absent from the trapping record. We postulated that animals persist by retreating to small patches of open woodland that are embedded within the grassland matrix and predicted that these patches would provide more food and shelter for small mammals than the surrounding grassland during dry periods but not at other times. We also predicted that capture rates and activity of the study species would be higher, and risk of predation lower, in the woodland than in the grassland during drought, and that after heavy rain the rate of return of small mammals to spinifex grassland would be correlated with proximity to patches of woodland. Sampling provided partial support for these predictions. Some food resources (seeds) were more abundant in woodland patches irrespective of environmental conditions, but others (invertebrates) showed no clear pattern; shelter resources were mostly invariant between habitats and times. Of the 3 study species, only the sandy inland mouse (Pseudomys hermannsburgensis) behaved largely as we had anticipated, but even this species was not confined to woodland during droughts. Both the spinifex hopping-mouse (Notomys alexis) and brush-tailed mulgara (Dasycercus blythi) continued to use grassland more than woodland during drought and nondrought periods, although N. alexis showed a tendency to increase its activity in woodland during dry conditions. Trappability remained relatively constant among species, habitats, and times, indicating that the temporal patterns of habitat use we uncovered were real and not artifacts of changes in animal behavior or susceptibility to capture. We conclude that woodland patches contribute importantly to the persistence of P. hermannsburgensis during droughts, whereas N. alexis and D. blythi either use other unidentified refugia at such times or occur at such low densities in grassland habitat that their chances of being captured are very small.

Extreme climatic events drive mammal irruptions: regression analysis of 100-year trends in desert rainfall and temperature

Extreme climatic events, such as flooding rains, extended decadal droughts and heat waves have been identified increasingly as important regulators of natural populations. Climate models predict that global warming will drive changes in rainfall and increase the frequency and severity of extreme events. Consequently, to anticipate how organisms will respond we need to document how changes in extremes of temperature and rainfall compare to trends in the mean values of these variables and over what spatial scales the patterns are consistent. Using the longest historical weather records available for central Australia – 100 years – and quantile regression methods, we investigate if extreme climate events have changed at similar rates to median events, if annual rainfall has increased in variability, and if the frequency of large rainfall events has increased over this period. Specifically, we compared local (individual weather stations) and regional (Simpson Desert) spatial scales, and quantified trends in median (50th quantile) and extreme weather values (5th, 10th, 90th, and 95th quantiles). We found that median and extreme annual minimum and maximum temperatures have increased at both spatial scales over the past century. Rainfall changes have been inconsistent across the Simpson Desert; individual weather stations showed increases in annual rainfall, increased frequency of large rainfall events or more prolonged droughts, depending on the location. In contrast to our prediction, we found no evidence that intra-annual rainfall had become more variable over time. Using long-term live-trapping records (22 years) of desert small mammals as a case study, we demonstrate that irruptive events are driven by extreme rainfalls (>95th quantile) and that increases in the magnitude and frequency of extreme rainfall events are likely to drive changes in the populations of these species through direct and indirect changes in predation pressure and wildfires.

A GRAPH THEORY APPROACH TO DEMOGRAPHIC LOOP ANALYSIS

A demographic analysis of the life-cycle graph can be used to quantify the separate contributions of different life-history types to the population growth rate. Loop analysis has been proposed (van Groenendael et al. 1994) as the appropriate method for partitioning the elasticity matrix to determine these contributions. However, in the analysis of complex demographic models it is difficult to derive the loops by simple inspection of the life-cycle graph. I show how graph theory can be used to describe a general and systematic procedure for deriving the loops from the structure of the life-cycle graph. I demonstrate that the concept of nullity (from graph theory) can be applied in this context to correctly determine the number of loops for any graph. Using examples from Campanula americana, Dipsacus sylvestris, and Caretta caretta, I illustrate the relationship of the loops to biologically relevant life-history contrasts. This relationship is crucial for the application of loop analysis to life-history evolution for the purpose of partitioning the separate effects on the population growth rate among different life-history components.

Evaluation of common methods for sampling invertebrate pollinator assemblages: net sampling out-perform pan traps.

Methods for sampling ecological assemblages strive to be efficient, repeatable, and representative. Unknowingly, common methods may be limited in terms of revealing species function and so of less value for comparative studies. The global decline in pollination services has stimulated surveys of flower-visiting invertebrates, using pan traps and net sampling. We explore the relative merits of these two methods in terms of species discovery, quantifying abundance, function, and composition, and responses of species to changing floral resources. Using a spatially-nested design we sampled across a 5000 km2 area of arid grasslands, including 432 hours of net sampling and 1296 pan trap-days, between June 2010 and July 2011. Net sampling yielded 22% more species and 30% higher abundance than pan traps, and better reflected the spatio-temporal variation of floral resources. Species composition differed significantly between methods; from 436 total species, 25% were sampled by both methods, 50% only by nets, and the remaining 25% only by pans. Apart from being less comprehensive, if pan traps do not sample flower-visitors, the link to pollination is questionable. By contrast, net sampling functionally linked species to pollination through behavioural observations of flower-visitation interaction frequency. Netted specimens are also necessary for evidence of pollen transport. Benefits of net-based sampling outweighed minor differences in overall sampling effort. As pan traps and net sampling methods are not equivalent for sampling invertebrate-flower interactions, we recommend net sampling of invertebrate pollinator assemblages, especially if datasets are intended to document declines in pollination and guide measures to retain this important ecosystem service.

Reproductive ecology of the Australian herb Trachymene incisa subsp. incisa (Apiaceae)

Within the Apiaceae, subtle variation in reproductive characters such as dichogamy, pollinator specificity and umbel density may cause cryptic specialisation and be responsible for the diversity of life histories and gender expression in the family. To address the paucity of information for Australian species we investigated the reproductive ecology of the native perennial herb, Trachymene incisa Rudge subsp. incisa. T. incisa exhibits protandry within flowers and umbels; however, an overlap of 3 days in male and female phases among umbels of consecutive orders permits geitonogamous pollination. There are 72 ± 2.0 (n = 74) white flowers per umbel and nectar is presented during the male and female phases. Apis mellifera appears to be the main diurnal pollinator. The pollen : ovule ratio is 1902 : 1, indicating that T. incisa is a facultatively xenogamous species. The long phase of pollen presentation and the low natural seed set of about 45% implies that many flowers are functioning as pollen donors only. Controlled pollination experiments showed that self-pollen led to lower seed set than cross, open and supplemental applications. Early and late-produced cohorts differed in days to emergence but not in seed mass or final percentage emergence.