Kirk A. Moloney

Finding the Missing Link between Landscape Structure and Population Dynamics: A Spatially Explicit Perspective

We construct and explore a general modeling framework that allows for a systematic investigation of the impact of changes in landscape structure on population dynamics. The essential parts of the framework are a landscape generator with independent control over landscape composition and physiognomy, an individual‐based spatially explicit population model that simulates population dynamics within heterogeneous landscapes, and scale‐dependent landscape indices that depict the essential aspects of landscape that interact with dispersal and demographic processes. Landscape maps are represented by a grid of \documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont} \usepackage{stmaryrd} \usepackage{textcomp} \usepackage{portland,xspace} \usepackage{amsmath,amsxtra} \usepackage[OT2,OT1]{fontenc} \newcommand\cyr{ \renewcommand\rmdefault{wncyr} \renewcommand\sfdefault{wncyss} \renewcommand\encodingdefault{OT2} \normalfont \selectfont} \DeclareTextFontCommand{\textcyr}{\cyr} \pagestyle{empty} \DeclareMathSizes{10}{9}{7}{6} \begin{document} \landscape

Detecting process from snapshot pattern: lessons from tree spacing in the southern Kalahari

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Fine-scale spatial and temporal variation in the demography of a perennial bunchgrass

\end{document} cells and consist of good‐quality, poor‐quality, or uninhabitable matrix habitat cells. The population model was shaped in accordance to the biology of European brown bears (Ursus arctos), and demographic parameters were adjusted to yield a source‐sink configuration. Results obtained with the spatially explicit model do not confirm results of earlier nonspatial source‐sink models where addition of sink habitat resulted in a decrease of total population size because of dilution of high‐quality habitat. Our landscape indices, which describe scale‐dependent correlation between and within habitat types, were able to explain variations in variables of population dynamics (mean number of females with sink home ranges, mean number of females with source home ranges, and mean dispersal distance) caused by different landscape structure. When landscape structure changed, changes in these variables generally followed the corresponding change of an appropriate landscape index in a linear way. Our general approach incorporates source‐sink dynamics as well as metapopulation dynamics, and the population model can easily be modified for other species groups.

Population Dynamics, Disturbance, and Pattern Evolution: Identifying the Fundamental Scales of Organization in a Model Ecosystem

SummaryA revision and extension of Vandermeers algorithm for choosing size categories from demographic data is presented along with an example of its use. The extension permits an exact consideration of sample populations which may have different underlying transition probabilities at different census periods and/or within different subpopulations. Once categories are chosen transition matrices may be computed to investigate the dynamics of the population.

PHENOTYPIC PLASTICITY OF NATIVE VS. INVASIVE PURPLE LOOSESTRIFE: A TWO-STATE MULTIVARIATE APPROACH

The spatial distribution of plants is often thought to be an indicator of underlying biotic and abiotic processes. However, there are relatively few examples of spatial patterns being analysed to detect an underlying ecological process. Using the spacing patterns being analysed to detect an underlying ecological process. Using the spacing of savanna trees in the southern Kalahari as an example, we applied methods of computer simulation modelling and point pattern analysis in an evaluation of their potential for identifying relevant pattern generating processes from snapshot pattern. We compared real tree patterns from the southern Kalahari, derived from aerial photographs, with patterns produced from computer simulation experiments in an investigation of the following questions: does the present pattern of tree distributions allow us to characterize (1) the relative importance of the major driving forces (e.g., competition for moisture, grass fire, herbivory), (2) the spatial dimensions and structures of the underlying processes, and (3) the actual dynamic status of the ecological system (a phase of decline, increase or constancy with respect to tree abundance)? The simulation experiments are based on a well established, spatially explicit, grid-based model that simulates the vegetation dynamics of the major life forms under a realistic rainfall scenario of the southern Kalahari and under the impact of grass fires, herbivory and the formation of localized clumps with increased tree seed availability. For a realistic range of parameters the simulation model produces long-term coexistence of trees and grasses with tree densities that correspond with long-term coexistence of trees and grasses with tree densities that correspond with densities observed in the field. Both real tree distributions derived from acrial photographs and tree pattern produced by the model are characterized by a tendency towards even spacing at small scales, clumping at intermediate scales and randomness or clumping at large scales. However, increasing the spatio-temporal correlation in the formation of seed patches in the model caused an increase in the tendency towards clumping in the tree distribution whereas an increase in seed patch numbers led to a decrease in clumping. Within single simulation runs the tree pattern could change in response to the variable rainfall sequences and the corresponding differences in grass fire frequency: periods of slightly increasing tree numbers caused by higher precipitation were characterized by an increase in tree clumping whereas periods of slightly decreasing tree numbers showed a tendency towards random or even tree spacing. Simulating the transition of an open savanna to a savanna woodland showed that the tree pattern in the transitional phase can be diagnostic of the underlying process: If the transition was caused by improved moisture conditions the transitional phase was characterized by increased clumping in the tree pattern. In contrast, a transition caused by an increase in the number of localized tree seed patches led to a characteristic even spacing of trees. Even though the simulated savanna clearly showed non-equilibrium dynamics, simulation results indicate that the tree population in the simulated area of the southern Kalahari is in a state of long-term tree-grass coexistence with the persisting structure of an open savanna system.

Analysis of fine-scale spatial pattern of a grassland from remotely-sensed imagery and field collected data

In this study, we examined the effects of water depth and temperature on seedling recruitment from a prairie wetland seed bank. We collected seed-bank samples from natural and restored prairie pothole wetlands in northwestern Iowa and combined them into a single sample. We examined seedling recruitment from this seed-bank sample in an experimental study using a factorial design of 4 temperature treatments (5° night and 15° day to 20° night and 30° day) and 3 water-depth treatments (0, 2, and 7 cm).Principal Components Analysis showed that both water depth and temperature had significant effects on the composition of the seedling community as measured by changes in relative stem density and biomass. Water depth had its strongest effects on stem density while temperature had its strongest effects on biomass.For the 22 most common species, stem density varied with water depth for 95% of the species and with temperature for 50% of the species. Most species with water depth responses had lower stem counts as water depth increased, and for the majority of species with temperature responses stem density increased with temperature.Total, annual, and perennial species richness was negatively correlated with water depth. Total and annual species richness was positively correlated to temperature, while perennial species richness was unresponsive to temperature. In addition, species found at low elevations as adults emerged at higher rates in the deep water treatments while species that occurred at higher elevations as adults had their highest emergence rates in the low water treatments.Our results suggest that differences in environmental conditions along coenoclines can affect the initial distribution of species emerging from the soil seed bank. Water depth sorted seedlings according to their adult water-depth tolerances, and temperature determined the proportion of annuals in the seedling community.

Pattern and scale in a serpentine grassland

Most life history studies assume that demographic transition rates within a population are temporally and spatially invariant. A violation of these assumptions may have important consequences in the analysis of long—term processes such as population growth and stability. To examine the potential problems, I studied the demography of a size—classified population of Danthonia sericea, a long—lived grass species, using standard matrix analysis techniques. The study was conducted for two June—to—June transition periods at five locations along a well defined soil/vegetation gradient in a field located in Durham, North Carolina. Demographic transition rates, and predictions of long—term population growth, differed markedly between transition periods and among locations, with the biggest differences being attributable to location. Elasticity analysis indicates that, in all cases studied, a large proportion of long—term population growth can be ascribed to demographic processes associated with individuals in the ...

Comparison of quantitative and molecular genetic variation of native vs. invasive populations of purple loosestrife (Lythrum salicaria L., Lythraceae)

We used auto‐ and cross‐correlation analysis and Ripleys K‐function analysis to analyze spatiotemporal pattern evolution in a spatially explicit simulation model of a semiarid shrubland (Karoo, South Africa) and to determine the impact of small‐scale disturbances on system dynamics. Without disturnities bance, local dynamics were driven by a pattern of cyclic succession, where ‘colonizer’ and ‘successor’ species alternately replaced each other. This results in a strong pattern of negative correlation in the temporal distribution of colonizer and successor species. As disturbance rates were increased, the relationship shifted from being negatively correlated in time to being positively correlated—the dynamics became decoupled from the ecologically driven cyclic succession and were increasingly influenced by abiotic factors (e.g., rainfall events). Further analysis of the spatial relationships among colonizer and successor species showed that, without disturbance, periods of attraction and repulsion between colonizer and successor species alternate cyclically at intermediate spatial scales. This was due to the spatial ‘memory’ embedded in the system through the process of cyclic succession. With the addition of disturbance, this pattern breaks down, although there is some indication of increasing ecological organization at broader spatial scales. We suggest that many of the insights that can be gained through spatially explicit models will only be obtained through a direct analysis of the spatial patterns produced.