Katriona Shea

THE INTERMEDIATE DISTURBANCE HYPOTHESIS: PATCH DYNAMICS AND MECHANISMS OF SPECIES COEXISTENCE

The intermediate disturbance hypothesis (IDH) has been used for several decades as an explanation for the coexistence of species in ecological communities. It is intuitively simple, but deceptively so. We show, via discussion and examples, that the IDH is not one mechanism of coexistence, but rather summarizes a set of similar phenomena that can arise from the action of several different coexistence mechanisms. These underlying mechanisms are defined by the various ways in which species differ in their response to disturbance-induced spatial and temporal variability in resources and environmental conditions. As an example, the original specification of the IDH required patchy disturbances for coexistence. However, because the underlying mechanisms of coexistence can also operate at the within-patch scale, patchy disturbances are not a necessary requirement for coexistence under intermediate-disturbance regimes. These conclusions are illustrated through the analysis of three models: a spatial within-patch model, a spatial between-patch model, and a purely temporal model. All three generate similar patterns of coexistence under intermediate disturbance, yet underlying that coexistence lie at least two quite-distinct mechanisms of species coexistence: the storage effect and relative nonlinearity. The results from our analyses suggest that, as a promoter of species coexistence, the IDH is both broader in scope and richer in detail than has previously been recognized.

Integrating the study of non-native plant invasions across spatial scales.

Non-native (alien, exotic) plant invasions are affecting ecological processes and threatening biodiversity worldwide. Patterns of plant invasions, and the ecological processes which generate these patterns, vary across spatial scales. Thus, consideration of spatial scale may help to illuminate the mechanisms driving biological invasions, and offer insight into potential management strategies. We review the processes driving movement of non-native plants to new locations, and the patterns and processes at the new locations, as they are variously affected by spatial scale. Dispersal is greatly influenced by scale, with different mechanisms controlling global, regional and local dispersal. Patterns of invasion are rarely documented across multiple spatial scales, but research using multi-scale approaches has generated interesting new insights into the invasion process. The ecological effects of plant invasions are also scale-dependent, ranging from altered local community diversity and homogenization of the global flora, to modified biogeochemical cycles and disturbance regimes at regional or global scales. Therefore, the study and control of invasions would benefit from documenting invasion processes at multiple scales.

Measuring plant dispersal: an introduction to field methods and experimental design

The measurement of plant dispersal is vital for understanding plant distribution and abundance at different scales. However, dispersal is difficult to measure and there is a lack of guidance for researchers new to the subject. In this paper we provide advice on methods for measuring dispersal in the field and approaches to experimental design. First, we encourage clear exposition of the aims of the dispersal study and the ultimate use to which the data will be put (e.g. local dynamics, invasion processes, etc). We outline the types of dispersal exhibited by plants and emphasise that many species are dispersed by multiple processes, which are not necessarily related to putative adaptations. Few studies properly address the full range of processes by which a species is dispersed. We review methods for measuring plant dispersal, summarising the type of dispersal measured and problems with each method. We then outline the major questions about effort to be considered in sampling protocols and present an optimisation algorithm for designing dispersal studies given a suite of options, and biological and resource constraints. We propose and demonstrate a simulation modelling approach to comparing the data quality obtained by alternative experimental designs. Integrating simulation models with pilot studies offers a rapid route to improved estimation methods. We then discuss functions commonly fit to dispersal data and recommend caution as none is a priori the best description of the dispersal process. Finally, we call for a better description and understanding of dispersal kernels by: a more rigorous approach to designing dispersal measurement; better targeting of dispersal studies to particular questions; and achieving a deeper understanding of the mechanisms underlying dispersal, so that we can move from descriptions of pattern to a grasp of process.

ACTIVE ADAPTIVE MANAGEMENT IN INSECT PEST AND WEED CONTROL: INTERVENTION WITH A PLAN FOR LEARNING

A major problem in insect pest and weed management is uncertainty. Managers are faced with three main types of uncertainty: uncertainty about biological and environmental processes, and observational uncertainty. Active adaptive management (AAM) is management with a deliberate plan for learning about the managed system, so that management can be improved in the face of uncertainty. We discuss the potential benefits of applying AAM to insect pest and weed control with reference to a number of examples. We first address the possible uses for AAM in biological control, with particular reference to agent selection and release. We also propose applying AAM methods to resistance management and to spatial strategies for pest control. We conclude with an overview of AAM, a discussion of some of the potential limitations to its use in pest management, and the possibilities for increased implementation of AAM in the future.

Management of populations in conservation, harvesting and control

Conservation, harvesting and pest control are three aspects of the same general problem: population management. All three involve intervention with the aim of regulating population size and growth in some way, yet the dissociation of these disciplines is pervasive. Recent developments and a comparison of approaches show the potential of a synthetic paradigm.

CONTEXT-DEPENDENT BIOLOGICAL CONTROL OF AN INVASIVE THISTLE

Carduus nutans (nodding or musk thistle) is an important invasive plant of Eurasian origin. Biological control of this species, using insects that attack rosettes or developing seed heads, has met with varied success in different parts of its invaded range. Here we develop and compare simple demographic matrix models for populations of this species in Australia and New Zealand, to explore reasons for these differences. In a New Zealand population, rapid population growth of C. nutans is driven by early life history transitions. In an Australian population, fecundity of C. nutans is of reduced importance, and survivorship of rosettes plays an increased role. These differences suggest how biocontrol agents that are successful at providing control in one situation may fail in another. Theoretical explorations of the models show which life history transitions drive the differences in matrix elasticities. We suggest that characteristics of the invaded community also play a role in invasion success of this spec...

Dispersal patterns, dispersal mechanisms, and invasion wave speeds for invasive thistles.

Understanding and predicting population spread rates is an important problem in basic and applied ecology. In this article, we link estimates of invasion wave speeds to species traits and environmental conditions. We present detailed field studies of wind dispersal and compare nonparametric (i.e., data‐based) and mechanistic (fluid dynamics model–based) dispersal kernel and spread rate estimates for two important invasive weeds, Carduus nutans and Carduus acanthoides. A high‐effort trapping design revealed highly leptokurtic dispersal distributions, with seeds caught up to 96 m from the source, far further than mean dispersal distances (approx. 2 m). Nonparametric wave speed estimates are highly sensitive to sampling effort. Mechanistic estimates are insensitive to sampling because they are obtained from independent data and more useful because they are based on the dispersal mechanism. Over a wide range of realistic conditions, mechanistic spread rate estimates were most sensitive to high winds and low seed settling velocities. The combination of integrodifference equations and mechanistic dispersal models is a powerful tool for estimating invasion spread rates and for linking these estimates to characteristics of the species and the environment.

How frequency and intensity shape diversity–disturbance relationships

Understanding the relationship between disturbance regimes and species diversity has been of central interest to ecologists for decades. For example, the intermediate disturbance hypothesis proposes that diversity will be highest at intermediate levels of disturbance. Although peaked (hump-shaped) diversity–disturbance relationships (DDRs) have been documented in nature, many other DDRs have been reported as well. Here, we begin to theoretically unify these diverse empirical findings by showing how a single simple model can generate several different DDRs, depending on the aspect of disturbance that is considered. Additionally, we elucidate the competition-mediated mechanism underlying our results. Our findings have the potential to reconcile apparently conflicting empirical results on the effects of disturbance on diversity.

Modeling the Mutualistic Interactions between Tubeworms and Microbial Consortia

The deep-sea vestimentiferan tubeworm Lamellibrachia luymesi forms large aggregations at hydrocarbon seeps in the Gulf of Mexico that may persist for over 250 y. Here, we present the results of a diagenetic model in which tubeworm aggregation persistence is achieved through augmentation of the supply of sulfate to hydrocarbon seep sediments. In the model, L. luymesi releases the sulfate generated by its internal, chemoautotrophic, sulfide-oxidizing symbionts through posterior root-like extensions of its body. The sulfate fuels sulfate reduction, commonly coupled to anaerobic methane oxidation and hydrocarbon degradation by bacterial–archaeal consortia. If sulfate is released by the tubeworms, sulfide generation mainly by hydrocarbon degradation is sufficient to support moderate-sized aggregations of L. luymesi for hundreds of years. The results of this model expand our concept of the potential benefits derived from complex interspecific relationships, in this case involving members of all three domains of life.

Competing harvesting strategies in a simulated population under uncertainty

We present a case study of the use of simulation modelling to develop and test strategies for managing populations under uncertainty. Strategies that meet a stock conservation criterion under a base case scenario are subjected to a set of robustness trials, including biased and highly variable abundance estimates and poaching. Strategy performance is assessed with respect to a conservation criterion, the revenues achieved and their variability. Strategies that harvest heavily, even when the population is apparently very large, perform badly in the robustness trials. Setting a threshold below which harvesting does not take place, and above which all individuals are harvested, does not provide effective protection against over-harvesting. Strategies that rely on population growth rates rather than estimates of population size are more robust to biased estimates. The strategies that are most robust to uncertainty are simple, involving harvesting a relatively small proportion of the population each year. The simulation modelling approach to exploring harvesting strategies is suggested as a useful tool for the assessment of the performance of competing strategies under uncertainty.