Forest Insect Population Dynamics, Outbreaks, And Global Warming Effects

By A. S. Isaev, Vladislav G. Soukhovolsky, O. V. Tarasova and 2 more

Research in insect population dynamics is important for more reasons than just protecting forest communities. Insect populations are among the main ecological units included in the analysis of stability of ecological systems. Moreover, it is convenient to test new methods of analyzing population and community stability on the insect-related data, as by now ecologists and entomologists have accumulated large amounts of such data. In this book, the authors analyze population dynamics of quite a narrow group of insects – forest defoliators. It is hoped  that the methods proposed herein for the analysis of population dynamics of these species may be useful and effective for analyzing population dynamics of other animal species and their effects and role in global warming.

What can insects tell us about our environment and our ever-changing climate?  It is through studies like this one that these important answers can be obtained, along with data on the insects and their behaviors themselves.  The authors present new theories on modeling and data accumulation, using cutting-edge processes never before published for such a wide audience.  This volume presents the state-of-the-art in the science, and it is an essential piece of any entomologist’s and forest engineer’s library. 

• Language: English
• Publisher: Wiley
• Release date: Mar 16, 2017
• ISBN: 9781119407492

Book preview

Introduction

An insect outbreak is one of the first critical events in ecological systems described in world literature (Exodus 10:12). Until now, however, prediction and control of insect populations damaging forest stands and agricultural crops has remained an unresolved issue. The current insect outbreak situation can still be described with the biblical quote: … When it was morning, the east wind had brought the locusts ….

Research in insect population dynamics is important for more reasons than just protecting forest communities. Insect populations are among the main ecological units included in the analysis of stability of ecological systems. Moreover, it is convenient to test new methods of analyzing population and community stability on the insect-related data, as by now ecologists and entomologists have accumulated large amounts of such data.

In this book, the authors analyze population dynamics of quite a narrow group of insects – forest defoliators. We hope, though, that the methods we propose for the analysis of population dynamics of these species may be useful and effective for analyzing population dynamics of other animal species.

Below is a brief description of each chapter in the book.

Chapter 1 is, rather predictably, a review of the literature on modeling forest insect population dynamics. Section 1.3 provides a brief description of the phenomenological theory of population dynamics (Isaev et al., 1984; Isaev et al., 2001).

Chapter 2 discusses the issue that is seldom addressed in the literature – the choice of the way of describing insect population dynamics. In our opinion, for each definite task in the analysis of insect population dynamics, there is a specific way of data presentation: as a time series, a phase portrait, the Lamerey stairs, or potential function. Therefore, we discuss different ways of presenting survey data, as related to the purposes of the analysis.

We think that a necessary condition for the successful analysis of processes occurring in forest ecosystems is a certain irreverence towards the field data. As field ecologists, we know very well how much effort it takes to carry on insect population surveys on the same plot in the forest for many years. On the other hand, we are aware of the inaccuracy of the field data and the inevitable errors in estimates of the density of population dispersed over a vast area. Survey data should not be regarded as something incontrovertibly true but rather as a basis for research activities. These activities should include repair and transformation of the field data, based on the theoretical concepts developed in this research. Before using the survey data for analysis, they need to be cleaned as much as possible, to remove the inevitable errors of surveys, without distorting the time series. Our experience shows that it is important not only to collect the data but also to treat them properly. Therefore, Chapter 2 gives a detailed description of field data repair and transformation. This chapter focuses on the methods used to process survey data and transform an arbitrary time series into the stationary time series, which can then be studied by using standard techniques of correlation and spectral analysis.

Chapter 3 is devoted to the analysis of weather effects on the development of outbreaks of taiga defoliating insects. This subject has been extensively discussed in the literature, especially in the last decades, as related to the possible global climate change. Here we present our understanding of these processes.

Chapter 4 analyzes spatial coherence of population dynamics of the same insect species in different habitats and the temporal coherence of population dynamics of several insect species in the same habitat. Such analysis can be used to reveal interactions between species associated with, for example, competition for food and to estimate possible responses of different species to external impacts such as changes in weather and geophysical parameters.

Chapter 5 describes parasite – host interactions for populations of forest insects and their parasites in different outbreak phases.

In Chapter 6, we present a model of food consumption by insects, which links population dynamics with food properties. We propose a quasi-economic approach to describing food consumption and introduce indicators of food consumption analogous to costs in economics. In this way, we relate the energy and population approaches to the description of the processes in the forest – insect system and approach evaluation of fecundity of individuals – very important parameters for analysis and forecast of insect population dynamics.

Chapter 7 is devoted to modeling time series of forest insect population dynamics by using autoregressive models. The chapter describes models of population dynamics of the larch bud moth and other species of the defoliating insect community in forests of the Alps, the pine looper in Europe, defoliating insects in the Siberian pine forests, the European oak leaf-roller in European Russia, and the gypsy moth in the South Urals. For autoregressive models, we introduce parameters of stability, stability margin, and robust stability, which are used to assess the risks of removal of the species from the community. These models serve as a basis for developing adaptive methods for short-term forecasts of forest insect population dynamics.

Chapter 8 deals with a new method of describing and modeling forest insect population dynamics, based on the presentation of critical events in the population as first- and second-order phase transitions. Using the models of phase transitions, we managed to introduce conditions of the occurrence of forest insect outbreaks, describe the patterns of insect migrations in the forest during an outbreak, and characterize the susceptibility of populations to weather effects.

We consider in Chapter 9 methods of short-, medium-, and long-term forecasting of insect population dynamics based on the approaches described in the previous chapters and methods of assessing the risk of the tree stand damage and death caused by insect outbreaks. In addition to that, Chapter 9 contains a brief discussion of the problems associated with controlling the risks of insect attacks and making decisions about extermination measures based on forest entomological monitoring. We may have given too little consideration to these issues, and they will need to be discussed more thoroughly in a future study.

Finally, in Chapter 10, we discuss the effects of possible global climate change on population dynamics of defoliating forest insects. We use ADL-models and phase transition models developed in this book to assess the risks of outbreaks under various scenarios of climate change.

We hope that this book will be useful to specialists in ecology, entomology, ecological modeling, and forest protection as well as to undergraduate and graduate students of ecology and entomology.

We are grateful to our former and current Ph.D. students – S. Astapenko, Y. Bekker, O. Bulanova, P. Tsikalova, T. Iskhakov, I. Kalashnikova, P. Krasnoperova, V. Kuznetsova, M. Meteleva – for their assistance in different stages of the research. We specially appreciate out deceased colleagues – Yuri P. Kondakov and Viktor M. Yanovsky, with whom we had studied forest insect population dynamics for many years.

Our studies were supported by very many grants of Russian Foundation for Basic Research No. 96-04-48340, 99-04-49450, 00-04-48990, 02-04-48769, 02-04-62038, 03-04-49723, 03-04-62037, 04-04-49821, 08-04-00217, 08-04-07052, 09-04-00412, 10-04-08236, 11-04-00173, 11-04-08064, 15-04-01192.

Chapter 1

Population Dynamics of Forest Insects: Outbreaks in Forest Ecosystems

1.1 Approaches to modeling population dynamics of forest insects

Populations of forest insects constitute one of the components of a community that includes the species we are interested in, species competing for various types of resources, parasites, predators, and host plants. All these species making up an ecosystem are influenced by weather and other external factors. In accordance with the basic theses of systems analysis, it is not correct to study only one ecological component, which, in our case, is a forest insect population. Analysis of the processes occurring in a complex system must be based on the systems approach and investigation of all ecosystem components.

However, the dogmas of the systems analysis collapse once the researchers face their object – a population of a certain species of defoliating forest insects. Long-duration measurements in the forest usually record only local population density. Sometimes, one manages to estimate the mass of individuals, their coloration, female fecundity, and the extent of parasitic infection. Other components, such as predators, usually remain unevaluated. Moreover, in a study of a certain insect species, surveys cannot be performed in every developmental stage of this insect. Thus, the system as a whole eludes the researcher.

In this situation, models of population dynamics serve as an integrator used to combine discrete observations and local experiments in a system, thus enabling the description of both individual characteristics of a forest insect species (the type of long-term population dynamics, intra-population relationships and interactions with parasites and predators, susceptibility to external effects on population dynamics) and general trends in the population dynamics of ecological groups of insects.

Thus, there may be two extreme approaches to constructing mathematical models of population dynamics of a single forest insect species. If the model of the population dynamics of a species takes into account all of its specific biological and ecological properties as well as effects of various external factors in its habitat, this approach will be too detailed and ineffective, as the modeler will need to know too many aspects of the species.

On the other hand, if the approach used for constructing the population dynamics model is too generalized, e.g., if the Lotka–Volterra model is used to describe population dynamics of absolutely all species of forest insects and if individual properties of the species can only be defined by using five coefficients of that unified model, this total approach will predictably be no more fruitful than the extremely detailed one.

The phenomenological approach was previously proposed for modeling forest insect population dynamics: only the main trends in changes of population density were studied, with various factors potentially capable of influencing the insects reduced to two types – modifying (density independent) factors and regulating (density dependent) factors (Isaev, Khlebopros, 1973; Isaev et al., 1984). Analysis of forest insect population dynamics by using phase portraits provided a basis for the qualitative model of outbreak development and classification of the types of forest insect population dynamics and outbreaks.

In this study, we use the systems approach in our search for the necessary generalization level in modeling population dynamics of a single insect species. The descriptions of population dynamics of all forest insect species will be based on the following notions:

– the notion of the existence of three stable states of the population that differ considerably in density.

The first state is degenerate; it characterizes the habitat in which population density of a given insect species is equal to zero. The other two states are characterized by nonzero values of population density. In one of these states (in the outbreak phase), population density is much higher than population density in the other state – in the stable state. Hence, there must be a system of negative feedbacks regulating the behavior of the system close to each of these stable states. If a certain number of insects of one species are introduced into the habitat with the stable population density x = 0, in some time, the reproduction coefficient of the invading population will be equal to zero, all insects will die, and the system will return to the state with the population density of this species equal to zero. Thus, it is very important to define the conditions under which the population will be in one of these stable states, in order to be able to predict population dynamics.

– the notion of the ability of the population to pass from one stable state to another and the attainability of different stable states under certain conditions.

Transitions may be regarded as the critical events in forest insect populations. The transition from the low population density – the sparse stable state – to the high population density – the outbreak phase – is of special interest for ecological theory and forest management, as insect outbreaks are among the main causes of tree damage and die-off of forests in the boreal ecosystem. It is also very important to understand why the population of a species passes from the zero-density state to the stable state with a nonzero density. In recent decades, invasions of new species of forest insects have become more and more frequent, and, thus, it is essential to identify the conditions under which the invading species, initially represented by just a few insects, successfully colonizes the new habitat.

– the notion of two types of factors influencing forest insect population – modifying and regulating factors.

Modifying factors are density-independent factors and changes in the population density do not influence the current value of the modifying factor. The converse is certainly not true, and the sensitivity of population parameters to a change in the value of the modifying factor may be nonzero.

Values of the regulating factors depend on the current and previous states of the population. At low values of population density, sensitivity to the external field is low, modifying factors do not influence the population, and population density variations are determined by the intra-population regulation. As the density grows, sensitivity is rapidly increased, and then weather effects may facilitate development of an outbreak. It is a difficult task to decide whether food is a modifying or a regulating factor. Under certain conditions (usually at a low density of forest insect population), food (tree needles or leaves for forest defoliating insects) may be a modifying factor. However, as the population density is increased and insects consume greater amounts of food, host trees respond by changing the quality of the food. In this case, food is also a regulating factor.

– the notion of the time lag between the impact of external factors and response of the population.

While evaluating the sensitivity of population parameters to the impacts of external factors, one needs to take into account that the time of the impact and the time when the response is first detected may not coincide. Then, there is a time lag between the impact and the response. The length of the time lag may vary, influencing the behavior of the population.

This book reports studies based on the principles of the system of regulating forest insect population dynamics. On the one hand, it describes properties of specific time series of forest insect population density (by using the abundant field data accumulated over the last few decades). On the other hand, the authors employ the theory of phase transitions – a universal approach to the description of critical events, validated by physicists – to describe universal properties of the processes occurring in forest ecosystems and leading to qualitative and quantitative changes in the state of these systems.

The vast majority of forest insects do not undergo any qualitative changes, and their densities remain low for an indefinitely long time, causing no significant alterations in other components of the ecosystem. Only a small number of species may change qualitatively and quantitatively and attain outbreak densities.

1.2 The role of insects in the forest ecosystem

The role of insects in the forest is determined by their specific interactions with the plant community. Phytophagous insects consume part of the vegetable matter as green mass, reproductive structures, cambial tissues, bark, and wood. Insects transform biomass in their vital processes and quickly return it to the soil, making up for the absorbed organic matter and carbon dioxide, which are necessary for successful functioning of the forest ecosystem. Thus, the ecosystem grants part of its biomass to insects for speedy transformation and in this way increases the overall stability of the system. By infesting weakened trees, insects improve the health of the forest. In all phases of tree stand formation, they take part in selection and self-thinning, contributing to the development of stable tree stands.

Every ecological group of insects has its own way of interacting with the plant community. Defoliating insects are external feeders and are, thus, more vulnerable to the impacts of biotic and abiotic factors. Xylophagous insects, which live in the wood and beneath the bark, are closely related to the tree as their habitat. The regulating effect of natural enemies is not a determining factor in the population dynamics of xylophages, as their successful development is mainly related to the availability of suitable food. Insects feeding on reproductive structures (fruits, cones, seeds) have specific ecological relations with their hosts. They infest tree stands at the fruiting time, and their populations do not fluctuate greatly. The qualitatively dissimilar interactions of different ecological groups determine the types of their feedbacks and population dynamics.

The great majority of forest insect species have densities corresponding to stable populations and represent consumers, which generally produce no adverse effects on growth and development of the forest community. Only a small number of species are capable of dramatic density variations and, under suitable conditions, may attain outbreak densities rather quickly (in two or three years). The outbreak does not usually last long (two to three years), and then, under the impact of regulatory mechanisms (parasites and predators, diseases, food shortage etc.), the population enters the critical and decline phases. After that, the population gradually resumes its sparse stable state, completing the outbreak cycle. Outbreaks are not, however, always