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- Herausgeber: John Wiley & Sons
- Kategorie: Wissenschaft und neue Technologien
- Serie: Conservation Science and Practice
- Sprache: Englisch
Part of the Zoological Society of London's Conservation Science and Practice Series, Applied Population and Community Ecology evaluates theory in population and community ecology using a case study of feral pigs, birds and plants in the high country of south-eastern Australia.
In sequence, the book reviews the relevant theory and uses long-term research over a quarter of a century on the population ecology of feral pigs and then community ecology of birds and plants, to evaluate the theory. The book brings together into one volume, research results of many observational, experimental and modelling studies and directly compares them with those from related studies around the world. The implications of the results for future wildlife management are also discussed. Intended readers are ecologists, graduate students in ecology and wildlife management and conservation and pest managers.
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Seitenzahl: 307
Veröffentlichungsjahr: 2012
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Table of Contents
Series Page
Title Page
Copyright
Preface
Chapter 1: Introduction
Population Ecology and Community Ecology Theory
People
Research Activities and Questions
Methods of Study
Feral Pigs as a Model System for Studying Applied Ecology
Structure of the Book
Chapter 2: Applied Population and Community Ecology
Distribution
Invasion
Dispersion
Population Density
Dynamics
Demography
Diminishing Returns
Species–Area Relationships
Abundance, Association and Size of Species
Conclusion
Chapter 3: Environment
Location
Climate
Vegetation and Land Use
Wildlife
Chapter 4: Population Ecology of Feral Pigs
Distribution
Invasion
Dispersion
Population Density
Dynamics
Demography
Movements
Diet
Predation
Parasites and Pathogens
Conclusion
Chapter 5: Ground Disturbance and Feral Pigs
Ground Rooting and Gradients
Damage (Ground Rooting) and Density (Feral Pigs)
Levy Walks
Extent and Dynamics of Ground Rooting
Ground Rooting as an Ecological Process
Conclusion
Chapter 6: Feral Pig Population Management
Warfarin Poisoning
Trapping
Hunting and Other Control
Control as an Ecological Process
Eradication
Conclusion
Chapter 7: Community Ecology
Ground Rooting and Plant Community Ecology
Bird Community Structure
Bird Species–Area Relationship
Frequency Distribution of Recording of Birds
Bird Community Dynamics
Birds and Exotic Pines
Birds and Feral Pig Control
Birds and Fox Control
General Bird Discussion
Conclusion
Chapter 8: The Future: Management Options
Feral Pig Dynamics
Modelling of Feral Pig Control
Lethal Control
Nonlethal Control
Ecological Interactions
Management Aims and Monitoring
Conclusion
Chapter 9: Conclusions
9.1 Applied Population Ecology
9.2 Applied Community Ecology
9.3 Predictions and Observations
9.4 Gaps in Our Knowledge
Appendix A: Long-term Data on Feral Pigs and Ground Rooting in Namadgi National Park, Australia
Appendix B: Association Matrix of Birds Observed at Study Sites in Namadgi National Park, Australia
References
Index
Conservation Science and Practice Series
Published in association with the Zoological Society of London
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Library of Congress Cataloging-in-Publication Data
Hone, Jim.
Applied population and community ecology : a case study of terrestrial vertebrates / Jim Hone.
p. cm.
Includes bibliographical references and index.
ISBN 978-0-470-65864-2 (cloth)
1. Feral swine–Ecology–Australia. 2. Biotic communities–Australia. 3. Vertebrate populations–Australia I. Title.
SF397.83.A8H66 2012
577.8′20994–dc23
2012008287
A catalogue record for this book is available from the British Library.
Wiley also publishes its books in a variety of electronic formats. Some content that appears in print may not be available in electronic books.
Front cover: Wild feral pigs near the Gulf of Carpentaria, North Queensland ©
iStockphoto.com/John Carnemolla.
Back cover: Feral pig in Namadgi National Park. Photo by Peter West.
Cover design by: Edge Creative
www.edgecreativestudio.com
Preface
A scientific study of any wildlife species that occurs over many years or decades inevitably creates a published literature that is scattered around the world. Such a dispersed literature is good for dissemination of results but hard to find or follow for scientists not involved in the original study. Studies of feral pigs and birds in the Australian high country have been published in many journals and books, and some data remains unpublished. The aim of this book is to bring such scattered literature together into one place, and to discuss the results in the context of the wider scientific discipline of applied ecology. New data will also be analysed and published.
My study in Namadgi National Park has been supported by the University of Canberra in many ways and the university is thanked for that support. Initially there was also support from the Australian National University. Field work was assisted by support from the manager and staff of Namadgi National Park. I thank Alan Crowden for discussions and Anne Milligan, Charles Krebs, Don Fletcher, Nick Dexter and Peter Caley for useful comments on draft chapters. Don Fletcher and Nicola Webb are thanked for discussions and data. Discussions with John Parkes and Mary Bomford assisted with the development of ideas on pest eradication. Glen Saunders assisted with references. Astrida Upitis assisted in many ways, from field work to discussions.
The Australian high country is a special place and decades of research therein were both productive and enjoyable.
Chapter 1
Introduction
Ecology is a broad scientific discipline dedicated to the study of individuals, populations and communities. It has many principles and concepts that are relevant to biodiversity conservation, sustainable harvest and pest control of species and communities. Its applications are growing and this book describes and evaluates such applications through use of a case study: the population and community ecology of terrestrial vertebrates, in particular feral pigs (Sus scrofa) and birds in the high country of south-eastern Australia.
The area known as the Australian high country is that part above about 900 m elevation in south-eastern New South Wales (NSW), north-eastern Victoria and the Australian Capital Territory (ACT). The highest part, Mt Kosciuszko, is 2228 m, which is high by Australian standards but obviously low in international terms. The focus of this book is on Namadgi National Park in the ACT and the northern parts of Kosciuszko National Park in NSW. Note that the spelling of ‘Kosciuszko’ has changed; in older books, maps and other publications it was spelled ‘Kosciusko’, but this was an error. ‘Kosciuszko’ is derived from the surname of a Polish patriot. ‘Namadgi’ is from an aboriginal language and refers to the range of mountains south and west of Canberra.
Readers from different countries may interpret the words ‘national park’ differently, so a few words on their meaning in this context are appropriate. In Australia, national parks are a form of conservation reserve with the aims of conserving biodiversity and providing recreation and research opportunities. Typically, national parks in Australia are administered by the local state or territory government, and they exclude agriculture, mining, forestry, large urban areas and recreational hunting. In other countries, some of these exclusions do not apply; for example, agriculture occurs in national parks in Britain and livestock grazing occurs in some parks in the Spanish Pyrenees (Bueno et al. 2009). The International Union for the Conservation of Nature (IUCN)'s definition of a national park is stated in the positive, and the absence of mining, agriculture or forestry is not mentioned (Sinclair 2008: Table 16.1).
Population Ecology and Community Ecology Theory
Population ecology is the study of the dynamics and internal workings of a population—a group of individuals of one species. In this book, the population focus is on the distribution, dispersion, density, dynamics and demography of a population of feral pigs. The responses to feral pig control will also reflect changing demographic rates, especially of survival. Population ecology is discussed in detail in Chapter 2 and then applied to feral pigs in Chapters 4, 5 and 6.
Community ecology investigates the plant and wildlife communities of an area and their interrelationships. In this book, the focus is on the plant (grasses and forbs) and bird communities, particularly in relation to feral pig foraging and the effects of feral pig control. Community ecology is discussed in detail in Chapter 2 and then applied to plants and birds in Chapter 7.
People
Humans have existed in the high country for at least 4000 years (Costin et al. 1979). Aboriginal people lived in and moved through parts of the area in search of food and shelter. Europeans first settled there in the 1800s and established agricultural activities, in particular livestock (sheep and cattle) grazing (Costin et al. 1979; Anon 1986, 2006). Some of that activity was associated with the clearing of woodlands and forests and the introduction of new plants and animals, the latter including feral pigs. Since the 1940s, land use in much of the high country has changed, with the creation of several national parks, in which land and resources are managed for biodiversity conservation, human recreation and water-supply management (Anon 1986, 2006, 2010). Humans are an important component of the story of feral pigs in the high country, because they introduced them. People today have a range of views towards the pigs, seeing them as destructive pests to be eradicated, a resource for hunting or a great species for scientific study.
Research Activities and Questions
Ecological study in the high country has been undertaken over several decades by government scientists, academics and postgraduate students. Much of the research has been by John McIlroy and staff (formerly of CSIRO), Glen Saunders and staff (of NSW Primary Industries), Mike Braysher (formerly of Environment ACT), Brendan Cowled and colleagues (of the Invasive Animals Cooperative Research Centre) and Jim Hone (University of Canberra). Each has different research interests and activities and has studied pigs in a variety of locations within the high country.
The original reason in the early 1980s for managing feral pigs in the high country was concern over impacts on biodiversity and visual impacts. Research began for those same reasons, and also as part of broader exotic disease contingency planning to answer questions such as: What threshold population density is needed for an exotic disease, such as foot and mouth disease, to establish? What might happen if an exotic disease established in feral pigs? and What level of feral pig control is needed to eradicate the exotic disease? Examples of studies of biodiversity, control and links to exotic diseases are described in this book.
Other research questions included: What are the major features of feral pig population ecology? Do feral pigs have an effect on vegetation (community ecology)? What effects do pig control have on the feral pig population? and Are there effects on the bird community? There was also opportunistic research resulting from these studies, for example into the effects of fox control and the effects of wildfire on the bird community. Hence the focus was on applied aspects of wildlife population ecology as well as on the broader topic of community ecology.
Methods of Study
The many studies of feral pigs in the Australian high country have used a range of research paradigms and methods (Table 1.1). Some, such as that of demography (Saunders 1993), have been observational. Others have been experimental, such as that comparing pig survival in areas poisoned and not poisoned (Hone & Stone 1989; McIlroy et al. 1989). Still others have used modelling approaches, such as one on poisoning (Hone 1992a). A few studies combined methods, such as one on pig ecology, impacts and control (Hone 2002), which used multiple working hypotheses (Platt 1964; Chamberlin 1965). The implications of the method of study on the results and inferences are important topics in science, including in ecology (Krebs 1988; McCallum 1995; McArdle 1996) and wildlife management (Walker 1998), and are examined in this book.
Table 1.1 Examples of the research paradigms (after Sibly & Hone 2002) and methods of study used during research on feral pigs and birds in the Australian high country.
In several chapters of the book, the theoretical bases of population and community ecology and feral pig control will also be described and evaluated, to demonstrate some of the ecological processes that can generate observed patterns. The link between patterns and processes is an important topic in science (Cale et al. 1989).
A methodological theme throughout the book is that there are two different, but related, views on analysis of ecological data. The first compares a variable with and without a treatment; for example, with feral pigs and without feral pigs (Figure 1.1a). The second focuses on the value of a variable (y) at different levels of the treatment (x); for example, levels of feral pig density or ground disturbance (Figure 1.1b). The first view focuses on tests of difference, such as a Student's t test or one-way analysis of variance, and the second view focuses of regression analysis and estimation of regression parameters. The first view is a subset of the second view. The second view has greater utility for wildlife management and for further analysis, including prediction, reflecting greater understanding of ecological processes, and is used frequently in this book. Evidence of association, such as a regression relationship, does not by itself establish causality. The latter requires time ordering, a mechanism and uniqueness (Cox 2007; Cox & Donnelly 2011).
Figure 1.1 Alternative themes in data presentation and analysis. A comparison of the values of a variable y (a) with and without a treatment x and (b) with different levels of the treatment x and with the fitted linear regression line.
Feral Pigs as a Model System for Studying Applied Ecology
Wild pigs occur as wildlife in many parts of the world, being naturally distributed as wild boar through much of Europe and Asia. Feral (domestic gone wild) populations also occur for example in parts of Australia, New Zealand, the USA, Britain and South America, and released wild boar are found in parts of the USA (Tisdell 1982). Fascinating aspects of the early history of pigs in the eastern USA, namely the conflicts between farmers who suffered crop damage by pigs and the ‘owners’ of the pigs, are described by Conover (2007). Wild boar became extinct in Denmark, Ireland and Britain (Wallis De Vries 1995), though the species has been reestablished in parts of Britain (Wilson 2004). In different places, the pigs are variously called ‘wild pigs’ (synonymous with ‘feral pigs’), ‘feral hogs’ and ‘feral swine’.
Feral pigs occur in much of northern and eastern Australia (Hone & Waithman 1979; Tisdell 1982; Hone 1990a; Strahan 1995; Choquenot et al. 1996). They were introduced to Australia with European settlement (Frith 1973; Tisdell 1982) and may be derived from pigs from both Europe and Asia (Gongora et al. 2004). Feral pigs now occur in parts of the high-elevation areas in south-eastern Australia, including in Namadgi National Park (Figure 1.2) and Kosciuszko National Park.
Figure 1.2 Feral pigs in subalpine woodland in Namadgi National Park. There are nine feral pigs in the photograph. The trees in the background are resprouting after a wildfire killed their aboveground branches. (Photo by Cameron Hall).
Feral pigs are of ecological interest because they are an environmental pest and cause problems in agriculture. They are also a resource, providing sustained meat harvest (Tisdell 1982). Hence some people want to control feral pigs, some to eradicate them and some to conserve them. A similar variety of social views occur for other species, especially native species that are the subject of conservation and pest-management actions (Woodroffe et al. 2005). Wild pigs are a disease host, being infected in parts of the world with a range of livestock diseases, such as bovine tuberculosis and foot and mouth disease. Feral pigs can be studied as herbivores as their diet is mostly green vegetation (Giles 1980), and can also be studied as carnivores, including as predators of lambs (Pavlov & Hone 1982). The pigs can be studied as a single species in population ecology and as part of an ecosystem in community ecology. People undertake intensive feral pig control, which is mostly lethal control, such as poisoning, trapping and shooting (Frith 1973; Tisdell 1982; Choquenot et al. 1996). Aspects of this control can be studied as analogous to the functional response of a predator, with the controller being the predator. Nonlethal control, such as fertility control, is a future possibility. Hence the studies of feral pigs can provide insights into a range of other species- and wildlife-management topics.
Over the duration of the studies described in this book, wild pigs have had a variety of labels or names. They have been described sequentially as noxious animals, feral animals, vertebrate pests and invasive animals. Their ecology, effects and control have not changed as a result of the relabelling.
Structure of the Book
This book is an attempt to bring together in one place many ecological and management results that are scattered through a variety of scientific journals and other publications in Australia and elsewhere. The book initially describes relevant theory in population and community ecology (Chapter 2). It then examines the high country environment (Chapter 3). Population ecology of feral pigs is discussed in Chapter 4, with an emphasis on the empirical results and how they illustrate principles and concepts in population ecology, such as density, dynamics and demography. The patterns and processes of ground rooting (disturbance) by feral pigs are examined in Chapter 5 and the management of feral pigs in Chapter 6. The latter topic has been the focus of much research. The effects of feral pigs on the plant community of subalpine grassland and the structure and dynamics of the bird community are then discussed in Chapter 7. Empirical results are emphasised in Chapters 5–7, with discussion of the principles and concepts of community ecology, especially the species–area relationship. Aspects of future research and management are discussed in Chapter 8. The advances in our knowledge of population and community ecology and management that have been outcomes of the research are summarised in Chapter 9. The book deliberately links to relevant ecological studies from around the world, as it is a book about science, not simply about killing feral pigs. A set of appendices documents some key data obtained over the past quarter of a century of study, to assist future researchers and managers of feral pigs and birds in the high country and beyond.
The core ecological chapters (4–8) have a common structure. Initially, the relevant principles and concepts in ecology are introduced. The study designs used to evaluate these ecological ideas are then described, followed by the key results. Finally, the implications are described, and the chapter is summarised briefly.
Chapter 2
Applied Population and Community Ecology
The topics of population and community ecology are broad. In this chapter, selected aspects of the application of topics in ecology are examined briefly in sequence. Empirical aspects of the same topics are then examined in Chapters 4–8.
Distribution
The distribution of a species can range from continuous—occurring everywhere—to patchy. In a patchy distribution the patches can range from being independent, such as in a set of relic populations, to non-independent because of movement of individuals between patches. The latter is a metapopulation, such as has been reported in some butterfly species (Hanski 1999).
Invasion
The central concept of invasions is that a species spreads from one location and community to another. The topic has a long history of empirical and theoretical development (Elton 1958; Drake & Mooney 1989; Davis 2009). Invasion is often divided into three phases or stages (Shea & Chesson 2002). The first stage is introduction, which is related to movements and scales. The second stage is establishment, which occurs when the net reproductive rate (R) is greater than 1.0 and is essentially population ecology. It is a function of the niche and niche opportunity (Shea & Chesson 2002). Community interactions are important because an invader needs to have lower resource requirements than its competitor(s) (Tilman 1982). The third stage of invasion is spread, which is again related to movements and scale.
Dispersion
Dispersion is the spatial pattern of individuals in a population. Individuals are often clumped (or ‘aggregated’) (Taylor 1961) in many animal and plant populations (Anderson et al. 1982). Rarely are individuals randomly or regularly dispersed within a population (Taylor 1961).
Population Density
Population density is defined as the number of individuals per unit area. Density can be estimated using a range of methods (Seber 1982; Krebs 1999; Lancia et al. 2005), usually involving counting individuals or groups, mark-recapture, or counting signs such as dung. The evaluation of indices has been encouraged (Engeman 2003, 2005) and debated (Anderson 2003). In this study, indices are evaluated, as described in Chapter 4.
A variety of density-estimation methods have been used in the Australian high country for feral pigs and birds. These include mark-recapture analysis (McIlroy et al. 1989; Pech & McIlroy 1990), dung counts (Hone 1988a) and area counts (Osborne & Green 1992; Hone 1995). These are described in more detail in Chapters 4–7.
Dynamics
Trends in wildlife density are of great interest, as they can be used to assess whether a population is increasing, decreasing or stable over time. Table 2.1 lists many measures of trend in use. The most common measures are percentage change, the finite population growth rate (λ) and the instantaneous population growth rate (r) (Table 2.1). Rates are commonly measured over 1 year. The percentage change is used in assessments of conservation status, such as those listed by IUCN: namely threatened, vulnerable and endangered (www.redlist.org). A short example illustrates the differing measures: A population that doubles from 1000 to 2000 over 1 year shows a 100% increase per year, an annual finite population growth rate (λ) of 2.0 and an annual instantaneous population growth rate (r) of 0.692. A population that halves over 1 year from 1000 to 500 shows a decrease of 50%, an annual finite rate of 0.5 and an annual instantaneous rate of −0.692. Note that the instantaneous rate is symmetric around 0 and the finite rate is not.
Table 2.1 Methods of estimating trends in wildlife population size (abundance) or density.
Method
Comments
1. Eyeballing data
Used to assess visually increases, decreases or stability as part of exploratory data analysis, though it is insufficient by itself
2. Percentage (%) change
Often used to summarise trends, e.g. 20% increase per year
3. Finite population growth rate (λ)
The ratio of abundance (or density) in successive time periods (Caughley 1980; Lande 1988) or the solution of the Euler–Lotka equation using demographic rates (Lande 1988) or mark-recapture analysis (Pradel 1996)
4. Average rate of change
The slope of the linear regression of abundance (or density) over time (Hatch 2003)
5. Log-linear population growth rate
The slope of the linear regression of the logarithm (to base 10) of abundance (or density) over time (Alford & Richards 1999)
6. Instantaneous population growth rate (
r
)
The slope of the linear regression of the logarithm (to base e) of abundance (or density) over time (Caughley 1980) or the solution of Euler–Lotka equation using demographic rates (Caughley 1980). Methods 5 and 6 have different estimated slopes, though Method 6 assumes exponential population growth and Method 5 does not
Is there an expected pattern in annual wildlife population growth rate (r)? In the long term, over years and decades, it is expected that there will be no trend. That is, the expected mean annual r is 0 (Caughley 1980; Sinclair 1997; Hone 1999; Sibly & Hone 2002; Sinclair et al. 2006). This requires discussion, as it is often misunderstood or misinterpreted. Obviously, populations increase in some years and decrease in others, and sometimes do not change. When one estimates the annual rate (r) for a population and collates all such estimates over time, the data set shows a frequency distribution with a mean and variance. The mean annual r has been reported for unmanipulated populations of species, namely the red fox (Vulpes vulpes), European rabbit (Oryctolagus cuniculus) and house mouse (Mus domesticus), in Australia (Hone 1999) and for a harvested population of the feral goat (Capra hircus) in Egmont National Park in New Zealand (Forsyth et al. 2003). Note that the taxonomy of house mouse in Australia has been a topic of review and the species is also classified as Mus musculus (Van Dyck & Strahan 2008). The maximum annual population growth rate (rm) of the nonharvested populations was highest for the smallest species (mouse) and smallest for the largest species (fox). The value of rm is negatively related to body mass across a wide range of organisms, including mammals (Duncan et al. 2007), as predicted by the metabolic theory of ecology (Brown et al. 2004).
Each of the first three species had a mean annual r that was not significantly different from 0, and examples of their frequency distributions are shown in Figure 2.1. The frequency distribution of annual r should be slightly skewed to the left: a population can decrease more than it can increase in any 1 year. For example, a feral horse (Equus caballus) population can decrease in density by 50% in 1 year (a drop from 2000 to 1000) but cannot increase by the corresponding amount (1000 to 2000) as horse populations have a maximum annual growth rate of about 25% (Eberhardt 1987), so can only increase over 1 year from 1000 to 1250. Populations were predicted to spend more time with a negative annual growth rate—that is, to be above carrying capacity (K) (Sibly et al. 2005)—and the prediction was supported empirically (Sibly et al. 2007). The frequency distribution of annual λ should be skewed to the right, as a population can increase by a certain amount with a biological limit. However, λ can only decrease to 0 in 1 year, and that corresponds to extinction in that year. An example of the distribution of λ values across bird species showed an apparently right-skewed distribution (Green & Hirons 1991).
Figure 2.1 The frequency distributions of annual instantaneous population growth rates (r) of the European rabbit (solid line), red fox (dashed line) and house mouse (dotted line) in Australia (modified from Hone 1999) and the feral goat (dashed and dotted line) in Egmont National Park in New Zealand (after Forsyth et al. 2003).
The change in density over time is the core business of population dynamics. Much has been written on this topic (Berryman 1999; Sibly et al. 2003), perhaps more than on many other topic in ecology. Here is not the place to describe it all, but a few points need to be made. First, ecologists have used different approaches or paradigms in studying dynamics. These reduce to studying the effects of density on dynamics (the density paradigm), the effects of demographic (birth and death) rates on dynamics (the demographic paradigm) and the effects of mechanistic factors such as food, predation, parasites and competitors on dynamics (the mechanistic paradigm) (Sibly & Hone 2002; Begon et al. 2006). These approaches can also be used in combination, such as studying the effects of demographic rates and also the effects of food on the effects of demographic rates, as is reported for barn owls (Tyto alba), for example Hone & Sibly (2002). Studies of feral pigs have used all three approaches (Table 1.1); for example, the density paradigm was used by Hone (2002, 2007), the demographic paradigm was used by Saunders (1993) and the mechanistic paradigm was used by McIlroy et al. (1989), Hone & Stone (1989) and Choquenot (1998).
Second, some attempts have been made to identify generalisations, or principles, of population dynamics. For example, one principle is exponential growth (Berryman 1999). The dynamics of large herbivores has been described as characterised by constant annual adult survival and variable juvenile survival (Gaillard et al. 1998; Eberhardt 2002). The colonisation of an area by large mammals, such as ungulates, has been hypothesised to show an eruptive (also written as ‘irruptive’) pattern in density, reflecting high initial fecundity and survival, followed by a decline in density as juvenile survival decreases and then fecundity decreases (Caughley 1970; Forsyth & Caley 2006).
Demography
The demographic approach has been the focus of much ecological research. Some links to dynamics were noted above. The connections between demographic rates of fecundity and survival are shown in Lotka's equation (2.1), relating age-specific fecundity (mx), age-specific survival (lx) for each age (x) class, and annual instantaneous population growth rate (r) (Caughley 1970).
2.1
Many examples of such analyses and data have been published (Caughley 1970; Krebs 2009). A simplified version assumes that survival rates are summarised as juvenile survival (l) and annual adult survival (s). The two-stage Lotka equation is:
2.2
where λ is the annual finite population growth rate, l is survival from birth to age at first reproduction (α) and b is annual fecundity (mean female young per adult female per year). Examples of use are for the northern spotted owl (Strix occidentalis caurina) (Lande 1988), barn owl (Hone & Sibly 2002), red fox (Berghout 2000), feral horse (Walter 2002; Dawson & Hone 2011) and eastern grey kangaroo (Macropus giganteus) (Fletcher 2006). Matrix models (Caswell 2001) are an alternative to the use of such equations.
A related analysis that estimates, formally, the effect on annual finite population growth rate (λ) of a unit change in a demographic rate is sensitivity analysis (Lande 1988). In some species, such as the red fox, juvenile survival has the greatest effect on annual finite growth rate (McLeod & Saunders 2001), while in others, such as the feral horse (Walter 2002), the greatest effects are caused by a change in annual adult survival. In other studies, different results do occur. Analysis of red fox populations showed highest sensitivity to annual adult survival in stable and increasing populations and highest sensitivity to juvenile survival in a declining population (Berghout 2000). Eberhardt (2002) examined the topic for many large mammals and reported a common pattern of annual adult survival having the highest sensitivity. The use of sensitivity analysis for wildlife management is limited by the capacity of wildlife managers to manipulate a particular demographic rate. For example, annual adult survival may have higher sensitivity than juvenile survival but be relatively costly for managers to manipulate, while juvenile survival might be easier (cost little) to manipulate.
Diminishing Returns
In production economics, a guiding principle is that for each unit increase in inputs, such as fertiliser on a crop, there is a progressively smaller increase in outputs, such as yield. This is the principle of diminishing returns (Blake 1968; Gans et al. 2009). In managing feral pigs, or other vertebrate pests, the principle can be applied to the response, such as yield or biodiversity, to different levels of pest-control effort, such as the number of traps used per night.
In wildlife management generally, diminishing returns can be expected but may not always be detected. For example, there was a positive, linear relationship between the annual population growth rates of both African elephants (Loxodonta africana) and black rhinoceros (Diceros bicornis) and efforts to control poachers in parts of Africa (Leader-Williams & Albon 1988). If poacher control had reached even higher levels, the positive relationship should be curved, concave down, reflecting diminishing returns and the existence of a maximum annual population growth rate.
Species–Area Relationships
Many hypotheses have been proposed to account for observed patterns in species richness in communities. These include effects of history, habitat complexity and disturbance (Begon et al. 2006; Krebs 2009). These have been expressed for plants, for example, as disturbance effects by feral pigs (Hone 2002), and for birds as effects of habitat change of various forms (Ford et al. 2001; Ford 2011), fire (Gill & Catling 2002; Keith et al. 2002; Lindenmayer et al. 2008a) and exotic species such as pines (Lindenmayer et al. 2008b).
In community ecology, a very common pattern is that large islands have more species than small islands (Preston 1962; Begon et al. 2006; Krebs 2009). This has been reported for taxa ranging from plants to butterflies to birds. The species–area relationship is part of the theory of island biogeography (MacArthur & Wilson 1967). A common equation used to describe the relationship is:
2.3
Figure 2.3 The species–area relationship for herbivorous marsupials on islands near Tasmania. (Modified from Caughley & Sinclair (1994: Table 15.1)) The solid line represents the fitted linear log10-log10 regression.
A prediction of the species–area relationship is that pests can do one or more of the following: kill (as a predator), compete with or share a pathogen with native species, so reducing species richness without changing the area of an island. An example of predation by an invasive species is that by brown tree snakes (Boiga irregularis) against forest birds on the Pacific Ocean island of Guam (Savidge 1987). An effect of such predation is that the species–area curve is shifted downwards. Control of such pests would be predicted to increase the curve.
