As we explained in Chapter 1, environmental variables, including the weather and climate, can only influence bird populations if they alter demography: the reproductive or mortality rates and, for the subdivided parts of a closed population, immigration and emigration as well. In the previous chapter we discussed the effects of phenological mismatch on bird populations, but that is just one of the ways in which climate change can have an impact on demography (Table 4.1). These other mechanisms are the focus of this chapter. We will not restrict ourselves only to studies of climate change impacts, but also review the wider range of studies which have looked at the relationships between bird population processes and temperature, precipitation and other weather variables. Whilst many of these will really be examining the effects of annual variation in the weather (as opposed to long-term trends in climatic averages), they may still be useful in helping us understand the impact of climate change upon bird populations in the future.
Long-term studies are necessary in order to adequately describe how populations respond to annual fluctuations in the weather, and especially to see how population size is affected by longer-term changes, including recent climate change (which we regard as a long-term change in those weather variables, ideally over a minimum 30-year period, although many studies putatively demonstrating impacts of climate change span shorter periods). One of the longest such studies is that of the annual heronry census, coordinated by the BTO since 1928, which has been used to demonstrate the sensitivity of grey heron Ardea cinerea populations to cold winter weather (North 1979; Reynolds 1979). The impact of severe winters can be clearly seen leading to periodic population declines, but in response to a run of mild winters from the late 1980s to late 2000s, the population remained high and stable (Figure 4.1). Such large-scale population monitoring programmes are now widely established across Europe and North America, and often use the observations of amateur ornithologists, collected using standardised methods (e.g. Anders & Post 2006; Gregory et al. 2009; Moller & Fiedler 2010) to deliver large-scale monitoring for the production of robust population trend estimates (van Strien et al. 2001; North American Bird Conservation Initiative, US Committee 2011). However, for many species and countries elsewhere, these annual monitoring data do not exist, which is an obstacle to scientific understanding and effective conservation action (Amano & Sutherland 2013).
This book is about the impact of global climate change on birds, especially on their populations and conservation status, and what can be done about it. Birds are widespread in their distribution and occur in almost all environments. People enjoy watching them and many are easy to observe. As a result, they have long been studied by both amateur naturalists and professional scientists and they are amongst the best understood group of organisms. Data exist on the migration of birds from ringing (banding) studies and the direct observation of arriving and departing individuals, on their historical distribution from museum specimens, archaeology, literary and other sources, and on the timing and success of their breeding from nest recording that span many decades, or in the case of museum specimens, over a century. More recently, quantitative counting and mapping techniques have provided up to 50 years of standardised population and distribution data collection (Møller & Fiedler 2010). The internet is now being used to collect millions of sightings from bird watchers every year, whilst recent technological advances allow almost real-time tracking of migrating birds. These data provide an unparalleled opportunity first to understand the relationship between climate and species distributions and populations, and second to document changes in those distributions and populations occurring as a result of climatic change. Critically reviewing and documenting these kinds of evidence and what they tell us about the impacts of climate change on birds is one of the main purposes of this book, covered in Part 1.
Unfortunately, popular as they are, many bird species and populations are under threat. Of the 10 064 bird species identified around the world, some 13% are regarded as threatened by extinction within the next 100 years. Another 880 species are near-threatened (BirdLife International 2012a). Populations of habitat-specialists and shorebirds are in particular decline (Butchart et al. 2010). The threat of extinction which these species face is a real one; 103 species have been lost forever during the last 200 years. There is an urgent need for effective bird conservation to halt these trends. Whilst there have been significant conservation successes, these have only slowed, rather than halted, global rates of biodiversity loss (Butchart et al. 2010; Hoffman et al. 2010). Conservationists are winning occasional battles, but seem to be losing the war.
Why timing matters
In the previous chapter we saw that many bird species have altered the timing of their migration and breeding as a result of recent climate change. At first sight, the amounts of change seem trivial, averaging 2–3 days advance per decade, but cumulatively they mean that over the last 30 years, birds may now be arriving or breeding a week earlier than they used to. This is important because in many bird species, the timing of these events has evolved to match the timing of peak resource requirements, and therefore any disruption in that timing may affect birds’ breeding success or survival. Most obviously, the time at which birds are rearing their young and need a lot of readily available food often coincides with the greatest abundance of that food. Tits (Paridae) breeding in oak woodland try to maximise the availability of caterpillars, particularly of the larvae of moths such as the winter moth Operophtera brumata, and Tortrix species at the time when nestlings are being reared (Visser et al. 2006; Both et al. 2009). Linked with this, sparrowhawks Accipiter nisus, which find recently fledged tits easy prey, time their reproduction so that the peak abundance of tit fledglings coincides with their own nestling period (Newton 1986; Nielsen & Møller 2006; Both et al. 2009). On the coast, many seabirds breed when large numbers of small fish are maximally available (Durant et al. 2004a, 2004b, 2005). Arctic and upland nesting waterfowl and waders time their breeding to fit in the narrow window of snow-free conditions on the ground, and when there is a superabundance of invertebrate prey (Pearce-Higgins et al. 2005; Meltofte et al. 2007a). The breeding of many tropical species is triggered by rainfall and timed to match peaks in invertebrate or seed resources (Section 2.7).
What is phenology and why does it matter?
We are all familiar with the changing seasons of the natural world around us.
At medium and high latitudes, spring is characterised by budburst, leaf growth and the flowering of many plants, the arrival of long-distance migrants and breeding of most bird species, and the emergence of adult stages of many insects. Autumn is signalled by the departure of long-distance migrants and leaf fall of deciduous plants. In both freshwater and marine environments, spring warming stimulates first phytoplankton and then zooplankton blooms which provide food for higher predators, influencing the timing of fish and bird breeding seasons. In the humid tropics, where temperature regimes vary less throughout the year, seasonality is often determined by predictable variation in rainfall, which then stimulates a flush in plant growth, flowering and fruiting, animal emergence and breeding. In the dry tropics, many organisms adjust their life cycles to the unpredictable arrival of rains or fires, which then lead to a pulse of growth and biological activity. Phenology is the study of the timing of these events.
So far in this book we have described the mechanisms by which climate change affects birds, reviewed potential future changes in their distribution and abundance caused by climate change and discussed the implications of these for conservation. It is clear that as the magnitude of warming increases, so will the likely severity of impacts on bird populations. The number of projected bird extinctions is modelled to increase by about 1.6 times from a 2 °C to 4 °C global warming scenario, and differ by approximately 2.5 times between a 2 °C and 6 °C scenario (Box 6.5), when it is estimated one-quarter of global bird species would be at risk of being committed to extinction as a result of climate change (Section 6.8.2). Clearly, addressing the causes of climate change will be an important way of reducing the likely detrimental impacts of climate warming on birds (Warren et al. 2013). However, attempts to do this through the reduction of greenhouse gas emissions or the removal of gases from the atmosphere (termed climate change mitigation) may also have a detrimental effect on bird populations.
The principal sources of anthropogenic greenhouse gas emissions are carbon dioxide from the burning of fossil fuels (coal, oil and gas), deforestation and other land use changes leading to the burning or decay of plant material, release of methane from refuse in landfill sites, gut fermentation by ruminant livestock and the farming of rice, and release of nitrous oxide caused by the use of fertilizers in agriculture (Section 1.5). Changes in the ways in which energy is obtained and used, the protection of forests and peatlands and changes in farming practice are therefore the main methods being considered for climate change mitigation. In addition, there might be ways in which the environment could be artificially adapted to remove greenhouse gases from the atmosphere at higher rates. All of these mitigation measures will change the natural environment and could therefore affect bird populations. In this chapter we examine some of these potential effects and assess ways in which potential adverse impacts of mitigation may be reduced, focussing particularly on renewable energy generation.
Preceding chapters have illustrated how temperature, precipitation and other climatic factors affect the breeding productivity, survival and abundance of individual bird species through a variety of mechanisms. As a result, the geographical ranges of species can frequently be well described by the climate, as illustrated with reference to the red grouse in Chapter 1, although that descriptive ability does not show for certain whether the climate has a direct influence, an indirect influence or no real influence at all on species’ distributions (Gaston 2003). There are plenty of examples of biotic factors such as prey availability (Koenig & Haydock 1999; Banko et al. 2002), competition (Terborgh 1985; Emlen et al. 1986; Gross & Price 2000) and predation (Pienkowski 1984; Dekker 1989) being the main proximate factor limiting species’ ranges, but of course, the distribution of many of those other species may also be affected by climate. For example, the northern limit of the distribution of the red fox, which is thought to restrict the range of some wader species (e.g. Pienkowski 1984), is determined by resource (food) availability and therefore ultimately determined by climate (Hersteinsson & Macdonald 1994). The northern limit of Hume’s leaf warbler Phylloscopus humei which feeds on arthropods in tree canopies, is limited by cold temperature, as this causes leaf loss and therefore reduces food availability (Gross & Price 2000). Climate is therefore often regarded as the ultimate determinant of species’ distributions and abundance, even though the precise mechanisms causing the limitation may be unclear (Huntley et al. 2007).
Climate change is anticipated to result in species shifting their distribution to higher latitudes and altitudes (Chapter 6), as has already been observed (Chapter 5). Changes to habitats, and the abundance of food organisms, predators, competitors, parasites and diseases, and the direct effects of climate will alter species’ demographic rates and abundance (Chapters 3 and 4). In parts of the range where population density increases, this is likely to result in an increasing number of dispersing individuals being available to colonise areas of habitat beyond the current range margin. At the retreating range margin, conditions are likely to become increasingly unfavourable, resulting in reduced fecundity and/or survival. Initially, as population density declines, negative effects of climatic change on a particular demographic rate may be at least partially compensated for by density-dependent improvements in other rates. The population in this part of the range may then stabilise at a lower level for some time. However, progressive change will eventually cause population declines, fragmentation of the distribution, local extinctions and finally loss of range. Between the expanding and retreating margins, the same mechanisms may lead to shifts in the distribution of areas with high population density, and changes to the composition of communities (Chapter 5).
Observations of impacts of climate change, and concerns over the impacts projected to come, have stimulated increasingly detailed thinking about what conservationists can do to counter negative impacts through what is termed climate change adaptation: interventions to reduce the vulnerability of species and their habitats to actual or expected climate change effects. Recent advances in conservation science have provided an increased understanding of the precise requirements of species and the impacts upon them of threats such as habitat loss and degradation, overexploitation, persecution and pollution, all driven by expanding human populations and their increased demands for food, recreation and commodities. This understanding has underpinned some successful conservation programmes that have reversed population declines and range losses of some species. We therefore start this chapter with a summary of the tools that conservationists have found to be effective in countering these threats to birds, before considering how they may be adapted for use in the face of climate change.
Impacts of recent climate change
In the first part of the book, we examined the impacts of climate change on bird populations. We found good evidence that significant changes have occurred in the timing of seasonal events within the annual cycle of birds (Chapter 2). In recent decades, both spring arrival of migratory species and egg-laying dates, as measured for the average individual in a population, have advanced consistently by some 2 days per decade across temperate and boreal latitudes. Phenological changes affecting the timing of the end of the breeding season, and autumn departure dates of migrants have varied much more between species, depending on migration, moult and breeding strategies. A wide range of correlative analyses, supported by a small number of studies of underlying mechanisms, have demonstrated that many of these changes are a consequence of warming. Recent climate change has therefore altered the seasonal pattern of avian life cycles. Although there is currently insufficient monitoring of birds in tropical areas to track their long-term phenological responses to climate change, the studies which have been conducted suggest that here, changes in precipitation, and not temperature, are likely to be the main determinant of the timing of commencement of the breeding season and the movement of individuals. Trends in tropical bird phenology are therefore likely to be related primarily to changes in rainfall patterns.
Having described the impacts that climate change has already had upon birds, their populations, distributions and communities, in this second part of the book, we look now at what can be done to reduce the negative impacts of current and future climate changes on birds. The first stage in attempting to do this is to predict what the consequences of future climate change will be for the conservation status of wild species and populations. Although there are many impacts of climate change which have been documented, few clearly demonstrate a current and urgent threat to particular populations or species. For most species, it is not the climate change which has occurred so far that is the problem, but the magnitude of climate change to come. In this chapter, we attempt to quantify the likely size of that future problem – how severe is the impact of climate change on birds likely to be?
This is not a simple question to answer. The foregoing chapters documented the complexity of the effects of climate and climatic change on reproductive and mortality rates of birds, which are the mechanisms by which climate affects their distribution and abundance. Given this complexity, it might be thought that any attempt to predict the effect of climate change on a bird species would require a detailed knowledge of how its demographic rates will be affected, in both the short and the long term. Such knowledge can certainly be very helpful as we shall see later in this chapter, but realistically, is only available for a handful of the 10 000 bird species on Earth. To make an assessment that will be widely applicable, we need to consider alternative approaches, which require less detailed information, to predicting the effects of climate change on bird species. Building on the role of climate in delimiting species’ distributions (Sections 1.8 and 5.2), the most widely used approach is to build a statistical model of geographical variation in the distribution or abundance of a species in relation to climatic and sometimes also to other environmental variables. The spatial association between a species and climate described by that model is then used to make future projections of the impact that climate change may have on that species’ distribution or abundance.
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