Introduction

Effective management and the maintenance of marine ecosystem services rely on a capacity to predict the ecological consequences of environmental change and potential management interventions (Chapter 1). Making these predictions is difficult because anthropogenic stressors do not produce uniform or consistent impacts on biodiversity and ecosystem functioning. Rather, their effects can be modified by a variety of factors that cause them to vary among locations and different points in time. Thus, the effectiveness of actions taken to manage environmental problems is likely to vary in a similar way: interventions that are sufficient to mitigate a stressor's impacts in one situation might be inadequate or excessive in others. Both sound science and efficient management require us to recognise that spatial and temporal variability are inherent to natural systems, and that the ecosystem complexity places inherent limits on our ability to predict future ecological conditions. However, many of the causes of this variability have been identified. Careful consideration of these factors will enhance scientific understanding, improve ecological prediction and enhance our efforts to optimise marine policy and management by reducing the uncertainty associated with the effects of stressors.

In this chapter, we examine three factors that cause anthropogenic activities to have inconsistent impacts on marine ecosystems. Both individual organisms and entire ecosystems tend to respond nonlinearly to gradients in stressor severity. Thus, we begin by discussing how differences in stressor intensity, as well as spatial and temporal characteristics of stressors' disturbance regimes can influence their impacts. Next, we discuss how characteristics of receiving ecosystems influence their sensitivity to stressors' impacts. These include biological characteristics such as the genetic structure of populations and community composition, as well as physical and chemical variables that may alter stressors' effects. Finally, we will discuss an issue of increasing concern among scientists and policy makers: the impacts of multiple stressors and the ways in which stressors can modify one another's effects.

Introduction

Marine ecosystems provide a range of essential benefits to society, including food and other products, waste assimilation, coastal protection and climate regulation as well as less tangible, but no less important cultural and aesthetic benefits (Chapter 2). To a considerable degree, those benefits are underpinned by ecosystem services dependent on the efficient functioning of the ecosystems, although detailed understanding of relationships between particular services and particular functional processes is currently being developed (Chapter 2). In turn, the efficient functioning of ecosystems has been linked to the number and identity of species present, as well as the prevailing environmental conditions (Chapter 5).

Society derives many of its benefits from ecosystems via sectoral activities and industries, such as fishing, construction, energy, shipping, leisure and tourism. These activities can impose pressures on ecosystems, such as removal of biomass, inputs of nutrients and other contaminants and the introduction of artificial structures and non-indigenous species (Chapter 3). Such pressures act through a range of mechanisms affecting different levels of biological organisation to modify biodiversity and ecosystem functioning (Chapters 3 and 4). The chapters in this book have reviewed current knowledge of how the spectrum of human activities and pressures (collectively referred to as stressors) affect biodiversity, ecosystem functioning and the provision of services and benefits to society. A key objective was to provide a synthesis of evidence for policy makers and managers to facilitate trade-offs between sectors of activity based on the benefits they provide weighed against the degree to which they may compromise the delivery of ecosystem services now and into the future (see Chapter 11).

In this chapter, we first synthesise and summarise the inferences presented in the book about the range of impacts of different activities and pressures on biodiversity and ecosystem processes. We then ask how this kind of knowledge can help policy makers and managers, particularly in the achievement of the international targets laid down in the Millennium Development Goals (Chapter 1) and whether the ecosystem approach and the concept of ecosystem services can provide an effective framework for facilitating the achievement of those goals.

Marine ecosystems

The Earth is a blue planet. Seas and oceans cover over 70% of the Earth's surface and, with an average depth of over 3.2 km, the total volume of marine ecosystems is vastly greater than that of terrestrial and fresh-water environments combined, comprising 98% of the total inhabitable space on the planet (Speight and Henderson, 2010). Marine ecosystems contain 31 of the 33 phyla of animals, each of which constitutes a unique and distinctive body plan, with 15 of those phyla occurring only in the sea (Angel, 1993). Approximately 250 000 marine species have been described, with an estimated 750 000 still to be discovered (Census of Marine Life, 2010). Marine creatures include the largest ever to live (blue whales), yet the energy to fuel these giants is mainly captured by microscopic plankton, rather than more substantial plants, one of a number of fundamental differences between marine and terrestrial ecosystems (Steele, 1985, 1991; Webb, 2012). Of the global annual net primary productivity, approximately 104.9 petagrams (1015 grams) of carbon per year, around half is produced by marine ecosystems (Field et al., 1998). Although coral reefs and beds of seaweed are, per unit area, among the most diverse and productive ecosystems on Earth, open oceans have levels of productivity akin to terrestrial deserts because life is so thinly spread, but nevertheless make the single greatest contribution to global productivity because of their size (Whittaker and Likens, 1975).

Archaeological records show that even the earliest humans exploited the marine environment for food, as a medium for transportation and as a repository for waste (Jackson, 2001; Jackson et al., 2001). Today, the marine environment provides approximately 80 million tonnes per year of fish and shellfish from capture fisheries, representing about 3% of our global animal protein supply. Marine aquaculture contributes an additional 20 million tonnes (FAO, 2012). Therefore, marine ecosystems can be seen as having an important role in global food security (Frid and Paramor, 2012).

Introduction

With the adoption of the Convention on Biological Diversity (United Nations1992), the sustainable management and protection of biodiversity shifted from being an option to an acknowledged necessity: sustainability is now the high-level goal of environmental management policy. Sustainability implies the ability for processes and activities to be able to continue indefinitely. Within the specific context of environmental management this has been taken to mean meeting the needs of the present without compromising the ability of future generations to meet their own needs (derived from the definition of sustainable development developed by the Brundtland Commission (World Commission on Environment and Development 1987)). More recently the concept has been expanded to explicitly include three elements, sometimes referred to as the three pillars of sustainability: environmental, social and economic (Table 7.1). For many policy makers, sustaining social structures and hence economic systems is the imperative, while many scientists would argue that you cannot have a sustainable economy without sustainable use of natural systems.

Introduction

The sustainable use, management and conservation of ecosystems, as promoted by the Convention on Biological Diversity's Ecosystem Approach (United Nations 1992), and recent initiatives such as the Millennium Ecosystem Assessment (United Nations 2005), emphasise the inter-dependence between ecological systems and human well-being. Healthy social systems demand healthy ecosystems and vice versa. This emergent world view is compelling and persuasive to conservationists, policy makers and managers alike, because it implies win-win solutions for nature conservation and for human development which relies on the continued provision of ecosystem goods and services. Ecosystem management within this context requires a holistic approach that acknowledges the need to work with and across a broad range of natural, physical, social and economic sciences. Whilst there are many successful programmes which have achieved this, mainstream ecologists who have so much to bring to the table have been slow to embrace such approaches. Jones and Paramor (this volume) consider many of the important cultural challenges. The view that humans are part of, not apart from, the biophysical system in which they are embedded has not always sat comfortably with academic researchers, who have traditionally seen their prime focus on, and responsibility to, either the natural system or to broader societal goals, but rarely both. In addition, there are misunderstandings and fears about what holistic ecosystem approaches really are, in turn due to the divergent pathways along which different sections of the ecological community have developed.