Managing for cumulative impacts in ecosystem-based management through ocean zoning

Abstract

Multiple activities affect the marine environment in concert, yet current management primarily considers activities in isolation. A shift towards a more comprehensive management of these activities, as with recent emphasis on ecosystem-based approaches to management, requires a means for evaluating their interactive and cumulative impacts. Here we develop a framework for this evaluation, focusing on five core concepts: (1) activities have interactive and cumulative impacts, (2) management decisions require consideration of, and tradeoffs among, all ecosystem services, (3) not all stressors are equal or have impacts that increase linearly, (4) management must account for the different scales of activities and impacts, and (5) some externalities cannot be controlled locally but must be accounted for in marine spatial planning. Comprehensive ocean zoning provides a powerful tool with which these key concepts are collectively addressed.

Introduction

The management of ocean resources is presently characterized by a sector-by-sector approach, with a few exceptions (e.g., Ref. [1]), where each human activity, such as coastal development, water management, and fisheries or energy production, is managed separately. The anthropogenic threats that result from these activities—including climate change, biodiversity loss, eutrophication, habitat damage and fragmentation, and invasive species—are numerous and particularly harmful to coastal ecosystems, which are influenced by activities on land (terrestrial and freshwater), along the coasts, and in the ocean. Furthermore, our understanding of the consequences of human activities on marine systems has focused on declines in endangered species or resources, communities (e.g., Refs. [2], [3]), habitats [4], or some combination of these [5], but these efforts generally assume additive effects of these activities. When management planning and implementation within a sector does consider the effects of an activity on habitat or other ecosystem components, it is only with respect to the overall goals for that sector. For example, U.S. fishery management plans are required to consider habitat impacts of fishing and other activities, but only insofar as they affect Essential Fish Habitat, defined by Congress as “those waters and substrate necessary to fish for spawning, breeding, feeding or growth to maturity” [6]. Other sectors such as coastal wetlands protection under the U.S. Clean Water Act or the Environmental Protection and Biodiversity Conservation Act 1999 in Australia may have requirements to reduce, minimize or mitigate impacts of particular activities, yet these typically do not incorporate specific goals and objectives to maintain ecosystem properties or functions.

It is a focus on ecosystem function and services within management that allows for a full appreciation of how human activities interact. Restoration and maintenance of the suite of benefits that ecosystems provide—ecosystem services—is a core goal of ecosystem-based management [7], [8]. These services are diverse and are influenced by a suite of ecological and social factors. For example, provisioning services such as the production of local, healthy seafood depend upon, among other factors: (1) fish and shellfish populations that are robust to fishing pressure, (2) availability of suitable habitat, (3) the presence of other ecosystem components such as prey or nursery habitat, (4) suitable water quality, (5) local fleet access, including accessible harbors, and (6) local markets. Clearly, all of these factors are influenced by anthropogenic activities other than the direct effects of fishing, but current fisheries management tends to focus disproportionately on population metrics for particular target species rather than the many other factors that affect population size.

Cumulative and interactive consequences of different human activities are largely ignored in management plans because of the piece-wise nature of current management. In simple terms, the cumulative impact across all of the sectors may be much greater, or in rare cases less, than the sum of individual impacts because of interactive or multiplicative effects. These different scenarios of how activities can interact are illustrated in Fig. 1 and described in greater detail below. If most activities do interact, and there is good evidence supporting this (e.g., Ref. [9]), then managing each activity largely in isolation will be insufficient to conserve marine ecosystems, or even to meet individual sector goals. Furthermore, some of these threats have direct effects on ecosystem components, as with fishing overharvest or damage to habitat caused by bottom trawling or anchors from recreational boats, while others have more indirect consequences, for example introduced species that compete with or prey on native species. These indirect effects in particular make detection and assessment of interactions more complex than simple cause–effect mechanisms. Importantly, these activities may also interact with natural temporal or spatial variability in environmental conditions, such as El Nino, multi-decadal oscillations, or the location of upwelling zones. Acting in concert, natural variability and anthropogenic perturbations (through both direct and indirect mechanisms) decrease the ability and predictability of marine ecosystems to deliver vital services to humankind, such as abundant seafood, clean water, recreational opportunities, and the protection of coastal areas from storm surge and waves [7], [10].

These issues can make it seem daunting if not impossible to manage for cumulative and interactive impacts. However, ecosystem-based management, of recent interest both nationally (e.g., U.S. Commission on Ocean Policy, Pew Oceans Commission) and internationally (e.g., Millennium Ecosystem Assessment, United Nations Environment Programme), provides a framework for addressing these issues through a more integrated approach to maintaining the ecosystem services that support human well-being rather than just sector-by-sector goals. Furthermore, active discussions about the use of marine spatial planning, and possibly the specific approach of ocean zoning, throughout the world seek to determine whether management could be made more spatially coherent across broad areas (e.g., Ref. [11]). Putting these approaches into practice requires clarity on several key issues. Conventional sector-by-sector management is largely ill-equipped to handle the full range of human activities that affect the ocean because it does a poor job of accounting for: (1) interactions among activities, (2) cumulative impacts of these activities over space and time, (3) the process by which activities (both singly and cumulatively) ultimately affect the delivery of ecosystem services, and (4) explicit tradeoffs among activities. Ecosystem-based management that includes marine spatial planning, in particular the specific application of spatial planning that is comprehensive ocean zoning, can explicitly deal with each of these issues.

Here, we are concerned with the cumulative impacts of human activities on the potential of marine ecosystems to provide the suite of services that we need and want. To address this issue we describe the key concepts concerning cumulative impacts of human activities in marine environments, the interactions between activities that compromise the ability of marine ecosystems to provide ecosystem services, the role of dominant stressors in degrading ecosystems, and how scaling of these effects must be carefully considered. We illustrate these concepts with examples from a variety of marine ecosystems, and offer general guidelines for how to incorporate the ideas into marine spatial planning, or where necessary comprehensive ocean zoning.