Migratory species and ecological processes.

AuthorReynolds, Heather L.
  1. INTRODUCTION II. OVERVIEW A. Ecosystem Services of Migratory Species B. Negative Effects of Migratory Species III. CASE STUDIES A. Serengeti Ungulates B. Pacific Salmon C. Long-Nosed Bats and Hummingbirds D. Manatees E. Migratory Birds IV. Conclusion I. INTRODUCTION

    Migration of animal species, defined as the periodic movement between two sites, (1) has been well recognized for millennia and subject to intense scientific interest for over 150 years. Most scientific attention has been focused on the physiology, behavior, or population dynamics of particular migratory species. (2) Yet like all species, migratory species are members of ecological systems (ecosystems)--assemblages of plants, animals, and microbes that interact with one another and with the physical and chemical environment. The capture and transformation of energy and nutrients by organisms within ecosystems, through photosynthesis, nitrogen fixation, feeding, defecation, pollination, and decomposition among myriad other processes and activities, results in an array of life-supporting goods and services without which human life could not be sustained. (3) Food, fiber, and timber production; recreational and aesthetic experiences; the provision of life-saving drugs and other pharmaceuticals; and the supply of fresh air and water are just a few of the ecosystem services provided by nature's "green infrastructure." (4) The activities of organisms in ecosystems can also result in disservices to humans, such as sustaining and spreading pests and disease.

    From this ecological perspective, migratory species are of interest as components of two or more ecosystems and vectors for energy and matter transfer between them. The distance and frequency that migratory species travel in space and time can vary widely, from the diurnal movements of tiny krill (e.g., Euphausia superba) through a few hundred vertical feet of ocean water (5) to the annual circumpolar navigations of Arctic Terns (Sterna paradisaea). (6) Despite these differences in scale, all migratory species share the distinction of being regular participants in multiple ecosystems encountered throughout the migration route.

    An important rationale for the conservation of species, including migratory species, is rarity. Rare species are more likely to go extinct because of demographic fluctuations or loss of critical habitat. (7) However, the ecosystem services provided by species may be a positive function of their abundance, such that the greater the number of individuals the greater amount of ecosystem services they provide. (8) Animal migrations provide especially clear examples of this relationship because they are often phenomena of abundance (9) and their periodic and often discrete nature facilitates measurement of species effects. For example, the more salmon migrate upstream to their spawning grounds, the more marine nutrients are transported inland to fertilize stream and forest ecosystems. (10) Likewise, the larger a population of migratory pollinators, the more plants will be pollinated. (11) Thus, valuation of migratory species based on their capacity to provide ecosystem services may be the inverse of their valuation based on rarity. This represents an alternative paradigm to rarity-based conservation and instead provides a strong rationale for conserving and protecting abundant migratory species because of the magnitude of their ecosystem services.

    Using the framework of ecosystem services and disservices our focus here is to consider the functional roles of migratory species as members of multiple ecosystems and to highlight novel implications for conservation policy that arise from this functional perspective. In Part II, we discuss the Millennium Ecosystem Assessment framework for ecosystem services and provide examples of the ecosystem services and disservices of migrating species. In Part III, we present case studies in order to explore more fully the services and disservices of migratory species and their relationship to abundance. In Part IV, we consider the implications that a functional perspective brings to conservation policy, with special attention to migratory species.

  2. OVERVIEW

    Ecosystem services are the life-supporting and life-enhancing benefits provided to people by the world's ecosystems, (12) or "natural capital." (13) Animals provide a variety of important ecosystem services with direct economic benefits to humans. For example, coffee, one of the world's most important export commodities, is highly dependent on pollination by natural insect populations. (14) An estimated one-third of global crop production is dependent on wild animal pollinators. (15) The economic value of commercial plus wild pollinator services is estimated to be $100-$200 billion annually on a global basis. (16) Food production from wild marine systems (overwhelmingly animal based) was estimated to have an annual worth of nearly one trillion dollars per year. (17)

    We use the Millennium Ecosystem Assessment (MA) classification of ecosystem services. (18) Initiated in 2001 by the United Nations, the MA is a working group of over 1300 leading scientists and other experts from more than 100 nations that provides scientific assessment of the health and condition of the world's ecosystems and ecosystem services and analyzes options for enhancing human well-being through the conservation and sustainable use of ecosystems and their associated services. (19) Associated services are categorized into four general groups: supporting, provisioning, regulating, and cultural. (20) Supporting services include biogeochemical processes that are foundational to all other services and may occur over time scales longer than those of human decision-making. (21) Examples of these supporting services include soil formation, soil renewal through nutrient cycling, and the development and maintenance of an oxygen-containing atmosphere. (22) Provisioning services are the products of ecosystems such as crops, game, timber, fiber, or fresh water and are typically well integrated into human economic markets. (23) Regulating services modulate the quality and quantity of environmental conditions within ranges that promote human well-being, and include disease control, pollination, water purification, and the regulation of climate at local to global scales. (24) Cultural services encompass the intangible benefits of ecosystems to human spirit, intellect, expression, recreation, and heritage. (25)

    From the perspective of human welfare, community and ecosystem effects of migrating species can be categorized as positive, insofar as ecosystem services are provided, or negative, insofar as human welfare is harmed. Examples of ecosystem services provided by migratory species are described in Part II.A. below. In Part II.B. we discuss ecosystem disservices of migratory species, such as dispersal of human pathogens and invasive species or the movement and accumulation of toxins.

    1. Ecosystem Services of Migratory Species

      Migratory species can provide all four categories of ecosystem services: supporting, provisioning, regulating, and cultural (Table 1). For example, migratory fish (e.g., salmon, (58) alewives (59)), seabirds, (60) and hoofed grazing mammals (ungulates) of the American plains or African Serengeti (61) are well known avenues by which large quantities of nutrients are recycled via excretion, predation, and decomposition, thus enriching the fertility of ecosystems and providing a key supporting ecosystem service. (62) Another supporting service is the maintenance of biodiversity. The wallowing and grazing activities of ungulates provide this supporting service by creating different types of microhabitats that sustain distinct assemblages of plant species. (63) As a provisioning ecosystem service, migratory species are fished and hunted, providing humans with food and materials. (64) Migratory species can also provide important regulating ecosystem services such as ecosystem engineering, modulation of climate, seed dispersal, pollination, and pest control. For example, migratory buds and bats are important pollinators and seed dispersers in many systems and thereby directly influence the structure and dynamics of plant communities, including agricultural crops and species of conservation concern. (65) Additionally, migratory species provide a variety of cultural services, often intertwined with fishing, hunting, or other food provisioning services. (66) The willingness of people to travel and expend significant amounts of money for things like bird watching, observing marine mammals and turtles, and African safari adventures is evidence of the important cultural services of migratory species. (67) Migratory species provide especially valuable cultural services because their migration pathways are typically predictable in time and space so that ecotourists are virtually guaranteed to see them. (68)

    2. Negative Effects of Migratory Species

      While most attention focuses on the various advantages accruing to migratory species and their resident ecosystems as a result of migrations, we also recognize that there can be significant negative consequences of migration for humans and ecosystems (Table 2). One of the most dramatic examples is the spread of infectious disease by migratory species. For example, West Nile Virus and Avian Influenza Virus have been distributed widely by birds, and have resulted in many human deaths. (69) The dispersal of pathogens by migratory species is more significant with longer distance migrations and where the pathogen has a broad host range, potentially infecting many species along the migration route. (70) Migratory species can also serve to move and concentrate toxic chemicals such as heavy metals or pesticides. (71) Likewise, invasive species, which can disrupt resident communities and ecosystems, can be dispersed by migratory species (72) or may be migratory themselves and...

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