The Need to Quantify Ecosystem Services Provided by Birds Author(S) :Daniel G

The Need to Quantify Ecosystem Services Provided By Birds Author(s) :Daniel G. Wenny, Travis L. Devault, Matthew D. Johnson, Dave Kelly, Cagan H. Sekercioglu, Diana F. Tomback, and Christopher J. Whelan Source: The Auk, 128(1):1-14. 2011. Published By: The American Ornithologists' Union URL: http://www.bioone.org/doi/full/10.1525/auk.2011.10248

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BioOne sees sustainable scholarly publishing as an inherently collaborative enterprise connecting authors, nonprofit publishers, academic institutions, research libraries, and research funders in the common goal of maximizing access to critical research. The Auk An International Journal of Ornithology Vol. No. January 

The Auk 128(1):1–14, 2011 The American Ornithologists’ Union, 2011. Printed in USA.

PERSPECTIVES IN ORNITHOLOGY THE NEED TO QUANTIFY ECOSYSTEM SERVICES PROVIDED BY BIRDS DANIEL G. WENNY,1,8 TRAVIS L. DEVAULT,2 MATTHEW D. JOHNSON,3 DAVE KELLY,4 CAGAN H. SEKERCIOGLU,5,9 DIANA F. TOMBACK,6 AND CHRISTOPHER J. WHELAN7

1Jo Daviess Conservation Foundation, 126 N. Main Street, Elizabeth, Illinois 61028, USA; 2U.S. Department of Agriculture, National Wildlife Research Center, Sandusky, Ohio 44870, USA; 3Department of Wildlife, Humboldt State University, Arcata, California 95521, USA; 4Biological Sciences, University of Canterbury, Christchurch 8140, New Zealand; 5Center for Conservation Biology, Department of Biology, Stanford University, Stanford, California 94305, USA; 6Department of Integrative Biology, University of Colorado Denver, P.O. Box 173364, Denver, Colorado 80217, USA; and 7Illinois Natural History Survey, c/o Department of Biological Sciences, University of Illinois at Chicago, 845 W. Taylor Street, Chicago, Illinois 60607, USA

What are birds worth—what is their actual dollar value to highlight recent steps toward quantifying those services, and, fi- human society? To most of us in the ornithological community, nally, suggest directions for future research. Overall, we empha- birds are invaluable. But in these times we need more specific ra- size that global efforts to conserve bird populations and sustain tionales to convince policy makers and business leaders to include avian biodiversity also preserve the diverse ecosystem services bird conservation in land-use and development decisions. Over provided by birds, thus contributing to human well-being. the past two decades, awareness of our dependence on a variety of ecosystem services (natural ecological processes that benefit DEFINITIONS AND BACKGROUND human society) and of their importance and prevalence has pro- gressed toward the goal of making conservation a mainstream Ecosystem services are divided into four categories (Millenium value (Ehrlich and Kennedy , Perrings et al. , Rands et al. Ecosystem Assessment ). Provisioning services refer to natu- , Sodhi and Ehrlich ). Building strategies for the protec- ral products that are directly used by humans for food, clothing, tion of ecosystem services into conservation and land-use plan- medicines, tools, or other uses. Cultural services provide recre- ning is essentially the promotion of human survival, although ational opportunities, inspiration for art and music, and spiri- many policy makers misinterpret conservation efforts as luxury. tual value. Regulating services include pest control and carcass Several previous reviews have identified ecosystem services that removal. Supporting services, such as pollination, seed dispersal, benefit human society (Costanza et al. , Daily , Pimentel water purification, and nutrient cycling, provide processes essen- et al. , Sekercioglu ). The challenge, however, is to calcu- tial for ecological communities and agricultural ecosystems. late the value of ecosystem services in meaningful and relevant The Millennium Ecosystem Assessment’s description of eco- ways that can be used to justify the protection of ecosystem ser- system services (Millenium Ecosystem Assessment ) is widely vices in land-use recommendations and policy decisions (Daily et cited, but considerable debate continues on what constitutes an eco- al. , ). As the case studies below illustrate, recent work system service and how each should be quantified (Boyd , Boyd on the ecosystem services provided by birds has made good prog- and Banzhaf , Matero and Saastamoinen , Nijkamp et al. ress toward this goal, but much remains to be done. Our objec- , Bartelmus , Farley and Costanza , Kontogianni et al. tives here are to describe the ecosystem services provided by birds, , Norgaard , Wainger et al. ). The main issues include

8Present address: Loras College, 1450 Alta Vista, Dubuque, Iowa 52004, USA. E-mail: [email protected] 9Present address: Department of Biology, University of Utah, 257 South 1400 East, Room 201, Salt Lake City, Utah 84112, USA.

The Auk, Vol. , Number , pages  . ISSN -, electronic ISSN -.  by The American Ornithologists’ Union. All rights reserved. Please direct all requests for permission to photocopy or reproduce article content through the University of California Press’s Rights and Permissions website, http://www.ucpressjournals. com/reprintInfo.asp. DOI: ./auk.. — 1 — 2 — WENNY ET AL. — AUK,VOL. 128

how to value nonmarket services, how to avoid double counting a although many raptors (both hawks and owls) consume rodents, process and its end product, and how to incorporate ecosystem val- we know of no study that has examined this predator–prey inter- uation into policy and land-use decisions. We do not advocate any action from the perspective of economic value or trophic cascades. particular method of valuation here, but we argue that a consistent A few studies have directly assessed birds of prey as agricultural methodology for calculating units of ecosystem services is needed rodent-control agents, and the results are somewhat ambiguous. (as with any system of weights and measures; Boyd ). Wood and Fee () reviewed measures to control rats in Malay- sian agroecosystems, including deployment of nest boxes to raise OVERVIEW OF ECOSYSTEM SERVICES PROVIDED BY BIRDS populations of Barn Owls (Tyto alba). They concluded that the evidence was inconsistent and that the effect of owls warrants further Birds are the best-known class of vertebrate animals, occur world- investigation. Kay et al. () reported that perches placed around wide in nearly all habitats, and provide many services (Sekercio- soybean fields in Australia increased the number of diurnal raptors glu a, b; Whelan et al. ). Thus, they are an ideal group to around and over the fields, which in turn decreased House Mouse examine for ecosystem service valuation. Yet, surprisingly, little (Mus musculus) population growth rate and maximum population ornithological research has been done in an ecosystem-services density in the fields. Perches placed  m apart were more effective context. Much ecosystem-services work has been focused on wa- than those placed  m apart. Other studies demonstrated that tersheds and insect pollination, perhaps because market value can providing artificial perches attracts various birds of prey, including readily be assigned to both fresh drinking water and agricultural American Kestrels (Falco sparverius), which also suggests that this crops that require pollination (Kremen et al. , Brenner et al. method may enhance or concentrate foraging in potentially ben- ). Similarly, economic aspects of some cultural and provision- eficial ways (Wolff et al. , Sheffield et al. ). Clearly, more ing services such as bird watching and hunting have been quan- research is needed on the potential for birds of prey to drive trophic tified (Sekercioglu , LaRouche , Leonard , Carver cascades in natural and agricultural ecosystems. ). Other historical and cultural aspects of birds have been re- The role of granivorous birds in control of agricultural weeds viewed and quantified in a general way (Diamond and Filion , is essentially unknown, but one example is suggestive. In New Podulka et al. , Mynott ). Most of the important ecologi- Zealand, a granivorous bird introduced for aesthetic reasons, the cal roles that birds fill, however, involve supporting and regulating European Goldfinch (Carduelis carduelis), destroyed × more services, such as insect pest control and seed dispersal, and these seeds of the aggressive pasture weed Carduus nutans than a weevil types of services are the most difficult to quantify (Farber et al. (Curculionidae: Rhinocyllus conicus) that was introduced to pro- ; Sekercioglu a, b; Whelan et al. , ). As we de- vide biological control of C. nutans (Kelly and McCallum ). In scribe below, many of the most important ecosystem services that fact, the % of seed destroyed by goldfinches at that site compares birds provide result from their foraging behavior. Through their favorably to the highest well-documented seed losses attributed to foraging, birds act as mobile links that transfer energy both within R. conicus (–%) at sites where the insect provided effective bi- and among ecosystems, and thus contribute to ecosystem func- ological control (Kelly and McCallum ). While it is likely that tion and resilience (Lundberg and Moberg ). We know that most species of avian granivores are beneficial in agroecosystems, birds are important ecologically; the challenge is to quantify that especially because most species also eat considerable quantities importance in terms that are currently meaningful to humans. of invertebrates during the breeding season, the most promi- Pest control.—The regulating and supporting services pro- nent studies of granivores are those on birds as agricultural pests vided by birds result mostly from foraging (i.e., consuming and (Weatherhead et al. , Elliott and Lenton , Dolbeer , processing resources; Table ). The prime example is insectivory, Basili and Temple , Avery et al. , Blackwell and Dolbeer which can provide the ecosystem service of pest control. More , McWilliam and Cheke , Cirne and Lopez-Iborra , than % of bird species are predominantly insectivorous, and Hagy et al. ). Future research should examine more fully the nearly % eat invertebrates at least occasionally (Sekercioglu costs and benefits of avian granivory in agricultural settings. b; Table ). The beneficial role of birds in consuming arthro- Bird–plant mutualisms.—The bird–plant interactions of pol- pods, and especially their responses to and influence on insect lination and seed dispersal have potentially large effects on ecosys- outbreaks (e.g., spruce budworms [Choristoneura spp.], cicadas tems. Nearly % of bird species disperse seeds, primarily through [Magicicada spp.], and Mormon Crickets [Anabrus simplex]), is fruit consumption, but also through scatter-hoarding of nuts and well documented (U.S. Biological Survey reports, summarized conifer seed crops (Vander Wall , Sekercioglu b). It is dif- by Whelan et al. ). Furthermore, numerous studies in both ficult to estimate the number of plant species dispersed by birds, natural and agricultural habitats show not only that birds reduce because of overlap with seed-dispersing mammals and incomplete herbivorous insect populations, but also that plants respond knowledge of many habitats. In the temperate zone (i.e., Europe, with higher growth rates or crop yields (see Whelan et al. : North America, Japan, and New Zealand), –% of woody flora Table ), a classic “trophic cascade” (Terborgh and Estes ). To are fleshy-fruited (Burrows ). Nonwoody species are less likely cite an anecdotal example, the  extermination campaign in to have fleshy fruit, so the average across whole floras is lower; for China against the Eurasian Tree Sparrow (Passer montanus) ulti- example, in New Zealand, fleshy fruits are found in % of trees mately contributed to insect pest outbreaks rather than rice yield (Kelly et al. ) and % of all woody species (Burrows ), increases, demonstrating indirectly that the sparrows’ control of but in only % of the whole flora (Lord et al. ). These tem- insects benefited the crop (Suyin , Becker ). perate-zone totals exclude dry (lacking a fleshy covering), scatter- Other trophic cascades that involve birds potentially ben- hoarded tree nuts and conifer seeds, which are common in the efit agriculture, but they have seldom been studied. For example, Northern Hemisphere (Tomback and Linhart , Vander Wall JANUARY 2011 — PERSPECTIVES IN ORNITHOLOGY — 3

TABLE 1. Relative importance of dietary categories among avian orders ( indicates primary item, – indicates less important item, blank indicates an item rarely or never eaten at family level within orders). List based on Gill and Donskar (2010). Diets from Harris (2009).

Terrestrial Aquatic

Order Families Genera Species Inverts Verts Inverts Verts Carrion Fruit Seeds Vegetation Nectar

Tinamiformes 1 9 47 –––– Struthioniformes 1 1 2 – – Rheiformes 1 2 2 – – Casuariiformes 2 2 4 –– –– Apterygiformes 1 1 5 Galliformes 5 83 297 Anseriformes 3 51 169 Gaviiformes 1 1 5 – Sphenisciformes 1 6 19 Procellariiformes 4 26 134 – Podicipediformes 1 6 21 Phoenicopteriformes 1 3 6 Phaethontiformes 1 1 3 Ciconiiformes 1 6 19 Pelecaniformes 5 35 111 – – –– Suliformes 4 8 55 – Accipitriformes 4 72 260 – Falconiformes 1 11 65 – – Otidiformes 1 11 27 Mesitornithiformes 1 2 3 –– Cariamiformes 1 2 2 – Eurypygiformes 2 2 2 –– Gruiformes 6 42 162 – – –– Charadriiformes 19 94 379 – –– – Pteroclidiformes 1 2 16 – Columbiformes 1 42 321 – Psittaciformes 3 86 373 – –– Opisthocomiformes 1 1 1 – Musophagiformes 1 6 23 – – Cuculiformes 1 32 146 –– Strigiformes 2 27 220 – – Caprimulgiformes 4 21 117 – Apodiformes 4 128 454 Coliiformes 1 2 6 Trogoniformes 1 7 42 Leptosomiformes 1 1 1 – Coraciiformes 6 35 157 – – Bucerotiformes 4 17 73 –– –– Piciformes 9 67 431 – – Passeriformes 120 1278 6237 –– – –

). Also, the tropics hold most plant species diversity, and trop- yet the dependence of these plants on birds for dispersal and the ical floras are disproportionately woody and fleshy-fruited (Howe anthropogenic influences on the seed-dispersal pathways are in and Smallwood , Willson et al. , Fleming ). Thus, many cases poorly understood. The great declines in abundance probably –% of all plant species have vertebrate-dispersed of large frugivorous birds and mammals have resulted directly or fruit (,–, species). Certainly, many tens of thousands indirectly from human activities, and some have been extirpated of plant species benefit from bird dispersal in terms of gene flow, from regions or have become extinct (Cordeiro and Howe , colonization of open sites, escape from predators, directed disper- Sekercioglu et al. , Peres and Palacios , Terborgh et al. sal to favorable sites, or enhanced germination (Vander Wall and ). Large-seeded plants are most at risk because they require Balda , Howe and Smallwood , Johnson and Webb , large-bodied dispersers, which are more vulnerable to anthropo- Tomback and Linhart , Jordano , Tomback ). genic effects (Hansen and Galetti , McKinney et al. ). As Birds disperse the seeds of many woody plant species with a result, the number of relatively large-seeded plants with few or direct value to humans for timber, medicine, food, or other uses; no dispersers is now rising, especially on islands, which have lower 4 — WENNY ET AL. — AUK,VOL. 128

diversity and less ecological redundancy than continental areas , ; Wassenberg and Hill ; Jennings and Kaiser ). (e.g., Kelly et al. ). Lower densities of frugivores may disperse Among passerines, corvids—especially American Crows (Corvus a smaller fraction of the fruit crop, which can result in fewer seed- brachyrhynchos) and Common Ravens (C. corax)—are the most lings or in seedlings being more concentrated under the parent conspicuous scavengers (DeVault et al. ). More research is plants (Cordeiro and Howe , Terborgh et al. , Cordeiro needed on the ecological consequences of obligate and faculta- et al. , Sethi and Howe , Sharam et al. , Chimera tive scavenging, particularly on how these processes are affected and Drake ). These effects generally result in changes in plant by contemporary human activities such as transportation (Dean community composition rather than the local extirpation of and Milton , Antworth et al. ) and commercial fisheries plant species (Wright and Duber ; Cordeiro and Howe ; (Britton and Morton ), which make many dead animals and Muller-Landau ; Wright et al. a, b; McKinney et al. ; byproducts available for scavengers. Sharam et al. ). It is unknown how these changes will affect Birds contribute to nutrient cycling in all habitats, but most plant populations, or even entire forest communities, that are im- impressively where aquatic birds nest colonially on islands (Po- portant to humans. More experimental work is needed to deter- lis and Hurd , Anderson and Polis ). Seabirds often nest mine the ecological processes involved and their outcomes. in dense colonies both in coastal areas and on islands where they Fewer bird and plant species are involved in bird-pollination process large amounts of food in small areas. In this manner, sea- mutualisms (~ bird species and ~% of regional floras; Stiles birds transport nutrients from the aquatic zone to the terrestrial , Nabhan and Buchmann ), but recent evidence suggests zone. Such large inputs of phosphate-rich guano can influence that bird-pollination failure still poses important risks. The rela- the structure and composition of plant communities (Ellis ). tionships tend to be more specialized than with seed dispersal, Conversely, removal of nesting birds after introduction of a preda- and the outcome of a failed mutualism is unambiguously negative tor fundamentally alters the plant community (Croll et al. , (failure to produce seed; Kelly et al. ). For some plants in New Bellingham et al. ). Zealand, insects were regarded as effective substitutes for missing Birds as ecosystem engineers.—Ecosystem engineering is birds, but data do not support this belief (Kelly et al. , Rob- the one supporting service provided by birds that does not re- ertson et al. ), even for some temperate-zone plant species sult from foraging but involves construction of nests that are later with apparently insect-adapted flowers (Anderson ). As with used by many other organisms. Nests vary greatly in building seed dispersal, plant extinction may not follow loss of pollinators, materials, structure, complexity, size, longevity, and usefulness but we have few good measures of such effects, especially in cases to other organisms. Examples include excavated cavities or bur- where birds have declined rather than become extinct. One re- rows, cup nests, platform nests, mud nests, and domed nests (see cent study provides a cautionary example: Anderson et al. () Ehrlich et al. ). Open-cup and domed nests, the most com- showed a terrestrial trophic cascade in New Zealand whereby mon nest types (Collias and Collias , Collias ), are often mammalian carnivores reduced densities of pollinating birds, re- taken over by small mammals (Gates and Gates ), overwinter- sulting in an % reduction in seed output of the bird-pollinated ing spiders (Otzen and Schaefer ), and bumble bees (Dame et shrub Rhabdothamnus solandri and a % reduction in shrub real. ). Many animals, including insects like beetles and social generation. The authors stress that gradual plant declines might wasps, rodents, lizards, snakes, frogs, and even other bird species, frequently pass unrecorded. Where comparisons have been made use the domed ground nests of tropical ovenbirds (Furnariidae; within a single region, bird-pollinated plants seem to be more pol- Remsen ). Woodpecker cavities are used by other birds and len limited than dispersal limited; thus, the effects of mutualism by many other animal species, including mammals, amphibians, breakdown may be greater and faster-acting for bird-pollination and arthropods (Conner et al. , Neubig and Smallwood , than for seed-dispersal systems (Kelly et al. , ). However, Monterrubio-Rico and Escalante-Pliego ). Nest burrows are where pollination is primarily by insects, seed dispersal is prob- excavated by many bird taxa, including penguins, seabirds, alcids, ably the mutualism more at risk (Corlett ). parrots, owls, kingfishers, and passerines. These nests alter soil Scavenging and nutrient cycling.—The ecological importance properties and thus affect nutrient cycling (see above), and, like of scavenging birds is often underappreciated. Despite the com- woodpecker cavities, they are used by many other taxa, includ- mon assumption that decomposers (i.e., microbes and insects) are ing birds, snakes, mammals, and amphibians (Casas-Criville and primarily responsible for recycling carrion biomass, DeVault et Valera ). al. () demonstrated that vultures and other vertebrate scav- Summary: Indirect services.—Birds are highly mobile, occur engers usually consume most available carcasses in terrestrial globally, fill many ecological roles, and respond rapidly to envi- ecosystems. Although vultures are one of the most recognizable ronmental change. As described in the overview above, bird ac- types of birds to non-ornithologists, this familiarity is often not tivities provide links within and between ecosystems and can have accompanied by appreciation of the services they provide. By scav- large effects on other species. The ecosystem services that birds enging, vultures and other carnivorous vertebrates contribute to provide are largely indirect and support or enhance other services. waste removal, disease regulation, and nutrient cycling (Houston For example, insectivory, pollination, seed dispersal, and nutri- , DeVault et al. ). ent cycling benefit plants that then produce oxygen, food, lum- In addition to vultures, many other bird species scavenge an- ber, medicine, flood and erosion control, aesthetics, recreation, imal carcasses at least occasionally, including raptors, seabirds, and other benefits for human society. Birds may act as density- gulls, herons, rails, shorebirds, woodpeckers, and passerines dependent consumers that exert strong top-down effects on food (DeVault et al. ). Seabirds, in particular, are accomplished webs, which can result in prey population regulation, pest control, scavengers, often feeding on fishery discards (Hill and Wassenberg and corresponding changes in plant communities. Therefore, in JANUARY 2011 — PERSPECTIVES IN ORNITHOLOGY — 5

the context of ecosystem services, population decline among birds al. ). As public goods, ecosystem services are susceptible to may lead to changes that cascade through ecosystems and cause “externalities,” such as uncompensated side effects from other us- subsequent declines in benefits to humans. ers of a common resource. For example, extensive habitat modifi- Because the services are usually indirect, neither birds nor cation by one landowner may negatively affect pollination or pest their services are generally included in ecosystem-valuation mod- control for an adjacent landowner. An additional complication is els. Therefore, birds are only indirect beneficiaries of any con- that the economic value in environmental markets that are driven servation actions advocated by economic models. This approach by regulations (e.g., those mandated by the Clean Water Act and implies an indicator-species model of conservation in which a Endangered Species Act) is not determined by production func- limited subset of species or other environmental indicators are tions or the value to the end users, as in traditional markets. In- the basis for conservation planning and land-use decisions. Such stead, regulators set the value. For all of these reasons, market indirect benefits may be sufficient for bird conservation in some failure (i.e., the failure of market value to reflect full social cost), cases, but to date, the indicator-species model has had inconsis- is more often the rule than the exception for ecosystem services. tent success in predicting abundance and diversity of other spe- These problems raise fundamental concerns about the ability of cies (Roberge et al. , Larsen et al. , Cushman et al. ). neoclassical economic theory to adequately address environmen- Data that enable valuation of bird services will improve the mod- tal issues (Hall et al. , Lux , Nadeau ), and in that els of ecosystem valuation and increase bird-conservation efforts sense ecosystem valuation is a step toward bridging the divide be- as well as the benefits to humans. At the same time, efforts to es- tween economics and natural sciences. tablish valuation will promote additional research on many fun- A variety of methods have been used in valuation of ecosys- damental and important ecological questions. tem services (Farber et al. , ). Here, we briefly review the methods that are useful for quantifying services provided by birds. QUANTIFYING ECOSYSTEM SERVICES All of these methods are conventional in that the output is an economic value and therefore represents the “marginal value” people The overall goal of determining the value of ecosystem services are willing to pay for an item or service. Non-monetary valuation can be divided into three components. First is the need to describe methods such as ranking or stakeholder analysis have promise for and quantify the services themselves at local and regional levels. community-level decisions, but they have not yet been applied to The goal of describing ecosystem services is largely accomplished the services discussed here. (Sekercioglu a, b; Whelan et al. ), and we know consider- The value of birds in pest control can be estimated as the able natural-history details that are relevant to many ecosystem costs avoided by using birds instead of pesticides. These valua- services. But we need more detail at local levels from a variety of tions have been determined for bats (Cleveland et al. ) and in- sites to make global comparisons; in this way we can minimize sects (Losey and Vaughan ). Data necessary for this estimate the problems associated with “benefit transfer” (i.e., assuming that include first the monetary loss (e.g., reduced crop yields) from value estimates from one site are equivalent to those at a similar herbivory under current conditions, and then, based on diet and habitat elsewhere; Farber et al. , Plummer , Eigenbrod natural history of both insects and insectivores, an estimate of the et al. ). Data from multiple sites will also allow an assessment additional loss that would occur with no birds present. Assum- of the extent and sources of variation in ecosystem services. Un- ing that pesticides could accomplish the pest-control function of derstanding the variation in services among sites will lead to more birds and would yield the same crop levels in the absence of birds, robust estimates of the value of ecosystem services and more ef- the cost of that amount of pesticides is the avoided cost and an fective conservation plans. estimate of the value of avian pest control. Note that this method Second, we need methods to quantify the direct or indirect works well for agricultural crops or timber species for which we values of ecosystem services provided, and to test these methods have both market values and natural-history data (e.g., Takekawa with case studies. Finally, we need to combine the information and Garton ), but not for most wild plants (e.g., Sharam et al. from multiple ecosystem services to form a metric or model of val- ). Also note that this is short-term costing, assuming no evo- ues to assess how ecosystem services can be maximized under dif- lutionary responses of the pests to the pesticides, whereas experi- ferent land-use scenarios or policy changes. We need this type of ence has shown that pests rapidly evolve resistance to pesticides modeling approach because a given service (e.g., seed dispersal) (Gassmann et al. , Bourguet et al. ) but have not yet man- will not be protected successfully by itself, but rather as part of a aged to do so to birds because birds also evolve. comprehensive conservation strategy. Several models incorporat- An alternative, but one that still requires some estimate of the ing some ecological input have been developed (Daily and Mat- market value of an end product, is production valuation in which son , Ranganathan et al. , Daily et al. , Nelson et al. value is assigned on the basis of the economic outcome that results ). from changes in services. For example, the value of scatter-hoard- Most of the supporting and regulating ecosystem services are ing of seeds by corvids could be based on the reforestation value of not traded in traditional markets, and in that sense they are public the species they disperse (see case study below). Similarly, replace- goods with approximately the same cost (usually “free”) and value ment costs reflect the value of replacing or recreating a missing to all users. However, the value of some public ecosystem services ecosystem service. The Biosphere  experiment, which created an or resources may decline with level of use. For example, intensive artificial habitable system and cost ~$ million per human inhab- birdwatching at a given site may disturb the birds to the point that itant per year, took this to an extreme (Avise ). they leave or alter their behavior, thus rendering the resource un- Finally, through surveys or polls, preference-based ap- available or less worthwhile for additional viewers (Blumstein et proaches can yield contingent values that are essentially the 6 — WENNY ET AL. — AUK,VOL. 128

willingness to pay for an ecosystem service (Bowker and Stoll pest-control services, a spatially explicit individual-based model ). Contingent values, along with travel and equipment costs, can simulate how changes in landscape composition can affect the are used to estimate the economic impact of tourism and other delivery of pest-control services (M. D. Johnson and S. F. Railsback recreational uses. For example, birdwatchers in the United States unpubl. data). This approach could be used by conservation plan- spend more than $ billion annually for travel and equipment ners to estimate the economic value of forested habitats within (LaRouche , Carver ) and would be willing to spend $ agricultural landscapes, and to provide economic estimates of to $ per day for birdwatching opportunities (LaRouche ). ecosystem services under proposed land-use scenarios. Swedish oaks.—The replacement costs for the seed-dispersal CASE STUDIES services of Eurasian Jays (Garrulus glandarius) in Stockholm Na- tional Urban Park were estimated by Hougner et al. (). The Coffee pest control in Jamaica.—Shade-coffee farms can be high- National Urban Park of Stockholm features one of the largest oak quality habitats for insectivorous birds, especially migratory gen- forests in Sweden. The Swedes recognize oaks as keystone species eralist species that do not rely on intact understory vegetation that support unique communities of insects, lichens, mosses, and (Tejeda-Cruz and Sutherland , Johnson et al. ). Bird fungi, as well as nesting birds and bats (Hougner et al. ). In foraging within farms is concentrated in the shade trees that the National Urban Park of Stockholm, many of the oaks (Quer- grow over the coffee shrubs (Wunderle and Latta ). The cof- cus robur and Q. petrea) are more than  years old. Nearly % fee shrubs are naturally chemically defended and comparatively of the oaks in the park most likely result from acorn dispersal, poor in insect abundance (Lepelley , Greenberg et al. ). primarily by Eurasian Jays. Given that an epidemic of lethal oak Nonetheless, many birds also forage, to some degree, on insects disease is spreading across Europe and that most of the oaks in on the coffee shrubs (Wunderle and Latta ). Bird exclosure the park are currently healthy, Hougner et al. () argued that experiments have confirmed that bird foraging reduces over- the natural seed-dispersal services of the jays will be especially all insect biomass on coffee shrubs in Guatemala (Greenberg et important for maintaining healthy forests through natural local al. ), Mexico (Philpott et al. ), Panama (Van Bael et al. seed-dispersal over time. The authors calculated the replacement ), Puerto Rico (Borkhataria et al. ), and Jamaica (John- value of one pair of territorial jays, using two approaches: the cost son et al. ). The Coffee Berry Borer (Hypothenemus ham- of manually planting acorns and the cost of planting sapling oaks. pei) is the world’s most damaging insect pest of coffee (Damon They used data from several references to quantify acorn disper- ). Recent experiments in Jamaica indicate that birds reduce sal by Eurasian Jays and the number of sapling oaks that arise pest populations, increase saleable fruit, and boost farm income from jay dispersal each year, estimating germination and survival (Kellermann et al. , Johnson et al. ). Calculations of the rates. Having computed the costs of manual reforestation, the au- benefits provided were obtained by documenting pest infestation thors concluded that the minimum replacement value of a pair of levels in the presence and absence of bird foraging (via exclosures) Eurasian Jays was about $, (conversion from SEK, based on and translating higher saleable crop yields in the presence of birds  values) if acorns are seeded, and about $, if saplings into a dollar figure using crop market prices. Birds boosted farm are planted. Given the area occupied by oak forest in the Park, income by $ ha– year– on high-elevation farms (Kellermann et these jays represent a value of $, to $, per ha for forest al. ) and by $ ha– year– on a mid-elevation farm (Johnson regeneration. et al. ; here and below, figures are in U.S. dollars). Nutcrackers and pines.—A similar example is the economic As agents of ecosystem services, birds are notably mobile and value of scatter-hoarding (caching) seeds of Whitebark Pine (Pinus capable of utilizing multiple habitats. Therefore, the delivery of albicaulis) by Clark’s Nutcrackers (Corvidae: Nucifraga columbi- ecosystem services by birds in some cases may depend strongly ana). The cones of Whitebark Pine do not open, so this conifer ob- on habitat configuration and landscape composition. To harness ligately depends on nutcrackers for dispersal (Tomback , ; economic forces for conservation of birds and their habitats in Hutchins and Lanner ). Pine squirrels (Tamiasciurus spp.) are agricultural landscapes, ornithologists must not only document important conifer-seed predators and compete with nutcrack- the economic value of the ecosystem services provided by birds, ers for Whitebark Pine seeds, but they contribute little or no seed but also clarify bird movements and relationships among agricul- dispersal (Siepielski and Benkman , McKinney et al. ). tural lands and surrounding natural habitats. Several models are Cronartium ribicola, an invasive fungal pathogen that causes white available for projecting ecosystem services over a changing land- pine blister rust, and regional outbreaks of native pine beetles have scape, such as InVEST (Daily et al. ) and individual-based produced precipitous declines in Whitebark Pine nearly rangewide; models (Grimm and Railsback ). Ongoing radiotelemetry this pine is currently being evaluated for federal listing as a threat- studies have shown that an important coffee pest predator, the ened or endangered species (Tomback and Achuff ). The U.S. Black-throated Blue Warbler (Dendroica caerulescens), commutes Forest Service has undertaken restoration programs that involve from diurnal foraging territories within coffee habitat to noctur- the planting of putative pathogen-resistant seedlings, grown from nal roosting sites within natural forests (Jirinec et al. ) and seeds harvested from screened parent trees (e.g., Schwandt et al. establishes foraging territories close to farm edges and patches , Tomback and Achuff ). The cost of these restoration ef- of uncultivated vegetation within farms (B. R. Campos and M. D. forts essentially represents the valuation of natural seed-dispersal Johnson unpubl. data). These results establish links between the activities throughout the range of the pine. provisioning of an economically valuable ecosystem service On the basis of figures obtained from two U.S. national for- and natural vegetation both within and outside coffee farms. By ests, D. F. Tomback (unpubl. data) calculated the costs of planting linking bird movements to maps of landscapes and estimates of upper subalpine terrain with a typical density of Whitebark Pine JANUARY 2011 — PERSPECTIVES IN ORNITHOLOGY — 7

seedlings ( ha–). Ironically, a large part of the cost of obtaining et al. ), or generate noise and droppings in residential areas seeds requires that trees be climbed and cones caged in early sum- (Gorenzel and Salmon ). Some research regarding birds as ag- mer to prevent nutcrackers from depleting the seeds and squirrels ricultural pests has shown that perceived damage can be greater from cutting down cones, and then climbed again in September than actual damage (Basili and Temple ) or that the damage to harvest the cones. These costs were reduced by assuming that can be minimized with appropriate management (Dolbeer ). adequate numbers of seeds for a -ha planting could be harvested Crop pests also have beneficial effects, such as insectivory (Dol- from only one tree, although restoration efforts would actually re- beer ), and research on potential pest species needs to exam- quire more genetic diversity. Other expenses, such as materials, ine all the ecological roles that a bird fills in order to evaluate the travel, and the cost of protecting trees each year from pine beetles economic costs and benefits. Although a few bird species cause were excluded, whereas the costs of growing seedlings, planting economic damage, at the ecosystem level the services provided by seedlings, and administrative oversight were included. birds are overwhelmingly positive (Sekercioglu a, Whelan et Replacing nutcracker seed-dispersal services costs the U.S. al. ). More generally, payments for ecosystem services (PES) Forest Service a minimum of $, ha– in two national forests. are receiving increased attention in natural-resource management Whereas this seedling planting could be accomplished within one practice and theory (e.g., Pagiola , Farley and Costanza , field season, D. F. Tomback (unpubl. data) used a study of postfire Sommerville et al. ), and the recognition of ecosystem disser- regeneration after the  Yellowstone fires to estimate the aver- vices (McCauley ) is also becoming more formalized (Lyyti- age number of new seedlings per year that germinated per hectare maki et al. , Dunn , Power ). Very little of that work, from natural seed caches (Tomback et al. ). She concluded that however, has been focused on organismal delivery of services and it would take a minimum of  to  years in the Yellowstone area costs (but see Nelson ), and it is important for ornithologists for nutcrackers to produce  Whitebark Pine seedlings per hect- to contribute to this line of research in the future. are. Although this is slower, spreading regeneration over several Pest control.—The key aspect of pest control in need of fur- years may yield benefits by spreading risks over time (e.g., reduc- ther study is the extent to which trophic cascades have measurable ing risks of failure in a dry season, higher genetic diversity by in- economic benefits in terms of increased plant growth or agricul- cluding parents seeding in different years). However, given that the tural production. We know that top-down effects of bird foraging nutcrackers would spread both pathogen-resistant and susceptible are widespread, but most studies are still restricted to two trophic genotypes, establishing  healthy trees per hectare under cur- levels: birds and their prey. More experiments involving all three rent conditions by way of nutcrackers would take additional time. trophic levels (e.g., Marquis and Whelan , Mols and Visser Vulture decline in South Asia.—The consequences of the re- , Johnson et al. ) are needed, especially in agroecosys- cent catastrophic decline of vultures (three Gyps spp.) in South tems. Similarly, Fayt et al. () concluded that woodpeckers can Asia because of toxic livestock chemicals vividly demonstrate the regulate populations of insect pests of northern temperate coni- vital role that vultures play in ecosystems (Pain et al. , Green fer forests, but no studies have explicitly examined the economic et al. , Oaks et al. ). In the near absence of vultures, cattle benefits to the timber industry of this interaction. Research exam- carcasses remained on the landscape for longer periods and were ining the consequences of bird consumption of pests (arthropod available to other scavengers. As a result, populations of feral dogs or rodent) for either crop yield or plant demographics would be and other human-commensal facultative scavengers increased, extremely useful if conducted within multiple agricultural eco- and diseases spread to humans and domestic livestock. Markan- systems to determine generality and variability. This is a prime dya et al. () estimated that human health costs attributable example of an area of research where repetition aimed at establish- to population crashes of vultures in India totaled $ billion over ing the generality of research, rather than aimed at “being first” the years –. Additional cultural costs to the Parsi sects, or “novel,” needs to be encouraged by funding agencies. Another which rely on vultures for corpse cleansing, totaled $. million aspect of pest control that avian ecologists (and funding agen- (Markandya et al. ). cies) should be poised to exploit is centered on unfortunate natural “experiments” like avian population declines in eastern North RESEARCH NEEDS America from West Nile virus (LaDeau et al. ). Research on potential consequences (e.g., increases in human diseases car- The overview of selected ecosystem services and case studies dis- ried by insects) of those population declines on ecosystem func- cussed above point to some very specific research needs that are tion and provision of ecosystem services would be very useful and outlined below. More generally, we lack basic information on all instructive. the ways that birds could contribute to ecosystem services that ul- Other useful avenues for research involve determining the fea- timately benefit humans. Although we know in general the types of sibility and effectiveness of habitat manipulations that boost either ecosystem services that birds provide, we often lack sufficient de- populations of key avian pest consumers (e.g., deploying nest boxes) tails of bird behavior and ecology to formulate models of ecosystem or their effectiveness as pest consumers (e.g., providing perches valuation in a broader framework relevant to human well-being. for foraging). These sorts of manipulations should be a standard In addition to the more specific research subjects noted below, component of any integrated pest management (IPM) plan. Ad- a topic that has received relatively little attention in the ecosystem- ditionally, we need cost–benefit analyses of the effectiveness of services literature is the economic costs of some bird activity. For such manipulations, at least in agroecosystems, and cost–benefit example, some birds may be crop pests (Elliott and Lenton , comparisons of bird control versus chemical control mechanisms Dolbeer , Basili and Temple ), disperse weed seeds (Wil- with large externalities (i.e., pesticides). Moreover, studies that ex- liams ), damage property or livestock (Lowney , Harding amine the effectiveness of such manipulations must incorporate 8 — WENNY ET AL. — AUK,VOL. 128

effects from the framework of “landscapes of fear” (Laundré et al. for carcasses among vertebrates, insects, fungi, and microbes ). For example, perches may alter the behavior of small rodents (DeVault et al. , ; Selva et al. ; Selva and Fortuna through their increased fear, thus restricting their foraging, even if ; Parmenter and MacMahon ). Habitat type, climate, rodent population size does not decline markedly. carcass type, composition of the vertebrate community, and other A few studies have suggested that avian granivores exert biotic and abiotic factors all influence competition for carrion weed control, but these studies need to examine the effects on (DeVault et al. ). In some situations, the competitive balance an agricultural crop or other plants. Exclosure experiments are for carrion is shifted naturally away from birds, toward insects needed to carefully document birds’ seed consumption in a va- and microbes (e.g., DeVault et al. ) or facultative mammalian riety of contexts, from natural communities to agroecosystems scavengers (e.g., Putman ). Future research aimed at identi- to restoration projects. In areas where birds are considered pests fying the conditions under which various taxa consume carrion (e.g., Basili and Temple ), careful documentation of trophic would be beneficial. Such work would help elucidate vital links function would be useful—birds may, for example, consume seeds between ecosystem health and the population status of various of crops, but may compensate via consumption of pest insects. vertebrates, such as the vulture–cattle carrion system in India. Dispersal and pollination.—The key remaining questions Future investigations into the scavenging ecology of birds would about dispersal and pollination are largely very hard to answer. also improve our understanding of disease ecology (Jennelle et We need more information on the mechanisms (preferably from al. ), nutrient transport across ecosystem types (Polis et al. manipulative experiments) over the whole life cycle of the plants. ), and the distribution of predators and their prey (Cortes- Unresolved topics include () how various factors, including fru- Avizanda et al. a, b). givores (birds and mammals), seed predators, pathogens, habitat fragmentation, and plant competitors, interact to determine plant CONCLUSIONS reproductive success; () how widespread density- and distance- dependent seed and seedling mortality effects are (so-called Janzen- Birds provide many ecosystem services, which by and large are Connell effects), both in the tropics and in the temperate zone invisible and underappreciated. Several sudden losses of such ser- (Packer and Clay ), because these greatly increase depen- vices (e.g., carrion scavenging in India, pest control in China when dence on dispersers; () the level of change in dispersal services sparrows were locally exterminated, forest plant pollination in and its impacts at the plant community level following hunting, New Zealand) provide a sense of the negative consequences should habitat fragmentation, disturbance such as fire, or other anthro- such services be lost. We suggest that ecosystem services be bet- pogenic change; () the extent of seed limitation, which deter- ter studied and valued properly to ensure that humans continue mines whether pollination limitation matters (Kelly et al. ); to receive the benefits, and that birds continue to provide them. and () more studies to determine whether the unexpected impor- The case studies presented here show promising lines of research, tance of bird pollination to plants with flowers that are apparently but much work remains to be done. Despite the huge role of birds suited to insect pollination in temperate New Zealand (Kelly et al. as insectivores, very little research has been done on insectivory ) applies in other areas. in an ecosystem-services context (pest control), and most of what Birds disperse seeds of native and non-native plant species has been done is on pest control in coffee plantations. Similarly, and in some areas play a role in the spread of invasive plants (Salla- the ecosystem service of seed dispersal has been quantified only banks , Vila and Dantonio , Renne et al. , Cordeiro for seed-caching corvids. Dispersal of woody plants by terrestrial et al. , Gosper et al. , Bartuszevige and Gorchov , frugivores and dispersal of aquatic plants by waterfowl have not been Milton et al. , Underhill and Hofmeyr ). Such dispersal is addressed. We are not aware of any ecosystem-services valuation not necessarily detrimental—the non-native plant species them- research on the role of birds in nutrient cycling or as ecosystem selves may provide ecosystem services, such as erosion control or engineers. Further research to better understand the economic aesthetics. The question becomes whether the benefits of seed dis- value of birds will enable better policy and restoration practices, persal outweigh their detrimental effects. The situation is compli- promote and justify bird conservation efforts, and ultimately dem- cated further when non-native plants are dispersed by non-native onstrate the vital connections among human well-being, intact birds such as European Starlings (Sturnus vulgaris) in North ecosystems, and the preservation of avian biodiversity. America and European Blackbirds (Turdus merula) in New Zea- land (Williams ). More work is needed on dispersal of non- ACKNOWLEDGMENTS native plant species in an ecosystem-services framework. As with other ecological processes, if we understand the specifics we will This paper is based, in part, on a symposium held at the joint AOU, be better able to develop realistic valuation models. COS, CSO meeting in San Diego in February . We thank the Scavengers.—Unfortunately, the value provided by ecosystem organizers of that conference for supporting our participation services is most apparent after their loss. The catastrophic eco- at the symposium. Thanks to W. Gibbons, N. Lichti, D. Shealer, logical and human-health ramifications created by the recent col- S. Zack, and the lab group of J. Brown for many helpful comments lapse of vulture populations in India (Markandya et al. ) have on the manuscript. D.K. is grateful for funding from the New revealed the importance of ecosystem services provided by carri- Zealand Foundation for Research, Science and Technology under on-feeding birds. It is clear that in some areas proper ecosystem contracts COX and COX. D.F.T. thanks M. Jenkins, function is dependent, in part, on scavenging birds. Even so, the U.S. Department of Agriculture (USDA) Forest Service, Flathead cycling of carrion biomass, whether by scavenging or decompo- National Forest, and E. Davy, USDA Forest Service, Bridger-Teton sition, is a complex process governed by an intense competition National Forest, for providing the costs associated with whitebark JANUARY 2011 — PERSPECTIVES IN ORNITHOLOGY — 9

pine restoration projects. C.J.W. gratefully acknowledges R. Marquis Brenner, J., J. A. Jimenez, R. Sarda, and A. Garola. . An for his contributions to past research and ongoing, critical discus- assessment of the non-market value of the ecosystem services sions on ecosystem services. C.S. thanks the Christensen Fund provided by the Catalan coastal zone, Spain. Ocean & Coastal for support. Management :–. Britton, J. C., and B. Morton. . Marine carrion and scaven- LITERATURE CITED gers. Oceanography and Marine Biology :–. Burrows, C. J. . The seeds always know best. New Zealand Anderson, S. H. . The relative importance of birds and insects Journal of Botany :–. as pollinators of the New Zealand flora. New Zealand Journal of Carver, E. . Birding in the United States: A demographic and Ecology :–. economic analysis. Report -. U.S. Fish and Wildlife Service, Anderson, S. H., D. Kelly, J. J. Ladley, S. Molloy, and J. Terry. 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