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252 BRAWN ROBINSON THOMPSON

INTRODUCTION

Disturbance was once viewed largely as an insult to the balance of nature and

synonymous with habitat destruction (see Botkin 1990). Certain forms of distur-bance, however, are now held by ecologists and conservation biologists to play a

fundamental and creative role in maintaining the natural heterogeneity in envi-

ronmental conditions that organisms experience through space, time, or both.

Much theory and empirical work has been devoted to understanding how such

heterogeneity, or patchiness, affects the evolution of life histories and key ecolog-

ical processes at the population, community, ecosystem, and landscape levels of

organization (Connell 1978, Pickett & White 1985, Southwood 1988, Alverson

et al. 1994). The importance of disturbance to the ecology of species and conser-

vation of biodiversity has gained widespread recognition (Connell 1978, Sousa1984, Pickett & White 1985, Petraitis et al. 1989, DeGraaf & Miller 1996a,

Askins 2000) and has been defined by Pickett & White (1985, p. 7) as any

relatively discrete event in time that disrupts ecosystem, community, or popu-

lation structure and changes resources, substrate availability, or the physical

environment.

In the 1970s and 1980s, a series of models appeared that related disturbance

to enhanced species diversity at the local (i.e., within-patch) scale. The posited

mechanism was generally the prevention of competitive dominance and equilibrial

conditions either directly (through density-independent mortality of organisms) orindirectly (through changes in habitat and resource levels). Empirical evaluation

of these models has been generally corroborative, but the majority of these studies

have been on space-limited, sessile organisms such as barnacles, mussels, and

plants (Sousa 1984).

More recent works have considered the role of natural disturbance on species

diversity at the landscape or regional scale (Angelstam 1998, Askins 2000). Now,

disturbance is viewed as a natural ecological process leading to a mosaic of habitats

or successional stages that may enhance both and diversity (Anglestam 1998).

Indeed, ecological disturbance is an assumption in models that consider spatiallystructured populations. Depending on the scale of disturbance and the scale at

which an organism perceives the environment (Vos et al. 2001), natural and,

increasingly, anthropogenic disturbance can form the landscape framework for

metapopulations and source-sink dynamics (Hanski 1991, Dunning et al. 1992).

At a more evolutionary time-scale, the frequency of habitat disturbance has been

hypothesized as the source of selective pressure underlying the evolution of dis-

persal strategies and other key life-history traits (Southwood 1988). In restoration

ecology, landscape ecology, and the concept of ecosystem management, natural

disturbance is now generally recognized as essential for maintaining biodiver-sity (Alverson et al. 1994, Askins 2000). Direct application of disturbance (e.g.,

prescribed burning or silviculture) or allowing the natural agents of disturbance

(e.g., wildfire or floods) to proceed, are becoming standard elements of local and

regional conservation strategies (Johnson et al. 1998).

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DISTURBANCE ECOLOGY AND BIRDS 253

This paper reviews how natural and anthropogenic disturbance can affect the

population and community ecology of birds, and it emphasizes the conservation

implications of these effects. The motivation for this review is twofold. First,

the ecology of disturbance with relatively mobile organisms is different, but lesswell understood, than that for sessile organisms (Sousa 1984). Second, evidence

is growing that some form of disturbance is required by the habitat needs of a

large segment of the worlds terrestrial avifauna (Askins 2000). Much has been

written on the loss and fragmentation of closed-canopy forests due to agriculture

or urbanization (e.g., Robinson et al. 1995), but less study has been devoted to the

role of managing ecosystems and successional processes through the application of

disturbance. We limit our review to bird species associated with terrestrial habitats

or ecosystems and emphasize species that breed in North America. Finally, we

feature the major agents of disturbance that we believe are most relevant to land-usepolicy or options for avian conservation: fire, silviculture, and floodplain dynamics.

Silviculture stands apart as a commercially driven source of habitat disturbance;

however, its effects are pervasive and potentially creative. Other important agents of

disturbance in terrestrial avian habitats include drought, hurricanes, and herbivory

owing to insect outbreaks or mammals (Rotenberry et al. 1995, R. A. Askins,

personal communication).

GENERAL BACKGROUND

Habitat Ecology of Birds

The proximate cues used by birds to discriminate among habitats probably include

factors that influence the availability of food, risk of predation (to adults or nests),

and availability of nest sites. These factors include vegetation structure, floris-

tic composition, densities of conspecifics and heterospecifics, and microclimate

(Hilden 1965, James 1971, Hutto 1985, Block & Brennan 1993, Martin 1998).

Cues at the landscape or regional scale may include the size, shape, distribution,configuration, and connectedness of different patch types (Wiens et al. 1993). Dis-

turbance likely affects all aspects of avian habitat quality and selection from the

microhabitat to regional scale.

Conservation Status of Species in Disturbance-MediatedHabitats and Ecosystems

In a review of the conservation status of birds of eastern North America, Askins

(1993) reported a significantly greater proportion of recent declines in estimatedabundances for species most often associated with disturbance (grasslands,

shrubland, or savannas) than with forest birds (also see Peterjohn & Sauer 1999).

Estimates of trends in abundances were derived from the North American Breeding

Bird Survey (BBS), a continental avian survey program that was initiated in 1966

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(Sauer et al. 2000). We expanded the comparative approach of Askins (1993) to

consider all native North American terrestrial species for which abundance trends

from 1966 to 1998 could be estimated with reasonable precision. We included 274

species and placed each into one of five habitat categories: grassland (n = 27),shrub-scrub (n = 79), open woodlands and savanna (n = 63), closed-canopy

forest (deciduous, coniferous, or mixed, n = 77), and generalist (n = 28). The

last category was used for species associated with two or more categories over all

or part of their range and species of urban or suburban habitats. We then consulted

Sauer et al. (2000) for estimates of population trends. The significance of these esti-

mated trends are placed by Sauer et al. (2000) into four categories: significant (p 0.10), upward, or downward.

The distribution of species in the four trend categories varied significantly

among the different habitats or ecosystems (p = .0004, exact test of proportions). Agreater proportion of species experienced significant decreases in all disturbance-

mediated habitats than in the forested or generalist categories (Figure 1). The

greatest proportion of decreases was within grassland birds where 56% of species

in our sample declined significantly, followed by shrub-scrub (39%), and open

woodlands (33%). Combining these three categories into a general disturbance

category revealed that 40% (n = 169) of North American species associated with

Figure 1 Status of North American birds in five habitat groups with respect to trends in

abundance from 19661998. Trends from the North American Breeding Bird Survey (see

text for explanation of survey, how species were assigned to habitat groups, and how trends

were estimated and significance was determined).

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some type of disturbance-mediated habitats were significantly decreasing between

1966 and 1998. Conversely, only 17% of the disturbance species experienced

significant increases, whereas 34% of the forest species in our sample significantly

increased.

EFFECTS OF FIRE

Under natural conditions, fire is the major force that governs the development and

function of most deciduous, coniferous, and mixed forest types at temperate lati-

tudes (Wright & Bailey 1982, Attiwill 1994, Frost 1998). Fire, along with other

factors, is key in maintaining entire ecosystems such as grasslands and savannas

at temperate and tropical latitudes, and Mediterranean-type shrublands (Moreno &Oechel 1994, McPherson 1997). In an extensive analysis of preEuropean

settlement forests in the United States, Frost (1998) found that fire played a

fundamental role in the structure and composition of all but the wettest, most

arid, or fire-sheltered forest types. The estimated fire frequency from lightning

strikes and Native Americans varied from 1 to 700 y; one half of the continental

United States burned every 1 to 12 y. Other historical analyses of forests world-

wide have also identified the pervasive role of fire on landscape diversity and the

composition, structure, and function of stands (Atiwill 1994, Angelstam 1998).

Suppression of fire and longer intervals between fires have emerged over the

past 150 y in nearly all human dominated ecosystems and habitats where fire was a

factor historically (Frost 1998, Smith 2000). Prescribed fire as a means of restoring

or maintaining biodiversity is becoming commonplace throughout North America

and elsewhere (Askins 2000), but long-term studies of periodic fire and birds are

few. We review four case histories: coniferous and boreal forests of the west-

ern United States and southern Canada, oak savannas of the Midwestern United

States, pine savannas of the southeastern United States, and North American grass-

lands. Other important systems in which fire has known effects on birds include

shrubsteppe in the Western United States (Knick & Rotenberry 1999), diverse

Australian ecosystems (Reilly 1991), Mediterranen-type ecosystems (Moreno &

Oechel 1994), and Cerrado in Brazil (Cavalcanti 1999).

Coniferous and Boreal Forests of the Western United Statesand Southern Canada

The conifer-dominated forests of the western United States are variable in species

composition and include ecoregions (sensu Ricketts et al. 1999) that are geograph-

ically widespread. The ecoregions include: Sierra Forests dominated by mixed-

conifer stands (Ricketts et al. 1999); Rocky Mountain Forests (split by Ricketts

et al. 1999 into several regions) dominated at different elevations by ponderosa

pine (Pinus ponderosa), spruces (Picea spp.), lodgepole pine (Pinus contorta),

and firs (Abies spp.); and mid-elevation forest in the southwest dominated by

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ponderosa pine. Fire was a common disturbance in these forests; estimates range

from 1 per 10 to 300 y (Telfer 2000), with stand replacement fires more common in

mixed-conifer forests at relatively high elevations. Historically, replacement fires

were uncommon in open ponderosa pine forests (Telfer 2000). Boreal forests aremost extensive in southern Canada, with elements found in northern sections of

the continental United States. These forestsdominated by spruce, jack pine (Pi-

nus banksiata), and quaking aspen (Populus tremuloides)experience relatively

frequent stand-replacing fires that occur every 40 to 100 y (Telfer 2000).

The effects of fire on birds in these ecoregions are comparatively well known

(Bock et al. 1978, Raphael et al. 1987). The most celebrated case history is the

dependence of the Kirtlands Warbler (Dendroica kirtlandii) on burned jackpine

habitat (Probst & Weinrich 1993). Hutto (1995) examined effects of stand replac-

ing fires on breeding bird communities in coniferous forests of the Northern RockyMountain Forests. Fifteen species of birds were markedly more common on re-

cently (i.e., within 13 y) burned habitats than in nearly all the other forest types

and successional stages sampled in the region. The Black-backed Woodpecker

(Picoides arcticus) was largely restricted to the recently burned stands (12 y after

fire). Overall avian community structure was distinctive in burned sites (either

12 y or 1040 y after fire) largely because of greater densities of woodpeckers,

flycatchers, or granivorous species. An open stand with many snags (i.e., dead or

dying trees) and open ground provides for the nesting and foraging requirements

of these species. Other studies in mixed-coniferous forests confirm that cavity-nesting birds in particular benefit from the action of fire disturbance that creates

nesting foraging substrate in snags (Caton 1998).

Information on the effects of wildfire and prescribed fire on birds in southwest-

ern ponderosa pine forests, although hampered by a lack of long-term replicated

studies (Finch et al. 1997), indicates that several species benefit from disturbance.

And in many cases, these are the same species or species groups that respond pos-

itively to disturbance in mixed-conifer burns (summarized in Finch et al. 1997).

Populations of woodpeckers, certain flycatchers, and certain ground foragers tend

to increase after fire.Boreal forests in southern Canada have provided opportunities to assess the ef-

fects of natural burn cycles. As with mixed-conifer sites, stand-replacement burns

host distinctive bird communities from the time of initial burns and succession

from brush/shrub to relatively mature forest (Helle & Neimi 1996, Imbeau et al.

1999, Drapeau et al. 2000, Schieck & Hobson 2000). Virtually all studies have ob-

served enhanced diversity with fire disturbance and greater landscape diversity.

The Black-backed Woodpecker, in particular, is a specialist in recent burns (Bock &

Bock 1974). Thisspecies forages on wood-boring beetles that inhabit dying conifers

for 23 y after exposure to fire (Murphy & Lehnhausen 1998).

Midwestern Oak Savannas and Woodlands

Oak savannas and woodlands were formerly a significant element of the landscapes

of the midwestern United States and Canada that covered an estimated 11106 ha

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(Nuzzo 1986, Anderson & Bowles 1999; important oak savannas are also found

in the southwest United States and California, but are not considered here). Oak

savannas and woodlands are distinguished from oak forests by a more open canopy

(10% to 70% closure) and a herbaceous understory. Dominant tree species varyaccording to soil type and geographic location but typically include fire-resistant

species such as bur oak (Quercus macrocarpa), blackjack oak (Q. marilandica),

pin oak (Q. palustris), and white oak (Q. alba) (McPherson 1997). Under natural

conditions, the understory is dominated by herbaceous plants that are typically

associated with prairies (McPherson 1997). Oak savannas and woodlands were

formerly transitional vegetation types between prairies and closed-canopy forests

(Anderson & Bowles 1999). Fire was the primary disturbance force in the region,

and savannas burned less frequently than prairies, but more often than areas of

forest.Because of development and lack of disturbance, only 1% of the former oak

savannas in the Midwest remain, and this habitat type is judged as globally signif-

icant (Ricketts et al. 1999). With fire suppression, shaded conditions develop, the

herbaceous understory is lost, and encroachment by mesophytic species such as

maples (Acerspp.) occurs (Packard 1993). Accordingly, prescribed fire, mechani-

cal cutting, and herbiciding for the control of exotics are now common throughout

the region in an effort to restore degraded savannas or maintain an open canopy

(Packard 1993). Extant savannas in the upper Midwest are characteristically small,

isolated remnants (Temple 1998).Studies of birds and oak savannas have emphasized within-patch effects of

prescribed burning. Overall, it is clear that bird communities of oak savanna reflect

the transitional status of this vegetation type. Temple (1998) concluded that most

bird species characteristic of oak savannas also have affinities with either prairies

or forests. In the midwestern United States, this group includes about 40 bird

species; of these, about 60% are decreasing significantly in the region (Brawn

1994). Whereas Temple (1998) concluded that there are no true savanna birds in the

Midwest, one possible exception may be the Red-headed Woodpecker (Melanerpes

erythocephalus)a species that is decreasing throughout much of its range (Saueret al. 2000). In Illinois upland habitats, this species is restricted to oak savannas

and habitats that mimic savannas such as parks (J. D. Brawn, unpublished data).

Bird communities in oak forest in the upper Midwest change with fire and

savanna restoration. Davis et al. (2000) reported that sites with a history of more

fires have fewer trees, higher densities of snags, and variable densities of shrubs.

Bird species richness over two breeding seasons was markedly greater in the burned

sites, and avian community structure varied systematically with the intensity of

restoration. Canopy foliage gleaners such as the Red-eyed Vireo (Vireo olivaceus)

and the Scarlet Tanager (Piranga olicavea) decreased with restoration, whereasomnivorous ground or shrub feeders and woodpeckers increased. Species favored

by restoration and disturbance included the Baltimore Oriole (Icterus galbula), the

Red-headed Woodpecker, and the Field Sparrow (Spizella pusilla). Many species

such as the Indigo Bunting (Passerina cyanea), Field Sparrow, and American

Goldfinch (Cardeulis tristis) breed regularly in oak savannas and nest or forage in

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shrubs (Sample & Mossman 1994, Davis et al. 2000) and are also characteristic of

edge habitats and successional habitats such as abandoned pastures. Frequent and

pervasive burns will eliminate shrubs; therefore, the frequency of fire disturbance

can significantly change the diversity of bird communities within restored oaksavannas and woodlands.

Southeastern Pine Savannas

The upland landscapes of the southeastern United States were once dominated by

open pine savannas with an herbaceous understory (Platt 1999). These savannas

covered an estimated 200 106 ha (Platt 1999) and were dominated by longleaf

pine (Pinus palustris) (McPherson 1997). At elevations below 75 m, these savannas

were nearly continuous along the Atlantic and Gulf coasts from North Carolina toTexas.

Frost (1998) estimated that lightning caused fires every 13 y over much of the

region. The pine savannas of the southeastern United States, therefore, are one of

the most pyrogenic vegetation types known. Not surprisingly, changing land use,

fire suppression, commercial logging, and grazing have had immense impact on

the vegetation of the region. By 1985, only 4% of the preEuropean settlement

savannas remained (Noss 1989, Platt 1999) with only 3000 ha in old-growth form.

The avifauna of the region includes three species that are essentially endemic;

the Red-cockaded Woodpecker (Picoides borealis), the Brown-headed Nuthatch(Sitta pusilla), and the Bachmans Sparrow (Aimophila aestivalis) (Jackson 1988).

The range and abundances of the Red-cockaded Woodpecker have decreased

dramatically; this species is classified as federally endangered. The Bachmans

Sparrow has also decreased significantly throughout much of its range, although

it can use early successional pine fields (Dunning & Watts 1990). Fire-disturbed

habitat characterized by large, well-spaced pines and lack of midstory trees

(especially encroaching hardwood species) is essential for the Red-cockaded

Woodpecker (James et al. 1997, Plentovich et al. 1998). Habitat requirements for

the Bachmans Sparrow can vary geographically, but this species was historicallyalso dependent on fire (Dunning & Watts 1990, Plentovich et al. 1998).

The significant historical role of fire in molding the upland landscapes of the

southeast, coupled with the endangered status of Red-cockaded Woodpeckers

has generated considerable interest in the study of fire and avian biodiversity

in the region (Wilson et al. 1995). A 15y study of fire suppression in a Florida

pine woodland documented progressive changes in habitat structure and floris-

tics as the herbaceous understory was lost and mesophytic hardwoods encroached

(Engstrom et al. 1984). Avian species composition changed concomitantly, with

open-habitat species such as the Bachmans Sparrow and Loggerhead Shrike (La-nius ludovicianus) disappearing within 9 y as species characteristic of hardwood

forest colonized the site.

Habitat management for Red-cockaded Woodpeckers, such as short intervals

between growing season fires and removal of midstory, has provided opportunities

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to assess how disturbance influences overall community structure. These studies

provide evidence that management for an obligate disturbance species can have

important collateral benefits for the regional avifauna. Burger et al. (1998) re-

ported that intense management for woodpeckers promotes local increases in sevenspecies that are declining regionally or nationally. These species occur regularly

in early successional habitat or mature pine-grassland habitat with an herbaceous

understory. Management units for Red-cockaded Woodpeckers in South Carolina

also attracted over 30 scrub and successional species soon after burning and thin-

ning of the canopy and midstory (Krementz & Christie 1999).

Grasslands

Before European settlement, about 162 106

ha of grasslands occurred in NorthAmerica in three major ecoregions: tallgrass prairie, shortgrass prairie, and mixed-

grass prairie (Samson & Knopf 1996, Herkert & Knopf 1998). Loss of native

grassland habitat has been nearly complete in all regions (Samson & Knopf 1996),

resulting from conversion to cropland, planting of exotics, overgrazing, and en-

croachment by shrubs (Vickery et al. 1999). The biogeographic history of North

American grassland is complex, but these ecosystems undoubtedly developed with

frequent fire from lightning strikes and fires set by Native Americans (see several

chapters in Samson & Knopf 1996). The frequency of fire and other disturbance

agents such as grazing or mowing (a detrimental form of anthropogenic disturbancedepending on when it occurs; see Bollinger et al. 1990) is a major determinant of

the structure and plant species composition of most types of prairie. Vickery et al.

(1999) identified 32 bird species that are obligate to temperate-zone grasslands

in North America, and they listed an additional 52 species that regularly occur in

grasslands or ecotonal grasslands such as savannas.

Managing disturbance is the foundation of grassland restoration and conserva-

tion (Leopold 1949). Surprisingly few studies have been designed to assess fire

effects on grassland birds, but it is evident that the temporal and spatial scales

of disturbance have important consequences for habitat suitability. A 23-y studyby Johnson (1997) in mixed-grass prairie in North Dakota assessed the effects of

a 35 y burn rotation on avian abundances. Burning initially rid sites of vegeta-

tion and created open ground or short herbaceous vegetation; succession then led

to taller vegetation and encroachment of woody vegetation. Avian responses to

fire disturbance and subsequent succession fall into three groups: 1. those species

that immediately colonized such as the Killdeer (Charadrius vociferous), Marbled

Godwit (Limosa fedoa), or Upland Sandpiper (Bartramia longicauda); 2. species

that used sites about 2 y after burning, but before woody encroachment, such as the

Bobolink ( Dolichonyx oryzivorus) and Grasshopper Sparrow ( Ammodramus sa-vannarum); and 3. species that required woody vegetation and relatively long-term

protection from disturbance such as Common Yellowthroats (Geothlypis trichas).

Similar patterns of turnover were found by Madden et al. (1999) in mixed-grass

prairies in North Dakota that burn frequently. Therefore, even within the constraints

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of what is a prairie or grassland, there is an important short-term successional cy-

cle that is disturbance-dependent, with significant turnover in avian community

structure. Fire is particularly important in mesic grasslands that are comparatively

productive and where woody encroachment proceeds rapidly (Madden et al. 1999).Time elapsed since fire disturbance also influences bird populations in tallgrass

prairie. Local abundances of Henslows Sparrows (Ammodramus henslowii), a

species of conservation concern throughout much of its range (Peterjohn & Sauer

1999), are highly sensitive to near-term fire history. In Illinois, newly burned sites

are generally avoided and greatest abundances are attained on sites 24 y postburn

(Herkert & Glass 1999) within a relatively specific range of vegetation heights and

litter depths.

EFFECTS OF FLOODPLAIN DISTURBANCE

Floodplains are among the most disturbance-prone ecosystems. Although flood

pulses tend to be predictably seasonal in most regions of the earth, the severity and

timing of the flood pulse vary greatly (Malanson 1993, Sparks 1995). Floods create

oxbow lakes and backwaters, and promote primary successional gradients as plant

communities form on newly created soils inside meander loops (point bars) and

on islands (Malanson 1993). By depositing silt on the floodplain, floods enhance

primary productivity (Johnson et al. 1994). Severe floods can scour most or all

vegetation from a floodplain; lack of severe flooding can allow woody encroach-

ment on low-lying sections (Johnson 1994). The flood pulse concept (Junk et al.

1989) recognizes the importance of seasonal floods in promoting high biodiversity

and productivity in the lateral floodplain. The connection between rivers and their

backwaters is crucial for wildlife, especially fishes (Sparks 1995) and waterfowl

(Bellrose et al. 1983). The intermediate disturbance hypothesis (Connell 1978)

has been implicitly recognized in studies of floodplain ecosystems (Conner & Day

1976). Floodplain disturbances create an extraordinary diversity of habitats in river

systems where floods are not constrained or altered and terrestrial bird communi-

ties of floodplains are some of the richest in the world (e.g., Terborgh et al. 1990).

The river continuum concept (Vannote et al. 1980) relates the magnitude and pre-

dictability of seasonal flood pulses to the size of the river or stream with maximal

diversity predicted in midsized rivers where levels of disturbance are intermediate.

Perhaps because of their high productivity and rich soils, floodplain habitats

are also among the most endangered ecosystems (Dynesius & Nilsson 1994).

Rivers have been dammed, channelized, straightjacketed by levees, subjected

to increasing agricultural runoff, and lost to agriculture and plantation forestry

(Hunt 1988, Sparks 1995). Floodplains have therefore lost much of their fish and

wildlife as once-productive backwater swamps, marshes, and lakes have been iso-

lated or subjected to excessive siltation when they do flood (Bellrose et al. 1983,

Hunt 1988, Sparks 1995). In North America, three of the four terrestrial bird

species that became extinct since European settlement depended largely on flood-

plain forests including the Ivory-billed Woodpecker (Campephilus principalis)

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Carolina Parakeet (Conuropsis carolinensis), and Bachmans Warbler (Vermivora

bachmanii) (Askins 2000). Riparian forest loss in western North America is usually

estimated at 95% or greater as a result of conversion to agriculture and alteration

of hydrology (Knopf et al. 1988).In this section, we review the linkages between disturbances and floodplain

bird communities in three regions in North America: desert rivers of the southwest

United States, big rivers of the central United States, and forested rivers of the

southeastern United States.

Riparian Forests of Southwestern North America

Western riparian forests provide important resources for birds (Grinnell & Miller

1944, Rice et al. 1984, Rosenberg et al. 1991). A typical southwestern riparian

successional gradient consists of sandbars in the frequently flooded river channel,

willows (Salix spp.) along the edge of the channel and on islands, cottonwoods

(Populus spp.) farther back from the river channel, and, in areas less frequently

flooded, more diverse hardwoods. Each plant community occurs in a well-defined,

often narrow, zone with abrupt borders reflecting the frequency of disturbance

(Rosenberg et al. 1991). These zones contain rich breeding bird communities

that may be associated with particular plant species for nesting and feeding.

Disturbance-dependent willows, for example, are strongly preferred by several

species and subspecies that are currently rare and endangered. Examples are South-western Willow Flycatchers (Empidonax traillii extimus), Arizona Bells Vireos

(Vireo bellii arizonae), and Least Bells Vireos (V.b. pusillus) (Askins 2000).

Alteration of the disturbance regimes in western riparian forests has threatened

this entire system (Carothers & Johnson 1975, Szaro & DeBano 1985, Knopf et al.

1988, Ohmart 1994). Damming of rivers has reduced the extent of the willow-

cottonwood plant communities by reducing the severity and frequency of flood

disturbance (Rice et al. 1983, Anderson et al. 1983), with the resulting decline of

the many bird species that depend upon these communities (Strong & Bock 1990).

Dams reduce seasonal flooding in undammed portions of the river, which driesout backwaters and oxbows. Willows and cottonwoods do not grow along newly

created reservoirs because water levels fluctuate too greatly to allow seed germi-

nation (Hunter et al. 1987). Invasive introduced salt cedars (Tamarix chinensis)

have taken over extensive areas of the floodplain in Arizona because damming

and irrigation projects have made the soil more salty and hence more favorable

to this species (Rosenberg et al. 1991). These dense homogeneous stands of salt

cedar have far fewer birds than do native woodlands (Anderson et al. 1977). After

much trial and error, some of these riparian zones have been restored successfully

(reviewed in Askins 2000), and flood pulses are being reintroduced to the ColoradoRiver in the Grand Canyon.

Large Rivers of Central North America

Large floodplain systems such as those found in the Midwestern United States were

historically among the Earths most productive ecosystems (Bellrose et al 1983

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Sparks 1995), largely as a result of seasonal flood pulses that enriched soils and

created diverse landforms and plant communities including backwaters, oxbow

lakes, willow/cottonwood forests, mixed hardwood forests on natural levees, and

more open sandbar habitats. In the Midwest, these floodplains supported hugepopulations of migratory waterfowl (Bellrose et al. 1983) and diverse communities

of terrestrial birds (Zimmerman & Tatschl 1975, Knutson et al. 1996).

Disturbance regimes within these systems have been altered by navigation dams,

levees, and agriculture (Bellrose et al. 1983, Dynesius & Nilsson 1994, Johnson

et al. 1994). Most backwaters are isolated hydrologically or they rapidly fill with silt

(Bellrose et al. 1983, Sparks 1995) during the summer. Levees increase the severity

of floods in the river channel (Johnson et al. 1994), and flood-intolerant trees are lost

in the remaining floodplain (Hosner 1958, Hunt 1988, Knutson & Klaas 1998).

As a result, remaining mixed floodplain forests are being replaced by skeletalforests of dead trees and willow thickets (Yin & Nelson 1995, Knutson & Klaas

1997). Before they fall, dead trees temporarily provide habitats for cavity-nesting

birds such as Red-headed Woodpeckers and Prothonotary Warblers (Protonotaria

citrea) (Knutson & Klaas 1997).

With reduced flood severity, many rivers in the Great Plains that formerly lacked

woody cover are lined with mixed hardwoods more characteristic of floodplains

east of the Mississippi (Knopf & Scott 1990, Johnson 1994). Eastern forest birds

use these new wooded corridors to invade western forests, activity that threa-

tens the natural integrity of regional bird communities (Knopf et al. 1988). The lossof sandbar habitat resulting from altered disturbance regimes has been implicated

in the declines of the endangered Interior Least Tern (Sterna antillarum) and

Piping Plover (Charadrius melodus) (Schwalbach et al. 1993), which nest most

successfully on island sandbars to which predators have little access (Kirsch 1996).

Southeastern North American Floodplain Forests

Southeastern floodplain forests have rich bird communities (Fredrickson 1980,

Knutson et al. 1996, Sallabanks et al. 2000) that appear to depend in part on vary-

ing levels of disturbance. Carolina Parakeets and Ivory-billed Woodpeckers may

have depended upon mature floodplain forest, a habitat that has been almost com-

pletely logged and converted to rowcrops (Askins 2000). At least two species, the

extinct Bachmans Warbler and the rare Swainsons Warblers (Limnothlypis swain-

sonii) historically inhabited dense stands of cane (Arundinaria gigantea) (Meanley

1966, Eddleman et al. 1980, Hamel 1986, Remsen 1986), a plant that thrives at

an intermediate level of floodplain disturbance and occasional fire. Human activ-

ities have altered disturbance regimes to the extent that cane stands are now rare

(Askins 2000). Several bird species depend largely upon tree species associated

with varying levels of disturbance (Gabbe et al. 2001). Yellow-throated Warblers

( Dendroica dominica), for example, depend upon bald cypress (Taxodium dis-

tichum) growing in backwaters (Gabbe et al. 2001). There is little evidence, how-

ever, that bird communities in this region change dramatically along successional

gradients as has been documented in rivers with unaltered floodplain dynamics

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in, for example, Amazonia (Dyrcz 1990, Terborgh et al. 1990, Robinson 1997,

Robinson & Terborgh 1997).

Riparian disturbances in southeastern floodplains create natural edges such as

the borders of oxbow lakes, natural levees, and the rivers themselves (Hupp &Osterkamp 1985). The natural edges may enhance both bird community diversity

and nesting success (Sallabanks et al. 2000, Knutson et al. 2000). The complex

vegetation structure along natural disturbances may provide dense cover in which

nests may be concealed (Knutson et al. 2000).

EFFECTS OF SILVICULTURAL DISTURBANCE

Nearly one half (1563 million ha) of the area of natural forests worldwide is con-

sidered available for timber harvest given current legal and economic restrictions

(FAO 1999). In 1991, total world wood production was 4410 million m3, 37% of

which was in the tropics and the balance mostly in northern temperate and boreal

conifer forests (Whitmore 1998). Timber harvest is likely a more extensive distur-

bance agent in developed countries than all forms of natural disturbance. From the

early 1980s to the early 1990s, for example, approximately 24% of the timberland

in Michigan, Wisconsin, and Minnesota was disturbed by forest management (tim-

ber stand improvement, harvesting, planting, etc.) and 13% by natural processes

(fire, windthrow, flooding, and pests) (Schmidt et al. 1999).

Silvicultural treatments are typically applied to stands or patches with sim-

ilar tree composition and structure, often on a definable ecological land type.

Regeneration treatments remove existing trees and establish tree reproduction.

The clear-cut, shelterwood, and seed-tree methods regenerate even-aged stands

and selection methods regenerate uneven-aged stands (Nyland 1996). Intermedi-

ate treatments modify existing forest stands (stand improvement) or control its

growth and provide early financial returns (thinnings) (Nyland 1996).

Uneven-aged and even-aged methods differ in the scale and intensity of distur-

bance. Uneven-aged methods maintain a mix of tree sizes or ages within a habitat

patch by periodically harvesting individual or small groups of trees. Even-aged

methods harvest most or all of the overstory and create a fairly uniform habitat

patch dominated by trees of the same age (Nyland 1996). At a landscape level,

however, both methods can remove a similar volume of trees and regenerate similar

areas of forest but the size and distribution of disturbance patches varies (Shifley

et al. 2000). Bird communities differ in their response to regeneration methods as

a result of these differences in scale and intensity of disturbance.

For brevity, we review mainly the effects of regeneration practices in natural

or semi-natural North American forests where conservation is at least a secondarygoal, and do not consider intermediate treatments, intensive culture, or agroforestry.

North American Forests, Within Patch Effects

EVEN-AGED METHODS Even-age regeneration usually leads to near complete

turnover in the breeding bird community (Webb et al 1977 Conner et al 1979

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Franzreb & Ohmart 1978). About 50% of neotropical migratory birds in central

US hardwood forests prefer disturbance-mediated early successional stands cre-

ated by harvest and 1020% prefer these habitats in southern pine forests (Dickson

et al. 1995).In central and northern hardwood forests in the United States, nearly all (n> 50)

species that previously bred in the mature forest abandon regeneration cuts

(Annand & Thompson 1997). Bird communities respond significantly to the dis-

turbance and ensuing successional changes. If residual live and dead trees are

left in cut-over stands, some species such as Scarlet and Summer Tanager (Pi-

ranga rubra), Great Crested Flycatcher ( Myiarchus crinitus), and Red-bellied

Woodpecker (Melanerpes carolinus) that use the canopy or tree boles may continue

to use the stand. Species that soon colonize and use residual snags, trees, slashpiles,

or a rapidly developing herbaceous layer include Eastern Bluebirds (Sialia sialis),Northern Flicker (Colaptes auratus), Winter Wren (Troglodytes troglodytes),

Carolina Wren (Thryothorus ludovicianus), Field Sparrow, and American

Goldfinch. By the second growing season, developing ground vegetation, low

shrubs, and young trees attract numerousspecies, including Common Yellowthroat,

Chestnut-sided Warbler ( Dendroica pensylvanica), Mourning Warbler (Oporor-

nis philadelphia), Swainsons Thrush (Catharus ustulatus), Yellow-breasted Chat

(Icteria virens), Prairie Warbler (Dendroica discolor), and Blue-winged Warbler

(Vermivora pinus). By 10 y after timber harvest, tree reproduction forms a closed

canopy, most early successional species abandon the site, and mature forest speciesbegin to colonize (DeGraaf & Chadwick 1987, Thompson et al. 1996). Overall,

densities of birds in young regenerating forests often are similar to or much greater

than those in mature or midsuccessional pole-sized forests (Conner et al. 1979,

Yahner 1986, Probst et al. 1992, Hagan et al. 1997, Hobson & Schieck 1999).

Moreover, species richness may be greater in regenerating stands in the eastern

United States (Conner et al. 1979, Yahner 1986, King et al. 2001).

Regeneration methods influence the future vegetation composition of a stand.

For example, use of shelterwood or selection methods as opposed to clear-cutting

will favor more shade-tolerant species over intolerant species, such as westernhemlock over Douglas fir (Pseudotsuga menziesii) in western conifer forests, or

sugar maple (Acer saccharum) and American beech (Fagus grandifolia) over oaks

in eastern deciduous forests. The use of the seed tree method can directly control

the seed source for regeneration and is often used in southern pine forests. Species

composition is sometimes directly manipulated by the planting of improved tree

stock and often occurs with Douglas fir in western forests and loblolly (Pinus

taeda) and shortleaf pine (Pinus echinata) in southern forests. These shifts in

floristic composition can alter an avian community because many bird species have

demonstrated preferences for various tree species as foraging substrates (Franzreb& Ohmart 1978, Holmes & Robinson 1981).

A few studies have reported reproductive rates for songbirds in forest patches

managed by even-aged management. In extensively forested northern New

England, predation rates on artificial ground and shrub nests were not different

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among successional stages (DeGraaf & Angelstam 1993). Elsewhere, Morse &

Robinson (1999) observed lesser nest predation of Kentucky Warblers in mature

forest than in recent clear-cuts. Annand & Thompson (1997) reported compara-

tively high nest success and low parasitism rates for several shrub nesting birdsin stands regenerated by the clear-cut or shelterwood methods in Midwestern US

hardwood forests.

Even-aged management regenerates an entire stand, so stand size determines

the size of the new habitat patch. Krementz & Christie (1999) found no evidence,

however, that species richness or reproductive success varied among stands grow-

ing after clear-cuts ranging from 2.8 to 56.7 ha in southeastern longleaf pine

forests.

UNEVEN-AGED METHODS Single-tree and group-selection regeneration methodsremove only a portion of the trees in a habitat patch and attempt to maintain a

constant tree size/class structure over time. As a result, the turnover in bird species

following a regeneration harvest, and during succession between harvests, is less

significant than that following even-aged regeneration harvests (Thompson et al.

1995).

In eastern deciduous forests, most mature forest species occur in similar or

slightly lower numbers in forests managed by the selection method than in un-

treated mature forest (Annand & Thompson 1997, Robinson & Robinson 1999).

Gap-dependent species such as Hooded Warblers (Wilsonia citrina), IndigoBunting, White-eyed Vireo (Vireo griseus), and Carolina Wren, or species de-

pendent on understory density such as the Worm-eating Warbler (Helmitheros

vermivorus), are more abundant in forests managed by the selection method

than in mature even-aged forests. In New Englands northern hardwood forests,

bird communities in uneven-aged forest managed by selection method are also

similar to those in mature forests (DeGraaf & Chadwick 1987). Notwithstand-

ing, there is some evidence that uneven-aged management may selectively fa-

vor red maple (Acer rubrum), a change that could result in less oak and beech

mast for birds and other wildlife (R. A. Askins, personal communication). Se-lection cutting also appears to have minimal impact on the composition and

abundance of bird communities in western conifer forest. Similarly, in mixed-

conifer forest in the western Sierra Nevada, there were few consistent differences

in bird communities between uneven-aged and mature even-aged forests (Morrison

1992).

Again, there is limited knowledge of reproductive success in forest patches

managed by uneven-aged methods. In forests managed by the selection method

in southern Illinois, reproductive success of four songbird species was not, on

average, greater in logged or nonlogged forest, and cowbird parasitism was onlygreater for two species in recent cuts (Robinson & Robinson 2001). Nest suc-

cess of early succession birds does not appear to differ between group selection

cuts and clear-cuts in eastern deciduous forests (King et al. 2001, Clawson et al.

2001).

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North American Forests, Landscape Level Effects

Landscapes managed by even-aged methods are often a mosaic of different-aged

stands, whereas those managed by uneven-aged methods are often less heteroge-

neous and composed of a range of tree sizes. Thompson et al. (1996) used a simple

model to demonstrate how these patterns in composition might affect diversity in

central hardwood forests. The biggest differences among landscapes managed by

these silvicultural methods was not in the abundance of species that nest in mature

forest, but in early succession species that favored either small gaps (created by se-

lection cutting) or large patches (created by clear-cutting). Thompson et al. (1992)

compared the abundance species in landscapes with and without even-aged man-

agement (clear-cutting). In general, the abundance of only a few species nesting in

mid- or late-successional forest differed significantly between landscapes, and

diversity and abundance of early successional species was greater in managed land-

scapes. These patterns of diversity vary with rotation time between harvests, but sil-

vicultural disturbance clearly has a potentially beneficial effect on avian diversity.

Nonetheless, timber harvest decreases the continuity of mature or old forest

in the landscape and is a source of habitat fragmentation. Forest fragmentation

by human dominated land-uses, such as agriculture or urban and suburban de-

velopment, is hypothesized to reduce reproductive success of forest songbirds by

elevating numbers of generalist predators and nest predation in the landscape or

by increasing the amount of edge and edge-related nest predation (Robinson et al.

1995). In one of the few controlled landscape-scale studies of the effects of regen-

eration practices, Clawson et al. (2001) estimated differences in abundance and

reproductive success of common forest birds in Midwestern oak forests before and

after logging. Densities of some forest species decreased while others increased

following logging; however, neither nest predation or cowbird brood parasitism

increased following logging.

Syntheses are hampered by differences in methods (especially artificial vs.

real nests), the potential effects of other uncontrolled local factors, or effects of

landscape context (i.e., Donovan et al. 1997). Manolois (2001) reviewed evidence

for edge effects resulting from timber harvest and concluded that many of the

studies that reported no edge effects did not have adequate statistical design or

power to detect effects if they were present, and 9 of 13 studies with adequate

power detected edge effects. General pooling of species may be misleading since

lower nesting success near edges may be specific to certain groups such as ground

nesters (Flaspohler et al. 2001). Overall, evidence is conflicting for the nature of

edge effects between mature forest and recently harvested stands (Yahner & Scott

1988, Morse & Robinson 1999, Manolis et al. 2001).

Conservation Implications and Research Needs

This paper and other analyses (e.g., Alverson et al. 1994, Askins 2000) summa-

rize evidence that ecological disturbance is fundamental to the conservation of

birds. Whereas outright habitat loss is the most direct threat to avian biodiversity

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worldwide, it is clear that significant numbers of bird species are associated with

habitats and ecosystems created by diverse forms of disturbance. We do not min-

imize the conservation significance of species associated with habitats that are

rarely subjected to large-scale disturbance (e.g., primary or old growth forests),and we are encouraged that forest birds have benefited by afforestation in the

eastern United States (Askins 2000). Rather, we believe that understanding and

applying ecological disturbance offers opportunities to conserve large and diverse

group speciesmany of which are declining as a result of habitat loss and succes-

sional changes in habitat structure.

The frequency and intensity of ecological disturbance is a determinant of the

presence of many terrestrial habitat types (or ecosystems) in North America that

support significant components of avian biodiversity (Figure 2). We believe this

offers significant management and research opportunities; if conservation strate-gies for birds are to be effective, some combination of these habitats (along with

tracts of mature forest) should be maintained within or across landscapes. Pro-

tected habitat should not be protected from processes that maintain or conserve

significant elements of biodiversity.

As shown here, the bulk of information about disturbance and birds is on varia-

tion in populations and communities resulting from the initial effects of disturbance

Figure 2 Selected terrestrial habitats used by birds in North America qualitatively ordinated

according to frequency and intensity of disturbance. Even-aged forest can result from fire,

wind throw, floods, or even-aged silviculture. Uneven-aged forest results from small-scale

wind throw or uneven-aged silviculture. Woodlands and savanna result from moderate to light

intensity frequent fires. Grasslands generally result from frequent and intense fire, grazing,

or both.

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or ensuing succession. These studies are highly informative to land managers, but

the need for comprehensive understanding of disturbance and birds motivates a

complex research agenda. A prevailing need is to design and implement long-term

and large-scale studies that monitor the effects of disturbance. Studies to date havegenerally been short-term, without controls, unreplicated, and unplanned (Finch

et al. 1997). Outlined below are the issues and questions that we believe are the

most pressing.

Associations Between Habitat Disturbanceand Population Viability

Studies of forest birds, habitat fragmentation, and edge effects clearly identify

the usefulness of information on annual reproductive success and other demo-graphic traits that affect population viability (Heske et al. 2001). Data on local

abundances are crucial but incomplete in the context of estimating viability and

assessing land-management options (Brawn & Robinson 1996). With few excep-

tions (e.g., Johnson & Temple 1990, Krementz & Christie 1999), little is known

about the demography of birds in disturbance-mediated or successional habitats.

Our expectation is that fecundity will change with successional changes in habitat,

especially in grasslands and early successional forests, but details on the time-frame

of this dynamic will be highly informative to managers in determining desirable

intervals between prescribed fire, silvicultural treatments, or other modes of dis-turbance. Comparative studies from various geographic locations will be highly

informative. In the Midwest, for example, prescribed fire and restoration maintain

oak savannas in a specific disturbance-mediated state, but long-term successional

changes are common following the less frequent stand-replacing fires in coniferous

forests.

The effects of patch size on disturbance-dependent birds also requires close

study. Numerous studies confirm that avian diversity and reproductive success

is, on average, greater in larger than smaller patches of forest and grasslands

(Thompson et al. 2001, Heske et al. 2001). The effects of patch size on birds in moretransitional, disturbance-mediated habitats such as early successional forests and

oak savannas are less clear. Early results from a study comparing nesting success

of forest birds in managed forests found little or no difference in nesting success

of disturbance-dependent birds in small selection openings vs. larger clear-cuts

(Clawson et al. 2001; see also King et al. 2001).

Disturbance and Landscape Context

Questions about the scale and landscape context of disturbance will be differ-

ent depending on the mode of disturbance and the ecoregion involved. In the

extensive coniferous forests of the western United States and southern Canada,

a primary question is how to apply or allow disturbance so that a natural or

natural-like mosaic pattern of cover types is maintained (Hunter 1993, Landres

et al. 1999). In isolated oak savanna restorations, the question is the effect of

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surrounding habitat; that is, what are the effects (if any) of nearby grassland, for-

est, or urban areas on the structure of savanna bird communities (Temple 1998)?

Specific issues include the dispersion and arrangement of disturbance patches.

Such studies will also serve to address questions from theory on ecological neigh-borhoods and the scale of ecological patterns or processes (Addicott et al. 1987,

Engstrom & Mikushinski 1998). The frequency and extent of natal and breeding

dispersal are key to determining the scale at which birds respond demographically

to landscape features (Dunning et al. 1992, Vos et al. 2001) and are a persis-

tent uncertainty for most species. Notable exceptions are the model studies on

the Bachmans Sparrow and the Red-cockaded Woodpecker that relate ecologi-

cal disturbance, landscape features, and the spatial structure of their populations

(Dunning et al. 1992, Engstrom & Mikusinski 1998, James et al. 1997). Ide-

ally, local conservation decisions regarding ecological disturbance should be madewithin a multi-scale framework that acknowledges landscape and regional goals

based on historical heterogeneity in ecological landtypes (Thompson & DeGraaf

2001).

Effects of Natural vs. Anthropogenic Disturbance

Future land management planning will have to consider a range of approaches to

balance economic needs and conservation objectives. Understanding how these

different approaches affect the size and viability of birds associated with distur-

bance is essential. For example, a stand-replacing fire and a clear-cut both remove

trees but the long-term implications for avian conservation are likely to be different

(Hobson & Schieck 1999, Imbeau et al. 1999). One approach to the conservation

of disturbance-dependent birds is to restore natural disturbance regimes, but this

is realistic only on large expanses such as those in the western United States. Else-

where, disturbance-dependent birds are likely to depend on human activities that

stem from direct management or commercial activities. Ways to optimize these ac-

tivities for conservation, renewable resources, and agriculture requires immediate

study. In forests with commercial value, for example, the range of options includes

(from most to least intensive): short rotation intensive culture, plantation forestry,

commercial forestry in natural stands, multiple-use or ecosystem management,

and wilderness management through natural disturbance. Another issue is the in-

tegrity of floodplain bird communities that depend upon restoration of flood-pulse

disturbances (Galat et al. 1998); at present, there is disagreement over whether the

flood pulse should be mimicked through moist soil management (Galat et al. 1998)

or promoted by directly allowing the river to flow into sections of the floodplain

during flood pulses.

Finally, we believe that the social implications of ecological disturbance require

exploration. Episodes of opposition to prescribed fire and silviculture, for example,

strongly identify the need to educate the public on the positive role that natural

and anthropogenic disturbance can play in the conservation of avian biodiversity

(Marynowski & Jacobson 1999).

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ACKNOWLEDGMENTS

We thank Bob Askins, Dick DeGraff, Jim Herkert, and David King for thought-

ful comments. The help of Kathy Jennings was instrumental in preparing the

manuscript. Research support to JDB and SKR was provided in part by the

National Science Foundation (DEB 99-04058).

Visit the Annual Reviews home page at www.AnnualReviews.org

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