Elements of ecology 9th by smith 2

www.downloadslide.net C h ap t er 16   •  Community Structure    369  Ecologists use various sampling and statistical techniques to delineate and classify communities Generally, all employ some measure of community similarity or difference (see Quantifying Ecology 16.1) Although it is easy to describe the similarities and differences between two areas in terms of species composition and structure, actually classifying areas into distinct groups of communities involves a degree of F HB SF SF GB WOC BG ROC S ROC OCF P CF ROC H H OH OCF OH OCH OCF OCH P OCF OCF OCH OH (a) 1981 (WO 1828 est stnu ite o Table mountain pine heath (P) 457 Flat draws Sheltered slopes Canyons Ravines Coves Wetter Chestnut oak– chestnut heath (OCH) Chestnut oak– chestnut forest (OCH) 610 Red oak– pignut hickory forest (OH) 762 Hem loc k fo 1067 914 (b) Wh che ak– Re res t (H ) 1220 Cove forests (CF) Elevation (meters) 1372 ak– stn ut f che ore st ( Beech forests Mesic type Sedge type t for RO C) 1676 1524 C) Grassy balds (GB) (Heath balds [HB]) Boreal forests (Red spruce [S], Frazier fir [F], Spruce–fir [SF]) subjectivity that often depends on the study objectives and the spatial scale at which vegetation is being described The example of forest zonation presented in Figure 16.15 occurs over a relatively short distance moving up the mountainside As we consider ever-larger areas, differences in community structure—both physical and biological—increase An example is the pattern of forest zonation in Great Smoky Mountains National Park (Figure 16.18) The zonation is a complex pattern related to elevation, slope position, and exposure Note that the description of the forest communities in the park contains few species names Names like hemlock forest are not meant to suggest a lack of species diversity; they are just a shorthand method of naming communities for the dominant tree species Each community could be described by a complete list of species, their population sizes, and their contributions to the total biomass (as with the communities in Table  16.1 or Figure 16.15) However, such lengthy descriptions are unnecessary to communicate the major changes in the structure of communities across the landscape In fact, as we expand the area of interest to include the entire eastern United States, the nomenclature for classifying forest communities becomes even broader In Figure 16.19, which is Pitch pine heath (P) Virginia pine forest (P) Ridges and peaks Drier Smoky Mountains National Park (a) Topographic distribution of vegetation types on an idealized west-facing mountain and valley (b) Idealized arrangement of community types according to elevation and aspect M16_SMIT7406_09_GE_C16.INDD 369 500 1000 km Braun’s Forest Regions Open slopes Figure 16.18  Two descriptions of forest communities in Great (Adapted from Whittaker 1954.) Mixed mesophytic Southeastern evergreen Western mesophytic Beech-maple Oak-hickory Maple-basswood Oak-chestnut Hemlock-white pinenorthern hardwoods Oak-pine Figure 16.19  Large-scale distribution of deciduous forest communities in the eastern United States is defined by nine regions (Adapted from Dyer 2006.) 04/02/15 9:38 PM www.downloadslide.net 370    Part FIVE   •  Co m m uni ty E c o l o g y Qua n ti f y in g Ec olog y 16   Community Similarity W hen we say that a community’s structure changes as we move across the landscape, we imply that the set of species that define the community differ from one place to another But how we quantify this change? How ecologists determine where one community ends and another begins? Distinguishing between communities based on differences in species composition is important in understanding the processes that control community structure as well as in conservation efforts to preserve natural communities Various indexes have been developed that measure the similarity between two areas or sample plots based on species composition Perhaps the most widely used is Sorensen’s coefficient of community (CC) The index is based on species presence or absence Using a list of species compiled for the two sites or sample plots that are to be compared, the index is calculated as: Number of species common to both communities CC = 2c> 1s1 + s2 Number of species in community Number of species in community As an example of this index, we can use the two forest communities presented in Table 16.1: s1 = 24 species s2 = 10 species c = species CC = 12 * 92 124 + 102 = 18 = 0.529 24 a broad-scale description of forest zonation in the eastern United States developed by E Lucy Braun, all of Great Smoky Mountains National Park shown in Figure 16.18 (located in southeastern Tennessee and northwestern North Carolina) is described as a single forest community type: oak-chestnut, a type that extends from New York to Georgia These large-scale examples of zonation make an important point that we return to when examining the processes responsible for spatial changes in community structure: our very definition of community is a spatial concept Like the biological definition of population, the definition of community refers to a spatial unit that occupies a given area (see Chapter 8) In a sense, the distinction among communities is arbitrary, based on the criteria for classification As we shall see, the methods used in delineating communities as discrete spatial units have led to problems in understanding the processes responsible for patterns of zonation (see Chapter 17) M16_SMIT7406_09_GE_C16.INDD 370 The value of the index ranges from 0, when the two communities share no species in common, to 1, in which the species composition of the two communities is identical (all species in common) The CC does not consider the relative abundance of species It is most useful when the intended focus is the presence or absence of species Another index of community similarity that is based on the relative abundance of species within the communities being compared is the percent similarity (PS) To calculate PS, first tabulate species abundance in each community as a percentage (as was done for the two communities in Table 16.1) Then add the lowest percentage for each species that the communities have in common For the two forest communities, 16 species are exclusive to one community or the other The lowest percentage of those 16 species is 0, so they need not be included in the summation For the remaining nine species, the index is calculated as follows: PS = 29.7 + 4.7 + 4.3 + 0.8 + 3.6 + 2.9 + 0.4 + 0.4 + 0.4 = 47.2 This index ranges from 0, when the two communities have no species in common, to 100, when the relative abundance of the species in the two communities is identical When comparing more than two communities, a matrix of values can be calculated that represents all pairwise comparisons of the communities; this is referred to as a similarity matrix Calculate both Sorensen’s and percent similarity indexes using the data presented in Figure 16.15 for the forests along the elevation transect in the Siskiyou Mountains Are these two forest communities more or less similar than the two sites in West Virginia? 16.10  Two Contrasting Views of the Community At the beginning of this chapter, we defined the community as the group of species (populations) that occupy a given area, interacting either directly or indirectly Interactions can have both positive and negative influences on species populations How important are these interactions in determining community structure? In the first half of the 20th century, this question led to a major debate in ecology that still influences our views of the community When we walk through most forests, we see a variety of plant and animal species—a community If we walk far enough, the dominant plant and animal species change (see Figure 16.15) As we move from hilltop to valley, the structure of the community differs But what if we continue our walk over the next hilltop and into the adjacent valley? We would most likely notice that although the communities on the 04/02/15 9:38 PM www.downloadslide.net C h ap t er 16   •  Community Structure    371 Species importance (abundance)  Figure 16.20  Two models of Association E Association D Association C C A B D E E D D E Transition zone Species importance (abundance) C Transition zone Environmental gradient B A D E F C H G I Environmental gradient hilltop and valley are quite distinct, the communities on the two hilltops or valleys are quite similar As a botanist might put it, they exhibit relatively consistent floristic composition At the International Botanical Congress of 1910, botanists adopted the term association to describe this phenomenon An association is a type of community with (1) relatively consistent species composition, (2) a uniform, general appearance (physiognomy), and (3) a distribution that is characteristic of a particular habitat, such as the hilltop or valley Whenever the particular habitat or set of environmental conditions repeats itself in a given region, the same group of species occurs Some scientists of the early 20th century thought that association implied processes that might be responsible for structuring communities The logic was that the existence of clusters or groups of species that repeatedly associate was indirect evidence for either positive or neutral interactions among them Such evidence favors a view of communities as integrated units A leading proponent of this thinking was the Nebraskan botanist Frederic Clements Clements developed what has become known as the organismic concept of communities Clements likened associations to organisms, with each species representing an interacting, integrated component of the whole Development of the community through time (a process termed succession) was viewed as development of the organism (see Chapter 18) As depicted in Figure 16.20a, the species in an association have similar distributional limits along the environmental gradient in Clements’s view, and many of them rise to maximum abundance at the same point Transitions between adjacent communities (or M16_SMIT7406_09_GE_C16.INDD 371 C A (a) (b) D community (a) The organismal, or discrete, view of communities proposed by Clements Clusters of species (Cs, Ds, and Es) show similar distribution limits and peaks in abundance Each cluster defines an association A few species (e.g., A) have sufficiently broad ranges of tolerance that they occur in adjacent associations but in low numbers A few other species (e.g., B) are ubiquitous (b) The individualistic, or continuum, view of communities proposed by Gleason Clusters of species not exist Peaks of abundance of dominant species, such as A, B, and C, are merely arbitrary segments along a continuum associations) are narrow, with few species in common This view of the community suggests a common evolutionary history and similar fundamental responses and tolerances for the component species (see Chapter 5 and Section 12.6) Mutualism and coevolution play an important role in the evolution of species that make up the association The community has evolved as an integrated whole; species interactions are the “glue” holding it together In contrast to Clements’s organismal view of communities was botanist H A Gleason’s view of community Gleason stressed the individualistic nature of species distribution His view became known as the individualistic, or continuum concept The continuum concept states that the relationship among coexisting species (species within a community) is a result of similarities in their requirements and tolerances, not to strong interactions or common evolutionary history In fact, Gleason concluded that changes in species abundance along environmental gradients occur so gradually that it is not practical to divide the vegetation (species) into associations Unlike Clements, Gleason asserted that species distributions along environmental gradients not form clusters but rather represent the independent responses of species Transitions are gradual and difficult to identify (Figure 16.20b) What we refer to as the community is merely the group of species found to coexist under any particular set of environmental conditions The major difference between these two views is the importance of interactions—­ evolutionary and current—in the structuring of communities It is tempting to choose between these views, but as we will see, current thinking involves elements of both perspectives 04/02/15 9:38 PM www.downloadslide.net 372    Part FIVE   •  Co m m uni ty E c o l o g y Ec o l o g i c a l Issues & Applications Restoration Ecology Requires an Understanding of the Processes Influencing the Structure and Dynamics of Communities As we have discussed in previous chapters, human activities have led to population declines and even extinction of a growing number of plant and animal species Landuse changes associated with the expansion of agriculture (Chapter 9, Ecological Issues & Applications) and urbanization (Chapter  12, Ecological Issues & Applications) have resulted in dramatic declines in biological diversity associated with the loss of essential habitats Likewise, dams have removed sections of turbulent river and created standing bodies of water (lakes and reservoirs), affecting flow rates, temperature and oxygen levels, and sediment transport These changes have impacted not only the species that depend on flowing water habitats (see Figure 9.15) but also coastal wetlands and estuarine environments that depend on the continuous input of waters from river courses (see Chapter 25) In recent years, considerable efforts have been under way to restore natural communities affected by these human activities This work has stimulated a new approach to human intervention that is termed restoration ecology The goal of restoration ecology is to return a community or ecosystem to a close approximation of its condition before disturbance by applying ecological principles Restoration ecology involves a continuum of approaches ranging from reintroducing species and restoring habitats to attempting to reestablish whole communities The least intensive restoration effort involves the rejuvenation of existing communities by eliminating invasive species (Chapter 8, Ecological Issues & Applications), replanting native species, and reintroducing natural disturbances such as short-term periodic fires in grasslands and low-intensity ground fires in pine forests Lake restoration involves reducing inputs of nutrients, especially phosphorus, from the surrounding land that stimulate growth of algae, restoring aquatic plants, and reintroducing fish species native to the lake Wetland restoration may involve reestablishing the hydrological conditions, so that the wetland is flooded at the appropriate time of year, and the replanting of aquatic plants (Figure 16.21) More intensive restoration involves recreating the community from scratch This kind of restoration involves preparing the site, introducing an array of appropriate native species over time, and employing appropriate management to maintain the community, especially against the invasion of nonnative species from adjacent surrounding areas A classic example of this type of restoration is the ongoing effort to reestablish the tallgrass prairie communities of North America When European settlers to North America first explored the region west of the Mississippi River, they encountered a landscape on a scale unlike any they had known in Europe The forested landscape of the east gave way to a vast expanse of grass and wildflowers The prairies of North America once covered a large portion of the continent, ranging from Illinois M16_SMIT7406_09_GE_C16.INDD 372 and Indiana in the east into the Rocky Mountains of the west and extending from Canada in the north to Texas in the south (see Section 23.4, Figures 23.14 and 23.15) Today less than percent of the prairie remains and mostly in small isolated patches, which is the result of a continental-scale transformation of this region to agriculture (see Figure 9.17) For example, in the state of Illinois, tallgrass prairie once covered more than 90,000 km2, whereas today estimates are that only km2 of the original prairie grassland still exists To reverse the loss of prairie communities, efforts were begun as early as the 1930s in areas of the Midwest, such as Illinois, Minnesota, and Wisconsin, to reestablish native plant species on degraded areas of pastureland and abandoned croplands One of the earliest efforts was the re-creation of a prairie community on a 60-acre field near Madison, Wisconsin, that began in the early to mid-1930s by a group of scientists, including the pioneering conservationist Aldo Leopold The previous prairie had been plowed, grazed, and overgrown The restoration process involved destroying occupying weeds and brush, reseeding and replanting native prairie species, and burning the site once every two to three years to approximate a natural fire regime (Figure 16.22) After nearly 80 years, the plant community now resembles the original native prairie (Figure 16.23) These early efforts were in effect an attempt to reconstruct native prairie communities—the set of plant and animal species that once occupied these areas But how does one start to rebuild an ecological community? Can a community be constructed by merely bringing together a collection of species in one place? Figure 16.21  Volunteers help National Oceanic and Atmospheric Administration (NOAA) scientists prepare seagrass shoots for planting in the Florida Keys The plantings help enhance recovery of areas where sea-grass communities have been damaged or large-scale die-off has occurred 04/02/15 9:38 PM www.downloadslide.net  (a) (b) Figure 16.22  Photographs of early efforts in the restoration of a prairie community at the University of Wisconsin Arboretum (now the John T Curtis Prairie) (a) In 1935, a Civilian Conservation Corps camp was established and work began on the restoration effort (b) Early experiments established the critical importance of fire in maintaining the structure and diversity of the prairie community Many early reconstruction efforts met with failure They involved planting whatever native plant species might be available in the form of seeds, often on small plots surrounded by agricultural lands The native plant species grew, but their populations often declined over time Early efforts failed to appreciate the role of natural disturbances in maintaining these communities Fire has historically been an important feature of the prairie, and many of the species were adapted to periodic burning In the absence of fire, native species were quickly displaced by nonnative plant species from adjacent pastures Prairie communities are characterized by a diverse array of plant species that differ in the timing of germination, growth, M16_SMIT7406_09_GE_C16.INDD 373 C h a p t e r   •  Community Structure    373 and reproduction over the course of the growing season The result is a shifting pattern of plant populations through time that provides a consistent resource base for the array of animal species throughout the year Attempts at restoration that not include this full complement of plant species typically cannot attract and support the animal species that characterize native prairie communities The size of restoration projects was often a key factor in their failure Small, isolated fragments tend to support species at low population levels and are thus prone to local extinction These isolated patches were too distant from other patches of native grassland for the natural dispersal of other species, both plant and animal Isolated patches of prairie often lacked the appropriate pollinator species required for successful plant reproduction Much has been learned from early attempts at restoring natural communities, and many restoration efforts have since succeeded Restored prairie sites at Fermi National Accelerator Laboratory in northern Illinois are the product of more than 40 years of effort and now contain approximately 1000 acres; it is currently the largest restored prairie habitat in the world Attempts at reconstructing communities raise countless questions about the structure and dynamics of ecological communities, questions that in one form or another had been central to the study of ecological communities for more than a century What controls the relative abundance of species within the community? Are all species equally important to the functioning and persistence of the community? How the component species interact with each other? Do these interactions restrict or enhance the presence of other species? How communities change through time? How does the community’s size influence the number of species it can support? How different communities on the larger landscape interact? As we shall see in the chapters that follow, ecological communities are more than an assemblage of species whose geographic distributions overlap Ecological communities represent a complex web of interactions whose nature changes as environmental conditions vary in space and time Figure 16.23  Curtis Prairie at the University of Wisconsin Arboretum Native prairie vegetation has been restored on this 60-acre tract of land that was once used for agriculture 04/02/15 9:38 PM www.downloadslide.net 374    Part FIVE   •  Co m m uni ty E c o l o g y Su m m ar y Biological Structure  16.1 A community is the group of species (populations) that occupy a given area and interact either directly or indirectly The biological structure of a community is defined by its species composition, that is, the set of species present and their relative abundances Diversity  16.2 The number of species in the community defines species richness Species diversity involves two components: species richness and species evenness, which reflect how individuals are apportioned among the species (relative abundances) Dominance  16.3 When a single or a few species predominate within a community, they are referred to as dominants The dominants are often defined as the most numerically abundant; however, in populations or among species in which individuals can vary widely in size, abundance alone is not always a sufficient indicator of dominance Keystone Species  16.4 Keystone species are species that function in a unique and significant manner, and their effect on the community is disproportionate to their numerical abundance Their removal initiates changes in community structure and often results in a significant loss of diversity Their role in the community may be to create or modify habitats or to influence the interactions among other species Food Webs  16.5 Feeding relationships can be graphically represented as a food chain: a series of arrows, each pointing from one species to another that is a source of food Within a community, many food chains mesh into a complex food web with links leading from primary producers to an array of consumers Species that are fed on but that not feed on others are termed basal species Species that feed on others but are not prey for other species are termed top predators Species that are both predators and prey are termed intermediate species Functional Groups  16.6 Groups of species that exploit a common resource in a similar fashion are termed guilds Functional group or functional type is a more general term used to define a group of species based on their common response to the environment, life history characteristics, or role within the community Physical Structure  16.7 Communities are characterized by physical structure In terrestrial communities, structure is largely defined by the vegetation Vertical structure on land reflects the life-forms of plants In aquatic environments, communities are largely defined by physical features such as light, temperature, and oxygen profiles All communities have an autotrophic and a heterotrophic layer The autotrophic layer carries out photosynthesis The heterotrophic layer uses carbon stored by the autotrophs as a food source Vertical layering provides the physical structure in which many forms of animal life live Zonation  16.8 Changes in the physical structure and biological communities across a landscape result in zonation Zonation is common to all environments, both aquatic and terrestrial Zonation is most pronounced where sharp changes occur in the physical environment, as in aquatic communities Community Boundaries  16.9 In most cases, transitions between communities are gradual, and defining the boundary between communities is difficult The way we classify a community depends on the scale we use Concept of the Community  16.10 Historically, there have been two contrasting concepts of the community The organismal concept views the community as a unit, an association of species, in which each species is a component of the integrated whole The individualistic concept views the co-occurrence of species as a result of similarities in requirements and tolerances Restoration Ecology  Ecological Issues & Applications The goal of restoration ecology is to return a ­community or ecosystem to a close approximation of its condition before disturbance by applying ecological principles Restoration ­ ecology requires an understanding of the basic processes influencing the structure and dynamics of ecological communities Stud y Q ue s ti o n s How is a rank-abundance diagram generated? What does it show? Distinguish between a dominant and a keystone species What is the advantage of species diversity indices over species richness? Are all carnivores top predators? What distinguishes a top predator in the structure of a food chain? M16_SMIT7406_09_GE_C16.INDD 374 What is the role of a keystone species in a community? Distinguish between guilds and functional types In Figure 16.18, the vegetation of Great Smoky Mountains National Park is classified into distinct community types Does this approach suggest the organismal or individualistic concept of communities? Why? 04/02/15 9:38 PM www.downloadslide.net C h ap t er 16   •  Community Structure    375  Further R eadi n g s Classic Studies Pimm, S L 1982 Food webs Chicago: University of Chicago Press Although first published more than 30 years ago, this book remains the most complete and clearest introduction to the study of food webs New edition published in 2002 Recent Research Brown, J H 1995 Macroecology Chicago: University of Chicago Press In this book, Brown presents a broad perspective for viewing ecological communities over large geographic regions and long timescales Estes, J., M Tinker, T Williams, and D Doak 1998 “Killer whale predation on sea otters linking oceanic and nearshore ecosystems.” Science 282:473–476 An excellent example of the role of keystone species in the coastal marine communities of western Alaska Falk, D A., M Palmer, and J Zedler 2006 Foundations of restoration ecology Washington, D C.: Island Press Overview of ecological principles and approaches to restoring natural communities and ecosystems Mittelbach, G G 2012 Community Ecology Sunderland, MA: Sinauer Associates Students  Go to www.masteringbiology.com for assignments, the eText, and the Study Area with practice tests, animations, and activities M16_SMIT7406_09_GE_C16.INDD 375 An excellent text that provides an overview of the study of ­ecological communities Morin, Peter J 1999 Community Ecology Oxford: Blackwell Science This text provides a good overview of the field of community ecology Pimm, S L 1991 The balance of nature Chicago: University of Chicago Press An excellent example of the application of theoretical studies on food webs and the structure of ecological communities to current issues in conservation ecology Power, M E., D Tilman, J Estes, B Menge, W Bond, L Mills, G Daily, J Castilla, J Lubchenco, and R Paine 1996 “Challenges in the quest for keystones.” Bioscience 46:609–620 This article reviews the concept of keystone species as presented by many of the current leaders in the field of community ecology Ricklefs, R E., and D Schluter, eds 1993 Ecological communities: Historical and geographic perspectives Chicago: University of Chicago Press This pioneering work examines biodiversity in its broadest geographical and historical contexts, exploring questions relating to global patterns of species richness and the historical events that shape both regional and local communities Instructors  Go to www.masteringbiology.com for automatically graded tutorials and questions that you can assign to your students, plus Instructor Resources 04/02/15 9:38 PM Chapter www.downloadslide.net 17 Factors Influencing the Structure of Communities Douglas-fir and western hemlock with an abundance of dead wood and decomposing logs—a setting characteristic of old-growth forests CHAPTER GUIDE 17.1 Community Structure Is an Expression of the Species’ Ecological Niche 17.2 Zonation Is a Result of Differences in Species’ Tolerance and Interactions along Environmental Gradients 17.3 Species Interactions Are Often Diffuse 17.4 Food Webs Illustrate Indirect Interactions 17.5 Food Webs Suggest Controls of Community Structure 17.6 Environmental Heterogeneity Influences Community Diversity 17.7 Resource Availability Can Influence Plant Diversity within a Community Ecological Issues & Applications Top Predator and Trophic Cascade 376 M17_SMIT7406_09_GE_C17.INDD 376 04/02/15 8:49 PM www.downloadslide.net C h ap t e r 17   •  Factors Influencing the Structure of Communities    377 17.1  Community Structure Is an Expression of the Species’ Ecological Niche As we discussed in Chapter 16, the biological structure of a community is defined by its species composition, that is, the species present and their relative abundances For a species to be a component of an ecological community at a given location, it must first and foremost be able to survive The environmental conditions must fall within the range under which the species can persist—its range of environmental tolerances The range of conditions under which individuals of a species can function are the consequences of a wide variety of physiological, morphological, and behavioral adaptations As well as allowing an organism to function under a specific range of environmental conditions, these same adaptations also limit its ability to equally well under different conditions As a result, species differ in their environmental tolerances and performance (ability to survive, grow, and reproduce) along environmental gradients We have explored many examples of this premise Plants adapted to high-light environments exhibit characteristics that preclude them from being equally successful under low-light conditions (Chapter 6) Animals that regulate body temperature through ectothermy (poikilotherms) are able to reduce energy requirements during periods of resource shortage Dependence on external sources of energy, however, limits diurnal and seasonal periods of activity and the geographic distribution of poikilotherms (Chapter 7) Each set of adaptations enable a species to succeed (survive, grow, and reproduce) under a given set of environmental conditions, and conversely, restricts or precludes success under different environmental conditions These adaptations determine the fundamental niche of a species (Section 12.6) M17_SMIT7406_09_GE_C17.INDD 377 The concept of the species’ fundamental niche provides a starting point to examine the factors that influence the structure of communities We can represent the fundamental niches of various species with bell-shaped curves along an environmental gradient, such as mean annual temperature or elevation (Figure 17.1a) The response of each species along the gradient is defined in terms of its population abundance Relative abundance Fundamental niche Abundance T he c o m m u n it y i s a gr o up of plant and animal species that inhabit a given area As such, understanding the biological structure of the community depends on understanding the distribution and abundance of species Thus far we have examined a wide variety of topics addressing this broad question, including the adaptation of organisms to the physical environment, the evolution of life history characteristics and their influence on population demography, and the interactions among different species Previously, we examined characteristics that define both the biological and physical structure of communities and described the structure of community change as one moves across the landscape (Chapter  16) However, the role of science is to go beyond description and to answer fundamental questions about the processes that give rise to these observed patterns What processes shape these patterns of community structure? How will communities respond to the addition or removal of a species? Why are communities in some environments more or less diverse than others? Here, we integrate our discussion of the adaptation of organisms to the physical environment presented previously with the discussion of species interactions to explain the processes that control community structure in a wide variety of communities (Parts Two and Four) E1 E2 E3 Environmental gradient (a) Relative abundance Realized niche Abundance  E1 E2 E3 Environmental gradient (b) Figure 17.1  (a) Hypothetical example of the fundamental niches (potential responses in the absence of species interactions) of four species represented by their distributions and abundances along an environmental gradient Their relative abundances at any point along the gradient (E1, E2, and E3) provide a first estimate of community structure (b) The actual community structure at any point along the gradient is a function of the species’ realized niches—the species’ potential responses as modified by their interaction with other species present 04/02/15 8:49 PM www.downloadslide.net 378    Part FIVE   •  Co m m uni ty E c o l o g y Although the fundamental niches overlap, each species has limits beyond which it cannot survive The distribution of fundamental niches along the environmental gradient represents a primary constraint on the structure of communities For a location that corresponds to a given point along the environmental gradient, only a subset of species will be potentially present in the community, and their relative abundances at that point provide a first approximation of the expected community structure (Figure 17.1a) As environmental conditions change from location to location, the possible distribution and abundance of species changes, which changes the community structure For example, Figure 17.2 is a description of the biological Species Number of Relative individuals abundance Carolina Chickadee 244 24.6 Red-eyed Vireo 139 14.0 Eastern Wood Pewee 90 9.1 Tufted Titmouse 64 6.5 Blue Jay 55 5.5 Yellow-billed Cuckoo 41 4.1 Hooded Warbler 39 3.9 Downy Woodpecker 36 3.6 Red-bellied Woodpecker 27 2.7 Blue-gray Gnatcatcher 25 2.5 White-breasted Nuthatch 24 2.4 Pine Warbler 22 2.2 Scarlet Tanager 21 2.1 Cardinal 20 2.0 Carolina Wren 16 1.6 Wood Thrush 15 1.5 Summer Tanager Cerulean Warbler Yellow-shafted Flicker 15 0.9 Acadian Flycatcher 0.7 Indigo Bunting 0.7 White-eyed Vireo 0.6 Yellow-breasted Chat 0.6 Ruby-throated Hummingbird 0.5 Hairy Woodpecker 0.5 Ovenbird 0.5 Kentucky Warbler 0.5 Yellow-throated Vireo 0.4 Worm-eating Warbler 0.4 Brown-headed Cowbird 0.4 Chipping Sparrow 0.4 Prairie Warbler 0.3 Rufous-sided Towhee 0.3 Pileated Woodpecker 0.2 Great Crested Flycatcher 0.2 Brown Thrasher 0.2 Ruby-crowned Kinglet 0.2 Warbling Vireo Black-throated Green Warbler Yellowthroat 2 0.2 0.2 0.2 992 100.0 Total M17_SMIT7406_09_GE_C17.INDD 378 structure of the breeding bird community on the Walker Branch Watershed in east Tennessee (species present and their relative abundances) The figure shows the maps of geographic range and population abundance of four of the bird species that are  components of the bird community on the watershed As we discussed previously, these geographic distributions reflect the occurrence of suitable environmental conditions (within the range of environmental tolerances; Chapter 8) Note that the geographic distributions of the four species are quite distinct, and the Walker Branch Watershed in east Tennessee represents a relatively small geographic region where the distributions of these four species overlap As we move from this site in east Figure 17.2  The structure Pine Warbler 1.06 μg/cm2 0.76–1.05 μg/cm2 0.46–0.75 μg/cm2 0.10–0.45 μg/cm2 23 46 69 Distance (m) Figure 17.16 ... 1.1 2. 2 2. 2–5.9 1.5 0.8 Ovenbird 42 of the breeding bird community on the Walker Branch Watershed in Oak Ridge, Tennessee (United States) expressed in terms of