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- By Rose Teteki Abbey, K. C. Abraham, David Tuesday Adamo, LeRoy H. Aden, Efrain Agosto, Victor Aguilan, Gillian T. W. Ahlgren, Charanjit Kaur AjitSingh, Dorothy B E A Akoto, Giuseppe Alberigo, Daniel E. Albrecht, Ruth Albrecht, Daniel O. Aleshire, Urs Altermatt, Anand Amaladass, Michael Amaladoss, James N. Amanze, Lesley G. Anderson, Thomas C. Anderson, Victor Anderson, Hope S. Antone, María Pilar Aquino, Paula Arai, Victorio Araya Guillén, S. Wesley Ariarajah, Ellen T. Armour, Brett Gregory Armstrong, Atsuhiro Asano, Naim Stifan Ateek, Mahmoud Ayoub, John Alembillah Azumah, Mercedes L. García Bachmann, Irena Backus, J. Wayne Baker, Mieke Bal, Lewis V. Baldwin, William Barbieri, António Barbosa da Silva, David Basinger, Bolaji Olukemi Bateye, Oswald Bayer, Daniel H. Bays, Rosalie Beck, Nancy Elizabeth Bedford, Guy-Thomas Bedouelle, Chorbishop Seely Beggiani, Wolfgang Behringer, Christopher M. Bellitto, Byard Bennett, Harold V. Bennett, Teresa Berger, Miguel A. Bernad, Henley Bernard, Alan E. Bernstein, Jon L. Berquist, Johannes Beutler, Ana María Bidegain, Matthew P. Binkewicz, Jennifer Bird, Joseph Blenkinsopp, Dmytro Bondarenko, Paulo Bonfatti, Riet en Pim Bons-Storm, Jessica A. Boon, Marcus J. Borg, Mark Bosco, Peter C. Bouteneff, François Bovon, William D. Bowman, Paul S. Boyer, David Brakke, Richard E. Brantley, Marcus Braybrooke, Ian Breward, Ênio José da Costa Brito, Jewel Spears Brooker, Johannes Brosseder, Nicholas Canfield Read Brown, Robert F. Brown, Pamela K. Brubaker, Walter Brueggemann, Bishop Colin O. Buchanan, Stanley M. Burgess, Amy Nelson Burnett, J. Patout Burns, David B. Burrell, David Buttrick, James P. Byrd, Lavinia Byrne, Gerado Caetano, Marcos Caldas, Alkiviadis Calivas, William J. Callahan, Salvatore Calomino, Euan K. Cameron, William S. Campbell, Marcelo Ayres Camurça, Daniel F. Caner, Paul E. Capetz, Carlos F. Cardoza-Orlandi, Patrick W. Carey, Barbara Carvill, Hal Cauthron, Subhadra Mitra Channa, Mark D. Chapman, James H. Charlesworth, Kenneth R. Chase, Chen Zemin, Luciano Chianeque, Philip Chia Phin Yin, Francisca H. Chimhanda, Daniel Chiquete, John T. Chirban, Soobin Choi, Robert Choquette, Mita Choudhury, Gerald Christianson, John Chryssavgis, Sejong Chun, Esther Chung-Kim, Charles M. A. Clark, Elizabeth A. Clark, Sathianathan Clarke, Fred Cloud, John B. Cobb, W. Owen Cole, John A Coleman, John J. Collins, Sylvia Collins-Mayo, Paul K. Conkin, Beth A. Conklin, Sean Connolly, Demetrios J. Constantelos, Michael A. Conway, Paula M. Cooey, Austin Cooper, Michael L. Cooper-White, Pamela Cooper-White, L. William Countryman, Sérgio Coutinho, Pamela Couture, Shannon Craigo-Snell, James L. Crenshaw, David Crowner, Humberto Horacio Cucchetti, Lawrence S. Cunningham, Elizabeth Mason Currier, Emmanuel Cutrone, Mary L. Daniel, David D. Daniels, Robert Darden, Rolf Darge, Isaiah Dau, Jeffry C. Davis, Jane Dawson, Valentin Dedji, John W. de Gruchy, Paul DeHart, Wendy J. Deichmann Edwards, Miguel A. De La Torre, George E. Demacopoulos, Thomas de Mayo, Leah DeVun, Beatriz de Vasconcellos Dias, Dennis C. Dickerson, John M. Dillon, Luis Miguel Donatello, Igor Dorfmann-Lazarev, Susanna Drake, Jonathan A. Draper, N. Dreher Martin, Otto Dreydoppel, Angelyn Dries, A. J. Droge, Francis X. D'Sa, Marilyn Dunn, Nicole Wilkinson Duran, Rifaat Ebied, Mark J. Edwards, William H. Edwards, Leonard H. Ehrlich, Nancy L. Eiesland, Martin Elbel, J. Harold Ellens, Stephen Ellingson, Marvin M. Ellison, Robert Ellsberg, Jean Bethke Elshtain, Eldon Jay Epp, Peter C. Erb, Tassilo Erhardt, Maria Erling, Noel Leo Erskine, Gillian R. Evans, Virginia Fabella, Michael A. Fahey, Edward Farley, Margaret A. Farley, Wendy Farley, Robert Fastiggi, Seena Fazel, Duncan S. Ferguson, Helwar Figueroa, Paul Corby Finney, Kyriaki Karidoyanes FitzGerald, Thomas E. FitzGerald, John R. Fitzmier, Marie Therese Flanagan, Sabina Flanagan, Claude Flipo, Ronald B. Flowers, Carole Fontaine, David Ford, Mary Ford, Stephanie A. Ford, Jim Forest, William Franke, Robert M. Franklin, Ruth Franzén, Edward H. Friedman, Samuel Frouisou, Lorelei F. Fuchs, Jojo M. Fung, Inger Furseth, Richard R. Gaillardetz, Brandon Gallaher, China Galland, Mark Galli, Ismael García, Tharscisse Gatwa, Jean-Marie Gaudeul, Luis María Gavilanes del Castillo, Pavel L. Gavrilyuk, Volney P. Gay, Metropolitan Athanasios Geevargis, Kondothra M. George, Mary Gerhart, Simon Gikandi, Maurice Gilbert, Michael J. Gillgannon, Verónica Giménez Beliveau, Terryl Givens, Beth Glazier-McDonald, Philip Gleason, Menghun Goh, Brian Golding, Bishop Hilario M. Gomez, Michelle A. Gonzalez, Donald K. Gorrell, Roy Gottfried, Tamara Grdzelidze, Joel B. Green, Niels Henrik Gregersen, Cristina Grenholm, Herbert Griffiths, Eric W. Gritsch, Erich S. Gruen, Christoffer H. Grundmann, Paul H. Gundani, Jon P. Gunnemann, Petre Guran, Vidar L. Haanes, Jeremiah M. Hackett, Getatchew Haile, Douglas John Hall, Nicholas Hammond, Daphne Hampson, Jehu J. Hanciles, Barry Hankins, Jennifer Haraguchi, Stanley S. Harakas, Anthony John Harding, Conrad L. Harkins, J. William Harmless, Marjory Harper, Amir Harrak, Joel F. Harrington, Mark W. Harris, Susan Ashbrook Harvey, Van A. Harvey, R. Chris Hassel, Jione Havea, Daniel Hawk, Diana L. Hayes, Leslie Hayes, Priscilla Hayner, S. Mark Heim, Simo Heininen, Richard P. Heitzenrater, Eila Helander, David Hempton, Scott H. Hendrix, Jan-Olav Henriksen, Gina Hens-Piazza, Carter Heyward, Nicholas J. Higham, David Hilliard, Norman A. Hjelm, Peter C. Hodgson, Arthur Holder, M. Jan Holton, Dwight N. Hopkins, Ronnie Po-chia Hsia, Po-Ho Huang, James Hudnut-Beumler, Jennifer S. Hughes, Leonard M. Hummel, Mary E. Hunt, Laennec Hurbon, Mark Hutchinson, Susan E. Hylen, Mary Beth Ingham, H. Larry Ingle, Dale T. Irvin, Jon Isaak, Paul John Isaak, Ada María Isasi-Díaz, Hans Raun Iversen, Margaret C. Jacob, Arthur James, Maria Jansdotter-Samuelsson, David Jasper, Werner G. Jeanrond, Renée Jeffery, David Lyle Jeffrey, Theodore W. Jennings, David H. Jensen, Robin Margaret Jensen, David Jobling, Dale A. Johnson, Elizabeth A. Johnson, Maxwell E. Johnson, Sarah Johnson, Mark D. Johnston, F. Stanley Jones, James William Jones, John R. Jones, Alissa Jones Nelson, Inge Jonsson, Jan Joosten, Elizabeth Judd, Mulambya Peggy Kabonde, Robert Kaggwa, Sylvester Kahakwa, Isaac Kalimi, Ogbu U. Kalu, Eunice Kamaara, Wayne C. Kannaday, Musimbi Kanyoro, Veli-Matti Kärkkäinen, Frank Kaufmann, Léon Nguapitshi Kayongo, Richard Kearney, Alice A. Keefe, Ralph Keen, Catherine Keller, Anthony J. Kelly, Karen Kennelly, Kathi Lynn Kern, Fergus Kerr, Edward Kessler, George Kilcourse, Heup Young Kim, Kim Sung-Hae, Kim Yong-Bock, Kim Yung Suk, Richard King, Thomas M. King, Robert M. Kingdon, Ross Kinsler, Hans G. Kippenberg, Cheryl A. Kirk-Duggan, Clifton Kirkpatrick, Leonid Kishkovsky, Nadieszda Kizenko, Jeffrey Klaiber, Hans-Josef Klauck, Sidney Knight, Samuel Kobia, Robert Kolb, Karla Ann Koll, Heikki Kotila, Donald Kraybill, Philip D. W. Krey, Yves Krumenacker, Jeffrey Kah-Jin Kuan, Simanga R. Kumalo, Peter Kuzmic, Simon Shui-Man Kwan, Kwok Pui-lan, André LaCocque, Stephen E. Lahey, John Tsz Pang Lai, Emiel Lamberts, Armando Lampe, Craig Lampe, Beverly J. Lanzetta, Eve LaPlante, Lizette Larson-Miller, Ariel Bybee Laughton, Leonard Lawlor, Bentley Layton, Robin A. Leaver, Karen Lebacqz, Archie Chi Chung Lee, Marilyn J. Legge, Hervé LeGrand, D. L. LeMahieu, Raymond Lemieux, Bill J. Leonard, Ellen M. Leonard, Outi Leppä, Jean Lesaulnier, Nantawan Boonprasat Lewis, Henrietta Leyser, Alexei Lidov, Bernard Lightman, Paul Chang-Ha Lim, Carter Lindberg, Mark R. Lindsay, James R. Linville, James C. Livingston, Ann Loades, David Loades, Jean-Claude Loba-Mkole, Lo Lung Kwong, Wati Longchar, Eleazar López, David W. Lotz, Andrew Louth, Robin W. Lovin, William Luis, Frank D. Macchia, Diarmaid N. J. MacCulloch, Kirk R. MacGregor, Marjory A. MacLean, Donald MacLeod, Tomas S. Maddela, Inge Mager, Laurenti Magesa, David G. Maillu, Fortunato Mallimaci, Philip Mamalakis, Kä Mana, Ukachukwu Chris Manus, Herbert Robinson Marbury, Reuel Norman Marigza, Jacqueline Mariña, Antti Marjanen, Luiz C. L. Marques, Madipoane Masenya (ngwan'a Mphahlele), Caleb J. D. Maskell, Steve Mason, Thomas Massaro, Fernando Matamoros Ponce, András Máté-Tóth, Odair Pedroso Mateus, Dinis Matsolo, Fumitaka Matsuoka, John D'Arcy May, Yelena Mazour-Matusevich, Theodore Mbazumutima, John S. McClure, Christian McConnell, Lee Martin McDonald, Gary B. McGee, Thomas McGowan, Alister E. McGrath, Richard J. McGregor, John A. McGuckin, Maud Burnett McInerney, Elsie Anne McKee, Mary B. McKinley, James F. McMillan, Ernan McMullin, Kathleen E. McVey, M. Douglas Meeks, Monica Jyotsna Melanchthon, Ilie Melniciuc-Puica, Everett Mendoza, Raymond A. Mentzer, William W. Menzies, Ina Merdjanova, Franziska Metzger, Constant J. Mews, Marvin Meyer, Carol Meyers, Vasile Mihoc, Gunner Bjerg Mikkelsen, Maria Inêz de Castro Millen, Clyde Lee Miller, Bonnie J. Miller-McLemore, Alexander Mirkovic, Paul Misner, Nozomu Miyahira, R. W. L. Moberly, Gerald Moede, Aloo Osotsi Mojola, Sunanda Mongia, Rebeca Montemayor, James Moore, Roger E. Moore, Craig E. Morrison O.Carm, Jeffry H. Morrison, Keith Morrison, Wilson J. Moses, Tefetso Henry Mothibe, Mokgethi Motlhabi, Fulata Moyo, Henry Mugabe, Jesse Ndwiga Kanyua Mugambi, Peggy Mulambya-Kabonde, Robert Bruce Mullin, Pamela Mullins Reaves, Saskia Murk Jansen, Heleen L. Murre-Van den Berg, Augustine Musopole, Isaac M. T. Mwase, Philomena Mwaura, Cecilia Nahnfeldt, Anne Nasimiyu Wasike, Carmiña Navia Velasco, Thulani Ndlazi, Alexander Negrov, James B. Nelson, David G. Newcombe, Carol Newsom, Helen J. Nicholson, George W. E. Nickelsburg, Tatyana Nikolskaya, Damayanthi M. A. Niles, Bertil Nilsson, Nyambura Njoroge, Fidelis Nkomazana, Mary Beth Norton, Christian Nottmeier, Sonene Nyawo, Anthère Nzabatsinda, Edward T. Oakes, Gerald O'Collins, Daniel O'Connell, David W. Odell-Scott, Mercy Amba Oduyoye, Kathleen O'Grady, Oyeronke Olajubu, Thomas O'Loughlin, Dennis T. Olson, J. Steven O'Malley, Cephas N. Omenyo, Muriel Orevillo-Montenegro, César Augusto Ornellas Ramos, Agbonkhianmeghe E. Orobator, Kenan B. Osborne, Carolyn Osiek, Javier Otaola Montagne, Douglas F. Ottati, Anna May Say Pa, Irina Paert, Jerry G. Pankhurst, Aristotle Papanikolaou, Samuele F. Pardini, Stefano Parenti, Peter Paris, Sung Bae Park, Cristián G. Parker, Raquel Pastor, Joseph Pathrapankal, Daniel Patte, W. Brown Patterson, Clive Pearson, Keith F. Pecklers, Nancy Cardoso Pereira, David Horace Perkins, Pheme Perkins, Edward N. Peters, Rebecca Todd Peters, Bishop Yeznik Petrossian, Raymond Pfister, Peter C. Phan, Isabel Apawo Phiri, William S. F. Pickering, Derrick G. Pitard, William Elvis Plata, Zlatko Plese, John Plummer, James Newton Poling, Ronald Popivchak, Andrew Porter, Ute Possekel, James M. Powell, Enos Das Pradhan, Devadasan Premnath, Jaime Adrían Prieto Valladares, Anne Primavesi, Randall Prior, María Alicia Puente Lutteroth, Eduardo Guzmão Quadros, Albert Rabil, Laurent William Ramambason, Apolonio M. Ranche, Vololona Randriamanantena Andriamitandrina, Lawrence R. Rast, Paul L. Redditt, Adele Reinhartz, Rolf Rendtorff, Pål Repstad, James N. Rhodes, John K. Riches, Joerg Rieger, Sharon H. Ringe, Sandra Rios, Tyler Roberts, David M. Robinson, James M. Robinson, Joanne Maguire Robinson, Richard A. H. Robinson, Roy R. Robson, Jack B. Rogers, Maria Roginska, Sidney Rooy, Rev. Garnett Roper, Maria José Fontelas Rosado-Nunes, Andrew C. Ross, Stefan Rossbach, François Rossier, John D. Roth, John K. Roth, Phillip Rothwell, Richard E. Rubenstein, Rosemary Radford Ruether, Markku Ruotsila, John E. Rybolt, Risto Saarinen, John Saillant, Juan Sanchez, Wagner Lopes Sanchez, Hugo N. Santos, Gerhard Sauter, Gloria L. Schaab, Sandra M. Schneiders, Quentin J. Schultze, Fernando F. Segovia, Turid Karlsen Seim, Carsten Selch Jensen, Alan P. F. Sell, Frank C. Senn, Kent Davis Sensenig, Damían Setton, Bal Krishna Sharma, Carolyn J. Sharp, Thomas Sheehan, N. Gerald Shenk, Christian Sheppard, Charles Sherlock, Tabona Shoko, Walter B. Shurden, Marguerite Shuster, B. Mark Sietsema, Batara Sihombing, Neil Silberman, Clodomiro Siller, Samuel Silva-Gotay, Heikki Silvet, John K. Simmons, Hagith Sivan, James C. Skedros, Abraham Smith, Ashley A. Smith, Ted A. Smith, Daud Soesilo, Pia Søltoft, Choan-Seng (C. S.) Song, Kathryn Spink, Bryan Spinks, Eric O. Springsted, Nicolas Standaert, Brian Stanley, Glen H. Stassen, Karel Steenbrink, Stephen J. Stein, Andrea Sterk, Gregory E. Sterling, Columba Stewart, Jacques Stewart, Robert B. Stewart, Cynthia Stokes Brown, Ken Stone, Anne Stott, Elizabeth Stuart, Monya Stubbs, Marjorie Hewitt Suchocki, David Kwang-sun Suh, Scott W. Sunquist, Keith Suter, Douglas Sweeney, Charles H. Talbert, Shawqi N. Talia, Elsa Tamez, Joseph B. Tamney, Jonathan Y. Tan, Yak-Hwee Tan, Kathryn Tanner, Feiya Tao, Elizabeth S. Tapia, Aquiline Tarimo, Claire Taylor, Mark Lewis Taylor, Bishop Abba Samuel Wolde Tekestebirhan, Eugene TeSelle, M. Thomas Thangaraj, David R. Thomas, Andrew Thornley, Scott Thumma, Marcelo Timotheo da Costa, George E. “Tink” Tinker, Ola Tjørhom, Karen Jo Torjesen, Iain R. Torrance, Fernando Torres-Londoño, Archbishop Demetrios [Trakatellis], Marit Trelstad, Christine Trevett, Phyllis Trible, Johannes Tromp, Paul Turner, Robert G. Tuttle, Archbishop Desmond Tutu, Peter Tyler, Anders Tyrberg, Justin Ukpong, Javier Ulloa, Camillus Umoh, Kristi Upson-Saia, Martina Urban, Monica Uribe, Elochukwu Eugene Uzukwu, Richard Vaggione, Gabriel Vahanian, Paul Valliere, T. J. Van Bavel, Steven Vanderputten, Peter Van der Veer, Huub Van de Sandt, Louis Van Tongeren, Luke A. Veronis, Noel Villalba, Ramón Vinke, Tim Vivian, David Voas, Elena Volkova, Katharina von Kellenbach, Elina Vuola, Timothy Wadkins, Elaine M. Wainwright, Randi Jones Walker, Dewey D. Wallace, Jerry Walls, Michael J. Walsh, Philip Walters, Janet Walton, Jonathan L. Walton, Wang Xiaochao, Patricia A. Ward, David Harrington Watt, Herold D. Weiss, Laurence L. Welborn, Sharon D. Welch, Timothy Wengert, Traci C. West, Merold Westphal, David Wetherell, Barbara Wheeler, Carolinne White, Jean-Paul Wiest, Frans Wijsen, Terry L. Wilder, Felix Wilfred, Rebecca Wilkin, Daniel H. Williams, D. Newell Williams, Michael A. Williams, Vincent L. Wimbush, Gabriele Winkler, Anders Winroth, Lauri Emílio Wirth, James A. Wiseman, Ebba Witt-Brattström, Teofil Wojciechowski, John Wolffe, Kenman L. Wong, Wong Wai Ching, Linda Woodhead, Wendy M. Wright, Rose Wu, Keith E. Yandell, Gale A. Yee, Viktor Yelensky, Yeo Khiok-Khng, Gustav K. K. Yeung, Angela Yiu, Amos Yong, Yong Ting Jin, You Bin, Youhanna Nessim Youssef, Eliana Yunes, Robert Michael Zaller, Valarie H. Ziegler, Barbara Brown Zikmund, Joyce Ann Zimmerman, Aurora Zlotnik, Zhuo Xinping
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References
- R. Norman Owen-Smith, University of the Witwatersrand, Johannesburg
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Frontmatter
- R. Norman Owen-Smith, University of the Witwatersrand, Johannesburg
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14 - An adaptive resource ecology : foundation and prospects
- R. Norman Owen-Smith, University of the Witwatersrand, Johannesburg
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Summary
In this concluding chapter, I review retrospectively the conceptual foundation established by preceding sections, and outline prospects for further developments of the resource-centred metaphysiological approach. I also look beyond the specific models developed for large mammalian herbivores in this book, to raise some general theoretical issues concerning the dynamics of consumer–resource systems. Thereby I introduce the vision of a broader Adaptive Resource Ecology.
Retrospective review
In starting this book, I set out to establish a conceptual framework for modelling herbivore–vegetation systems that would accommodate spatiotemporal variability in the resource base. The approach entailed integrating models of adaptive resource use by individual herbivores into a metaphysiological formulation of population dynamics. The basic principle was not to develop an elaborate, multi-level simulation, but rather to incorporate the functional outcome of lower-level processes, such as adaptive behaviour, into higher-level dynamics.
The starting foundation was Caughley's (1976a) modification of the classical Lotka–Volterra equations for herbivore–vegetation systems. I extended Caughley's analysis by recognizing how variation in food quality could cause the consumer gain function to deviate from the intake (‘functional’) response to changing resource abundance. Furthermore, I specified mortality losses as being non-linearly dependent on nutritional gains (following Getz 1991, 1993). In addition, I allowed for physiological expenditures as an independent biomass loss. The GMM label captured mnemonically the basic processes governing consumer biomass dynamics, at all levels from individuals to populations: Growth, Metabolism and Mortality.
Contents
- R. Norman Owen-Smith, University of the Witwatersrand, Johannesburg
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8 - Resource production : regeneration and attrition
- R. Norman Owen-Smith, University of the Witwatersrand, Johannesburg
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Summary
Vegetation constitutes a renewable resource that can be harvested through its consumption by herbivores. However, in seasonal environments the production of edible plant material does not take place continuously. Plants regenerate much of their above-ground biomass at the start of a growing season (early summer or rainy season), partly by reallocation from below-ground reserves. Later when conditions become adverse (too dry or too cold), they cease growth, and progressively shed the senescent parts. Superimposed on this seasonal cycle of growth and attrition are changes in plant populations, either through vegetative growth of new ramets, or via the recruitment of new individuals (genets) from seeds (Harper 1977).
Vegetation growth is not only phased seasonally, but also fluctuates in response to variability in weather during seasonal periods. Plants are a renewing resource for only a portion of the year, and a depleting resource for the remainder. Hence, no balanced equilibrium between production and consumption is maintained, except perhaps transiently. Because senescent parts are less nutritious than actively metabolizing leaves, herbivores must respond to seasonal changes in food quality as well as quantity. Different plant types and parts have distinctive patterns of growth and attrition in biomass and nutritional value.
In this chapter, an appropriate production function for the annual dynamics of the vegetation resource, accommodating this temporal and nutritional variability, is developed. The basis for this production, in terms of fluxes in sunlight, available moisture and mineral nutrients in soils, will not be considered explicitly.
7 - Resource allocation : growth, storage and reproduction
- R. Norman Owen-Smith, University of the Witwatersrand, Johannesburg
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Surplus resources gained from the environment while foraging, in excess of basic maintenance needs, can be allocated to biomass increase in different ways. Consumers can (a) grow in individual body size, (b) add to stored body reserves, or (c) divert resources to nurture the growth of offspring. The relative advantages of these alternative investments depend on the age, size and current body condition of individual animals, and also on timing within the seasonal cycle of resource abundance. The patterns of allocation by different individuals govern how food gains become transformed into population biomass dynamics. At times resource gains may be insufficient to cover physiological losses, so that population biomass declines.
In a variable environment, tradeoffs need to be made between surplus gains at one time and deficits incurred at a later stage. Evaluating the outcome requires a dynamic approach to optimization. In particular, we must consider the consequences of allocation decisions for the future body state of individual animals over some extended period. The time frame thus expands to an annual cycle or longer, ultimately up to individual lifespans.
In this chapter, we consider first how resource gains become transformed into a biomass growth potential, taking into account losses to maintenance metabolism. The simplest decision is then considered, whether and when to allocate surplus resources to fat stores, for fully grown animals that have no growth potential. The benefits of having body reserves need to be balanced against the costs of acquiring and carrying them.
Index
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4 - Resource distribution : patch scales and depletion
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In the preceding chapter, we recognized a distinction between within-patch feeding and between-patch searching. However, patches may be identified at a diversity of scales. To a large extent they are generated by the feeding responses of herbivores to particular forms of heterogeneity in the vegetation. Plants occur underfoot almost everywhere, except in desert regions. Herbivores selectively consume certain plant parts and species, and forage preferentially in particular vegetation types and landscape regions (Senft et al. 1987; Bailey et al. 1996). By rejecting the intervening plant material as food, herbivores define the gaps separating patches, thereby accentuating what might be somewhat fuzzy patterns in plant distribution (Arditi and Dacorogna 1988). Even in a uniform grass sward, herbivores create a patch structure with the first bite that they take, depleting food in the spot where they fed relative to the surrounding matrix.
The handling time required to deal with food that is patchily distributed determines the asymptotic form of the classical ‘Type II’ intake response (Holling 1959, 1965). If food arrived in the form of fine particles that could be swallowed without pause, as for filter feeders like barnacles or whales, the intake rate would rise linearly with food density until digestive capacity was satiated. The clustering of plant parts into bite-sized entities causes herbivores to divert their attention from searching, even if only momentarily, to deal with the material plucked.
Acknowledgements
- R. Norman Owen-Smith, University of the Witwatersrand, Johannesburg
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3 - Resource abundance : intake response and time frames
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For herbivores, food resources change in amount both intrinsically through vegetation growth and decay, and as a result of consumption. Such changes in food abundance influence the rate of food intake obtained by each individual herbivore. The relationship between food intake rate and resource abundance has conventionally been termed the ‘functional response’ (following Solomon 1949). However, because this is just one of several response functions considered in this book, I will call it specifically the intake response.
Not all of the standing vegetation biomass that a botanist would measure is effectively available for consumption by a specific herbivore. Some plant material is inaccessible, e.g. underground plant parts for most antelope and deer. Inaccessibility may be temporary; for example, leaves high in tree canopies may later be shed. A portion of plant biomass is inedible, e.g. woody trunks of trees for most ungulates. But inedibility is relative; vegetation components rejected as food at one time may later be eaten when little else is available. In effect, different plant species and parts vary over time in their acceptability to consumers. I will use the term potential food to encompass all vegetation components that are eaten at some time or other. Currently edible, accessible and acceptable material constitutes available food.
9 - Resource competition : exploitation and density dependence
- R. Norman Owen-Smith, University of the Witwatersrand, Johannesburg
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Demographic theory is permeated by the concept of density dependence, i.e. a decline in relative growth rate as population density increases towards some ‘carrying capacity’, where net population growth becomes zero. Density dependence arises fundamentally from intraspecific competition for limiting resources. As population density rises, each individual gets a smaller share of the resource supply. Competition may be expressed directly, through aggressive interactions, or other forms of interference with resource acquisition; or indirectly, simply via resource depression. Density dependence can also arise in other ways, e.g. through rising predation losses or parasite spread with increasing density, but these will not be the subject of this chapter.
For large herbivores, overt interference with foraging is rare. Exceptions occur in situations where high-quality food is locally concentrated, e.g. fallen fruits under a tree canopy. But generally food resources are widely dispersed, so there is little to be gained by displacing another animal from a feeding area. Competition occurs largely as a consequence of resource exploitation. As a result, the effects of competition may not be experienced immediately, but only at some later stage when less food remains as a result of the feeding impacts of conspecifics. In a seasonal environment, resource depression may be minor during the growing season when vegetation resources are renewing, but intensifies over the course of the dormant season when resource depletion is progressive.
2 - Consumer–resource models : theory and formulation
- R. Norman Owen-Smith, University of the Witwatersrand, Johannesburg
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This chapter outlines the theory, principles and concepts that underlie models of consumer–resource dynamics. We establish first the foundation in terms of the fundamental biological laws that apply to all organisms, and clarify the distinction between processes occurring in ecological versus evolutionary realms. This relates in particular to the currency to be used for the state variables in the models.
The kinds of consumers we have in mind in this book are large mammalian herbivores, with the resource constituted by vegetation. Starting from the simplest phenomenological models, we work upwards towards models of interactive herbivore–vegetation dynamics. Caughley's (1976a) classical model is elaborated further, drawing upon metaphysiological modelling concepts developed by Getz (1991, 1993). The three functional components contributing to herbivore biomass change -resource gains, metabolic attrition and mortality losses – are distinguished in formulating the GMM metaphysiological model of herbivore dynamics. The potential of this model to accommodate seasonal variability in resource production, as well as other environmental influences on herbivore population changes, will be outlined.
Some of the mathematical terminology and functions that will be adopted in the remainder of the book will be introduced. This sets the stage for the chapters that follow, which will explore in more detail the processes underlying population responses to spatial and temporal variability in environments.
12 - Resource partitioning : competition and coexistence
- R. Norman Owen-Smith, University of the Witwatersrand, Johannesburg
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- Adaptive Herbivore Ecology
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The importance of competition in structuring herbivore species assemblages is widely surmised, but rarely demonstrated. One view is that past competition over evolutionary time has resulted in niche divergence, such that competition has only a minor influence on extant populations (Owen-Smith 1985), or at least is manifested only intermittently (Owen-Smith 1989). Sinclair (1985) suggested that risk of predation has an overriding effect on species associations. However, Prins and Olff (1998) considered competition to be pervasive within grazing ungulate assemblages, such that species of closely similar size rarely coexist.
Part of the problem is that competition among large herbivores arises largely indirectly via vegetation modification or ‘sward capture’ (Murray and Illius 1996, 2000), rather than through overt interference. Smaller ungulates have the potential to out-compete larger species by depressing vegetation biomass below that needed to meet the greater absolute food requirements of the latter (Illius and Gordon 1987). On the other hand, the vegetation impacts of the bigger species can alter vegetation structure such that habitat conditions are changed for other species. However, the habitat modification need not be detrimental. A reduction in grass height could improve food access and dietary quality for smaller species better adapted to exploit short grasslands, leading to interspecific facilitation rather than competition (Vesey-Fitzgerald 1960; McNaughton 1976; Prins and Olff 1998). Nevertheless, despite the short-term gains in nutritional intake that may result for these small species, consequent increases in population abundance have not been observed (Sinclair and Norton-Griffiths 1983).
Acronym and symbol conventions
- R. Norman Owen-Smith, University of the Witwatersrand, Johannesburg
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- Adaptive Herbivore Ecology
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1 - Conceptual origins : variability in time and space
- R. Norman Owen-Smith, University of the Witwatersrand, Johannesburg
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- Adaptive Herbivore Ecology
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- 27 June 2002, pp 1-12
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The motivation to write this book stems from a long-felt uneasiness with the theoretical models that throng the ecological literature. Having originally studied the physical sciences at university, I fully recognized the power of fundamental theory in science. But the mathematical models that I found in the ecological literature were discordant with the world that I encountered as a field biologist. Their conventional assumptions of uniform environments and near-equilibrium conditions seemed far divorced from the continually changing environments that I experienced in African savannas. Biological persistence seemed to be more a matter of coping with variability than balancing around some equilibrium state. The vast superabundance of green vegetation at one time of the year contrasted sharply with the sparse dry remnants at a later stage. Conditions were also spatially heterogeneous, with certain localities retaining green glades at times when only standing brown hay remained elsewhere. Furthermore, conditions fluctuated widely between years, some years being quite benign, others severely adverse. Animals persisted by responding to this variability in numerous ways: adjusting what they ate, the habitats they occupied, and when they reproduced. Conventional mathematical models omitted the basic features that distinguish biological from physical systems: temporal variability, spatial heterogeneity, and adaptively changing responses over different time scales.
10 - Resource-dependent mortality : nutrition, predation and demography
- R. Norman Owen-Smith, University of the Witwatersrand, Johannesburg
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- Adaptive Herbivore Ecology
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Losses in population biomass through mortality are the integrated outcome of many influences. Resource deficiencies can eventually deplete body reserves to critical levels of malnutrition, but only rarely can deaths be ascribed simply to starvation, at least for wild herbivores. Extreme weather heightens metabolic costs and restricts foraging activity, draining body reserves towards critical levels. Animals that are weakened through food shortfalls become more susceptible to being killed by predators. Malnourished animals are also more vulnerable to parasite and disease infestations, amplifying their susceptibility to predation (Sinclair 1977; White 1983). Food shortages cause animals to spend more time foraging, raising physiological expenditures and increasing exposure to predation and other sources of accidental death. Risk of predation affects the habitats that can be exploited securely, exacerbating food limitations (McNamara and Houston 1987b; Sinclair and Arcese 1995). Where surface water is a limitation, animals must concentrate their movements around drinking points, where predators lie in wait. Social strife over inadequate resources may lead directly to mortality, or to injuries pre-disposing animals to other sources of mortality.
Susceptibility to predation depends on the types of predator present and their hunting methods. Adaptively foraging predators should preferentially seek those prey that are malnourished and hence most easily captured and killed. Nevertheless, healthy animals may sometimes have the misfortune to become victims of predators lying in ambush.
5 - Resource quality : nutritional gain and diet choice
- R. Norman Owen-Smith, University of the Witwatersrand, Johannesburg
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- Adaptive Herbivore Ecology
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For herbivores, the nutritional gain depends not only on the rate of food intake, but also on the quality of the material consumed. Accordingly, in this chapter the focus shifts from the intake response to the biomass gain response, represented by the function G in Equation 2.14, and hence to factors determining the value of the conversion coefficient c from plant to herbivore biomass.
The nutritional quality of herbage consumed is governed by its composition, firstly in terms of the proportion of cell wall fibre relative to cell contents, and secondly by concentrations of protein, soluble carbohydrates, mineral elements and other nutrients in the cell contents. These factors, as well as the composition of the cell wall, particularly its degree of lignification, determine the potential nutritional yield in the form of metabolizable energy and material substrates. However, the cell wall content also slows down the rate of digestive processing, restricting the rate at which these energizing and material nutrients become available, particularly for ruminants. In addition, many plant species contain various secondary chemicals, which may function as potential toxins, or impede digestion in various ways, reducing nutrient availability. Diet selection thus becomes largely a matter of balancing different nutritional benefits against various costs and rate restrictions. Immediate gains may need to be balanced against longer-term consequences. Quality as governed by nutritional yields must be traded against quantity as determined by the rate of intake obtained, over different time frames.
11 - Habitat suitability : resource components and stocking densities
- R. Norman Owen-Smith, University of the Witwatersrand, Johannesburg
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- Adaptive Herbivore Ecology
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- 27 June 2002, pp 232-263
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This chapter initiates the third section of this book, illustrating applications of the GMM model to particular issues in herbivore ecology and management. We address first the assessment of habitat suitability, as manifested by the abundance or performance of a herbivore population in a particular region. Wildlife managers usually think of the habitat ‘carrying capacity’, or maximum population that can be sustained. Livestock ranchers seek the optimal stocking density that would yield the highest production of meat, wool or other products. Caughley (1976b) termed the former ‘ecological carrying capacity’, the latter ‘economic carrying capacity’. Theoretical ecologists symbolize the zero growth density by the constant K in the logistic equation (May 1981), while acknowledging its vague reality. Ecological analysis has focussed largely on the feedbacks regulating populations around some density (Sinclair 1989). Less attention has been paid to the environmental determinants of the density attained.
The basic utility of the ‘carrying capacity’ concept was questioned by McLeod (1997) for real-world environments where population abundance fluctuates widely over time. Is it represented by the mean density? The peak density attained between disrupting events? Or by some remote upper level, rarely reached (cf. Ellis and Swift 1988)? Densities also differ widely regionally, and change numerically with enlargements in the scale of the area encompassed (Pastor et al. 1997).
Many factors contribute to habitat suitability, including not only the availability of suitable food and other resources, but also shelter from extreme conditions, and security against predators and other hazards.
Adaptive Herbivore Ecology
- From Resources to Populations in Variable Environments
- R. Norman Owen-Smith
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The adaptation of herbivore behaviour to seasonal and locational variations in vegetation quantity and quality is inadequately modelled by conventional methods. Norman Owen-Smith innovatively links the principles of adaptive behaviour to their consequences for population dynamics and community ecology, through the application of a metaphysiological modelling approach. The main focus is on large mammalian herbivores occupying seasonally variable environments such as those characterised by African savannahs, but applications to temperate zone ungulates are also included. Issues of habitat suitability, species coexistence, and population stability or instability are similarly investigated. The modelling approach accommodates various sources of environmental variability, in space and time, in a simple conceptual way and has the potential to be applied to other consumer-resource systems. This text highlights the crucial importance of adaptive consumer responses to environmental variability and is aimed particularly at academic researchers and graduate students in the field of ecology.