2012 COURSE DATES: AUGUST 4 – 17, 2012 - Sirenian International

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POPULATION CONTROL AND COMPETITION Thus, although rigorous proof that herbivores are generally controlled by predation is lacking, supporting evidence is available, and the alternate hypothesis of control by weather leads to false or untenable implications. The foregoing conclusion has an important implication in the mechanism of control of the predator populations. The predators and parasites, in con- trolling the populations of herbivores, must thereby limit their own resources, and as a group they must be food-limited. Although the populations of some carnivores are obviously limited by territoriality, this kind of in- ternal check cannot operate for all carnivores taken together. If it did, the herbivores would normally expand to the point of depletion of the vegetation, as they do in the absence of their normal predators and parasites. There thus exists either direct proof or a great preponderance of factual evidence that in terrestrial communities decomposers, producers, and predators, as whole trophic levels, are resource-limited in the classical density- dependent fashion. Each of these three can and does expand toward the limit of the appropriate resource. We may now examine the reasons why this is a frequent situation in nature. Whatever the resource for which a set of terrestrial plant species compete, the competition ultimately expresses itself as competition for space. A community in which this space is frequently emptied through depletion by herbivores would run the continual risk of replacement by another assemblage of species in which the herbivores are held down in numbers by predation below the level at which they damage the vegetation. That space once held by a group of terrestrial plant species is not readily given up is shown by the cases where relict stands exist under climates no longer suit- able for their return following deliberate or accidental destruction. Hence, the community in which herbivores are held down in numbers, and in which the producers are resource-limited will be the most persistent. The development of this pattern is less likely where high producer mortalities are inevitable. In lakes, for example, algal populations are prone to crash whether grazed or not. In the same environment, grazing depfetion is much more common than in communities where the major producers are rooted plants. A second general conclusion follows from the resource limitation of the species of three uophic levels. This conclusion is that if more than one species exists in one of these levels, they may avoid competition only if each species is limited by factors completely unutilized by any of the other species. It is a fact, of course, that many species occupy each level in most communities. It is also a fact that they are not sufficiently segregated in their needs to escape competition. Although isolated cases of nonoverlap have been described, this has never been observed for an entire assemblage. Therefore, interspecific competition for resources exists among producers, among carnivores, and arllong decomposers. It is satisfying to note the number of observations that fall into line with the foregoing deductions. Interspecific competition is a powerful selective force, and we should expect to find evidence of its operation. Moreover, the evidence should be most conclusive in trophic levels where it is neces- 423
424 THE AMERICAN NATURALIST sarily present. Among decomposers we find the most obvious specific mechanisms for reducing populations of competitors. The abundance of antibiotic substances attests to the frequency with which these mechanisms have been developed in the trophic level in which interspecific competition is inevitable. The producer species are the next most likcly to reveal evidence of competition, and here we find such phenomena as crowding, shad- ing, and vegetational zonation. Among the carnivores, however, obvious adaptations for interspecific competition are less common. Active competition in the form of mutual habitat-exclusion has been noted in the cases of flatworms (Beauchamp and Ullyott, 1932) and salamanders (Hairston, 1951). The commonest situation takes the form of niche diversification as the result of interspecific competition. This has been noted in birds (Lack, 1945; MacArthur, 1958), salamanders (Hairston, 1949), and other groups of carnivores. Quite likely, host specificity in parasites and parasitoid insects is at least partly due to the influence of interspecific competition. Of equal significance is the frequent occurrence among herbivores of ap- parent exceptions to the influence of density-dependent factors. The grass- hoppers described by Birch (1957) and the thrips described by Davidson and Andrewartha (1948) are well known examples. Moreover, it is among herbivores that we find cited examples of coexistence without evidence of competition for resources, such as the leafhoppers reported by Ross (1957), and the psocids described by Broadhead (19>8). It should be pointed out that in these latter cases coexistence applies primarily to an identity of food and place, and other aspects of the niches of these organisms are not known to be identical. SUMMARY In summary, then, our general conclusions are: (1) Populations of producers, carnivores, and decomposers are limited by their respective resources in the classical density-dependent fashion. (2) Interspecific competition must necessarily exist among the members of each of these three trophic levels. (3) Herbivores are seldom food-limited, appear most often to be predator-limited, and therefore are not likely to compete for common resources. LITERATURE CITED Andrewartha, H. G., 1957, The use of conceptual models in population ecology. Cold Spring Harbor Symp. Quant. Biol. 22: 219-232. Beauchamp, R. S. A., and P. Ullyott, 1937, Competitive relationships be- tween certain species of fresh-water triclads. J. Ecology 20: 200- 208. Birch, L. C., 1957, The role of weather in determining the distribution and abundance of animals. Cold Spring Harbor Symp. Quant. Biol. 22: 2<strong>17</strong>-263. Broadhead, E., 1958, The psocid fauna of larch trees in northern England. J. Anim. Ecol. 27: 2<strong>17</strong>-263.
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ECOLOGY, BEHAVIOR & CONSERVATION OF
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discomfort without proper protectio
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ABOUT BELIZE Research Site: The pro
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FIELD TRAINING AND ASSIGNMENTS Duri
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ENVIRONMENTAL CONDITIONS SBCRC is s
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HEALTH INFORMATION Routine Immuniza
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• Consult the CDC for immunizatio
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Oil or oil-based repellant (e.g. ol
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Ecology, Behavior, and Conservation
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MINIMUM / MAXIMUM CLASS SIZE: 6-24
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COURSE FEE & ADDITIONAL INFORMATION
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Ecology, Behavior & Conservation of
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2012 Field Course Policy & Liabilit
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2012 Field Course Policy & Liabilit
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FIELD JOURNALS You are required to
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OUR INVESTIGATION Figure 2. G. crut
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OUR INVESTIGATION Introduction The
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OUR INVESTIGATION Introduction On a
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Method We selected a patch reef loc
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OUR INVESTIGATION Introduction We f
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OUR RESEARCH FINDINGS Introduction
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area on the north part of the islan
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Introduction Our Investigation Befo
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Introduction Sea stars have long be
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INVESTIGATION Introduction Retracti
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Introduction Shells serve as a limi
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Introduction Lunar reproductive rhy
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TRIP SUMMARY SHEET (page 1) Day of
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RECORD OF EFFORT DATA SHEET Event (
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MANATEE SIGHTINGS AND SCANS Page 2
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MANATEE SIGHTINGS AND SCANS Page 4
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Date: Page 2 of ____ Sight/Scan Rec
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Date: Page 4 of ____ Sight/Scan Rec
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Sighting Log Continued from Page 1
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2010 VT: Scholar, BANNER, Safe Zone
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presentation to Horn Point Environm
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• Developed and managed education
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LaCommare, Katherine S. November 20
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• Comparative Vertebrate Anatomy,
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B. Ouranos, C. Bursey, and H. Kalb.
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2006. Holy Redeemer Day Camp. Conta
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18 NEWS AND VIEWS By 1995, in respo
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4842 J. M. BLUMENTHAL ET AL. are co
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4844 J. M. BLUMENTHAL ET AL. templa
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4846 J. M. BLUMENTHAL ET AL. Fig. 2
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4848 J. M. BLUMENTHAL ET AL. Fig. 4
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4850 J. M. BLUMENTHAL ET AL. mixed
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4852 J. M. BLUMENTHAL ET AL. Formia
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The Elusive Manatee -- An ethologic
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during the summer months while othe
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polyandrous - often mating with sev
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in the summer. Chessie, a male Flor
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ecords indicate that manatees have
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Florida manatee. National Biologica
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parturition: the action or process
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Manatees on the Belize Barrier Reef
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(a) (b) Manatees on the Belize Barr
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direction, and (e) when ‘travelli
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Season signi cantly aVected mean se
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male travelled at least as far as B
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55. Ontario Fisheries Research Labo
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NEWS OF THE WEEK MEETINGBRIEFS>> 10
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Functional Ecology 2008, 22, 284-28
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286 S. R. Noren Fig. 3. Swimming ki
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288 S. R. Noren Kelly, H.R. (1959)
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630 MARINE MAMMAL SCIENCE, VOL. 23,
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MARINE MAMMAL SCIENCE, 15(1): 102-1
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104 MARINE MAMMAL SCIENCE, VOL. 15.
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Page 236 and 237: $1.2 million (Moore et al. 2009a).
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Anim Cogn (2009) 12:43-53 47 Table
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Anim Cogn (2009) 12:43-53 49 Table
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Anim Cogn (2009) 12:43-53 51 the pr
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Anim Cogn (2009) 12:43-53 53 L, Rei
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(Halodule wrightii and Syringodium
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Diurnal and Nocturnal Scans Four of
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Figure 3. The number of scans (whit
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in 2006. Although this difference i
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manatus in Costa Rica. Biological C
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574 D. J. Semeyn et al. and natural
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576 D. J. Semeyn et al. Fig. 1 Refe
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578 D. J. Semeyn et al. Fig. 3 Refe
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580 D. J. Semeyn et al. Fig. 4 Kern
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582 D. J. Semeyn et al. biologists
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Intraspecific and geographic variat
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III. RESULTS Manatees in Florida an
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Genetica (2011) 139:833-842 DOI 10.
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Genetica (2011) 139:833-842 835 dur
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Genetica (2011) 139:833-842 837 Tab
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Genetica (2011) 139:833-842 839 Man
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Genetica (2011) 139:833-842 841 edu
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Fish Sci (2011) 77:795-798 DOI 10.1
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Fish Sci (2011) 77:795-798 797 was
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Low genetic variation and evidence
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Reduced Belize manatee dispersal an
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and respond to human encounters by
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tourist vessels are present, and th
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that approach behaviour may be habi
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to structure almost completely the
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MARINE MAMMAL SCIENCE, 27(3): 606-6
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Journal of Zoology The lesser of tw
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E. M. Arraut et al. & Sparks, 1989)
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E. M. Arraut et al. Frequent cloud
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E. M. Arraut et al. rising-water we
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E. M. Arraut et al. (Link, 1795) an
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The Journal of Experimental Biology
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Fig. 1. Rescue locations of manatee
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The present study is the first to c
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Bearhop, S., Waldron, S., Votier, S
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Table 1. Results from the GLM relat
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28
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Figure 1. Map of the Drowned Cayes
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Figure 3. Histogram of number of ma
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a. b. c. Power Power Power 100 80 6
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2012 COURSE DATES: AUGUST 4 – 17, 2012 - Sirenian International

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