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Zeitschrift für Jagdwissenschaft

, Volume 46, Issue 3, pp 139–166 | Cite as

Population regulation in large northern herbivores: evolution, thermodynamics, and large predators

  • W. T. Flueck
Abhandlungen

Summary

Understanding population regulation of large northern herbivores like cervids has important practical and theoretical implications. Corrective measures for high densities of cervids must be based on theory and thus necessitate analysis of contradicting views of top-down and bottom-up population control. The former considers cervids incapable of self-regulation and hence that they need external factors like predation to achieve effective population regulation. The latter claims that cervids exhibit the capacity to adjust their numbers to the food supply as shown by physiological responses. However, these phenomena are not an expression of evolutionary adaption, they are a predictable physiological reaction to reduced food. In addition, studies were often done in man-modified environments or without considerations of modulating effects through predators. It is unlikely to study pristine relationships between large herbivores and predators in the future through field work, and insights from other fields need to be heeded. Considerations from evolution, thermodynamics, food webs and nutrient cycling indicate that the development of biological systems is unidirectional due to irreversible processes and leads toward optimal order and optimal accumulation of energy and nutrients. Large predators are thus not just a luxury development of evolution, but a necessary sequel to natural laws and they increase efficiency of the system to capture solar energy. It explains why analogous ecomorphs, like saber-tooth “cats” (placental and marsupial) have re-evolved independently at least 5 times. As a group, large predators developed traits allowing self-regulation including territoriality, intra- and interspecific killing, prey-switching, and dispersal. However, in man-modified environments, herbivore densities can reach such high levels that even an intact predator community will no longer exert regulation as there is an upper limit of predator density determined by social mechanisms. As kill success rates are very low, predators also affect herbivores by largely determining spacial distribution and behavioral adaptions, all of which modify herbivore-plant interactions. Cervids on the other hand exhibit traits all indicative of absence of a capacity to self-regulate. Predictable physiological responses to reduced food intake thus operate so late that the typical population response is an irruption with subsequent major dieoff and leads to a reduction in system performance including loss of biodiversity. Therefore, the claim that there exists “natural” regulation in such situations is an erroneous term for what is better called forced starvation. Two conclusions can be drawn:
  1. 1.

    Herbivore densities above a critical level will inevitably cause shifts in community functioning by altering plant and animal species composition, nutrient and energy flow patterns: the system will effectively be forced to a less complex and hence, less productive level, an evolutionary step backwards.

     
  2. 2.

    By having modified ecological parameters necessary for the welfare of herbivores, we need to take on the responsibility to guarantee their future welfare by actively replacing missing factors. For cervids it may mean implementing harvesting such that population densities permit natural plant rejuvenation and optimal biodiversity. Where the predator community is still intact, it should be protected by all means, and where still possible, predator communities should be restored. A basic tool is to disseminate the best available information, namely that large northern herbivores do not exhibit self regulation.

     

Keywords

Large Predator Herbivore Density Predator Community Behavioral Adaption Interspecific Killing 
These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

Bestandesregulierung von nördlichen Großherbivoren: Evolution, Thermodynamik und Großraubtiere

Zusammenfassung

Das Verständnis über Bestandesregulierung von nördlichen Groflherbivoren wie Cerviden hat wichtige praktische und theoretische Bedeutung. Lösungen für hohe Dichten von Cerviden müssen auf einer Theorie basieren und verlangen deshalb eine Analyse der sich widersprechenden Ansichten von top-down und bottom-up Regulierung. Die Erstere erkennt, dass Selbst-Regulierung unter Cerviden nicht besteht und dass Prädation für Bestandesregulierung notwendig ist. Die Letztere räumt den Cerviden ein, dass sie die Kapazität besitzen, ihren Bestand der Nahrungsquelle anzupassen. Dabei wird übersehen, dass dies keiner evolutionären Strategie, sondern vorhersagbaren physiologischen Reaktionen auf Nahrungsmangel entspricht. Zudem sind Studien oft in anthropogen modifizierter Umwelt oder ohne Berücksichtigung der modulierenden Effekte der Prädatoren gemacht worden. Durch die gegebene Schwierigkeit, zukünftig die ursprüngliche Beziehung zwischen nördlichen Großherbivoren und -raubtieren durch Feldstudien zu klären, ist es angebracht, sich auf andere relevante Wissensgebiete zu beziehen. Evolution, Thermodynamik, Analysen von Nahrungsketten und -zyklen deuten alle darauf hin, dass organische Entwicklung durch irreversible Prozesse richtungsspezifisch ist, sodass Ökosysteme optimale Ordnung und Anhäufung von Energie und Nährstoffen erreichen. Großraubtiere sind nicht nur luxuriöse Erscheinungen der Evolution, sondern sind Folge der genannten Gesetze und erhöhen somit die Wirksamkeit des Systems im Abfangen solarer Energie. So wird verständlich, dass Evolution in analoger Weise Raubtiertypen mehrere Male und innerhalb verschiedener Taxa hervorbrachte (z. B. 4 mal als Säbel-Säuger, aber auch als Säbel-Beuteltier). Zudem sind unter den Großraubtieren mehrere, für Selbst-Regulierung wichtige Charakteristika weitverbreitet: Territorialität, intra- und interspezifisches Töten, Prey-Switching und Dispersion. Im modifizierten Ökosystem können Dichten der Cerviden jedoch so hoch werden, dass auch eine intakte Räubergemeinschaft keine Regulierung mehr erreicht, weil die obere Grenze der Prädatorendichte durch Sozialverhalten bestimmt wird. Da die Wirksamkeit beim Beutefang sehr gering ist, beeinflussen Raubtiere zudem die Verbreitung und Verhaltensweise der Herbivoren, welche die Herbivore-Pflanzen Beziehung beeinflussen. Cerviden reagieren physiologisch auf Nahrungsmangel, prinzipiell unter extremen Bedingungen und so spät, dass die Pflanzengesellschaft schon schwer beschädigt ist und der Bestand ein Massensterben erleidet. Die Behauptung, dass in solchen Situationen „natürliches Selbst-Regulieren“ existiert, ist ein irrtümliches Konzept, das besser als forciertes Verhungern bezeichnet werden sollte. Die Integration der Herbivoren im Ökosystem entstand unter ständiger Bestandeskontrolle durch die Großraubtiergemeinschaft, d. h. die Beziehung zwischen Pflanzengemeinschaft und Herbivoren wurde prinzipiell durch Raubtiere moduliert. Nur in der Kulturlandschaft finden sich Bestände von Herbivoren, die ausschliefllich durch das Nahrungsangebot reguliert werden. Daraus lassen sich zwei Schlüsse ziehen:
  1. 1.

    Die Dichte von Herbivoren über ein Limit hinaus führt unweigerlich zu Verschiebungen in der Pflanzen- und Tierartenzusammensetzung, und des Musters der Stoff- und Energieflüsse: das System wird effektiv auf einen weniger komplexen Stand und zu abnehmender Produktivität gebracht, ein evolutionärer Rückschritt.

     
  2. 2.

    Die anthropogene Modifizierung der für das Wohlergehen der Herbivoren nötigen ökologischen Parameter verlangt, dass wir mit entsprechender Verantwortung dieselben ersetzen, sodass das zukünftige Wohlergehen der Herbivoren garantiert wird. Für Cerviden bedeutet das die Durchführung der Jagd, sodass die Bestandesdichte optimale Biodiversität und natürliche Verjüngung erlaubt. Wo die Raubwildgemeinschaft immer noch intakt ist, sollte sie mit allen Mitteln geschützt werden, und wo immer noch möglich, soll die Gemeinschaft wieder instand gesetzt werden.

     

Régulation de population chez les grands herbivores de l'hémisphère Nord : évolution, thermodynamique et grands prédateurs

Résumé

La compréhension de la régulation de populations des grands herbivores de l'hémisphère Nord tels que les Cervidés comporte des implications pratiques et théoriques importantes. Des mesures visant à corriger de fortes densités en Cervidés doivent être basées sur la théorie et nécessitent donc une analyse d'approches contradictoires du contrôle de population du sommet vers le bas et de la base vers le haut de la pyramide écologique. La première considère les Cervidés incapables d'auto-régulation, nécessitant dès lors, pour assurer une régulation de population effective, l'intervention de facteurs externes tel que la prédation. La seconde prétend que les Cervidés possèdent la capacité d'ajuster leur nombre aux ressources alimentaires par des réponses physiologiques. Cependant, on perd de vue que ces phénomènes ne sont pas une expression d'une stratégie évolutive, mais qu'ils correspondent plutôt à une réaction physiologique prévisible vis-à-vis d'une réduction des ressources alimentaires. En outre, les recherches ont souvent été réalisées dans des milieux altérés par l'homme ou sans tenir compte des effets modulateurs causés par les prédateurs. Compte tenu de cette difficulté d'expliquer dans le futur, par des études de terrain, la relation originelle entre les grands herbivores et les grands prédateurs de l'hémisphère Nord, il s'impose de s'en référer à d'autres sources pertinentes de connaissances. Evolution, thermodynamique, analyses des chaînes et des cycles alimentaires concluent toutes au fait que le développement de systèmes biologiques procède par des processus irréversibles et dans une direction bien déterminée, pour aboutir à une allocation optimale de l'énergie et des composants alimentaires. Les grands prédateurs ne sont donc pas comme une sorte de luxe de l'évolution mais une conséquence nécessaire des lois naturelles en question, augmentant de la sorte l'efficacité du système en ce qui concerne la mobilisation de l'énergie solaire. Ceci explique pourquoi l'évolution a, à différentes reprises et de façon analogue, développé des écomorphes au sein de différents taxons; ainsi les « chats » à dents de sabre (placentaire et marsupial) ont-ils ré-évolués indépendamment au moins 5 fois. Comme groupe, les grands prédateurs ont développé des caractères autorisant l'auto-régulation, tels que la territorialité, la prédation intra- et interspécifique, l'alternance de proies et la dispersion. Cependant, dans les milieux modifiés par l'homme, les densités d'herbivores atteignent de telles proportions que même une zoocénose intacte de prédateurs ne serait plus en mesure d'exercer une régulation du fait que des mécanismes sociaux définissent une limite supérieure à la densité des prédateurs. Comme les taux de mortalité par prédation sont très faibles, les prédateurs agissent également sur la dispersion et le comportement des herbivores, lesquels influencent à leur tour les rapports herbivores — végétation. La réponse physiologique des Cervidés à un manque de nourriture n'intervient en principe que lorsque les conditions sont extrêmes et fort tardivement, c'est-à-dire lorsque le tapis végétal est déjà fortement endommagé et lorsque la population subit des mortalités massives. L'affirmation selon laquelle intervient, dans de telles situations, une « auto-régulation naturelle » est un concept erroné qui devrait plutôt être désigné sous le nom d'épuisement suite à une famine forcée. L'intégration des herbivores dans l'écosystème trouve son origine dans un contrôle permanent du niveau de population par la communauté des grands carnivores, c'est-à-dire que la relation entre la végétation et les herbivores est en principe modulé par les prédateurs. Ce n'est que dans le paysage cultivé que l'on trouve des herbivores qui sont exclusivement régulés par les ressources alimentaires. De tout ceci on peut tirer deux conclusions :
  1. 1.

    une densité dépassant un certain niveau conduit inexorablement à des glissements vers des compositions floristique et faunistique ainsi que vers des flux de l'énergie et de la matière : le système est effectivement ramené à un niveau moins complexe et à une productivité moindre, ce qui correspond à une régression évolutive;

     
  2. 2.

    la modification anthropogène des paramètres écologiques, nécessaires à l'épanouissement des herbivores, exige que nous procédions au remplacement des mêmes de façon responsable, de telle sorte que le bien-être futur des herbivores soit assuré. Pour les Cervidés, cela signifie la poursuite de la chasse de manière à ce que la densité de population permette une biodiversité optimale et une régénération naturelle. Là où la communauté des prédateurs est toujours intacte, il convient de la protéger par tous les moyens et, là où la chose est toujours possible, cette communauté de prédateurs doit à nouveau être restaurée.

     

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© Blackwell Wissenschafts-Verlag 2000

Authors and Affiliations

  • W. T. Flueck
    • 1
  1. 1.Consejo Nacional de Investigaciones Cientificas y TecnologicasBariloche

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