Herbivores

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Doe with calf grazing

Herbivores are as primary consumers one of the groups into which the ecology , the consumer divides. The next higher trophic level is represented by the carnivores , which the herbivores hunt and eat and are referred to as secondary consumers. In the case of herbivores, irrespective of the systematics , one speaks synonymously mainly of herbivores ( lat. Herba 'herb' and vorare 'devour'), ​​whereby small organisms tend to be called phytophagous ( Greek phyton 'plant' and phagein 'eat').

properties

Herbivores include all animals that mainly feed on plants , especially ruminants . Depending on the definition, species are added that in some cases also feed on fungi , protists or bacteria , i.e. feed on the biomass of lower trophic levels . Exclusively or specialized fungal-eating species ( mycetophage ) or bacteria-eating species (bacteriovore: the term bacteriophage is avoided because of the risk of confusion with bacteriophages , i.e. viruses) belong to the saprophage food chain and are not regarded as herbivores. In the past, however, this distinction was not common because fungi and bacteria were considered part of the plant kingdom for a long time. Species that feed on both animal and plant biomass are known as omnivores (or omnivores).

When looking at herbivores, two levels of observation can be distinguished:

  1. In terms of the individual species, herbivores are species with a special adaptation to the uptake of plant matter. This applies to morphological adaptations (e.g. specially developed teeth), physiological adaptations (e.g. symbiosis with bacteria in the digestive tract), behavioral adaptations (e.g. in food choices).
  2. In terms of the ecosystem, herbivores are species of the second trophic level (primary consumers) within the live-eater subsystem, i.e. H. only with consumption of living biomass. Species that z. B. feed on dead leaves on the forest floor, belong to the saprobionts and are not herbivores.

So herbivores are the primary consumers . On the other hand, since they also serve as food for carnivores and omnivores ( carnivores or omnivores ), they can also be called secondary producers . (more precisely: secondary producers of the first level / the first trophy level). The only difference here is the viewing angle or the viewing plane.

Classifications

Herbivores exist in a variety of different specializations and adaptations. Depending on the type of diet, they can be divided into different groups:

According to the diet:

  • Ectophagus are biting or chewing species that z. B. Bite off leaves. The expression is v. a. common in invertebrates,
  • Filter feeders are aquatic species that either passively (by taking advantage of the current) or actively (by creating a water current themselves) filter particles out of the water and feed on them. Strictly speaking, phytophage filter feeders would only be those that feed exclusively or predominantly on unicellular algae (phytoplankton),
  • Gall formers release hormonally acting substances that stimulate the plant to form growths ( plant galls) from whose tissues they live,
  • Miners are species that live in the interior of plant tissue and z. B. eat burrows in leaves, leaving the outer shell (epidermis) intact,
  • Plant teats pierce living plant tissue. Can be further subdivided depending on the tissue used (xylem sucker, phloem sucker, parenchyma sucker),
  • A bison - a typical pastor
    Pastors are species that graze flat lawns or mats of plant species.

After specializing in plant organs:

  • Leaf eater (phyllophage),
  • Flower visitors with a diet of pollen and nectar. This includes symbiotic pollinators, but also numerous other
  • Wood eater (xylophage). Wood includes living and dead matter. Because it is difficult to digest, many wood-eaters can only break it down with the help of special types of fungus or bacteria, with which they often form symbiotic communities. They are therefore a special case within herbivores and are often considered separately,
  • Seed eaters feed on v. a. of seeds or fruits and
  • Root eater (rhizophagus).

After specializing in plant species:

In general, species with a narrow specialization in one or a few plant species ( monophage ), species with medium specialization, e.g. A distinction is made between plant families ( oligophage ) and generalists ( polyphage ). Special strategies are e.g. B. grass-eating species (Graminivore).

The categories mentioned are combined to characterize the ecological niche of a species. Examples:

  • "Wireworm" (larva of the click beetle Agriotes lineatus ): omnivorous, rhizophagous, polyphagous.
  • Cattle ( Bos primigenius taurus ): polyphagous pasturage,
  • Black bean aphid ( Aphis fabae ): ectophagus sucking (phloem suckers), polyphagous host changer (i.e. different generations suckle on different species),
  • Strip bug ( Graphosoma lineatum ): sucking, on plant seeds, oligophagous on umbellifers (Apiaceae),
  • Tobacco bladder foot ( Thrips tabaci ) sucking on the ectophagus (parenchyma suckers), polyphagous, leaf sucking.

However, some combinations are preferred in nature, others do not occur at all or only as extreme exceptions: filter feeders, pastures and wood eaters are e.g. B. (almost) always polyphagous. Very many species do not differentiate between living and dead biomass or belong to the secondary and tertiary trophic level of the food chain. Aquatic grazers usually graze on biofilms that contain not only algae but also bacteria and fungi without differentiating between them.

Phytophagous species make up a significant proportion of the earth's biodiversity. Most significant for the total number of species are the insects because of their generally high number of species . The role of monophagous and oligophagous specialists on tropical plant species is considered to be particularly important. Depending on how high the proportion of this (very poorly researched) group is estimated, there are dramatically different estimates for the total number of species. In mammals, species-rich orders and superordinates such as the ungulates, the hare-like (Lagomorpha) or the rodents (Rodentia) are exclusively or predominantly herbivores. The (super) order of primates to which humans belong includes a particularly large number of omnivores (such as humans themselves). In addition, there are almost purely phytophagous species such as the gorillas .

Influence of body weight on ecology

In herbivorous mammals, the body weight is a decisive factor for the ecology and thus for the behavior of the animal ( allometry ). A simple application of body weight is, for example, the calculation of the basal metabolic rate with the help of Kleiber's law . Based on this, scientists have found further dependencies of herbivores on their body weight. Examples are:

  • Selection of plants with different nutrient content (quality) : Small herbivorous mammals mainly choose plants (parts) with a high nutrient content as food. With increasing body weight, the herbivores also tolerate plants with a lower nutrient content, but then require much more feed mass. This is also known as the Jarman-Bell principle .
  • Influence of predators : As the body mass of a herbivore species increases, the number of its predator species decreases. Megaherbivores ( adult weight more than 1000 kg; e.g. elephant and rhinoceros) have almost no natural enemies.

Regardless of body weight, the digestive system ( ruminant or rectal fermenter ) can also have a major impact on the ecology and behavior of a herbivore. So have z. B. Zebra (rectal fermenter) and Gnu (ruminant), despite their similar body weight, have completely different geographical distribution patterns and selection mechanisms for their feed.

denture

The herbivorous mammals are characterized by wide incisors , with which they cut off the plants, reduced to missing canines , molarized premolars and molars , with which they grind the food. Transversal chewing movements are possible (also because of the missing canine teeth) and necessary for grinding. There are also toothless sections and often a frontally elongated skull.

Because of the rough fibers of grass, grass-eating mammals have specially trained, hypsodontic teeth that are taller than other herbivore teeth and can grow back for life. In this way, it is possible for the grass-eaters to use grass as their main source of nutrition over the long term, despite the fact that their teeth are abraded by their food. While hypsodontia is a common characteristic of grass-eaters, since this type of tooth can also be found in other herbivores, this is not a clear indication.

The hypsodontic teeth contrast with the brachydontic teeth of many leaf eaters , which are rather small and not regrowing. This bit is designed to grind tree leaves and twigs. If a brachydont herbivore consistently consumed rough grass fibers, its teeth would wear out over time.

See also

literature

  • Danell, Kjell; Bergström, Roger; Duncan, Patrick; Pastor, John: Large Herbivore Ecology, Ecosystem Dynamics and Conservation . Cambridge University Press, Cambridge 2006, ISBN 978-0-521-53687-5 , pp. 522 .

Individual evidence

  1. ^ Mya Breitbart, Forest Rohwer: Here a virus, there a virus, everywhere the same virus? . In: Trends in Microbiology . 13, No. 6, August, pp. 278-284. doi : 10.1016 / j.tim.2005.04.003 .
  2. ^ Mark L. Wickstrom, Charles T. Robbins, Thomas A. Hanley, Donald E. Spalinger, Steven M. Parish: Food Intake and Foraging Energetics of Elk and Mule Deer . In: The Journal of Wildlife Management . 48, No. 4, 1984, pp. 1285-1301. doi : 10.2307 / 3801789 .
  3. ↑ Gary E. Belovsky: Optimal foraging and community structure: The allometry of herbivorous food selection and competition . In: Evolutionary Ecology . 11, No. 6, 1997, pp. 641-672. doi : 10.1023 / A: 1018430201230 .
  4. ^ RHV Bell: The use of the herb layer by grazing ungulates in the Serengeti National Park, Tanzania . University of Manchester, 1969.
  5. ^ PJ Jarman: The Social Organization of Antelope in Relation to Their Ecology . In: Behavior . 48, No. 3/4, 1974, pp. 215-267.
  6. ↑ a b Anthony RE Sinclair, Simon Mduma, Justin S. Brashares: Patterns of predation in a diverse predator-prey system . In: Nature . No. 425, 2003, pp. 288-290. doi : 10.1038 / nature01934 .
  7. ↑ a b R. Norman Owen-Smith: Megaherbivores: The influence of very large body size on ecology . Cambridge University Press, Cambridge.
  8. ^ R. Norman Owen-Smith: Niche separation among African ungulates . In: Species and speciation . 4, 1985, pp. 167-171.
  9. ↑ a b John Damuth, Christine M. Janis: On the relationship between hypsodonty and feeding ecology in ungulate mammals, and its utility in Palaeoecology . In: Biological Reviews . 86, August, pp. 733-758. doi : 10.1111 / j.1469-185X.2011.00176.x .