Digestion and excretion (pathway cause)
Don't need the entire report?
Generate a print friendly version containing only the sections you need.Generate report
PicturesTop of page
IdentityTop of page
Preferred Scientific Name
- Digestion and excretion (pathway cause)
International Common Names
- English: Endochory; Endozoochory
- Spanish: Digestión/excreción
- Portuguese: Digestão/excreção
OverviewTop of page
The digestion and excretion pathway is probably the single most important means for plant dispersal or spread within an area once introduced, established and naturalized. Livestock are common vectors in managed systems especially grazing land, whereas wild animals, mostly birds and mammals, often play a critical role in natural or semi-natural systems such as forests. However, there are examples where it has been considered as the cause for international introductions, either from the migration of birds or large herbivores, or the long-distance transport of livestock either by ship, or overland.
DescriptionTop of page
Summary of organism types or species introduced
Concerning the animal vectors themselves, most invasions via endozoochoric pathways are by large herbivores or birds, though not exclusively so. Many plants are also dispersed by a variety of organism types, though some have a specialized mutual relation with one animal species or a single organism type. Snails consume and disperse seeds from a few species; earthworms are dispersal agents for many more. Some fish also eat the fruits/seeds of fruits that fall on the water surface and aid in dispersal. There are specialized terms for dispersal by digestion and excretion by specific organism types, such as mammaliochory (by mammals), ornithochory (by birds), saurochory (by reptiles) and chiropterochory (by bats). Humans could also be responsible, as demonstrated by the introduction of the tomato as an invasive species on some uninhabited islets in the Galapagos, as they may have been excreted by visiting researchers (if they were not merely dropped).
As for the species introduced and/or spread, there are far too many invasive plants that have digestion and excretion as a pathway to list exhaustively; it is estimated that 64% of all gymnosperm species and 27% of all angiosperms produce fruit that attract animals (Traveset et al., 2007). Endozoochory is a widely studied area, but a thorough review of the current state of knowledge would be beneficial, though this may require dividing into manageable sub-units for thoroughness.
The process is, as described in the pathway title, digestion and excretion. However, in addition to the geographic movement of seed by animals, other processes may be advantageous to the plant. The first occurs during digestion, as chemicals and/or physical action are widely reported to break the dormancy of the seeds of many species (Traveset et al., 2007). For example, with the hard-coated seed of leguminous plants, germination percentages may be many times more than the control. The second occurs during excretion, with the increased chance of the seed being placed in a favourable micro-environment, i.e., near to a water source, and being enclosed in a ready source of plant nutrients.
Geographical routes and corridors
Long-distance introduction would have followed migratory routes (natural), or trade routes, overland livestock movements following grazing and water, or to a port or railhead (human-induced). Local spread could be considered to be random, though may be more prevalent where animals are likely to collect, such as roosts, sources of drinking water, etc.
The only anthropogenic role in digestion and excretion as a pathway involves the raising and movement of domesticated livestock. Long-distance endozoochory is thought to have been responsible for some plant introductions, in livestock transported by ship or overland. However, this has been mainly responsible for the spread of invasive plants on managed and natural grasslands, and extensive grazing land such as steppes, savannas and other dry areas, even deserts. Significant areas in the Americas were invaded by native legume trees in the late 1850s, e.g. Texas and the southwestern USA, and the Chaco of Argentina and Paraguay, when cattle ranching was introduced, disturbing the ecosystem and providing native trees with an environment where they 'densified' into impenetrable monocultures. Many other examples have since been identified, with cattle being the predominant vector, though sheep, goats and camels are also implicated.
Man has also been indirectly responsible via the introduction of animals into an environment, and which became feral, or naturalized, and these then became vectors for the spread of invasive plants; such as feral pigs, goats and even rabbits and other small mammals, also birds. The invasion of wild fig trees in Australia, which had a restricted range for many decades, is thought to have been triggered by the introduction of the myna bird, which proved to be a very effective dispersal agent. The spread of the fire tree in Hawaii is also considered to be have been induced, but by multiple triggers. Trees had been naturalized for many years, but were few in number, as were feral pigs: the trigger was the introduction and establishment of European earthworms. The pigs now had a sufficient source of food and multiplied, and the ground disturbance from digging for this food provided a very suitable environment for the spread of the fire tree.
Species Transported by CauseTop of page
ManagementTop of page
Controlling the movement of free-ranging livestock and wild mammals is only feasible with high cost and high maintenance fencing, and may only be practical in small and localized cases such as protected areas. Preventing the movement of birds would, of course, be impossible. Eradication of the animal vector would be effective if possible, but the costs on biodiversity are likely to outweigh any benefits in such circumstances and ethical or moral issues would also need considering.
Quarantine would be the only possible means of reducing the risks associated with human-mediated animal movement, such as the sale of cattle through town markets, international trade in live animals, or the translocation of wild animals such as rogue bull elephants from one reserve to another. Enforcing a quarantine period would ensure that any previously digested seeds are excreted before they are moved into uninfested areas.
ReferencesTop of page
Traveset A; Robertson AW; Rodríguez-Pérez J, 2007. A review on the role of endozoochory in seed germination. In: Seed dispersal: theory and its application in a changing world [ed. by Dennis, A. J.\Schupp, E. W.\Green, R. J.\Westcott, D. A.]. Wallingford, UK: CABI, 78-103. http://www.cabi.org/CABeBooks/default.aspx?site=107&page=45&LoadModule=PDFHier&BookID=369
ContributorsTop of page
6/30/2009 Original text by:
Nick Pasiecznik, Consultant, France