In ecology, a tramp species is an organism that has been spread globally by human activities. The term was coined by William Morton Wheeler in the bulletin of the American Museum of Natural History in 1906, used to describe ants that “have made their way as well known tramps or stow-aways [sic] to many islands". The term has since widened to include non-ant organisms, but remains most popular in myrmecology. Tramp species have been noted in multiple phyla spanning both animal and plant kingdoms, including but not limited to arthropods, mollusca, bryophytes, and pteridophytes. The term "tramp species" was popularized and given a more set definition by Luc Passera in his chapter of David F. William's 1994 book Exotic Ants: Biology, Impact, And Control Of Introduced Species.
Definition
Tramp species are organisms that have stable populations outside their native ranges. They are closely associated with human activities. They are disturbance-specialists, and are characterized by their synanthropic associations with humans as their primary mode of expansion is human-mediated dispersal. That being said, tramp species are not limited to anthropogenically disturbed habitats, they have the potential to invade pristine habitats, especially when established in a new area. For example, Anoplolepis gracilipes was able to invade undisturbed forest ecosystems in Australia after being introduced and having an established population in northeast Arnhem Land. While some tramp species are invasive, the majority of them are not. Some can exist alongside native species without competing with them, simply occupying unfilled niches, such as is the case with some populations of Tapinoma melanocephalum and Monomorium pharaonic, who rarely interfere with native species outside human settlement areas.
Ants
Ants have a more rigid list of criterion to be considered "true" tramp species. The most cited body of work outlining these traits comes from Luc Passera. His primary and most important criterion is that the distribution of the species must be linked to human activities, what he refers to as "anthropophilic tendency". He also lists the following traits as being likely common to all tramp species: small size, monomorphism of worker ants (worker ants having only one phenotype), high rates of polygyny, unicoloniality, strong interspecific aggressiveness, worker ant sterility, and colony reproduction by budding. These traits may appear with more or less intensity among considered tramp species, and in fact, literature does not currently require a tramp species to possess every single one of these attributes. Ant tramp species in particular can be ecological indicators on the susceptibility of an ecosystem to become invaded or ecological instability.
Causes and distribution All tramp species are distributed globally by as a result of human transportation. As such, they are almost always present in urban or human-settled environments, and have colonizing mechanisms that are well adapted to human cohabitation, referred to as possessing "anthropogenically reinforced dispersal biology". The globalization of trade and travel have contributed significantly to the dispersal of tramp species worldwide. Trade activities involving the importation and exportation of cargos on ships (often containing plants, soil, wood and other biological mediums) are noted as being an especially important methods of introduction. These often repeated introductions (as oftentimes shipments will come from the same place) contribute to fortifying the genetic variability and initial population sizes of newly transplanted tramp species, which facilitates their establishment in novel environments. After their human-mediated introductions, tramp species can also benefit from human disturbance to the environment. Anthropogenic forces (such as construction and agriculture) can dramatically impact local fauna and flora, weakening the environment and making the area more susceptible to the encroachment of tramp species. This phenomenon is noted as a particularly tough issue in Tropical Asia, where monocropping practices of local rubber plant farms have decimated indigenous species assemblages and habitat structures, allowing the establishment of many problematic tramp species. Another example is the Thousand Islands Archipelago in Indonesia, where the small tropical islands are especially vulnerable to human disturbance, which facilitated the establishment of multiple tramp species. The range expansion of tramp ants is projected to increase with weather pattern changes due to climate change. As many tramp species are well adapted to disturbances in their native habitat, they are particularly resilient to large-scale, unpredictable weather events (such as floods, wildfires and monsoons), which are set to increase in frequency as anthropogenic activity continues to affect global systems.
Effects on local environments
Tramp species can have similar effects to invasive species, and in some literature the term "tramp" species is used as a synonym for invasive. As such they can outcompete and displace local fauna, decreasing species richness. They can also have direct impacts on human health, such as is the case with Solenopsis geminata and Brachyponera senaarensis. Both of these venomous species have been known to bite humans, often times causing severe anaphylactic reactions; this has made them known public health hazards in the regions they are found. Tramp species can also be nuisance pests, damaging housing structures and crops. However, it is important to note that tramp species are not always invasive, and can cohabitate without harming local environments or species assemblages.
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