Question: Which is generally considered an introduced species, Centranthus  or Arabis?
Context: Centranthus is a flowering plant genus comprising herbs and subshrubs native to Southern Europe.  It is in the Caprifoliaceae family.  There are about twelve species in the genus.  Some "Centranthus" are known as introduced species in other parts of the world, including "Centranthus ruber" in the western United States and "Centranthus macrosiphon" in Western Australia. Propagule pressure (also termed introduction effort) is a composite measure of the number of individuals of a species released into a region to which they are not native.  It incorporates estimates of the absolute number of individuals involved in any one release event (propagule size) and the number of discrete release events (propagule number).  As the number of releases or the number of individuals released increases, propagule pressure also increases.  Propagule pressure can be defined as the quality, quantity, and frequency of invading organisms (Groom, 2006).  Propagule pressure is a key element to why some introduced species persist while others do not (Lockwood, 2005).  Species introduced in large quantities and consistent quantities prove more likely to survive, whereas species introduced in small numbers with only a few release events are more likely to go extinct (Lockwood, 2005).  Propagule pressure is a composite measure to the number of individuals released into a non-native region (Lockwood, 2005).  Three approaches are used to study and measure propagule pressure.  One approach introduces a specific amount of propagules into controlled plots.  A second approach allows introduced species to mature and colonize naturally while observing native and non-native species during the colonization.  The final approach used to study and measure propagule pressure utilizes past records of the numbers of individuals introduced, including natural introductions and intentional introductions (Colautti et al., 2003). Lappula squarrosa is a species of flowering plant in the borage family known by several common names, including European stickseed, bur forget-me-not, bluebur, and bristly sheepbur.  It is native to Europe and Asia, where it is common, and it is an introduced species in much of North America and Africa.  It is well known as a noxious weed where it is naturalized and also in many parts of its native range.  This is an annual herb producing an erect stem often with sprays of many long, bending branches, its form varying in different regions and climates.  The plant may approach a meter in height.  The stems are lined with linear to oval leaves up to 5 centimeters long and coated in whitish hairs, and the herbage emits a scent generally considered unpleasant.  The inflorescence is a long, leafy raceme of tiny flowers near the ends of the branches.  Each flower is 2 to 4 millimeters wide with five light blue corolla lobes.  White-flowered plants are occasionally seen.  The fruit is a cluster of four nutlets which are coated in hooked prickles.  The seeds are dispersed when the prickles get caught on animal coats and human clothing, and when they are moved by wind. This is a list of reptiles which are found in the U.S. state of Florida.  This list includes both native and introduced species.  Introduced species are put on this list only if they have an established population (large breeding population, numerous specimens caught, invasive, etc.).  Three out of the four orders of reptiles can be found in Florida, with the Tuatara order being absent.  Though many sources has different amounts (due to introduced species), this lists 118 species, which is about right. An introduced species (alien species, exotic species, non-indigenous species, or non-native species) is a species living outside its native distributional range, which has arrived there by human activity, either deliberate or accidental.  Non-native species can have various effects on the local ecosystem.  Introduced species that become established and spread beyond the place of introduction are called invasive species.  The impact of introduced species is highly variable.  Some have a negative effect on a local ecosystem, while other introduced species may have no negative effect or only minor impact.  Some species have been introduced intentionally to combat pests.  They are called biocontrols and may be regarded as beneficial as an alternative to pesticides in agriculture for example.  In some instances the potential for being beneficial or detrimental in the long run remains unknown. Arabis , or rockcress, is a genus of flowering plants, within the family Brassicaceae, subfamily Brassicoideae. Arabis glabra, commonly known as tower rockcress or tower mustard, is a tall, slim, grey-green plant with small creamy flowers at the top of the stem.  It usually grows on poor chalky or sandy soils, in open situations.  It is native to Europe, Asia, and North Africa, and it is widespread in North America where it is also probably native.  It can be found in many other parts of the world as an introduced species. Chorispora tenella is a species of plant in the mustard family known by several common names, including purple mustard, blue mustard, musk mustard, and crossflower.  This mustard is native to Eurasia but is well known in other parts of the world, particularly in temperate regions, as an introduced species and a noxious weed.  This is an annual herb reaching a maximum of half a meter in height and covered abundantly in sticky foliage.  The four tiny flower petals emerge from a loose tube of sepals and spread into a corolla about a centimeter wide.  The flowers are lavender in color and a field heavily infested with purple mustard can take on a distinct lavender wash.  The plant has a strong scent which is generally considered unpleasant.  The fruits are long upturned cylindrical capsules about 4 centimeters long containing round, reddish-brown seeds.  This is a tenacious weed which can be troublesome in agriculture.  It reduces yields in grain fields and when it is consumed by dairy cattle it gives their milk a bad taste and odor.  This plant reproduces by seed, so any control method preventing the plants from setting seed is effective. A complete list of introduced species for even quite small areas of the world would be dauntingly long.  Humans have introduced more different species to new environments than any single document can hope to record.  This list is generally for established species with truly wild populations—not kept domestically—that have been seen numerous times, and have breeding populations.  While most introduced species can cause negative impact to new environments they reach, some can have positive impact, just for conservation purpose. Seabirds include some of the most threatened taxa anywhere in the world.  For example, of extant albatross species, 82% are listed as threatened, endangered, or critically endangered by the International Union for Conservation of Nature.  The two leading threats to seabirds are accidental bycatch by commercial fishing operations and introduced mammals on their breeding islands.  Mammals are typically brought to remote islands by humans either accidentally as stowaways on ships, or deliberately for hunting, ranching, or biological control of previously introduced species.  Introduced mammals have a multitude of negative effects on seabirds including direct and indirect effects.  Direct effects include predation and disruption of breeding activities, and indirect effects include habitat transformation due to overgrazing and major shifts in nutrient cycling due to a halting of nutrient subsidies from seabird excrement.  There are other invasive species on islands that wreak havoc on native bird populations (e.g. brown snakes on Guam), but mammals are by far the most commonly introduced species to islands and the most detrimental to breeding seabirds.  Despite efforts to remove introduced mammals from these remote islands, invasive mammals are still present on roughly 80% of islands worldwide.
Answer: Centranthus