Issues In Ecology Number 5 Spring 2000 Figure 3-Invaders often alter drastically the ecosystems they occupy,over-turning native species composition,as well as changing the fire frequency.soil chemistry and hydrology.The Florida Everglades have been much altered by the eeffects of invasive plantsn com m across much are composed of small for f by Brazilian pepper has radically transformed these ecosystems into virtual monocultures of the invasive tree with devastating effects on the native biota. invaders such as the zebra mussel have arrived as sels in the Great Lakes may go through only a brief lag accidental contaminants in ship ballast phase,or none at all.On the other hand,many immigrant In contrast,most invasive vertebrates,principally fish species do not become abundant and widespread for de cades,uring which time they may of the ertebrate invaders,how Brazilian ever,have been spread accidentally:rats,brown tree ninet ely noticeabl snakes,sea lampreys. until the early 1960's.They are now established on more Some invasive plants have been accidentally introduced than 280.000 hectares in south Florida.often in dense as contaminants among crop seeds and other cargo. stands that exclude all other vegetation (Fig.3). Many,if not most,plant invaders have been deliberately During the lag phase,it can be difficult to distin introduced includina some of the worst pests:wate guish doomed populations from future invaders.Most ex hyacinth.melaleuca tre rick migrant pop of deliberately introduced spe yet the dynamics of such a population are cies that later become biotic invaders emphasizes that guishable from those of a future invader,which is growing not all pests arrive unheralded and inconspicuously:many slowly but inexorably larger.This similarity in the size and are the product of deliberate but disastrously flawed hu- range frustrates attempts to predict future invaders while man forethouaht(Fia 1) they are few in numbers and presumably controllable. Whether most invasic sendure lag phases,and The Transformation from Immigrant to Inva why they oc main conje ral.Any lag in popu The progression from immigrant to invader ofter lation growth and range expansio n for a pot ntial invade entails a delay or lag phase.followed by a phase of rapid most likely results from several forces and factors oper exponential increase that continues until the species ating singly or in combination: reaches the bounds of its new range and its population The number and arrangement of infestations of immi- arowth rate slackens (Fia.2).This simplified scenario has many variants,of course.First,some invasions such as those by Afric canized bees in the Americas and zebra mus single larger one Issues in Ecology Number 5 Spring 2000 4 invaders such as the zebra mussel have arrived as accidental contaminants in ship ballast. • In contrast, most invasive vertebrates, principally fish, mammals, and birds, have been deliberately intro￾duced. Some of the worst vertebrate invaders, how￾ever, have been spread accidentally: rats, brown tree snakes, sea lampreys. • Some invasive plants have been accidentally introduced as contaminants among crop seeds and other cargo. Many, if not most, plant invaders have been deliberately introduced, including some of the worst pests: water hyacinth, melaleuca trees, and tamarisk or salt cedar. The prominence of deliberately introduced spe￾cies that later become biotic invaders emphasizes that not all pests arrive unheralded and inconspicuously; many are the product of deliberate but disastrously flawed hu￾man forethought (Fig. 1). The Transformation from Immigrant to Invader The progression from immigrant to invader often entails a delay or lag phase, followed by a phase of rapid exponential increase that continues until the species reaches the bounds of its new range and its population growth rate slackens (Fig. 2). This simplified scenario has many variants, of course. First, some invasions such as those by Africanized bees in the Americas and zebra mus￾sels in the Great Lakes may go through only a brief lag phase, or none at all. On the other hand, many immigrant species do not become abundant and widespread for de￾cades, during which time they may remain inconspicuous. Brazilian pepper trees were introduced to Florida in the nineteenth century but did not become widely noticeable until the early 1960’s. They are now established on more than 280,000 hectares in south Florida, often in dense stands that exclude all other vegetation (Fig. 3). During the lag phase, it can be difficult to distin￾guish doomed populations from future invaders. Most ex￾tinctions of immigrant populations occur during the lag phase, yet the dynamics of such a population are often indistin￾guishable from those of a future invader, which is growing slowly but inexorably larger. This similarity in the size and range frustrates attempts to predict future invaders while they are few in numbers and presumably controllable. Whether most invasions endure lag phases, and why they occur, remain conjectural. Any lag in the popu￾lation growth and range expansion for a potential invader most likely results from several forces and factors oper￾ating singly or in combination: • The number and arrangement of infestations of immi￾grants. Usually invasions proceed fastest among many small, widely separated infestations compared with a single larger one. Figure 3 igure 3 - Invaders often alter drastically the ecosystems they occupy, over-turning native species composition, as well as changing the fire frequency, soil chemistry and hydrology. The Florida Everglades have been much altered by the collective effects of invasive plants, including Schinus terebinthifolius (Brazilian pepper). A) The potential natural com￾munities across much of the Everglades are composed of small forested hammocks in a matrix of marshes. B) Invasion by Brazilian pepper has radically transformed these ecosystems into virtual monocultures of the invasive tree with devastating effects on the native biota. Photo by Richard Mack. Photo by Richard Mack