An Overview of the Plant Invasion Process

    Given a plant species with certain life history traits and a vulnerable local opportunity space, the invasion process consists of four component processes: dispersal of the species into that locality, followed by colonization and enduring occupation of the habitat, ultimately ending in extinction .  The invasion is successful only when the first three of these are accomplished.  Most invading species probably fail to complete all three steps, and there is little experimental information estimating the failure rate.

The Invasion Matrix.  he processes (invasion, colonization, enduring occupation, extinction), life history activities (dispersal, recruitment, establishment including reproduction, and several modes of enduring occupation) and examples.  

Invasion is community and speciation.  Plant invasions are events in the ecology of community assembly and succession, as well as in the evolution of niche differentiation by speciation.  There is not meaningful difference between the invasion process and these processes except the scale of attention humans bring to their observations.  In all these processes disturbance is a prime motivator of change.  Habitat disturbance as a direct or indirect consequence of human activity is of central importance.  The scale of habitats in time and space is continuous; and all communities are inter-related.

DISPERSAL  
The first activity in invasion is successfully introducing propagules (seeds, vegetative buds, etc.) into a candidate opportunity space. 

dispersal        
1:  the act of scattering, spreading, separating in different directions (Anonymous, 2001)
2:  the spread of animals, plants, or seeds to new areas (Anonymous, 1979)
3:  outward spreading of organisms or propagules from their point of origin or release (Lincoln et al., 1998)
4:  the outward extension of a species' range, typically by a chance event (Lincoln et al., 1998)
Herein dispersal is defined:
5: the search by plant propagules (e.g. seeds, buds) for opportunity space

COLONIZATION  
The process of colonization includes three activities: recruitment, establishment and reproduction at the new locality.

colonization      1:  (of plants and animals) to become established in (a new environment) (Anonymous, 1979)
                          2:  the successful invasion of a new habitat by a species (Lincoln et al., 1998)
                          3:  the occupation of bare soil by seedlings or sporelings (Lincoln et al., 1998)

recruitment      1:  seedling and bud shoot emergence
                         2:  the influx of new members into a population by reproduction or immigration (Lincoln et al., 1998)

establishment  1:  growing and reproducing successfully in a given area (Lincoln et al., 1998)

ENDURING OCCUPATION OF A LOCALITY
Several modes of long-term presence at a locality are possible.  An invading species can have an enduring presence for more than one generation in the same locality.  This long-term presence is often facilitated by plant traits that allow the formation of soil propagule (e.g. seed) pools.  A species present in one locality can also expand its range into new localities.

EXTINCTION

extinction    
1:  the process of elimination, as of less fit genotypes
2:  the disappearance of a species or taxon from a given habitat or biota, not precluding later recolonization from elsewhere

All local populations become extinct.  The important considerations for an individual species are on what spatial and time scale these extinction events occur.  Many of our most common crop field weeds (e.g. Setaria) have been around for thousands of years.  But every local population goes extinct at some point.  For example, many susceptible biotypes have disappeared from local fields and were replaced by either resistant biotypes of the same species, or by other species in that locality.  Within any field an individual weed species is spatially located in patches.  These patches can change from year to year, on this spatial scale extinction occurs continuously with re-invasion of adjacent areas.  Most weed species do this process of patch changing continuously on many spatial and temporal scales.  Plant community succession is a series of invasions and extinctions.  As the colonizers become established they create opportunity space for later successional plant species.  On and on it goes.

Local selection and adapted phenotypes
    Once a species successfully occupies a local site of some time period, the action of selection pressures result in local adaptation in favor of particular genotypes and phenotypes.  The selection pressures these populations experience in the invasion and occupation phases derives from both biological, abiotic and human selection pressures.  This local selection also acts on the variable phenotypes of that invading species and selects adapted biotypes that occupy that space into the future.  Some of the consequences of this local evolution and adaptation include increases in locally-adapted phenotypes, range expansion beyond the locality, and population shifts in the local community as a consequence of altered neighbor interactions.  

©jdekker-2005