Data collection on the early phases of invasion is generally retrospective and therefore limited. In this study, we highlight three sources of data that may be useful in understanding the processes that operate in the particularly poorly characterized early stages of invasion: introduction, establishment, and lag phases. Understanding these early phases of biological invasions is necessary to understand the factors involved in species colonizations and to better predict and reduce the negative impact of invasive species. We discuss the data available on biological control organisms, horticultural introductions, and natural history collections, and how these data can be used to understand intersections between ecological and evolutionary processes in the early phases of biological invasions. Not all introduced species become invasive; yet the failure of introductions is poorly understood. It has been hypothesized that of the species introduced into novel habitats, 10% or less become established and of those, approximately 10% become invasive , though the actual success rate may depend on the taxonomic group . Failure to persist beyond the establishment and lag phases has been explained as the result of ecological factors such as inappropriate climate, intense competition, unsuitable disturbance regime, predation, or disease . In addition, stochastic processes or failure to reach a spatial or numerical critical patch size have been implicated in invasion failure. An expansion may be incited by an environmental change, such as a shift in abundance of a mutualist species or a change in disturbance frequency . Adaptive evolution to conditions in the nonnative range may be critically important in the transition to the expansion phase of invasions . Evolutionary considerations such as effective population size and maladaptation have been shown to play a role in the ability of an introduced species to reach the expansion phase.

In some cases, however,drainage collection pot expansion may ultimately depend on the intrinsic biology of the invading species and may not be influenced by evolutionary forces during the early phases of invasion . For example, woody tree species with a long generation time simply may require time to reach reproductive maturity before expansion can occur . For some species that become invasive, the rate of spread after introduction is constant, and the species expand directly after being introduced . Often, however, the rate of spread is slowed after establishment, and a lag in population growth is experienced prior to range expansion . The slow population growth rates that define the lag phase appear to vary among species and may be either positive or negative. Lag times with low but non-negative growth rates can be explained by purely spatial dynamics or stochasticity . Even without taking into account ecological factors, lag times can also be explained by simple logistic population growth . The lag phase of invasions is often attributed to negative density dependence , as negative population growth rates are often observed when a species is at low densities . While Allee effects are commonly cited as an explanation for the lag phase, lag times can occur as a result of neutral processes. Long periods between reproductive events , spatial heterogeneity , and variable connectivity all can contribute to the ending of the lag phase and the start of the expansion phase. Furthermore, introduced species may experience evolutionary consequences of small population sizes that contribute to the lag phase . Upon introduction, invading species often have low genetic diversity and may be more susceptible to genetic drift. Nevertheless, adaptive evolution has been documented to occur even after genetic bottlenecks , and such evolution may be critical for overcoming the consequences of Allee effects . Studies that investigate ecological andevolutionary processes during the early phases of invasion are difficult because they often require retrospective datasets , yet these studies are critical for understanding biological invasions . Biological control organisms, horticultural introductions, and natural history specimens are tractable datasets for evaluating the importance of evolutionary processes on successful introduction and establishment before invasive expansion.

These sources of data provide information on species introductions that may or may not result in successful establishment. Moreover, for established species, data are useful for both invasive and noninvasive species. All three sources of data potentially provide demographic, phenotypic, and genotypic data at multiple time points during the invasion process, including immediately after introduction and before invasive expansion . Although correlative data alone are insufficient to establish causality, these data form a critical foundation of knowledge that can be used to guide and inform future manipulative experiments. As such, these three sets of data offer an opportunity to formulate and investigate hypotheses related to ecological and evolutionary factors that may facilitate understanding of the early stages of species invasions. Ecological niche modeling also is an important research tool that can be used to predict the non-native range of invasive species, but because it does not inform the evolutionary processes involved in the early stages of invasion, we do not cover it in detail here . Nevertheless, ecological niche models are an important complement to the ideas we propose here for increasing the use of each of these datasets, and ecological models should be used to predict the potential range of successfully spreading bio-control and horticultural species. Results from these models then can be used to hone investigative strategies into evolutionary processes in specific invasive taxa; for example, invasive species that have expanded beyond their predicted non-native range may be selected for study as species that potentially evolved in their nonnative range. Similarly, species that match their predicted range may be explored in detail as species that potentially invaded successfully with little evolution.The linkage between invasion biology and biological control has long been recognized , and biological control practice has the potential to serve as a testing ground for several ecological and evolutionary theories regarding species invasions . As introduction of a bio-control agent is deliberate, bio-control provides one of the few opportunities to observe the dynamics that occur during the initial phases of invasion. bio-control releases can be viewed as ecological and evolutionary experiments testing successful establishment in different habitats, with different bio-control agent populations, and with different propagule pressure.

Thus, more emphasis can be placed on the evolutionary processes at the early stages of invasion, given there is an appropriate focus on individual- and genetic-level record keeping during the releases.For example, the ladybird Harmonia axyridis was introduced as a biological control agent to Europe and North America. In Europe, it is now considered an invasive species and is being used to study invasion mechanisms . Brown et al. studied the early stages of invasion by tracking the distribution changes of the ladybird from the first year of its arrival in Great Britain,round plastic pot and Lombaert et al. compared laboratory bio-control versus invasive populations of H. axyridis to assess differences in adaptive phenotypic plasticity, which they found for some of the metrics measured. These studies indicate that biological control may contain fruitful and untapped information resources that can address the role of evolution in the establishment and spread of invasive populations. In addition to establishment and expansion successes, the failure of a bio-control agent may provide insights into the factors that distinguish successful invaders from those that are unable to become established. bio-control organisms have been chosen for certain criteria, both their own and of their hosts , which may affect the likelihood of establishment and spread. Often these criteria have not been consistent ; however, the criterion of host specificity is fundamental to regulation and safety and thus bio-control agents represent only this subset of possible invaders. When multiple, geographically separated, native-range source populations are used as collection sites for potential bio-control agents, the opportunity exists for investigating functional genetic differences among the sources. These genetic differences may influence establishment success depending upon preadaptation to the release sites . In addition, large-scale laboratory rearing may incite evolution in laboratory conditions that has the potential to influence success when the bio-control agents are released in the field . Genetic diversity and genetic change in the laboratory have the potential to answer questions about necessary preadaptation and the effects of uniting historically allopatric populations in the field once releases begin . Because bio-control organisms are susceptible to potential Allee effects , they may make particularly good models for the study of how different types of Allee effects may constrain or even promote evolution in invasions. Lack of persistence of small populations due to Allee effects may actually buffer the entire species from drift processes , while persistent small populations are vulnerable to genetic drift.

The outcome of the interaction between demographic and evolutionary effects may be influenced by the component Allee effect that is exhibited. For example, Allee effects due to initial over dispersal may have different outcomes than reproduction that is dependent on sociality . While bio-control organisms present the potential to study a wide range of Allee effects, they also provide the opportunity to study invasions free from Allee effects altogether, as parasitoid wasps, a large group of bio-control organisms, are unlikely to experience any density-dependent dynamics at low densities . There is a paucity of work addressing the evolution of bio-control organisms in their introduced range , but there is emerging evidence that evolution is associated with establishment . We argue that data from bio-control organisms provide the necessary set of information to understand the role of evolution in invasion success .There are three main stages in the process of releasing a biological control agent in the USA that may provide useful data for understanding the early phases of biological invasion: importation followed by quarantine clearance; first environmental release; and redistribution of agents from established populations within a state and across state borders . Each of these stages requires permitting by the United States Department of Agriculture Animal and Plant Health Inspection Service Plant Protection and Quarantine and related documentation, except the within-state transfer of non-quarantine organisms . The first and second stages usually involve USDA Agricultural Research Service state quarantines, which strictly follow the permitting process. In the third stage, numerous private, local, state, and regional agencies, as well as universities become involved in the redistribution of biological control agents. Each agency has different standards for regulation, and interstate movement of biological control organisms probably occurs without proper permits. The documentation of biological control releases and establishment of agents depends on the various participating agencies and institutions. Prior to 1980, eachARS quarantine facility, mostly involved in stages one and two, had their own forms and protocol for documentation . The need for standardized documentation was realized as the number of quarantine facilities increased . A serious attempt was made in 1982 to standardize biological control record keeping by the establishment of the USDA ARS Biological Control Documentation Center . The BCDC developed paper forms for recording each of the above-mentioned three stages of biological control practice to set up a uniform documentation system. The BCDC also maintains extensive records on biological control activities, mostly within the USDA, including published and unpublished reports, reprints, correspondence, journals and books relating to biological control dating back to the 1930s . One of the BCDC’s greatest accomplishments was the launch of an online electronic database named ROBO . This program attempted to integrate information from participating US agencies and quarantines conducting classical biological control programs. ROBO currently provides records on importation/ exportation and transfer of biological control organisms and non-indigenous pollinators for the years 1979–2008. Individual files may contain information on the original collection , initial and subsequent releases , availability of voucher specimens, and much more or less depending on the given organism . Similar databases have been, or are being, developed by various state and local agencies, universities, and individual biological control quarantine facilities or scientists. These projects differ greatly in magnitude among institutions. The BIRLDATA is an example of one of the most comprehensive databases, containing computerized records on importation, transfer and release of biological control agents received at the ARS Beneficial Insects Research Laboratory at Newark, Delaware from 1933 to present .