Criteria for Determining Establishment of Exotics
The TBRC is charged with maintaining a Texas State List which covers the bird species that have been documented to occur in the state which are considered to be naturally occurring. Naturally occurring indicates that the individual(s) are free-flying non-captive birds that were born and survive in the wild on their own. For most native species that are encountered in Texas, there is rarely a question about their provenance/origin and most native species documented in Texas are part of the Texas State List.
Where the lines get a bit more fuzzy is when numbers of a particular introduced/exotic species are documented in the state for a period of time. Such species typically originated from older generations that were clearly at one point in the past part of a captive population. Individuals from these populations were either released into the wild or escaped from captivity and started at some point to reproduce and possibly thrive in small areas. Depending on where you live, you may see these exotics often and wonder why they aren’t on the Texas State List. The reason is most likely because they have not been accepted as established by the TBRC.
For the TBRC to consider an exotic to be established, members of the committee must review documentation provided to them based on the criteria stated below and agree that the documentation supports that criteria. The TBRC’s criteria closely mirrors the American Birding Association’s (ABA) own Criteria for Determining Establishment of Exotics.
The EIGHT criteria that must be met for an exotic species to be considered established follow:
1) Physical documentation must be made available to the TBRC. This requirement is no different than any other species on the Texas State List. Species identification must be confirmed by photo(s), video(s), audio recording(s) and/or specimen(s).
2) There is a more-or-less-contiguous population of interacting or potentially interacting individuals, rather than a scattering of isolated individuals or pairs. Most exotics present in Texas occur in and near metropolitan areas. For persistence, it is vital that exotic birds in these areas are not isolated from each other but rather occur in sufficient proximity to allow interaction and therefore gene flow.
3) The population is not currently, and is not likely to be, the subject of a control program where eradication may be a management goal that is likely to succeed. Some exotics (e.g., Mute Swan) present a clear danger to native species or habitats, or to agriculture or commerce, in some areas, and listing these species as established may create a conflict between some birders and land management personnel.
4) The population is large enough to survive a routine amount of mortality or nesting failure. We cannot provide a stand-alone numerical threshold for determining when an exotic species is established. The reason for this is hopefully clear: No single number would be adequate for populations as varied as large, long-lived parrots with low reproductive potential and small, short-lived finches with high reproductive potential. Demographic characteristics such as habitat preferences, lifespan, reproductive output, dispersal frequencies and distances, and genetic viability will be considered separately for each species. Members of the TBRC will critically review each species based on the documentation provided and will make a judgment based on the best available evidence. Much attention will be given to factors such as population size, distribution, and, particularly, evidence of successful breeding. However, we recognize that some number of individuals is preferable as a baseline to judge when a species may be established. It is preferable that this baseline number be a census of well over a hundred individuals but the TBRC can and will consider populations with lesser numbers. In almost all cases, populations numbering only dozens of individuals may be too small to be considered established. Additionally, information should be provided to indicate that there is little or no evidence that ongoing releases play a substantial role in population maintenance. For species whose numbers may be artificially supplemented from time to time, evidence should be provided that these releases are not necessary to maintain population size or persistence.
5) Sufficient offspring are being produced to maintain or increase the population. Such criteria will vary from species to species, according to factors affecting the population, both natural (competition from other species; effects of hurricanes) and artificial (recapture for the pet trade; culling by hunters). Certainly, a species whose numbers are increasing and whose range is expanding is a better candidate for establishment than a species whose numbers and range are stable. Species with declining numbers and/or contracting range should have a much greater evidentiary threshold to meet before being considered established.
6) The population has been present for at least 15 years. Previous criteria considered by the ABA used a 10-year persistence threshold. Based on several cases, the ABA determined 10 years is an insufficient period to judge the likelihood that an exotic will persist.and increased the persistence criteria to 15 years. The TBRC will use this same 15 year minimum though we realize that 15 years may still be insufficient in some cases to determine establishment; populations of many exotics follow a “boom and bust” cycle over several decades. With long-lived species (e.g., Amazona parrots) or when other exotic populations are regularly subsidized, one could argue that persistence should be for 30 or more years for genuine trends in the population to become obvious. Our point here is that like numerical criteria, no simple formula of the number of years for persistence can apply to all species. Flexible persistence criteria (“at least 15 years”) and lack of numerical criteria will allow TBRC members to exercise their own judgment in potentially uncertain or controversial cases, but only in the context of strong biological evidence and with the intention that the final judgment be a conservative one.
7) The population is not directly dependent on human support. Although somewhat subjective, this criterion is meant to exclude from consideration those exotics that rely on direct human support for their ongoing survival and/or persistence (reliance on bird feeders; periodic releases of additional individuals).
8) A publication, ideally in a peer-reviewed journal or book, describes, how, when, and where the above seven criteria have been met. A publication will streamline the voting process by clearly presenting evidence of establishment. In the absence of a publication, the TBRC may still officially consider adding an exotic to the Texas State List if such evidence has been gathered by a Committee member or other interested individual. In all cases, the TBRC can provide guidance/assistance in getting the documentation published in the TOS Bulletin.
Other aspects might certainly play a factor in the TBRC’s decision to add/not add a species to the Texas State List; the more documentation that can be provided, the stronger the case will be. The TBRC holds neither a generic “for” position in favor of the addition of exotic species the state list nor do we hold a generic “against” position where exotics are seen as unwelcome additions. In addition, the TBRC is not the entity that will tell you if an exotic species (or any species) you may see is “countable” or not. Countability of any given species is a personal decision that may or may not follow TBRC decisions and may or may not follow what is or is not on the Texas State List. For exotic species, it is the goal of the TBRC to weigh the merits of the documentation provided in view of the criteria listed above in regard to a species being established and to make a judgement based upon that data.
There are a few exotics already on the Texas State List that did not go through this same process using this criteria outlined above. Several of them (House Sparrow, Eurasian Starling, Rock Pigeon, etc.) have long histories in the state and are so widespread that they are as abundant and conspicuous as many of our native species. A smaller number are not as obviously abundant and the TBRC acknowledges that they are “grandfathered in” without having gone through the process/criteria outlined above. The most obvious examples of this are Green Parakeet and Red-crowned Parrot. Neither had as formal or as detailed a criteria for being added to the state list though both were considered by the TBRC along very similar principles around what is considered established as per the content presented here. Please see the 1995 Annual Meeting Minutes that covers the background that lead to both species being added to the Texas State List.
The TBRC is willing and able to help out any individuals or groups that wish to present a case for an exotic species to be added to the state list but it will most likely not be the case that TBRC member(s) will be the driving force behind such efforts. Such efforts may best be left up to individuals and groups that live in areas where they can regularly see and document any emerging population(s).