natserv: 'NatureServe' Interface

#' @section National (N) and Subnational (S) Conservation Status Ranks:
#' \itemize{
#'  \item NX,SX - Presumed Extirpated - Species or community is believed to be
#'  extirpated from the nation or state/province. Not located despite intensive
#'  searches of historical sites and other appropriate habitat, and virtually
#'  no likelihood that it will be rediscovered.
#'  \item NH,SH - Possibly Extirpated (Historical) - Species or community
#'  occurred historically in the nation or state/province, and there is some
#'  possibility that it may be rediscovered. Its presence may not have been
#'  verified in the past 20-40 years. A species or community could become NH
#'  or SH without such a 20-40 year delay if the only known occurrences in a
#'  nation or state/province were destroyed or if it had been extensively and
#'  unsuccessfully looked for. The NH or SH rank is reserved for species or
#'  communities for which some effort has been made to relocate occurrences,
#'  rather than simply using this status for all elements not known from
#'  verified extant occurrences.
#'  \item N1,S1 - Critically Imperiled - Critically imperiled in the nation or
#'  state/province because of extreme rarity (often 5 or fewer occurrences)
#'  or because of some factor(s) such as very steep declines making it
#'  especially vulnerable to extirpation from the state/province.
#'  \item N2,S2 - Imperiled - Imperiled in the nation or state/province because
#'  of rarity due to very restricted range, very few populations (often 20 or
#'  fewer), steep declines, or other factors making it very vulnerable to
#'  extirpation from the nation or state/province.
#'  \item N3,S3 - Vulnerable - Vulnerable in the nation or state/province due
#'  to a restricted range, relatively few populations (often 80 or fewer),
#'  recent and widespread declines, or other factors making it vulnerable
#'  to extirpation.
#'  \item N4,S4 - Apparently Secure - Uncommon but not rare; some cause for
#'  long-term concern due to declines or other factors.
#'  \item N5,S5 - Secure - Common, widespread, and abundant in the nation or
#'  state/province.
#'  \item NNR,SNR - Unranked - Nation or state/province conservation status
#'  not yet assessed.
#'  \item NU,SU - Unrankable - Currently unrankable due to lack of information
#'  or due to substantially conflicting information about status or trends.
#'  \item NNA,SNA - Not Applicable - A conservation status rank is not
#'  applicable because the species is not a suitable target for conservation
#'  activities.
#' }
#'
#' @section Breeding Status Qualifiers:
#' \itemize{
#'  \item B - Breeding - Conservation status refers to the breeding population of
#'  the species in the nation or state/province.
#'  \item N - Nonbreeding - Conservation status refers to the non-breeding
#'  population of the species in the nation or state/province.
#'  \item M - Migrant - Migrant species occurring regularly on migration at
#'  particular staging areas or concentration spots where the species might
#'  warrant conservation attention. Conservation status refers to the
#'  aggregating transient population of the species in the nation or
#'  state/province.
#' }