Since 1988, recovery efforts have produced additional information to test whether the original objective provides for a "self-sustaining population." In particular:
The following principles will guide future recovery efforts for the Atlantic Coast piping plover population:
- Attain sufficient population size and productivity to provide >95% probability of persistence for 100 years.
All populations face varying probabilities of extinction due to stochastic events that affect survival and productivity. At given average rates of survival and productivity, and variability around these averages, large populations have lower probabilities of extinction than small ones. Population viability analysis is a form of risk analysis applied to the issue of population extinction. It is a structured and systematic analysis of the interacting factors, including abundance, rates of survival and productivity, demographic and environmental stochasticity, and catastrophes, that determine a population's risk of extinction. In recent years, PVA's have been used as tools in establishing recovery goals for threatened and endangered species such as the northern spotted owl and the desert tortoise. Information about the Atlantic Coast piping plover PVA is provided in Appendix E. Modeling was conducted to estimate probabilities of extinction, as well as probabilities that the population would fall below thresholds of 50, 100, and 500 pairs. The results of this modeling are the basis for the revised quantitative delisting objectives.
- Assure that population increases are evenly distributed throughout the plover's Atlantic Coast range.
This principle was reflected in the 1988 recovery objective stipulation that the population increase had to be achieved "while maintaining the current distribution." Dispersal of the population across its breeding range serves as a hedge against catastrophes, such as hurricanes, oil spills, or disease that might depress regional survival and/or productivity. Maintaining robust, well-distributed subpopulations should reduce variance in survival and productivity of the Atlantic Coast population as a whole, facilitate interchange of genetic material between subpopulations, and promote recolonization of any sites that experience declines or local extirpations due to low productivity and/or temporary habitat succession.
To facilitate an even distribution of the population, the recovery team has established four recovery units (Atlantic Canada, New England, New York-New Jersey, and Southern) and assigned a portion of the population target to each. These units are large enough that their overall carrying capacity should be buffered from changes due to natural habitat formation processes at individual nesting sites, while still assuring a geographically well-distributed population.
Current information indicates that most Atlantic Coast piping plovers nest within their natal region, that regional population trends are related to regional productivity, and that intensive regional protection efforts contribute to increases in regional piping plover numbers (see Breeding Site Fidelity and Dispersal, page 28). However, at least low levels of dispersal are on-going within the Atlantic Coast piping plover population, and recovery units do not represent biologically distinct population segments, as defined in the U.S. Fish and Wildlife Service's policy regarding the recognition of distinct vertebrate population segments under the Endangered Species Act (USFWS 1996a).
A premise of this plan is that the overall security of the Atlantic Coast piping plover population is profoundly dependent upon attainment and maintenance of the minimum subpopulations for the four recovery units. Any appreciable reduction in the likelihood of survival of a recovery unit will also reduce the probability of persistence of the entire population.
- Prevent loss of genetic diversity over the long term.
Small populations risk loss of genetic diversity through inbreeding and random genetic drift. In the short term, such a loss may reduce individual fitness and productivity. Over the long term, loss of genetic diversity may erode the evolutionary potential of a population or species, reducing its ability to adapt to changes in its environment, and thereby increasing its risk of extinction. An Ne of 500 was cited by Franklin (1980) and Frankel and Soulé (1981) as the minimum effective population size necessary to maintain long-term genetic fitness and evolutionary potential. Since no formal estimates of Ne/N are currently available, and the species' sparse distribution results in highly non-random breeding that may pose a barrier to gene flow, the revised delisting criteria require the U.S. Fish and Wildlife Service to verify that the target population is sufficiently large to maintain long-term genetic fitness.
- Provide mechanisms to prevent a reversal of population increases following delisting under the Endangered Species Act.
All of the piping plover protection mechanisms devised to date are labor-intensive activities that are effective only if implemented annually. While increasing piping plover numbers will reduce the probability of extinction, these gains will be quickly eroded if actions to mitigate threats from predation and human-caused mortality, disturbance, and habitat degradation are not continued. The PVA shows that even a population that is substantially larger than that provided in the 1988 recovery objective must sustain high productivity and survival and low variance in those parameters in order to persist over the long term. This will require continued intensive management to ensure high productivity and maintenance of wintering and breeding habitat quantity and quality.
While protection of piping plovers and their habitat will require a significant long-term commitment, the benefits go beyond survival of this one species. Protection of piping plovers and their habitat responds to the stated purposes of the Endangered Species Act (Section 2(b)), by "provid[ing] a means whereby the ecosystems on which endangered species and threatened species depend may be conserved." Since 1988, two more species that share the piping plover's beach habitat over parts of its range, the northeastern beach tiger beetle and seabeach amaranth, have been added to the list of threatened species. This and the observed response of other beach-nesting birds to piping plover protection efforts has increased biologists' awareness of the piping plover as an indicator of the health of the fragile beach ecosystem.
