Distribution and Abundance of the Interior Population of the Least Tern (Sternula Antillarum), 2005; A Review of the First Complete Range-Wide Survey in the Context of Historic and Ongoing Monitoring Efforts

Abstract

Abstract:The interior population of the Least Tern (Sternula antillarum) was added to the U.S. Fish and Wildlife Service (USFWS) list of threatened and endangered species in 1985 because of suspected low numbers and concerns about breeding season habitat loss or degradation on large interior rivers. Range-wide survey data were incomplete when Interior Least Terns (ILT) were originally listed. Although many ILT counts have been conducted over the past 20 years, regular survey coverage is still incomplete across the large breeding range of ILT, limiting the ability to assess the conservation status or trends for this population. During the last two weeks of June and the first week of July 2005, over 140 participants contributed to the first complete range-wide survey for ILT (see acknowledgments). The primary objectives of this survey were 1) to provide a minimum count of the number of adult ILT occurring in North America during the breeding season, 2) to document the range-wide distribution of nesting colonies, and 3) to describe the types of habitats that are being used for nesting. Survey crews covered ~4,700 river miles, 22 reservoirs, 62 sand pits, 12 industrial sites, 2 rooftop colonies, and over 16,000 acres of salt flats, counting a grand total of 17,591 ILT in association with 489 different colonies. Just over 62 percent of all adult ILT were counted on the "Lower" Mississippi River (10,960 birds on 770+ river miles). Four additional river systems accounted for 33.3 percent of the remaining ILT, with 11.6 percent on the Arkansas River system (including the Canadian and Cimarron Rivers and the Salt Fork of the Arkansas River), 10.4 percent on the Red River system, 6.9 percent on the Missouri River system, and 4.4 percent on the Platte River system. Lesser numbers of terns were counted on the Ohio River system (1.0 percent), the Trinity River system in Texas (1.0 percent), the Rio Grande/Pecos River system in New Mexico and Texas (0.8 percent), the Wabash River System (0.6 percent), two reservoirs in East Texas (0.3 percent), and the Kansas River system (0.3 percent). A majority of adult terns were counted on rivers (89.9 percent), with much smaller numbers at sand pits (3.6 percent), reservoirs (2.5 percent), salt flats (2.3 percent), industrial sites (1.4 percent), and rooftops (0.3 percent). This report discusses the results of the 2005 survey at three different spatial scales: 1) the entire breeding range for ILT and adjacent breeding populations on the Gulf Coast; 2) regional analyses by major river systems; and 3) individual survey segments (some of which have been combined into geographic segments comprised of more than 1 similar survey segment). Results of the 2005 survey are also compared with historic survey data from 1986 through 2004. The value of historic data for local, regional, and range-wide analyses of population trends is evaluated in the context of this first complete picture of the breeding distribution of ILT. Recommendations are made to 1) increase annual survey coverage for ILT to in...

Interior Least Tern (Sternula antillarum) breeding distribution and ecology: implications for population‐level studies and the evaluation of alternative management strategies on large, regulated rivers

“…). Figure illustrates the cumulative contribution of the top 16 (of 48) subpopulations to the total ILT population based on counts from 2005, the only year for which count data are available for all populations (Lott ).…”

“…We summarized the following characteristics for each of the populations and subpopulations identified in the analysis above: (1) the number of colony sites within each subpopulation (from a randomly chosen year among those with data available between 2002 and 2011); (2) the continuous linear extent (in km along river centerlines) of occupied habitat; (3) the median intercolony distance (ICD, in km) for all pairs of colonies within a subpopulation, arranged linearly along the channel; and (4) the maximum count between 2002 and 2011 as a percentage of the range‐wide count of 17,571 adult ILT in 2005 (Lott ) (Table ). We view these percentages as coarse estimates, as 2005 survey data came from multiple sources, none of which employed protocols to estimate detection probabilities, which are known to vary with observer, sampling methods, and environmental conditions (Lott ; Nichols et al. ).…”

“…The USFWS defined the ILT population as all Least Terns nesting on or adjacent to the major rivers of the Great Plains and the Lower Mississippi Valley (north of Baton Rouge, LA), and along Texas rivers >80 km from the Gulf of Mexico coast (USFWS ). ILT breeding populations are distributed from the Missouri River in Montana south through the Mississippi River Valley and its large western tributaries, with large distributional gaps along this river network due to the presence of multiple reservoirs and the channelized Lower Missouri River Navigation System (Funk and Robison ; Johnson ; USFWS ; Lott ) (Fig. , Table ).…”

“…Max% ILT, max. count between 2002 and 2011 divided by 17,591, the number of adult ILT counted during the only range‐wide survey for this population (Lott ).…”

“…Much of this initial habitat loss and degradation will be permanent without dam removal or drastic changes to water management policies that have been institutionalized, and tested in court, for decades (USFWS ; USDOI ). The current distribution of ILT is a patchwork of populations distributed across dike‐field sand deposits within navigation systems (USACE , ; USFWS ); relict river segments, sensu Johnson (), below large multipurpose dams (USFWS , ; Lott ); and anthropogenic sites along altered river channels (Sidle and Kirsch ) (Fig. , Table ).…”