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Monthly Changes in Population Size and Body Composition of Eared Grebes (Podiceps nigricollis) on Great Salt Lake, Utah
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Eared grebes ( Podiceps nigricollis ) are colonial‐nesting waterbirds that breed in Canada and northern United States. Great Salt Lake (GSL), Utah, is vital to the survival of this species because all eared grebes in North America stage in the fall either on the GSL or Mono Lake, California. The importance of GSL and its surrounding wetlands for breeding eared grebes is unknown. We studied eared grebe nesting status and chronology in the freshwater wetlands around GSL and found over 4,280 nests distributed among 35 colonies during 2018 and 5,794 nests among 23 colonies during 2019. We also located the 2 largest colonies of this species ever recorded (902 and 1,492 nests). Mean clutch size differed between years and was 2.4 eggs during 2018 and 2.0 during 2019; clutch sizes were lower at GSL than in colonies located in more northern latitudes, perhaps due to a local paucity of invertebrate prey during the egg‐laying period. Grebe nests around the GSL were constructed with, and anchored to, growing Stuckenia pectinate . Eared grebes near GSL started laying eggs in the first week of June during 2018 and a week later during 2019. The number of incubated nests per colony peaked on 27 June during 2018 and 9 July during 2019. Nests continued to be incubated into August in both years. These dates are later than those reported in more‐northern colonies. The later nesting in GSL colonies could be due to the birds' need to wait for Stuckenia pectinata to form mats at the water surface. This plant species needs a water depth of 38 to 45 cm to thrive, and increasing amounts of freshwater from the GSL watershed are diverted for agriculture and human development. If this trend continues, there may not be enough water to maintain the required water depth for dense stands of Stuckenia; the loss of which may prevent the grebes from nesting. © 2021 The Wildlife Society.
Utah’s Great Salt Lake (GSL) is so saline that the only invertebrates that survive in the open water are brine fly larvae and brine shrimp. In the absence of competition from other invertebrates, they are incredibly abundant. Only a few avian species can take advantage of their abundance because a bird cannot eat them without also ingesting salt. Moreover, brine shrimp and brine flies are so tiny that only a few avian species can consume the massive number of brine shrimp and brine flies required to meet a bird’s nutritional needs. For example, eared grebes need to consume 28,000 adult brine shrimp each day to survive. To achieve this, an eared grebe has to spend 7 h daily foraging and needs to harvest one shrimp per second during foraging.
Great Salt Lake (GSL) is a critical migratory stopover location for Eared Grebes (Podiceps nigricollis), but the factors influencing the timing of their fall migration have not previously been investigated. We used archived Doppler radar data to visualize the nocturnal departures of Eared Grebes from GSL over 16 years, from 1999–2015. We used generalized linear models (GLMs) to examine interannual variability in the timing of migration of Eared Grebes in relation to prey availability and lake temperature, as well as variation in the nightly departure likelihoods of Eared Grebes in relation to weather. On average, Eared Grebes departed from GSL over a period of 31 days each year, with departures occurring on 17 of those days. We did not find any trend toward earlier departures over the years of the study. Departures typically began earlier in the year when densities of brine shrimp adults (Artemia franciscana) were higher than average, densities of brine shrimp cysts were lower than average, and lake temperatures were warmer than average. The span of migration departures was most strongly related to the day of first departure, with longer spans occurring in years with an earlier migration initiation day. We also found that likelihood of departure from GSL was greater on nights with high barometric pressure and low lake temperature. High barometric pressure was related to low wind speeds and low incidence of precipitation. We concluded that Eared Grebes depart when they have gained sufficient mass to successfully migrate rather than lingering at GSL for as long as possible, and that migration departure is most likely on nights with fair weather.
Most North American Eared Grebes (Podiceps nigricollis) undertake a post-breeding migration to two hypersaline lakes in the USA: Mono Lake in California and Great Salt Lake in Utah. Single air photo surveys were conducted in mid-October at Mono Lake from 1996-2012 and multiple fall surveys were conducted from 2013-2018, the latter to determine variation in abundance patterns within and across years. In four of the six years with multiple fall surveys, peak abundance occurred in mid-October as expected. However, in 2014 and 2015, Eared Grebe numbers declined dramatically soon after arrival, coinciding with low levels of their primary food, brine shrimp (Artemia monica). Abundance remained low from 2016-2018, and this could have been due to a shift to Great Salt Lake or to a massive mortality event. In 2017 and 2018, Eared Grebes breeding in south-central British Columbia, Canada were marked with Very High Frequency (VHF) radio transmitters and light-level geolocator (GLS) tags. Contrary to 1996, when the majority of VHF-tagged birds were molting/staging on Mono Lake, our 2017-2018 telemetry data indicated that most individuals were on Great Salt Lake. Our study provides insight into the variable abundance patterns at Mono Lake and novel information on Eared Grebe migration patterns.
Great Salt Lake (GSL) is a hypersaline terminal lake and has varied historically in salinity from 6 to 28%. Because the lake’s salinity is much greater than in marine environments (~3.5%), salinity is often assumed to be the driving factor for GSL benthic and pelagic food webs. Certainly, many species cannot live in a hypersaline environment (e.g., fish), and the diversity of species capable of coping with hypersaline conditions is limited. However, the GSL’s benthic and pelagic food webs are adapted to these extreme saline conditions, and their dynamics (primary and secondary production, species abundances, etc.) respond in a complex fashion to the interplay of salinity, temperature, and nutrient availability. Therefore, focusing solely on salinity is not appropriate. In this chapter, we first explore historically how GSL food webs have been reported to change and found salinity to have limited impact. We next demonstrate that in recent years (1994–2018) GSL food webs varied far less with salinity than might be expected, even though salinity varied by 8.2–17.5%, because temperatures and nutrient availability covaried with salinity and showed more impacts than salinity alone. Finally, we employ the observations on the interplay of salinity, temperature, and nutrients to project how future climatic changes in the GSL watershed will affect primary producers and consumers and impact GSL food webs. These future climatic changes will have profound effects on GSL food web dynamics.
Terminal lakes are highly susceptible to climate change impacts since water that enters through precipitation, runoff, and groundwater must be balanced with water that leaves through evaporation. A change in this equation can lead to a decline in elevation, which can be tragic for the ecosystem, particularly if the closed basin is shallow. Great Salt Lake faces many threats that will impact the volume of water in the depression of the Bonneville Basin where it resides. If the lake’s level declines, salinity increases, and wetlands are altered. Salinity is a driver of microbial diversity and, as this foundation of the ecosystem is altered, so will be the rest of the food web, affecting large numbers of avian migrators along the Pacific and Central fly-ways. Human population growth and water diversions for agriculture have put a strain on Great Salt Lake, resulting in a terminal lake whose trajectory is downward in surface area. How might anthropogenic climate change impact this scenario? Alterations in temperature can influence the timing of snowmelt and change evapotranspiration. As temperatures increase and droughts persist, climate change will amplify the decline in lake elevation, creating more dust from the exposed lakebed. Dust blowing into inhabited valleys will worsen air quality with particulates and may be laden with the pollutants collected by the lake. Early melting of the snowpack in the Wasatch Mountains due to higher temperatures would be further impacted as airborne dust from the dry shorelines is deposited during storms and can reduce the albedo of snow, altering groundwater recharge of the watershed. The current status of Great Salt Lake, with no water rights of its own and increasing pressures for water use upstream, does not bode well for the survival of this critical ecosystem given climate change predictions for the southwestern United States.