Marine Ecosystem: Interconnected Variables
PRBO's Farallon Island data sets are among the longest continuous time series on seabirds anywhere in the world. Such data sets provide the ecological backdrop important for conservation planning. Shown here, over 33 years, are annual mean spring sea-surface temperatures (SST) and--for the Cassin's Auklet and Common Murre--annual mean dates of egg-laying and annual reproductive success.
|The Cassin's Auklet closely tracks sea surface temperature in timing its annual breeding on the Farallon Islands. © Mike Danzenbaker, www.avesphoto.com|
Cooler spring-summer ocean temperatures indicate upwelling, which transports nutrients closer to the ocean surface, resulting in greater ocean productivity (increased food resources). SST in March-April (pre-breeding period) rose from the mid-1970s to mid-1990s but has fallen in recent years. The pattern of timing of breeding for Cassin's Auklets, which eat mostly krill, was similar to that seen in SST, becoming later from the mid-1970s to mid-1990s and earlier in recent years, likely in response to a recent shift to cold-water (favorable) conditions. For Common Murres, which eat krill and fish depending on the time of year, breeding dates became earlier through time.
Murre reproductive success has been relatively constant, with near failures (possibly due to low food supplies) in the warm El Niño years 1983 and 1992. Auklet reproductive success was also low in 1983 and 1992, but has increased considerably since 1997, again possibly in response to a colder ocean-climate regime and increased ocean productivity.
--Christine Abraham, Marine Ecology Division Biologist
Western Songbirds: Population Declines
A 20-year study at the Palomarin Field station, monitoring bird populations through mist-netting, reveals strong trends and suggests some explanations for them. Fully 52% (16 of 31) of species we captured during fall were declining; none were increasing; in the most recent decade, when the rates of decline were greater, twice as many species declined. To identify possible causes, we grouped species into life-history categories such as nest height, likelihood of cowbird parasitism, and wintering location. Unfortunately, almost all groups underwent significant declines (as exemplified in the figures at right*), making interpretation difficult and suggesting that many factors or something more systemic, such as large-scale climate factors or habitat loss, is involved.
|Chestnut-backed Chickadee is one of the 31 landbird species whose populations show 20-year declines. PRBO photo|
The Pacific Decadal Oscillation (PDO), characterized by warm and cool phases on a scale of decades or greater, is closely linked to spring temperatures in western North America: positive PDO values are associated with warmer and earlier springs, negative values with colder and later springs. Our study fell mostly within a warm PDO, which should have been favorable for songbird populations by providing a longer breeding season as well as milder winters and early springs for winter residents. Yet we still saw many long-term declines, implicating something other than large-scale climate changes as the primary factor. These results lead us to ask, Will the shift to a cold PDO result in even more severe declines for western songbirds?
Ongoing monitoring at Palomarin Field Station will enable PRBO to continue investigating change on the time scale necessary to understand population dynamics.
--Grant Ballard and Tom Gardali, Terrestrial Ecology Division Biologists
*The y-axis values in these graphs--"natural log-transformed"--closely approximate percent change. Log transforming numbers smaller than one (1.0) results in negative numbers.
Plover Lessons: the Long-Term Perspective
How important is each site in a region to a threatened population of Snowy Plovers? Is the plover's reproductive success sufficient to sustain, or even increase, the population? Are there actions we can take to improve suitable habitat? Reliable answers call for data from many years: shorter-term findings may be deceptive.
PRBO has monitored plovers for 27 years at the Pajaro River mouth on Monterey Bay. For the first 9 years, 8-26 pairs of plovers nested there, but in only 3 years did they fledge enough young to avoid population declines. This reduced the number of adult birds arriving (maturing) to breed at this site: during the next 9-year period, the number of nesting pairs declined dramatically. The few that remained produced less than a third the number of young required for population maintenance every year (minimum fledglings per male).
|The number of chicks that each male Snowy Plover can fledge varies widely at a single nesting area. Photo by Jack Haverty|
Cooperative work then began to protect plover eggs and chicks (see page 11); it paid off. The number of nesting plovers at the river mouth began a climb to their present 50-plus pairs, and reproductive success met or exceeded the minimum for population maintenance in 6 of the last 9 years.
The middle third of this study would suggest that Pajaro River mouth is not very important to Snowy Plovers in the region: birds there produced less than 3% of the fledged young from Monterey Bay beaches in 1987-1995. In the last 9-year period, though, plovers on this relatively small portion of shoreline have produced up to a third of the young from Monterey Bay beaches. Thanks to the last 9 years of a long-term effort, we are discovering the key actions to improve the quality of the habitat. With active management and human sensitivity, it should be possible to recover this threatened species.
Lynne Stenzel, Wetland Ecology Division Biologist