Species are limited to specific geographic area by historical contingency and an interaction between extrinsic abiotic and biotic factors and intrinsic dispersal abilities and adaptive traits. Species-occupied areas immediately adjacent to species-absent territories are defined as range margins, which often are associated with poor quality habitat and declining species fitness (reproductive success) and therefore a lower population density. However, if limiting environmental conditions change at a range margin, the distributions of species may expand or retract in response to that change. Hargrove and Rotenberry 2011 investigated whether the breeding success and the abundance of the desert species Amphispiza bilineata, the black-throated sparrow, was reduced at its range margins and if these reductions were linked to a potential climate-induced elevational shift. The authors compared the abundance and breeding success (at the nest level, territory level, and population level) of A. bilineata at two spatially separate sites over a period of three years. The first site was a mesic environment (characterized by chapparal vegetation) within the sparrow’s upper elevation margin. It was located near site two, a desert habitat (characterized by desert scrub vegetation) at lower elevations that was more commonly occupied by the sparrow. The results show that although sparrow abundance was greater at the drier site, the species’ reproductive success at the mesic site significantly outperformed the reproductive success of the sparrows inhabiting the drier site during the two driest years of the study. However, the reproductive success of the birds inhabiting the desert scrub environment improved during a year with higher precipitation levels. Despite these observations, there was little indication of an upward elevational shift in the sparrows’ distribution over a 26-year period although a warming trend and drier conditions were observed, suggesting the presence of an “ecological trap” within the system. Ultimately, this “ecological trap” could prevent or delay sparrow climate-induced range shifts.—Megan Smith
Hargrove, L., and John T. Rotenberry, 2011. Breeding Success at the Range Margin of a Desert Species: Implications for a Cliamate-Induced Elevational Shift. Oikos 120: 1568–1576.
The black-throated sparrow (Amphispiza bilineata) breeds territorially throughout the desert of the southwest United States and northern Mexico. It is non-territorial in winter. The study area was located at the species’ western range margin in San Diego County, California. There, the Peninsular Mountains create a rain barrier to the eastern Colorado Desert. There is a strong ecological gradient that varies with elevation along the eastern slopes of these mountains. This ecological gradient is also correlated with temperature and precipitation. Within this area, the sparrows are abundant in desert habitat east of the Peninsular Mountains but are rare or absent at higher elevations and coastal areas. The species’ distributional margin occurs along a plant community transition at mid-elevation between desert scrub and chaparral.
Large-scale sparrow distributions and abundance were calculated using point counts at 90 locations along the full elevational gradient (150–1850 m in elevation over a distance of 30 km) from desert scrub to chaparral vegetation. Point counts lasting 15 minutes were conducted between the 2005–2008 breeding seasons. They were repeated 2–3 times a year using distance sampling with a single observer. The relationship between mean birds per point count and elevation was compared using statistical analysis. Local-scale bird abundance and breeding activity was recorded by establishing 10 study sites along the margin of the sparrows’ distribution limits at 1150–1450 m and 6 study sites in lower-elevation desert scrub at 150–650 m. The higher elevation sites were characterized by chaparral vegetation while the lower elevation sites were characterized by desert scrub. Mean distance between the desert scrub and chaparral sites was 13 km, while mean within-habitat distance of sites was 6.7 km. The study sites were approximately 24 ha in area (1200 X 200 m). Local-scale bird abundance was estimated based on territory density estimates at each of the 16 sites using weekly territory mapping during the breeding seasons. The locations of birds and their behaviors were plotted weekly and territories were identifiable based on male singing and aggressive interactions exhibited with neighbors and pairs that foraged closely together. Territory density was calculated for each study site as the maximum number of territories at any point in time. A territory was defined by the presence of a single male during three consecutive weeks. Differences between desert scrub and chaparral sites were tested using a statistical analysis.
Breeding success was calculated by conducting weekly censuses and nest monitoring at each of the 16 study sites during the breeding seasons of 2005–2008. Breeding activity was monitored for each territory from vantage points that were unlikely to cause disturbance. The locations and numbers of all adults and fledglings were mapped weekly at each study site, and all nesting activity was monitored. Survival probability was estimated daily through nest checks. Nests were avoided if they were being constructed or if sparrows were laying eggs. Clutch size and final nest outcome were estimated if possible. Additionally, the authors calculated an index of relative productivity using the fledgling ratio (proportion of total fledglings relative to adults across the season) based on weekly observations of adult and fledgling numbers. Fledglings were identified through begging calls and by visually observing their limited movement. Tail length and mobility were used to approximate fledgling age. Differences in fledgling ratios between desert scrub and chaparral sites were tested using statistical analyses.
A site-specific breeding index was generated by scoring each territory based on the highest stage of progression observed during the breeding season: 1) territorial male alone, 2) adult pair, 3) nest construction, 4) nest with eggs, 5) nest with nestlings, 6) fledglings, and 7) fledglings plus a second nest attempted. Differences in territory breeding index scores were tested using a statistical analysis. Mean clutch size was estimated using nests for which the final clutch size was determined with certainty. Daily nest survival probability was estimated using nests for which the final outcome was known. A maximum-likelihood estimate of daily nest survival probability for each habitat type and year assuming constant daily survival rate was generated using Program MARK, version 5.1. Differences between the two vegetation sites for clutch size and daily nest survival probability were tested using statistical analyses.
The authors used weather data from PRISM Group, Oregon State University to create an approximation of environmental conditions during the study period. This data was compared to 40-year means. The cumulative precipitation during the July to June rain-year, and the mean monthly minimum and maximum temperatures during spring months (March to June, when nesting occurred) were calculated for each site.
Hargrove and Rotenberry found that between 2006 and 2008, the mean monthly maximum temperature during the spring season was 16.2°C greater at the lower-elevation desert scrub sites compared to the higher-elevation chaparral sites, and that the mean minimum temperature was 8.5°C greater at the desert scrubs sites. Over a period of 40 years, the mean monthly maximum temperature in spring increased 2.4°C at the low-elevation desert scrub sties and 2.7°C at the higher-elevation chaparral sites while the mean monthly minimum temperature in spring increased 0.4°C at the low elevation desert scrub sites and 2.1°C at the higher elevation sites. Precipitation levels were lower at the low-elevation desert scrub sites in comparison to the higher-elevation scrub sites. Across the three years of the study, the desert scrub sites experienced 79% less precipitation than the chaparral sites. All the sites and years between 2006 and 2008 were below the 40-year precipitation means. In 2007, the area experienced record drought conditions while 2008 came close to the 40-year mean precipitation levels. A figure displaying the cumulative precipitation during July to June rain-year at desert scrub sites and chaparral sites between 2006 and 2008 was constructed.
The authors also found that mean black-throated sparrow abundance declined toward the upper elevation margin across all three years of the study. Overall abundance was 157% greater in the 150–650 m elevation range (desert scrub sites) than in the 1150–1450 m elevation range (chaparral sites). Sparrows were absent from the 1450–1850 m elevation range. Similarly, territory density between 2006 and 2008 was 81% greater at desert scrub sites than at chaparral sites. A figure displaying the mean black-throated sparrow abundance along the elevation gradient was constructed.
The results show that productivity (ratio of fledglings) was greater at the chaparral study sites across all three years. The most productive year was 2008 followed by 2006, with 2007 being the least productive year. No fledglings were observed at the desert scrub sites in 2006 or 2007, demonstrating a 100% reproductive failure at these sites during the two driest years of the study despite the greater density of birds found at these sites. However, in 2008 (the wetter year), the fledgling ratio was equivalent to the fledgling ratio found at the chaparral sites in 2008. Breeding success measured at the territory level was significantly lower at desert scrub sites than at chaparral sites between 2006 and 2008. The breeding index three-year average at chaparral sites was 4.0 while the breeding index in 2006 and 2007 for the desert scrub sites were 2.4 and 2.0, but was 5.1 in 2008. There were a small number of nests at the desert scrub sites in 2006 and 2007. Although clutch size and daily survival probability could not be estimated in 2007 for desert scrub sites, the authors proposed that the overall pattern was similar to other breeding success measures, suggesting a reduction in both clutch size and nest survival probability at desert scrub sites in the two driest years (2006 and 2007). A reversal of that pattern was seen in 2008. Clutch size was greater at chaparral sites than at desert scrub sites in 2006, but there was no difference between sites in 2008. Lay dates were earlier at desert scrub sites than at chaparral sites. A figure displaying the territory-level breeding success of black-throated sparrows at deserts scrub and chaparral sites between 2006 and 2008 was constructed.
Within southern California, the desert regions are predicted to become warmer and drier within the next 100 years while events such as floods and droughts are expected to increase. Therefore, if the sparrows breeding success improves at the distribution margin, breeding distributions are expected to expand unless there are other fitness-related factors interacting within the system. However, although sparrow-breeding success was greater at the upper elevational margin (since drought was the probable cause of reduced reproduction at the desert scrub sites), the birds showed little sign of any upward shift in their elevation distribution. Higher sparrow abundance persisted at the desert scrub sites even during the direst years of the study and the upper elevational limit did not experience a shift either. The authors additionally did not find evidence for an upward elevational shift for the sparrows at another site despite strong warming trends and drier conditions.
Hargrove and Rotenberry proposed that the sparrows’ observed stasis could be attributed to tradeoffs—such as increased survival rates at lower elevations—that increase overall species fitness within the desert scrub habitat. Additionally, the life-history strategy of the sparrow may explain its range stasis. For example, the sparrow species could take advantage of wet years for reproduction while reproductively stagnating during dry years. Finally, desert scrub habitat may have a greater suitability historically for the black-throated sparrows, indicating that the species has evolved a preference for desert scrub vegetation over that of chaparral. Therefore, the greater density of sparrows at sites with reduced reproductive success that are close to sites with lower density and greater reproductive success signals the presence of an ecological trap for this species. Ecological traps can drive a population to extinction and occur when low-quality habitat is preferred over high quality available habitat. So, even if marginal areas have greater climatic suitability, sparrows may still retain an inherited preference for less-suitable central habitat, leading to their extinction. Ultimately, the local biotic interactions of these sparrows outweigh the effects of climate change, thereby inhibiting range shifts.