Linked paper: Demographic analysis demonstrates systematic but independent spatial variation in abiotic and biotic stressors across 59 percent of a global species range by K.J. Ruskin, M.A. Etterson, T.P. Hodgman, A.C. Borowske, J.B. Cohen, C.S. Elphick, C.R. Field, R.A. Longenecker, E. King, A.R. Kocek, A.I. Kovach, K.M. O’Brien, N. Pau, W.G. Shriver, J. Walsh, and B.J. Olsen, The Auk: Ornithological Advances 134:4, October 2017.
Ecologists have long hypothesized that the factors that affect a species vary over its geographical range. For example, cold climates may limit survival at higher latitudes, while competition with other species may be more important at lower latitudes. Scientists have proposed that this sets up a tradeoff for each species, favoring individuals that are physiologically hearty to harsh abiotic conditions at higher latitudes and individuals that are good competitors at lower latitudes.
With the help of 14 coauthors scattered across the northeastern U.S., I collected demographic data on Saltmarsh Sparrows to test whether this pattern was supported. Our team, known as the Saltmarsh Habitat and Avian Research Program (SHARP), conducted coordinated demographic research on Saltmarsh Sparrows at 23 sites in 7 states from Maine to New Jersey. We searched for nests, revisited them every few days throughout the breeding season, and classified each as successful or failed due to various causes.
Saltmarsh Sparrows breed exclusively in high marsh habitat, which is the zone of tidal marshes that typically floods monthly during the astronomical high tides. Saltmarsh Sparrows build their nests in the short grasses of the tidal marsh, just a few inches above the ground. As a result, nests often fail due to flooding during the high monthly tides. Most nest failure in Saltmarsh Sparrows is caused either by this nest flooding, or by depredation.
Footage captured by University of Connecticut graduate student Samantha Apgar.
Using monitoring records from 837 nests collected across our study sites, we observed patterns in the factors that limit nest survival that varied predictably across hundreds of kilometers. We found that the biotic stressor, nest depredation, increased toward lower latitudes, which is consistent with the Asymmetric Abiotic Stress Limitation (AASL) hypothesis. AASL proposes that populations are limited by biotic stressors like nest depredation at the lower latitudes of their range, while abiotic stressors such as climate limit populations at higher latitudes. Conversely, we observed that the abiotic stressor, nest flooding, did not vary with latitude. Instead, nest flooding was best predicted by indicators for regular monthly flooding as well as irregular flooding events, which varied independent of latitude. Our results suggest that stressors to Saltmarsh Sparrow reproductive success vary systematically across its range, but independently from each other. Therefore, we did not observe the tradeoff between physiological heartiness at higher latitudes and competitiveness at lower latitudes that is predicted by the AASL hypothesis.
In addition to the insight this example provides into how different stressors limit species across their ranges, the patterns of biotic and abiotic stress that we observed provide information relevant to conservation of the Saltmarsh Sparrow. The Saltmarsh Sparrow is considered threatened by the International Union for the Conservation of Nature, and SHARP researchers have found that the Saltmarsh Sparrow population is small, declining, and expected to go extinct this century. For example, our results suggest that predator control may be an effective method for improving Saltmarsh Sparrow fecundity toward the low latitudes of its range, but not farther north.
This new article in Auk: Ornithological Advances is the latest in a series we have written about the Saltmarsh Sparrow and other tidal marsh birds found in northeastern North America, many of which are facing population declines and habitat change. Learn more about tidal marsh birds and SHARP’s research at our website (www.tidalmarshbirds.org) and Facebook page (www.facebook.com/tidalmarshbirds).