PARENT SESSION
Oral Session # 93: Vegetation Change and Response.
Presiding: B Parry Hecht
Friday, August 8. 8:30 AM to 12:00 PM, SITCC Meeting Room 105.

Seemingly gradual changes in climate provoke punctuated biological responses.

Harley, Christopher^*,1, Hunt, Luke¹, Paine, Robert², ¹ Hopkins Marine Station, Pacific Grove, CA, USA² University of Washington, Seattle, WA, USA

ABSTRACT- Biotic response to changing climate can occur gradually in small increments, or suddenly in large jumps. Because limited sampling through time may detect change in the former case, but not in the latter, distinguishing between these two patterns is central to our ability interpret available data and forecast future biological change. Unfortunately, the high resolution, long-term datasets required to make such distinctions are rare. Here, we use photographic reconstruction and direct measurements to describe changes in the vertical distributional limits of two intertidal algae in response to changing sea level. In Pacific Grove, CA, sea level has been rising 1-2 mm/yr for the past several decades. In approximately 1960, the upper limit of Endocladia muricata shifted upshore by 30 cm within a few years. Following this rapid shift, Endocladia's upper limit has remained remarkably constant to the present. On Tatoosh Island, WA, where tectonic uplift outpaces sea level rise by 1.4 mm/yr, the upper limit of Mazzaella parksii was relatively stable for at least 15 years. In the 1990s, the upper limit of Mazzaella shifted downwards by 25 cm within three years to a new level where it has remained since 1995. In both cases, distributional changes were sudden and could not have been predicted by prior trends in the data. These results caution against the use of stasis in available data as an indicator of future stability.

Key words: sea level, distributional shifts, climate change, upper limits