Sunday, April 04, 2010

Temporal Resolution of The Fossil Record

The effects of temporal resolution on species turnover and on testing metacommunity models. 2010. A. Tomašových and S. Kidwell. The American Naturalist 175, pp. 587–606.

Devil Dinosaur © Marvel Comics
The amount of variability seen among successive fossil assemblages tends to be low compared to that ecologists see over shorter time periods. This suggests that communities are extremely resilient or resistant to change over decades to centuries.

Scientists used data on living and fossil communities to explore where the low variability in fossil assemblages comes from. Does it come from the natural "time averaging" of skeletal remains that occurs after death, or from biological processes that act as a stablizing force over time?

Processes in the latter category include strong species preferences for particular niches that may enable them to avoid extinction owing to high growth rates at small population sizes, or the buffering of population growth during unfavorable times.
On most seafloors, lake bottoms and land surfaces, however, sediment accumulates quite slowly compared to the rates at which local animal populations generate skeletal remains. Multiple generations of skeletal remains may therefore become mixed within a single fossil assemblage.

Simulating the effects of time-averaging on living communities from a Texas lagoon and an east African lake found that low temporal variation in species composition can be produced or reinforced by time averaging alone. Typical paleoecological data will thus seem to support models of local community stability, whether those communities have actually been stable.

Although many fossil assemblages will be too time-averaged to discriminate variability generated by biological processes operating at small spatial scales (approximately a meter) —such as competition or predation-prey interactions—they excel at capturing the larger picture, such as the identities and relative abundances of species in the regional diversity pool that local communities draw upon.

The composition of that pool is determined by speciation, extinction and biogeographic processes that are usually extremely challenging to measure using the limited spatial and temporal scope of conventional biological sampling. "It's a matter of scale," Kidwell said. "The fossil record can't give us everything we want at the fine scale, but for some questions, we can get what we need." link