SPECIES REDUNDANCY AND FAUNAL STABILITY IN THE FOSSIL RECORD
Our study system is part of the Middle Devonian Hamilton Fauna of New York State, which represents an ecosystem that exhibits long-term faunal stability. We focused on the epifaunal bivalve assemblage from the silty shallow water facies of the Skaneateles Formation. We estimated the frequency of shell-crushing predation for the pterioid bivalve assemblage during this time of faunal stability and compared this ecological proxy to relative abundance data.
Out of 538 specimens from eight localities representing three stratigraphic units on average 21% of epifaunal bivalve specimens display evidence of unsuccessful attacks by unidentified shell-crushing predators. Repair frequency remains stable throughout the study interval. Factors such as taxonomic identity and stratigraphic position have no effect on the calculated frequency of repair. Contrary to the stability of the biotic interaction, abundance data for the epifaunal bivalve assemblage fluctuates throughout the time interval. This discrepancy between the repair frequency and abundance data may be explained by species redundancy; in other words, the ability of ecologically equivalent taxa to functionally compensate for each other within an ecosystem without an effect on the net function of the whole system. In our study system, the fluctuating abundance of the bivalve taxa did not influence biotic interactions, which suggests that these pterioid bivalve taxa represent redundant species that are functionally interchangeable among each other within the ecosystem.
The difference in the results for abundance and biotic interactions raises the question of whether abundance data alone can meaningfully inform us about the function and structure of ecosystems. Relative abundance is silent on functional differences or commonalities among species and does not capture the full depth of functional diversity of species and their importance for ecosystem function.