Je. Duffy et al., Grazer diversity, functional redundancy, and productivity in seagrass beds: An experimental test, ECOLOGY, 82(9), 2001, pp. 2417-2434
Concern over the accelerating loss of biodiversity has stimulated renewed i
nterest in relationships among species richness, species composition, and t
he functional properties of ecosystems. Mechanistically, the degree of func
tional differentiation or complementarity among individual species determin
es the form of such relationships and is thus important to distinguishing a
mong alternative hypotheses for the effects of diversity on ecosystem proce
sses. Although a growing number of studies have reported relationships betw
een plant diversity and ecosystem processes, few have explicitly addressed
how functional diversity at higher trophic levels influences ecosystem proc
esses. We used mesocosm experiments to test the impacts of three herbivorou
s crustacean species (Gammarus mucronatus, Idotea baltica, and Erichsonella
attenuata) on plant biomass accumulation, relative dominance of plant func
tional groups, and herbivore secondary production in beds of eelgrass (Zost
era marina), a dominant feature of naturally low-diversity estuaries throug
hout the northern hemisphere. By establishing treatments with all possible
combinations of the three grazer species, we tested the degree of functiona
l redundancy among grazers and their relative impacts on productivity.
Grazer species composition strongly influenced eelgrass biomass accumulatio
n and grazer secondary production, whereas none of the processes we studied
was clearly related to grazer species richness over the narrow range (0-3
species) studied. In fact, all three measured ecosystem processes-epiphyte
grazing, and eelgrass and grazer biomass accumulation-reached highest value
s in particular single-species treatments. Experimental deletions of indivi
dual species from the otherwise-intact assemblage confirmed that the three
grazer species were functionally redundant in impacting epiphyte accumulati
on, whereas secondary production was sensitive to deletion of G. mucronatus
, indicating its unique, nonredundant role in influencing this variable. In
the field, seasonal abundance patterns differed markedly among the dominan
t grazer species, suggesting that complementary grazer phenologies may redu
ce total variance in grazing pressure on an annual basis. Our results show
that even superficially similar grazer species can differ in both sign and
magnitude of impacts on ecosystem processes and emphasize that one must be
cautious in assuming redundancy when assigning species to functional groups
.