L. Gough et Jb. Grace, Effects of environmental change on plant species density: Comparing predictions with experiments, ECOLOGY, 80(3), 1999, pp. 882-890
Ideally, general ecological relationships may be used to predict responses
of natural communities to environmental change, but few attempts have been
made to determine the reliability of predictions based on descriptive data.
Using a previously published structural equation model (SEM) of descriptiv
e data from a coastal marsh landscape, we compared these predictions agains
t observed changes in plant species density resulting from field experiment
s (manipulations of soil fertility, flooding, salinity, and mammalian herbi
vory) in two areas within the same marsh.
In general, observed experimental responses were fairly consistent with pre
dictions. The largest discrepancy occurred when sods were transplanted from
high- to low-salinity sites and herbivores selectively consumed a particul
arly palatable plant species in the transplanted sods. Individual plot resp
onses to some treatments were predicted more accurately than others. Indivi
dual fertilized plot responses were not consistent with predictions (P > 0.
05), nor were fenced plots (herbivore exclosures; R-2 = 0.15) compared to u
nfenced plots (R-2 = 0.53). For the remaining treatments, predictions reaso
nably matched responses (R-2 = 0.63).
We constructed an SEM for the experimental data; it explained 60% of the va
riance in species density and showed that fencing and fertilization led to
decreases in species density that were not predicted from treatment effects
on community biomass or observed disturbance levels. These treatments may
have affected the ratio of live to dead biomass, and competitive exclusion
likely decreased species density in fenced and fertilized plots. We conclud
e that experimental validation is required to determine the predictive valu
e of comparative relationships derived from descriptive data.