Species composition reflects a combination of environmental and histor
ical events at a site; hence, changes in species composition can provi
de a sensitive measure of ecologically relevant changes in the environ
ment. Here, we consider the analysis of species composition when multi
ple sites are followed through time. Analyses of temporal trends in sp
ecies composition either summarize species composition into a few metr
ics (indices or axis scores) or analyze the similarity among sites. We
develop and illustrate the similarity approach. Each pair of samples
represents a pair of replicates, a pair from the same site at differen
t times, a pair from different sites at the same time, or an unrelated
pair. Differences among times can be estimated by comparing average t
emporal dissimilarity to average replicate dissimilarity. Temporal tre
nds can be described by one of three statistics that measure progressi
ve change, the correlation of temporal dissimilarity with the length o
f time between samples. These methods are illustrated using data on ch
anges in a South Carolina zooplankton assemblage following disturbance
, and changes in bird species composition on Skokholm Island, Wales. I
t is difficult to define and interpret temporal trends. Some definitio
ns of interesting trends, like increasing divergence from another set
of sample plots, place additional requirements on the sampling design.
Including replicate samples or clustering sample plots and including
''control'' plots for comparison with sentinel sites would contribute
to an understanding of changes in species composition.