Investigating long-term stability of community structure in a desert ecosystem

Background/Question/Methods

Studies of animal community dynamics are often confined to short time periods due to the scarcity of available data.  However many of the proposed mechanisms for community change (e.g. climate shift, species invasions) operate on time scales of many years to decades.  To investigate questions of temporal scale in community dynamics, we use a dataset of small mammal censuses spanning 38 years, during which time significant changes in species composition have been observed.  We used clustering algorithms to group sampling instances based on similarity of relative species distribution observed at each snapshot in time.  These clustered groups were then used to investigate potential abiotic influences on animal community structure, such as precipitation and temperature.  

Results/Conclusions

Clustering analysis suggests that stable coexistence of species in the small mammal community persists about 10-15 years before the community re-organizes into a different “regime.”  This broad level of organization can be connected to a change in vegetation structure at the site: shrub cover increased significantly from the late 1970s to the present, and later regimes are increasingly dominated by shrubland-affiliated rodent species.  Further sub-division of these decade-long community regimes revealed a seasonal component to species composition, which has strengthened in recent decades.  This seasonal signal is likely driven by the tendency for some small-bodied rodents to enter torpor during winter to conserve energy, and such small-bodied species constitute a much higher percentage of the community in more recent years than past years.  We hypothesize that this seasonal level of community organization is influenced by the availability of resources during the winter, which is correlated to seasonal precipitation.