No consistent small-scale negative native-exotic richness relationship in natural communities

Background/Question/Methods

The role of biodiversity in habitat invasibility has been a major subject in biological invasion and ecosystem functioning.  For example, it has frequently been claimed that, while native and exotic richness are positively related on large scales, those on small scales are often negatively related.  However, while large-scale studies that are almost exclusively observational seem to support the positive correlation, most negative relations over small scales are actually based on experiments, and those in natural settings seem more complex and controversial.  The inconsistencies in small-scale studies may be mostly because (1) in the same community, different measures at a given time can yield different results, and similarly the same measure used over different times can yield different results, and (2) species identity may play a major role.  To test whether the long-claimed negative relations are consistent, and to propose reasons if not, I compared experimental and field data from multiple sites across North Dakota, Santa Monica Mountains in California, and numerous others sites worldwide.  I specifically examined the studies that use richness as the independent variable. 

Results/Conclusions

The new synthetic results show that (1) there is no consistent negative relation between native and exotic richness, and the frequently claimed negative native-exotic richness relations rarely exist in natural and mature communities, (2) most of the negative relationships were from using biomass, cover, survivorship, or density of exotics, rather than exotic richness, and (3) significantly more experimental and theoretical studies revealed negative relations than field observations.  Also, in both experimental and natural communities, the native-exotic relations switch with time, i.e., positive in early development and negative in late (or mature) stages.  Thus, while the positive native-exotic correlations over large scales seem more consistent, those on small scales depend on the variables used, community type (i.e., native vs. experimental), and the time factor.  This study thus calls for greater attention to be paid to the differences between experimental and natural communities (particularly when applying experimental results to natural settings), the distinction between correlation and relations, and the role of the time in assessing invasibility.  Because many other factors besides native richness determine habitat invasibility, comparative studies examining the relative importance of multiple factors in different ecosystems are needed.