Shifting community composition and habit prevalence over three decades in a disturbed aquatic system

Background/Question/Methods

Modern ecological change occurs due to a combination natural successional processes, disturbance regimes, and human-induced disturbances.  Systems that are highly disturbed by humans are particularly vulnerable to accelerated rates of species immigrations and extirpations.  We used a highly disturbed system of ponds in Indiana Dunes National Lakeshore to investigate aquatic plant community composition changes over three decades.  These ponds formed within the past 3000 years due to glacial retreat, and first experienced major human-induced disturbance during European settlement. Since that time, increasing industrialization and an influx of invasive species have continued to change pond successional dynamics.  Pond vegetation was originally sampled in 1982.  In 2010 we resampled vegetation in three rows of ponds ranging in age from 2000-3000 years.  We sought to identify and explain changes in plant community composition over 28 years. Particularly, we were interested in investigating the impacts of hydrological changes and invasive species influxes on communities in ponds of varying size, age, and fragmentation history.

Results/Conclusions

We found that tall emergent species have increased in prevalence consistently across pond types over the past 28 years.  Three of these species are non-native: reed canary grass (Phalaris arundinacea), common reed (Phragmites australis), and purple loosetrife (Lythrum salicaria).  One-fifth of quadrats sampled included at least one of these invasive species. The native shrub species Cephalanthus occidentalis (buttonbush) also increased substantially in frequency.  Cattails (Typha sp.) were present in 40% of sampled areas in both time periods, but a substantially higher dead-to-live Typha stem ratio in 2010 is consistent with very recent population decline.  We found that pond size has been more important than age in determining the type of species shifts.  The increasing prevalence of emergent species in this system is consistent with a concomitant trend of decreasing water levels, including particularly shallow water in the mid 2000s that allowed extensive Typha growth.