DORMANCY AND DISPERSAL IN A POPULATION MODEL WITH CORRELATED LANDSCAPE DISTURBANCES

DORMANCY AND DISPERSAL IN A POPULATION MODEL WITH CORRELATED LANDSCAPE DISTURBANCES

By Andrew Bryant Johnson

Thesis Co-Advisors: Dr. David E. Hiebeler and Dr. Francis A. Drummond

A Lay Abstract of the Thesis Presented

in Partial Fulfillment of the Requirements for the

Degree of Master of Science

(in Ecology and Environmental Science)

December, 2010

            Populations of organisms require a means of escape from poor conditions in order to persist.  Dispersal and dormancy provide means of escape in space and time, respectively.  The value of each escape strategy can depend on characteristics of a population’s habitat.   This study examines the interaction of dispersal and dormancy, and determines how their value is affected by habitat characteristics.  My goal is to determine the amounts of dormancy and dispersal that make a population most successful when facing various levels of habitat availability and habitat disturbances of various sizes and frequency.  A computer model was developed to simulate a theoretical population of sedentary organisms with a potential dormant state, such as plants that produce dormant seeds.  The success of simulated populations is observed over time as individuals reproduce and die, offspring disperse, individuals emerge from dormancy then reproduce, and habitat and recovery events transition landscape patches between suitable and unsuitable conditions for survival.

I determined that rare large disturbances are more harmful to locally dispersing populations than frequent small disturbances.  As the dispersal ability of a population increases, the effects of disturbance size decrease provided the per-site disturbance rate is held constant.  Furthermore, populations that experience large habitat disturbances benefit substantially from producing offspring in a dormant state.  This is also the case for populations with small amounts of suitable habitat on the landscape.  However, dispersal also helps populations survive in these poor habitat conditions, and as a population’s dispersal increases, the value of dormancy generally declines.  The model shows that dormancy provides a means of rapidly recolonizing disturbed regions of landscape, and this benefit becomes increasingly important as disturbances become larger.  When disturbances are small, emergence from dormancy should be immediate in suitable conditions, but as disturbance size increases, delayed emergence maximizes population density by maintaining a form of seed bank that can recolonize the disturbed areas.  In addition, the model provides an explanation for the development of density-dependent emergence from dormancy in natural populations, and it provides a useful tool for determining the effects of changes to a population’s dispersal ability and emergence rate.