Electronic Theses and Dissertations

Date of Award

2011

Document Type

Dissertation

Degree Name

Ph.D. in Biological Science

First Advisor

J. Stephen Brewer

Second Advisor

Marjorie M. Holland

Third Advisor

Marc Slatter

Abstract

Some communities may resist invasion because they are dominated by ecosystem engineers. If these dominant species are removed by some catastrophic disturbance, they can be replaced by invasive species that will dramatically alter the ecosystem in a way that may prevent its recovery. I tested several hypotheses related to this idea within barrier island scrub communities dominated by Florida rosemary (Ceratiola ericoides Michx.). Hurricanes represent a major source of disturbance to coastal Florida rosemary communities, and individual rosemary shrubs in three populations recently impacted by major hurricanes were not protected from the storm surge at higher elevation. Unlike inland populations of rosemary more likely to be affected by fire, some individuals in coastal populations resprouted following hurricane damage and may reproduce at an earlier age. After Hurricane Katrina, which caused widespread mortality of rosemary and other dune species on Horn and Petit Bois Islands, Mississippi, an invasive grass, Panicum repens L. (torpedograss), began encroaching upon the dunes from its previous habitat in adjacent swards. Previous studies have shown that rosemary inhibits the growth of native bunchgrasses via allelopathy, and I demonstrated in two greenhouse experiments that rosemary may be capable of mild alleopathic inhibition of torpedograss aboveground growth. Comparing pre-storm data to torpedograss abundance and Florida rosemary recruitment after the hurricane revealed that after an initial increase, torpedograss density on the dunes declined as the native community recovered. Torpedograss did not threaten rosemary seedling recruitment, perhaps due to persistent soil effects associated with the pre-storm presence of adult rosemary shrubs; therefore it appears that Florida rosemary's removal by the hurricane contributed to encroachment by torpedograss, but that torpedograss did not sufficiently alter the dune habitat to prevent community recovery. An increase in hurricane frequency or intensity could change the outcome of this interaction. Although it is an aggressive wetland invader, torpedograss may play a more opportunistic role in xeric systems. During a fourteen-month study at Archbold Biological Station, populations of torpedograss adjacent to pasture were increasing, while those adjacent to scrub were declining. Roadside populations of torpedograss therefore represented propagule sinks rather than sources at this location.

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