Date of Award
Master of Science
Background : Old fields are abandoned agricultural areas that are undergoing secondary succession. A variety of factors influence the assembly, invasibility, and species turnover during secondary succession in old fields. Old fields are important ecological research locations as an increasing amount of agricultural land is abandoned worldwide. Objectives : 1) To determine if exotic and native "invaders" display differences in invasion success in an old field, and 2) To observe how native and exotic species abundances change over the course of secondary succession under predictable disturbance regimes and resource levels with regards to the Core-Satellite Species (CSS) hypothesis. Methods : An old field located in southern Illinois subject to three levels of fertilization (control, annually fertilized, and fertilized every 5 years) crossed with three levels of mowing treatments (control, spring mowed, and spring/fall mowed) established in 1996 was used to test objectives Seedlings of C3 grasses (native and exotic) were planted within one fertilizer treatment to compare invasion success. Species composition data (species occurrence and abundance at three heights) in all blocks were previously collected from 1996 to 2002 and the sample plots were resampled in 2013. Species sampled in 2013 were assigned to core or satellite categories per the CSS hypothesis. Results : Invading exotic seedlings had higher survivorship than invading native seedlings from the same functional group within the same fertilizer level, and added fertilizer increased seedling survivorship. The number of core and satellite species varied early after experiment establishment in 1996 but has since leveled off, and the ratio of exotic to native species cover at heights <1m ratio was affected by an interaction between mowing treatment and time. The species occupancy distribution of all species in 2013 was not bimodally distributed, but the occupancy distribution of exotic species in 2013 was bimodally distributed.The community composition of the plots varied among blocks, but was affected by an interaction between fertilizer treatment and mowing. Conclusions : Exotic invading grass seedlings in a successional community were relatively more successful than native invading grass seedlings. However, overall survivorship of all invading grass seedlings was low, a typical finding in grasslands. The identity of core and satellite species continued to fluctuate through time following 17 years of secondary succession. Resource availability and disturbance regime affects on community composition throughout secondary succession, and may result in many alternative stable states. This research supports the `CSS' and `parallel dynamics' hypotheses and indicates that presence of exotic species within a community does not necessarily facilitate invasion by new exotic species.
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