| Understanding the maintenance of species diversity is a central question in community ecology. Ecological processes explaining the maintenance of species diversity, including density-dependence, interspecific competition, and disturbance, should be able to elucidate not only the causes of the maintenance of species diversity, but also its decline. This study investigated how these three processes contributed to a decline in species diversity, associated with the rise of one dominant species, Acer saccharum (Sugar maple), in a deciduous forest in central Illinois, U.S.A. during 1939–2001. First, the impact of density-dependence, interspecific competition, and disturbance on mortality and recruitment of tree species was evaluated. Second, neighbor-specific transition matrices and integrated megamatrices were used to evaluate the relative importance of different neighbor interactions for the great population expansion of A. saccharum. Finally, the causes of spatial patterns of disturbance in three species forest fragments in central Illinois were evaluated at both forest- and individual-tree levels. All trees with diameter at breast height (DBH) ≥ 7.6 cm in eight discrete quadrats (48 x 68 m) and one large quadrat (216 x 260 m) were mapped in Brownfield Woods in 1939, 1951, 1988, and 2001. The results indicated that the decline in species diversity in Brownfield Woods was caused by the demographic advantage of A. saccharum, while rare species were reduced by interspecific competition with A. saccharum. Disturbances, including Dutch elm disease, did not help to maintain the rare species, but facilitated recruitment of A. saccharum. The population of A. saccharum grew at an accelerated rate during the 62 years, despite the operation of density dependence of its mortality and recruitment. The neighborhood analyses indicated that the demography of A. saccharum depends on specific neighborhoods. Its density-dependent mortality was compensated by disturbance-facilitated recruitment. Finally, spatial patterns of disturbance did not reflect the environmental variation at the forest level, including forest edge, aspect, and soil drainage, but rather corresponded to the distributions of preexisting gaps. Disturbances were more likely to occur adjacent to canopy gaps. This study indicates the importance of multiple-scaled and process-based approaches in studying the maintenance/failure of species diversity. |
