Legacy effects of sika deer overpopulation on ground vegetation and soil physical properties

Abstract

Long-lasting overpopulation of deer can prevent or retard the recovery of ground vegetation of forests even after overpopulation would have abated. This persistent impact, or legacy effect of deer, would be explained by hypothetical mechanisms such as limitation of seed sources, dominance of competitors that are unpalatable to deer, and negative feedback from soil conditions degraded by direct and indirect effects of deer. So far, however, little evidence is available for existence of such legacy effects. To detect the legacy effects caused by sika deer (Cervus nippon) and reveal their possible mechanisms, we studied ground vegetation and soil condition of Japanese natural broad-leaved forests and cedar plantations, which have records of vegetation surveys of the past 11 years. Foliar cover and species composition of ground vegetation were analyzed together with the deer density index (DDI), which represents mean deer density of the last 9 years before the survey. The relationship between vegetation cover and deer density had significantly changed between the survey years, as was proved by a regression analysis. The cover of ground vegetation did not increase during the 11 surveyed years even at most study sites whose DDI value had decreased, thereby indicating the existence of legacy effects. This delay of recovery was not attributable to the dominance of unpalatable plants, which had slightly decreased during the 11 surveyed years. Palatable plants did not increase in most study sites where DDI decreased, suggesting seed source limitation. Negative indirect effects of DDI on soil physical property in the present, such as increased bulk density and decreased coarse porosity, were detected through structural equation modeling (SEM) analysis. In the case of broad-leaved forests, the SEM model that included the 2005 DDI values showed better performance than that using present DDI values, indicating a strong legacy of past deer impact. These facts suggest the existence of legacy effects, which possibly have been caused by seed source limitation and degraded soil conditions. Moreover, above statistical analyses showed that canopy closure, which can be caused by delayed management practices (e.g., gap creation and thinning), further limited recovery of vegetation. Ground vegetation of broad-leaved forests were suggested to show lower resilience than those of cedar plantations, requiring conservation activities such as canopy gap creation.