Abstract
Effects of high deer herbivory in North America on populations of favored plant browse species have been well-documented, however since less palatable plants now dominate the understory, we asked whether these species could be vulnerable as well, and if so, what symptoms might signal that this was occurring? Using American ginseng (Panax quinquefoliusL.) as our representative less palatable understory plant, we compared two subpopulations within a single natural population that were differentially exposed to browse; one isolated from deer by growing atop a large, flat-topped boulder, and a browse-exposed subpopulation in the surrounding low-lying area. We tested the hypothesis that deer effects would be manifested in all parts of the life history; through reduced growth, survival and reproduction. In turn, we hypothesized that browse would reduce population growth rates, and that differences in stage structure of the population would be produced. Taking advantage of a 20 year record of formal demographic censusing, we showed that browse effects were manifested primarily in reduced size-specific growth, while size-specific fertility and survival were relatively unaffected by exposure to browse. Demographically, these differences in growth were sufficient to drive population size reductions of 4.5%/y in the off rock subpopulation while the on rock plants slowly increased in number. High browse off the rock resulted in high proportions of plants in a stunted juvenile state in the off rock population relative to the on rock plants. A high proportion of juveniles is therefore a clear symptom of an understory subjected to chronic overbrowsing, providing land managers a rapid way to assess whether deer could be impacting understory biodiversity. The sharp demographic contrasts we observed between browsed and unbrowsed subpopulations also implies that promotion of refugia within managed lands will likely become increasingly important management tools for biodiversity preservation as long as unchecked deer populations persist.
Highlights
“Since I have lived to see state after state extirpate its wolves
It has been argued that the absence of keystone predators along with mismanagement of their populations resulted in white-tailed deer being recognized as keystone species in its own right (Waller and Alverson, 1997) due to their broad-ranging impacts on the entire ecosystem
Ecologists have been concerned that common understory plant species may be gradually going extinct due to overbrowsing (Carson et al, 2014)
Summary
“Since I have lived to see state after state extirpate its wolves. I have watched the face of many a newly wolfless mountain, and seen the south-facing slopes wrinkle with a maze of new deer trails. Exclusion experiments permit a rigorous, spatially controlled test of browse effects and result in an entire community response; they are well-suited to examining rates of recovery following release from high browsing rates Unless such experiments are very long-term, allowing re-colonization by a full complement of understory plants, this approach primarily captures the initial recovery period, rather than a continuous condition of low browse rates. A few studies have used naturally-occurring refugia within populations exposed to browsing (Rooney, 1997; Carson et al, 2005); if the refugia have been in existence for a long time, plant demography in the refugium presumably reflects performance, including the whole-community effects such as increased competition, that may counter some positive effects of no browsing. In extreme refugia, browse rates may be zero, which could be unrealistically low relative to a desirable, or generally achievable, rate
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