RESEARCH NEWS: Gaps Made By Matriarchial Group Removal Do Not Last in High Deer Density Populations

Commentary on Miller et al. (2010). Tests of localized deer management for reducing deer browsing in forest regeneration areas. Journal of Wildlife Management 74: 370-378.

Managing deer on very small spatial scales has traditionally been problematic, but efforts to remove matriarchal social groups of deer may hold promise for reducing browsing impacts for 10-15 years. This could possibly create a spatial hole in the deer population, thereby allowing a sufficient window-of-opportunity for regeneration. The effectiveness of this approach depends on how accurately the "rose petal hypothesis" actually characterizes population expansion. The rose petal theory suggests that within a group, matriarchal does are located near the center and younger individuals establish home ranges that overlap radiating outward. In other words, removing matriarchal social groups will only work if deer exhibit low female dispersal distances, high female survival rates, and high philopatry.

Miller et al. tested the rose petal hypothesis in Randolph County, West Virginia (eastern North American deciduous forest). Deer densities were considered high (and in excess of sustainable numbers), estimated at 12-20 per square km, with a very skewed ratio typical of traditionally exploited deer populations (6-15 males: 100 females).

The authors first collected movement data (via telemetry) on 224 animals. A social group was identified and targeted for removal in a 1.1 square km area in 2002. A total of 51 deer were removed, 39 were female. This was estimated to be 80% of the animals in the 1.1 square km target area. Vegetation monitoring consisted of examining browsable units and actual browsing on tree regeneration. A second removal was conducted in 2005, with 26 of 31 removals being females.

After the 2002 removal, browsing dropped from 15% to 5% after removal and persisted at this level for 3 years. Telemetry data indicated that deer from surrounding areas did gradually shift their home range and fill in the void. Animals removed in 2005 were not closely related genetically to the 2002 removal group.

The authors were not sanguine about the effectiveness of this approach. It did provide a short-term benefit to the vegetation, but the duration of the benefit was brief and unlikely to translate to increased regeneration.

PEER REVIEWED RESEARCH: Limited Benefit of Woody Debris on Regeneration

Deer can limit forest regeneration following logging. Foresters use woody debris to limit deer browsing impacts to new seedlings; it is one tool of many. The idea is this: woody debris creates a physical barrier that protects seedlings from browsing. It is not 100% effective, but it is thought to boost seedling survival by a few percent. In reforestation, a few percent can mean the difference between regeneration success and regeneration failure.

Kruger and Peterson (2009) investigated this technique in northwestern Pennsylvania. They carefully quantified the size of woody debris--this is seldom done in other studies. They found no benefit of leaving woody debris. Beneath and adjacent to woody debris, they found lower seedling densities. Instead, a recalcitrant understory of ferns developed. They argue that woody debris limits light, and this is more important than browsing. This study contradicts conventional wisdom and some of the published literature.

Woody debris retention works in some places at some times. It seems to me that the benefit of woody debris retention will be influenced to a great extent by context dependence. Kruger and Peterson have conducted a careful study and produced some counter-intuitive results. It is incumbent upon other researchers to see how well these results hold up in other locations.

Source: Kruger, L.M. and C.J. Peterson. 2009. Effects of woody debris and ferns on herb-layer vegetation and deer herbivory in a Pennsylvania forest blowdown. Ecoscience 16: 461-469.