Deer as a Facilitator of Exotic Species Invasion in Mature Forests of Virginia
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This document summarizes a study on the role of white-tailed deer in facilitating the invasion of exotic plant species in mature forests of Virginia. The study found that deer preferentially browse on native plant species over exotic species, limiting the recruitment of native woody seedlings. A series of experimental plots were established to examine the effects of removing invasive plants, reducing deer numbers through fencing, or both. After 4 years, only plots that received both treatments saw an increase in native oak saplings and other native species. The results suggest that to increase native plant diversity and recruitment, both deer densities and invasive species must be controlled.
Deer as a Facilitator of Exotic Species Invasion in Mature Forests of Virginia
- 1. Deer as a facilitator of exotic species invasion in mature forests of Virginia William McShea Conservation Ecology Center, Smithsonian Conservation Biology Institute, Front Royal, VA
- 2. Appalachian Mountains /Ridge and Valley • World-class temperate forest both for diversity and extent SNP & M,GW&J NFs • Large blocks of public forest • Primarily deciduous forest • Mature 2° forest (>100 yrs) • Close to urban centers
- 3. Stressors on eastern deciduous forests • Forest loss, fragmentation, and parcelization • Loss of critical species (American chestnut, wolves, cougars) • Climate change (temperature, storm severity and frequency) Invasive plant and animal species White-tailed deer – you know the story – you know the story
- 4. Browsing by deer alters forest succession pathways and sustainability
- 5. When Should We Worry About “Overabundant” Species • Cause significant economic loss • Lower diversity across the landscape • Unsustainable demographics of rare species • Alters pathways for productivity or succession
- 6. A species that, other than as a result of an introduction, Native Species historically occurred, or currently occurs, in a particular ecosystem. Any species, including its seeds, eggs, spores or other Exotic Species biological material capable of propagating that species, that is not native to a particular ecosystem. Invasive Species An exotic species whose introduction does, or is likely to, cause economic, human health or environmental harm. In US – 20,000 native plant sp.; 4,000 alien plants; 400 invasive plants Lets agree to worry about invasive (native or exotic) first & exotic later
- 7. Abundant ground cover – little of it native species
- 8. Hidden native species are denied light but also invisible to deer??
- 9. The problem – low recruitment and diversity of native plants • White-tailed deer prefer browsing native vegetation compared to exotic species, so native woody species not only compete with invasive species for light, but must survive preferential herbivory by deer. The Solutions • Remove invasives. But - when invasive plants are removed - do plant populations increase or does herbivory increase? • Reduce deer numbers. But - when deer are removed - do natives increase or do invasive plants respond more quickly?
- 10. Direct consumption (-) (-) (+) Competition for light (-) and nutrients Can you increase recruitment Predictions: of woody seedling or species richness 1) Reduced invasives of herbaceous plants by fixing only will increase herbivory one 2) Reduced herbivory problem??? will increase invasives Are you promising something you can not deliver when you try only one??
- 11. Project Details • Co-Investigators – Norm Bourg, Chad Stewart • C & O Canal and SCBI – 330 Plots • 4 x 4 m plot with 4 1x1 m subplots within each • ID and count all native herbaceous and woody plants < 30cm high, within each sub plot. All saplings ( > 30 cm) within each plot. For grasses and ferns estimate % cover. • Identify all exotic species, count and estimate % cover. • Calculated species richness, species diversity (Shannon index, H’), and number of individuals per plot in 2005 (pre) and 2007 and 2009
- 12. Do Nothing 179 plots NN 53 plots PN 45 plots NF 49 plots PF
- 13. Measured all plots in summer 2005 and select management category
- 14. Pulled Invasives twice each year: may and august
- 16. Timeline • 325 plots established 2005 (+ 8 in 2006) • Invasives pulled twice each year & fences maintained • Plots resurveyed in 2007 and 2009
- 17. After 4 years: Woody Species (> 30 cm) 14 Mean Number of Stems 12 10 Recommended stocking rate for native (all) 8 seedlings native (trees) 6 invasives (all) 4 2 0 Control Pulled Fenced Pull/Fence Treatments 2009 Sapling Data Fencing (not pulling) increased native species (total or just trees) Invasive species highest when deer excluded Neither treatment achieved needed stocking rate after 4 years
- 18. 1 Red Maple 0.9 No treatments increased numbers 0.8 0.7 Number of Individuals/Plot 0.6 0.5 0.4 0.3 0.2 0.1 0 Control Fenced Only Pulled Only Pulled and Fenced Treatment 2005 2009
- 19. 1 0.9 Quercus Species Only combined treatment increased numbers 0.8 Number of Individuals/Plot 0.7 * 0.6 0.5 0.4 0.3 0.2 0.1 0 Control Fenced Only Pulled Only Pulled and Fenced 2005 2009 Treatment
- 20. Oak Saplings in 2009 Treatment N of Plots N of Saplings Density Control 183 4 0.02/plot Pulled 55 0 0.00/plot Fenced 50 6 0.12/plot Pull/Fence 48 16 0.33/plot
- 21. Forestry matters: decline of oaks in eastern forests JWM 2007, McShea et al. based on FIA data • Decline in proportion of intermediate-sized oaks in eastern forests: 32% in 1989 down to 21% in 2000 •stem density of maples doubled from 660 (+ 201) in 1989 1303 (+246) in 2000 • nosignificant change in mast production yet • maples are immune to deer and invasives and (lack of fire), while oaks are not
- 23. Setting the Stage for this Study Study took place on SIGEO Plot at SCBI SIGEO Origins Smithsonian Tropical Research Institute (STRI) In 1980, Steve Hubbell and Robin Foster established a 50- hectare (124-acre) forest plot in Panama. The survey methodology was unprecedented in scale and scope. Every tree greater than or equal to 1cm diameter was identified, measured, tagged, and mapped. All trees are re-censured every 5 years. A “base layer” of tree community is overlaid with multiple explanatory and ancillary layers to understand forest community structure Barro Colorado Island
- 24. SCBI - SIGEO Plot Location 26 ha plot is created in 2008 With 20 x 20 m grid cells with each post marked with row and column Due to be resurveyed in 2013
- 25. Tree Measurements - all stems > 1cm dbh measured, mapped, tagged & identified to species Layers subsequently added: DEM, Downed woody debri; Soil nutrients and carbon; Seedlings; Small mammals; Invasive worms; Invasive exotic plants
- 26. SCBI - SIGEO Forest Dynamics Plot Initial census of 25.6 ha completed: - 40,180 stems (29,914 living); 26 Families, 38 genera - 62 species of free-standing trees & shrubs, 54 of which are native; Composition: - Mature secondary Eastern mixed deciduous forest – dominant species: Canopy: Understory: - Liriodendron tulipifera Asimina triloba - Carya glabra Carpinus canadensis - Carya tomentosa Cornus florida - Quercus alba Cercis canadensis - Q. rubra Amelanchier arborea - Nyssa sylvatica - Fraxinus americana Shrubs: Lindera benzoin (>17,000 stems) - Succeeding to oak-hickory forest (8 spp. oaks, 4 spp. hickory + 2 Juglans spp.)
- 27. Added Twist to Plot 26 ha site includes 4 ha deer exclosure erected in 1990 8 ft fence with 4 ft of American wire fencing (set with larger mesh on bottom) and 6 strands of high tensile wire (4 ft) 8 ft 2”x2” spacers with 6 slots (10o angle) hold high tensile fence in place
- 28. Idea for study came mainly from distribution of Microstegium in plot relative to deer exclosure 4 ha deer exclosure erected in 1990 prior to invasion of most exotic species 1) Are invasive plant distributions shaped by distribution of deer 2) Underlying mechanisms for impact Protocol for invasive species study • All 20 x 20 m grid cells examined in 2010 • All invasive species recorded as present/absent • All individual clusters counted except Microstegium (5 abundance classes based on %) • Distributions matched with data layers
- 29. Common invasive plants in forest Wineberry Rubus phoenicolasius Japanese barberry Berberis thunbergii
- 30. Common invasive plants in forest Japanese stiltgrass Microstegium vimineum Multiflora rose Rosa multiflora
- 31. Experimental Design Four step process 1) Use canonical correlations to determine tree communities within each grid cell and select section of grid (reference plot) that “matched” deer exclosure 2) Insure that the range of all parameters within deer exclosure and reference plot are encompassed by the remaining squares (Phosphorus issue) 3) Permutation trees (conditional inference tree; party in R) were constructed using all squares outside of deer exclosure and reference plot (p < 0.99 to prune tree) 4) Repeated process using significant variables identified in first tree - using gird cells from deer exclosure and reference plot while adding “Fence” as variable
- 32. SIGEO study area (20 x 20 m2 quadrats) Colors reflect community types, created using a cluster analysis based on basal area of the 10 dominant tree species in each unit (80%). Reference Plot (4 ha) matching communities Deer Exclosure (4 ha) erected 1990
- 33. Norm Bourg Jenny McGarvey and Xiaoli Shen
- 34. Min Max Mean SD SPECIES ABUNDANCE ROMU individual number (clumps) 0 33 1.01 2.96 BETH individual number (clumps) 0 138 6.44 11.79 RUPH individual number (clumps) 0 125 6.33 12.96 %MIVI cover in category 0 4 1.76 1.2 0 = absent; 1 = 1-25%; 2 = 26-50%; 3 = 51-75%; 4 = 76-100% Comfrey individual number 0 116 10.99 18.83 PREDICTOR VARIABLES No. of deer fecal pellet groups 0 52 1.93 4.99 Distance from quadrat center to forest edge (m) 21 523 282.89 115.5 Transformed aspect† -1 1 -0.008 0.72 Slope (degrees) 1.9 21.4 10.34 3.81 Topographic convergence index‡ 0 280.8 34.45 30.35 Woody species richness 3 20 10.16 2.85 No. of woody stem 7 582 60.37 71.22 pH 3.96 6.11 5.09 0.47 ECEC (estimated cation exchange capacity, cmol/kg) 6.98 41.26 18.13 5.65 Nitrogen (mg/kg) 2.13 9.09 4.3 1.16 Phosphorus (mg/kg) 10.31 35.05 20.1 5.82 % canopy openness (hemispherical photo) 3.69 20.89 9.07 2.16
- 35. Test Case – known deer-dispersed plant Wild Comfrey Cynoglossum virginianum 11.39 1.17 38.49 4.48 4.15 0.79 21.79 2.20 6.21 1.71 0.01 0.01
- 36. Multiflora rose Rosa multiflora More abundant in exclosure Higher nitrogen levels increase abundance 1.00 + 0.15 4.58 + 1.01 0.29 + 0.10 1.55 + 0.43 0.22 + 0.05
- 37. Japanese barberry Berberis thunbergii Soil cations important (organic content) Twice as abundant outside of exclosure 2.17 + 0.39 5.32 + 0.80 11.16 + 1.0 3.26 + 0.25 1.80 + 0.33
- 38. Wineberry Rubus phoenicolasius High nitrogen and low shrub density increases abundance - but in cells with low nitrogen more abundant with deer 3.25 + 0.51 20.9 + 2.68 9.82 + 1.28 2.25 + 0.36 0.22 + 0.09 7.11 + 1.29
- 39. Japanese stiltgrass Microstegium vimineum Highest abundances found with low density of shrubs and high pH or high Levels of P and understory openness Absence found almost only within deer exclosure 0 = absent; 1 = 0-25%; 2 = 26-50%; 3 = 51-75%; 1.76+0.13 2.60+0.1 1.39+0.09 1.67+0.16 2.33+0.12 4 = 76-100%. 1.58+0.14 2.20+0.12 0.49+0.06
- 41. Hard Seed Crops Critical for Wildlife Populations in Eastern Forests • In order to produce seed we need mature trees • In order to produce mature trees we need: – Lower densities of deer – Control of invasive plants – Periodic light reaching forest floor – disturbance • Fire, harvest, hurricanes, insect outbreaks These are not problems with seedling establishment but problems with the transition from seedling to sapling
- 42. A “Perfect” World • Overall Lower Density of Deer • Removal of invasive, exotic species • Create Spatial and Temporal Heterogeneity • Increase the Movement of Ungulates with Respect to Resources (Predators) • Increased Avenues for Flow of Productivity and thereby Increased Biodiversity
- 43. Virginiathis all mean? What does Working Landscapes • Deer do play a role in establishment of some exotic species in this forest • For one species (multiflora rose) the distribution of the plant was not influenced by deer – bird-dispersed seeds • Mechanism may not be the same for each exotic species • Mechanisms for facilitation are not clear from this study 1 1 • Deer actively spread seeds through feces or on coat – possible but unlikely Bill McShea , Maria Van Dyke , and James • Deer activity changes soil characteristics and nutrient levels – possible (N, ECEC, P lower) 2 • Absence of deer reduces leaf litter disturbance - possible Barnes • Native plants established before arrival of most invasives (1990 fence) - possible 1. Smithsonian Conservation Biology Institute, Front Royal, VA 2. Piedmont Environmental Council, Warrenton, VA Mission: To encourage the sustainable use of Virginia landscapes for biodiversity www.vaworkinglandscapes.org
- 44. Develop a Conservation Network: Landowners
- 45. Grassland Research: Measuring biodiversity on grasslands in the Northern Shenandoah Valley and Piedmont Regions 1. Richness and Abundance of: • Birds • Pollinators • Native Plants 2. Effects of Management programs on these three biota 3. Conversion of introduced (CSG) to native grasslands (WSG)
- 46. Wildlife WSG All Fields = 20 Acres Not hayed Not grazed New and/or Managed WSG Hayed or ≤ 2 yrs old Wildlife CSG Not hayed Not grazed Managed CSG Hayed or grazed
- 47. Citizen Science Birds Survey Protocol Techniques Field Identification Training Native Bees and Butterflies Surveys Protocol Techniques Ecology and Life History Identification Training Plants Survey Protocol Techniques Field and Microscope Identification
- 48. Fruit -eating birds Ground-nesting Under-story birds seedlings Non-mast seeds mesopredators Rodents Humans Ticks Gypsy moths Pathogens Mast-eating Pathogens birds DEER Mast Production Oak seedlings Oak Trees Fire Sunlight Soil Nutrients Soil Moisture
Editor's Notes
- #45: Networking (~2 slides) Who else we want to attract to our work – who we hope to attract Partners – each has their roleThroughlandowner participation and networking with contractors and state and non-governmental agenciesProviding education and community centered opportunities to discuss all aspects of conservation on working landscapes and those adjoining them.Research management strategies to determine the effectiveness, cost-benefit (environmental and economic), and bmp’s
- #46: We are attempting to determine best management practices and their costs, benefits (environmental and economic) and effectivness
- #47: Discuss that Farms come in all different sizes we standardized to 20 acres.Discuss the Four Management Types
- #48: 50+ Citizen Scientists trained by professional birders, botanists and pollinator ecologists. I d like to point out that while the pollinator surveyors need not bee professional amateur bee taxonomists we do require that amateur experts lead each bird and plant survey. Any citizen scientist that is not an expert assists the amateur expert until the feel comfortable in taking the lead.***In training Plant and Bird citizen scientists are simply being trained in running our specific protocolsWhich is different from the poll’s and then we rely on prof for ID’s Not enough expertise in poll to be able to …