cervids density and the ecological integrity of forest
TRANSCRIPT
Cervids density and the ecological integrity of forest ecosystems: are deer impacts directly proportional to their density?
NSERC - Produitsforestiers AnticostiIndustrial ResearchChair
1. Cervids as global disturbance agents
Cervids are key components of forest ecosystems, yet, as their density increases, they also compromise forest integrity (Côté et al. 2004). Depending on the level of alteration imposed by cervids, forests may lose their capacity to absorb disturbance and to maintain their composition, structure and functions. This is a concern in ecosystems where cervids populations are uncoupled from their natural regulating factors such as density-dependent predation or competition for resources and now act as global agents of disturbance (Tremblay et al. Invited revision).
Understanding the functional relationships between forest ecosystems integrity and deer density is essential for ecological-based management of deer-forest systems. In our effort to achieve this, we:
• Investigated the relationships between the integrity of the forest ground layer and deer density
• Assessed white-tailed deer (Odocoileus virginianus) densities compatible with the conservation of the integrity of eastern balsam fi r (Abies balsamea) – white birch (Betula papyrifera) forests of Anticosti Island, Québec, Canada.
On Anticosti, a large introduced deer population has been exerting dramatic impacts on native plant communities which culminated with the conversion of balsam fi r stands to white spruce (Picea glauca; Potvin et al. 2003).
0 10 2030
4050
Time
Deer density(deer/km2)
low
Potentialrecovery paths
High integrity dynamic regime
Low integrity dynamic regimemaintained by browse tolerant/resistant species
Irreversiblethreshold
-
+
high
Leve
l of a
ltera
tion
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Grasses
Deer density (deer/km2)
Abo
vegr
ound
bio
mas
s (g
/m2 )
00 10 20 30 40 50
100
200
300
cutover 2004
forest understory 2004
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Abies balsamea
Deer density (deer/km2)
Abo
vegr
ound
bio
mas
s (g
/m2 )
0
2
4
6±1 se max
2002 2003 2004
cutvover 2004
cutover 2003
Epilobium angustifolium
00 10 20 30 40 500 10 20 30 40 50
10
20
30
40
cutover 2002
cutvover 2004
cutvover 2003
cutover forest understoryforest understory
Deer density
Pla
nt r
espo
nse
The classical linearsuccession hypothesis
Nonlinear, potentiallymultiple equilibriums,
hypotheses
x 3 replicated blocks x 3 years
10 ha0 deer/km2
40 ha x 3 deer7.5 deer/km2
20 ha x 3 deer15 deer/km2
In situ density (unfenced)27 to 56 deer/km2
cutover forest understory
6. Concluding remarks: an ecosystem-based management model inspired from the catastrophe theory (Scheffer et al. 2001)
2. Three types of functional relationships proposed as research hypotheses
3. A controlled browsing experiment designed to monitor functional
relationships between forest integrity and deer density
4. Results
Exponential decay functions link biomass of preferred seedlings and forbs to deer density in cutover
Exponential growth of grasses biomass as deer density increases in cutover
Negative sigmoid functions in the morphological responses of Maianthemum canadense to deer density in cutover
5. Summing up: several functional relationships within a simple deer-forest system
• Exponential decay functions dominate in cutover after 2-3 years• Usually no response thresholds or thresholds <7.5 deer/km2
• Impacts of deer on forest integrity at low density levels• Maximum impact reached at ≥ 15 deer/km2
• Positive response of browse tolerant (e.g. grasses) or resistant (e.g. white spruce) species• No short term response in forest understory
7. References cited
Côté, S. D., T. P. Rooney, J.-P. Tremblay, C. Dussault, and D. M. Waller. 2004. Ecological impacts of deer overabundance. Annual Review of Ecology, Evolution, and Systematics 35: 113-147.Potvin, F., P. Beaupré, and G. Laprise. 2003. The eradication of balsam fi r stands by white-tailed deer on Anticosti Island, Québec: a 150-year process. Ecoscience 10: 487-495.Scheffer, M., S. R. Carpenter, J. A. Foley, C. Folke, and B. Walker. 2001. Catastrophic shift in ecosystems. Nature 413: 591-596.Tremblay, J.-P., I. Thibault, C. Dussault, J. Huot, and S. Côté. 2005. Long-term changes in the availability of white-tailed deer (Odocoileus virginianus) winter forage in the boreal forest and the potential of litterfall as an alternate food source. Canadian Journal of Zoology: Invited revision.
The model implies that:• Deer act as a global disturbance slowly undermining forest resilience and may force
regime shifts• Browse tolerant or resistant plants gain a competitive advantage over preferred species
and contribute to maintain the system into an alternate dynamic forest regime through positive feedbacks
• Reduction of deer density >3 years after a canopy disturbance may not be suffi cient to restore forest integrity
• Restoration of altered deer-forest systems may require other direct intervention such as scarifi cation
The perspective of catastrophic regime shifts call for a precautionary approach in the management of deer-forest systems, an ecosystem-based perspective and better link between science and management (adaptive management).
Rapid inhibition of sexual reproduction as deer density increases in cutover
[email protected] Tremblay1,2,3, Jean Huot1,2,3 and François Potvin1,2,4
1NSERC – Produits forestiers Anticosti Industrial Research Chair, 2Département de biologie, Université Laval, 3Centre d’études nordiques, 4Ministère des Ressources naturelles et de la faune