Deer density and plant palatability predict shrub cover, richness, diversity and aboriginal food value in a North American archipelago

Abstract

Aim Trophic cascades are a common consequence of herbivore outbreak and in the absence of hunting can cause the local extinction of native plant species and communities. We compared plant communities at 66 island and mainland sites to test the hypothesis that deer (Cervidae) determine species cover, richness and diversity and that palatable species become rare at high deer density. We validate a region-wide index of deer density and impact on plant communities in a region where culturally significant food plants maintained by aboriginal people prior to European contact helped to define baseline plant communities.Location Gulf and San Juan Island archipelagos and North American mainland.Methods We conducted surveys of 49 native, 10 exotic and 15 culturally significant plant species and deer sign at 66 sites on 35 islands and mainland to determine deer abundance and plant species cover, richness and diversity. We identified culturally significant food plants facilitated by aboriginal people using ethnobotanical knowledge, quantified plant palatability via cafeteria trials and characterized shrub architecture.Results Native and culturally significant shrub cover, richness and diversity were 52-85% lower at sites with abundant deer (0.9-2.8 ha À1 ) versus no deer. However, these values were also 38-82% lower at sites in the lowest deer density class (< 0.08 ha À1 ) versus sites with no deer present. Palatable cover was 92% lower where deer were abundant versus absent and 28% lower in low-density versus deer-free sites. Shrub architecture provided an easily applied index of native and culturally significant plant cover and deer density.Main conclusions We provide comparative examples of endangered plant communities to demonstrate that, contrary to the intermediate disturbance hypothesis, any positive effect of deer on plant diversity on islands in the Pacific north-west of North America occurs at densities < 0.08 ha À1 , if at all. This detailed example of trophic downgrading highlights the need and provides the methods to identify herbivore densities compatible with the persistence of all native species in conserved landscapes.

When anthropogenic‐related disturbances overwhelm demographic persistence mechanisms

“…Densities of vertebrate herbivores in our system appear to be at historically unprecedented levels (MacDougall ; Arcese et al . ), which is a commonly observed response to the loss of apex predators in terrestrial and aquatic systems (e.g. Jackson et al .…”

“…In this region, negative impacts on trees recruitment by deer browsing have been described at densities of >0.8–4 individuals ha −1 ; estimated densities are currently >15 individuals ha −1 (Gonzales & Arcese ; Arcese et al . ). Although we lack exact data on deer densities, experimental exclosures at our study site have shown that native shrubs known to be highly sensitive to deer browsing have difficulty recruiting (e.g.…”

“…; Arcese et al . ), with oak recruitment known to be sensitive to herbivory by vertebrates and insects, grass competition and drought (Fuchs, Krannitz & Harestad ; MacDougall, Duwyn & Jones ). As is often common with declining populations, however, the demographics of rarity for this species are unknown.…”

Tax Shifting and Incentives for Biodiversity Conservation on Private Lands

“…Prior to European colonization the CDF occurred as uneven‐aged forest (often >300 years old) dissected by shallow and deep soil meadows and woodlands (Meidinger & Pojar ) and was maintained in part by aboriginal land management practices designed to enhance hunting opportunities and root and fruit harvests (Arcese et al . ). The region also constitutes one of the most desirable places to live in Canada, which is reflected by high real estate values, averaging $5.2m (Canadian) per hectare, but still including lower‐valued parcels in rural areas (8% of parcels are valued less than $247,000 per hectare, assessment year: 2011).…”

Direct and Indirect Interactions between Landscape Structure and Invasive or Overabundant Species

“…Many ecosystem engineers are underrecognized for their roles. For example, introduced mammals on islands are well known for their disastrous effects as predators, but many also are ecosystem engineers, as demonstrated for nutria (Myocastor coypus), beaver, gray squirrel (Scurius carolinensis) [36] and deer [79]. Non-native invasive bivalves, such as zebra mussels, also act as aquatic ecosystem engineers, altering the structure and function of within-lake landscapes [80].…”