Deer: Beautiful, destructive and driving evolutionary change

deerFor many individuals, seeing deer in the wild provides joy and a sense of connection to nature. However, deer also cause hundreds of millions of dollars in damage to gardeners, farmers and insurance companies and dramatically change forest and prairie ecosystems
through trampling, browsing and grazing. In environments where deer numbers increase beyond the environment’s capacity to support them (carrying capacity), which can occur in the absence of native predators or human hunting, deer often starve, suffer increased disease and rarely raise offspring to maturity. Given these outcomes, and despite their beauty, deer populations may often require stewardship to maintain ecosystem health and ensure the persistence of diverse communities of all native species.

Cora Skaien, a PhD student working with Dr Peter Arcese (FRBC Chair in Applied Conservation Biology, Department of Forest and Conservation Sciences), is studying how deer impact species in threatened Garry oak and maritime meadow ecosystems in Southwestern British Columbia. This ecosystem has been reduced to 5% of its original extent at the time of European settlement, making its conservation a key goal of local, provincial and federal recovery planners. Habitat loss, fragmentation and degradation are key threats to oak and maritime meadows, arising through the cessation of aboriginal burning (mid-1800s), conversion of meadows to agriculture and housing, the invasion of exotic forbs, grasses and shrubs, and extirpation of native predators. Without predators and with the decline or prohibition of hunting over much of the region, deer populations now reach densities as high as 170 deer / km2 on some of the Southern Gulf Islands, or nearly 20 times the densities likely to have occurred under First Nations land management. As a consequence, many forest and meadow communities have lost most of their native understory and herbaceous species, which have been replaced by unpalatable native grasses, forbs and shrubs.

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On Sidney Island, BC, Cora and Dr Arcese worked with local land owners and the Islands Trust to establish two 750 m2 exclosures to investigate deer impacts experimentally. The exclosures prevented deer from accessing a degraded maritime meadow habitat dominated by exotic grass and herbaceous species, but also contained many iconic natives at very low densities. After only 3 years of protection, the
cover of natives inside the exclosures roughly doubled despite no change outside. And strikingly, the cover of 2 Brodiaea species, iconic meadow species promoted by coastal First Nations peoples as a food plant, was 4 times higher inside exclosures than outside and also flowered extensively inside exclosures. Conversely, there was no change in exotic species cover despite widespread belief that exotics will thrive and outcompete natives in these critically endangered shallow-meadows if the top-down control of herbivores is removed. These results offer clear suggestions to managers that reducing deer densities, and thus deer herbivory, prior to attempting exotic species removal is an effective management strategy when endeavoring to restore degraded Garry oak and maritime meadow ecosystems.

garyoakCora and Dr Arcese have also used these exclosures to study the role deer play in the evolution of Garry oak species, focusing on seablush (Plectritis congesta), a ‘winter annual’ that is highly susceptible to herbivory in winter and spring. With its bright green leaves and large pink inflorescences, seablush is a striking plant that welcomes wildflower lovers to Garry oak and maritime meadows each spring, but which differs dramatically in appearance and stature on islands with or without deer. Specifically, on Southern Gulf Islands without deer, seablush can reach 100 cm in height, with branches originating up to 4 cm or higher above the ground, and express `winged` seeds on ~90% of plants. In contrast, on islands with deer, plants tend to be short (~15 cm), with branches low to the ground (< 1 cm) and express wingless seeds (~90% of plants). On many islands with high deer densities, seablush is no longer present despite an historical occurrence. These observations led Cora and Dr Arcese to ask why these differences existed and whether deer were responsible.

To find out, Cora and Dr Arcese surveyed >300 sites from 2005-2014. They observed patterns throughout the Georgia Basin to discover that the observations made in the Southern Gulf Islands remained true; on small islands without deer seablush created beautiful pink and green mosaics of plants reaching up to 1 m tall, and on the mainland or islands with over-abundant deer, seablush was typically absent or represented by tiny plants missed by the average observer. Cora and Dr Arcese then collected seed from plants on each of 12 islands, 6 with and 6 without resident deer, and grew them in and outside the Sidney Island exclosures. This experiment allowed them to compare survival of plants with different morphological characteristics in contrasting environments with respect to deer. They found that plant survival was dramatically reduced in the presence of deer, but also that plants originating from islands without deer suffered a much larger decline in survival (23% survival relative to plants inside exclosures) than plants originating from islands with deer (56% survival relative to plants inside exclosures). Remarkably, plants exposed to deer only survived when less than 10 cm tall, whereas those inside exclosures grew to 80 cm and produced 5-10 times more seeds per plant on average.

To better inform conservation planning for seablush and other species iconic to the Garry oak and maritime meadow ecosystem, it is important to understand that deer are influencing native populations through preventing many native species from flourishing. It is also important to remember that we have created an unnatural environment where deer densities are far greater than they were historically due to predator extirpation and prohibition of deer hunting. Thus, we encourage land owners in southwestern British Columbia to fence remnant Garry oak and maritime meadow patches on their properties to prevent access by deer, plant historically present and culturally significant species (eg Camas, seablush, Brodiaea, chocolate lily, etc) within fenced areas, and to avoid increasing deer densities via feeding deer.

The relationship between deer and seablush also provides an excellent novel system in which to study evolutionary change in patchy environments. Deer have acted as a selective pressure to cause evolutionary change in the physical appearance of plants in seablush populations, resulting in smaller and less obvious seablush plants in areas where deer are present. Moving forward, Cora will investigate the heritability and underlying genetic architecture of the plant traits discussed in this article to further elucidate on how deer have acted as a selective pressure to shape plant morphology in seablush, and to address the management question of how to best re-establish seablush populations in habitats with and without deer.