Eagleridge:
the ecology of a rare urban rock outcrop

by Gary Lewis B.Sc.

During the late spring of 2000 I carried out research on the vegetation and ecology of a rock outcrop at Eagleridge, North Vancouver. My project was designed to fulfill the requirements of my undergraduate thesis project at UBC and also to support the work of Kella Sadler, a Ph.D. student at UBC studying the rock outcrops of the Georgia Basin. My comparatively small project presents many interesting findings about the ecology of Eagleridge and rock outcrops in general, and underlines their importance as rare ecosystems in BC. The completion of Kella's work - the first study of its kind in our region - will provide us with further insights into rock outcrop ecology.

Despite the prevalence of rock outcrops in the northwest of North America (BC, Washington and Oregon), these ecosystems have rarely been studied in our region. The lack of scientific attention paid to these ecosystems is, however, unrelated to their importance. These rare habitats contain rare species and support uncommon vegetation associations.

Rock features in the landscape, such as cliffs, ridges, inselbergs and outcrops, are some of the most rigorous habitats occupied by plants. Outcrop plant species must generally contend with microenvironmental conditions which are much more extreme than surrounding habitats. These various stresses include shallow, poorly developed soil, full exposure to the sun, high evaporation rates, elevated summer temperatures, extreme summer drought, wide diurnal temperatures, exposure to the elements, vertical substrates, rapid runoff of rainfall, and alternately wet and dry seasonal conditions. Rock features also present unfavourable conditions for colonization due to long dry summers, high temperatures and limited suitable substrates.

The various conditions which characterize rock outcrops are set up by their topography which might include single or multiple domes, rock pavements, fugitive outcrops (rock just beneath a shallow soil layer) and tumulus or boulders. Each feature could exist at diverse scales from one metre squared to the size of a mountain.

Some major findings of previous research, especially from Australia and the southeastern US, include: 1. Soil depth appears to be a major factor influencing the diversity of outcrop vegetation. Greater soil depth often results in greater species diversity. 2. Slope steepness, due to its impacts on the variables of soil depth and water and nutrient availability, is also an important variable influencing plant diversity and distribution on outcrops. 3. The degree of heterogeneity of the outcrop - its various rock configurations - is thought to control species diversity and to determine the complexity of the ecosystem. 4. Outcrops often support an "azonal vegetation" - a species assemblage distinct from the surrounding vegetation. 5. The major processes driving succession, the ecological process that begins with bare soil, lichens and mosses and ends with forests, is thought to be weathering and decomposition of rock.

Eagleridge

Eagleridge lies at the urban fringe of West Vancouver along the Upper Levels Highway and within the Coastal Douglas-fir (CDF) biogeoclimatic zone. The Gulf Islands and southeastern Vancouver Island contain the largest components of the CDF which occupies a mere 0.3% of the province. A very small percentage of this figure is occupied by Garry oak and rock outcrop ecosystems. In the Lower Mainland the CDF is restricted to West Vancouver and to Richmond, Delta, Ladner and Surrey. Due to this distribution and the lack of outcrops in the Fraser delta municipalities, the outcrop complex at Eagleridge and the smaller outcrops in adjacent Lighthouse and Whytecliff Parks and above the Horseshoe Bay Ferry Terminal are most likely the only CDF rock outcrops in the Lower Mainland.

Eagleridge contains a large complex of rock outcrops of diverse sizes which are scattered amongst a matrix of douglas-fir (Pseudotsuga menziesii) and arbutus (Arbutus menziesii) forest. Aerial photographs from 1926 to 1996 show remarkable stability of outcrop boundaries over this time period. The 1926 photo clearly shows that prior to the construction of the Upper Levels Highway an outcrop of nearly one hectare was present just below the current complex.

Species diversity, soil depth and slope steepness

I found soil depth and slope steepness to be strong variables affecting species diversity at Eagleridge. Total species numbers (vascular plants and mosses) and total vascular plant numbers increased with greater soil depth but decreased as slopes became steeper. Mosses alone showed the opposite trend with maximum diversity on fairly steep, soilless surfaces. Soil depth and the related variables of water and nutrient availability, and soil organic content will control the number of species capable of surviving on an outcrop. The results also underline the dominance (and importance) of mosses early in succession when soil accumulation is negligible. As soil depth increases, vascular plants appear to out compete mosses.

Microhabitats and plant assemblages

Various multivariate computer techniques happily crunched the data and confirm the presence of distinct plant assemblages on the outcrop each related to different aspects of the microtopography. Gravel (mixed with soil) on relatively flat areas supports a diverse assemblage whose major species are Zygadenus venenosus (death camas), Prunella vulgaris (self-heal), Eriophyllum lanatum (Oregon sunshine), Cryptogramma crispa (parsley fern) and grasses. Bench areas, relatively flat areas with soil accumulation, support Racomitrium canescens (roadside rock moss), Eriophyllum lanatum and grasses. Rock surfaces with active water seepage support a Bryum miniatum (red bryum), Racomitrium heterostichum and Selaginella association. Steep rock surfaces support Racomitrium lanuginosum, Racomitrium canescens, Polytrichum juniperinum (juniper haircap moss) and Selaginella. More moderately sloped rock areas are covered by Racomitrium canescens, Polytrichum piliferum (awned haircap moss), Selaginella and grasses.

Each outcrop microhabitat supports a distinct grouping of plants. While some species occur on various microhabitats such as the moss Racomitrium canescens, other species occur in only one microhabitat, such as Mimulus guttatus (monkey flower) which occurred only on gravel. Other species occur almost entirely in one microhabitat such as Cryptogramma crispa (parsley fern) which occurred 88% of the time on gravel. Diversity on the outcrop appears to be highly controlled by the diversity of the microtopography of the rock - the greater the heterogeneity of the outcrop, the greater the number of distinct microhabitats, plant assemblages and species that will occur in a given area.

A comparison with the adjacent forest shows nearly double the species diversity on the outcrop as under the canopy of Douglas-fir and arbutus trees which surround it - 76 species of vascular plants and mosses on the outcrop versus 43 in the forest. Of all forest and outcrop species recorded only seven percent are shared between each habitat. These findings suggest that outcrops are highly diverse systems possessing a significant azonal vegetation. It is also important to note that 17% of Eagleridge's outcrop species are moderate to well-established introduced weeds while only five percent of the forest species are exotic and these are uncommon and poorly established. The outcrop appears to be much more susceptible to invasion than its surrounding forest.

The outcrop complex at Eagleridge represents a unique and diverse ecosystem in the Lower Mainland and in British Columbia. The diverse topographic and environmental characteristics of these outcrops maintain rare vegetation associations and rare species. The ecological processes which occur on outcrops are also unique. Successful maintenance of biodiversity within BC will depend on the protection of rock outcrops. The Eagleridge outcrop complex is unique in the Lower Mainland and should be protected from further destruction and degradation from development and other human activities.

Image © copyright Gary Lewis

Click image for a larger view