When returning to sites that we restored it was clear that in some of them the plants had survived but they were not forming communities. We were essentially landscaping with native plants rather than restoring natural areas. This raised the question of was a planting a functioning system and when was it just ornamental. On reflection it was clear that a functional community had an identity and could maintain itself in the face of disturbance. In other words, it was resilient. I explored a number of approaches to identify this threshold which I am presenting here.
Sun et al. (2014) looked at ecological memory in community development and discovered that in a terrestrial forest ecosystem, in order to move from disturbance to a grassland, the first stage of succession, 46.5% of ecological memory needed to be in place.
Bauer and Loeffelholtz (2004) concluded that an impervious surface area of more than 60% in an urban watershed did not support a community and the stream was merely drainage. There had to be at least 40% permeable surface to maintain some kind of water quality.
A functioning community implies that there are healthy symbiotic relationships between species to maintain some balance. Schaefer and Bocking (2014) looked at soil microarthropods as grazers of mycorrhizae, the assumption being that the mycorrhizae would not be established in an ornamental landscape. They discovered a threshold where there were significantly more of these grazing microarthropods, especially mites, in natural areas compared to urban and transitional sites.
Urban Habitat Quality Index
In order to more comprehensively assess habitat along an urban-natural area gradient to identify a resilience threshold I developed an Urban Habitat Quality Index where I identified 15 parameters that were each qualitatively rated on a Likert Scale for 0-5 (Schaefer 2018). The study was done in Oxford and the results are shown in the table below. Aston’s Eyot was the most disturbed, most urban site and the other sites represented a gradient with Wytham Woods being the most natural. Expressed as a percentage, there appeared to be a threshold of 40% between the most urban, ornamental sites and the transitional and natural sites.
|Habitat/ Indicator||Burgess Park 8.5 ha||Aston’s Eyot
|Port Meadow 120 ha||Magdalen College
|5. Patch Size||0||1||3||2||5|
|7. Habitat Diversity||3||3||4||5||5|
|9. Ponds and Marshes||0||0||5||2||1|
|10. Woodlands >5 ha||1||0||5||5||5|
|12. Invasive Species||3||2||4||4||5|
|13. Intensity of use||1||0||0||2||5|
|14. Age >60 years||0||0||5||5||5|
|Urban Habitat Quality Index = Total / 75||29||30||47||52||68|
|Index as a Percent of Total Perfect Score of 75||39||40||63||69||91|
A minimum requirement of 40% ecosystem function for ecological resilience was similarly found by Cantarello et al. (2017) in a temperate forest landscape.
The above examples would seem to indicate that there is a threshold between an urban habitat that is merely ornamental compared to one that is functional. That threshold is at about 40% functionality and this represents the point at which a community has become resilient.
Bauer, M.E. and B. Loeffelholz. 2004. Estimation, mapping and change analysis of impervious surface area by Landsat remote sensing. In: ASPRS Annual Conference. Proceedings:23–28 May 2004, Denver, CO
Cantarello, E.; Newton, A.C.; Martin, P.A.; Evans, P.M.; Gosal, A.; and M..S. Lucash, 2017. Quantifying resilience of multiple ecosystem services and biodiversity in a temperate forest landscape. Ecol. Evol. 7:9661–9675.
Schaefer, V.H. 2018. A qualitative assessment of urban ecosystem resilience using a habitat quality index. Society for Ecological Restoration Conference: Restoration for Resilience. Burnaby, BC. Proceedings. p. 16-17
Schaefer, V. and M. Bocking. 2015. Detecting the Threshold between Ornamental Landscapes and Functional Ecological Communities: Soil Microarthropods as Indicator Species. Urban Ecosystems 18:1071-1080.
Sun, Z., Ren, H., Schaefer, V., Lu, H., Wang, J., Li, L. and N. Liu. 2014. Using ecological memory as an indicator to monitor the ecological restoration of four forest plantations in subtropical China. Environmental Monitoring and Assessment 8229-8247.