Connectivity of green and blue infrastructures: living veins for biodiverse and healthy cities

Structural heterogeneity

What makes me enjoy certain gardens more than others?

This afternoon I was walking in some parks of Paris and as I was on my way homewards images came to my mind of two magazine articles that I had read who-knows-when with beautiful photographs of gardens, each by a different landscape designer. I couldn’t remember who either of them was, but with those strange associations that memory is made of, and the wonder that is internet search engines, I rapidly rediscovered both of them.

One was Miranda Brooks, the English landscape architect: The other was Chilean landscape architect Juan Grimm:

What I remembered liking about the images I saw of their gardens, and what I appreciate now as I learn more about them, is their use of structure and shape, texture, and colours. Both take advantage of the furry, jagged, mottled, frizzy, dappled, crumpled effects of plants allowed to grow in what resembles a natural way, massed together in burgeoning clouds, mixed together in apparently random coexistence. Various sizes, forms and textures converge artfully to resemble… something.

What exactly did I think they resembled? I decided that my eye has been trained by my experience researching community ecology and degradation. There is no good general definition of degradation, so it has become a “you know it when you see it” phenomenon. I am not even sure that degradation is any one phenomenon, but rather a whole bunch of different things that are possibly not related. One thing that looks degraded to me, though, is some kind of habitat where all the plants are only of one or two sizes and shapes. This suggests that something has gone wrong at some point— it suggests that the main species have not been able to reproduce for a long time, or the propagules for the next successional phase have not been able to arrive, or some invasive plant has out-competed all the other ones, or a high level of some kind of disturbance or an unusual perturbation has set environmental conditions to a state excluding most of the local biodiversity. Normally, there should be small trees waiting under the canopy of the tall trees to grow taller; there should be baby shrubs, medium shrubs and big senescent shrubs; or a variety of grass forms and heights. Geographical and geological heterogeneity—big rocks, narrow valleys, streams—add to ecological diversity by adding many microhabitats and niches. I am delighted by a structurally diverse habitat, even if I have to kneel down in the grass to see the diversity, and somehow made uncomfortable by a monotonous one.

This is what I react to when I see gardens like those of Miranda Brooks and Juan Grimm. Even though they are gardens, they remind me of a complex, diverse habitat that is growing, changing and moving. This is equally what I think Eliot Porter captured so beautifully in his nature photography:

This notion of structural heterogeneity and its links to biodiversity have not escaped ecological study. Some researchers have discussed how irregularly shaped and slowly changing structures, like isolated trees, decaying logs, or coral reefs, form key habitats and niches. And these issues have also been examined in urban parks, a little bit. Larger urban trees have been associated with greater bird diversity. Pedro Pinho, from the Lisbon team, is lead author on a paper showing that vegetation density and functional diversity (the diversity of different ecological roles or strategies of the plants) are related to urban forest biodiversity (Pinho et al. 2016). Most research on ecological heterogeneity, however, only asks about the number and kinds of habitat patches, not the structural diversity within those habitats. In fact, the idea of 3-D structural heterogeneity is largely assumed to be a qualitatively rich but quantitatively intractable “natural history” issue, and there is relatively little work on it compared to 2-D “patch” concepts of heterogeneity, which are highly amenable to modelling and GIS (geographical information system, using images from satellites) analysis. Structural heterogeneity forms, as Byrne (2007) suggests, the scenery or backdrop to the “action” of ecology, and is thus often taken for granted. It seems, however, to be an obvious object of enquiry in urban parks as well as natural areas under different environmental conditions or management regimes. Advances in LiDAR (laser imaging) technology and analysis (which remains expensive) are one way to make this topic tractably quantifiable. For example, researchers have used LiDAR to predict locations of optimal spider habitat in forests.

Research within BIOVEINS also investigates whether vegetation structural heterogeneity in parks provides more ecosystem services. As Cadenasso et al. (2007) propose in their assessment scheme for urban habitats, “The type of vegetation, surface material and buildings are hypothesized to influence ecosystem function because of their differential influence on the amount and distribution of organisms, materials and energy.” Do these ecosystem functions produce more ecosystem services available to users in those or other habitats or land-cover areas? That is, does a greater diversity of structures, shapes and sizes translate into a habitat (or park) that is more active, undergoes more ecological processes, or exports a greater flux of processes? This question reflects a hunch I also have, that habitats undergoing succession, or that could do so even if currently kept in stasis, are able to export or subsidize ecosystem processes and services in the surrounding landscape (see my paper Root-Bernstein & Jaksic 2015). But while successional processes will usually create structural heterogeneity, I am not sure whether artificial creation of structural heterogeneity in a park or garden also exports ecosystem services. This is because I am not sure if functional diversity or heterogeneity per se can give birth to ecosystem processes, or whether vice versa, ecosystem processes are the origin of diversity and heterogeneity: or anyway if the ecosystem services in these two cases would both be of the same kind and magnitude. These kinds of snake-biting-its-own-tail issues are at the heart of applied ecology.

Tews, J., Brose, U., Grimm, V., Tielbörger, K., Wichmann, M. C., Schwager, M., & Jeltsch, F. (2004). Animal species diversity driven by habitat heterogeneity/diversity: the importance of keystone structures. Journal of biogeography, 31(1), 79-92.
Buhl‐Mortensen, L., Vanreusel, A., Gooday, A. J., Levin, L. A., Priede, I. G., Buhl‐Mortensen, P., ... & Raes, M. (2010). Biological structures as a source of habitat heterogeneity and biodiversity on the deep ocean margins. Marine Ecology, 31(1), 21-50.
Manning, A. D., Fischer, J., & Lindenmayer, D. B. (2006). Scattered trees are keystone structures–implications for conservation. Biological conservation, 132(3), 311-321.
Stagoll, K., Lindenmayer, D. B., Knight, E., Fischer, J., & Manning, A. D. (2011). Large trees are keystone structures in urban parks.
Byrne, L. B. (2007). Habitat structure: a fundamental concept and framework for urban soil ecology. Urban Ecosystems, 10(3), 255-274.
Bell SS, McCoy ED, Mushinsky HR (1990) Habitat structure: the physical arrangement of objects in space. Chapman & Hall, London
Lovett GM, Jones CG, Turner MG, Weathers KC (eds) (2005) Ecosystem function in heterogeneous landscapes. Springer, Berlin Heidelberg New York
Martinuzzi, S., Vierling, L. A., Gould, W. A., Falkowski, M. J., Evans, J. S., Hudak, A. T., & Vierling, K. T. (2009). Mapping snags and understory shrubs for a LiDAR-based assessment of wildlife habitat suitability. Remote Sensing of Environment, 113(12), 2533-2546.
Vierling, K. T., Bässler, C., Brandl, R., Vierling, L. A., Weiß, I., & Müller, J. (2011). Spinning a laser web: predicting spider distributions using LiDAR. Ecological Applications, 21(2), 577-588.
Pinho, P., Correia, O., Lecoq, M., Munzi, S., Vasconcelos, S., Gonçalves, P., ... & Lopes, N. (2016). Evaluating green infrastructure in urban environments using a multi-taxa and functional diversity approach. Environmental research, 147, 601-610.
Cadenasso, M. L., Pickett, S. T., & Schwarz, K. (2007). Spatial Heterogeneity in Urban Ecosystems: Reconceptualizing Land Cover and a Framework for Classification. Frontiers in Ecology and the Environment, 80-88.
Root-Bernstein, M., & Jaksic, F. 2015. Ecosystem process interactions between central Chilean habitats. Global Ecology and Conservation (GECCO) 3, 776-788.

images: Interview with Juan Grimm at his private garden, Apartamento issue 21.

--Meredith Root-Bernstein, 7 February 2019

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