What's the role of ornamental and cultivated garden plants in cities?
Updated: Jul 30, 2020
Native species are often recommended as the best choice for ecologically sound outcomes of greening projects. But the truth is, the ecology of plants that have been hybridized or bred through "artificial" selection is far less studied than the “naturally” occurring species that are the product of natural selection. So how much do we really know about the environmental impacts and ecological roles of cultivated and domesticated plants, including common garden plants?
In my opinion, we don’t know very much – at least in an urban context. Given that horticulturally bred plants (sometimes known as cultigens) dominate urban greenspaces, this is a major gap in the science of urban greening.
There’s a lot to explore to understand the possible roles and value of cultigens vs. naturally occurring plants in urban green spaces. Today, I’m just going to tackle one small part of this issue: How can we understand the ecology of cultivated organisms?
This post is a non-technical discussion of a forthcoming paper in Philosophical Topics - my first publication in a philosophy journal! Many thanks to the special issue editor Stefan Linquist for helping me articulate my ideas to philosophers of ecology.
If you’re not a philosopher and just want to explore issues in urban greening, let’s dive in.
To keep this manageable, I’m going to break up the discussion into two posts. In this one I’ll summarize some of the common approaches for studying the ecology of cultigens, and why I think each has some major limitations. In my next post I’ll highlight some methods that are less commonly used in ecology, but could be very promising for understanding the role of cultigens in cities and greenspaces.
Option 1: Study cultigens in exactly the same way as organisms that are the product of natural selection.
We could make no distinction at all between cultigens and natural species. This is the approach advocated by philosopher Mark Sagoff. I think Sagoff’s overall analysis of the role of cultivated organisms is correct – even though they occupy a tiny space in the ecological literature, there really aren’t any great justifications for excluding cultigens from the study of the earth’s ecosystems. Sagoff points out that ecology is a peculiar discipline in its preference for studying “natural” vs. human-created organisms and ecosystems, and that few other sciences work this way. Physiologists, for example, make no distinction between the photosynthesis of a natural occurring plant vs. an artificially selected corn cultivar.
Ecologists absolutely should study cultivated landscapes and the organisms they contain, but I don’t agree with Sagoff that ignoring the human origins of these systems is a useful approach. It happens – there are many ecological studies that overlook human imprints on the landscape either past or present, and consider only the non-human processes that have shaped ecosystems.
But ecology is not just the study of ecosystems as they are now - it’s also the study of how ecosystems came to be. Community assembly, for example, is an important aspect of ecology that refers to the process of how particular groups of organisms came to co-exist. In ecosystem science, my own sub-discipline, we look for causal explanations of the flows and cycles of energy, water, carbon, and other materials through ecosystems, and these explanations often rely on the history of interactions between organisms, or between organisms and their environment.
When those organisms include humans, knowledge about human actions, including how and why those actions occurred, might be very important in answering ecological questions. So studying cultigens might be fundamentally different than studying natural selection: for cultigens, there is specific information we need about humans, human activities, and human agency that may not be as important for non-cultigens.
Option 2: Coupled natural-human systems (CNH)
Urban ecologists and many other scientists have agreed that understanding various human dynamics is very informative in modern ecology. A common solution to this problem is to use a binary model that depicts ecosystems as having two distinguishable but interlocking parts: a “natural” component driven by non-human processes, and an identifiably different component driven by social, economic, cultural, and political processes.
This approach was used by the National Science Foundation in their former CNH funding program, originally part of the Biocomplexity in the Environment solicitation, and eventually becoming a stand-alone program. Over the years NSF funded many interdisciplinary teams of researchers from various natural sciences, including ecology, and social sciences. (Full disclosure: my first major grant was funded by this program around 2001).
In these projects, natural scientists would study “their” ecosystem component using the standard methods from their field, and social scientists would study the human component. Ultimately, these teams would try and find ways to integrate their results to some extent, often with some kind of computational model, or two separate computational models that would be mathematically connected to each other.
The conceptual diagram of how this is supposed to work looked like this:
Given the many studies that used this approach (including my own), we must have learned quite a lot about cultigens, right? Unfortunately, the CNH model is very difficult to apply to these organisms.
Take one of the most common cultigens in the world: Kentucky bluegrass (Poa pratensis). This species has lived with and been modified by humans for so long, no one really knows where it’s originally from and whether and when it was imported to North America (i.e. it’s probably not from Kentucky). It’s now a ubiquitous lawn grass and the dominant plant species in many cities.
What do you think – is a lawn natural? Or human created? Does it belong on the left or right side of the CNH diagram? In that kind of binary model, the individual parts of ecosystems have to be broken down into either a human sub-system or a natural one.
Eventually, many scientists pointed out that separating between the natural and human parts of landscapes is extremely difficult, and possibly even counter-productive in the quest to understand how cultivated ecosystems work.
To study the way lawns and people interact, does it really matter whether Kentucky bluegrass goes in the natural or the human system? I don’t think so. Instead, we might look at some more integrated models for studying cultigens and their role in cities and ecosystems.
Option 3: Ecosystem Services (ES)
Most scientists would probably call ES an object of scientific study and not a method. But I think it’s also a way of conceptualizing some of the interactions between people and nature, and a possible way to study cultigens.
ES are the benefits that people derive from nature. There are many different types of ES and different ways of categorizing them, but for our purposes, if we were to ask the question: why do people breed and cultivate plants? To say that it’s because these plants provide people with benefits would be a very reasonable answer.
This suggests that explanations for the various features of cultigens – their morphology (e.g. the size, shape, and color of leaves, flowers, and fruits), physiology (growth rates, water use, and nutrient requirements), and ecology (e.g. how they interact with other species like pollinators) might be found in their benefits for people. Cultigens are probably bred to maximize these benefits, so studying ES in our quest to understand cultigen ecology makes sense.
In a way, ES is a more integrated model than CNH for studying organisms that are strongly influenced by artificial selection. The ES concept doesn’t distinguish between natural and human features of ecosystems, and it also provides a means of comparing cultivated and non-cultivated systems. As a method, it prompts researchers to quantify various benefits (and costs) of different types of plants, ecosystems, and landscapes.
But having said all that, ES is still a limited model for fully understanding the ecology of cultigens. As many have pointed out, some ecosystem benefits remain stubbornly difficult to quantify. And more broadly, categorizing human-nature interactions as either a “cost” or a “benefit” can be very limiting. Are these the only ways that people interact with nature: as a transaction that results in a cost or a benefit?
Going back to the example of turfgrasses, people (especially Americans) put enormous care into lawns, including significant time, money, and resources. Why? Do they get some benefits in exchange for this transaction? Do lawns provide ecosystem services? Yes, but I don’t think these services entirely explain the intricate historical, cultural, and human-plant dynamics around lawn care and cultivation in cities.
In my next post I’ll highlight some factors that might provide a more complete, and ecological, explanation for the role of cultigens in greenspace - and why these explanations are essential for a more comprehensive science of urban greening.