Succession-Legacy Effects, Path Dependence

Ecological studies are often limited for practical reasons to single time snapshots or very short periods (Callahan 1984). However, the history of land-use in and surrounding UA is an undeniably important aspect of its ecology (Lane 2015, Crumley 2018, Isendahl 2018. Isendahl et al. 2018). The extensive use of impervious surface (e.g. concrete, asphalt) in urban areas is hypothesized to have strong, perhaps long-term effects on the space, form and functioning of UA (Rudd et al. 2002, Goddard et al. 2010, Martellozzo et al. 2014). Many urban soils are compacted, and contain contaminants dangerous to human health (De Kimpe and Morel 2000, Li et al. 2001). Brownfields represent a particularly extreme example of contaminated urban soils. These are sites that have a history of poor management practices leading to environmental contamination, which require significant remediation before usable as a city resource for human habitation or food production if converted to UA (Groffman and Tiedje 1988. Reddy Krishna R. et al. 1999, Browm and Jameton 2000, Pouyat et al. 2010, 2015). The study of brownfield sites can therefore be particularly useful for understanding legacy effects in urban soils more generally. Understanding how urban soils and their associated communities change over time and whether they can be managed to create safe, productive agricultural products are questions directly related to the concept of ecological succession.

Ecological succession considers the change in species composition of a community over time. First used by Cowles to describe the changes in vegetation structure and communities he observed along transects running inland from the shores of Lake Michigan, the concept of succession has since been used to explore the role of deterministic and stochastic processes in shaping communities (Cowles 1899, Clements 1916, Gleason 1926, Tansley 1935). Current research on ecological succession stresses the importance of stochasticity in conjunction with legacy effects, which can create a path dependence in the community structures that develop (Connell and Slatyer 1977, Bazzaz 1996). This means that the structure of a community is dependent on past processes including for example, the order of colonization events or how soils developed in the area (Fukami 2015). This notion is particularly relevant for UA where the history of urbanization is integral to its form (Isendahl 2018). However, few studies have explicitly looked at succession in UA systems to date (Table 12.1).

There is a rich literature on urban soils and the communities of microorganisms that inhabit them, though these studies are not typically conducted in UA (Craul 1999. De Kimpe and Morel 2000). Studies involving bio-remediation, the use of microorganisms to clean up contaminated sites, are highly relevant to UA but rarely discussed in conjunction with food production (Adhikari et al. 2004, Malik 2004. Irani et al. 2011). This is unfortunate since many UA sites occupy abandoned land (Taylor and Lovell 2012, Gardiner et al. 2014). The danger to public health in UA is widely acknowledged, yet some growers may not test soils before cultivation, or testing may not continue long-term (Brown and Jameton 2000,

Summary of Urban Agriculture Natural Science Methodologies

Target Taxon or Abiotic Factor	Sampling Method	Types of Questions	Pros	Cons	Special Considerations	Alternatives	References
Invertebrates General	Sweep netting	Abundance and diversity	Efficient method for sampling across taxonomic groups; can be non-lethal	Often destructive to vegetation;not selective in type of invertebrate collected			Vollhardt et al. 20081
	Targeted visual survey	Abundance and diversity	Low impact; good for quantifying abundance of uncommon species or species not attracted to traps	Only feasible for relatively large, obvious species (e.g. lady beetles, caterpillars, ants); high potential for observer bias			Edwards 2016
	Sticky traps	Abundance and diversity	Low effort; efficient for sampling range of flying arthropods ; especially useful for sampling adult parasitoids	Lethal; difficult to identify specimens to high taxonomic resolution; traps may be disturbed by garden/ farm users			Egerer et al. 2018b; Lowenstein & Minor 2018
Herbivores	Sentinel plants	Abundance and diversity; impacts of herbivory	Effective way to assess herbivory pressure while controlling for plant quality	Plants may require care	Requires permission to place plants in garden/farm;		Egerer et al. 2018a
Predators and Parasitoids	Sentinel pests	Abundance and diversity; biological control	Most accurate way to quantify predation pressure		must rear or buy pest species; requires placing food plant as well; requires permission		Lowenstein et al. 2017; Philpott & Bichier 2017
Ants (Formicidae)	Tuna baits	Diversity; competitive relationships	Low effort / Allows for non-lethal sampling / Easily adapted to small spaces	Baits may attract unwanted organisms or may be viewed as undesirable by gardeners/ farmers; sampling biased towards certain species			Edwards 2016; Uno et al. 2010
Ants (Formicidae)	Pitfall traps	Diversity and activity level	Samples broad taxonomic and functional group range	Traps may be disturbed by garden/farm users; requires disturbing soil			Edwards 2016

	Mini-Winklers	Abundance and diversity	Samples broad taxonomic and functional group range	High effort; involves collecting soil and/or litter			Savage et al. 2015*
	Tanglefoot exclusion	Effectiveness of biocontrol	Can clearly demonstrate effect of ants on pests	Sticky substance may be mildly annoying to garden/ farm users			Reimer et al 1993т; Morris et al. 2015+;
Pollinators (Hymenoptera, Diptera, Lepidoptera)	Pan/bowl traps	Abundance and diversity; diet and pollination potential (from pollen collected from bodies)	Low effort / Samples species often missed in netting	Lethal / Traps may be disturbed by garden/farm users; standard protocol requires access to large area			Matteson et al. 2008; Quistberg et al. 2016; Glaum et al. 2017
	Aerial netting	Abundance and diversity; diet and pollination potential (from capture location and/or pollen)	Complements pan/bowl traps; can be non-lethal; can catch flying bees; adaptable to any space	Can damage vegetation; biased towards larger-bodied bees		Ziplock bag 'net’	Quistberg et al. 2016
	Ziplock bag 'net'	Abundance and diversity; diet and pollination potential (from capture location and/or pollen)	Complements pan/bowl traps; can be non-lethal; not destructive of plants; adaptable to any space	Generally biased towards large-bodied bees ; can only catch bees when landed		Aerial netting	Fitch et al. 2019
	Sentinel plants	Pollination effectiveness; pollinator foraging behavior; competitive interactions	Allows estimation of pollinator visitation to particular species	Time intensive; only feasible for small number of plant species	Requires permission to place plants in garden/farm		Fitch 2017; Lowenstein et al. 2014
	Sentinel colonies	Pollinator diet; pollinator population dynamics; pollinator pathogen and parasite prevalence	Allows researcher to address a range of questions that are otherwise difficult to answer	Relatively expensive and difficult to maintain			Williams et al. 201 2t; Vaidya et al. 2018
	Trap nests	Abundance and diversity of trap-nesting bees; population dynamics; diet; pathogen and parasite prevalence	Allows researcher to address a range of questions that are otherwise difficult to answer	Often low rate of colonization			Maclvor et al. 2014; Maclvor and Packer 2015;
Soil invertebrates	Pitfall traps [see under Ants (Formicidae)]						Gardiner et al. 2014; Burkman & Gardiner 2015
Soil invertebrates	Mini-Winklers [see under Ants (Formicidae)l

(Continued)

Target Taxon or Abiotic Factor	Sampling Method	Types of Questions	Pros	Cons	Special Considerations	Alternatives	References
	Baermann funnels	Abundance and diversity	Useful for sampling nematodes	High effort; requires removing soil from site			Sliarma et al. 2015
	Sentinel pests [see under Predators and Parasitoids]						Yadav et al. 2012
Vertebrates General	Mesh exclosures / fencing	Effectiveness of vertebrate biocontrol; vertebrate herbivore effects	Can clearly demonstrate effect of particular guilds of organisms	High effort; often requires large amount of space and may interfere with use; may be disturbed by garden/farm users			Karp et al. 20l3t
Birds	Point counts	Abundance and diversity	Well-established protocol facilitates comparison across studies	Time-intensive; sensitive to researcher bias	Ability to identify birds by sight and sound needed		Rottenborn 1999; Paker et al. 2014
Birds	Mist-netting	Abundance and diversity; diet (from excrement); nutritional status and health; population genetics	Not sensitive to researcher bias; allows for collection of tissue samples	Requires expensive materials and training; highly obvious and potentially disruptive to other use of sites	Requires special permits		Evans et al. 2009; Hamer et al. 2012
Mammals	Live-trapping (Sherman and/or Longworth traps)	Abundance and diversity; diet (from excrement); nutritional status and health; population genetics			Requires special permits		Munshi-South and Kharchenko 20 Ю; Wilson et al. 2016
Mammals	Camera traps	Abundance and diversity	Allows assessment of occurence/abundance of uncommon and secretive species	Equipment expensive and likely to be tampered with			Widdows et al. 2015; Gallo et al. 2019
Plants General	Transect surveys	Abundance and diversity; spatial arrangement				Quadrat surveys
Plants General	Quadrat surveys	Abundance and diversity; spatial arrangement				Transect surveys	Ahrne et al. 2009;
Crop yield	Sentinel plants	Yield of particular crop type; response to treatment	High degree of control by researcher	Limited to a small number of plants and species			Potter & LeBuhn 2015; Bennett & Lovell 2019

	Gardencr/user data	Larger-scale yield data for one to many crops	Direct estimate of plot- or site-scale yield	Difficult to assess accuracy of data; requires training of and coordination with site users
Microbes	Soil microbe DNA extraction	Microbial diversity	Allows for characterization of soil microbial community	Sample processing can be costly		Yan et al. 2016* Wang et al. 2017*
Abiotic factors Climate and temperature	Data logger	Site-level temperature, humidity, insolation	Low-effort; fine-scale data	Can be expensive; data can be lost if loggers are tampered with or removed	Publically- available weather station data	Glaum et al. 2017
Abiotic factors Climate and temperature	Publically- available weather station data	Local temperature, humidity, precipitation	Low effort; free	Resolution poorer than data logger, esp. for sites in close proximity	Data logger
Soil characteristics	Soil physical analysis	Soil texture; soil aggregate structure; soil moisture content				Beniston et al. 2016; Pennisi et al. 2016
	Soil chemical analysis	Soil nutrient content (e.g. C. N, P); soil heavy metal content; soil pH	Accurate, precise estimates of soil properties; many labs will accept samples for processing	Requires specialized equipment or ability to pay for sample processing		Beniston et al. 2016; Harada et al. 2019
	Litterbags	Decomposition rates; role of soil biota in decomposition; nutrient fluxes	Relatively low effort; may not require expensive equipment	Requires leaving litterbags for an extended time		Pavao- Zuckerman & Coleman 2005*; Tresch et al. 2019
	Ion-exchange resin bags	Leaching rates for nutrients and heavy metals	Accurate estimates of leaching rates	Requires leaving resin bags for an extended time; requires some processing		Harada et al. 2018a; Harada et al. 2019
Atmosphere characteristics	Atmospheric particulate collectors	Deposition rates for atmospheric particulate matter (e.g. heavy metals)	Accurate estimates of deposition rates	Requires leaving (often large) collectors for an extended time; requires some processing		Harada et al. 2018a; Harada et al. 2019
Vegetation biochemistry	Vegetation biochemical analysis	Nutrient content of crop plants; heavy metal content of crop plants	Accurate estimates of nutrient and/or contaminant concentrations in crops	Requires specialized equipment or ability to pay for sample processing		Harada et al. 2018a; Pennisi et al. 2016

Study conducted in urban area but not UA.

' Study conducted in agricultural area but not UA.

De Kimpe and Morel 2000. Minca et al. 2013. Beniston et al. 2016). Ecological study of succession in UA could pull techniques from these fields to study the change in soil communities over time and still more, the potentially irreversible effects of past land-use choices on these communities (Huang et al. 1998, Fukami 2015, Ong and Vandermeer 2018, Vandermeer and Perfecto 2019). The idea of historical legacy is tied to the physical concept of hysteresis, where the path forwards is different from the path back (Mayergoyz 2012). All cities were built on top of some natural ecosystem at some point in history, and that land-use history is likely to have a strong deterministic effect on agricultural productivity (Bellemare et al. 2002). UA is unique however, in the extensive translocation of soils (Craul 1992). Many gardens are built from scratch, transporting organic material from a variety of distance sources including bogs, city composting facilities or other farming operations (Beniston and Lai 2012, Beniston et al. 2016). This process is expected to have important effects on the communities that assimilate in UA yet extracting the effects of soil and land-use histories on community composition requires more than observations of above and below ground biodiversity. Observational studies need to be combined with more experiments and theory in order to tease apart mechanisms of community assembly and its impact on ecosystem services in UA.

One potential avenue for exploring these questions utilizes UA’s use of both perennial and annual systems and techniques (Bernholt et al. 2009, Lin et al. 2017). UA can be managed to include both perennial (multi-season) and annual (single season) plants, though some sites or plots in allotment-style gardens may specialize on one or the other, depending on the use and extent of mechanical tilling at the site. Perennial sites or plots offer opportunities to study long-term impacts of UA on ecological processes especially when compared to annual sites or plots. Annual sites and plots are tilled every season to aerate soils and minimize weeds whereas perennial sites and plots refrain from disturbing soils. Mechanical tilling has dramatic effects on soil communities in rural agriculture systems, though the effects on UA are less well known (Giller et al. 1997, Hoflich et al. 1999). Perennial crops are woody, which commonly include fruit trees and bushes, while annual crops are herbaceous, presenting potential confounding issues for comparative studies. However, some perennial crops, including many herbs, are managed as annual crops and could be used to tease apart effects of soil disturbance on above and below ground community composition.

Succession-Legacy Effects, Path Dependence

Path dependence and “lock-in” – long-term effects of not using money

Path dependence

Path Dependency and Development Trends of Munich and Its Metropolitan Area

Between Multiple Transformations and Systemic Path Dependencies

Transformation and Path Dependencies in Fergana Valley's Post-Soviet Agriculture

Transformation and Path Dependencies in Fergana Valley's Post-Soviet Irrigation System

Path Dependency?

Convergence or Path Dependence?

Path Dependency Hypotheses