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

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

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*

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*

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

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

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.

 
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