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Article
Citizen Science as a Tool in Biological Recording—A
Case Study of Ailanthus altissima (Mill.) Swingle
Barbara Sladonja ID and Danijela Poljuha * ID
Institute of Agriculture and Tourism, Karla Huguesa 8, 52440 Poreˇc, Croatia; barbara@iptpo.hr
*Correspondence: danijela@iptpo.hr; Tel.: +385-52-408-336
Received: 24 November 2017; Accepted: 10 January 2018; Published: 13 January 2018
Abstract:
Non-native invasive species frequently appear in urban and non-urban ecosystems and
may become a threat to biodiversity. Some of these newcomers are introduced accidentally, and others
are introduced through a sequence of events caused by conscious human decisions. Involving the
general public in biodiversity preservation activities could prevent the negative consequences of
these actions. Accurate and reliable data collecting is the first step in invasive species management,
and citizen science can be a useful tool to collect data and engage the public in science. We present
a case study of biological recording of tree of heaven (Ailanthus altissima (Mill.) Swingle) using a
participatory citizen model. The first goal in this case study was to develop a cheap, widely accessible,
and effective inventory method, and to test it by mapping tree of heaven in Croatia. A total of
90.61 km of roads and trails was mapped; 20 single plants and 19 multi-plant clusters (mapped
as polygons) were detected. The total infested area was 2610 m
2
. The second goal was to educate
citizens and raise awareness of this invasive species. The developed tool and suggested approach
aided in improving invasive risk management in accordance with citizen science principles and can
be applied to other species or areas.
Keywords: biological recording; citizen science; invasive species; mapping; mobile tools
1. Introduction
Scientists have devoted significant energy to determining the principles of alien species
invasions [
1
]. Alien species invasion is recognised as one of the most severe and demanding
global environmental threats [
2
]. Therefore, the impacts of these species should be comprehensively
assessed [
3
]. Human activities have caused ecological conditions in the urban flora that render them
suitable for alien species settlement [
4
]. Generally, invasive alien species are considered a consequence
of globalisation, and they may spread into new environments in many ways, some intentional and
some accidental. Even if a large share of invasive species was transported unintentionally, such as via
ship ballast waters, systems of canals and dams, or cargo holds, many other cases have been caused
by a sequence of events based on conscious human decisions. Many recently introduced invasive
species were originally introduced as ornamental or erosion control plants—these include tree of
heaven (Ailanthus altissima (Mill.) Swingle) [
5
] and beach vitex (Vitex rotundifolia L. f.) [
6
]. In some
cases, invasive species were introduced in an attempt to control other invasive species—for example,
several species of mongoose [
7
]. On the other hand, some species are imported to different places
intentionally, but released by mistake—these include Caulerpa taxifolia (M. Vahl) C. Agardh [
8
] into
the Mediterranean region, and lionfish (Pterois spp.) [
9
] and python (Python spp.) in Florida [
10
].
Although certainly not all human acts leading to invasions have been performed with the goal of
causing larger ecological and biodiversity damage, but are simply the consequences of a lack of
information or education, they still have global repercussions. Such decisions would probably have
been made to a lesser extent if the actors had been better informed. The fact that a limited number of
Forests 2018,9, 31; doi:10.3390/f9010031 www.mdpi.com/journal/forests
Forests 2018,9, 31 2 of 14
scientists are well-informed about the possible consequences does not help the general picture where
non-professionals are responsible for concrete environmental changes. The involvement of the wider
public in science is known as “citizen science”. This approach, described as the participation of the
general public in scientific research, dates back to the 19th century [
11
]. Citizen involvement in data
collection and monitoring offers the public an appealing opportunity to participate in research, and
allows them to benefit from the learning experience [
12
,
13
]. Citizen science is a tool of participatory
conservation that has numerous and differently named models, and has been identified as the most
acceptable sustainable environmental approach [
14
]. Efficient urban ecosystem management requires
bringing science, policy, and citizen participation together [
15
]. The continuing evolution and spread of
the Internet and other communication technologies in recent years has increased the number of citizen
science projects in many disciplines. Environmental education within these projects, in particular,
those concerning invasive species, makes the public aware of their role in this conservation issue and
consequently encourages them to support future initiatives for the prevention of the further spread
of invasive species [
16
–
18
]. Public education is one of the best methods of citizen involvement in
the process of democratic decision making and policy creation, especially in fields of ecology, new
technologies, and other sciences, and could also be a successful tool in preventing future bad decision
making (Figure 1).
Forests 2018, 9, 31 2 of 14
The fact that a limited number of scientists are well-informed about the possible consequences does
not help the general picture where non-professionals are responsible for concrete environmental
changes. The involvement of the wider public in science is known as “citizen science”. This approach,
described as the participation of the general public in scientific research, dates back to the 19th
century [11]. Citizen involvement in data collection and monitoring offers the public an appealing
opportunity to participate in research, and allows them to benefit from the learning
experience [12,13]. Citizen science is a tool of participatory conservation that has numerous and
differently named models, and has been identified as the most acceptable sustainable environmental
approach [14]. Efficient urban ecosystem management requires bringing science, policy, and citizen
participation together [15]. The continuing evolution and spread of the Internet and other
communication technologies in recent years has increased the number of citizen science projects in
many disciplines. Environmental education within these projects, in particular, those concerning
invasive species, makes the public aware of their role in this conservation issue and consequently
encourages them to support future initiatives for the prevention of the further spread of invasive
species [16–18]. Public education is one of the best methods of citizen involvement in the process of
democratic decision making and policy creation, especially in fields of ecology, new technologies,
and other sciences, and could also be a successful tool in preventing future bad decision making
(Figure 1).
Figure 1. Invasive species introduction, management, and response through citizen science.
To be efficient, citizen involvement should not be time consuming or boring, require
complicated or expensive tools, or be expensive to perform. Its direct results are relevant, but they
are only a part of the complete list of benefits. The following case study describes the current status
of an alien plant invasion caused by the unintended consequences of bad decisions and is an
example of citizen science with a goal of improving the environment and possibly preventing future
mistakes in invasive alien species management in Croatia.
Case Study—Biological Recording of Ailanthus altissima
Tree of heaven (Ailanthus altissima) is one of today’s most invasive plant species, present on all
continents except Antarctica [19–21], preferring urban and highly localised disturbed areas [1]. It
was introduced from China to Europe (Paris) in the 1740s primarily as an ornamental tree [22]. There
are no written records of the exact year of introduction in Croatia, but it is certain that it was planted
in gardens as a decorative plant [23]. Although there are many studies focused on various aspects of
A. altissima around the world [22], only a few studies have been conducted on biological recording
Figure 1. Invasive species introduction, management, and response through citizen science.
To be efficient, citizen involvement should not be time consuming or boring, require complicated
or expensive tools, or be expensive to perform. Its direct results are relevant, but they are only a part
of the complete list of benefits. The following case study describes the current status of an alien plant
invasion caused by the unintended consequences of bad decisions and is an example of citizen science
with a goal of improving the environment and possibly preventing future mistakes in invasive alien
species management in Croatia.
Case Study—Biological Recording of Ailanthus altissima
Tree of heaven (Ailanthus altissima) is one of today’s most invasive plant species, present on all
continents except Antarctica [
19
–
21
], preferring urban and highly localised disturbed areas [
1
]. It was
introduced from China to Europe (Paris) in the 1740s primarily as an ornamental tree [
22
]. There are
no written records of the exact year of introduction in Croatia, but it is certain that it was planted in
Forests 2018,9, 31 3 of 14
gardens as a decorative plant [
23
]. Although there are many studies focused on various aspects of
A. altissima around the world [
22
], only a few studies have been conducted on biological recording
models [
24
–
26
], and, as far as we know, there have been none that connect participatory concepts with
this invasive species.
In Croatia, A. altissima is established in all regions, especially coastal areas; it is spreading in many
urban areas, on islands, and in protected areas [
27
,
28
]. Despite this, there has been no systematic
biological recording, monitoring, or management of this species. On the national level, there are
activities planned that are aimed at effective management of its population in the future [
23
]. Sporadic
biological recording has been performed so far by ground-based visual methods, and there is no
continuous long-term monitoring [
23
,
29
,
30
]. In the city of Poreˇc, this plant was introduced as an
ornamental and planted in some private and public gardens. To date, sporadic activities for raising
awareness of this and similar ecological issues have not engendered much interest and a new approach
is needed. Moreover, the real number and area of occurrences was not known, and data were needed
to form the basis for an invasive risk management plan.
Invasive species biological recording, mapping, and monitoring are prerequisites for successful
biological invasion risk management [
31
–
33
] and to prevent invasive species expansion into large areas,
where eradication becomes very difficult and costly [
1
]. Among invasive plants, trees are generally
easiest to map because of their size [
34
], although there is currently no methodical, reasonably priced
protocol for the estimation of tree invasions that can be used in invasive species management or
scientific research [
35
]. Conducting invasive plant inventories is a critical component of an integrated
approach to invasive plant management [
26
]. One of the first, crucial steps in an effective management
strategy is the estimation of invasive species abundance and distribution [
1
]. Inventory data often
provide the information necessary to evaluate the extent of plant invasion, allowing land managers to
prioritise management efforts; however, these data are often expensive to collect [36].
New communication technologies have popularised citizen science and allowed for instant
transmission of data, the combination of electronic sensor data with observation logs, and the validation
of observations in real time. The most commonly used mapping systems and tools in plant management
are traditional ground-based visual methods, handheld GPS (Global Positioning System), aerial and
other remote methods [
37
], and innovative high-definition video systems [
38
]. Several applications
(apps) for tracking invasive species with mobile smart phones have been developed in the USA
(including iMapInvasives, IPAlert, IveGot1, IceGot1, Southeast Early Detection Network SEEDN,
EDDMapS, What’s Invasive, and Stink Bug Scout), and are used as invasive species management
tools (http://ecospotinvasivespecies.weebly.com/mobile-apps-used-to-track-invasive-species.html).
These apps, although free to use, full of useful features, and handy, were created to include the general
public in ongoing biological recording of selected species and locations in the USA, and cannot be used
for other species or localities outside the USA. Also, in Europe, a number of apps have been developed
for the same purpose, among them EASIN (European Alien Species Information Network) developed
by JRC (Joint Research Centre of EU Commission, Brussels, Belgium), RINSE That’s Invasive! app,
KORINA app, Plantas Invasoras app, and IASTracker app. The potential usefulness of wearable
sensors and smartphones has recently been described by Savage (2015) [
39
]. It has been stated that
virtual globes, particularly Google Earth, are readily available, free, and handy, and have enormous
potential in invasion science. Their practical use as tools for identification and monitoring of plant
invasions and data presentation has been recognised [36].
To involve the local people in A. altissima data acquisition, we developed a cheap, widely
accessible, and effective mapping method, and tested it with a case study of A. altissima. We mapped
urban and suburban areas of the City of Poreˇc, Croatia, South-East Europe (Figure 2), using a
participatory citizen model. We expected that this and similar emerging mobile tools would enable
more efficient data collection for invasive species, assure more efficient invasive species management,
and educate the people and consequently prevent future bad decisions regarding invasive species.
Some of these results may be seen in future years.
Forests 2018,9, 31 4 of 14
Forests 2018, 9, 31 4 of 14
Figure 2. Geographical location of study area.
2. Materials and Methods
2.1. Case Study—Biological Recording of Ailanthus altissima
Smartphones equipped with GPS, camera, the TrackMyTour app (TMT; https://trackmytour.com/),
and the LocaToWeb app (LTW; https://locatoweb.com/) were used to collect data on A. altissima
presence. Smartphone GPS-based solutions are highly accurate, with accuracy within an estimated
3–5 m under ideal conditions [40]. Both apps are available for free at the app store. The
TrackMyTour app is a travel-tracking app that allows the creation of an online map and a microblog
of movement. It can record the latitude, longitude, and altitude of each waypoint, and has an option
to describe particular waypoints (i.e., infestation sites) and to upload photos related to each
waypoint. TMT is not a real-time tracker, but waypoints can be created offline and submitted later,
when a connection is possible. Moreover, it allows for direct upload from several smartphones to a
common web account. Maps are exported in kml file format. TMT was not used for tracking the path
because it connects the recorded points linearly instead of following the real trajectory. LTW,
however, can track the real trajectory and calculate distance and time covered by car or bike or on
foot. For that reason, we used both apps. At the start of recording we activated both apps: LTW for
trajectory tracking and TMT for data recording. Data recorded offline were later submitted online in
gpx format. All data (both kml and gpx files) were imported into the downloadable free software
Google Earth Pro (http://www.google.com/earth/download/gep/agree.html). The data from both
apps were merged into a kmz format file. Data processed and saved in Google Earth Pro can be
edited, so tracks were highlighted (Figure 3), and different colours, sizes, photos, and comments
were assigned to each waypoint (Figures 4 and 5), and implemented on the web, enabling a second
step of public access. All data were collected from August to November 2015.
Figure 3. The followed biological recording routes inside 10 sub-areas (coloured) of the City of Poreč.
Route numeration corresponds to Table 1.
Figure 2. Geographical location of study area.
2. Materials and Methods
2.1. Case Study—Biological Recording of Ailanthus altissima
Smartphones equipped with GPS, camera, the TrackMyTour app (TMT; https://trackmytour.
com/), and the LocaToWeb app (LTW; https://locatoweb.com/) were used to collect data on
A. altissima presence. Smartphone GPS-based solutions are highly accurate, with accuracy within
an estimated 3–5 m under ideal conditions [
40
]. Both apps are available for free at the app store.
The TrackMyTour app is a travel-tracking app that allows the creation of an online map and a
microblog of movement. It can record the latitude, longitude, and altitude of each waypoint, and has
an option to describe particular waypoints (i.e., infestation sites) and to upload photos related to each
waypoint. TMT is not a real-time tracker, but waypoints can be created offline and submitted later,
when a connection is possible. Moreover, it allows for direct upload from several smartphones to
a common web account. Maps are exported in kml file format. TMT was not used for tracking the
path because it connects the recorded points linearly instead of following the real trajectory. LTW,
however, can track the real trajectory and calculate distance and time covered by car or bike or on
foot. For that reason, we used both apps. At the start of recording we activated both apps: LTW for
trajectory tracking and TMT for data recording. Data recorded offline were later submitted online
in gpx format. All data (both kml and gpx files) were imported into the downloadable free software
Google Earth Pro (http://www.google.com/earth/download/gep/agree.html). The data from both
apps were merged into a kmz format file. Data processed and saved in Google Earth Pro can be edited,
so tracks were highlighted (Figure 3), and different colours, sizes, photos, and comments were assigned
to each waypoint (Figures 4and 5), and implemented on the web, enabling a second step of public
access. All data were collected from August to November 2015.
Forests 2018, 9, 31 4 of 14
Figure 2. Geographical location of study area.
2. Materials and Methods
2.1. Case Study—Biological Recording of Ailanthus altissima
Smartphones equipped with GPS, camera, the TrackMyTour app (TMT; https://trackmytour.com/),
and the LocaToWeb app (LTW; https://locatoweb.com/) were used to collect data on A. altissima
presence. Smartphone GPS-based solutions are highly accurate, with accuracy within an estimated
3–5 m under ideal conditions [40]. Both apps are available for free at the app store. The
TrackMyTour app is a travel-tracking app that allows the creation of an online map and a microblog
of movement. It can record the latitude, longitude, and altitude of each waypoint, and has an option
to describe particular waypoints (i.e., infestation sites) and to upload photos related to each
waypoint. TMT is not a real-time tracker, but waypoints can be created offline and submitted later,
when a connection is possible. Moreover, it allows for direct upload from several smartphones to a
common web account. Maps are exported in kml file format. TMT was not used for tracking the path
because it connects the recorded points linearly instead of following the real trajectory. LTW,
however, can track the real trajectory and calculate distance and time covered by car or bike or on
foot. For that reason, we used both apps. At the start of recording we activated both apps: LTW for
trajectory tracking and TMT for data recording. Data recorded offline were later submitted online in
gpx format. All data (both kml and gpx files) were imported into the downloadable free software
Google Earth Pro (http://www.google.com/earth/download/gep/agree.html). The data from both
apps were merged into a kmz format file. Data processed and saved in Google Earth Pro can be
edited, so tracks were highlighted (Figure 3), and different colours, sizes, photos, and comments
were assigned to each waypoint (Figures 4 and 5), and implemented on the web, enabling a second
step of public access. All data were collected from August to November 2015.
Figure 3. The followed biological recording routes inside 10 sub-areas (coloured) of the City of Poreč.
Route numeration corresponds to Table 1.
Figure 3.
The followed biological recording routes inside 10 sub-areas (coloured) of the City of Poreˇc.
Route numeration corresponds to Table 1.
Forests 2018,9, 31 5 of 14
Forests 2018, 9, 31 5 of 14
Figure 4. Global view of the satellite map of the A. altissima infestation in Poreč City as shown by
Google Earth Pro software. Pins (yellow) and polygons (orange, red) indicating infestation are
shown. Transparent circles represent locations of source A. altissima trees and distribution of
invasion sites.
Figure 5. Example of an infestation site with comments assigned to the waypoint. PO-01-1 = Poreč,
mapping area 1, infestation site 1; Polygon orange = dense infestation areas (2–5 plants/m2);
PS = polygon size; TD = type of damage, F (functional), E (environmental), A (aesthetic);
LD = location of damage, U (urban).
Figure 4.
Global view of the satellite map of the A. altissima infestation in Poreˇc City as shown by
Google Earth Pro software. Pins (yellow) and polygons (orange, red) indicating infestation are shown.
Transparent circles represent locations of source A. altissima trees and distribution of invasion sites.
Forests 2018, 9, 31 5 of 14
Figure 4. Global view of the satellite map of the A. altissima infestation in Poreč City as shown by
Google Earth Pro software. Pins (yellow) and polygons (orange, red) indicating infestation are
shown. Transparent circles represent locations of source A. altissima trees and distribution of
invasion sites.
Figure 5. Example of an infestation site with comments assigned to the waypoint. PO-01-1 = Poreč,
mapping area 1, infestation site 1; Polygon orange = dense infestation areas (2–5 plants/m2);
PS = polygon size; TD = type of damage, F (functional), E (environmental), A (aesthetic);
LD = location of damage, U (urban).
Figure 5.
Example of an infestation site with comments assigned to the waypoint.
PO-01-1 = Poreˇc,
mapping area 1, infestation site 1; Polygon orange = dense infestation areas (2–5 plants/m
2
);
PS = polygon size; TD = type of damage, F (functional), E (environmental), A (aesthetic); LD = location
of damage, U (urban).
Forests 2018,9, 31 6 of 14
2.2. Citizen Science in Biological Recording
The recording methods involved recorders on foot, bike, or car, who inputted A. altissima
infestation data into handheld smartphones. Recorders were recruited via personal contacts, social
networks, emails, or telephone calls. We offered no material incentives. We decided to select
environmentally educated or highly motivated recorders and to invest time and money in training and
motivating. If recorders were able to undertake the hiking or biking, or had an available car, and could
attend the training session, they were accepted. Recorders attended one half-day training session
led by the authors. Training included theoretical and practical information related to all aspects of
mapping methodology (identification, mobile application handling, data recording, and processing).
Recorders collected data individually, in pairs, or in the case of young students, in groups of four to
five. We assigned each recorder, whether hiking, biking, or driving, a specific stretch of trail or road on
which to record the presence and abundance of A. altissima. Recorders scanned for the target species
in two 20-m zones, one on each side of the trail or road. Recording zones were chosen because of its
shade intolerance and thus preferences for open areas along roads, trails, abandoned lots, and parks.
We instructed recorders to not leave their assigned route. Biological recording was performed by
different categories of citizens, according to age, education, and occupation, with a total of 62 recorders:
4 scientists, 4 teachers, 46 students (21 high school students and 25 elementary school students),
3 university students, and 5 volunteers (3 retirees and 2 adults), which was optimal for efficient
recording organization. Infestation was recorded by visual estimation and exact measurements.
Owing to the significant population dynamics and areas of high plant density, counting all single
trees was not possible, and therefore data was simplified by using polygons—a four-squares drown
approximately following per the natural plant distribution.
Ailanthus altissima plants were censused, mapped, and measured as follows: (1) Each located
plant or cluster of up to five plants was plotted with a virtual pin at the corresponding position on the
map in TMT; (2) Plants were classified into four groups (D1–D4) based on the tree diameter (d): D1,
d < 3 cm; D2, d = 3–8 cm; D3, d = 8–18 cm; D4, d > 18 cm). For each plant in group D4, the diameter at
breast height (DBH) was measured using a handheld meter; (3) If the infestation included more than
five plants, it was recorded as a polygon; (4) Polygon areas were measured and clusters were marked
with a pin in the centre of the drawn polygon; (5) To indicate plant density, polygons were placed
into one of three categories (represented by colours for online imaging) representing the population
density: yellow, for areas with sparse plants (corresponding to no more than 1 plant/m
2
); orange,
for dense areas (2–5 plants/m
2
) or mix of sparse and very dense areas, and red, for very dense areas
(more than 5 plants/m2); (6) Other notes and specific site descriptions were recorded as comments.
We used multiple methods to ensure the accuracy of recorder-generated data. Our recording
methodology led to low-quantity but high-quality records. Quality assurance methods were based on
previous training, education in situ, and accurate route planning. Many possible biases were avoided by
the fact that participants recorded only one species in a limited area, which permitted control of every
site. A short training and demonstration about specific morphologic features, including the presence
of extrafloral nectaries, enabled correct and reliable species identification with a misidentification
rate of zero. All data were additionally checked by reviewing photo documentation taken by the
recorders. When we suspected that a misidentification occurred, we verified the identification in situ.
In order to eliminate biases in route selection, participants did not select their own survey locations
but were instructed to follow assigned routes. Routes were planned by dividing the city map into
sectors with exact roads and trails marked. Double counting was avoided by linking each record with
its exact location.
2.3. Mapped Areas
Ailanthus altissima plants were monitored in urban and suburban areas of the City of Poreˇc, Croatia
(Figure 2). Routes were examined in many different types of urban environments: the old town, public
Forests 2018,9, 31 7 of 14
green areas, parks, roads, streets, walking paths, tree-lined avenues, uncultivated areas, bus stations,
parking places, industrial areas, landfills, tourist resorts, and residential areas.
2.4. Harmfulness Assessment
The effects caused by A. altissima were estimated, recorded, and classified according to
type (functional, environmental, aesthetic, health, and safety) and site (urban, suburban, and
industrial). Effects estimations were based on either field records or subjective assessment based
on theoretical knowledge. In the cases of specific effects (e.g., damage to architectural, historic, artistic,
or archaeological items), experts were consulted. Estimates of invasiveness were calculated as the
ratio of the area covered by A. altissima to the total examined area, as estimated using the route length
plus a buffer of 20 m on each side. Alien species invasibility was calculated in terms of percent plant
cover [41].
3. Results and Discussion
3.1. Surveyed Areas
The Poreˇc City area was divided into 10 sub-areas and corresponding routes (Table 1, Figure 3).
The whole area was inspected using nearly 100 km of roads and trails. Along these routes, 39 infestation
sites were found; there were 20 singular plants and 19 polygons (Table 1, Figure 4). Although the
simplifications made by the introduction of polygons did not allow for detailed and precise data
analysis, it also did not hinder the aim of this case study, which was to gain some insight into the
infestation rate in the examined area. All invasive plants and site descriptions (Figure 5) were included
in an online mapping database.
Table 1. Route descriptions and infestation sites.
Route Type Length (km)
Infested Areas (Number) Infested Area (m2)
Plants (Number) * Polygons (Number) **
D1 D2 D3 D4 Sparse Dense Very Dense
1 Urban 4.2 0 0 0 0 0 1 0 24
2 Urban 8.82 0 0 0 0 1 1 0 8
3 Urban 5.02 0 0 0 0 0 0 0 0
4 Urban 5.63 0 0 0 0 0 0 1 300
5 Urban 8.38 2 1 2 3 3 3 1 655
6 Urban 8.73 0 0 0 0 0 0 0 0
7 Sub urban 23.10 0 0 0 0 0 0 0 0
8 Sub urban 11.38 1 1 2 1 0 1 2 284
9 Industrial 10.92 0 0 0 1 0 0 1 80
10 Industrial 4.43 3 2 1 0 1 3 0 1259
Total 90.61 6 4 5 5 5 9 5 2610
* Plants categories related to trunk diameter (d, cm): D1, d < 3 cm; D2, d = 3–8 cm; D3, d = 8–18 cm; D4, d > 18 cm.
** Polygon categories related to plant density: sparse, <1 plant/m
2
; dense, 1–5 plants/m
2
or mix of sparse and very
dense areas; very dense, >5 plants/m2).
3.2. Harmfulness
Effects caused by A. altissima presence were determined visually or assumed and described.
In total, there were 22 different types of damage or effects on different types of structures; these are
presented in Table 2.
Forests 2018,9, 31 8 of 14
Table 2.
Classification, site, and number of effects caused by Ailanthus altissima (Mill.) Swingle presence
in urban and suburban areas.
Type of Effect Site * Number of Findings
Functional
Infestation of driveways and parking places U/SU 2
Infestation of roads SU/I 2
Walkways obstruction SU/I 2
Difficulty in visualisation of road signs SU/I 7
Reduced visibility on the road
U/SU/I
8
Physical and chemical damage to architectural,
historic, artistic, and archaeological objects U 1
Damage to building construction I 1
Damage to power lines I 1
Damage to fences and railings U/I 5
Difficulty in maintenance of green areas U 2
Difficulty in maintenance of facilities U/I 5
Disorders in horticulture planning in tourist areas U 1
Environmental
* Loss of biodiversity U 6
* Habitat alteration U/I 18
* Habitat degradation SU 1
Safety and health
Hazards caused by the reduction of drivers’ views U 1
Hazards caused by reduced visibility on crossroads and roundabouts SU 6
Damage to pavements and footpaths SU/I 2
* Appearance of allergy symptoms SU/I 39
Aesthetic
Aesthetic damage of archaeological sites U 1
Damage to public and private green areas
U/SU/I
21
Vegetation anthropisation—landscape monotony I 1
Aesthetic damage to tourist or residential areas U 1
Site of effect recorded: U, Urban area; SU, Suburban area; I, Industrial area. * Effect assumed on the basis of literature
data and in situ estimation.
Ailanthus altissima invasibility in Poreˇc was calculated to be 0.072% of the total area surveyed.
This result implies low invasion when compared to some other related published data [
41
,
42
], and it
also indicates that eradication programs in this particular area may be successful.
The main focus of a high-quality invasive risk management plan is the knowledge of invasive plant
distribution and abundance, which can therefore be used in identification of endangered areas with a
high risk of invasion, evaluation of ecosystem impacts, continuous long-term invasion monitoring,
early detection of new invasive species or locations, planning further management activities, and
raising public awareness about invasive species [
36
]. The analysis of the urban appearance of invasive
species is a complex topic that is still under investigation. Urban areas are characterized by complex
ecological relationships and influenced by modified environmental conditions, as well as anthropogenic
factors [
43
]. In these circumstances, invasive species with a wide ecological tolerance enjoy a significant
advantage, and therefore it is very difficult to predict their future spread. Because of this, continuous
biological recording in particular areas is recommended. Biological recording is the first step in an
invasive risk management plan, and the methods for such recording should be easily available and
efficient. Ecological inventories can have additional advantages: detailed surveys provide basic data for
monitoring programs, whereas inventory maps provide important documentation of population status
and management actions, maintenance of long-term continuity of staff knowledge, and justification for
requested or approved funding. Additionally, inventory maps can be used as outreach and education
tools for government personnel and policymakers and to improve public knowledge. Biological
recording produces valuable datasets to scientists, governments, and land managers and can be
successfully based on volunteer contributions, activities known as “citizen science”. In some countries,
Forests 2018,9, 31 9 of 14
such as the United Kingdom, citizen science has a long tradition that started as biological recordings
performed by thousands of people across the country [
44
]. The contribution of volunteer experts to
ecology and conservation has been very important in Britain and Ireland over the past 50 years [
44
–
46
].
In practice, however, there can be some difficulties in obtaining and analysing data gathered by citizen
scientists, including double-counting [
47
]. In our case, we had delayed recording due to personal
reasons of some volunteers, as well as technical issues related to the use of the apps (especially with
older participants) and to compiling data in the final interactive map. In the present recording we
have still not sufficiently involved adults and especially retirees. This post-working population has
valuable potential in citizen science activities, and could be well-engaged volunteers who can dedicate
appropriate time and commitment to the common good.
On a global level, maps of invasive species distributions are usually created by gathering data
from other sources such as herbaria, zoological collections, and research institutions [
46
], whereas at
the local scale they typically result from extensive field work and mapping. There are complex and
advanced systems for invasive species recording and distribution, including remote sensing (RS) [
48
],
geographical information systems (GIS) [
49
–
52
], and geospatial technologies [
53
], but their application
is expensive and reserved for the scientific community or highly skilled technicians, and are not of
practical use at the local scale. Ailanthus altissima is mentioned among other invasive species to be
tested for satellite remote sensing biodiversity evaluation [
37
], but the available parameters for imagery
sources and classification techniques of remote sensing imagery related to vegetation mapping of this
species are still in development.
The mapping system developed within the present study satisfied the requirements for a
successful biological recording and created a foundation for long-term monitoring at both local
and global scales. It allowed for easy acquisition of basic data for the creation of an A. altissima map
along with the assignment of additional data such as photos and comments. Moreover, the tools
used will allow spatial monitoring over time. They were very inexpensive, involving the use of basic
equipment that is already widely available to the public. Downloading and upgrading the apps and
Google Earth Pro was free. Because each location was corroborated with photos, it was possible to add
additional data afterwards, and the apps can be used simultaneously by many participants. Timely
reporting of results is often an important aspect of a monitoring scheme. In our case, it was enabled
by automatic uploads of data. The small-size equipment (smartphones) makes recording handy and
able to be performed during participants’ regular daily activities. The created tool is not exclusive to
a certain species or specific area. Thus, it can be applied to any zone or species, both in Croatia and
beyond. Ailanthus altissima is a highly invasive species with significant negative effects on ecosystem
services: it leads to decreased biodiversity [
54
,
55
] and other ecological impacts [
56
,
57
], can be an
allergen, and also has other effects on human health and structures [
58
,
59
]. The presence of A. altissima
was previously screened in Croatia by Idžojti´c and Zebec [
27
] and their results showed a high level of
infestation in all regions. This earlier biological recording was conducted using herbaria examples and
field work.
Other isolated attempts at A. altissima management have been performed on islands (Cres) or
protected areas (National Park Brijuni) (data not published). Despite its evident propagation and the
need for action, to date, A. altissima has not been systematically managed in Croatia; this study was
one of the first attempts to systematically map and manage A. altissima in this region. In Istria, the
Croatian north coastal region, where our study was conducted, A. altissima is present both in coastal
areas and inland, along the roads and inside cities. It is not known when it was introduced, but its
presence in Istria was mentioned in the literature in 1936 in the context of its possible use as firewood
and for land reclamation [60].
The present study revealed the presence of A. altissima in all screened urban, suburban,
and industrial areas of the City of Poreˇc (Table 1). Poreˇc is a tourist city with important historical and
cultural heritage, and A. altissima infestations affect archaeological sites and ancient walls. Moreover,
it disturbs landscapes, causing aesthetic damage to natural and historical vistas (Table 2). Its presence
Forests 2018,9, 31 10 of 14
is hazardous for the sites, representing an important economic and environmental problem due to the
use of herbicides and the difficulty of eradicating it [
25
,
61
,
62
]. Eradication of A. altissima is known to
be complex, and usually requires the application of combined measures, such as mechanical removal,
burning, biocontrol, or chemical treatment [
63
,
64
], and therefore it is recommended to be performed or
supervised by professionals. In our particular case, we suggest responsible authorities to initiate the
eradication at locations recorded inside the urban areas with major risk of damage, and citizens to
address all new infestations to the Invasive Species Centre Poreˇc (in establishment; http://civ.iptpo.hr).
The vigour of this species is particularly evident along roads and near recent construction sites, where
it thrives because of its shade intolerance, wide tolerance to different soil types, and high reproductive
capacity [
5
,
65
]. It has previously been stated that A. altissima is competitive in urban areas [
4
]. It is quite
easy to identify the source of an infestation by identifying the oldest trees in the area. In the Poreˇc area,
there are 5-year-old A. altissima trees, introduced for ornamental purposes or during larger construction
events, which are most likely the source of the infestation. These individually non-problematic trees
are inside private gardens, a part of urban horticulture, or are at recent construction sites, and could
silently be the cause of many new infestation sites in the area, shown as conspicuous nuclei in Figure 4.
The mean diameter of these five A. altissima trees was 44.83 cm based on the strong correlation between
A. altissima tree DBH and age [
65
]. The estimated age of the oldest trees in the Poreˇc area is about
40 years, which corresponds to historical data on urban horticulture in Poreˇc (personal communication).
These deliberate, but unintentionally harmful, introductions are still potentially dangerous events
that can be linked to the phenomenon of the “tyranny of small decisions”. The phrase was coined in
1966 by the American economist Alfred E. Kahn and originally applied to social and economic aspects
of human society [
66
]. It basically represents a situation where many small and short-term decisions
cumulatively result in neither an optimal nor a welcome outcome. In 1982, Odum [
67
] applied this
concept to environmental issues. He explained that the accumulation of small environmental decisions
can have serious consequences, and he feared that the lack of a truly holistic perspective would impede
the ability to find solutions for the “tyranny of small decisions”. His eventual solutions involved
developing a better understanding of whole-system processes and better education for researchers,
teachers, planners, and politicians. Cooper et al. [
68
] identified the direct involvement of citizen
participants in monitoring and active management of residential land as one of the most efficient
ways to confront the “tyranny of small decisions”. Zisenis [
4
] highlighted the need for individual
evaluation of all such cases of non-native species introduction, which is possible only by performing
comprehensive and reliable data collection. This study was performed according to the main principles
of participatory conservation or citizen science, and the first goal was achieved: the inventory of
A. altissima in the Poreˇc area was completed and assessment is continuing along with other invasive
species additions and long-term monitoring.
Participatory conservation must be properly managed and involve all relevant stakeholders
and the coordination of participatory mechanisms within and between participants [69]. The present
biological recording was initiated by scientists who engaged the local community and by a participative
ecological project involving local bodies of government and educational institutions. Data collection
involved citizens, including students, in monitoring and data processing. Volunteers have been
previously recommended as reliable data recorders that can provide accurate datasets with minimal
variability [
47
]. Some studies have shown that out-of-classroom environmental teaching increases
achievement relative to the still-prevalent theoretical classroom-based teaching [
70
,
71
]. Participatory
biological recording has multiplicative effects: it has increased knowledge, changed attitudes and
behaviours, sensitised people to nature through increased connection to nature, fostered participation
in local community activities, and led to practical environmental actions, resulting in a better
understanding of the value of biological records and environmental monitoring [
72
]. The effects
of the activities performed here on future citizen actions and decisions related to invasive species
management remains to be tested and confirmed with a larger data set. Environmental issues have
the best prospects for citizen science monitoring, which can be followed by sustainable management
Forests 2018,9, 31 11 of 14
models. This is possible only with the collaboration of local authorities, scientists, citizens, and
teachers, and its success is measured through their interest and involvement in future monitoring and
management actions. In our case, the study resulted in the involvement of an extended stakeholders’
consortium in the preparation of a long-term regional invasive risk management plan in the urban area.
Citizen science has a huge potential to confront significant problems. Coordination of the
local population will create new approaches for garden-use practices that will result in long-term
environmental improvements and ultimately prevent unintended negative ecological consequences.
4. Conclusions
We proposed that citizen science, which can operate over large scales and has many positive effects
on the general public, can be used to create a new tool in invasive species management. Within a case
study of citizen science in Croatia, we proposed a new tool for alien invasive plant biological recording
based on citizen science principles and new widely accessible technologies. Biological recording
is the first step in an integral invasive risk management plan and the methods for its conduction
should be easily available and efficient. We developed a universally applicable and affordable tool
for invasive plant species mapping, and tested it in terms of its cost, time, and accuracy in mapping
A. altissima. We expect that proposing this and similar emerging mobile tools will enable more efficient
data collection for invasive species and consequently assure more efficient invasive risk management.
Acknowledgments:
This study was supported by the Municipality of Poreˇc. Much appreciation to volunteers for
their help in data acquisition.
Author Contributions:
B.S. and D.P. contributed equally to the manuscript. B.S. and D.P. both conceived, designed,
and performed the survey, analysed the data, and wrote the paper.
Conflicts of Interest: The authors declare no conflict of interest.
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