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1 Introduction
One of the basic questions concerning biological in-
vasions is: What makes a species able to survive (or even
prosper) within a foreign environment? The ‘biological
foreignness’ of a species to an ecosystem can obstruct a
potential invader in many different ways: The abiotic
conditions may not be suitable, e.g. due to inappropri-
ate temperatures; there may be competitors hindering
the establishment, or necessary mutualists may be miss-
ing. Thus, most of the species arriving in a foreign area
fail to establish (e.g. M
ACK et al. 2000). The systematic
search for problems encountered by a foreign species in
a new area might help to reveal the principal factors de-
termining invasion processes (TUCKER a. RICHARDSON
1995; RICHARDSON et al. 2000). Several current papers
address the mechanisms of invasions (e.g. HIGGINS a.
RICHARDSON 1998; REJMÁNEK 1999; SHER a. HYAT T
1999; DAVIS et al. 2000; KEANE 2002; SHEA a. CHES-
SON
2002), and some of them emphasize the impor-
tance of dissecting the process into several stages (e.g.
MACK 2000; RICHARDSON et al. 2000; KOLAR a.
LODGE 2001).
The rapid spread of the South African neophyte
Narrow-leafed Ragwort (Senecio inaequidens DC.) in Cen-
tral Europe is well documented (e.g. ERNST 1998 for the
Netherlands, D
ANCZA a. KIRÁLY 2000 for Hungary,
B
ÖHMER et al. 2001 for Germany, PYS
ˇ
EK et al. 2002 for
the Czech Republic). Based on a review of existing
knowledge (with a focus on Germany), we show the
species’ characteristics and environmental conditions
that help to avoid or overcome potential obstacles. For
the systematic analysis of the invasion process we use a
conceptual model, the “model of invasion steps and
stages” (
INVASS model, HEGER 2004) which takes up
the idea that ‘biological foreignness’ is the crucial fea-
ture of biological invasions. As a basis for this analysis,
we suggest a scheme that visualizes the invasion process
as a “staircase”. The model postulates that a plant must
overcome several steps to advance from one invasion
34 Erdkunde Band 59/2005
THE INVASION OF CENTRAL EUROPE BY SENECIO INAEQUIDENS DC.–
A COMPLEX BIOGEOGRAPHICAL PROBLEM
With 5 figures, 4 tables and 1 photo
TINA HEGER and HANS JÜRGEN BÖHMER
Zusammenfassung: Die Einwanderung des Schmalblättrigen Greiskrautes (Senecio inaequidens DC.) nach Mitteleuropa –
Analyse eines komplexen biogeographischen Problems
Das aus Südafrika stammende Schmalblättrige Greiskraut (Senecio inaequidens
DC.) hat sich seit den 1970er Jahren, von
Westen (Raum Aachen) und Nordwesten (Bremen) einwandernd, sehr schnell in Deutschland ausgebreitet und dringt weiter
ins östliche Mitteleuropa vor. Bevorzugte Wuchsorte sind Ruderalstellen an Verkehrswegen (Bahnanlagen, Autobahnen),
Stadtbrachen, Abraumhalden, Baustellen und weitere anthropogen gestörte Standorte, auf denen die Pflanze vor allem im
Spätsommer aspektbildend in Erscheinung tritt. Die außergewöhnliche Ausbreitungsgeschwindigkeit und die möglicherweise
zunehmende Konkurrenzfähigkeit in verschiedenen Pflanzengesellschaften geben Anlass, dieser invasiven gebietsfremden Art
besonderes Augenmerk zu widmen. Wir analysieren den Einwanderungsprozess mit Hilfe eines konzeptionellen Modells zur
Erklärung von biologischen Invasionen, dem
INVASS-Modell (“Model of Invasion Steps and Stages”).
Summary: Since the 1970’s, the neophyte Narrow-leafed Ragwort (Senecio inaequidens
DC.) has spread rapidly in Central
Europe, particularly in Germany. The species grows on roadsides and railway embankments, on urban wasteland, slag heaps,
construction sites and other disturbed locations. This study reviews literature on the invasion of S. inaequidens in Central
Europe with a focus on Germany, and analyses the factors determining the species’ success. The analysis is based on a con-
ceptual invasion model, termed the “model of invasion steps and stages” (
INVASS model). Using an organism-centred
approach, the model names problems which may arise during a plant invasion. The present study discusses factors which may
have influenced the spreading of S. inaequidens. Species characteristics and favourable conditions contributing to the success of
the invasion steps are named. Unresolved questions are identified, and major subjects for further research are recommended.
stage to the next. The invasion steps, which are
partly similar to elements of classic succession theories
(CLEMENTS 1916; MACARTHUR a. WILSON 1967; cf.
M
UELLER-DOMBOIS 2001), are designed in a way to en-
compass the main problems that may arise for a plant
continuing in an invasion process (Fig. 1, for a definition
of the steps and stages see below, and in detail H
EGER
2001; a discussion of other proposed schemes can be
found in HEGER a. TREPL 2003).
The INVASS model systematically analyses the main
factors influencing the course of an idealized invasion
during each of the four steps (i.e. transportation,
growth, and reproduction of the first individuals in the
new area; population growth, and colonization of new
localities). An organism-centred approach explores
problems that might cause a crucial obstacle for the in-
vader during each invasion step, and which species
characteristics or features of the new environment
might enable the plant to avoid or overcome these ob-
stacles. The suggestions of the model, summarized in
four tables describing the potential problems as well as
favourable species characteristics and environmental
conditions (H
EGER 2004), are appropriate as a frame-
work for the analysis of invasion cases. In the following,
they are used as checklists leading step-by-step through
the investigation of the invasion of Central Europe by
Senecio inaequidens. It is shown that, with the help of the
tables, this complex invasion process can be described
in a clear and transparent manner, highlighting the im-
portant factors which led to the success of the species.
2 Senecio inaequidens in Central Europe and South Africa
Senecio inaequidens is a perennial shrubby herb about
60–80 cm tall. The lemon-yellow flower heads contain
80 to 100 flowers (GUILLERM et al. 1990), leading to a
production of up to 29,000 achenes per plant (E
RNST
1998). The plants – as are all Asteraceae – are protan-
drous. They are highly self-fertile (ERNST 1998); clonal
reproduction has not been observed. The achenes are
small (3 mm) with relatively long pappus hairs (5 mm),
helping them not only to be transported by wind, but
also to stick to many different structures.
The species is native to South Africa’s “Highveld”;
its original range covers approximately the provinces
of North-West, Northern Province, Gauteng, Mpuma-
langa, Free State and Kwazulu-Natal, at elevations be-
tween 1,400 and 2,850 m (WERNER et al. 1991;
MEUSEL
a. JÄGER 1992). Thus, the original range lies
within the region of summer rainfall: more than 80%
of the annual precipitation occurs between October
and March. Winter (during the European summer) is
sunny and dry, with daily maxima of temperatures
around 20°C. The temperature does usually not fall
below –4.5°C (but locally reaches –11°C; J
ACKSON
1961; THOMPSON 1965; LOW a. REBELO 1996; MUR-
REY et al. 1996).
Occurring within grassland and savannah biomes
(LOW a. REBELO 1996), S. inaequidens originally colo-
nizes skeletal sectors on steep, moist and grassy slopes,
as well as sandy and gravelly banks of periodic streams
(HILLIARD 1977). A noteworthy phenomenon is that
S. inaequidens has extended its range in South Africa to
anthropogenic locations with weak competition (partic-
ularly on the verges of roads and areas damaged by fire,
but also on coastal dunes; HILLIARD a. BURTT 1987).
The species is currently colonizing a wide ecological
range of areas, from dry to humid habitats, stone to
clay soils, exposed to shaded locations (WERNER et al.
1991).
The first occurrence in Europe was detected near
Hannover in 1889 (E
RNST 1998), followed by a finding
at the port of Bremen in 1896 (KUHBIER 1977). Since
the early 1970s, the species spread rapidly in Germany
(see Fig. 2 a-d). The wave of its eastward propagation
from the centre of dispersal at Liège (since 1955) at-
tained the western border of Germany at Aachen
around 1970, followed by the first reports in West Ger-
many. It also spread from an older population near
Tina Heger and Hans Jürgen Böhmer: The invasion of Central Europe by Senecio Inaequidens DC. 35
Steps of an invasion
Stages of an invasion
1 Transportation
2 Independent growth and
reproduction of at
least one individual
3 Population growth to MVP
4 Colonisation of
new localities
Presence in the
home range
1 Presence in the
new area
2 Spontaneous
establishment
3 Permanent
establishment
4 Spreading
in the new area
is completed
Fig. 1: Chronological discrimination of an idealised invasion
process into steps and stages. Different stages are reached
by overcoming a sequence of steps in the course of an
invasion. The height of each step depends on the ability of
the species to overcome environmental limitations.
MVP:
Minimum viable population (from H
EGER 2001)
Chronologische Abfolge der Schritte und Stufen eines
idealisierten Invasionsprozesses. Neue Phasen werden
durch die Überwindung einer Reihe von Hürden erreicht.
Die Höhe einer Stufe hängt von der Fähigkeit einer Art
ab, limitierende Umweltfaktoren zu überwinden.
MVP:
Mindestpopulationsgröße (nach H
EGER 2001)
36 Erdkunde Band 59/2005
2a 2b
2c 2d
Düsseldorf, and reports of sites east of the Rhine be-
came frequent a little later. The plant has been reported
from the Cologne area since about 1980. OBERDORFER
(1983) was the first to term the plant as naturalized.
It has colonized the eastern Ruhr area (around
Dortmund) since about 1990. At this time, its German
range was mostly restricted to the western part of
North Rhine-Westphalia and the area around Bremen
(HAEUPLER a. SCHÖNFELDER 1989). In 1985, S. inae-
quidens appeared in northern Hessen on railway gravel.
The gap in distribution between the Liège and Bremen
areas was closed by the mid-1990s. Isolated occur-
rences were reported by the early 1990s from the upper
Rhine Valley, the Neckar region, and Bavaria. ADOLPHI
(1992) detected the first occurrence of S. inaequidens on
the Baltic island of Rügen, and KÖNIG (1995) in Berlin.
BRENNENSTUHL (1995) reported isolated pioneers in
Saxony-Anhalt, eastern Thuringia and the area of
Lower Lausitz, and HENKER (1996) in Mecklenburg-
West Pommerania. IHL (1997) judged that S. inaequidens
had become established in Saxony. The latest reports
have come from Thuringia, from Greifswald, and from
Bavaria, so that it may be assumed that the range of the
species is spread throughout Germany, at least along
railway tracks (for a detailed review of literature on the
invasion process see BÖHMER et al. 2001).
In Europe as well as in South Africa, stony sites are
preferred (E
RNST 1998; BÖHMER et al. 2001). Senecio
inaequidens can be found up to elevations of 1,000 m
within the Alps (B
ÜSCHER 1989). It appears early in the
course of succession (ASMUS 1988), but usually is dis-
placed during later stages. Nevertheless, the species – in
Central Europe as well as in South Africa – is able to
build up large dominant stands (A
DOLPHI 1997).
2.1 ‘Foreignness’ of the species
As a basis for the forthcoming analysis it is important
to know how ‘foreign’ the new environment actually is
to S. inaequidens, i.e. what important differences exist
between the home range and the new range. Those
differences are:
– postponement of the growing season due to a
transportation from the southern to the northern hemi-
sphere
– less extreme seasonal differences in precipitation
in Central Europe than in South Africa (for instance no
serious drought in the Central European winter)
– lower temperatures during the year in Central Eu-
rope, especially during winter
– biocoenoses composed of different species and
genera; this is indicated
(1) by the different inhabited vegetation types
(2) by the different biogeographical realms of the
areas (Paleotropica as opposed to Holarctica).
3 The first step: transportation to Europe
In any invasion, the first obstacle faced by the
prospective invader is to leave its home range and to
reach a new area; it has to overcome a barrier to dis-
persal (cf. R
ICHARDSON et al. 2000). In the case of S. in-
aequidens, the propagules either have to pass the whole
continent of Africa as well as the Mediterranean Sea,
or surmount a distance of about 11,000 km across the
Atlantic Ocean. Senecio inaequidens overcame this diffi-
culty by transportation of sheep wool from South
Africa to Europe. Other modes of transportation are
also conceivable, but every one of the reported five pri-
mary centres of origin are connected to the wool pro-
cessing industry (i.e. Mazamet, France; Calais, France;
Verona, Italy; Liège, Belgium and Bremen, Germany;
WERNER et al. 1991), which supports this assumption.
Tina Heger and Hans Jürgen Böhmer: The invasion of Central Europe by Senecio Inaequidens DC. 37
Fig. 2a–d: Documentation of the spread of Senecio inaequidens
in Germany, as an example for its rapid range expansion in
Central Europe
Dokumentation der Ausbreitung von Senecio inaequidens in
Deutschland als Beispiel für die rasche Ausbreitung der Art
in Mitteleuropa
Fig. 2a: Distribution of the species in 1979 (after R
ADKO-
WITSCH 1997)
Verbreitung im Jahr 1979 (nach R
ADKOWITSCH 1997)
Fig. 2b: Distribution in 1989 (after R
ADKOWITSCH 1997)
Verbreitung im Jahr 1989 (nach R
ADKOWITSCH 1997)
Fig. 2c: Distribution in 1997 (after R
ADKOWITSCH 1997)
Verbreitung im Jahr 1997 (nach R
ADKOWITSCH 1997)
Fig. 2d: Documented range extensions of S. inaequidens until
2003 (data taken from http://www.floraweb.de, W
EISS
1999, ZAHLHEIMER 2000, 2001, MEIEROTT 2001, BORN-
KAMM 2002, LANG a. WOLFF 2002, WOLFF a. LANG 2002,
K
UHBIER 2003, expanded by own observations as well as
written notifications by H.-U. P
IONTKOWSKI, Eckernförde
and H.-E. SALKOWSKI, Vallendar)
Dokumentierte Vorkommen von S. inaequidens im Jahr
2003 (Daten aus http://www.floraweb.de, WEISS 1999,
Z
AHLHEIMER 2000, 2001, MEIEROTT 2001, BORNKAMM
2002, LANG a. WOLFF 2002, WOLFF a. LANG 2002, KUH-
BIER 2003, ergänzt um Beobachtungen von H.-U. PIONT-
KOWSKI, Eckernförde, und H.-E. SALKOWSKI, Vallendar)
Table 1 summarizes the factors that drive uninten-
tional transportation according to the INVASS model,
and indicates which of them have contributed to the
success of S. inaequidens during this step. In the following
section, the statements of the table will be discussed in
detail.
3.1 Availability and accessibility of a pathway
Transportation of S. inaequidens from South Africa to
Europe was possible because sheep wool has been im-
ported to Europe since the 19
th
and the beginning of
the 20
th
century (see R
EEKEN 1996). The question why
such a pathway exists is a cultural one, which cannot be
answered here. A main problem S. inaequidens had to
overcome is establishing contact with exported wool in
South Africa.
To achieve this, five factors are implicated (cf. ISPM
11, 2001). The first one is the volume and frequency
of movements along the pathway. Since the 1840s,
Germany developed from a wool exporting to an im-
porting country, and South Africa became one of the
main exporters. Within two decades, sheep wool
became a very important commodity world-wide
(R
EEKEN 1996). It is not known how much wool has
been imported from South Africa to Europe per year
during this time and how many sheep were needed to
produce this amount of wool, but it is clear that the
pathway was exploited quite frequently and to a large
extent during this decade. The transportation of ach-
enes by this pathway therefore was quite likely.
A second question is whether there was a seasonal
timing of diaspore production and movements along
the pathway, increasing the probability of a transporta-
tion. Since grazing of the South African grassland and
bush savannah took place during southern summer
(W
ELLINGTON 1955) and S. inaequidens flowers mainly
from October to February, a coincidence of the avail-
ability of diaspores and sheep grazing was also likely.
An association of diaspores with a pathway is more
likely if the species is dominant in its area of origin. As
mentioned above, S. inaequidens is able to build up large
dominant stands, but this seems to happen only locally.
38 Erdkunde Band 59/2005
Table 1: Main factors determining the first step of invasion according to the
INVASS model, differentiated into potential problems, favourable species
characteristics and favourable environmental conditions (after H
EGER a. TREPL 2003). Highlighted are those factors which might have caused a
failure in the case of Senecio inaequidens in Central Europe. “?”: further research needed. For more details see text
Entscheidende Faktoren der ersten Stufe einer biologischen Invasion gemäß dem INVASS-Modell, unterteilt in potentielle
Probleme, günstige Arteigenschaften und günstige Umweltbedingungen (nach H
EGER a. TREPL 2003). Hervorgehoben sind
jene Faktoren, die im Falle von Senecio inaequidens eine Erschwernis verkörpert haben könnten. „?”: Weitere Forschung nötig.
Nähere Erläuterungen im Text
Step 1: Determining factors
Trans- Potential problems Did Favourable species characteristics Were Conditions favourable Were
por- for an invader it they for transportation they
tion occur? given? given?
Lack of suitable pathways no Adaptations for long-distance yes Suitable pathway is available yes
No association with a potential no dispersal Pathway is used with big volume yes
pathway Production of many descendants yes and high frequency
Transport and storage are difficult no Seeds with high longevity compared yes Transportation starts during yes
to survive to duration of transport fruiting time
Pest management procedures may no Robust seeds not Predominance in the area (no?)
harm the species neces- of origin
No possibility to be transferred no sary Occurrence on sites in contact
to a suitable habitat to a potential pathway yes
Transportation lasts shortly no
No extreme conditions during yes
transportation
No pest management procedures yes
Associated commodity is imported yes
to many destinations
Points of entry, transfer, and yes
destination are near to suitable sites
Time of entry is during a period not
suitable for germination and neces-
establishment sary
Associated commodity is no
intended for planting
Presumably, this has not contributed to a frequent asso-
ciation of the species with sheep. It is supposed that
usually only a few achenes would be found in the wool
of a flock of sheep (cf. E
RNST 1998).
The probability of dispersal is further increased
if the species occurred at sites with a connection to
the potential pathway. The original sites inhabited by
S. inaequidens are steep and stony, and thus not very
suitable for agricultural utilization, except for sheep
grazing. Thus, an association of the species with sheep
wool was likely.
A last important question is how it was possible for
diaspores of S. inaequidens to become attached to the
wool of grazing sheep. The essential characteristic for
overcoming this obstacle is the morphology of the ach-
enes described above, i.e. originally an adaptation for
long-distance dispersal which was useful for human-
mediated transportation in this case. Moreover, the
species is able to produce many seeds (see above), which
also increases the probability of transportation.
3.2 Survival during transport and storage
The transportation of wool from South Africa to
Europe took place by ships and afterwards by railway
(REEKEN
1996). It can be assumed that transportation
took up to several months. Nevertheless, the conditions
during transportation presumably were not extreme. It
can also be assumed that there were no special treat-
ments of the wool (such as freezing) during storage or
transportation. Therefore, the duration of the trans-
portation might have been the only problem connected
to this sub-step. From the reviewed literature, longevity
of seeds is not apparent and has to be explored in
future investigations, but E
RNST (1998) observed a dor-
mancy lasting several months. Therefore, the longevity
seems to be great enough to survive a period of at least
several months.
3.3 Survival of existing pest management procedures
A crucial situation for the achenes arriving with
sheep wool would have arisen if there had been pest
management procedures during the process of harvest-
ing and importation of wool. Before spinning, the wool
was washed and combed, both of which presumably
took place without the use of chemicals, thus providing
no preclusion of the invading species.
3.4 Transfer to suitable habitats
The last problem for the arriving achenes of S. inae-
quidens during the first step of invasion was reaching a
suitable habitat. Within the new area, transportation
could have taken place due to adaptations of the
achenes to anemochory, but even more probable was a
transfer due to human-aided transportation (hemero-
chory). The wool imported to Europe was sent to dif-
ferent wool processing factories in Germany, France,
Italy, and the Netherlands. Obviously, the seeds would
have been teased out of the wool and deposited in the
vicinity of these factories, in many cases finding condi-
tions suitable for growth (see below). Therefore, heme-
rochory favoured the transfer to a high degree.
3.5 What is the likelihood of overcoming the first step
of invasion?
The summary in table 1 shows that none of the po-
tential problems of the first step of invasion actually
arose during the transportation of S. inaequidens to Eu-
rope. This is due to some favourable species character-
istics, and especially to several favourable conditions
during transportation. Therefore, overcoming the first
invasion step was apparently not a singular, rare event;
instead, it is likely that the transportation of S. inae-
quidens from South Africa to Europe took place repeat-
edly.
4 The second step: growth and reproduction
Similar to the ‘ecesis’ coined by C
LEMENTS (1928) for
successions, the second step of invasion is the process of
growth and establishment of a single individual arriv-
ing at a new site. The founder individuals, which have
no chance to adapt to the new environment, have to
germinate, mature and reproduce; these three sub-
steps again resemble the ones proposed by C
LEMENTS
(1928).
4.1 Dormancy and germination
Problems during dormancy and germination of the
diaspores result from seed predators, and from adverse
abiotic conditions that hinder germination or the sur-
vival of dormancy. Some species, moreover, need a
trigger to end dormancy and start germination. As a
third problem, this might not be available in the new
environment (see Tab. 2).
Thus, the first question to be answered is whether
there are seed predators harming S. inaequidens. In
Europe, several indigenous species of birds and phy-
tophagous insects (e.g. species of the genera Nysius and
Stictopleurus) have been observed feeding on the species’
fruits (W
ERNER 1994; ERNST 1998; SCHMITZ a.
W
ERNER 2000). Some of these insects are specialists for
Tina Heger and Hans Jürgen Böhmer: The invasion of Central Europe by Senecio Inaequidens DC. 39
the genus Senecio, and some are generalists (SCHMITZ a.
W
ERNER 2000). So, it can be assumed that the first ar-
riving achenes already had some generalist enemies
which might have eaten them. On the other hand, spe-
cialist predators would be lessened due to the ‘biologi-
cal foreignness’, which might have been an advantage
for the plant (see e.g. KEANE 2002). An open question is
whether the achenes have some possibility to defend
against generalist predators (e.g. due to alkaloids that
can be found in adult plants, see E
RNST 1998). The pro-
duction of vegetative propagules, which provides an
escape from seed predators, is not a possible solution
for S. inaequidens.
Dormant achenes of S. inaequidens encounter lower
winter temperatures in Europe than in South Africa.
Surprisingly, the achenes are able to cope with this
problem, as E
RNST (1998) observed achenes (of Euro-
pean individuals, which might have adapted) that sur-
vived two European winters and frost of –15°C.
The next question is why the first achenes to arrive
were able to germinate in the new environment. Not
much is known concerning this question. Further re-
search (with South African achenes) will have to clarify
the conditions required for germination and whether a
trigger is necessary to end dormancy. The requirement
of a trigger (as e.g. a certain daylength) could explain
the ability of the species to adapt its life cycle to the
postponed growing period in Europe. It is known that
during germination, competitors may inhibit the ap-
pearance of seedlings (E
RNST 1998). S. inaequidens is
thus a weak competitor during this phase which re-
quires a suitable microhabitat to meet its demands;
unfortunately, the habitat of the first successfully ger-
minating achenes is unknown.
40 Erdkunde Band 59/2005
Table 2: Main factors determining the second step of invasion in the case of Senecio inaequidens, see Table 1
Entscheidende Faktoren während der zweiten Stufe der Invasion von Senecio inaequidens, vgl. Tabelle 1
Step 2: Determining factors
Potential problems Did Favourable species characteristics Were Favourable conditions which might Were
for an invader it they be given in the new area they
occur? given? given?
a) Dor-
mancy
and ger-
mination
Seed predators
Environmental conditions are not
suitable to survive dormancy /
for germinantion
Trigger necessary for germination
is not available
Defence against generalist predators
Production of vegetative
propagules
Robust diaspores
Broad ecological amplitude
concerning necessary conditions for
germination
Competitive strength
Dormancy with no necessary /
easy available trigger
No specialized seed predators
because of ‘foreignness’ of the plant
New environment is similar
to the home range concerning
climate
Favourable microhabitat
yes
no
?
?
no
yes (?)
?
no
?
yes
no
yes
b)
Growth
to
maturity
Competitors
Predators
Necessary mutualists are not
available
Unfavourable abiotic
conditions
Shortage of resources
Competitive strength
Defence mechanism against
generalist predators
Ability to tolerate loss of tissue
No mutualists necessary
Mutualism with a cosmopolitan
partner
Spread of mutualist partner
with diaspore
Broad ecological amplitude
Existence of a safe site
Few specialist predators because
of ‘foreignness’
Absence of related species
Presence of potential mutualists
New environment climatically
similar to the home range
Origin and new area have similar
disturbance regimes
no
yes
?
yes
no
no
yes
yes
?
?
?
limited
yes
yes
no
?
no
in
places
c) Inde-
pendent
repro-
duction
Trigger or necessary conditions for
production of flowers not available
Lack of sexual partners
Lack of suitable pollinators
Fruiting and ripening of seeds is
not possible
Vegetative reproduction
No special demands concerning
fruiting and flowering
Dispersal leading to big
founder population
Autogamy or agamospermy
Monoecism
Self-fertility
Hermaphroditism
Unspecific pollinators
Attractive flowers
Longevity of the plant
Prolonged flowering period
New environment is similar
to the home range concerning
climate
Large founder population
due to favourable conditions
of transportation
Presence of generalist pollinators
no
yes
no
no
no
yes
no
no
no
yes
yes
yes
yes
?
yes
no
no
yes
4.2 Growth to reproductive maturity
During the growth to reproductive maturity, crucial
situations for the invader may arise due to competi-
tors, predators, a lack of mutualists, adverse abiotic
conditions, and a shortage of requisite resources (see
Tab. 2).
Seedlings of S. inaequidens – as well as growing and
adult individuals – do not seem to be strong competi-
tors (E
RNST 1998). This assumption is based upon the
observation that the species requires open sites with
little competition for light and nutrients (see above).
Thus, the success of the species is favoured to a high
degree by the abundance of safe sites due to anthro-
pogenic disturbance. It is assumed that S. inaequidens
met an underexplored niche: its preferred sites are
poorly occupied by indigenous competitors (BÖHMER
et al. 2001, and references therein).
Up to now, 62 phytophagous insects have been ob-
served feeding on S. inaequidens in Europe (SCHMITZ a.
WERNER 2000). Of these, 11 only feed on flowers and
fruits, therefore 51 affect the growth of the plants.
Three of these species are specialists for the genus
Senecio, and three for Asteraceae. These numbers would
have to be compared to the phytophagous complex of
the species in its home range, but a comparison with the
indigenous species Senecio jacobea (96 phytophagous in-
sects) supports the assumption that S. inaequidens again
has an advantage due to its ‘foreignness’. The presence
of closely related species moderates this: specialized
predators had (and will have in the future) the possibil-
ity of switching to the new host (S
TRONG et al. 1984;
but see FRENZEL et al. 2001).
The impact of phytophagous insects on the growth
of S. inaequidens in Europe seems negligible until now.
This may be due to the ability of the plant to produce
alkaloids (ERNST 1998). Another favourable species
characteristic especially concerning grazers is the
ability of the species to regenerate lost tissue (see
GUILLERM et al. 1990).
Not much is known concerning non-insect predators
of S. inaequidens. An alien rust (Puccinia laenophorae Cooke)
grows on the plant, and an indigenous fungus (Coleospo-
rium senecionis (Pers.) J. J. Kickx) has also been observed
(SCHMITZ a. WERNER 2000). An open question is what
influence these and other pathogens have on the plant
in Europe and the home range.
There is also a lack of knowledge concerning pos-
sible mutualists S. inaequidens might need during growth
to maturity. Further research is required to clarify
whether the species is mutualistic in South Africa, and
whether a problem might have occurred because of its
biological foreignness in Europe.
Concerning the abiotic conditions, during the period
of growth to maturity, problems may arise especially
due to the differences in temperatures between the
European and the South African range of the species
(see above). A shortage of resources may not be a prob-
lem, because water supply is usually better in Europe
than in its home range, especially during winter.
A crucial obstacle to the establishment of seedlings is
low winter temperatures (E
RNST 1998). This is a prob-
lem for individuals germinating in late autumn (i.e. dur-
ing South African spring), which seems to happen
rarely (G
UILLERM et al. 1990; ERNST 1998). In addition
to the demand for suitable temperatures, the seedlings
seem to need a certain degree of moisture (E
RNST 1998).
The plants’ demands during their growth to maturity
are not well studied. Most of the knowledge is con-
cluded from physical conditions predominating the
occupied sites. As described above, these cover a wide
range, suggesting a broad ecological amplitude for
the species (e.g. concerning available nutrients). Never-
theless, there are some limitations. The species prefers
open sites, which indicates a demand for light (MOLL
1989; K
EHREN 1995; E
RNST 1998). It is discussed
whether the species requires high temperatures during
summer (average minimum temperatures during July
not below 12°C, R
ADKOWITSCH 1997; cf. WERNER et
al. 1991; A
DOLPHI 1997). Further studies will have to
prove this assumption, and it has also to be examined
whether this demand could be met by a choice of suit-
able microsites (despite possibly lower average temper-
atures). The woody stem base as well as the ability to re-
generate from the main roots (GUILLERM et al. 1990)
contribute to the species’ ability to tolerate frost (KUH-
BIER 1977; GRIESE 1996; ADOLPHI 1997). Nevertheless,
the extreme frosts in winter 2002/2003 in Central Eu-
rope caused a noticeable loss of S. inaequidens popula-
tions, e.g. in the City of Leipzig (P. Gutte, oral comm.).
Successfully invading species sometimes profit from
an adaptation to a disturbance regime which resembles
the one in the new area (G
ROVES 1991). In some re-
spects this also applies to S. inaequidens. Within the
South African range, fire and grazing are the main fac-
tors driving the ecosystems (LOW a. REBELO 1996),
both being unusual for the sites occupied in Central
Europe. But, from the viewpoint of the organisms,
these disturbances may resemble those present in some
of the European sites in some respect: It has variously
been observed that S. inaequidens is promoted by mow-
ing (B
ÖHMER et al. 2001). The plant is also able to resist
the application of herbicides and to tolerate heavy met-
als in the soil (W
ERNER et al. 1991; HARD 1993), but
these characteristics are not obviously connected to the
disturbance regime of its home range.
Tina Heger and Hans Jürgen Böhmer: The invasion of Central Europe by Senecio Inaequidens DC. 41
4.3 Reproduction
The next range of problems faced by plant invaders
is connected to their reproduction. The question is:
How did the first individuals in Europe manage to re-
produce despite a lack of reproductive partners and
without prior adaptation to the new conditions? Senecio
inaequidens is unable to reproduce vegetatively, which
would have been one way of easily overcoming this
problem.
For alien species lacking this possibility, several prob-
lems may emerge (see Tab. 2). One of them is the pro-
duction of flowers. Since climatic conditions differ be-
tween the Central European and the South African
ranges, and the growing season is postponed, the pro-
duction of flowers might have been hindered in Eu-
rope. This has not been the case, and the question is
why. In Kwazulu-Natal S. inaequidens flowers from Oc-
tober to February, but some plants may flower through-
out the year (A
DOLPHI 1997). In Europe, the species has
two main periods of flowering, one starting in July and
one in September (B
ÖHMER et al. 2001). Its flowering
period locally lasts to December or even January
(G
ERSTBERGER 1978; GUILLERM et al. 1990;
M
AZOMEIT 1991; KUHBIER 1996; ADOLPHI 1997;
E
RNST 1998). Apparently, the species has almost kept
its South African flowering period (which in Central
Europe now takes place in autumn and the beginning
of winter) and has added a second one during the
European spring. This second flowering period seems
to be caused by an ongoing adaptation, as the onset of
flowering has continuously shifted „forward” during
recent years to the beginning of May (B
ÜSCHER 1989;
M
OLL 1989). To sum up the existing knowledge, S. in-
aequidens seems to require no particular daylength, no
vernalisation, and to have no special demands con-
cerning the abiotic conditions.
Pollination of an alien species may be hindered due
to a lack of sexual partners, or of suitable pollinators.
The first problem does not arise if the founder popula-
tion is large, due to favourable conditions of transporta-
tion. As discussed above, this has apparently not been
the case for S. inaequidens. One reason for this is the con-
dition of transportation, another is the missing ability to
disperse clustered seeds (e.g. with fruits containing many
seeds). Further possibilities to explain the crucial lack of
partners are autogamy, agamospermy, or monoecism,
which are not applicable in the case of S. inaequidens.
Nevertheless, even single individuals are able to repro-
duce because of its protandric (thus hermaphrodite)
flowers coupled with high self-fertility (see E
RNST 1998).
In Europe, there are several generalist pollinators,
which probably facilitated a pollination of even the first
flowers produced by this insect-pollinated species. Sev-
eral syrphids and other insects have been observed to
visit the flowers (E
RNST 1998), and the species does not
seem to depend on specialist pollinators. An alien
species starting with a small population has a higher
probability of being pollinated if its flowers are espe-
cially attractive for some reason (cf. C
HITTKA a.
S
CHURKENS 2001). Few flowers are available to insects
in Europe during autumn, and the late flowering of
S. inaequidens provides a competitive advantage. Pollina-
tion becomes even more probable, if the species has a
long life span (further research has to clarify this for
S. inaequidens) or if it is – like S. inaequidens – flowering for
a long period (B
ARRET a. RICHARDSON 1986).
A last problem which might have hindered the
reproduction is the ripening of seeds under foreign cli-
matic conditions. Flowers that pollinated after the end
of November are not able to produce viable achenes
(E
RNST 1998). Therefore, again the crucial question is
how the species managed to postpone its life cycle ac-
cording to the European growing season. The ripening
of seeds during the European summer and autumn
seems to provide no problem for the plant.
4.4 What is the likelihood of overcoming the second step
of invasion?
Compared to table 1, the summary in table 2 indi-
cates more factors which might have caused a problem
during the second step of invasion. Senecio inaequidens
lacks some convenient characteristics, e.g. competitive
strength, the ability to reproduce vegetatively, and also
a broad amplitude of light requirement. Moreover,
several environmental conditions were unfavourable,
above all the climatic differences between Europe and
the home range. Senecio inaequidens nevertheless was suc-
cessful in overcoming this step, which can be attributed
mainly to the following factors:
– the self-fertility of the species coupled with its
protandrous flowers,
– the flexibility in the beginning of flowering, and
particularly,
– the availability of abundant sites with low compe-
tition (‘empty niches’) due to anthropogenic changes,
coupled with the ability of the species to occupy those
sites despite the locally extreme conditions.
5 The third step: population growth to a minimum viable
population
After overcoming the problems associated with
growth and reproduction in Europe, the founder indi-
viduals of S. inaequidens faced the next invasion step:
42 Erdkunde Band 59/2005
attaining a critical self-supporting population size (the
minimum viable population) to minimize the probabil-
ity of extinction due to demographic, genetic or envi-
ronmental stochasticity (see Tab. 3). An examination of
the species’ invasion history in Europe reveals that sev-
eral problems actually did emerge during population
growth: ERNST (1998) for instance, reports that several
populations of S. inaequidens persisted for some years in
the vicinity of Dutch sites with wool processing, but
later disappeared again. According to A
DOLPHI (1997),
the plant has been ephemeral in Germany for many
decades. It has been introduced repeatedly, but started
establishing itself not before the 1960s. Many small
populations of S. inaequidens have obviously been unable
to overcome the third step of invasion.
5.1 Problems for single plants within the growing population
Population growth is only possible if each individual
of the population manages to overcome the problems
of growth and reproduction. These problems have
been discussed above, but there are some differences for
the generations descending from the founder popula-
tion.
As discussed above, the first arrivals of S. inaequidens
in Europe had to face many different problems. It could
therefore be assumed, that the first settlement of some
individuals was due to a coincidence of rare events
(“good luck”), and an establishment was only possible
due to genetic adaptation (but see HARPER 1982). This
possibility is plausible especially because of the sup-
posed small population at the beginning of the inva-
sion, which might have led to a founder effect (cf.
M
AC
ARTHUR
a. WILSON 1967).
Unfortunately, no genetic comparison between dif-
ferent populations of S. inaequidens within Europe or
South Africa has been undertaken as of yet. Neverthe-
less, some observations seem to support the assump-
tion. One was already mentioned above; the continu-
ous shift of the beginning of flowering, which could be
interpreted as an adaptation to European climate
(B
ÖHMER et al. 2001). Another notable fact is that the
achenes of S. inaequidens show quite a complex pattern
of dormancy. Achenes ripening in early summer have
low dormancy and are able to establish a new genera-
tion in the same year, whereas those of late autumn
have high dormancy and germinate the next spring.
ERNST (1998) suggests that this pattern has evolved in
adaptation to the climatic conditions in Europe, be-
cause low dormancy of early summer achenes is not
suitable in the South African climate. It would be inter-
esting to know whether the species has a special genetic
predisposition which might have led to a quick genetic
change in the beginning of the invasion.
The growing population is more attractive to preda-
tors than single plants (or achenes), and the longer
the species is present in the area, the more predators
might switch to the new host. This seems to be the case
for S. inaequidens, since some specialized insects can be
found in the flower heads (SCHMITZ a. WERNER 2000),
and it can be assumed they did not feed on the first
arriving achenes. Again, the presence of congeners
from the genus Senecio in Europe seems to be a disad-
vantage for the plant (SCHERBER et al. 2003).
Tina Heger and Hans Jürgen Böhmer: The invasion of Central Europe by Senecio Inaequidens DC. 43
Table 3: Main factors determining the third step of invasion in the case of Senecio inaequidens, see Table 1
Entscheidende Faktoren während der dritten Stufe der Invasion von Senecio inaequidens, vgl. Tabelle 1
Step 3: Determining factors
Potential problems Did Favourable characteristics Were Favourable conditions which Were
for the plant it of the plant they may be given in the new area they
occur? given? given?
Popula-
tion
growth
First settlement was due to
“good luck“
New Predators because higher
attractivity or long period of
presence
Control measures
Lack of suitable safe sites in
the immediate vicinity of
the founder individuals
Small size of initial population
leads to demographic, genetic,
and environmental stochasticity
Ability for genetic adaptation
Adaptations for directed
short-distance dispersal
Persistent seed bank
Production of many seeds
Short reproduction cycle
High genetic variation of
the population
Self-fertility
Absence of specialized
predators despite a longer time
of presence in the new area
Absence of related species
No control measures
Presence of generalists facilitating
directed short-distance dispersals
Suitable sites in the vicinity of
the founder individuals
Large founder population
due to favourable conditions
of transportation
Repeated secondary introductions
?
yes
no
no
yes
?
(yes)
?
yes
yes
?
yes
no
no
yes
not nec-
essary
yes
no
yes
Alien plants may not be able to establish in a new
area if effective control measures hinder the growth
and reproduction of individuals. This was not the case
for S. inaequidens in Europe.
An aggregation of individuals, which is one require-
ment of population growth, can be achieved through
directed short distance dispersal or dispersal by wind,
the latter being the case for S. inaequidens. Achenes
blown over an open site have a good chance to aggre-
gate at some higher structure at the edge of the site. An-
other possibility to assure aggregation is the assembling
of a persistent seed bank. Whether this is the case for
S. inaequidens has to be determined in future studies.
Population growth is in any case not possible if the
descendants do not find suitable sites for establishment
in the vicinity of the founder individuals. For S. inae-
quidens, the preference of anthropogenic sites has led to
an abundance of appropriate locations with low com-
petition (such as railway tracks and stations, industrial
sites, and sites along highways).
5.2 Problems of the population
Small populations, such as those most likely pro-
duced by S. inaequidens, have to face a set of problems
resulting from demographic, genetic, and environ-
mental stochasticity (e.g. S
HAFFER 1981; SOULÉ 1987;
SIMBERLOFF 1988). Since S. inaequidens is now estab-
lished in Central Europe, some populations must have
been able to grow fast enough to survive. One prerequi-
site for a fast population growth is the ability to produce
many descendants. Individuals of S. inaequidens produce
even more descendants in Europe than in South Africa.
This is remarkable if we consider the seemingly adverse
climatic conditions, and it might be due to the absence
of enemies or a prolonged flowering period.
A reproductive cycle of only 96 days has been ob-
served for single individuals. So, an individual is capa-
ble of producing two generations within a vegetation
period (W
ERNER et al. 1991; ERNST 1998). This is an-
other characteristic favouring fast population growth.
The successful establishment of S. inaequidens was fur-
ther supported to a high degree by repeated introduc-
tions of diaspores. Without these, the small founder
populations would have vanished and the species would
not have been able to establish (cf. K
OWARIK 2003).
Additionally, the probability of becoming extinct
due to some kind of stochasticity is lessened if the
populations are genetically variable. For S. inaequidens
some morphological features hint at such variability
(A
DOLPHI 1997), but further research will have to clar-
ify this. The high self-fertility of the species has proba-
bly also minimized the problems connected to genetic
stochasticity (see OOSTERMEIJER et al. 1996).
5.3 What is the likelihood of overcoming the third step
of invasion?
The determining factors for the step of population
growth are summarized in table 3. An obviously crucial
problem for the species was the small size of the initial
population. This obstacle has been overcome mainly
due to
– the ability of S. inaequidens to produce many propag-
ules quickly, and (probably even more decisive),
– secondary introductions of diaspores, providing new
founder populations again and again over multiple
decades.
The latter circumstance made overcoming the third
step of invasion quite probable.
44 Erdkunde Band 59/2005
Photo 1: Senecio inaequidens DC
. in Fuerth/Bavaria (near Nu-
remberg, 2001); photo: H. J. BÖHMER
Senecio inaequidens DC
. in Fürth/Bayern (bei Nürnberg,
2001); Aufnahme: H. J. B
ÖHMER
6 The fourth step: colonization of new localities
Starting from single established populations at wool
processing sites, S. inaequidens managed to spread in
Europe (for details see B
ÖHMER et al. 2001). Thus, it
had to overcome the fourth step of invasion, which is
the colonization of new localities. Again, an alien
species must overcome a series of obstacles during this
step (see Tab. 4). They have to be differentiated into two
groups: necessary conditions (if they are not fulfilled a
colonization of new localities is impossible), and factors
which influence the extent and velocity of the spread-
ing.
6.1 Necessary conditions
A colonization of new localities is impeded if the
alien species is not able to produce enough descen-
dants. As discussed above, this is not the case with S. in-
aequidens. Dense stands (cf. e.g. B
RANDES 1993; REIDL
1995) produce “enormous seed quantities” (ADOLPHI
1997), creating great colonization pressure, even in un-
usual habitats (e.g. lawns, and the façade of the cathe-
dral at Cologne). Although, in the beginning of the in-
vasion the production of just a few descendants might
have protracted the spread.
Apart from this, the spread of an alien species is hin-
dered if no suitable habitats are within reach. This, in
turn, may be due to (a) a lack of suitable habitats or (b)
an inability of the propagules to be transported to a
suitable location. As mentioned repeatedly, there is no
lack of suitable sites for S. inaequidens in Central Europe,
although climatic conditions differ from the home
range. This is not only due to the high abundance of
anthropogenic sites (e.g. B
ORNKAMM 2002a, b), but also
to a broad ecological amplitude, enabling the species to
colonize e.g. woodland clearings. On the other hand,
the demand for high light limits its spread. Further
studies will have to reveal whether, additionally, a ge-
netic adaptability of the species or a high genetic vari-
ation within and between the European populations
favoured the colonization of new localities.
Since S. inaequidens is adapted to dispersal via wind, a
lack of dispersal means has probably never been a
problem. Its eastward propagation from Belgium and
the Netherlands was obviously facilitated by the pre-
vailing westerly winds (GERSTBERGER 1978; WERNER et
al. 1991). A further means of dispersal within Europe
might be animals; the ability of the achenes to stick to
fur has been demonstrated. Transportation by water
may also be possible. Nevertheless, the availability of a
dispersal means does not guarantee an effective trans-
portation, i.e. having a high probability of leading to a
site suitable for the growth of the species. None of the
modes of dispersal mentioned fulfills this precondition.
However, why might S. inaequidens have been able to
spread despite this deficiency? Two favourable condi-
tions can give an answer to this question. At first, re-
peated introductions to different wool processing sites
Tina Heger and Hans Jürgen Böhmer: The invasion of Central Europe by Senecio Inaequidens DC. 45
Table 4: Main factors determining the fourth step of invasion in the case of Senecio inaequidens, see Table 1
Entscheidende Faktoren während der vierten Stufe der Invasion von Senecio inaequidens, vgl. Tabelle 1
Step 4: Determining factors
Potential problems Did Favourable characteristics Were Favourable conditions which Were
for the plant it of the plant they may be given in the new area they
occur? given? given?
Coloni-
zation
of new
localities
Not enough descendants are
produced
Lack of suitable sites
Lack of appropriate means of
dispersal
Transportation has low
chance to reach suitable site
Spread is hampered due to
competitors, increasing appearance
of enemies or missing mutualisms
Change of abiotic conditions
has negative effects on
the performance of the species
Control measures may hamper
the spread
Ability to produce many seeds
or vegetative propagules
Broad ecological amplitude
Genetic adaptability of
the individuals
High genetic variation within
and between populations
Adaptations to hydrochory
Adaptations to dispersal
by generalist animals
Competitive strength
Climatic conditions similar
to the home range
Anthropogenic changes create
suitable sites
Natural disturbances create
suitable sites
Presence of generalist dispersers
Repeated secondary introductions
Human aided dispersal within
the new range
Predators which may switch
to the new host are absent
Mutualists which may switch to
new hosts are present
Changes of abiotic conditions have
positive effects
No control measures
no
no
no
yes
?
in the
past: no
no
yes
limited
?
?
no
no
no
no
yes
not nec-
essary
not nec-
essary
yes
yes
no
?
in the
past: yes
yes
all over Europe led to different focal points of spread,
thus stimulating the expansion (cf. M
OODY a. MACK
1988). Recently, the species has been recommended for
planting by beekeepers (A
DOLPHI 1995); such a usage
would intensify this effect. Secondly, human-aided
transportation within Europe provides a very effective
means of dispersal. An indicator for this is the observ-
able pattern of expansion, since in Europe the plant is
found mainly along linear anthropogenic structures
such as railways and highways (RADKOWITSCH 1997;
ADOLPHI 1998). A transportation of achenes sticking
to trains, in the profile of tyres or on transported goods
are some possible modes of hemerochory (cf. B
RANDES
1993; GRIESE 1996; RADKOWITSCH 1997), others are
listed in BÖHMER et al. (2001).
6.2 Factors influencing the extent and velocity of spreading
The factors mentioned until now are those which
may have totally prevented the spread of S. inaequidens
in Central Europe. Additionally, it could be asked
which factors determine how fast and to what extent
the species is spreading. The arguments listed above
favour the suggestion that a genetic change has acceler-
ated the expansion. Further research is necessary to
establish whether this was the case, and to explore the
probability of future change (e.g. due to hybridization,
cf. ELLSTRAND a. SCHIERENBECK 2000).
Extent and velocity of spread can, moreover, be
strongly influenced by the effect of competitors, preda-
tors or mutualists. What is known about the effect of
these biotic factors up to now has been stated above. It
is hypothesized that phytophagous insects (especially
seed predators) may have a stronger effect on S. inae-
quidens in the future (ERNST 1998; SCHMITZ a. WERNER
2000). As the species is not a strong competitor, the ex-
tent of the spread is limited – and might be strongly af-
fected if a strong competitor suddenly dominates its
preferred sites. But summing up, not very much is
known about the influence of biotic factors on the
process of spreading in the past, at present and in the
future.
Abiotic conditions, especially climatic ones, may
strongly influence the process of spreading. It has been
stated repeatedly that the rapid expansion of S. inae-
quidens was parallel to a period of dry and warm sum-
mers and mild winters in Europe (B
ÜSCHER 1989;
W
ERNER
et al. 1991; BÜSCHER a. LOOS 1993; HARD
1993; R
ADKOWITSCH 1997; ERNST 1998). It is probable
that climate shows a strong positive (or negative) influ-
ence on the extent and velocity of the species’ spread,
but its demands concerning temperature and humidity
are not known in detail.
6.3 What is the likelihood of overcoming the fourth step
of invasion?
Table 4 shows that the lack of effective natural dis-
persal (i.e. leading to directed dispersal) was the only se-
rious problem which might have prevented the species
from spreading. This deficiency was overcome due to
hemerochory. Another very important favourable con-
dition for step 4 is, again, the anthropogenic creation of
suitable sites. Extent and velocity of spreading more-
over seem to be influenced strongly by climatic condi-
tions. It remains to be explored whether the rapid ex-
pansion during the 1970s can be ascribed to climatic
factors alone, or whether other factors (genetic or biotic
ones) also contributed.
7 Conclusions
The invasion of S. inaequidens in Central Europe has
not been an uncomplicated and highly probable inci-
dence. Instead, the process is quite complex and a num-
ber of crucial obstacles have occurred, which had to be
surmounted. During the four steps of invasion pro-
posed by the
INVASS model, different species charac-
teristics and favourable conditions of the new environ-
ment (or during transportation, respectively) helped to
avoid potential problems, or to overcome emerging cru-
cial situations. Therefore, the reason for the success of
the species in Europe is the coincidence of a range of
different factors, indicated in tables 1 to 4.
It has been shown that a conceptual model can offer
a useful framework for the description and explanation
of the complex interplay of factors determining inva-
sion cases. Using the scheme of steps and stages as a ba-
sis, the four tables can be used as a guideline for a de-
tailed analysis. Thus, the INVASS model provides a
heuristic tool to systematize case studies of plant inva-
sions.
Although the invasion of Central Europe by S. inae-
quidens seems to be studied quite well, some interesting
questions remain to be answered. Perhaps the most ur-
gent one is if a genetic adaptation has occurred, and
whether the species is genetically variable in Europe
and South Africa. To be able to assess future trends for
the ongoing spread of the species it would be helpful to
know more about the conditions necessary for germi-
nation, and about the influence of predators in the
home range as well as the European range.
Still, almost all reports on S. inaequidens are records of
range extensions. The number of detailed ecological
research is correspondingly small; not much is known
about the ecological consequences of the invasion. In
recent years, it has been observed that S. inaequidens is
46 Erdkunde Band 59/2005
probably threatening indigenous species of importance
to nature conservation (e.g. blue lettuce, Lactuca perennis;
Adolphi, oral comm.). The spreading to cereal cultures
(e.g. wheat) may entail another problem, not just due to
the species’ competitive capacity, but particularly be-
cause of its poisonous quality. According to B
ROMILOV
(1995), S. inaequidens is a crop weed in South Africa and
repeatedly finds its way into bread. In Germany, S. in-
aequidens has been found on fallow fields (according
to A
DOLPHI
1997), but it has not yet been detected on
cultivated surfaces. The poison has repeatedly been
detected in milk as well, even though S. inaequidens is
usually avoided by grazing animals. Therefore, a mon-
itoring program is advisable in those parts of the range
of Senecio inaequidens where the species has either begun
to exert massive colonization pressure on locations out-
side the ruderal sites preferred in the past or is capable
of doing so (B
ÖHMER et al. 2001).
Acknowledgements
We thank the German Federal Environmental
Agency (Umweltbundesamt, Berlin) and the German
Federal Foundation for the Environment (Deutsche
Bundesstiftung Umwelt) for financial support. We thank
Klaus Adolphi, Curtis Daehler, Ulrike Doyle, Peter
Gutte, Gerhard Hard, Stephen Higgins, Alexandra
Leibstein, Lenz Meierott, Dieter Mueller-Dombois,
Hans-Ulrich Piontkowski, Daniel Prati, Annemarie
Radkowitsch, Hans-Erich Salkowski, Matthias Sea-
man, Christoph Scherber, Alison R. Sherwood, Bern-
hard Spachmüller, Ludwig Trepl, Frederick R. Wars-
hauer, Dietrich J. Werner, and Annett Winter for
assistance, valuable information and helpful comments.
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