Download 2.3 Can we predict whether a species will become invasive?

Introduced tree species in European forests
2.3
Can we predict whether a species
will become invasive?
Tina Heger
Life is complex, and so are invasion processes. Since the beginning of research on invasions the aims were to identify a set of traits which characterize invasive species, or
a set of factors characterising areas that are invaded (Crawley 1987, Rejmánek 1995).
Unfortunately, the more research that has been done, the more evident it has become
that these aims cannot be achieved (Williamson 2001, Rejmánek et al. 2005). Common
characteristics of invasive species in one region can differ significantly from the characteristics identified as being common in invasive species of a different region. As an
example, invasive plants in the British Isles have been found to produce significantly
larger seeds than native species (Crawley et al. 1997), whereas in Ontario invasive plant
species produced smaller fruits than native species (Cadotte and Lovett-Doust 2001).
Efforts to create lists of traits indicating invasiveness have so far only been successful
for relatively small groups of similar species. Invasibility of an area, i.e. its vulnerability
to invasion, at the beginning of research into invasions has been thought to strongly
depend on disturbance, and it was thought that undisturbed ecosystems are stable
and resistant to invaders. This notion has also proven untrue. To give an example, in
protected near-natural forests in New Jersey introduced tree species are common, with
Norway maple (Acer platanoides L.) (native in central Europe, but introduced and invasive in North America) reaching high abundances especially in the innermost, remote
and undisturbed parts (Webb et al. 2000).
What is the reason for these observations? Are the given examples only minor exceptions of a rule, or do they indicate some inherent mechanism complicating explanations and predictions? In this chapter, I will argue that the latter seems to be the case.
It is an obvious fact that every organism and every species is unique, and the same is
the case for every ecosystem, and every near-natural or human-altered area. With this
in mind it is easier to think about what makes a species invasive if a specific, potentially
invasive species is considered, along with a specific area that might be invaded. Certain characteristics of this species will significantly increase its potential to be invasive
in one specific ecosystem (e.g. an open grassland), but these same characteristics may
not increase its potential to be invasive in a different situation (e.g. a dense forest). Vice
versa, the abiotic characteristics (e.g. soil and climate) and the biotic community in this
specific area may make it a perfect spot for one invading species, but not for a different
one. Species traits and abiotic as well as biotic characteristics of a site are, therefore,
linked to each other like a key and a lock (Heger and Trepl 2003).
� Invasiveness and invasibility cannot be used as absolute categories. No species
is invasive everywhere, and no area is invasible for every species.
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2.3 Can we predict whether a species will become invasive?
Which species characteristics are favourable to become a successful invader depends on
which ‘problems’ the potential invader has to solve during its specific invasion process
(see Box 8). For a potentially invasive tree, a seed with a thick coat may be very helpful
in the case where at the site to be invaded, seed predators exist which can be repelled
by this feature. If the seed has been transported to a site without seed predators, this
feature is not necessary. Depending on the situations that occur during the invasion, a
species may thus pass through the whole process even if it does not have any of the traits
that are considered favourable for an invasive species. Nevertheless, having many such
favourable traits will increase the probability to become a successful invader. Similarly,
favourable conditions at a site can increase the probability that an introduced species
will be able to establish. But the crucial mechanism to keep in mind is: favourable species traits or favourable environmental conditions are necessary conditions for invasion
success only if the corresponding ‘problems’ actually do occur (see Box 8; Heger and
Trepl 2003). This circumstance has been termed context-dependence and it increases the
above mentioned complexity, which we have to understand and handle when trying to
explain, and – even more difficult – predict invasion processes.
Box 8. The model of invasion steps and stages (INVASS model)
From the viewpoint of an invading species, the invasion process can be dissected into different steps and stages, each connected to a certain set of problems that need to be overcome.
This is the basic idea of the model of invasion steps and stages (INVASS model; Heger and
Trepl 2003, Heger 2004). In Figure 20, each invasion stage describes a status that a species
can reach during its invasion process. To get from one invasion stage to the next, an invasion
step needs to be taken, which means that a specific set of problems needs to be solved (Figure
19).
Figure 20. Chronological dissection 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. MVP: Minimum viable population. From
Heger and Trepl (2003), with permission of Springer.
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Introduced tree species in European forests
To give an example, seeds of a cultivated tree may have been transported to a nearby site
outside the plantation, e.g. by a bird. During invasion step 2, this seed will now have to grow
without being cultivated, and the mature plant needs to reproduce. A first problem that could
potentially occur is a seed predator. A favourable condition at the site, from the viewpoint
of the invader, would be the presence of fewer predators than could be expected, e.g. as a
consequence of enemy release (Keane and Crawley 2002). A favourable trait would be to have
a general defence mechanism, e.g. a thick seed coat. With this approach of “taking the viewpoint of the invader”, potential problems that may hinder the invasion process can be named
for every invasion step, and conditions as well as species traits favourable to overcome every
one of these problems can be identified. Respective lists can be found in Heger (2004) and
Heger and Böhmer (2005).
Figure 21. Flow chart of an idealised invasion process according to the INVASS model. Invasion stages are shown as
orange rectangles; the major problems species have to overcome during each invasion step are shown as grey diamonds. “Reproduction?” means “Is the species able to reproduce without being cultivated?”, “New sites?” means
“Are other suitable sites available, and can they be reached by the species?” (from Heger 2004).
In addition to species traits, abiotic characteristics of a site and the composition of the
resident species community, there are more factors influencing whether a species will be
able to invade. The most important of these factors seems to be the so called ‘propagule
pressure’, i.e. the number of times a species is introduced into an area, and the number of
organisms (seeds, root fragments, adult organisms or other) that are present. It has been
shown several times that propagule pressure is a very good predictor of invasion success:
the more often a species is transported into a certain area, the more likely it is that this
species will be able to establish.
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2.3 Can we predict whether a species will become invasive?
Propagule pressure adds a new dimension to the set of factors influencing invasion processes. How often a species is transported into an area is determined by socio-cultural
processes, such as occurrence of trading routes, planting frequencies, or number of visitors to a national park. The high importance of this factor in determining the fate of an
invasion shows how strongly biological invasions are driven by non-ecological factors.
Socio-cultural factors are the main reason why invasions occur, and they can influence
invasions during every phase of the process. This is why it is not possible to explain
and predict the course of an invasion taking into account only biological features of the
species or its environment. Socio-cultural factors make predictions about invasions challenging: invasion science has to leave the realm of natural sciences to improve abilities to
explain and predict (Küffer 2010).
� Not only ecological, but also socio-cultural factors, e.g. trends and trading routes,
determine the course and the outcome of invasion processes.
An additional aspect complicating explanation and prediction of biological invasion results from the fact that during an invasion, species get transported into areas where they
did not evolve. In the new area, these species in many cases are confronted with abiotic conditions they have not experienced before (e.g. different soils, different humidity)
and biotic conditions (presence or absence of competing and predatory organisms). As
a positive effect from the viewpoint of the invaders, many predators within the native
range are left behind, and in exchange, they encounter species they did not have any
experience with in their native range. In almost every invasion case, the invading species
meet novel predators and novel competitors. They may also not find the mutualists they
have co-evolved with in the novel range. Much of the ‘eco-evolutionary experience’ they
gathered in their native range is no longer applicable. Vice versa, the resident species are
confronted with a species they did not encounter before (Saul et al. 2013). The outcome of
an interaction which has never occurred before (e.g. of an invading tree and the resident
insect herbivores) is difficult to predict.
Furthermore, no species is a stable, unchanging object, but instead, species are evolving constantly. Evolution has been perceived as a process taking place very slowly, and
evolutionary change as something that happened long time ago. Meanwhile it has become clear that evolutionary change can happen quite fast, i.e. in just a few generations,
and that species are constantly confronted with selective pressures eventually causing
changes in species traits (e.g. Erfmeier 2013). It has been shown that many invasive species did change genetically in the new range (Maron et al. 2004), and also that resident
species sometimes change in response to an invader (Leger 2008, Lau 2012). Also, hybridisation of introduced and native species can lead to the formation of novel species
which are sometimes even more vigorous than their parents (Ellstrand and Schierenbeck
2000). It may be possible to predict such general patterns (e.g. that closely related species
are likely to hybridise), but concerning the probability of establishment and spread of
introduced species, the possibility for evolutionary change is another factor challenging
precise predictions.
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Introduced tree species in European forests
� Novel
interactions and evolutionary changes are some of the processes
challenging predictions of the outcome of invasions.
Despite these inherent difficulties which make precise predictions difficult or even impossible, there is an urgent need to assess the risk from potentially invasive species. Prevention of biological invasions has been named as a prime aim in the Convention on Biological Diversity (CBD 1992) and the EU regulation No 1143/2014 on the prevention and
management of the introduction and spread of invasive introduced species (EC 2014). In
order to apply efficient measures for preventing the introduction of potentially invasive
species into a region, it is necessary to distinguish high risk species from those that are
very unlikely to become invasive in that area. Several risk assessment schemes have been
developed and applied to reach this aim. Many of these tools achieve good results, and
it can be expected that further improvement of the procedures will lead to even better
results in the future.
Nevertheless, due to the factors named above, i.e. complexity, context-dependence, the
influence of socio-cultural factors, novel interactions and evolution of invaders and resident species, risk assessment schemes will never be able to correctly classify the analysed invasion cases all the time. Risk assessment procedures will become better and
better in predicting the probable invasion cases, but the less likely an event is, the more
difficult it is to predict. This so called ‘base rate effect’ has been identified as a problem
also for weather forecasts and the prediction of earthquakes (Matthews 1996, 1997). As
an example, during times when the weather forecast has not been as accurate as it is
today, someone going for a walk for an hour in England was better off taking an umbrella for the walk instead of looking at the forecast – because the event of ‘no rain for one
hour’ is so unlikely that the forecast in most cases did not make a correct prediction. For
invasions this means that we may get better and better in predicting very likely invasion
cases, but there will always be a base rate of unlikely events which are unpredictable
(Lonsdale 2011).
Risk assessments for obvious reasons are based on data reflecting current states. Data on
current distributions of species oftentimes can be a good basis for predictions: whether
a species is invasive elsewhere has proven to be a very good predictor of future invasion
risk (Reichard and Hamilton 1997, Herron et al. 2007). Most risk assessment schemes
require judgment on how establishment and spread may be influenced by predicted climate change. In many cases, this is done by assessing whether the species in question
may benefit from a warmer climate or not. Sometimes ecological models are used to improve these predictions. In any case, what cannot be taken into account by current expert judgment or modelling techniques is the fact that species may change genetically
in response to a changed climate. Also, the long-term effects of invasions are far from
understood: the abundance and impact of invasive species in their new environment may
change drastically over time, due to many interacting ecological and evolutionary processes (Strayer et al. 2006).
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2.3 Can we predict whether a species will become invasive?
� Predictions of the risk of establishment of introduced species have already reached
acceptable accuracies. Much harder to predict is the future development of the
established populations, especially under the influence of changing climate and
novel interactions.
In conclusion, there are many reasons why precise predictions of biological invasions will
always be difficult or even impossible. Nevertheless, the prediction of ‘common cases’ is
possible. Future research should try to push the limits of predictability, and to improve
existing risk assessment methods and predictive models as much as possible. The aim
should be to improve predictability also of the less likely cases, and to put more research
effort into long-term effects of invaders in the face of likely ecological and evolutionary
changes. Concerning management approaches and also the evaluation of invasive species, we will have to stay flexible. The future will show how many species will decrease
in abundance again after some generations, even without management, and how well
resident species will be able to respond to their novel neighbours in the long run.
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