COMMENTARY To what degree are invaders drivers or passengers of

Journal of Vegetation Science 25 (2014) 1311–1312
COMMENTARY
To what degree are invaders drivers or passengers of
phylogenetic community structure?
Jean H. Burns
Abstract
Bennett et al. (2014, Journal of Vegetation Science 25: 1315–1326), test the biotic
resistance hypothesis and show that the phylogenetic structure of the invaded
community responds to an invader, rather than driving invasion patterns. More
such field experiments are needed, including those that manipulate the phylogenetic structure of communities at multiple spatial scales, to disentangle phylogenetic biotic resistance from other drivers of invasion patterns.
Bennett et al. (2014) provide a novel experimental test of
the biotic resistance hypothesis, demonstrating that the
phylogenetic structure of the native plant community has
little explanatory power in driving the invasion of Bromus
inermis Leyss. This is inconsistent with ‘Darwin’s naturalization hypothesis’, that competition from close native relatives might limit invasion (Darwin 1859; Strauss et al.
2006). Instead, they find that B. inermis decreases the frequency of occurrence of species from common lineages,
decreasing species richness and increasing standardized
phylogenetic diversity in invaded communities. Without
experimental evidence like this, such a pattern might have
been mistakenly interpreted as evidence that B. inermis
invasion is driven by its evolutionary distinctiveness relative to the invaded community. Instead, the invader is
probably more a driver of community phylogenetic patterns than a passenger.
Some observational studies have supported Darwin’s
naturalization hypothesis, for example among California
grasses (Strauss et al. 2006). If closely related species compete more strongly than distantly related species (e.g.
Burns & Strauss 2011; but see e.g. Venail et al. 2014), then
evolutionary distinctiveness could mean escape from competition for the invader. Other mechanisms could also
influence phylogenetic patterns in invasion, such as
greater enemy escape with longer phylogenetic distance
from the native species (Hill & Kotanen 2009). Darwin also
proposed the alternative hypothesis, that habitat filtering
might drive increased success of introduced species that
share traits with close native relatives (Darwin 1859), and
this pattern has also been found (reviewed in Procheş et al.
2008). However, there are very few experimental tests for
the mechanisms governing community assembly, and
many authors have pointed out that phylogenetic patterns
should not be interpreted as conclusive evidence for mechanism (e.g. Mayfield & Levine 2010).
Regional species pool
A
A
B
C
D
E
H
F
G
H
I
Before invasion
Local scale
Commentary
Regional scale
Burns, J.H. (corresponding author,
jbm122@case.edu): Department of Biology,
Case Western Reserve University, 10900 Euclid
Ave, Cleveland, OH 44106, USA
(a)
even
Less
invasive
introduced
species
Highly
invasive
introduced
species
After invasion
D
(c) reduced
richness
D
F
H
B
(b) clustered
B
C
D
F
(d)
novelty
A
B
D
H
Fig. 1. Spatial scale can influence phylogenetic dispersion patterns
leading to: (a) phylogenetically even local or (b) clustered regional
communities (Cavender-Bares et al. 2009). (c) In the study of Bennett et al.
(2014), a highly invasive species (H) reduced local species richness and
increased phylogenetic diversity, compared to pre-invasion (a). Other
studies have found that (d) more evolutionarily novel introduced species
(H) are more invasive than species with close native relatives (A), e.g.
Strauss et al. (2006).
Journal of Vegetation Science
Doi: 10.1111/jvs.12221 © 2014 International Association for Vegetation Science
1311
Commentary
J.H. Burns
Characteristics of the native community influence
invasions, but invaders also have large effects on native
community structure, so cause and effect can be difficult
to determine with purely observational studies. By
experimentally removing the local plant community and
adding an invasive species to paired removal and control
plots, Bennett et al. (2014) provide a direct test of the
strength of biotic resistance to invasion. One limitation
to this approach is that it does not directly manipulate
phylogenetic distance to the target community. Thus
phylogenetic distance is confounded with characteristics
of the native community such as nutrient availability.
Studies that manipulate phylogenetic community structure could directly test the phylogenetic component of
the biotic resistance hypothesis, and would be valuable.
Comparisons in the same system of the invasibility of
naturally occurring and experimentally manipulated
phylogenetic community structure are also needed, to
determine how much real-world variation experimentally identified mechanisms are likely to explain (Price &
P€artel 2013).
Many community phylogenetic patterns, such as
Darwin’s naturalization hypothesis, are dependent on
spatial scale (reviewed in Procheş et al. 2008; CavenderBares et al. 2009), perhaps because different mechanisms
may act at different scales (Fig. 1). Further, multiple
mechanisms are likely occurring at any given scale. Thus,
ecologists should use caution in extrapolating the results
of small-scale field experiments, such as the results of
Bennett et al. (2014), to larger-scale community patterns,
such as those found in Strauss et al. (2006). Future experiments that test mechanisms governing scale dependence
are needed (Fig. 1). In addition, comparative studies of
species that differ in their invasiveness (sensu Burns et al.
2013) would help determine the mechanisms governing
invasion patterns like Darwin’s naturalization hypothesis.
Bennett et al. (2014) provide an important step forward
in our understanding of the mechanisms driving invasion
patterns, including Darwin’s naturalization hypothesis. As
they point out, invaders may both respond to community
phylogenetic structure and also change phylogenetic structure after invasion. Thus experimental field tests with
before and after invasion sampling are essential to disentangle cause and effect. Further, the mechanisms that
drive community phylogenetic patterns are not well
understood, and experiments designed to determine
mechanisms underlying patterns across spatial scales are
1312
essential. In addition to providing a novel experimental
result, Bennett et al. (2014) can serve as a call to action for
community ecologists to conduct more field experiments
on the mechanisms governing community phylogenetic
patterns (see also Cavender-Bares et al. 2012).
References
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Journal of Vegetation Science
Doi: 10.1111/jvs.12221 © 2014 International Association for Vegetation Science