Discussion
One of our main findings is that locally diverse ant assemblages
resisted W. auropunctata in both Mexico and Puerto Rico. We
identified several dominant species from the local ant assemblage in the
native and introduced habitats and compared their competitive
interactions with W. auropunctata. In particular, we examined
resource and interference competition across short and long-term time
scales. We found that in Mexico the only the ground-foraging ant
S. geminata displayed superiority against W. auropunctata,
while S. picea, P. synantropica, and P. protensa
also lost against W. auropunctata at baits. Furthermore, the
long-term competitive interactions in the laboratory showed
that W. auropunctata was superior against S. picea. In
Puerto Rico, another exotic ant S. invicta outcompeted W.
auropunctata in areas where they overlapped. Laboratory experiments
with the native ant L. iniquum showed that W. auropunctata
was superior at baits across the short and long-term competition
experiments.
Exotic ants can displace native ant species through interference and
resource competition(Sakai
2001). While these traits have been implicated in the success of exotic
ants, they don’t always explain why exotic ants excel at displacing
native ant species (Holway 2002a).
Within native communities, competitive trade-offs between behavioral
dominance and resource discovery are thought to promote species
coexistence (Fellers 1987,
can violate the discovery-dominance trade-off in the introduced range,
in the case of Argentine ants, resulting in a competitive advantage over
native species (Holway 1998). While the
breaking of the discovery-dominance tradeoff has been linked with the
invasive success of Argentine ants, we lack such information on other
major invasive ant species. Laboratory experiments involving W.
auropunctata and native ant species, showed that W. auropunctata
had the poorest foraging abilities in terms of resources discovery
compared competitive trade-offs and found that W. auropunctata
was less efficient in discovering and recruiting worker to baits in
their introduced habitat of Puerto Rico relative to their native habitat
in Mexico, suggesting that competitive trade-offs are not indicative of
invasive success (Yitbarek et al. in press).
The organization of exotic species within local species assemblages in
their native ranges allows us to understand why some exotic species
thrive in their introduced
ranges (Calcaterra 2016). Here we
sought to understand how W. auropunctata ranks relatively to
locally dominant native ant species. In Mexico, several native ant
species from the local species assemblage were able to withstand W.
auropunctata. The tropical ant S. geminata showed high levels of
aggressiveness towards W. auropunctata at baits. Although
W. auropunctata was initially able to dominate resources,
S. geminata’s high recruitment and interference ability led to
the displacement of W. auropunctata. Although S. geminata
is frequent victim to attacks by phorid flies which can interfere with
its ability to rapidly discover
resources(Brown 1991,
phorid flies at our site which greatly enhanced S.
geminata’s dominance at baits. In comparison, P. protensa is a
slow moving ground forager ant with many small nests that are widely
distributed. Although W. auropunctata was initially dominant at
baits, P. protensa was able to overtake the baits for some time
after which W. auropunctata regained its dominance again. These
cyclical patterns are illustrative of the transient nature of
competitive interactions over time.
The arboreal ant S. picea is a minute ant that primarily forages
on coffee where its nests can be found under the bark on the trunk of
the coffee tree. S. picea was initially more dominant but as
W. auropunctata recruited its workers from the leaf litter and
arboreal nests to the baits it started to regain its dominance. The
ability to forage and nest on the ground and the trees enables W.
auropunctata to hold a competitive advantage over native species. We
confirmed this pattern in the long-term experiment were W.
auropunctata interference ability enabled it to invade S. picea nests.
The ground-foraging ant P. synanthropica nests in the ground but
forages both in the leaf litter and in the coffee trees. P.
synanthropica was competitively dominant over W. auropunctata as
it quickly discovered baits and vociferously defended against W.
auropunctata intruders. P. synanthropica’s dominance has also
been observed in competition experiments against S. geminata and
P. protensa (Perfecto 2013).
In Puerto Rico, the exotic ant S. invicta was superior to
W. auropunctata at ground baits. S. invicta reached high
population densities at baits which forced W. auropuncatata to
being pushed out at baits. While S. invicta attained a numerical
advantage over W. auropunctata, the extent of their colonies was
limited to areas of the farm that received more sun, which allowed
W. auropunctata to retain its dominance in other areas where
S. invicta did not occur. Laboratory experiments showed an
oscillatory trend between W. auropunctata and L. iniquum
competition. We found that L. iniquum workers were faster at
discovering resources but were less efficient at guarding baits against
W. auropunctata workers. As W. auropunctata recruited more
workers it slowly attained dominance. These patterns are reflected
across the long-term experiment were we found higher numbers of live
W. auropunctata workers. W. auropunctata displaced
L. iniquum by raiding their nest and killing its workers and
brood, in some cases moving its entire nest into the invaded species.
Our long-term experiments show that nest invasions are a mechanism by
which W. auropunctata displaces native ant species. Previous
laboratory experiments showed that W. auropunctata displaced the
native ant species Monomorium subupacum by invading its nest and
consuming workers and brood (Vonshak
2011). Similar evidence for nest raiding of ant colonies has been found
for the invasive ants L. humile and P. megacephela in
their introduced ranges(Holway 2002,
Recent evolutionary studies have shown that W. auropunctata
invasion displays a polymorphism in its reproductive system
reproductive structure first occurred within the native range of
W. auropunctata from low-density populations in natural habitats
(i.e. sexual populations) to high-density workers (i.e. clonal
populations) in anthropogenic
habitats(Fournier 2005a,
human modified habitats suggests that W. auropunctata’s invasive
success is associated to its clonal reproductive
structure(Hufbauer
2012). It remains unclear, however, to what extent clonality in the
introduced range contributes to ecological dominance in W.
auropunctata populations. Our study compared the invasion dynamics of
W. auropunctata populations in agricultural ecosystems. W.
auropunctata populations occurring in these anthropogenic sites are
mostly likely clonal (Orivel
2009). Within our most densely populated site in Mexico we found that
W. auropunctata was competitively superior against several
dominant ant species, contrary to Puerto Rico where the most dominant
ant L. iniquum was able to withstand competitive pressure by
W. auropunctata .
Although direct competitive interactions can structure local
assemblages, indirect interactions can alter competitive
outcomes(Hsieh 2012).
Trait-mediated-indirect effects (TMII) have been found to play an
important role in structuring ecological communities
can be highly host specific, by using ant pheromones, which as a
consequence reduce the number of ant foragers at
resources (Feener Jr 2000). This
can have important ramifications in ecosystems when phorids reduce
competitive hierarchies (Hsieh 2012).
Several studies have reported that Pseudacteon phorids affect resource
retrieval by S. geminata species
didn’t observe phorids during direct encounters involving S.
geminata and W. auropunctata but that may be heightened by
biogeographical disparities
reductions in foraging activities of Linipethema species have
been recorded in the presence of phorid
species(Orr 2003). Future studies should
evaluate whether L. iniquum species get attacked by phorid flies
in the field and how this indirect effect impacts competitive
interactions with W. auropunctata in Puerto Rico.
A central question in invasion biology is to what extent exotic species
are limited by their competitors in their native ranges and how release
from interspecific competition can explain their invasive success in the
introduced range. We found that W. auropunctata was able to
outcompete dominant ants in Mexico, but that it was restricted by native
competitors in its introduced range of Puerto Rico. These findings
suggest that release from interspecific competition does not explain the
invasive success of W. auropunctata in novel environments. An
alternative explanation for W. auropunctata’s invasive success is
the escape from natural enemies in the introduced
range (Torchin 2003). Recent work
suggests that a purge from endosymbiotic bacteria might have contributed
to W. auropunctata’s invasion success because of lower parasite
pressure in the introduced
range (Rey 2013). The loss of
parasites during invasion has been reported for several exotic ant
species (Reuter 2004,
endosymbiotic bacteria within W. auropunctata populations between
Puerto and Mexico. In particular, experiments that seek to determine how
changes in W. auropunctata’s microbiota influence interspecific
interactions with native species.
\label{section-3}
\label{section-4}
\label{section-5}
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