Urbanized landscapes are the theater of multiple simultaneous biological invasions likely to affect spread dynamics when
co-occurring introduced species interact with each other. Interactions between widespread invaders call for particular atten
-
tion because they are likely to be common and because non-additive outcomes of such associations might induce negative
consequences (e.g., enhanced population growth increasing impacts or resistance to control). We explored the invasions
of two widespread invasive taxa: the Japanese knotweed species complex
Fallopia
spp. and the invasive garden ant
Lasius
neglectus
, in the urban area of Lyon (France). First, we investigated landscape habitat preferences as well as co-occurrence
rates of the two species. We showed that
Fallopia
spp. and
L. neglectus
had broadly overlapping environmental preferences
(measured by seven landscape variables), but their landscape co-occurrence pattern was random, indicating independent
spread and non-obligatory association. Second, as
Fallopia
spp. produce extra-floral nectar, we estimated the amount of
nectar
L. neglectus
used under field conditions without ant competitors. We estimated that
L. neglectus
collected 150–321 kg
of nectar in the month of April (when nectar production is peaking) in a 1162
m
2
knotweed patch, an amount likely to boost
ant population growth. Finally, at six patches of
Fallopia
spp. surveyed, herbivory levels were low (1–6% loss of leaf surface
area) but no relationship between ant abundance (native and invasive) and loss of leaf surface was found. Co-occurrences of
Fallopia
spp. and
L. neglectus
are likely to become more common as both taxa colonize landscapes, which could favor the
spread and invasion success of the invasive ant

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