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DOUG LEVEY: There are two social forms of fire ants. They're what
are called polygynes and monogynes. Polygyne literally
means many queens, monogyne means a single queen. Polygynes
have colonies in which there are many queens that are laying
eggs simultaneously whereas monogynes are generally smaller
colonies because there's a single queen that lays. There
are two important ecological differences between monogynes
and polygynes. One of them I've already mentioned, the polygynes
occur at much higher densities. They're simply larger nests,
more queens. The other one is that they disperse very
differently. The monogynes will have mating flights in which the
males and females will fly high up into the air, a hundred
meters or so often higher than that. They'll mate and then the
females will essentially rain down all over the place and
start new colonies. In contrast the polygynes will disperse
either on foot or in mating flights that are just a meter or
two off the ground. And so they're what we call disperse
limited, they don't spread as rapidly as the monogynes.
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When we went into this study then we predicted
that the monogynes would be less influenced
by our corridors simply because in our case they
fly well over the canopy of the forest that we were
studying and the scale of corridors really doesn't make
much difference to them because they're dispersing so high up
over such great distances but, that in contrast, corridors
would make an effect on how well the polygynes disperse. So what
we found was that indeed, that was the case, that in the areas
where there are polygynes that polygynes are much, much more
abundant they're already abundant, but they're much more
abundant on average in patches that are connected by a corridor
than in patches that are not connected by a corridor. And
that strongly suggests that the facilitation of movement
provided by the corridor allows for this increase in population
size. Monogynes on the other hand were not affected by
corridors. The presence of the corridor makes no difference to
them at all. Taking this one step further, monogynes we
found, were associated with a significant decrease in the
species diversity of the native ants so where the polygynes
occurred native ant species diversity, it was about 23%
lower in connected patches than in unconnected patches. So that
facilitation of movement by the polygynes does appear to have a
significant impact on the native species, whereas there was no
impact of the monogynes on the species diversity
of native ants.
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The important take home message here is that, in
general, corridors are an important and an affective
conservation tool. They make a big difference for species that
are not able to disperse very well. It turns out that most
invasive species like fire ants are good dispersers almost by
definition invasive species get around very well and fire ants
are no exception and for the vast majority of fire ants, the
monogynes, which tend to be more common, the corridors don't make
any difference at all. Where it gets interesting is that the
polygynes are essentially the exception that proves the rule.
They are an invasive form that is dispersal limited. They're
not very good at dispersing and so if you provide a corridor for
them, that corridor does benefit them and they are able to move
around. You can't have it both ways. You can't have an
ecological facilitation of just one type for just one type of
organism without it having similar effects on other types
of organisms. So for better or worse corridors will facilitate
the spread of some invasive species such as the polygynes.
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