Human history is paved with dreams of promised lands, often handed
by a godly figure, and offered to mankind for colonization. Generally speaking,
these promised lands are void of previous human presence. When they are not, it
generates long lasting problems which are not the focus of today’s post, quite
the opposite in fact.
Invasion biology typically deals with the promised land
concept: discover, settle and multiply in a new environment. For fish, the
notion of promised land may vary, but it generally already includes other fish
species presence, may they be competitors, predators, or food. This is often
factorised under the “invasibility” term. And this is interacting with
“invasiveness” that characterizes a species intrinsic ability to invade a
system. The respective boundaries of these two concepts are somewhat difficult
to describe, and they usually strongly interact, so it would be cool to be able to somehow isolate one of these two
to study the other properly.
The Promised Land ?
Such a setup would require a fish species to be invading an
ecosystem without any other prior fish species, which would remove a large part
of the invasibility effect. The almost perfect incarnation of this setup is
provided by the introduction of various salmonid species in the remote
Kerguelen Islands, at the convergence of Indian and Antarctic oceans. The freshwater
ecosystems of these islands are initially virgin of fish species, and present a
very poor and limited specific diversity. This is almost perfect, because
several species were released by “the godly hand” since 1954. However,
the success of the invasion by brown trout (Salmo trutta L.) far eclipsed the other
introduced species: between 1962 (the first successful natural spawning) and
2012, 42 systems were invaded, and only 9 of these invasions were the
consequence of a direct man-made introduction.
A typical Kerguelen river system, usually presenting an intricate arrangement of river arms, ponds, lakes, and sea.
Our lucky lab has been monitoring this colonization process
since the beginning. We therefore have a relatively precise idea of the
colonization dynamics. This offered a unique opportunity to study
metapopulation dynamics of the invasion process, as it is shown by the figure 1
below.
Figure 1: Brown trout colonization progress since 1960
as monitored by our lab. The black dots represent either observed colonizations,
or deduced colonizations using scale readings.White dots are for virgin patches.
We built a metapopulation-like invasion model, accounting
for various possible distance matrices between patches, patch size, but also
accounting for possible temporal trends in the dispersal kernel as well as in the
colonization function, allowing to test for possible change in the invasion
process along time. Because we had uncertainties on some colonization dates for
some patches, we also integrated this uncertainty in the model. Below, a plot
of trajectories obtained from the model, compared to the observed data.
Figure 2: A projection of simulated trajectories obtained from
the model (full grey lines), compared to observed number of colonized rivers
along time (full black line). The interrupted line also indicate what could be
the maximum number of colonized patches, accounting for the fact that we could
not check each system each year. This uncertainty was included in the model
estimates, and generates a part of the variation observed between trajectories.
The main results were satisfying – at least for a modeler. The
model actually predicted that colonization could occur (cool!) and that lack of colonization was not probable (equally cool!). It also predicted
a slowing pace of the number of river colonized per year, hereby meaning that a peak in the
invasion dynamics had been reached in the 1980’s. Now, that is for the general
pattern, but the underlying details are of much more interest:
- First, while we did not observe any extinction in our data, we still tried to fit a model accounting for patch extinction. It did not fit the data any better than a model without extinction, hinting that in the elapsed time, all colonization events were successful.
- The most likely dispersal pathway followed a coastal pattern: fish seemed to stick to the coast, instead of crossing bays or large fjords (> 1km).
- Large patches (large rivers, often connected to lake ecosystems) did not produce more dispersers, but they attracted more colonizers.
- The invasion process changed over time, which was translated in the model by an increasing dispersal distance and a decreasing colonization probability over time, whatever patches spatial arrangement and sizes.
Additionally, several cool things can be imagined
based on these results. First, assuming that the estimated parameters apply, we
can try to predict what would have happened if man only made one successful
release of brown trout in 1962. The figure 3 below shows that quantitatively
speaking, half the number of actually colonized rivers in 2012 would still be
virgin.
Figure 3: The same plot of projected trajectories, but now in French,
and assuming that only one man-made introduction happened in 1962, in one of
the central river system (Rivière du Chateau). So not only man intervention was
a prerequisite for salmonid invasion of these remote islands, but the
introduction effort highly influenced the general dynamics of invasion.
Second, if the process goes on, it means that colonization
rate will decrease, but colonizations might happen now farther from the
invasion core. Which is a problem, since brown trout is considered as an
invader in these ecosystems. However, the available virgin patches are
now mostly located on the western part of the archipelago, and some of
them are under the influence of several glaciers - currently melting.
Coming back to the Promised Land, despite the lack of fish
prey, fish competitor or fish predator in rivers, brown trout thrived in these
ecosystems, perhaps demonstrating a high invasiveness potential. Yet, the
colonization process is slowing down. And man-made introductions are nowadays
totally forbidden too. It is somehow puzzling to see how successful this
species can be out of its initial distribution area, in a wide range of invasibility
conditions (South America, New Zealand, Kerguelen).
Everywhere you go, brown trout might be already there.
We certainly hope that this study will contribute to
understand and to forecast how this species will now behave in the numerous ice-melting
boreal landscapes.
If you read French, and want to know more about our work in
Kerguelen Islands, you can also have a peek at our recent report on the last expeditions supported by the French Polar Institute.
References :
Lecomte, F.; Beall, E.; Chat, J.; Davaine, P.; Gaudin, P. 2013. The
complete history of salmonid introductions in the Kerguelen Islands, Southern
Ocean. Polar Biology, 36 (4) : 457-475.
Labonne, J.; Vignon, M.; Prévost, E.; Lecomte, F.;
Dodson, J.J.; Kaeuffer, R.; Aymes, J.C.; Jarry, M.; Gaudin,
P.; Davaine, P.; Beall, E. 2013.
Invasion dynamics of a fish-free landscape by brown trout (Salmo trutta). PLOS One, 8 (8) : e71052.
