When an invasive species leaves its habitat, it takes its predators and any other species that may have contributed to its success with it.
When invasive species invade a new area, they don't just lose their natural predators, but also the beneficial interactions that helped them spread to their new home.
Professor Angela Moles of the School of Biological, Environmental, and Earth Sciences at UNSW Sydney led a meta-study that looked at the "friendship networks" of invasive species (such plant pollinators) before and after they departed their native habitat and published their findings in Biology Letters. The research showed that invasive species interacted with roughly twice as many native friendly species than they did in invaded areas, at least for plants. This mutualistic species interacted with one another 2.3 times more frequently in their home region.
Prof. Angela Moles states, "In this paper, we show that many introduced species leave not only their adversaries but also their allies behind." About sixty percent of introduced species never get established, and this could explain why.
The "Missed Mutualist Hypothesis" proposes that invading species cause the extinction of native mutualists, or species whose associations are beneficial to both parties. This is the first meta-study to quantitatively investigate this hypothesis. The Missed Mutualist Hypothesis highlights the benefit offered to invading species when they lose their native predators, but it has been missed in favor of the more well-tested and recognized Enemy Release Hypothesis.
According to Prof. Moles, "I think many ecologists were more interested in understanding what made introduced plants so successful," as opposed to the elements that prevented them from invading a new range. However, knowing both the positive and negative selective pressures introduced species experience is necessary for comprehending the forces acting upon them, which in turn shape the success of colonization and the evolutionary trajectory of these species.
Animals that no longer have mutualists
The Missing Mutualist Hypothesis suggests that by taking into account the interspecific interactions of introduced populations, we can improve our biosecurity measures by accounting for both lost adversaries and missed mutualists.
According to Professor Moles, "for example," introduced pines can be stunted in their growth if there are insufficient fungus in the soil. However, the use of fungus as a growth aid in pine plantations is nearly entirely unregulated around the world.
The tree tobacco that has become an environmental weed in Australia has twice as many pollinators in its native region of South America as it does in one of its other introduced ranges, South Africa.
Biocontrol and management may be strengthened with a continued focus on, and attention to, mutualistic connections.
Even though clownfish (Anemone) are all the rage in North American aquariums, they have failed to catch on in the Caribbean or the Mediterranean. Professor Moles speculates that this is because anemones, a type of mutualist, are exclusively found in the Indo-Pacific.
The Port Jackson Fig, or Ficus rubiginosa, is pollinated by five different fig wasp species in Australia, but only one in the Northern Hemisphere where it has been imported.
Friend-making species that relocate to new areas
The article reveals that the disappearance of mutualists does not necessarily mean doom for species that venture into new settings.
When introduced to a new ecosystem, invasive species face choices somewhat different to those we do when we uproot our lives and relocate far away from our loved ones. To compensate for the loss of their mutualistic partners, invading species may develop novel mutualistic partnerships or face increased selective pressure to become autonomous.
Without mutualists, "species that transition to strategies may reallocate energy from mutualism toward enhanced competitive capacity," as prof. Moles puts it.
Flowers, for instance, produce nectar to reward pollinators and aroma to lure them, but these resources could be put to better use expanding leaf size or pollen production.
A third possibility and potential outcome involves introduced species creating new ties with new (local) mutualists, such as an invasive plant co-opting a local pollinator.
It is possible to implement management strategies that aim to disrupt fresh associations when introduced species acquire new mutualists in the introduced range. To control introduced legumes, which can colonize new environments by acquiring and employing local rhizobacteria or root-assisted bacteria, we could, for instance, interfere with the establishment of this connection, as Professor Moles suggests.
This paper's authors hope that the Missed Mutualist Hypothesis will be more widely cited and used, and that the whole suite of a species' relationships will be taken into account when trying to comprehend and manage an invasive species.
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Cell Biology
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