A story from the AP is suggesting that the recent swarm of bird deaths in Michigan are the indirect result of the proliferation of invasive species in the Great Lakes. The birds are washing ashore at Lake Michigan by the dozen, with the cause of death determined to be avian botulism (type "E"), one of several episodes to occur over the past few years. The botulism itself is not considered invasive, but scientists are speculating that it is getting to the birds through a food chain of invaders: Zebra and quagga mussels (Dreissena polymorpha and D. rostriformis bugensis) containing the bacteria that causes botulism (Clostridium botulinum) are either consumed by birds or consumed by invasive round gobies (Neogobius melanostomus), which are then consumed by birds.
I am not sure if invasive species are specifically to blame here, or if it is just that the invaders are so numerous that they now make up a significant portion of the diet of birds that frequent the Great Lakes. Are zebra and quagga mussels the only ones that can filter botulinum bacteria? Do round gobies harbor more of the bacteria than native fish? The AP article glosses over the surface of the full story, so I did some investigating and found the following hypotheses, summarized from this well-written article from Michigan Sea Grant and this one from Wisconsin Sea Grant:
- Invasive mussels create a type of habitat that is more favorable for the growth of Clostridium botulinum.
- Zebra and/or quagga mussels filter at a much faster rate than native mussel species, allowing the toxins and pathogens that they filter out to build up to a greater degree than similar-sized native species.
- The filtering power of these invasive mussels clears the water, allowing the algae Cladophora to grow. When this algae reaches sufficient numbers and then dies, the resulting anaerobic water conditions encourage the growth of Clostridium botulinum.
It is important to note that none of these hypotheses have yet been proven to be an underlying cause of avian botulism. Research continues in both the US and Canada in order that we might one day understand the cycle of bird deaths and stop it from repeating. Until then, the invasive mussels may seem the most obvious target of blame but it could turn out that the situation is much more complicated.