Great Egret

Great Egret

Wednesday, 14 September 2016

Passenger Pigeon, Japanese Barberry and.....Lyme Disease

September is the month when we mark the anniversary of an avian tragedy: it was on September 1, 1914, when the last remaining Passenger Pigeon died. A species that went from an estimated 3 billion to zero in the space of a human's lifetime. Not that the population was always so was a species whose population was believed to fluctuate dramatically, as described in this article in Scientific American.

Passenger Pigeons (PAPI) fed on the seed produced by various trees in the eastern and mid-western forests, specifically acorns from various oaks, nuts from the American Beech as well as chestnuts from the American Chestnut. American Chestnut was once one of the most common trees in eastern North American forests, but a serious blight that arrived in the early part of last century devastated the forest of that species. It is a legally endangered species due to its extremely limited occurrence in Canada.

Open oak forests were particularly attractive to PAPI, since not only did the acorns provide a valuable source of food, but the large and sturdy spreading branches were safe places to roost and/or build their nests.

In this first photo is a mix of acorns of the Red Oak and beech nuts of the American Beech.

 This next image is a close-up of an opened chestnut bur, showing the seeds in side the husk.

American Chestnut leaves

White Oak

These tree species are known to produce large quantities of seed for one or two years, and then very small quantities of seed for the next few years. Years of high seed production are known as 'mast' years. If several tree species produce mast in successive years, it might result in very high production of young PAPI causing the population to get to the estimated 3 billion. But that wouldn't last, and the productivity would then decline. This process is certainly beneficial for the trees. If the seed production maintained a consistent level over the years, those wildlife species that fed on them would maximize their production, and most seed would likely be consumed. By having most years of seed production at lower levels, the wildlife species would remain at a relatively lower level, so that when seed production increased briefly, more of it would survive to germinate and produce an age class of trees for the future.

Regardless, when the population of PAPI was reduced to zero, it resulted in a lot of seed not being consumed. And this was a boon for other species of wildlife that fed on those seeds that would normally be competing with the PAPI, such as White-tailed Deer, squirrels, mice, Wild Turkey, etc.

The numbers of those remaining species of wildlife would likely have increased substantially, now that there weren't a billion or more PAPI to compete with.

Japanese Barberry
Japanese Barberry (Berberis thunbergii) was introduced to North America in the late 1800s as an ornamental shrub. It leafs out early and has many flowers. It also has sharp spines along its stems.

It is a prolific seed producer, and birds love the bright red berries. But when the birds travel and defecate elsewhere, the seeds are spread far and wide. The sharp spines ensure that browsing by deer is minimal, so there are virtually no controls on the spread of this species. As a result, Japanese Barberry is one of the more insidious invasive species in our eastern forests. It is especially distinctive in the fall when their leaves turn a deep red.

The density at which Japanese Barberry can grow unchecked in a forest is great for mice! Their normal predators, such as owls, hawks and foxes will not penetrate the dense, prickly stand of barberry, so it is a safe haven. Numbers of mice where barberry is plentiful are known to be higher than areas where barberry is low to non-existent.

And that brings us to the unfortunate result of the loss of PAPI and the invasion of barberry. You see, mice are critical hosts to ticks, both the American Dog Tick (a.k.a. Wood Tick) and the Black-legged Tick (a.k.a Deer Tick). It is the latter species that is the prime vector of the spirochete which causes Lyme Disease. I have written several blog posts on Lyme Disease, specifically related to my several experiences with it. The most informative one can be read here.
Deer Tick
Ticks thrive where there are high populations of their deer and mice hosts. The blood meal, which is critical for the development of both nymph and adult ticks, is readily available from each of those mammal species when the host populations are high.  Also, where there is a dense stand of barberry, the relative humidity remains higher, which is also beneficial for the ticks.

It takes ticks two years to reach the adult stage when they will produce young. It is interesting to note that two years following a year of mast production of the acorns and seeds mentioned at the outset of this post, there is a greater abundance of ticks.

In simplistic terms, now that the billion or so PAPI are long gone, a high production of acorns and beech nuts attracts greater numbers of deer and mice. Mice are predominant hosts to the nymphal stage of the ticks, and deer are hosts to the adult stage of the ticks. So two years after the mast year, are more ticks, and the greater the chance of encountering a tick capable of transmitting Lyme Disease!

On a related note, I have read where in rural areas, free-range chickens, guinea fowl etc, are great at controlling ticks in their immediate area. It is possible that Wild Turkeys have a similar effect, as they feed on numerous small vertebrates and invertebrates alike. Wild Turkeys were extirpated from southwestern Ontario many decades ago, but beginning in 1984 were successfully re-introduced in several areas of the province. A decade or so after their initial re-introduction, saw Wild Turkeys move in to Rondeau Provincial Park, and they seem to be doing well. Perhaps they will help control ticks in areas of the park that they spend much of their time in.

 There are many sources of information on parts of what I have written. Some of the more useful references I found and used include:
The last one was particularly interesting, and is a pdf of an article that appeared in Birding magazine several years ago, entitled "Passenger Pigeons, Lyme Disease and Us: the unintended consequences of the death of a species" by David E. Blockstein. I couldn't find the article in the Birding archives, but managed to locate it via this online source.

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