Thursday, May 30, 2013

Ecological Importance of Beavers

An American Beaver gnawing through a tree
In addition to being prey animals for several mid-sized predators, the two extant beaver species (American Beaver, Castor canadensis, and Eurasian Beaver, Castor fiber) are keystone species whose presence in an area improves wetland health and increases biodiversity. Beavers are most renowned for their construction activities, particularly in the felling of trees to be used in the building of dams. Their permanently sharp, chisel-shaped incisors are ideal for gnawing through wood. Trees 5cm (2in) in diameter are felled in minutes. When felling trees, beavers gnaw through the tree trunks just enough so that the tree remains standing. They then retreat and allow the wind to complete the task. This technique requires months to perfect and young beavers are occasionally injured or killed in the process. The visible damage to trees may seem detrimental, but the trees that beavers favor (aspens, poplars, cottonwoods, and willows) are characterized by rapid growth and beaver pruning of these trees stimulates reinvigorated growth the following spring. 

Dam Structure
Dam building is most intense in the spring and fall, although lighter construction activities may go on at other times of the year. Beaver dams are preferably built across streams less than 6m (20ft) across and less than 1m (3.3ft) deep. However, the animals continually add new material to the structure resulting in dams that may be 100m (330ft) long and 3m (10ft) high over a period of years. Wood and stones are bound together by tightly compacted mud, which is applied by hand and not with their flattened tails contrary to popular belief. The mud, stones, sticks, and branches that beavers use to build their dams make for a very robust and solid structure, behind which a substantial pond is formed. By impounding a large body of water they effectively surround their home with a moat, increasing their security from predators. Moreover, a bigger lake means that the resident beavers have wider access by water to distant feeding areas.

An abandoned beaver dam with new vegetation growing
from the built up sediment within.
Benefits to Other Species
In addition, the large areas of wetland that dams create reduce erosion damage and improve biodiversity. By slowing the flow of rivers and streams, beaver dams boost sediment deposition, a natural filtration system that removes potentially harmful impurities from the water. Increased sedimentation also brings more nutrients to the soil in and around the water resulting in abundant and healthy aquatic and riverside vegetation. Small aquatic animals such as larval fishes and frogs, in turn, use this vegetation as shelter, and of course herbivores use these plants as a food source. Beaver ponds provide refuge and habitat for waterfowl, wading birds, fish, amphibians, turtles, and aquatic mammals. Eventually, the buildup of sediment around the dam becomes too great overtime and the resident beaver colony is forced to move on. The abandoned, silted-up beaver dam creates the basis for a rich, new ecosystem, developing into a wetland meadow whose soil, enriched by decaying plant matter and beaver feces, support reeds, sedges, and eventually large trees. Also, trees that are drowned by rising water levels caused by beaver dams become ideal nesting sites for birds such as owls and woodpeckers.

Canal Building
Of all the construction activities carried out by beavers, canal building is the least complex. They use their hands to loosen mud and sediment from the bottom of shallow streams and marshy trails, pushing it out of the way to the sides. The resulting channels enable the beavers to stay in the water while moving between ponds and feeding areas. This behavior occurs more often in summer when water levels are low. Other aquatic animals benefit from this activity as it can be a lifeline during drier periods. When one pond dries up, they can use the beaver channels to travel to another.

An example of a beaver lodge.
Burrowing Activity
Beavers are highly efficient diggers, usually excavating multiple burrows within the family territory. Hollowed into the bank of a stream or pond, a burrow may be a single tunnel or a whole maze ending in one or more chambers. In many beaver habitats, beaver families use burrows as the primary residence. Abandoned beaver burrows can in turn, be utilized by other animals such as otters or minks. 

Beaver Lodges
Alternative riverside accommodation is provided by the lodge, a conical pile of logs and branches sited on the bank or isolated in the middle of a beaver pond. Lodges average 3-4m (10-13ft) in diameter, with rooms measuring 1-2m (3.3-6.6ft), and always incorporates a living chamber above water level. Sometimes there is also a dining area nearer the water. The entrance to the lodge is underwater out of sight of potential predators and usually includes a back exit. Occasionally, aquatic birds will set up their nests on top of beaver lodges, taking advantage of the structure's high vantage point and distance from the shore and any terrestrial predators. 


A Canada Goose sits on its nest on top of a American
Beaver's lodge.
The beaver lodge is, in actuality, a highly specialized burrow composed mainly of organic materials rather than earth. Clearly, beavers are not far removed from their subterranean ancestors as environmental engineers. The extinct Miocene genus Palaeocastor, which were analogous to extant prairie dogs (Cynomys), were themselves keystone species whose digging activity helped to channel rainwater into the water table to prevent erosion. These animals would have also changed the local soil composition, and subsequently effected the flora and fauna of the area.

Beavers & Humans
True to their reputation, beavers are highly industrious animals whose construction work not only ensures their own survival, but the health of other organisms and the environment as a whole. When removed from an area, the aquatic habitat undergoes a noticeable loss of biodiversity. Unfortunately, intense hunting for their fur has lead to the extirpation of beavers on both sides of the Northern Hemisphere, with the Eurasian species in particular suffering a critical drop in numbers. Fortunately, hunting of these animals has largely ceased and populations are restoring themselves. The American species has regained much of its former range in the midwestern and eastern states, and its Eurasian cousin has grown from eight relic populations with an estimated 1,200 individuals in the early 1900s to over 600,000 today, revitalizing their ecosystems as their ranges expand. Sadly, this comeback ha not been without its backlashes. Beavers and humans come into conflict when beavers convert agricultural land into wetland. Beavers are a keystone species in wetland habitats and it remains for humans to acknowledge their environmental contributions and develop strategies that allow both humans and beavers to share the same landscape.

References & Further Reading
Macdonald, David W. "The Princton Encyclopedia of Mammals. Princeton", New Jersey: Princeton University Press, 2009. 142-145


Catalogue of Organisms. “Beaver Fever” (2012). Retrieved November 3, 2012 from http://coo.fieldofscience.com/2012/03/beaver-fever.html

Beaver Pictures and Facts. Retrieved November 3, 2012 from http://fohn.net/beaver-pictures-facts/index.html#Introduction  

Photo Credits
  1. Beaver gnawing tree: D. Gordon E. Robertson, 31 January 2010, Gatinaeu Park, Quebec, Wikimedia Commons
  2. Whitefish Channel beaver dam: Fungus Guy, 2 July 2006, Wikimedia Commons
  3. Abandoned beaver dam: July 2008, Wikimedia Commons
  4. Beaver lodge: Paulyang, 28 July 2007, Wikimedia Commons
  5. Canada Goose on beaver lodge: Frederic C. Brenner, 1960, Wikimedia Commons

Wednesday, May 15, 2013

Tasmanian Thylacine (Thylacinus cynocephalus)

The Tasmanian Thylacine (Thylacinus cynocephalus) was the last survivor of the Thylacinidae, a family of predatory marsupials that first appeared about 25 million years ago. It is well-known as the largest predatory marsupial in recent times and was the unfortunate victim of one of the worst genocidal assaults upon any land mammal.

A stuffed Thylacine at the Natural History Museum at Oslo. Wiki

Etymology
The name Thylacinus comes from the Latin word thylaco, meaning "pouch" or "sac", and the Greek cynos, meaning "dog". The species name is a combination of the Greek cynos and kephale, the latter meaning "head", referencing the superficially fox or jackal-like head of the animal. This species is also commonly referred to as the Tasmanian Tiger for the vertical stripes running down its back.

Habitat & Distribution
Tasmanian Thylacines roamed freely across mainland Australia, Tasmania, and New Guinea for at least 4 million years, making it among the oldest mammalian species in the world. They survived into historic times on the island of Tasmania, and became extinct there due to human persecution. Remnant populations of it have been rumored to still exist on mainland Australia. Their preferred habitat were forests and woodlands, as well as the more open, drier savannas and scrublands that once dominated northern, eastern, and southern Australia.


Physical Attributes
At first glance one might mistake the Tasmanian Thylacine for a type of wild dog. Although its head is somewhat reminiscent of that of a dog, its body was more cat-like. Its body was long, muscular, and flexible with sturdy limbs and semi-plantigrade feet. Its tail was long, stiff, and muscular to keep the animal balanced while running and leaping. These were stalk-and-ambush predators, unable to run fast for long periods like a dog. Its forelimbs and paws were built to seize and hold its prey after a surprise attack. Its most distinctive feature was the row of vertical stripes down its back. Thylacines also had the ability to prop themselves up on their hindfeet and tail in a manner similar to a kangaroo, a posture that enabled them to peer over tall grass or other obstacles when searching for prey. They have even been reported to hop in a kangaroo-like hop for short distances.

Ecology & Behavior
According to eyewitness accounts, Tasmanian Thylacines were solitary while hunting although small family groups may have shared the same home range. Breeding could apparently take place at anytime of the year with peak breeding seasons in winter and spring. Females could produce up to 4 pups at a time. They were primarily nocturnal or crepuscular.


Tasmanian Thylacines were mid-sized predators that specialized in taking prey smaller than themselves. Detailed biomechanical analysis of the animal's skull reveals that its narrow face was ill-suited to handle relatively large prey items on a regular basis, thus limiting it to animals its own size or smaller. Furthermore, the anatomy of its limbs is reminiscent to those of modern ambush hunters like cats and civets. In this regard, the Maned Wolf (Chrysocyon brachyurus) of South America,  another small-prey specialist which hunts by stalk-and-ambush, is perhaps the best ecological analogue for the Tasmanian Thylacine. 

The regular prey menu of the Tasmanian Thylacine included reptiles, birds, echidnas, possums, bandicoots, and wallabies up to about 10kg. Although its sense of smell was acute, its senses of sight and hearing were more important to it during a hunt. Once prey was detected, it would stalk it in a cat-like fashion before springing upon it once within range. Larger prey was dispatched with a bite to the back of the head while smaller prey would simply be pinned down and crushed in its jaws. Specializing in smaller prey in this manner means that the Tasmanian Thylacines would not have competed with the larger Australian predators of the Pleistocene such as Megalania (Varanus prisca), Marsupial Lion (Thylacoleo carnifex), or the Powerful Thylacine (Thylacinus potens), although its prey selection did overlap with those of Australia's smaller predators like Tasmanian Devils (Sarcophilus harrisii) and Spotted-tailed Quolls (Dasyurus maculatus).


Extinction
Thylacine numbers appear to have declined sharply between 77,000 and 69,000 years ago, a date that roughly corresponds to the eruption of the Toba supervolcano on Sumatra. An event which led to mass megafauna die-offs elsewhere in the world and is the likely cause for the genetic bottlenecks found in many of today's larger mammals. Nonetheless, the highly adaptable Thylacine persisted and coexisted peacefully alongside humans which arrived there about 50,000 years ago. The animal is even represented prominently in Australian rock art dating back thousands of years.  After the dingo was introduced to Australia around 4,000 years ago, Thylacines suffered another decline under the pressure of this new competitor. Because Dingos tend to target the larger kangaroos and wallabies, however, there would have been minimal dietary overlap and Thylacines could still thrive on the mainland. The island of Tasmania, which had been connected to the Australia until 19,000 years ago, became a stronghold for this species.

A "bagged" Thylacine alongside Mr. Weaver who
apparently shot it in 1869. Wiki

For a time Thylacines were the top predators on Tasmania. The arrival of Europeans, however, would prove to be their downfall. Not long after livestock were introduced to the island, a mass genocide of this species followed. Eventually bounties were placed on this species by the government and the animals suffered a very steep drop in population. Despite this obvious decline, it was not given official protection by the Tasmanian government until two months before the last known individual died at Hobart Zoo on September 7, 1936. 

As a result of being isolated on an island for 20,000 years, Tasmanian Thylacines had rather limited genetic diversity at the time of their extirpation, even less so than the Tasmanian Devil. It is possible that had they persisted to the present day they would still be endangered unless they had the ability to interbreed with their mother population on Australia.
Benjamin, the last known Tasmanian Thylacine shown in his enclosure at
Hobart Zoo in 1933. Here, he is shown adopting a classic mouth-agape
threat display at some off-camera annoyance. Wiki
Possible Existence on Mainland Australia & New Guinea
Throughout mainland Australia, multiple eyewitnesses going back since European occupation first began have claimed to have seen living Thylacines. Based on these eyewitness accounts it seems possible that Australia could still support a population of Thylacines which have managed to remain elusive, unlike their counterparts on Tasmania. A few anecdotal accounts even exist of Thylacines being shipped from Tasmania and being released on mainland Australia prior to their extirpation. Other reports from New Guinea reveal that the animals may still exist in the dense forests of the island which have remained largely unexplored by non-native peoples. Local tribes speak of a dog-like animal with a thick tail and a mouth that can open its mouth extremely wide. Of course, definitive evidence is needed to confirm these claims (e.g. a fresh carcass, live specimen, or clear video footage), but it would not be the first time a supposedly extinct animal was found to still persist. If still present, their long-term survival would hinge on the presence of viable populations and undisturbed habitat.

References & Further Reading
Prowse TAA, Johnson CN, Lacy RC, Bradshaw CJA, Pollak JP, Watts MJ, Brook BW (2013). "No need for disease: testing extinction hypotheses for the thylacine using multi-species metamodels". Journal of Animal Ecology 82(2): 355-364 <Full Article>

Menzies BR, Renfree MB, Heider T, Mayer F, Hildebrandt TB, Pask AJ (2012). “Limited genetic diversity preceded extinction of the Tasmanian tiger”. PLoS ONE 7(4): e35433 <Full Article>


Attard MRG, Chamoli U, Ferrara TL, Rogers TL, and Wroe S (2011). “Skull mechanics and implications for feeding behaviour in a large marsupial carnivore guild: the thylacine, Tasmanian devil and spotted-tailed quoll”. Journal of Zoology 285(7): 292-300 <Full Article>

Heberle G. (2004). “Reports of alleged Thylacine sightings in Western Australia”. Conservation Science Western Australia 5(1): 1-5 <Full Article>

Wednesday, May 8, 2013

The Truth About the Killer Birds

Most of today's flightless birds like emus, ostriches, cassowarys, and rheas are herbivores with small heads set on long, slender necks. These birds eat things like tender leaves. grasses, fruits, flowers, and the occasional insect or small lizard. Throughout the Cenozoic though there have been other types of flightless birds. Many of these had huge heads and powerful, crunching jaws. At first glance many people have thought that these scary-looking animals were powerful predators whose wicked beaks were frequently turned on the small mammals with which they lived. For some species this may have been an accurate description, but looks can be deceiving.

Gastornis: The "Horse-eater"
Head of Gastornis giganteus

The well known fossil bird Gastornis (formerly known as Diatryma), was a giant 2 meter (7ft) tall animal that lived in the forests of Europe and North America from the late Paleocene to middle Eocene. For over 150 years it has been depicted in art and media as a fast-running carnivore that hunted down the ancestors of modern horses. But things just do not add up. On closer inspection though, certain aspects of this bird's body stand out and point to the contrary.
Body profile of Gastornis giganteus. Note
the shortened foot region and hoofed toes.

Its beak, though undoubtedly powerful, was heavy, blunt, and could not have delivered the swift, precise blows needed to dispatch prey quickly. The beak also lacked the hooked tip found on those of most modern predatory birds. Its eyes are set on the sides of its head as seen in most plant-eating animals, an adaptation which provides an expanded field of vision. Predators typically possess forward-facing eyes, providing an overlapping field of vision which enables them to judge distances accurately when attacking prey. Each toe of Gastornis ended in a blunt hoof instead of a claw. Carnivorous birds have sharp talons on their feet to hold on to prey as they kill or eat it. 

The Demon Ducks of Australia
Head of Bullockornis planei (the Demon Duck of Doom)

In the forests and savannas of Australia lived another group of giant birds called the Dromornithidae, also known as the "thunderbirds" or the "demon ducks". The latter of the two nicknames refers to the group's close relation to ducks and geese. These animals first appeared during the Oligocene and died out shortly after the arrival of humans to Australia during the late Pleistocene. As with Gastornis, it has long been thought that many of these birds were carnivores, and that their huge beaks were used to crush prey and crack open bones. 

The larger species within this family, such as Stirton's Thunderbird (Dromornis strirtoni), the smaller Southern Thunderbird (D. australis), and the Demon Duck of Doom (Bullockornis planei) in particular had head and body proportions very similar to that of the earlier Gastornis. They had the same heavy, crushing beaks, long necks, robust bodies, hoofed toes, and short foot bones and long tibia that show they were not particularly swift runners (ironic given that the name Dromornis actually means "swift-running bird" in Greek). 


Studies have shown that the side-facing eyes of Bullockornis and Dromornis left a considerable blind spot directly in front of their faces, up to 40°. If these were predators they would have needed to take extra care to avoid tripping or crashing into trees when on the attack. Also, gastrolithes (stomach stones) are often found in association with skeletons, which are swallowed by herbivorous birds to digests tough plant matter. Gastrolithes have been found in herbivorous dinosaurs as well, such as the long-necked sauropods and horned ceratopsians. Crocodiles swallow stones too, but these function more as buoyancy aids. The highly acidic nature of the crocodile stomach is sufficient enough to break down any bone or keratin that they ingest.

The best known and last species within this group, the late Pleistocene Mihirung (Genyornis newtoni), has been confirmed to be a herbivore. Chemical analysis of fossil eggshell fragments reveal that these birds were browsers that selectively fed on the fruits, leaves, and twigs of a wide variety of plants. 

The argument is that these birds must have been predators, why else would they need such huge and powerful beaks? But consider this, some browsing herbivores today do have at least moderately powerful jaws, usually the result of including more wood and bark into the diet as opposed to just the softer leaves and fruits. Forest Elephant, Black Rhino, and Common Eland are good examples. Those animals that regularly feed on roots and tubers also have well-developed jaw muscles. Gorillas and Pandas have immensely powerful jaws (stronger than those of many comparably sized carnivores), which evolved to process extremely tough plants like bamboo stalks. Gastornis and the Australian thunderbirds very likely evolved to exploit similar niches. Their massive beaks were used to break branches, crush thick plant stalks, and strip bark instead of for tearing through flesh, although females could have potentially eaten bones on occasion for the calcium within (the main component of bird egg shells). These giant birds were more akin to giant flightless parrots than ferocious land-hawks.

The Terror Birds of South America

The Phorusrhacids, commonly known as terror birds, were a group of flightless birds that evolved in South America during the early Paleocene and lasted there until the Pleistocene. One species, Waller's Terror Bird (Titanis walleri), even invaded the southern United States 3 million years ago. There is no doubt that these birds were active predators. They ranged in size from small carnivore/omnivores probably the ecological equivalent of foxes and jackals, to truly massive predators that would have been the big cats of South America!
Head profile of the terror bird Phorusrhacos longissimus.

Like Gastornis and the thunderbirds, the terror birds had massive skulls (as long as the main body in some species). Unlike them, their beaks were not just deep, but narrow from side-to-side and built to withstand great impacts. Powered by powerful neck muscles, the skull and beak would have been wielded like the blade of an axe, driven downward with enough force to sever the prey's spinal column. This is the same killing method that was once used by predatory dinosaurs like Allosaurus! Crucially, the tip of the beak has a large hook for tearing off large chunks of meat. 


The limb and foot proportions were clearly those of a predator. Some species had short, sturdy legs for ambushing prey, some had longer, more slender legs for long chases or short sprints. All species had 3-toed feet with sharp talons on the toes that could be used for traction while running or for anchoring carcasses to the ground while feeding.


The manner in which these animals attacked and killed their prey must have been very similar to that of large theropod dinosaurs. Large mammalian prey would have been most likely stalked and rushed upon from behind. To minimize the risk of injury and ensure a swift death, most predators today target the head or neck of their prey and these birds were likely no exception. The momentum of the attack and the hammer-blow from the neck and open beak would inflict considerable impact damage. Death or paralysis would result from crushing or severing of the prey's vertebrae.


References
Murray, P. F. & Megirian, D. (1998) "The Skull of Dromornithid Birds: Anatomical Evidence for their Relationship to Anseriformes (Dromornithidae, Anseriformes)". Records of the South Australian Museum 31: 51-97.

Rich, P. (1979). "The Dromornithidae, an Extinct Family of Large Ground Birds Endemic to Australia". Bulletin of the Bureau of Mineral Resources, Geology and Geophysics 184: 1-190.


Chandler, R.M. (1994). "The Wing of Titanis walleri ((Aves: Phorusrhacidae) from the late Blancan of Florida". Bulletin of the Florida Museum of Natural History, Biological Sciences 36: 175–180.