Friday, July 20, 2012

This Island Earth Part II



In the previous installment of This Island Earth I detailed the cascade of events triggered by the introduction of feral pigs on Santa Cruz island. A major reorganization of the islands ecology was underway with new predatory golden eagles moving in to feed on the piglets as well as decimating the native island foxes. While the subsequent eradication of pigs has proven successful it was not without its critics. However this quote from Adrien M. Wenner, Professor Emeritus of Ecology and Marine Bio at UCSB succinctly makes the point for the pig eradication:

"As a biologist I have had extensive experience on the island and can report first hand about the pig situation there, Feral pigs on the island number in the thousands. In good years, they reproduce to their full ability and soon exceed their food source. As they run out of easily obtainable food. such as acorns, they desperately plow up the ground in search of bulbs, roots, and tubers, leaving the soil to be washed away in future rains, and thereby exterminating native plants. They then eat non nourishing grass as they starve. During the 1988 and 1989 droughts, for example, perhaps nine-tenths of the pigs died of starvation. But pigs don't starve immediately; as the weaker ones succumb, they get attacked and eaten by stronger pigs. At those times we could hear the squeal of pigs in such fights. By the end of 1989 nearly every pig I encountered was nothing more than a  bag of bones that could hardly move. When they noticed us, they most often fell over as they tried to move. Even in good years pigs suffer. Last week we grabbed a piglet for examination. Dozens of black legged ticks- vectors of Lyme disease, fleas, and lice lived on its soft underside. Island feral pigs, when they overpopulate, cannot migrate to greener pastures, they starve. is it more humane to let these feral pigs continue their overpopulation, starvation, and cannibalism or eliminate a few thousand from the island now, before untold thousands die in the future during such cycles?"





Pretty broootal eh? So clearly feral pigs, especially when left unchecked on a limited amount of land with a native flora/fauna not adapted to them can wreak all kinds of havoc. In fact one can make the argument that pigs, save human(zees) themselves, have more capacity to rework an ecosystem than any other extant mammal. More destructive capabilities than say, even an elephant?


....for it was a type of elephant that was the last large terrestrial herbivore on Santa Cruz Island and the other Channel Islands before the introduction of the pigs.


Now, granted the Channel Islands Pygmy Mammoth is a pygmy it still topped out at about a tonne in weight. If alive today it would still be considered a large animal. It certainly is larger than any of the feral pigs that nearly wiped out the native vegetation of the island. Yet the Pygmy Mammoth only went extinct about 10, 000 years ago- geologically speaking that is a blink of an eye. Vegetation wise the islands may have been a little wetter but all the plant species that live their now would have been the same.

But our pygmy friend did not drive the currently threatened Channel Islands Gooseberry to extinction.

CI Gooseberry. Ribes thacherianum

Nor did it trample and browse to death the last stand of Giant Coreopsis.

Giant Coreopsis on Santa Cruz Island. Coreopsis gigantea.

Or greedily gobble up the last acorn of a Valley Oak or girdle the last Torrey Pine.



I mean if the Pygmy Mammoth was anything like its modern elephant relatives it was probably a pretty robust ecosystem engineer? It is true that African Bush Elephants can radically alter their environment. But a growing body of evidence is suggesting that the dominant paradigm of elephant topples/eats trees and shrubs clearing the way for pasture is too simple. Elephants are now thought to be a little more choosier than once thought in their eating habits and that their habitat modifications can actually increase biodiversity. Much of our interpretation of "normal" elephant browsing behavior may be skewed by elephants acting unnaturally in man-made, fenced park environments. All in all, a growing picture is emerging of elephant modification actually creating a multiplicity of habitats.

Was the Pygmy Mammoth like the African Bush Elephant- I don't know. But I think it is safe to say it was more like its modern day relatives than it was like the feral pig in ecology. Let's look at what some of these differences are that allowed this island endemic to thrive for 47, 000 years...

Reproduction wise the feral pig is definitely on different playing field than the mammoth . As detailed in the boom/bust scenarios above feral pigs reproduce so quickly they can outstrip their food supply in a single season. Mammoths, like modern elephants, probably had exceedingly long gestation periods with prolonged mother/infant bonding periods. Clearly the mammoth is better set up for island sustainability in this regard than the pig...

One can imagine that the initial stock of full size mammoths eventually ran into food shortages like the pigs did. When this happened they did what they did to get there- they swam away. Island hopping or simply going back to the mainland would have been no sweat for these guys. Remember the closest relative of modern elephants are manatees. Eventually the winnowing effects of evolution would have produced a pygmy sized mammoth. And given the mammoths sea-faring skills I have no problem imagining mammoths would take advantage of the fastest growing plant known to man- Giant Kelp- which grows all around the islands. Granted I have no evidence of kelp consumption by mammoths but I can't help but imagine these intelligent herbivores would not take advantage of this resource in times of stress (or maybe just cuz it tastes good). So if things got bad, the mammoth could move onto greener pastures- the pig we know can not. Advantage mammoth.


 
Everything about the pig is designed for ground level feeding. Their efforts will be concentrated at the ground and, in large numbers, the effects on seedling recruitment, root/tuber health, and soil integrity can be dramatic. Any gardener worth a damn knows soil is the most important factor to plant health. But with enough pigs that soil is being lost with the rain. Elephants, although they do eat roots, have a much more diffuse spectrum from which to choose their dinner. They can browse at low, medium, and high levels, strip bark, dig up roots, graze- the point is that they are not focused on any one feeding tactic and the plant community can absorb the losses. Greater feeding envelope- advantage goes to the mammoth.


And lets not forget the feet...

Notice the fatty pad that cushions the impact of the foot on the ground. Ever hear stories about how easily elephants can sneak up on you in the bush-that's why! Now contrast the elephants padded sneaker with the pigs' steel toed workboot:


Sharp pointy hooves, all of the animals weight concentrated on those pointy tips- you can imagine what kind of damage those cloven hooves do, especially multiplied by the thousands. Remember the pigs has only one mouth but it has four feet each with two cloven hooves. Much of the debate surrounding herbivore effects on plants centers around what the mouth is doing to the plant but it is my contention that how the feet interface with the ground has an equal or even more profound effect on vegetation patterns. And it is the combined effect of those hoofs and mouths that turn diverse ecosystems into fennel barrens.

Invasive weed Fennel
Despite being several orders of magnitude larger that the feral pigs, Pygmy Mammoths appear to have coexisted in stability with the native flora and flourished as island endemics. I argue this is chiefly because of low birth rate, exceptional long distance swimming ability, large feeding envelope, and soft padded feet. I do not feel it a coincidence feral pigs fail in these comparisons.

Now, granted I have painted with some pretty LARGE strokes here and there are numerous variables that effect this comparison- but I still think the crux of my argument is valid. One can not always assume larger animals have a more profound effect on a habitats vegetation patterns than relatively smaller animals.

More to come in This Island Earth Part III- going back in deep time now...



Cheers!!!

Pertinencia

Santa Cruz Island Primary Restoration Plan. www.csun.edu/~hmc60533/CSUN.../Ward_RestorationPlan.pdf


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