Carrying Capacity - Is it 'Us' versus 'Them'?

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While the exact origin of the term 'Carrying Capacity' is not known for certain, it may have been first used in an 1845 by the US Secretary of State to the Senate. (Sayre, 2007)

In the time that has intervened, it has come to be understood to mean the following:

Carrying capacity in ecology refers to the level of land or resource use both by humans or animals, that can be sustained over the long term by the natural regenerative power of the environment.

Current growth and consumption patterns are placing increasing stress on ecosystems. Environmental degradation, biodiversity loss, deforestation, and the breakdown of social and economic systems are a few of the signs which indicate that ecosystems are stressed.

Carrying capacity assumes that there are a finite number of people who can be supported without degrading the natural environment and social, economic and cultural systems and, as such, "is an indirect measure of the maximum level of stress that the ecosystem can maintain". (Barbier, Burgess and Folke 1994).

In a system where humans do not intervene, the carrying capacity is regulated largely by the environment and the interactions of the resident flora and fauna, seldom static and ever dynamic, at times favoring some species over others, and with ingress and egress of populations in response to these changes.

The Yukon has several seasonal and cyclical events upon which life in these parts depends.

Approximately 57% of the Yukon is covered in Boreal Forest, and Pine is the dominant tree species, because it quickly regenerates in burned areas. The extreme heat of forest fire is actually required to open the cones of the Pine tree, allowing it to complete it's cycle of reproduction. In the aftermath of forest fire, the land rejuvenates with a diversity of grasses and undergrowth, providing food and shelter for small mammals and birds, which in turn compose a large part of the diet of intermediate predators.

As the forest grows and matures, it shades and chokes out the previous lush ground cover growth and the number of species declines until only a few species remain, among them the red squirrel, Arctic varying hare, mice, voles, foxes, coyotes and several species of birds that are resident year round, including the Raven and the Gray Jay.

With each cycle of burning, the species diversity increases and then subsequently declines, as nature has determined.



As the human population has moved ever northward and started to 'manage' the Yukon from the interests of our species, we have had serious impacts on this cycle by means of both our use of the habitat and by our practice of fire suppression.

In starting this thread, it is my desire to further explore the changes which human habitation has brought to the Yukon and I also invite others to participate with their experiences from their own regions of the globe.

My suggestion is that our expanding populations and proprietary interests are having effects on the carrying capacity of most habitats and that we may indeed be forcing an 'us or them' circumstance upon many populations of species that held tenure prior to our arrival and interventions.

The exploration is intended to be one of observation and not judgment.

That role, I suggest, is best left to Nature.

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The Snowshoe Hare is at the heart of an important cycle for wildlife, as it is the preferred menu choice for many, including some species that one would not commonly associate with predation.

The hare cycle occurs on average every 7-10 years, although it has been observed to crash as early as five years into a recovery or to maintain an increase for as long as 17 years dependent on food and habitat availability, weather and predation, among other factors.

Winters with deep snowfall are advantageous to this species, as they have large webbed hind feet allowing them to travel rapidly on the surface of even powder snow, giving them the opportunity to escape larger earthbound predators, although they are still vulnerable to birds of prey. Only the Lynx has also evolved large padded paws, and remains the large predator of greatest effectiveness.

The deep snowfall also allows the Snowshoe Hare to reach food sources that would otherwise not be available to it, so several deep snow winters in succession will often see a dramatic increase in the population of the species, which may increase as much as 100 fold from decline to peak of a cycle.

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The cycle of the Snowshoe Hare is closely related to the cycle of fire, as the regrowth forest in it's early to mid-stages provides optimum habitat for this species.

Major variables in habitat quality include average visual obstruction and browse biomass. Snowshoe hares prefer young forests with abundant understories. The presence of cover is the primary determinant of habitat quality for snowshoe hares and is more significant than food availability or species composition.[19] From Wiki

Considerable study has been given to the species as it is such a key player in the food chain. The following list of determinants would describe preferred to ideal habitat for Snowshoe Hares.

Cover requirements

Snowshoe hares require dense, brushy, usually coniferous cover; thermal and escape cover are especially important for young snowshoe hares [12]. Low brush provides hiding, escape, and thermal cover. Heavy cover 10 feet (3 m) above ground provides protection from avian predators, and heavy cover 3.3 feet (1 m) tall provides cover from terrestrial predators. Overwinter survival of snowshoe hares increases with increased cover.[19] A wide variety of habitat types are used if cover is available. Base visibility in good snowshoe hare habitat ranges from 2% at 16.5 feet (5 m) distance to 0% at 66 feet (20 m). Travel cover is slightly more open, ranging from 14.7% visibility at 16.5 feet (5 m) to 2.6% at 66 feet (20 m).

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The following article and photo taken from research done in the Yukon.

Column 463, Series I • March 10, 2006 • by Sarah Locke

Snowshoe hares might have something to teach us about stress

We humans might think that we have a lock on issues around stress, and have written the book on living in a stress-filled world. But animals also have their worries, and they might go far beyond the fear of being eaten, or worries about finding their next meal.
Snowshoe hares might pass their stress levels on to their kids.
(photo: Government of Yukon)


It is known that stress can lower rates of reproduction in some animals, and -- here in the boreal forest -- some researchers think stress just might drive the ten-year cycle of snowshoe hares -- a phenomenon that is one of the major lynchpins and great mysteries of life in this part of the world.

An earlier study on hares found that they are more stressed when their populations are declining than when they hit the bottom of their population cycle. This most likely occurs because -- with fewer bunnies around -- the remaining ones are more at risk of being eaten by a hungry lynx or coyote.

Michael Sheriff, a UBC doctoral student, wants to take this work one step further to see whether stressed-out hares have fewer babies -- an effect that has never been documented in this species. Studying what is known as the "stress axis," he predicts that increasing stress levels among hares will cause reproduction to decrease. He also wants to study whether mother hares pass their stress levels on to their kids -- before they are even born.

Populations of both hares and predators have hit their lows and are starting to climb again, so once again hares must be constantly looking over their shoulders, on guard against ever-hungry predators. Sheriff is live-trapping hares in Kluane right now, trying to see whether their stress levels are also on the upswing.

And how, one might well ask, does one go about determining whether a snowshoe hare is stressed? Not so long ago, researchers had to take blood samples from the hares -- knowing that this very act could increase the animals' stress levels.

Sheriff is relying on a new technique known as fecal sampling. Instead of drawing blood, he collects the hare's fecal pellets, which are always in ready supply. Another advantage is that the pellets indicate the animal's stress levels 24 hours before they were trapped, when life was normal and so were their stress levels.

The main indicators of stress are glucocorticoid metabolites, which are produced by the stress hormone cortisol. "It's the hormone responsible for the "fight or flight response," explained Sheriff. While animals such as rats and mice product cortisone when stressed, both humans and hares produce cortisol -- which means that the hare research might also shed some light on human physiology.

Since the early 1970s researchers have been trying to figure out what drives the snowshoe hare cycle. They've looked at predators such as lynx and coyotes, whose numbers rise and fall along with those of the hares. They've tried to figure out whether food is the issue -- perhaps the bunnies eat themselves out of house and home.

But even when hares were kept in enclosures where they had plenty of food, and were protected from predators, the cycle still continued. Another mystery is the lag in the cycle; no one can explain why the number of hares stays low for a few years, even after the number of predators has dropped, and the vegetation has had a chance to replenish itself.

"Neither of these theories sufficiently explain the driving force of the cycle or why the population remains at a low level; there should not be a lag," said Sheriff.

He hopes that something called "maternal pre-programming" might explain the mysterious lag. Basically this means that mother hares pass on the dubious gift of stress to their unborn babies: if a mother hare is stressed, her hormones affect her fetus, and she gives birth to a stressed youngster.

Sheriff plans to compare the offspring of stressed and unstressed mothers to see whether stress levels are passed from one generation to the next.

"This has never been shown before," he said. "It is a brand new theory."

If his research shows that stressed hares have fewer babies, and that stressed mothers give birth to stressed young ones, this finding could explain the lag in the hare cycle. "It may be due to the young having a reproductive output that would be good for the environment their mothers experienced," he explained.

His thinking goes like this: during downswings in the hare cycle, when hares are more at risk of being eaten, female hares might be much better off trying to avoid predators than trying to have lots of babies. If these stressed mothers pass their high levels of cortisol on to their kids, the next generation might also have a lower rate of reproduction, even if the hares are born at the bottom of the hare cycle when there are fewer predators.

If this is true, then bingo -- there is a lag in the cycle.

Sheriff says the study of stress levels is important because of the essential role that hares play in the food web of the boreal forest. If industrial development such as mining and logging continue to increase in this region, the stress levels of hares living in the boreal could increase as well. "If stress levels turn out to be a major determinant of reproduction, this could alter the whole ecosystem as hares are a keystone species in the boreal forest," he said.

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Snowshoe Hare Research Reveals Role of Stress in Survival

Apr 30, 2005
by Paul Fraumeni, Edge Magazine, University of Toronto
Rudy Boonstra



It is a conundrum that has fascinated professor Rudy Boonstra and his colleagues for years. The boreal forest wreathes the top of Canada, covering some five million square kilometres. Every ten years, the dominant herbivore in this forest – the snowshoe hare – cycles like clockwork.



In the southwestern Yukon, where Boonstra studies this species, their numbers peak at 200-300 per square kilometre and then rapidly decline to about seven over a period of two or three years. “We know that the cause of the decline is their predators – they are the primary food of the Canada lynx, coyotes and great horned owls," says Boonstra, a physiologist and zoologist at UTSC.


“Eventually, this predator population, which also cycles, builds up and kills off the majority of the hare population, causing the decline. What baffled us is why the subsequent low phase for hares then lasts between two and four years."



After all, he notes, snowshoe hares can live up to the reputation rabbits (to which hares are related) have of breeding quickly and in great numbers. Why, then, does it take them so long to recover from the decline, following the death of most of their predators, and repopulate the boreal forest?


Boonstra’s hypothesis, now widely accepted as fact by the scientific community, is that the snowshoe hares are chronically stressed during the decline because of high predation risk. The effects of this chronic stress may then impair both the reproduction of survivors and of their offspring, delaying recovery during the low phase.



“When humans experience prolonged, intense stress from, for example, a divorce or the death of someone very close, you don’t eat as much, you don’t sleep well, and you lose interest in sex. As a result, reproduction cannot take place. The same happens with the hares. In their increase and peak years, they produce as many as 19 babies in a summer. During the declining years, the number drops to seven."


Boonstra showed through a series of studies that hares breeding during the decline suffer from a kind of post-traumatic stress syndrome. “The overwhelming anxiety caused by years of being hunted constantly by the lynx programs this stress syndrome into their brains." He believes that this anxiety affects the hares for a few generations afterward, as it is passed on from mothers to their offspring.



This type of research has been central to Boonstra’s 30-year focus on ecology, which also includes field work on the Arctic coast of the Beaufort Sea, in the Rocky Mountains and in southern Ontario. It addresses biological questions that are essential to the broad issue of environmental change: How are populations of mammals regulated? Is stress related to the aging process? What are the environmental forces acting on mammals?


“If we are going to understand the changes in the earth’s environment, such as global warming and the melting of the polar ice caps, then we need to understand how these changes impact species on the planet. The world is one big ecosystem. Everything is connected. The shifts in the boreal forest and the reproduction rates of northern mammals such as snowshoe hares may seem trivial, but they are essential in understanding life on this planet."

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Considerable studies related to the Snowshoe Hare have been conducted and are ongoing in the Yukon. The domino effect of the hare cycle is quite amazing to observe first hand. The most dramatic evidence that I observed was during the height of the cycle of 1987, when training the dogteam, I saw that the deep snow was enabling the hares to feed on tree bark of a variety of species of tree that are not their preferred food, yet there was little else available to them. The trunks of sapling pine and spruce were stripped bare from the level of the snow to as high as the hares could reach, resulting in an unusual landscape of denuded trees, which subsequently died, and the hares crashed soon after.

From the study of tree rings, comes another hypothesis related to the mystery of the cycle, which is still not entirely understood.

Can the solar cycle and climate synchronize the snowshoe hare cycle in Canada? Evidence from tree rings and ice cores / Sinclair, A.R.E. Gosline, J.M. Holdsworth, G. Krebs, C.J. Boutin, S. Smith, J.N.M. Boonstra, R. Dale, M.
(American naturalist, v.141, no. 2, Feb. 1993, p. 173-198, ill.)
(Kluane Boreal Forest Ecosystem Project contribution, no. 24)
References.
ASTIS record 53366.
Languages: English
Libraries: ACU

Dark marks in the rings of white spruce less than 50 yr old in Yukon, Canada, are correlated with the number of stems browsed by snowshoe hares. The frequency of these marks is positively correlated with the density of hares in the same region. The frequency of marks in trees germinating between 1751 and 1983 is positively correlated with the hare fur records of the Hudson Bay Company. Both tree marks and hare numbers are correlated with sunspot numbers, and there is a 10-yr periodicity in the correlograms. Phase analysis shows that tree marks and sunspot numbers have periods of nearly constant phase difference during the years 1751-1787, 1838-1870, and 1948 to the present, and these periods coincide with those of high sunspot maxima. The nearly constant phase relations between the annual net snow accumulation on Mount Logan and (1) tree mark ratios, (2) hare fur records before about 1895, and (3) sunspot number during periods of high amplitude in the cycles suggest there is a solar cycle-climate-hare population and tree mark link. We suggest four ways of testing this hypothesis. (Au)

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While it has yet to be conclusively proven what the cause(s) of the Snowshoe Hare cycle may be, it has been long standing tradition among the people who live on the land that during the decline one must not eat hares nor even feed them to their dogs. One should only eat healthy animals and a species that is declining en mass does not qualify as healthy.

There has been research that indicates that plants actually have evolved their own defense against the hare and other herbivores.

Abstract
The plant defense guild hypothesis for the evolution of plant secondary chemicals predicts that plant species defend themselves against generalist herbivores such as the snowshoe hare (Lepus americanus) in the Canadian boreal forest by evolving unique antifeedant chemicals. Plant species may coevolve in an ecosystem by presenting an array of chemicals to herbivores. We report further evidence for this idea from the presence of 2,4,6-trihydroxydihydrochalcone in the CH2Cl2 extracts of Populus balsamifera juvenile twigs. These extracts, added to rabbit chow, were offered to hares in choice tests. The bioassay established that the chemical acted as an antifeedant for hares.

Abstract
Overwintering mountain hares (Lepus timidus) fed selectively on the shoots of a number of northern willow (Salix spp.) species. The hares preferred certain species over others and mature shoots over juvenile ones. There was a negative correlation between the phenolic glycoside concentration in the twigs and the hares' feeding. This correlation was substantiated by feeding experiments in which oat grains treated with purified phenoglycoside and catechin fractions of willow bark were offered along with untreated control oat grains to free-ranging mountain hares. Both fractions in concentrations normally found in willow twigs inhibited hare feeding. The results suggest that these phenolic compounds play a crucial role in the building up of resistance patterns among the willows. The decline in resistance in mature shoots of tall willows indicates that the juvenile resistance can be, perhaps secondarily, an adaptation against mammals browsing from ground level. Accordingly, low willow species retain a high level of resistance also in maturity.

Browsing hares may even contribute to the sex ratio of certain plants.

Abstract
To evaluate the general extent to which sex-related differences in palatability occur in boreal dioecious woody plants, males and females of five dioecious woody plant species were presented to free-ranging mountain hares (Lepus timidus) during winter. Hares strongly preferred branches from male plants when feeding on Populus tremula and Salix caprea and weakly preferred male S. pentandra. However, they did not show any sex-related preference when feeding on the other two species studied (Myrica gale and Juniperus communis). Nitrogen concentration and, to some degree, digestibility showed strong relationships with hare food preferences. By contrast, the concentration of phenolics was only weakly related to feeding preference. Phenolics could, nevertheless, still be important if only one or a few specific compounds deter hare feeding. These results indicate that sex-related differences in plant palatability in the boreal forest might be more widespread than previously believed, particularly for species of the family Salicaceae. Thus, herbivores might be responsible for the female-biased sex ratios found in willow populations in northern Scandinavia (e.g. Elmqvist et al. 198.

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That the Snowshoe Hare declines cyclically is a fact, regardless whether we understand all of the factors involved.

What then, is the effect on the other species that depend upon the hare as their major food source?

The Lynx is the species that is high profile in this relationship of predator to prey.

After considerable searching, I found the following excellent photograph that clearly shows how well adapted is the Lynx for hunting the Snowshoe Hare. Examine the length of limb and the large round pads of the paws, which offer the Lynx the same buoyancy in deep snow that the large hind feet of the hare afford.

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The following information taken from Wikipedia and coincides with the observations of elders and others living on the land.

Canada lynx feed predominantly on snowshoe hares, which typically comprise 60% to 97% of their diet; as a result, the size of the lynx population tends to run parallel to the 10 year long rise and decline of hare's numbers. However, especially in summer, they will also eat rodents and birds, and sometimes hunt larger prey such as deer. Like many cats, they will eat carrion when it is available.

Canada lynx will hunt every one to two days and consume around 600 to 1,200 grams (1.3 to 2.6 lb) of food per day. They hunt both by ambush and by actively seeking out prey, varying their tactics depending on the terrain and relative abundance of prey species. If food is scarce and the lynx comes upon a large number of prey, it may go on a spree, killing as many prey as possible, then storing the kills. They do not have stamina; whilst they are fast over short distances, they lack the ability to keep up speed for more than a few feet. Instead, they use their large ears and eyes to seek out prey. If the lynx does not catch its prey within the first few seconds, it will generally give up the chase to conserve energy.

If the lynx kills or scavenges a larger animal that it cannot consume all in one sitting, it will drag it to a hiding area such as a bush or under a rock and then will cover the dead animal with leaves and return to consume it later. Such behavior is particularly common when prey are abundant.

The breeding season in Canada lynx lasts only for a month, ranging from March to May, depending on the local climate. (Late February-Early March in Yukon ) Females come into oestrus only once during this period, lasting for three to five days.The female attracts a mate by leaving some of her urine where the male has marked his territory, and by repeated calling. Mating can occur six times in one hour. The female lynx will only mate with one male each season, but the male may mate with multiple females.

Gestation lasts around 64 days, so that the young are born in May or early June. Before birth, the female prepares a maternal den, usually in very thick brush, and typically inside thickets of shrubs or trees or woody debris. The dens are generally situated mid-slope and face south or southwest.

Litters contain from one to eight cubs, and tend to be much larger when prey are abundant. This suggests a greater degree of reproductive flexibility than in other cats, and females often do not mate at all when prey are scarce. When cubs are born in lean years, however, infant mortality may be as high as 95 percent.

Canada lynx cubs weigh from 175 to 235 grams (6.2 to 8.3 oz) at birth, and initially have greyish buff fur with black markings. They are blind and helpless for the first fourteen days, and weaned at twelve weeks. When their eyes open, they are a bright blue color, but as they mature, the eyes become a brown-hazel color. The mother brings the food to her cubs and allows them to play with it before eating it, thus training their hunting skills.

Cubs leave the den after about five weeks, and begin hunting at between seven and nine months of age. They leave the mother at around ten months, as the next breeding season begins, but do not reach the full adult size until around two years old. Female reach sexual maturity at ten months, although they often delay breeding for another year, while males reach maturity at two or three years. Canada lynx have been reported to live for up to fourteen years in captivity, although the lifespan is likely much shorter in the wild.

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http://www.youtube.com/watch?v=zNKvrxVgfns&feature=fvst Eddie Vedder :::: Society