MICHIGAN FORESTS FOREVER TEACHERS GUIDE

WILDLIFE ECOLOGY BASICS

Forest wildlife ecology, in many ways, is synonymous with forest ecology.  These basics can be reviewed on the "Forest Ecology Basics" webpage.  Wildlife are dependent upon the vegetation that supports them, so changes in vegetation significantly impact wildlife populations.  "Succession" is a key principle in wildlife ecology and should be remembered with considering wildlife populations.  With each change, there are "winners" and "losers".  Humans often equate "good" management and "bad" management to the species of wildlife they "like" or "don't like".  Natural resource management increasingly tends to take a landscape perspective to ensure a balance of habitats for all species of wildlife.  However, this does not always happen as well as it could because both public and private natural resource policy is not always driven by science!  More commonly, it is driven by public opinion as expressed and interpreted by legislators and other decision-makers.  

What is Wildlife?
Habitat
Population Dynamics
Cycles
Winter Adaptations

What is Wildlife?

The word "wildlife" is almost a uniquely North American term.  Equivalents in other languages are hard to find, but the concept has spread to other countries.  Concern about wildlife in America began in earnest in the last half of the 1800s, although the scientific roots can probably be traced to game management of the royal ownerships in Europe.  Widespread and uncontrolled logging occurred throughout most of the eastern forests over the course of a century.  The rise of conservation included both wildlife and timber resources, as well as water supplies..

The definition of "wildlife" often includes only vertebrates, particularly popular species such as those that are hunted, trapped, cause problems, or are endangered.  Vertebrates are animals with backbones, including birds, mammals, fishes, amphibians, and reptiles.  A more expansive definition of wildlife would include all animal lifeforms in an ecosystem, in this case, a forest system.  For the most part, this Teacher Guide will view wildlife in terms of vertebrate species.

The term "game species" refers to an animal that is either hunted or trapped.  Michigan lists 115 animals as game species.  "Non-game species" are all the other animals.  "Endangered and threatened species" are a special group of non-game species whose populations are low in either Michigan or the United States.  In Michigan, there are ten endangered or threatened forest wildlife species.  There is a "Michigan List" and a "Federal List", both contain many more than ten species, but they also include species typical of non-forest habitats.   The Michigan Natural Features Inventory keeps track of all these species and designations.  An "extinct" species is one that can no longer be found anywhere in the world.  An "extirpated" species is one that no longer occurs in a place (such as a state or region) where it once used to.  Woodland caribou have been extirpated from Michigan. 

Michigan has about 575 species of vertebrates.  Each has a unique set of habitat preferences and requirements.  These habitat preferences and requirements change during the life stages of most species, and many change with the season.  Computing all the variables would result in an almost limitless number of habitat combinations.   Managing for every species in every conceivable situation is an impossible task, even assuming we knew all the variables, which we do not.

For the most part, and more practically speaking, wildlife are managed by communities.   Single species are not usually the primary focus of a management plan in an area or region.  Notable exceptions include species such as white-tailed deer, Canada geese, ruffed grouse, bald eagles, and Kirtland's warblers.  Populations of many game species are directly managed through hunting and trapping regulations.  Habitat for some endangered and threatened species is sometimes managed exclusively for that species.  In other cases, restrictions on activities are prescribed by law, such as the prohibition of certain practices during a certain time period (usually breeding season), within a stated distance or area. 

Rather than direct management of a species population, habitat is managed for as much diversity as possible, with the explicit assumption that by providing as many alternatives as possible, each species of wildlife will find what it needs to maintain a viable population.  Because habitat management is largely a matter vegetation management, forest management is also wildlife management.  Actions by foresters, farmers, and other folks have affected wildlife populations more than professional wildlife managers.  However, wildlife biologists now co-manage many public and private lands.  This is one of the primary reasons why habitat considerations, at both the stand level and landscape level, are so important in forest management.

Does this "habitat diversity" management technique work?  Probably.  Since forest management became common in Michigan (perhaps within the last 50 years), no wildlife species has been lost from Michigan's forests . . . that we know of.

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Habitat

Many schools introduce the idea of "habitat" at the fourth or fifth grade level.  Habitat features can be expressed in five categories.

• Site Quality
• Space and Home Range
• Food & Water
• Shelter
• Variability

"Site quality" incorporates factors such as soil, topography, climate extremes, precipitation, and drought frequency.  Each of these factors are fairly easy to measure and evaluation.   Productivity of a given site will vary according to the specific combination of site factors.  Sites that produce high amounts of biomass, will generally support more wildlife species and larger populations than poor quality sites.

Every species has a minimum "space" requirement.  Space is needed to obtain life's necessities.  A large predator, such as a wolf, needs more space in which to meet it's needs than a field mouse.  "Home range" is the area within which an animal will feel comfortable, and is some cases, actively defend.  The amount of space and the size of a home range of a particular species will vary with the season and sometimes by gender.  This is particular true during the breeding season and during winter vs. summer months.  Many species acquire basic necessities from multiple forest and other vegetation types.  The relative proximity, or spatical arrangement, of these different types is an important habitat feature. 

"Food and water" are obvious needs of every living thing.  During the course of a year, availability and quality of food and water can change dramatically.  Wildlife, not just birds, will often migrate to avoid lean times.  Others might hibernate or undergo other metabolic changes.  White-tailed deer have different sets of digestive enzymes to accommodate the changes in browse.  Frogs may  spend the winter buried in pond muck.   During frozen months, locating liquid water can be a challenge. 

"Shelter" is needed for a variety of purposes.  The first to come to mind is protection against adverse weather.  Shelter is also needed to escape predators.  Young grouse and beaver are expelled from their parents' home range in the fall when food begins to grow scarce.   The young animals must find new suitable habitat.  This search usually makes them much more vulnerable to predation and severe weather.  Shelter or specific habitat conditions are also needed for courtship displays, nesting, rearing young, and roosting or loafing.  Male woodcock have fairly specific requirements for their dancing grounds and performing their sky dance.

"Variability" of habitat quality and habitat needs makes management difficult, as discussed above.  Wildlife needs vary with the season and life stage of a species.  Additionally, all species have preferred habitat and minimum habitat conditions.  For example, a population may do best in a collection of aspen types, but if adequate amounts of aspen are unavailable, the species will use alternatives and get by, until some environmental extreme occurs, such as a very cold winter.  A species that is flexible in its habitat adaptability is sometimes called a "generalist".   A species with a rather narrow and specific range of requirements may experience severe population fluctuations with changes in the environment.  These species are referred to as "specialists".  Species that are very sensitive to certain environmental changes are sometimes used as "indicator species".  These species clue wildlife managers into subtle environmental conditions and changes that would otherwise be difficult to measure or assess.  For instance, the presence and abundance of stonefly and caddisfly larvae in streams will tell us much about the conditions in that stream and the adjacent upland habitat. 

In forest management, wildlife habitat ranks high on the list of desired objectives.  However, the area of some of the most endangered forest habitats, young early successional forests, is declining.  These habitats also harbor some of the rarest forest wildlife species.  Keep in perspective, of course, that most threatened and endangered species are from non-forest habitats.  Many people tend to place emphasis on older forests with big trees . . . usually later stages of succession.  The decline of young early successional forests (not just forests of young trees) with its complement of shrubs, understory flora, and small early successional tree species have begun to catch the eye of researchers and ecologists.  These tend to be "waste places" and not regarded as valuable habitat by many viewers.  However, these kinds of forests are becoming increasingly important refugia for a number of special species, such as the Golden-Winged Warbler.  Because research has yet to yield much of the habitat requirements for all wildlife, forest managers tend to think in landscape terms and provide as much diversity in composition and structure as possible.  Beauty is often in the eye of beholder and, unfortunately,  some valuable habitat types rank low on the visual scales of many forest visitors. 

Activity Suggestion PLT The Forest of S.T. Shrew PLT The Fallen Log

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Population Dynamics

A wildlife population is a group of individuals of the same species that have some basis of commonality.  We can talk about the population of white-tailed deer in a geographic region; or the population of sticklebacks in a particular stream.  Populations can be linked to a feature in the landscape, to other populations, a time period, or other criteria.

Wildlife populations have inherent qualities that help in defining the welfare of various species. 

Age Structure:  The proportional amounts of young and old age classes reveal much about a population.  There should be some kind of balance among the classes and the "proper" balance will vary by species and season.   Generally, the age structure can be depicted by a triangle, with the numerous young on the bottom and the very few oldsters at the top.  "Age" might be measured in years, weeks, or days, depending upon the species considered.  At the end of the food-rich season, the youngest age classes are usually swollen.  The winter will kill many individuals, but usually the young and very old experience the highest mortality rates.  Humans sometimes have a strong impact on the age structure of a population.  White-tailed deer have few animals beyond 4.5 or 5.5 years largely because of hunting pressure, although an individual is capable of living a decade or more.   A heavily fished lake may reduce the number of sizable (aka older) adults to the point where breeding might be reduced.

Lifespan:  Obviously, different species have different lifespans.   Most insects complete their life cycles during the warm season.  Some have multiple generations during that time.  Other species live for years and individuals must have adaptations and adequate habitat to survive regular periods of food-shortages and inclement weather.  Species toward the end of food chains are usually much longer-lived that those in the beginning.  Long-lived species have strategies that favor the survival of fewer individuals.  Shorter-lived species generally utilize the opposite strategy.  The combination of lifespan and age structure reveal much about the general health of a population, either a wildlife population or a stand of trees.

Sex Ratio:  Each species has an "ideal" sex ratio.   Usually this is somewhere around 50:50, but not necessarily.  Honeybees, for example, have almost no males.  A particular sex ratio will help maximize "fecundity", or the ability of a species to produce new individuals.  Males of some species will mate with as many females as possible.  Other species, such as swans and geese, tend to be more monogamous. 

Natality and Mortality:  Natality is the inherent ability of a population to increase in numbers.  Mortality deals with the level of death within a population.   These terms are usually expressed as rates that reflect pressures to increase and decrease population size.  The size of a population is impacted by many factors, which vary over time.  At a particular point in time, natality factors or mortality factors may dominate, causing a population to increase or decrease.  Some factors are fairly predictable, such as the average clutch/litter size or the onset of winter.  Other factors, such as extreme weather events or disease epidemics, can have great impacts but are not predictable.

Interspecific Dynamics:  These are relationships among or between species.  The predator-prey relationship is a well-known example of an interspecfic dynamic.  Interspecific dynamics can be antagonistic or beneficial.  Lichens are two species (an algae and a fungus) working in concert to the benefit of both.  This is called a "mutualistic" relationship.  A "commensal" relationship is where one species requires another, but the host is relatively unaffected.    Another kind of interspecific relationship would be parasitic.   Mosquitoes draw blood essential to the completion of their life cycle, at the expense of another species.  Species that require something from another species are termed "obligate".  When the relationship is beneficial but not required, it is termed "facultative".  

Intraspecific Dynamics:  There are relationships among individuals of a population.   Competition for food, shelter, and other requirements are common examples.   Mating and establishing territories are other examples.  A species might be colonial in nature or live primarily as individuals.  There are many life strategies.

Territoriality and Home Range:  An individual or population of a species may actively mark and/or defend a particular area.  The male robin that challenges anything resembling another male robin is expressing "territoriality".   A "home range"  is the amount of space an animal needs in order to acquire the resources to meet its needs.  A predator such as a wolf may have a home range of many square miles, while an earthworm has almost none.  The amount of area for either a territory or home range is not necessarily constant.  It often varies with the season.  After the breeding season, male robins resume a gregarious nature.  Ruffed grouse will "expel" their young before the onset of winter because winter home ranges are larger than summer home ranges.  The young animals must seek their own new habitat. 

Migration:  Winters and dry seasons result in less available food and water.  Animals have a wide range of strategies to accommodate these seasonal fluctuations.  Migration is one such strategy.  Autumn bird migration is the most familiar.  Many species of birds fly south more because of food shortages, rather than cold temperatures.  Bald eagles, which normally migrate, will remain as a winter resident if a food source is available.  Other animals, besides birds, will migrate.  Deer and other "ungulates" move between winter and summer ranges.  Many fish species will seek out different waters with the season.  Monarch butterflies move to Mexico.  Not all migrators leave Michigan for better climates.   Some actually come to Michigan!  Chickadees, snow buntings, and great gray owls regularly arrive from more northern latitudes to become winter residents in our state.

Carrying Capacity:  The physical and biological resources of an area, varying with the season, will support only so many individuals.  This maximum amount called the "carrying capacity".  When most species approach their particular carrying capacity, mortality factors overtake natality factors and the population growth declines.  For some species, this ecological balancing-act is fairly regulated without great fluctuations.  With other species, there is a normal "boom and bust" cycle.  Ruffed grouse and snowshoe hare populations are good examples.  There are a few species that can maintain high population densities long enough to actually damage their habitat and substantially reduce the carrying capacity.  Deer and moose are classic examples of species than can damage their habitat.  Humans may very well fall into this category, as well.

Activity Suggestion PLT Dynamic Duos PLT Web of Life WILD Hunting

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Cycles

The size of wildlife populations sometimes display cycles of highs and lows over time.  There are four basic cycles to consider.

1.  The Theoretical Growth Curve

All populations have a tendency to increase.  Continued increases are checked by environmental limiting factors.  Many populations, especially small animals such as insects or rodents, increase exponentially.  Population size will then reach a plateau or crash.  The numbers of bacteria in a petri dish will increase slowly at first, then expand very rapidly until the food is gone or toxins produced reach lethal levels.  This curve is often referred to the "S-curve".  

2.  Annual Cycles

Most species will experience annual population highs and lows based on the seasons.  The winter season or dry season are when food and water become most limiting.  The end of the breeding season is when populations tend to peak. 

3.  Short and Long Term Cycles

For a complex of reasons, sometimes not understood, populations display regular cycles over a number years.  Ruffed grouse are well-know for their ten-year cycles.  Trapping records suggest that Canada lynx and snowshoe hare have parallel ten-year cycles, although some biologists have questioned the data analysis.   Lemmings are famous for their four-year cycles with such large numbers during peaks that farms and towns become invaded.  Every 10-12 years forest tent caterpillars will defoliate large expanses of hardwood forests in Michigan.  On longer cycle, roughly 30-40 years, the same relationship is found between spruce budworm and balsam fir. 

4.  Irregular or Irruptive Cycles

These are major changes in the population levels of a species that occur without any particular pattern.  Strong weather or climate events, such as a hurricane or drought, may cause a crash.  In other cases, the causes cannot be known for certain.  Why did Canadian raccoon populations peak at six times their normal, steady-state level in 1867?  Why did a South American rodent population inexplicable crash, upsetting the ecology of large region?  Why did the mule deer population of the Kaibab Plateau in Arizona go through the roof around 1920, emaciate the habitat, reduce the carrying capacity of the land, then crash?  Even more peculiar, why did mule deer populations near the Kaibob not experience such an event?

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Winter Adaptations

In the north, winter is the period of lean resources and a challenging environment.  Animals survive by employing one or more adaptive strategies; 1)  migration & movement, 2)  dormancy, or 3)  toughing it out.  The complexity and variation of adaptations is tremendous.  A more thorough study of how animals respond to their environment is an incredible journey into chemistry, morphology, physiology, and many other fields of study.  For more information of winter adaptations, click here. 

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