Ecosystems-The Big Picture
Fragmentation is occurring, to various degrees, in nearly all of the major habitat types found throughout the world. These habitats (ecosystems) include, in part, rain forests, hardwood and pine forests, grasslands, and wetlands.
Rain forests, which comprise 6% of the world's land area and which contain at least 50% of the world's total species, are being cleared/fragmented at a rate far exceeding all other types of habitat. Currently, 200,000 sq. kilometers (an area the size of the British Isles) of rain forest is lost each year throughout the world. Brazil, which has the most rain forest, has the highest rate of deforestation (700 times the world average). Since the beginning of this century, 25% of rain forests have disappeared worldwide and another 25% has been seriously degraded. Most of this has occurred in the last 30 to 40 years.
Forest lands in this country are now recovering from a long history of logging. We currently have approximately 70% of the total forest cover that was present in the 1700's. However, the current composition of our forests is very different than what was present historically. We now find many forests which are much more fragmented, and whose composition consists of planted monocultures providing little diversity of nesting cover for birds and mammals. Also, many forests are now close to urban areas which further increases stress and the likelihood of continued fragmentation and degradation.
Grasslands in the United States have nearly vanished in their conversion to agricultural lands. Before the 1800's, large expanses of prairie covered the middle United States. Now, however, less than 10% of this total remains and, like forest lands, are found throughout their historical range in small isolated fragments.
Finally, one of the most highly productive habitat types in the world, wetlands, continue to be lost despite federal and state laws designed for their protection. The amount which has already been lost varies between states. California, for example, has lost 91% of its wetlands. Michigan has lost of 50% (equal to the national average).
Collectively, all the native habitat types mentioned above continue to disappear. The remainder, in general, is consistently more fragmented and stressed by human encroachment. There are limits to this stress where we find organisms no longer able to survive within their ecosystem.
Adapt, migrate, or die
Organisms, when faced with stress, have three options: they can remain in the area and adapt to the changes, they can migrate to an area without the stress, or they can go extinct. This extinction may be local, state-wide, nation-wide, or world-wide depending on the organism and circumstances. The method of extinction is also relevant here. Organisms go extinct because they are either hunted to extinction (the passenger pigeon, for example), have lost an adequate amount of habitat which is needed for breeding, have been displaced by exotic species (European starling, sea lamprey, etc.), or by natural causes such as disease. Presently, hunting, habitat alterations and competition from exotic species account for 80% of current extinction events world-wide.
Can organisms adapt to habitat fragmentation and survive? The success varies with the species. Some animals found in Michigan, although few, can survive well even when in close association with humans. These are usually generalists, with flexible requirements. Examples include the opossum, raccoon, starling, and mallard. A high tolerance of human activity explains why these animals are commonly found in the urban settings. Most animals, however, cannot adapt. Their genetic programming has determined that they only are able to reproduce under specific conditions-conditions which are very exclusive of this human activity. For example, the Kirtlands's warbler, an endangered species found in Michigan, needs isolated stands of 6-15 year-old jack pine trees to nest in. The Wood thrush, also found in Michigan, needs large, undisturbed tracts of hardwood forests in which to nest and forage. Eliminate access to these specific requirements and the species does not breed (goes extinct-at least locally).
Can an organism migrate and avoid extinction? Again this depends on the species. The common problem experienced by all species is that as adequate habitat becomes more scarce and fragmented, there are fewer areas to migrate to. A significant number of songbirds found throughout the United States are declining in number because of a lack of adequate nesting habitat. Also, many of these birds, when they return from the tropics during the spring, find the habitat they used last year no longer exists. Finally, road building, in addition to fragmenting habitat, interrupts the migration of many species of mammals and herptiles (frogs, snakes, and turtles). The roadways during spring and fall lend adequate testimony to this phenomenon.
Living on the edge
When a habitat (usually woodlands) is fragmented, the amount of habitat edge that is exposed increases. With this increase brings an increase in predators, many of which hunt along this edge. The brown-headed cowbird, a Michigan species, acts as a parasite by laying its eggs in an other individual's nest. The cowbird young then hatch and out compete with the other individuals for food and space in the nest. This parasitism is a significant contributor to the decline in songbirds throughout the United States. The sightings of cowbirds has increased 900 times since 1900. The cowbird is an 'edge' species which increases in number as the amount of habitat edge is increased as a result of habitat fragmentation.
Getting your needs met
In summary, all species need adequate space for nesting, foraging, and dispersal. Most species are very intolerant of the changes brought by human activity and find that one or more of the above requirements must be compromised. However, there is little room for compromise because most species are limited by their genetic programming. In addition to this, these same individuals may very well face increased competition from exotic species, and an increase in predation. Historical and current trends show that when these factors impact an ecosystem it is the individual, and ultimately its species, which disappear.
A matter of priorities--local and global
An impoverishment of the natural world is certain if habitat loss and fragmentation continue. Biodiversity has decreased virtually everywhere since the modern human species has existed. Loss of species, and entire ecosystems occur both locally and on a global scale. For example, the Michigan Natural Features Inventory, as of 1992, lists a total of 208 animal and 362 plant species as being either endangered, threatened, or of special concern in the state. (See sidebar, page 4). Six fish, 3 mammal, and 33 plant species are now extinct state-wide. Michigan at one time saw wolverines, cougars, and even bison inhabiting its natural areas. Also, Grand Traverse County has lost all significantly-sized representatives of 12 historically-occurring natural plant community types since the 1800's (see listings).
As this region, and other areas of the state are confronted by an increase in urban sprawl and population size, it forces individuals to examine their priorities with regards to their lifestyle and the natural world. If priorities are properly placed, growth in any region can be accommodated within an effort to maintain the integrity of the natural world. However, herein lies a significant challenge. This challenge includes finding a consensus between needs which sometimes conflict, regional governmental agencies, and the public. It is difficult work to find this consensus but well worth the effort when one considers the rate at which we are losing natural communities. This process is made even more difficult by its occurrence within a culture which promotes an ever-expanding use of space and natural resources. Consumption and uncontrolled growth are the messages of our society and until these are addressed, ultimate success in preserving the integrity of our natural world will be elusive.
Again it is simply a matter of priorities. Do we, as individuals and as a society, value the other species with which we coexist, or do we simply ignore our impact on this world until we find ourselves bereft of natural beauty and ecological stability?
Greg LaCross is an instructor of Biology and Chemistry at Northwestern Michigan College.
Return to the Index of Synapse 35, Spring Equinox 1996