AREA EFFECTS AND HABITAT FRAGMENTATION
We briefly discussed the species/area effect in Notes 2. Remember that the relationship:
S = cAz, where S = # of species, A = area, and c and z are constants.
In the simplest case, we can assume a direct relationship between area and the number of species which can survive. However, there have been cases where species losses have been greater than what had been predicted based on the total area of reserves set aside. It is now generally accepted that the species/area effect is in fact multicausal, and not a simple linear relationship.
Be that as it may, let us start with the original model of MacArthur and Wilson for Island Biogeography. They envisioned an equilibrium situation, based on the equation above, in which an uncolonized area would begin to fill up with species until it reached a maximum amount, based upon the area of the island. Once that maximum was reached every successful new colonization of the island must be balanced by an extinction (i.e., an equilibrium is established). Now consider the figure below.
Here we see the colonization curves for such an island. Note that to the left the number of species on the island is low (assume close to zero), and that the number increases to the right across the graph. At first (left side of the graph) the colonization rate will be high because there are lots of open ecological niches on the island that can be easily filled, and all the rapid dispersers on the mainland (source area) will assumedly find this island quickly. [NOTE: The two curves represent Near Islands and Far Islands relative to the source area. The near island curve indicates a higher rate of species colonization because it will simply be easier for animals to migrate out to a near island. The far island rate is lower because the farther from the source area an island is, the more difficult it will be for species to reach it.]
As the number of species on the island grows, the rate of colonization will decrease because there will be less niches available (competition will increase), and the number of source area species not on the island assumedly decreases.
Now consider the next figure:
Here we have the Extinction Curves for our islands. The first thing to note is that we are no longer considering distance from the source area (near vs. far). In stead, we now consider island size (Large vs. Small). At this point we are taking into account the area effect. Since a large island will hold more species, a large island will have relatively low rates of extinction compared to a small island which will theoretically suffer higher rates of extinction due to lesser area and resources for species.
Now put the two curve sets (Colonization Rate and Extinction Rate) together, and we get the figure below.
Here we see the predicted equilibrium points for four separate cases (SFS = Species # for a far, small island; SNS = species number for a near, small island; SFL = Species number for a far, large island; SNL = Species number for a near, large island). What these equilibrium points represent is the stable number of species for the given islands. Consider that the curves trend upwards from each of these four points. As an equilibrium, the visual image is of a ball rolling along these curves - move to the left of the equilibrium point (lower species #) and the rate of colonization increases (we move back to the right); move to the right beyond the equilibrium point and the extinction rate increases (we move back to the left). As a result, we stay at our equilibrium point unless the area of the island changes, or the island moves farther from its source area. Both of these changes can be viewed as due to geologic events which will require a long time span - sea level changes and plate tectonic movement. We will only consider sea-level changes which will occur on a much shorter time span than significant plate tectonic movement.
It is important to differentiate Land Bridge Islands from Oceanic Islands because they develop quite differently. In the figure below, we start with a mountainous area during a sea level low stand (Time 1). At Time 2, sea level has risen significantly, flooding the lowlands and changing the mountains into islands. This has been a common event over the past 20,000 years as the glaciers have receded and sea level has risen.
The most obvious change is the loss of land area available to species, and the isolation of species that cannot cross bodies of water. This is an example of a Land Bridge Island.
Oceanic Islands, on the other hand, are those like the Hawaiian Islands which rose out of the sea due to volcanic activity.
The change in species composition is markedly different, as illustrated by the figure below. Graph A represents Land Bridge Islands which start with a high species diversity, and
species through time due to the loss of area. This reduction in species diversity is referred to a relaxation, and has been used as an example of what will happen as large wilderness areas are removed and only reserves are left behind.
Graph B represents the Oceanic Islands which start with zero species as they emerge from the sea, but are gradually colonized from the closest land mass. Note that in both cases an equilibrium point is eventually reached - through colonization for the Oceanic Island, but through major extinction on the Land Bridge Island.
Another aspect of area we have to consider is the boundary between the habitat we are interested in and adjoining habitats. This brings up the question of edge effects, since the boundary is generally gradational, not sharp. The figure below illustrates the point in a simple fashion. Even though we may have entered the habitat
we are interested in, the outer edges (zone of edge effects) will have some conditions which are different from those well within the habitat. If we are talking about the boundary between the forest and a field for instance, the edge effects can include greater degrees of sunlight at the edge of the forest, greater wind speeds, and drier and less shady conditions. These edge zones can act as "ecological traps" in that they represent the desired habitat for some animals, but may be liable to greater amounts of predation from animals inhabiting the adjoining habitat. Among birds, common problems along edge zones include nest predation and brood parasitism by species such as the cowbird.
The figure above illustrates the results of an experimental attempt to define the thickness of a forest edge zone. Experimental bird nests were placed at various points within the forest and the degree of predation was determined.
It is also important to note that the greater the degree of structural contrast between adjoining habitats, the greater will be the edge effects.
HABITAT FRAGMENTATION VERSUS HETEROGENEITY
The first consideration is the patchyness of the environment. As area increases, so do habitat diversity and resources. This means that "homogeneous" forest is actually very patchy when viewed closely. This patchyness is often maintained by disturbance (fires, blowdowns, ice storms, etc.). Note the patchiness of the areas illustrated in the figure below.
The diversity of habitats contributes to the diversity of animals, remember the concept of metapopulations and source and sink areas. In some cases a diversity of habitat is required by a species. During the summer I often camp near Tupper Lake at a state camp ground. I always wondered why frogs move from the pond at night and try to cross the campground drive where they are commonly run over by vehicles. When I got a chance, I asked a specialist in amphibians about it, and found out that while frogs spend the day in the water along the shore of the pond, at night they move into the uplands away from the pond to hunt. The roadway has simply become a barrier which divides too very different environments which together make up the habitat of the frogs.
Setting aside reserves by area with no consideration of the heterogeneity of the environment can often cause major losses in habitat diversity leading to major species loss.
Other factors which must be considered are the ranges of animals. Large animals often have very large ranges which they use for hunting and mating. Cougars have ranges in excess of 400 km.2; while grizzly bears have ranges greater than 900 km.2. Any "reserve" must be large enough to contain such creatures within their boundaries if they are to survive. Further, there are some "area sensitive" species. The figure below illustrates the area sensitivity of some birds. In this figure you can see that there appears to be a boundary point of approximately 10 hectares, the probability of these birds being present begins to decrease when the area is below that number - in some cases drastically. The exact cause or causes of this area sensitivity are not known, but edge effects may contribute.
We must also understand the difference between natural patchyness and habitat fragmentation.
1. Naturally patchy landscape has rich internal patch structure - treefall gaps, logs, different layers of vegetation; but fragmented landscape has simplified patches - parking lots, cornfields, clear cuts, mono-specific tree farms.
2. Because of (1) natural landscape has less contrast (less pronounced structural differences) between patches, and therefore, less edge effects.
3. Certain features of fragmented landscapes (roads, various human activities and structures) pose specific threats to population viability.
CONSEQUENCES OF FRAGMENTATION
Habitat fragmentation can immediately cause a loss of species which are endemic to portions of the landscape which are destroyed. Remember the patchyness of the environment. If all patches of a certain type are removed, any species requiring that patch type will go extinct. This affects many rare types of endemic species in forests. For example, in Columbia and Ecuador national parks do not include ranges of most birds species which are unique to those countries. Elimination of the habitat outside of the parks will cause these bird species to go extinct.
Barriers and Isolation
Remember my frog example above. Animals which require a mix of different habitats for distinct resources can be lead to extinction if a barrier separates one habitat from another. Also, species restricted to a certain kind of habitat will be endangered when there is no single patch which is large enough for a viable population. Such species will require a number of small habitat patches in order to survive. If there are barriers separating these patches the species will be endangered. Note, in the figure below, that when corridors are maintained for the white footed mice the population grows rapidly. When there are no
corridors population growth is restricted.
When patch fragments are initially formed there is an increase in population size of species within an isolated patch. This is refereed to as "crowding of the ark". These events are generally followed by a population collapse due to limited resources and intense competition among members of the same species.