(This exercise is based on With, K. A., S. J. Cadaret, and C. Davis. 1999. Movement responses to patch structure in experimental fractal landscapes. Ecology 80: 1340–1353.)
(Note: The reference above links directly to the article on the journal’s website. In order to access the full text of the article, you may need to be on your institution’s network [or logged in remotely], so that you can use your institution’s access privileges.)
Habitat fragmentation can disrupt the movements of individuals within the range of a population as well as among populations. Disruption of the latter can lead to diminished gene flow among populations, which in turn can result in population differentiation (the establishment of genetic differences among populations) and possibly even speciation. A more negative consequence of decreased movements among populations is an increase in the risk of population extinction (see Chapter 10 of the textbook).
Different organisms do not necessarily perceive the environmental landscape in the same way. As an extreme example, an ant “sees” the world at a much smaller grain than does a bear; what is a large distance for the former may be practically no distance for the latter. Animals that perceive the same grain size of the landscape may also differ in what they perceive as habitat; good habitat for one species may be inaccessible to another. Moreover, suitable habitat for foraging may not be suitable for long-distance movements. These differences in grain perception and habitat preference all affect the decisions animals make in moving across the landscape.
Kimberly With, then at Bowling Green University in Ohio, and her colleagues examined the movements of computer organisms—“cybercrickets”—that follow particular rules of movement across different landscapes. The researchers next examined the movements of actual crickets in experimental landscapes. By comparing the real crickets to the cybercrickets, With and colleagues attempt to address how real crickets perceive their landscape.
The researchers first simulated various types of landscapes. Each landscape was divided into a grid, the cells of which could represent either sand or grass habitat. The grain of the landscape was equal to the size of the grid cells. The proportion of grid cells that were sand or grass varied among the different landscapes. In addition, in some landscapes the habitat cells were clumped (that is, cells of each type were more likely to be adjacent to other cells of the same type) and others were patchy (cells of one type were less likely to be adjacent to cells of the same type). Habitat fragmentation tends to produce landscapes that are more patchy.
Figure 1 The patterns of grass (dark cells) and sand (light cells) habitat across landscapes that have different proportions of grass and sand. The landscapes also differ with respect to the degree of clustering; some are clumped and some are patchy. (Click image to enlarge.)
Question 1
Describe the difference between the patchy and clumped landscapes that are 50% grass.
Question 2
What differences are seen between the clumped and patchy landscapes that are 80% grass?
Figure 2 The squares that cybercrickets obeying either Rule 1 or Rule 3 could travel to on their next move. (Click image to enlarge.)
Question 3
The researchers looked first at how simulated “organisms” (aka cybercrickets) behave in the landscape. These cybercrickets could move according to two sets of rules (Rule 1 and Rule 3). Under Rule 1, a cybercricket can go to any square that is directly adjacent to the square it is on (excluding diagonals). Under Rule 3, the cybercricket is able to cross gaps of a square and thus is able to connect to more squares (see lower part of Figure 2). What is the maximum number of different squares that a cybercricket could go to if it obeys Rule 1? If it follows Rule 3?
Question 4
The connectivity of the landscape is a measure of how much of the landscape an organism can reach. In this case, it not only depends on qualities of the landscape itself but also on properties of the organism, including its habitat preference and its ability to move through the landscape (Rule 1 versus Rule 3). Figure 2 also shows how the connectivity changes as habitat changes from being primarily sand to primarily grass. On the left are results for clumped landscapes and on the right are the results for the patchy landscapes. “Grass-1” refers to cybercrickets that travel on grass and follow Rule 1, “sand-1” refers to those that travel on sand and follow Rule 1, and so on. For clumped landscapes, where is the transition between low connectivity to high connectivity for grass cybercrickets that follow Rule 1? Is it different from those that follow Rule 3 (can cross gaps)? If there is a difference, provide a possible reason for it.
Question 5
For patchy landscapes, where is the transition between low connectivity to high connectivity for grass cybercrickets that follow Rule 1? Is it different from those that follow Rule 3 (can cross gaps)? If there is a difference, provide a possible reason for it.
Figure 3 Mean and standard errors for path length and net displacement, for paths taken by cybercrickets. In this figure, equal numbers of clumped and patchy landscapes were used and the results for these different landscapes were averaged.
Question 6
The researchers examined the paths that the cybercrickets follow. In each replicate, a single individual cybercricket was placed randomly in the center of a landscape. The crickets moved at random into cells that they could occupy (in accordance with their travel rules). The simulation ended after either 50 moves or when the individual cybercricket reached an edge of the landscape. Various metrics were measured including path length (the total distance traveled) and the net displacement (the linear distance between the starting and ending points). For cybercrickets that travel on sand and follow Rule 3, what happens to their mean path length and net displacement as the proportion of grass in the habitat increases?
Question 7
For cybercrickets that travel on sand and follow Rule 1, what happens to their mean path length and net displacement as the proportion of grass in the habitat increases?
Figure 4 The data for the real crickets juxtaposed with the data from cybercrickets.
Question 8
Next, the researchers examined how crickets of the species Acheta domesticus move across experimental landscapes. What happens to path length and net displacement of the real crickets as the proportion of grass habitat increases?
Figure 5 The results for cybercrickets and real crickets in the clumped and patchy landscapes. (Click image to enlarge.)
Question 9
Figure 4 showed the results for clumped and patchy landscapes averaged together. Let’s take a closer look at each of these types of landscapes separately. What difference, if any, is seen between the clumped and the patchy landscapes for the cybercrickets that travel on sand and follow Rule 1?
Question 10
Are there any differences in the behavior of the real crickets in the clumped versus patchy landscapes?
Question 11
Based on the results thus far, what conclusions can you draw about the movement behavior of the real crickets?
Figure 6 Net displacement of small and large crickets at different levels of grass in the landscape (clumped and patchy treatments were combined). Means and standard errors are shown.
Question 12
Crickets varied considerably in size. The researchers were interested in whether large crickets (defined as those that were greater than 26 mm long) and small crickets (between 15 and 25 mm long) behaved differently. What are the effects of grass versus sand habitat on net displacement rates for the large crickets? For the small crickets?
