Question

Species-area relationships were assessed for reptile species on two sets of islands in different regions of the Indian Ocean. The estimated parameters were as follows: Region 1: c=1.6, z=0.25; Region 2: c=1.8, z=0.35. It is expected that 40% of the reptile habitat will be lost to development pressure over the next decade.

In which region do you expect the loss of reptile species to be greatest, and by how much? Show your work and explain your answer. (3 pts)

b) A different group of species is then assessed on islands in the first region, and the estimated parameters for this species group are c=2.5, z=0.29. What can you conclude about:

i) species richness of this group relative to the reptiles in this region? Why? (1 pts)

ii) the range size of this group relative to the reptiles in this region? Why? (1 pts)

As the conservation officer for Region 2, you were the person tasked with determining the species-area relationship of reptiles in the region. Explain how you came up with the estimates of c=1.8 and z=0.35 (3 pts)

Answer 1

Species-area relationships, or the correlation between the number of species and the island area is represented by the equation :

S = cA^Z

which can also be represented in the linear form by taking logarithms on both sides :

log₁₀S = log₁₀c + z log₁₀A

Here, S= no. of species

A= Area of the island

z= slope of the species-area relationship in the log₁₀A versus log₁₀S graph.

c=constant, which dpends on the unit of area measurement, and is numerically equal to the number of species that would exist in the island if the area was one square unit.

This representation of species-area relationship is based on the "equilibrium model of biogeography", according to which, bigger the island, greater the number of species. At equilibrium, each of these islands or habitats are considered to have equal rates of immigration and extinction. Therefore, if there are two islands with similar immigration rates, and at equilibrium, then the bigger island will have a lesser extinction rate than the smaller one because the bigger island has a greater number of species on it.

The straight line graph representing the species-area relationship shows that, in:

log₁₀S = log₁₀c + z log₁₀A

log₁₀S = no. of species (on the y-axis)

log₁₀A = island area (on the x-axis)

In this regard, we can say that the slope (z) of the species-area relationship graph is equal to the ratio of the vertical change() to the horizontal change (). Or in other words, greater the change in the y-axis, greater the value of 'm' and steeper the slope of the graph. In this case, it would mean that greater the change in the number of species, greater the value of z.

According to the data provided in the question, we can see that Region 2 has a higher value of z (0.35) than Region 1. Therefore must have been higher in Region 2 than Region 1. Therefore species number in Region 2 has increased more rapidly than in Region 1, or the immigration rate is higher in Region 2. This may be due to a suppsoed proximity of Region 2 to the mainland, or a lesser extinction rate due to bigger size, which can be hypothesized from this data. Therefore, form the given data, the loss of reptile species can be shown to have been higher in Region 1.

It has also been mentioned that 40% of the habitat area will be lost over the next decade.

Therefore, = x - (40% of x) = 60% of x = 0.6x.

and, m (Region 1) = 0.25, m (Region 2) = 0.35.

Therefore, (in Region 1 in a decade) = 0.25 * 0.6x = 0.15x

and (in Region 2 in a decade) = 0.35 * 0.6x = 0.21x

Dividing the two values, we find that the percentage difference in immigration rate in Region 2 as compared to region 1 is : [(0.21x - 0.15x)/ 0.21x] * 100 = 28.6%

Therefore conversely, the loss of reptile species will be greater in Region 1 by 28.6% than Region 2.

b) i) Species richness is defined as the number of different species present in an ecological community, regardless of their distribution pattern in the community. The parameters for this new species has been provided as c= 2.5 and z= 0.29. Now we know, that c is a direct function of the number of species present and it increases along with the increase in the number of species in a particular habitat. Therefore, a higher value of c for this new population indicates higher species richness of this population as compared to the earlier reptile populations.

ii) The range-size of a species is the size of the geographical area where the species can be found. In this case, both the parameters, c and z, for the new species is higher than the earlier reptilian populations. This indicates that the population number is higher in this new species than both the earlier reptilian populations. However, the assessment has been carried out in Region 1, which is the same region as for the first reptilian population considered in this study. Therefore, the new species represents a denser population than the earlier reptilian population in Region 1, and would hance have a higher range-size than the reptilian population in Region 1.

c) As the conservation officer for Region 2, the species area relationship of the reptiles in the region was determined by measuring the number of reptiles present in the island and the area of the reptile habitat. The nuber of species was plotted in the y-axis of the species-area relationship graph and the habitat area was plotted on the x-axis, since the latter is independent of the species population. By plotting data points acquired over time, the graph was plotted, and the slope of this graph has been represented the parameter 'z'. It is numerically equal to the ratio of the change in species number to the change in habitat area. The 'c' represented the y-axis intercept and is equal to the number of species existing in the habitat if the habitat was one square unit in area. The value of 'c' was determined using the length of the y-intercept on the graph.