Suppose you survey the plant communities on several islands. You find that the
smallest island contains the most plant species, in seeming contradiction to the speciesarea relationship. Using your knowledge of landscape ecology, how can this apparent
paradox be explained?
An example of an A answer:
“One way the smallest island can support the most species is from greater proximity to
the mainland (species source/source of colonists). In other words, the smallest island
must be closer to the mainland than the larger/other islands surveyed. This pattern was
developed by MacArthur and Wilson (1967) in their seminal book on the theory of island
biogeography, and was demonstrated empirically in the field by Simberloff and Wilson
for mangrove islands and invertebrate communities off of the coast of Florida. Although
extinction rates would be expected to be higher on the small island, near-continual
colonization from the mainland would offset this process, leading ultimately to greater
richness. Another possibility is that the smaller island is more suitable in terms of
habitat quality than the other/larger islands; Summerville and Crist showed that habitat
quality can override the influence of patch size.”
This answer is factually correct, detailed (with citations), well-organized,
and thorough. It provides multiple possible explanations of the pattern. This
answer is clearly head and shoulders above the others (below).
Examples of B answers:
“This island, while the smallest, is probably the closest to the mainland. This means
that it is also most likely to be colonized and have greater species richness, more so
than a larger island that is farther away (IB theory).”
“Although position relative to the coast was not stated, the smallest island may contain
the most species due to its distance from the coast. IB theory states that size of the
island and distance from the mainland play roles in the number of species. In other
words, it may be the smallest but nearest.”
These two answers, although correct in what they say, are sparse: each
provides only one possible explanation, and neither provides much explanative
detail in that. The use of “IB” without spelling out what the abbreviation means is
likewise indicative of the student’s desire to make a hasty rather than a thorough
and correct answer.
An example of a C answer:
“This would be indicative that patch size is not the only factor contributing to the number
of species. The smallest island may have much better habitat than the larger ones and
can therefore support more species.”
This answer is a minimalist one. The answer is actually only one
sentence, as the first sentence is simply a restatement of the pattern. The answer
provides only one explanation for the pattern, and does not even provide the
most robust/obvious one (from island biogeographic theory) at that. The patch
quality vs. size argument is flimsy (lacks detail).
An example of a D answer:
“On the basis of island biogeographic theory, this pattern can be possible due to
proximity to the mainland: though the island is the smallest, the despotic distribution can
play a role to show how the island is suitable to sustain lots of plant species.”
Erroneous extraneous material is tacked on to part of the correct answer,
indicating that the student is confused in his/her thinking. Although this student
has tried to provide more than one possible explanation, the second explanation
is irrelevant and is also incorrect (the despotic distribution refers to abundance
patterns, not species richness).
Examples of F answers:
“Summerville studied species in parks that exhibited SS > SL. There could be many
reasons for this. In the absence of a keystone species, many species could be equally
viable to profligate simultaneously. Plants, being stationary, have been known to
flourish in SS > SL some times. Some species could also thrive on the edge effects
between the patch island and ocean matrix. I also know the shifting mosaic model is on
the rise for validity and could give key insight that the outside matrix effects could cause
yet unknown consequences on S and P.”
This answer is filled with inaccuracies and ambiguities. To what do SS, SL,
S, and P refer? Moreover, Summerville was not involved in SLOSS work.
Inclusion of the SLOSS debate, keystone species, and shifting mosaic theory are
indicative that the student is grasping for straws and is trying to include as many
buzzwords as possible in hopes that the correct answer may be found
somewhere therein. This answer is irrelevant and wordy B.S.
“The results vary depending on whether you are looking at richness or evenness. This
small island has plants that do well in small areas, and large islands have species that
need more space, so smaller islands will have a richer diversity of organisms.”
This answer is factually flawed: the question of richness vs. evenness is
irrelevant to the question that was asked (evenness is not even an issue here),
nor does it incorporate any degree of landscape ecology. It is clear that this
student does not demonstrate any degree of mastery of the material. Moreover, it
is clear that the student has failed to understand a key component of the
question, namely, the species-area relationship. This answer is profoundly
wrong.