Billy West
GEO 4300
Lit Review 3
Stankiewicz, J., Thiart, C., Masters, J. C. and de Wit, M. J. 2006. Did lemurs have
sweepstake tickets? An exploration of Simpson's model for the colonization of
Madagascar by mammals. Journal of Biogeography, (2006) 33: 221–235
Stankiewicz, Thiart, and de Wit are from the Geological Sciences department of the
African Earth Observatory Network and University of Cape Town, Rondebosch.
Thiart is also associated with the Statistical Sciences department of same. Masters is
with the Natal Museum, Pietermaritzburg and the School of Biological and
Conservation Sciences at the University of KwaZulu-Natal in Scottsville (South
Africa).
2) Research Question/Problem
Is G.G. Simpson’s model of “sweepstakes dispersal” (i.e., the spatial spreading
by random chance of a species into a usually isolated area) valid with regard to
lemur colonization of Madagascar?
3) General Background Knowledge for the Study
Simpson in 1940 devised two binomial models to find the probability that
one individual from a population of one million attempted dispersals (i.e., attempted
crossings of the barrier, such as open sea, into the new region) has a .63 chance of
making it, and that in a million years successful dispersal also has a .63 chance of
success; these were specifically for the “African-Malagasy Sweepstakes,” referring to
the population of mammalian fauna (but only those small enough for “rafting”
across the ocean) from the African mainland to the island of Madagascar. The
authors of the study suspect that this classic model is inaccurate.
4) Methods
The assumptions Simpson made in his model were analyzed and challenged,
based on the requirements for the binomial distributions he described. Following
that, models were designed to investigate how a raft might cross from Africa to
Madagascar based on prevailing winds and currents, and then to study how an
extreme event such as a cyclone might facilitate dispersal from the mainland to the
island.
5) Results of the Study
The assumptions in Simpson’s models were found to be unrealistic: the
successful crossing of an individual lemur precludes the possibility of establishing a
breeding population, while the probability of a group of lemurs making a successful
crossing is vanishingly low; genetic evidence suggests that lemur colonization of
Madagascar was a single event, occurring once and ergo not readily conformable to
the binomial distribution; and the probability of dispersal, in any case, would not be
constant from year to year.
The authors found that assuming the absence of currents, a raft crossing to
Madagascar would be too slow and result in the starvation of its passengers, while
assuming prevailing current patterns would result in the beaching of the raft on the
continent instead of on the island. Transport by an extreme weather event was
calculated to be more unlikely than transport by rafting.
6) Weaknesses of the Study
The study was largely theoretical, and no direct observations or empirical
measurements were taken, basing the results on assumptions (quite reasonable
ones, but assumptions nonetheless). The study only undertook to challenge the
sweepstakes dispersal model for lemur colonization of Madagascar; alternative
hypotheses to how such an event occurred are beyond the scope of the study and
not offered by the authors.
7) What we learned new from the study
The sweepstakes dispersal model by Simpson can be ruled out as an
explanation for how lemurs populated Madagascar (but not, it should be said,
necessarily in other contexts). We learned that we do not know how lemurs came to
populate Madagascar.