Discussion Topic: Pattern/population-level consequences of Dispersal
Seminar in Dispersal Evolution & Ecology
March 3rd, 2011
Scribe: Abby Lawson
This week we broke from our traditional discussion format and tried something completely
different: speed dating! As usual there was a group reading assigned (Ronce and Promislow
2010) as well as individual readings assigned to pairs of people. The class was then divided into
two groups (US and THEM) in which pairs that had read a similar individual reading were in
different groups. The class period then consisted of seven 10-minute “dates” between US and
THEM group members (within group dating was strictly verboten). This enabled each person to
discuss their individual reading with their counterpart in the opposite group (that was assigned
the same reading), and discuss all the other individual readings on dates with other classmates
in the opposite group.
Date #1: Steve
Steve and I were assigned the same individual reading for the week (Massol et al. 2010). This
paper challenged the notion that environmental fluctuations lead to disruptive selection on
dispersal (which eventually results in a diversification of dispersal rates) by simulating
conditions in which disruptive selection could evolve in a stable environment by varying rates of
kin competition and cost of dispersal in their model. The paper was largely theoretical and
simulation based, though they applied their model to several real world examples. As a “takehome” conservation message, the authors suggested that when designing refuges for wildlife, it
is best to include patches of varying sizes that will promote disruptive selection and encourage
dispersal to available patches.
Date #2: Chad
Chad’s reading (Reed et al. 2011) was an applied paper that examined gene flow among
populations of wolf spiders in Mississippi, which use a highly specialized form of dispersal
(ballooning). This paper found increased rates of inbreeding in patches than previously
reported, which the authors argue is likely due to increased habitat fragmentation
(urbanization), relative to conditions in which the original dispersal strategy evolved, which is
an interesting demonstration of a highly specialized form of dispersal no longer being suitable
in a changing environment.
Date #3: Sara
Sara’s reading (Russo et al. 2006) was a bit of a precautionary tale regarding the need for
adequate knowledge of seed-disperser (spider monkeys in this paper) behaviors to adequately
model seed-shadow distributions— the discussion definitely made me re-think some of the
previous papers I’ve read on seed dispersal. The authors then compared their seed shadow
distribution predictions between models that included monkey behavior with those that did not
and found models that included animal behavior appeared to be more accurate. Much of the
seed-disperser patterns that were incorporated into the models were done so using stochastic
elements, though the authors caution that spatial scales need to be large enough to account for
long-distance migration patterns.
Date #4: Joy
Joy’s paper (Lachish et al. 2011) examined devil-facial tumor disease in Tasmanian devils and its
effects on genetics and dispersal pattern. This paper revealed that prior to infection, many
populations did not exhibit genetic structure, though the opposite pattern was apparent in
infected populations. This is likely due to the increases in inbreeding and relatedness within
populations that are shown to occur in just a few generations post-infection. Was an interesting
paper on a very current topic that demonstrated how prevalence of a virulent disease can
drastically alter dispersal patterns which are reflected in changes in genetics on a relatively
short time scale.
Date #5: Nick
Nick’s reading (Mendizabal et al. 2011) examined the genetic basis of dispersal in European
Gypsy populations, possibly the first paper we’ve discussed in class examining human dispersal
patterns. The authors attempted to construct a phylogeny of the different groups and
determine a common area of descent. The study also took into account genetic anomalies such
as the prevalence of congenital glaucoma in certain populations as well as cultural variation in
behavior— some populations more willing to accept non-gypsy females than others. Overall an
interesting discussion on dispersal patterns in our own species, in which discussion of dispersal
patterns is usually reserved for anthropology classes.
Date #6: Stephanie
Stephanie’s reading (Ovaskainen et al. 2008) examined dispersal traits in butterfly populations
of different ages occurring in varying habitat types: continuous and fragmented. Newer
populations had higher metabolism and dispersal rates in fragmented landscapes when
compared with older populations in large continuous landscapes, which is possibly due to
accumulation of non-dispersing genes in such populations. However, new populations
established in continuous landscapes also had lower rates of dispersal, which suggests that
environmental factors also may influence dispersal patterns.
Date #7: Kevin
Kevin’s reading (Petrovskii and Morozov 2009) was a technical/theoretical paper focused on
persistent patterns of “fat tails” in dispersal data— the rate of decay in population density over
long distances is usually described as a normal distribution though field data often show much
lower rates which result in a “fat tail”. However, this paper shows that fat-tailed long-distance
dispersal is a consequence of the observation individuals of the same species are not identical,
furthermore, the authors applied this notion to real data (which is often lacking in such
theoretical papers!) to back their theoretical predictions produced by their model.
Date #8: Cynthia (absent… or speed-dating cold feet?)
I did not discuss the Nathan and Muller-Landau (2000) paper due to an unforeseen classmate
absence. Though from the abstract it appears that the paper is a synthesis that examines
factors that affect spatial patterns of dispersal in plants and draws on recent mechanistic
models to describe such patterns to promote a deeper understanding of consequences of
dispersal processes.
GROUP SESSION
The global reading assigned (Ronce and Promislow 2010) wasn’t on the original topic assigned
for this week, but seemed appropriate given it discusses dispersal as a life history trait and was
a nice follow up last week’s fruitful discussion on senescence. Ronce and Promislow (2010) and
some of the individual reading assigned for this week (e.g., Petrovskii and Morozov 2009) were
a bit “mathy” and the class had a brief discussion on how to approach such papers. Dr. Forister
offered some helpful advice that it is a helpful and worthwhile exercise, at least once during our
graduate careers, to dissect such a paper and translate every formula presented into our own
words and understanding from start to end.
It was mentioned that in several of the graphs (Fig. 1,2; Ronce and Promislow 2010) that many
of the patterns in a species with partial vs. complete dispersal were quite similar, with the
notable exception being effective fecundity vs. age (Fig. 1b). In this pattern, the effective
fecundity pattern of the two strategies is similar up until the age of 35 where they drastically
depart. This suggests that in longer-lived species dispersal patterns do have a large impact on
effective fecundity. The consensus among the class for the main point expressed in the paper
was that if potential reproductive output of an older individual is less than that of potential
output from a recruit, then longer-life is no longer selected for.
Perhaps the biggest critique of this paper, which was brought up on several of my “dates” as
well as the group discussion, was that phenomena such as kin cooperation, experience raising
young, or mutualisms were not included in this model. The prevalence of such effects could
drastically alter several of the graphs presented in this paper and would be an interesting “next
step” to a follow up paper. It was also stated later that such kin cooperation patterns (e.g., the
grandmother effect in humans) could be a potential cause as to why humans are now selected
to live past the age of reproduction (i.e. enter menopause). Another criticism was that the
paper seemed like a lot of argument with relatively little data. Unfortunately future work
attempting to incorporate ideas like the grandmother effect are likely to be muddied by
modern phenomena such as modern medicine.
Class thoughts on the speed-dating format?
PROS
CONS
+ Required thorough understanding of
individual readings, well enough to explain
them to other classmates
+ Encouraged all class members to actively
participate in discussions
+ Allowed individuals assigned the same
reading to compare notes
- Exhausting! Maybe shorten the duration or
number of dates?
- Format requires advanced planning and does
not allow for last-minute absences
Literature Cited
Lachish, S., K.J. Miller, A. Storfer, A.W. Goldizen, and M.E. Jones. 2011. Evidence that diseaseinduced population decline changes genetic structure and alters dispersal patterns in the Tasmanian
devil. Heredity 106:172-182.
Massol, F., A. Duputié, P. David, and P. Jarne. 2010. Asymmetric patch size distribution leads to
disruptive selection on dispersal. Evolution 65(2):490-500.
Mendizabal, I., C. Valente, A. Gusmão, C. Alves, V. Gomes, A. Goios, W. Parson, F. Calafell, L.
Alvarez, A. Amorim, L. Gusmão, D. Comas, M.J. Prata. 2011. Reconstructing the Indian origin and
dispersal of the European Roma: a maternal genetic perspective. PLoS ONE 5(1): e15988.
Nathan, R. and H.C. Muller-Landau. 2000. Spatial patterns of seed dispersal, their determinants
and consequences for recruitment. TREE 15:278-285.
Ovaskainen, O., A.D. Smith, J.L. Osborne, D.R. Reynolds, N.L. Carreck, A.P. Martin, K. Niitpeõld,
and I. Hanski. 2008. Tracking butterfly movements with harmonic radar reveals an effect of population
age on movement distance. Proceedings of the National Academy of Sciences 105(49):19090-19095.
Petrovskii, S. and A. Morozov. 2009. Dispersal in a statistically structured population: fat tails
revisited. The American Naturalist 173(2):278-289.
Reed, D.H., V-h. Teoh, G.E. Stratton, and R.A. Hataway. 2011. Levels of gene flow among
populations of a wolf spider in a recently fragmented habitat: current versus historical rates.
Conservation genetics 12:331-335.
Ronce, O. and D. Promislow. 2010. Kin competition, natal dispersal and the moulding of
senescence by natural selection. Proceedings of the Royal Society 277:3659-3667. [GLOBAL READING]
Russo, S.E., S. Portnoy, and C.K. Augspurger. 2006. Incorporating animal behavior into seed
dispersal models: implications for seed shadows. Ecological Society of America 87(12):3160-3174.