Discussion Topic: Pattern/Population-Level Consequences of Dispersal

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 “take-home” 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 / - Exhausting! Maybe shorten the duration or number of dates?
+ Encouraged all class members to actively participate in discussions / - Format requires advanced planning and does not allow for last-minute absences
+ Allowed individuals assigned the same reading to compare notes /

Literature Cited

Lachish, S., K.J. Miller, A. Storfer, A.W. Goldizen, and M.E. Jones. 2011. Evidence that disease-induced 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.