Lecture 3

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Lecture 3
Dispersal – can be an important process in accounting for distribution of a species. Particularly in explaining
why some organisms have a limited distribution while others occur over a wide distribution.
Definition: spreading of individuals away from one another. All organisms disperse, if not as adults then as
young or propagules (seeds, larvae, etc.)
Locally important process to reduce negative effects of inbreeding.
In practice it is difficult to actually measure how much a species disperses, especially when it occurs within the
range of its distribution.
In evolutionary studies one is often interesting in determining the processes of genetic divergence that lead to
speciation (e.g. via isolation of populations within species), here one examines the genetic distance between
population – these studies can reveal whether dispersal is occurring and how often. Prolonged isolation can lead
to allopatric speciation.
Generally, dispersal is effective in the absence of barriers such as physical impediment, resources, competitors,
Exceptions: e.g. Burton 1994 study on high intertidal copepods along west coast of US indicate very limited
gene flow over as short a distance as 0.5 km.
Other examples: Grosberg 1987, studied dispersal potential in a tunicate (colonial marine invertebrate) Botryllus
using genetic markers.
Males release sperm, females brood eggs and release larvae which settle on the substrate. By fertilizing eggs
from sperm of male with unique genetic patterns – able to show that dispersing larvae were limited to within a
1m radius of origin.
Implications – susceptibility of local extinction is high, but perhaps high rate survival.
Opposite extreme: spread of microbial or viral pathogen into population of the long-spined sea urchin Diadema
Historical factors:
Examples of how barriers have impeded dispersal come from inadvertent introductions of species.
e.g. include:
Starlings introduced in 1890 from Europe by 1950 occurred throughout North America.
Fire ants introduced from Brazil 1930 – rapid dispersal.
Africanized honey bees – 1960’s to South America now in the lower Rio Grande, Texas. (Krebs chapter 4).
 For all these species, distribution previously limited by dispersal ability.
Dispersal can be effected in three different processes:
1. Diffusion - gradual movement of populations over favorable terrain, dispersal occurs over time period of
2. Jump dispersal - movement of individuals across great distances, usually over unsuitable habitat (e.g.
colonization of island or other patches of habitat. Usually restricted to species that have specific
attributes favoring dispersal. e.g. reptiles and birds in Galapagos. Or, unusual events favoring dispersal
e.g. ENSO events.
3. Secular migration - diffusion over evolutionary time. Populations spread at rate where differentiation
and/or speciation occurs. Important in explaining patterns of biogeography, e.g. human populations.
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Why disperse?
1. Crowded conditions - survival of offspring low where current adult population exists.
2. Unpredictable environment – probably most important reason for marine species. Probability of local
extinction high.
3. Avoid inbreeding - deleterious effects resulting from homozygosity, outcrossing results in higher fitness.
4. Social interactions
Greenwood’s study:
In birds, females tend to disperse, males are territorial and monogamous (usually), males choose females.
In mammals, males disperse, and compete for females. Males are territorial but polygamous.
Survey results:
Males disperse
Females disperse
no difference
Example of ecological and evolutionary implications of dispersal in marine species.
Modes of larval development:
Planktotrophic - weeks to months in developmental stage. Usually species have wide geographic range. Taxa
represented by fewer species, persist long over evolutionary time.
Nonplanktotrophic – larval development is direct or rapid with short time in plankton. Taxa represented by
many species, usually with limited geographic range.
Planktotrophic larval development found in 70% of all invertebrate marine organisms and in 99.9 % of tropical
marine fishes.
Over ecological time nonplanktonic forms may be favored if there is higher fitness with less dispersal since
there is much higher mortality of propagules in planktotrophic species.
Over evolutionary time however planktotrophic forms may be selected over nonplanktotrophic forms.
Resources: All things consumed by an organism.
Fixed resources - e.g. shelter (nonrewable)
Expendable resources – nutrients, mates (renewable)
Key point: resources represent quantities that can be reduced by the activity of the organism.
Here we review some of the ways that resources can affect patterns of distribution and abundance.
How individuals in a population are distributed with respect to their resources is dependent on two things:
1. Accessibility (presence or absence of barriers).
2. Supply (quantity or quality available)
Case 1: Resources continuous - vary in supply but equally accessible to all individuals in the population.
Individuals will distribute themselves according to the distribution of the resources. Known as the "ideal-free
distribution model" (Fretwell and Lucas 1970).
Ideal: no confound limiting distribution of resources
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Free: continuous distribution
In this model fitness among individuals is equal, variance in fitness among individuals is low and independent
of the density of the population.
Variance can arise as a result of natural selection for better efficiency at garnering resources.
Variation in distribution of individuals at different densities comes about because individuals are able to select
where they will be with respect to available resources – referred to as habitat selection.
Case 2: Resources partitioned by physical isolation.
Resources vary in supply but are not equally accessible.
Variance in fitness moderate to high – dependent on density of individuals.
Example: marine organisms recruiting to benthic habitats, particularly sessile species.
Strong selection for habitat selection at low density (e.g. evolution of cues for settlement). Little or no selection
at high densities.
Examples: Keough - preferential settlement exhibited by bryozoans and tunicates where resource quality is
affected by presence of space competitor. Here "space" is the resource.
Bryozoans - poor space competitor, good colonizer. Prefers small discrete substrates.
Tunicates - good competitors, poor colonizers. Fitness proportional to size of colony. Prefers to settle on larger
Grosberg – showed that differential settlement driven by the presence or absence of superior competitors.
Case 3: Resources partitioned by behavioral isolation. i.e., territoriality – the defense of a resource against
other consumers for that resource.
Fitness is a step function, variance in fitness is low to high and density-dependent.
Case 1 - no variance in fitness
Cases 2&3 – variance in fitness high to low,depending on density.
Case 2 - selection for habitat selection
Case 3 – selection for territorial behavior (competitive ability), as well as territorial behavior.
Variance means opportunity for selection.
Under what conditions will territoriality evolve?
Can expect territoriality to evolve only when the benefits (gains) from defence of a resource is greater than the
costs associated with defense.
Can express it as: Cl + Cd < Ew/od + E w/d
Cl = basic cost of living
Cd = cost of defense
Ew/od = energy gained w/out defense
E w/d = additional energy gain with defense.
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If Cd high and Ewd low then territoriality will not be favored.
e.g. 100 + 50 < 110 + 45, here anything >40 units of energy gained with defense will favor territoriality.
Conditions that influence territoriality:
1. Resource quality and distribution in space.
2. Resource distribution in time.
3. Intensity of competition for resources
Will not evolve when:
Resource quality is homogeneous and very abundant, or sparsely distributed. When sparse, need to travel from
place to place – not economical to defend if resources are easily depleted.
If resources are heterogeneous and clumped and competitor pressure is low or moderate, then it may pay to
Usually as one factor changes so does the other. The more food available the more competitors there are.
Example from Rufus Hummingbird:
Defends territory – size defended depends on resource distribution and abundance.
If nectar per flower < 3ul then territorial
If nectar per flower > 3 ul then not territorial
Size of territory directly related to floral density. Small when high, large when low.
In conclusion: a general theory for the evolution of territoriality.
Economic defensibility
Requisites for reproduction
Population density
Proximate factors : competition, aggressiveness, economic defensibility and population density
Ultimate factors: requisites for reproduction, population density