Comments on mutualism and its role in population dynamics.

[ParcBob at aol.com]

Zoology 369 Basic Ecology
L.E. Gilbert


Zoology 369 Gilbert's Lecture 3 and 4

Comments on mutualism and its role in population dynamics.
When ecologists attempt to understand why a particular species' ("X") 
population occurs at a given density or exhibits certain changes in numbers 
through time, they tend to investigate other species that compete with or 
consume species X. In other words, the focus is on factors that would 
counteract exponential growth of species X's population as for example the 
negative feed-back of a predator population might do. Mutualism as a factor 
in explaining the observed features of populations is rarely considered to be 
important. One reason for this may be the simplistic way that mutualism is 
incorporated into population models (as described in your text). By 
specifying that the benefit exchanged by mutualists is essentially an 
increase in carrying capacity for each mutualist, the models predict 
unlimited expansion of each population until the support system for both is 
greatly over-taxed and both populations crash. Consequently, theorists have 
considered mutualism to be a destabilizing force.Research on ecological 
systems in tropical habitats suggest that mutualism has been misinterpreted 
and underplayed as a determining factor in population dynamics.
 
A. I discussed several major categories of mutualistic interaction:

1. Plant-pollinator relationshipsExample: Orchid bees (euglossine) in new 
world rainforests. <A HREF="http://uts.cc.utexas.edu/~gilbert/teaching/zoo369/graphics/euglossinebee.jpg">Male euglossines</A> of a given species gather odors from 
specific species of <A HREF="http://uts.cc.utexas.edu/~gilbert/teaching/zoo369/graphics/euplusia.jpg">orchid</A> and are the only pollinators of that particular 
species. Females and males visit a variety of <A HREF="http://uts.cc.utexas.edu/~gilbert/teaching/zoo369/graphics/calatheabees.jpg">nectar producing plants</A> and are 
important pollinators for some. <A HREF="http://uts.cc.utexas.edu/~gilbert/teaching/zoo369/graphics/orchidbee.jpg">Females gather pollen</A> to provision <A HREF="http://uts.cc.utexas.edu/~gilbert/teaching/zoo369/graphics/eulemanest.jpg">brood 
chambers</A> from still other plants, some of which rely specifically on these 
insects to set seed. Still other plants <A HREF="http://uts.cc.utexas.edu/~gilbert/teaching/zoo369/graphics/dalechampia.jpg">provide resin</A> which euglossine 
females collect for <A HREF="http://uts.cc.utexas.edu/~gilbert/teaching/zoo369/beenest.html">nest construction</A> and pollinate the provider plant in the 
process. To understand the population biology of a given species of 
euglossine bee you would need to understand the specificity and strength of 
connection to as many as twenty different resource plants in its environment. 
Conversely to understand whether the population trends of a particular plant 
in this system are related to changes in euglossine populations you would 
need to know the specificity and strength of connection this plant has to 
particular bee species. Since some orchid bees are specific pollinators for 
some orchids, the extinction of the bee in an area would ultimately spell the 
loss of the plant by ending juvenile recruitment. However, since orchids can 
live many decades, it could require long-lived and persistent ecologists to 
monitor the orchid's decline. Care to make a mathematical model of such a 
system?
 
2. Plant-seed disperser mutualism Adult perennial plants grow where 
conditions were suitable for seed germination and seedling survival in the 
past but not when they reproduce. For examples, seed and seedling predators 
or parasites may build up under a tree dooming babies that try to develope 
below the parent, or the parent's shade may inhibit growth of its own 
offspring. Dispersing seeds away from the death zone around the parent and to 
areas of new disturbance (sunny open ground) is required for many plant 
species. Fruits are devices which help enlist mobile animals in the dispersal 
of seeds contained in the fruits. As in pollination mutualisms, the degree of 
specificity is important in understanding how influential such mutualism 
might be to driving the dynamics of interacting plant and animal populations. 
In some extreme case, the loss of a fruit bat species could mean the ultimate 
loss of a tree species from a Malaysian forest (or vice versa). However most 
such mutualisms are more diffuse (i.e. trees that rely on several bat 
species, and <A HREF="http://uts.cc.utexas.edu/~gilbert/teaching/zoo369/graphics/batngurania.jpg">bats that rely on fruits</A> of many tree species) and do not boil 
down to obligate species pairs. Those tend to go extinct! 

3. Plant defense mutualisms. Many plants provide food (food bodies or 
extra-floral nectar) and/or shelter (hollow stems, petioles, or thorns) to 
ants, wasps and other predators and parasites in return for protection from 
plant-feeding animals (herbivores). I gave examples of <A HREF="http://uts.cc.utexas.edu/~gilbert/teaching/zoo369/facultative.html">facultative</A> systems 
(eg. passionvines and various predaceous ants and parasitoid wasps) and and <A HREF="http://uts.cc.utexas.edu/~gilbert/teaching/zoo369/obligate.html">
obligatory ant-plant mutualism</A>, ant acacia and Pseudomyrmex ants, neither of 
which exist without the other. The acacia provides carbohydrates 
(extra-floral nectar), proteins and oils (beltian bodies on new leaflets) and 
domicile (hollow, stipular thorns). The Pseudomyrmex ants protect the host 
acacia from vertebrate and insect leaf-feeders as well as from other 
competing plants (they bite, sting, and kill the growth points). This plant 
drops out coming north in Mexico where cold periods are sufficient to inhibit 
ant defenses and deer, goats, and cattle are able to feed on the leaves and 
stems. 

4. Cooperative education of predators by aposematic insectsMullerian mimics 
are distasteful, warningly colored species that evolutionarily converge on 
similar warning signals. This is a form of mutualism since the per-capita 
probability of death to predation within each species' population is lower 
because they use the same rather that different warning signals. Since no 
deception is involved, this phenomenon should not technically be termed 
"mimicry." <A HREF="http://uts.cc.utexas.edu/~gilbert/teaching/zoo369/graphics/helimimics.jpg">Heliconius</A> butterflies provide a nice example since species from 
different clades look more alike within an area than do different populations 
of the same species from different regions.

 B. I discussed the interesting case of <A HREF="http://uts.cc.utexas.edu/~gilbert/teaching/zoo369/graphics/ethillapollen.jpg">Heliconius ethilla</A> in Trinidad whose 
population dynamics cannot be explained without considering several ways that 
mutualisms directly or indirectly influence the probabilities of birth, 
death, and survival of the insect's various life stages . 
1. Eggs are placed on new shoots of Passiflora vines. Eggs hatch in 4 days. 
If all goes well for the <A HREF="http://uts.cc.utexas.edu/~gilbert/teaching/zoo369/graphics/ethilla5.jpg">larva</A> that subsequently hatches, 5 larval stages or 
instars spend about 15 days consuming the passionvive leaves. A pupal stage 
of 9 days ends with the eclosion of a teneral adult. Adults marked in the 
first day of emergence have been recovered over 6 months later ranking 
Heliconius as the longest lived butterfly known (excluding species that 
diapause as adults). To recover a mobile butterfly after 6 months in the same 
place indicated a remarkable faithfulness to "place" which is associated with 
birds and mammals, but not insects. Mark-release-recapture studies showed 
that for two years, through dry season and wet, two ethilla populations 
showed remarkable <A HREF="http://uts.cc.utexas.edu/~gilbert/teaching/zoo369/graphics/ethsubpops.gif">constancy of numbers</A>. (We don't say "stable" because in 
population biology and in mathematics stability has a specific technical 
meaning. Experiments would be required to demonstrate "stability.") 

2. How does mutualism help explain the observed constancy of Heliconius 
populations?
 
a. Plant-pollinator mutualism. Heliconius adults feed on the pollen of 
certain plants especially the rainforest cucumber vines (<A HREF="http://uts.cc.utexas.edu/~gilbert/teaching/zoo369/graphics/ethgurania.jpg">Gurania</A> and <A HREF="http://uts.cc.utexas.edu/~gilbert/teaching/zoo369/graphics/psiguria.jpg">Psiguria
/Anguria</A>). This is a tight plant-pollinator relationship in which the 
butterflies are major pollinators for the plants and the plants major food 
resources for adult maintenance and egg production. 80% of a female's egg 
production come from amino acids that come from pollen she collects. Only 20% 
comes from that acquired by the caterpillars feeding on passionvines. Note 
that in most butterflies and moths, 100% of eggs derive from the efforts of 
the larval stage and eggs are laid in a quick pulse after adult emergence. In 
Heliconius, eggs are laid as they are manufactured over the adult's long 
lifespan. The butterflies learn the locations of pollen plants and establish 
home ranges based on pollen foraging routes. It appears that the pollen 
plants are more significant than larval host in determining a female's 
assessment of the habitat. Thus, as long as she knows the locations of a 
network of pollen plants, she will stay in the area, even during periods when 
new shoots of passionvine hosts are temporarily not available due to weather 
or defoliation by Heliconius or other competing herbivores. So while most 
herbivorous insects disperse away when suitable ovipostion sites are scarce, 
Heliconius females are content to stay put as long as the mutualist plant 
produces pollen (which is year-around). Moreover the pollen promotes a long 
reproductive life so that females can wait many weeks for the opportunity to 
resume <A HREF="http://uts.cc.utexas.edu/~gilbert/teaching/zoo369/graphics/ismoviposit.jpg">egg-laying</A>. In summary, this mutualism reduces the probability of 
local extinction due to adult dispersal, increases the probability of 
individual survival and reproduction across periods when natural 
perturbations of the environment reduce many other species of insect in the 
same habitat. Moreover, since pollen supply is 1) limiting and 2) does not 
increase if the adult butterfly population increases, egg numbers cannot 
increase as a simple linear function of butterfly population increase. Egg 
numbers are thus always more constant than number of adult females. 

b. Plant defensive mutualisms <A HREF="http://uts.cc.utexas.edu/~gilbert/teaching/zoo369/nectaries.html">Extra-floral nectaries</A> of passionvines 
encourage a variety of predators (ants, wasps) and parasitoids (flies, wasps) 
to patrol the shoots and leaves of the vine. Each life history stage, from 
egg to pupa is exposed to several natural enemies. In the ethilla population 
over 90% of eggs were killed by <A HREF="http://uts.cc.utexas.edu/~gilbert/teaching/zoo369/eggparasitoids.html">egg parasitoids, tiny wasps</A> that feed on 
extrafloral nectar and hang out on the new shoot. Larvae from eggs that 
survived wasps face many similar problems in their 15 day developmental 
period. <A HREF="http://uts.cc.utexas.edu/~gilbert/teaching/zoo369/graphics/predant.jpg">Ants</A> in particular are important because they patrol whether or not 
larvae are present (i.e. they are not limited by larvae). Consequently, 
increases or decreases in caterpillar numbers do not set off predator-prey 
oscillations as occur when predators have a delayed population response to 
changes in prey populations. In the end, each female ethilla butterfly 
averages one surviving female offspring from the approx. 1000 eggs deposited 
in her life. The net effect of the entire Passiflora mutualistic defensive 
system in this case, is to prevent the runaway herbivore outbreak that I 
described for owl butterflies in the banana plantations of Costa Rica. 
Otherwise the adult population would not show such constancy as was observed. 


c. Mullerian mimicry. When adult ethilla populations decline to very small 
numbers, there can be more <A HREF="http://uts.cc.utexas.edu/~gilbert/teaching/zoo369/graphics/jacamarwbfly.jpg">predaceous birds</A> than adult butterflies. Even 
though ethilla is bad tasting and aposematic, if each bird tested a butterfly 
to refresh its memory on what patterns to avoid, the butterfly population 
would quickly be extinct. It is during these time of rarity that companion 
aposematic species sharing the same color pattern signals can provide 
continuing education of local birds and save mullerian partners fron local 
extinction. 

d. Plant-seed disperser mutualism On a longer time frame the resource plants 
for the ethilla population are maintained by mutualists of these plants such 
as pollinators and animals required for effective <A HREF="http://uts.cc.utexas.edu/~gilbert/teaching/zoo369/graphics/batngurania.jpg">seed dispersal</A>. These 
interactions do not account for constancy on the order of a few months in the 
case of large perennial vines that require years to establish. However the 
long term trends of Heliconius populations ultimately depend on the 
mutualistic support of all the key resource plants.

e.<A HREF="http://uts.cc.utexas.edu/~gilbert/teaching/zoo369/graphics/helipopdyn.pdf">Summary diagram</A> showing the interactions discussed    (See Nature Potpourri 
FILES section|



Bob Parcelles, Jr.
Pinellas Park, Florida
Repley to: ParcBob at aol.com






Bob Parcelles, Jr.
Pinellas Park, Florida
Repley to: ParcBob at aol.com
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