FORESTS, FALL 2018
Question set THREE
DUE 8 November

QUESTIONS MOSTLY ABOUT NATURAL SELECTION (and one to review competition/niche theory): 

1. This figure shows the distribution of two species of cat-tails – Typha latifolia and Typha angustifolia – over a range of depths of water. Negative depth means out of the water. (Cat-tails are the dominant plant in the wetlands around the Dickinson Pond; both of these species occur on campus). The upper graph shows situations where both species occur together (in sympatry); the lower graph shows distributions in situations where only one of the two species occurs (allopatric). The vertical axis is a measure of abundance (don’t worry about different units; it's the relative abundance of the two species that's of interest here).
            Interpret the patterns observed in terms of fundamental and realized niches for the two species, indicating the implied competitive relationships. If the observed differences between the two graphs are a result of interspecific competition, you might hypothesize that competition is for either light or mineral nutrients (since these are perennial wetlands, it’s presumably not about water!). Cat-tails are rooted in the sediments, and presumably obtain mineral nutrients through their roots. Briefly, lay out an experiment to attempt to test these hypotheses. Explain your methods, and what you would expect if the relevant hypothesis is correct.

ANSWER 3 OF THE FOLLOWING 4: (In all of these use evolutionary/selective arguments carefully, making sure you put things in terms of how individuals with different traits are likely to differ in reproductive success as a result of different selective 'regimes'). 

2.  Insects are the primary herbivores affecting forest plants (and most plants that aren't in grazing/grassland ecosystems), and a wide range of chemical defenses has evolved in plants.  They come in two types:
    'Qualitative defenses' are outright poisons -- insecticides (many of our agricultural insecticides are modeled after these).  They tend to be effective in small amounts, and they're often quite small molecules (e.g., cyanide).   
    'Quantitative defenses' are indigestible, often bitter, compounds that dilute the food value of the plant tissue and often make it hard to digest (tannins are an example); these chemicals are typically large molecules, and they have to be present in high concentrations to be effective.
    Discuss the trade-offs -- selective advantages and risks or disadvantages -- involved for the plant in each of these defense 'strategies'.  Make sure you put your arguments in appropriate selective terminology.  Also consider how the evolutionary/selective response of insect herbivores to these two types of defenses might differ.   (Here is a CLUE: large, long-lived plants  like trees tend to employ quantitative defenses, while smaller or short-lived plants are more likely to employ qualitatitve toxins.  See if you can explain why this makes sense in light of your consideration of trade-offs.)

3. Insect populations exposed to regular applications of insecticides typically show evolution of genetic resistance quite quickly; 5-10 years of intensive use is about all a new insecticide is good for.  This is a simple (directional) selection scenario; strong toxins impose strong selection if there's any heritable (genetic) variation in tolerance.  Individual insects who are even slightly more tolerant of the toxin will have  higher fitness -- reproductive contribution to subsequent generations -- when the toxin is a major cause of mortality.
   a) If the insecticide is removed from the environment, it is usually the case that insecticide-resistant genotypes in insect population have lower fitness than the normal or 'wild-type'.  Offer a hypothesis explaining this phenomenon.  Predict what would happen, in this case,  if the insecticide were applied only in episodes separated by a number of years.
   b) It's also frequently the case that resistance does NOT evolve when several different types of insecticide (that is, ones that work by different means) are used in combination. (NOTE that resistance is frequently a single-gene trait -- i.e., conferred by a single mutation to a gene related to whatever physiological pathway the insecticide poisons).  Propose a reason for this phenomenon.    
    (A SIDE NOTE:  this is  precisely parallel to what occurs when pathogens are treated with antibiotics or antivirals; the second scenario corresponds to modern treatment of HIV infection with 'cocktails' of multiple anitviral drugs)

4. Leaves of deciduous trees start ‘shutting down’ (senescing) in the fall, recovering materials from their foliage and then shedding it,  in response to a combination of dropping temperatures and shortening day length (the precise 'triggers' vary).  Losing leaves is generally seen as an adaptation to reduce loss of water from the plant during the winter when below-freezing temperatures make it impossible for trees to acquire water from frozen soils or transport it through frozen tissues.  ASSUME that the 'triggers' for leaf senescence are genetically controlled (heritable). 
    a) Hypothesize about selective trade-offs involved in the timing of leaf senescence; what would be the primary selective costs and/or benefits of holding leaves longer? of dropping them sooner?
    b) Day-length is often an important part of the triggering process; why would this be a particularly selectively advantageous 'cue' for the plant if the primary adaptive value of losing leaves is relate to cold temperatures (i.e., why not respond simply to cold temperatures)?  Offer at least one hypothesis suggesting a 'fitness' value for responding to an 'indirect' cue like day-length.
    c) Some species (like beech and sugar maple) have very broad latitudinal ranges, including areas with very different seasonal timing (beech in its southernmost range may see only a month or two of 'winter' with freezing temperatures possible while in its northernmost range, freezes are possible for more like 7 months).  What would you predict about the genetic/heritable triggers for leaf senescence across such a range (what would happen to a beech tree from Georgia transplanted to Vermont)? (I suggest you talk about stabilizing and directional selection dynamics in your answers.)

 5.  Wildebeest in the Serengeti of East Africa have a very restricted calving season.  All females give birth within a 3 week period.  This is a pretty common phenomenon among mammals and birds that breed in dense populations.  It has been hypothesized that this is an 'adaptive' mechanism to reduce loss of calves to predators by "saturating" the predator populations briefly (this is similar to the notion that masting in trees saturates seed predators so that some seeds survive...).  In other words, having calves at the same time as all the other individuals in the herd increases relative reproductive success (fitness), and any heritable tendency to do this would be selected for What kind of observations and data could you collect to test this selective hypothesis (be clear how these data would allow you to assess predictions of the predator saturation hypothesis and differences in fitness within the wildebeest population)?