FORESTS, FALL 2019
Question set THREE
DUE 


TO REVIEW, ANSWER TWO OF THESE THREE QUESTIONS ABOUT NICHE RELATIONSHIPS AND NATURAL SELECTION..

1.  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) (compared to individuals not so synchronized).  If this is so, selection would favor any increase in heritable tendency tiwards synchronization. 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)?  (Make sure you are clear what the  hypothesis is...)

2. Increased movement of people and materials around the world has, in recent decades, led to the introduction and establishment of many non-indigenous species in our area (for example, about 1/3 of the approximately 2100 plant species growing 'wild' in Vermont are the result of human introduction).  Introduced species include plants, animals, fungi, and microorganisms (including some that are pests or pathogens).  Many conservationists view such introduced organisms as potential environmental threats, but views on this issue differ.  
     A. Using what you have learned about species interactions (competition and niche theory), suggest two or three ways in which introduced PLANTS might affect regional natural communities and their diversity.  Under what what circumstances would you regard introduction of a non-indigenous species as an environmental 'threat' ? Explain your reasoning (you may need to introduce some subjective valuations here; that's okay).  (You can think in terms of any taxonomic group or guild of organism -- plants, animals, parasites, predators, etc. -- for purposes of argument/illustration; if your arguments might apply more to some groups than others, say so...)
    B. All of the species that live in our area have dispersed here from further south over the last ~14,000 years; before that this area was ice-covered.  Consequently,  ecological communities have gone through long series of transformations as species expanded ranges following the retreat of the ice (and, later, may well have 'contracted' their range northward).  How are these changes similar to or different from the  consequences of introductions of species from other parts of the world by human agency?

3. 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 (above) 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 values between the graphs; 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.

cattail


II. ANSWER THESE THREE QUESTIONS ABOUT ECOSYSTEM PROCESSES

4.  Ecosystems come in all scales.  A compost pile is a decomposer-driven ecosystem, where the energy input is 'extrinsic'; energy comes into the ecosystem in organic matter from 'outside', and becomes available to compost-pile organisms through decomposition (break-down) of that organic matter by decomposer organisms. The compost pile has internal trophic dynamics, food webs, nutrient cycles, etc.  A gardener might want their compost pile-ecosystem to break down organic wastes (thus liberating nutrients tied up in organic material to return to the garden) as quickly as possible; thus, they would want the pile to support large populations of decomposers with high rates of metabolism.  There are two main groups of decomposers -- fungi and bacteria.  Bacterial decay tends to be faster, while a fungus-dominated compost pile works more slowly.  In practice, people have long noticed that compost piles too full of some kinds of material become fungus-dominated and very slow to break down fully; these types of materials include, for example, wood-chips and dry tree leaves of some types.  Fast, bacterial decay can be sustained when there is a high proportion of green plant material (vegetable scraps, grass clippings, etc.) or animal wastes.  Think about this is in terms of ecosystem dynamics (READ THE HINTS below) and
a) come up with a hypothesis for what might drive a 'switch' between fungal and bacterial dominance in the compost-pile ecosystem.
b) Sometimes a compost pile can get TOO active, leading to breakdown of materials so fast that nutrients are lost before the compost is added back to the garden (or even getting so hot it catches fire!).  Given your hypothesis offer a suggestion for how you might cool/slow down a compost pile (perhaps by limiting bacterial dominance) -- and why it should work.  
    HINTS: The energy source for all decomposers is the break-down of chemical bonds between carbon atoms in organic molecules (especially carbohydrates -- which are 'pure' caron/hydrogen/oxygen).  But, of course, other nutrients/minerals are required to build organisms and support their function.  Wood and dry leaves have a very high carbon concentration (they're made up almost entirely of combinations of carbohydrates).  Green plant materials and animal wastes have much higher concentrations of PROTEINS (remember that proteins are made of nitrogen-containing amino acids).  CONSIDER that there is a parallel here with the Lake Washington story.
    (A small additional question: small, forest streams are often primarily 'decomposer-dominated' systems as well, with very little primary production IN the stream.  Yet these streams have complex and 'lively' food chains.  What is the most likely source of the energy (organic matter) input that fuels such systems?)

 5. Many New England forests have been harvested at intervals of 70-80 years for over 200 years. The following graph shows general living biomass trends for such a forest over this time period, with four logging episodes.  You can think of this curve as showing the accumulation of NPP -- the excess of GPP over respiration and decay, accumulating as biomass (or as carbon). The sharply descending parts of the curve show removal of biomass (wood) in logging. Describe any other patterns or trends you see over the several cycles of logging and regrowth. Offer a hypothesis explaining the dominant patterns in terms of ecosystem processes; use terms and concepts from ecosystem ecology (e.g., you might need to refer to gross and/or net production, respiration, limiting resources,...). There's an appearance of unsustainability here; offer two possibilities (derived from your hypothesis) for ‘improving’ the situation; how might you change things to keep the biomass available for harvest from becoming less each time?

forest_growth
















6. Human burning of fossil fuels injects large amounts of carbon dioxide into the atmosphere.  CO2 is, of course, the source of carbon for photosynthesis and so an essential resource for primary production (by photosynthetic autotrophs).  It has been suggested that added CO2 should, therefore, act as a fertilizer, increasing plant growth and NPP.  If this were the case, there is the potential that ecosystems would become 'carbon sinks', removing (some of) the excess CO2 from the atmosphere and sequestering it in added biomass, thus reducing the rate at which this most important 'greenhouse gas' builds up in the atmosphere.  This would be a desirable thing. However, as we have seen, the regulation of ecosystem productivity is complex, and this outcome depends on several other things. Describe at least one assumption --  in terms of ecosystem process and properties -- of this hypothesis; that is, what would have to be true before this  CO2 fertilization effect (increased NPP resulting in increased sequestration of carbon) could take place?  Imagine you were in charge of things; think of some practices you could implement to INCREASE the likelihood that vegetation would take up more  CO2 as it became available through increased rates of NPP.