ECOLOGY FINAL EXAM STUDY GUIDE This study guide covers the major topics covered in Lecture following Midterm 2 Lectures 23 32 Good luck on the Final everyone FOOD WEB ECOLOGY Types of food webs most links of links Connectance web Links derived from observation all links are weighted equally Energy flow web Connected using estimates of energy transfer medium number Functional web Constructed using experimentation only the links that are important to community structure are highlighted least links Direct Interactions Many food webs consist of a few species that have strong direct interactions e g consumption by Experimental Example 1 Herbivorous Snails and the Predation Hypothesis The Predation Hypothesis is analogous to the IDH The number of seaweed species has a unimodal hump shaped relationship with periwinkle herbivorous snail density In other words the most diversity occurs with moderate grazing pressure At low periwinkle density and thus low predation pressure only fast growing species competitive dominants exist Low diversity At high periwinkle density only slow growing predator resistant tolerant species exist Low diversity At moderate periwinkle density all species can persist High diversity Types of indirect interactions Indirect Interactions Interaction modification Species C modifies the interaction between Species A and B Interspecific competition Species B competes with A and C removal of A results in domination of C and removal of C results in domination of A Apparent competition Species A and C are competing for resource B depletion of resource B by Species C affects Species A via resource depletion Indirect Mutualism Interference Species B and C eat prey A and D and competition between B and C indirectly benefits prey A and D Ecologists neglected indirect effects for a while because They require more species They take longer to develop They require more tedious experimental designs They are more difficult to detect Experimental Example Evidence for and against bird predation controlling limpet abundance in a series of three experiments Experiment A Light limpet L digitalis L strigatella and dark limpet L pelta Counted limpet abundance location in plot and the different types of habitat of mussels barnacles and algae in caged and uncaged plots Results showed that L digitalis survives better without birds caged plots L strigatella does better with birds uncaged plots and there was no difference between plots for L pelta Why are these trends being seen Experiment B Evidence for crypsis When the light limpet L digitalis was on barnacles it reduced the effect of predation and when the dark limpet L pelta was on mussels it reduced the effect of predation Both the light and dark snail densities were higher in areas that had a higher percent cover of their refuge barnacles or mussels respectively meaning that these limpets sought out these areas Experiment C Compared plots where barnacles were experimentally removed to control plots at the start of the experiment and after 5 months Percent cover of mussels dramatically increased after five months This revealed that competition with barnacles was suppressing mussel populations Barnacle removal also increased algal percent cover due to release of space and indirectly increased limpet abundance because limpets eat algae Summary The dark barnacle is most susceptible to predation However its abundance remained constant This is because mussel abundance increased after a release from interspecific competition as a result of birds also preying on barnacles The light barnacle s abundance decreased in the presence of birds due to predation The third barnacle Bottom up vs Top down Control More Indirect Interactions Bottom Up Control Driven by nutrient input Top Down Control Driven by dynamics at the highest trophic level Community structure Outward appearance of a community Top down interactions and trophic cascades in lake systems Introducing primary carnivores increases algal abundance and biomass Results can differ if we re looking at biomass vs the rate at which algal biomass is produced o For algal biomass we see trophic cascade story System piscivore planktivore herbivore phytoplankton As you increase piscivore biomass planktivore down herbivore up and phytoplankton down o For production unimodal relationship between piscivore biomass and production at each lower trophic level More complications Lag effects o Predatory ontogeny Small piscivorous fish eat algae and indirectly affect the abundance of planktivorous fish sharp blip o Predatory inertia the ones that are left are still eating If there is a fishkill and all the piscivorous fish die off feeding rates of remaining fish increase However eventually the effects of smaller population size will manifest and result in lagged effects but less pronounced then effects of predatory ontogeny Takes awhile for the prey population to respond Experimental Example Salt Marshes Treatments Cage and no cage With cages salt marsh grasses were positively effected Experimental Example Wolves and elk from textbook Experimental Example Trophic cascades in kelp forests Urchins heavily graze kelp forests leaving only crustose coralline algae urchin barrens Otters are the top predator in this system Islands with otters Only had small urchins 40mm tests max and a very low biomass Islands without otters Have a larger size range 80 mm tests and biomass was much higher Otters are preferentially consuming urchins with larger tests But over time otter abundances is declining at some islands the decline is patchy some islands maintain consistent otter abundance Experimental Example More otters Estes et al 1998 Otter abundance is dependent on the presence of orcas This creates a four level trophic cascade Presence of orcas decreases otter biomass increases urchin biomass increases urchin grazing intensity how much kelp is consumed per unit time decreases kelp density Experimental Example Prey switching in orcas Orcas were initially preying on whales then harbor seals then fur seals then sea lions then otters Biomass of each declined significantly before prey switching Experimental Example Kelp forests in LA Interaction food web More complicated multiple species at each trophic level Food web complexity dampens the effect of trophic cascade Species diversity and functional redundancy decreases the effect of species loss Food web complexity tends to increase with decreasing latitudes therefore the