Unit 4 Ecology

• Zero growth isoclines: when the population sizes are not changing
• Zero Growth Isoclines would have dN/dT = 0
• The Mathematical modes are based on assumptions, that help secribe the world

  • Compare modeled predictions to the natural world and look for difference
  • Outcomes give indications of the importance of intra- vs interspecific competition
  • Trains out intuition of how competition works
  • Highlight basics that can be compared to more complicated scenarios (or real world)
• Each speices has a unique isoline which its population size doesn't change

  • An infinite number of isoclines are possible,
  • plotting isoclines on same graph helps us see outcomes of competition
  • There are only 4 quantitatively different outcomes to competition
• Competitive Exclusion Principle:

  • Outcomes of competition reflect both intraspecific and interspecific interactions
  • Two species that use a limiting resource in the same way cannot coexist indefinitely
  • Therefore, competing species are more likely to coexist when their resources differ
• Competitive exclusion results in character displacement and resource partitioning

  • Niche differentiation when competitive exclusion results in differences in resource use
  • Over time competition can result in speciation
• What does the PREY isocline mean, and what must be true about the dN/dt for the prey population?

  At a particular density of predators, the density of prey is not changing (dNprey/dt = 0), vice versa for predator isocline
• dNprey/dt = 0 at the peaks of the prey isocline; also applies for dNpred/dt
• Lotka-Voltera Predator-Prey Model Assumptions

  • No crowding
  • Equal chance encounter between predators and prey
  • Prey is the only food for the predator
  • Predation is the only major cause of prey death
  • No handling time
  • No immigration/emigration
• Food Chains and Food webs show the movement of energy and trophic efficiencies
• Food webs are complex and represent competiton between different species
• Keystone vs. Dominant: keystone species are species that are important to the ecosystem but are not the most abundant species
• Dominant species: have large effects on identity and diversity of other species in community because of abundance/large biomass
• Species Composition: identity of species present in a community

  • Documenting species composition allows us to detect changes over time
  • We can identify important members of the community
• Density-mediated indirect interactions (DMII): change in density due to positive or negative interactions. Ex. Trophic cascade: rate of consumption by predator affects lower trophic levels (top-down effect)
• Bottom Up Trophic cascade: changes in the population of primary producers (plants) trigger a chain reaction of changes in the populations of all other trophic levels (herbivores, carnivores, etc.) within an ecosystem
• Bottom up: positive effect from bottom up (nutrients -> plants -> herbivores -> carnivores)
• Top-down: Carnivores indirectly benefit plants as they eat herbivores
• Community: a group of interacting species in the same place at the same time
• A regional species pool is formed by evolutionary processes, physiological constraints, dispersal ability, and habitat selection
• A regional species pool creates species interactions and ultimately a local community structure (regulated by environmental variation and disturbance and stress)
• An example of primary succession is a volcanic eruption - after a big disturbance, the soil is disturbed and plants can't necessarily grow, low-frequency event, high-intensity disturbance
• Secondary disturbance - some ecological communities are adapted to certain kinds of disturbance, but only with a particular range of disturbance intensity and frequency; when plants and animal recolonize an area after a disturbance; medium-frequency, medium-intensity disturbance
• An example of Low-intensity, low frequency is a tree falling
• An example of high frequency, high intensity is extreme weather conditions
• r selected: weedy/pioneer species that arrive straight after a disturbance

  • exponential growth
  • density-independent
  • type 3 survivorship
  • unstable environment
• K selected: species that arrive later than r-selected species after a disturbance

  • logistic growth
  • density-dependent
  • type 1 or type 2 survivorship
  • stable environment
• How succession works

  1. Pioneer arvie after.a disturbance (in the colonizing stage)
  2. Other species arrive, possibly see several stages of community development
  3. Theoretically reach a "climax community" under a stable environment
  4. Disturbance can "reset" succession to early stage of community composition
• Succession occurs via 3 different mechanisms:

  • Tolerance
  • Inhibition
  • Facilitation
• Inhibition: Some species arrive earlier and make it harder for species to invade earlier. Succession proceeds as short-lived species are replaced by longer-lived ones
• Tolerance: No critical interactions; early successional species have no impact on the establishment of later-successional species
• Intermediate Disturbance Hypothesis: r and k selected species are found during intermediate disturbance. Sousa's findings supported this hypothesis as he measured the difference in algae diversity between stable and unstable rocks

  Both unstable and stable rocks saw an increase in diversity during intermediate disturbance
• why is Ulva preferentially removed?

  • Extreme stress: low survival during extreme low tides
  • Selective Herbivory: shore crabs prefer to consume Vlva over Gigartina in lab experiments, crabs accelerate succession
• Wht processes dominate the successional trajectory?

  Ulva is an early-succession species that inhibits Gigartina ; ulva is more susceptible to environmental stress and consumers, and once ulva is removed, the late-successional Gigartina dominates -> which matches the inhibition mechanism
• Resilience: the ability of a community to return back to its original state after a disturbance; returns to the same state composition after disturbance
• Resistance: The ability of a community to withstand a disturbance; little change to the community when disturbed
• Return time: the time required to equilibrium after a certain distance
• If a community crosses a critical threshold, the community becomes ultimately different (alternate stable state)
• What is biogeography and what determines the patterns we see?

  the study of when and where organisms are found

  • climate
  • geographic area
  • isolation
  • evolutionary history
• Islands are an area where a lot of individuals can live and is surrounded by a habitat where those individuals can't live

  • an area of suitable habitat surrounded by inhospitable "matrix"
  • these include islands, alpine zones, ponds, and isolated forests