Freshwater ecology - Biotics

debate on density dependance and population regulation

1. those that emphasized the relative constancy of populations and the
   need to look for stabilizing forces (e.g., competition) to explain why populations are bounded

2. those that emphasized population fluctuations and the importance of
   external forces (e.g., weather) in explaining changes

+/-/0 interactions : competition, predator and prey, amensalism/commensalism, mutualism

• competition: -/-

• predator and prey: +/-

• amensalism: -/0

• commensalism: +/0

• mutualism: +/+

niche

• The total range of environmental variables where an organism can survive,
  grow, and reproduce

• an abiotic frame

fundamental niche

• Describes the range of environmental conditions that a species can
  occupy in the absence of biotic interactions

realized niche

• Describes the range of environmental conditions that a species can
  occupy in the presence of biotic interactions

• coexisting species often have overlapping niches = competition
  

competition

the negative effect which one organism has upon another by consuming, or controlling access to, a resource that is limited in availability

what is a resource

• Substances or factors that result in increased growth as the availability is increased

• nutrients, light, food, mates, spawning grounds, and space

  
  

two types of competition

1. exploitation competition

2. interference competition

exploitation competition

• one organism exploits a resource more efficiently (indirect)

• ex: one fish filters water more efficiently than another

interference competition

• direct interference of one organism with a competitor

• ex: aggressive behavior

interspecific competition

• Niche overlap between species (white spotted charr vs dolly varden)

• the more different organisms are the less competition there is between them (ex: phytoplankton rarely compete with fish)

intraspecific competition

Niche overlap within species (big dolly varden vs small dolly varden)

competitive exclusion principle

• Organisms that overlap completely in use of a limiting resource are
  not able to coexist

• If the overlap can be reduced (i.e., resource partitioning) coexistence can be possible

character displacement

Strong competitive interactions among coexisting organisms can
result in evolution of phenotypic differences in traits that are related
to the exploitation of the limiting resource

2 ways organisms can adapt to tolerate low amounts of a limiting resource

1. size

2. luxury uptake

relation between size and tolerating low amounts of resources

remaining small means you can get by with fewer resources, so in a
limiting situation can out compete a larger individual or species

relation between luxury uptake and tolerating low amounts of resources

storing a limit resource when it is prevalent to use later (e.g.,
many primary producers store phosphorous as a large polyphosphate
molecule)

Lotka-Volterra Model

• predicts potential outcome when two species are in competition for the same resources

alpha is what in the Lotka-Volterra model

• α is the effect of species 2 on species 1’s growth rate (and
  vice versa)

α 12 < 1 means…

the effect of species 2 on species 1 is less than the effect of species 1 on it’s own members (intraspecific more important)

α 12 > 1 means…

the effect of species 2 on species 1 is greater than the effect of species 1 on its own members (interspecific more important)

two species two resources model

• called Resource-Ratio Hypothesis (R* rule)

• A graphical model based on the supply rate of two resources

what does 1 indicate

Supply rate of resources is not sufficient for any of the species

what does 2 indicate

species B can survive but not species A

what does 6 indicate

Species A can survive but not species B

what does 3 indicate

Species B competitively displace Species A because B will make resource x
limiting for A

what does 4 indicate

Two species will coexist because an individual of one species will have a
greater effect on its own relatives than on members of the other species

what does 5 indicate

Species A displaces Species B because A will make resource y limiting for B

periphyton vs phytoplankton example

• Periphyton have dibs on nutrients (biomineralization is higher at sediment)

• Phytoplankton have dibs on light (they can move up in the water column)

  
  

crucian carp example

• the only fish in shallow ponds b/c they can survive periods of low oxygen (intraspecific effects)

• low body condition, low recruitment of young into mating, low glycogen in high density populations

• do better in low density populations

carnivory, herbivory, and parasitism are…

consumption of living organisms

4 principles of predation and herbivory

1. Consumption is a major source of mortality in many freshwater organisms
   and thus is an important structuring force in lakes and ponds

2. Direct lethal effects can cause local extinction

3. Predators can change behaviour of prey including changes in habitat-use,
   activity patterns, and foraging...thus, there can be indirect effects on prey
   growth and reproduction rates.

4. Can be a strong selective force over evolutionary time

predation cycle

• search

• encounter

• attack

• capture

• ingestion

adaptions that impact the predation cycle

• Defence adaptations have evolved in prey species (goal is to interrupt predation at any stage)

• Counter adaptations in predators (goal is to go from search to ingestion quickly)

  
  

lake victoria example of predator adaptations

• invasive nile perch indroduced

• the nile perch adapted big mouths and bellies to be better predators

• decrease in prey fish = change ecosystem

2 basic foraging strategies

1. ambush/ sit and wait

2. searching

ambush/ sit and wait strategy

• Predator remains inactive

• Often within complex microhabitat

• Prey passes by and is detected

• Can involve fast starts and attacks (uses energy)

• Capture success is high

searching strategy

• Move around in search of a prey item

• Uses energy for moving around

• Capture success can be low

choosing between basic foraging strategies

• depends on environment - if there is refuge from predators

• depends on organisms place in food chain - if at top they don’t need refuge to sit and wait

adaptions to help detect prey

• Visual, Mechanical and/or Chemical cues

• Size, shape, colour, contrast against background and movement can affect
  the distance at which prey is sighted

• Being able to react in a manner that optimizes encounter rate

• Environmental effects like turbidity and brownification

selective feeding

• Predator feeding on a prey organism more than is expected from its relative availability in the environment

• because prey is optimal diet choice - aim of a forager should be to consume prey types that achieve maximum net energy gain to maximize lifetime reproductive success

• changes prey community

defence

Adaptations that reduce the risk of being a victim of predation

primary defences

operate before a predator has detected the prey and decreases the probability of an encounter with a potential predator

secondary defences

• come into action after the prey has been encountered by a predator

• Increases the probability of surviving after being detected by a predator

2 types of primary defences

1. avoiding encounters

2. crypsis

avoiding encounters

• 2 types: spatial and temporal refugia

• Many prey organisms are poor at escaping predation so they aim to reduce encounters

• often leave systems with predators or breed in systems without predators

spatial refugia

• requires habitat complexity

• Provides space to be hidden and to escape to

• decreases interactions between predator and prey

3 types of temporal refugia

1. diel migration

2. seasonal migration

3. diapaus

temporal refugia

Reducing activity when prey detected reduces likelihood of being detected

diel migration

• diurnal vertical migration

• cost energy (less energy for growth and reproduction) but also protects from predation and UV damage

• in shallow lakes movement is horizontal instead of vertical

daphnia response to water flea predation

• when predator is not around daphnia are closer to surface where more nutrients and warmth is (increase growth)

• when predator is present daphnia stay closer to sediment and only go to surface at night

diapause

• a transient interruption in growth or development

• ex: produce resting eggs that only hatch when predators are absent or at low activity

• rely on predators periods of low activity

crypsis

• dull, colorless appearance that allows prey to blend into their environment

examples of secondary defences

• Erratic movements

• Hide in burrow/shell/tubes

• Spines

• chemical defense (extremely distasteful or anaesthetic effect)

  
  

prey chemical cues

• disturbance cues

• alarm cues

• predator confusion

• future vigilance

predator chemical cues

• attractant signal

• predator interference

• dietary attractant signal

• kairomones

• dietary alarm cues

constitutive defences

ex: spines on sticklebacks, shells of snails

They are present whether a predator is or isn’t there

inducible defences

defences that are phenotypically induced when predators are present

4 ways inducible defenses are promoted

1. There is a variability in predation pressure (if a predator is always present, a
   constitutive defense would be more advantageous)

2. The prey organisms have reliable cues of detecting the presence of the predator in the habitat

3. The prey benefits by having the induced morphology (it should increase the
   probability of surviving an encounter)

4. The inducible defence structure incurs a cost to the prey organisms

   
   

predator offence

• Predators may have inducible offence adaptations that increase foraging efficiency

• ex: salamanders will develop a wider gape to facilitate swallowing of tadpoles

HSS hypothesis

• argued that herbivore populations are limited by predators and thus cannot expand to densities where they can limit plants

• predators and plants in turn are limited by competition for resources

evidence of HSS hypothesis

• decrease fish = increase large zooplankton = decrease algae biomass = increase water clarity

• abundant fish = less zooplankton = increase algae biomass

size-efficiency hypothesis

• large zooplankton are dominant when fish density is low

• small zooplankton are dominant when fish density is high (fish don’t eat them)

• predicts when algal biomass will be high/low based on the presence of fish

what 2 hypotheses were used to develop the Cascading trophic interaction concept

• HSS hypothesis

• size-efficiency hypothesis

cascading trophic interaction concept

• a way to explain variation in lake productivity not explained by phosphorous levels

• A trophic cascade is an indirect interaction characteristic of linear food chains where a predator species A has an indirect positive effect on a plant species C by reducing the abundance of the herbivore species B

wolves as stream animals example of cascading trophic interactions: without wolves

• elk browse woody species causing decrease in woody plants near streams

• decreases beaver abundance

• decreases riparian ecosystem functions

• loss of food web support

• loss of wetlands

• channel widening and incisions

• loss of hydrologic conductivity between streams and floodplains

wolves as stream animals example of cascading trophic interactions: with wolves

• elk under higher predation risk = adjust foraging and movement patterns

• increased abundance of woody species

• riparian functions recover

• beaver recolonizes the area

• recovery of food web

• channels stabilize

• wetlands recover

• hydrologic conductivity recovers

Bottom-up :Top-down theory

• Combines influence of predators from the top and resource availability from the bottom

• Trophic levels near the base of the food chain will be primarily affected by bottom- up processes, these effects weaken as trophic level increases

• Top-down effects only important for upper trophic levels

• Fish will have no influence when there is high nutrient concentrations, so fish only have an impact on biomass in oligotrophic systems

Fretwell-Oksanen model

• Predicts that systems with low nutrient loads can only support primary producers

• The more primary producers that can be supported, the more trophic levels will be found in the system

• Control of abundance at the levels flips between resource competition and predation

trait-mediated indirect effects

• strong cascading effects often focus on the lethal effects of predators but indirect effects are also important

• ex: sunfish prefer the pelagic zone for foraging when large mouth bass are not present, but when they are present sunfish feed in the littoral zone because there is vegetation and they can eat benthic macroinvertebrates

microbial loop

• Bacteria retrieve carbon from phytoplankton, zooplankton and
  fish wastes and decaying particles (DOC/DOM)

• Bacteria then eaten by ciliates and heterotrophic flagellates,
  which feed zooplankton, rotifers, small fish, (releases nutrients back into ecosystem)

consumer release of nutrients effects

• Higher Organisms excrete DOC AND other nutrients like N and P

• when planktivorous fish become abundant they not only reduce the
  grazing pressure from zooplankton by predation (top-down)

• they also promote phytoplankton growth by increasing nutrient abundances
  (indirect bottom up effect)

criticism of trophic cascades

• strong trophic cascades only seem to be present in species poor systems where keystone species are present ... they also seem to be stronger in aquatic systems

possible issue with trophic cascades

• do trophic levels exist

• predator identity and multiple predators

• omnivory

• ontogenetic niche shifts and size-structured populations

• subsidized consumers and food chain coupling

do trophic levels exist

• food webs are not linear : species feed at different levels, species linked to multiple resources

• species grouped into guilds using isotopes

• species may be in a different level depending on resources and competition

issue of multiple predators

• predators interfere with other predators (eat the same prey)

• prey may increase with multiple predators because of strong competition

• prey may decrease with multiple predators because they have an additive impact

issue of omnivory

• organism feeds at more than one trophic level

• may feed on both animal and plant matter

• may feed on both carnivores and herbivores

• may feed on a different resource than the consumer

• may be cannabalistic

3 reasons omnivory is advantageous

1. Flexibility to feed at a lower trophic level where there are higher
   resource levels (protects against periods of low food availability)

2. Flexibility to feed higher in the trophic spectrum to obtain higher
   quality food and limiting resources

3. Omnivores are more likely to be in higher trophic levels (larger –
   better able to exploit wider range of prey sizes and species)

why is omnivory more common in the tropics

more diversity and abundance of fish

issue of ontogenetic niche shifts

• not all individuals in a population use the same trophic level

• as an organism ages they eat different food

issue with subsidized consumers and food chain coupling

• ponds, lakes, and streams are not closed systems

• open transport of resources across habitat boarders (nutrients, detritus, prey, and consumers)

• additions and deletions

allochthonous sources of energy

• Energy in the form of leaves, plants, pollen, twigs, branches, trunks get blown and washed into streams

• Smaller and softer items provide energy for food web

• Larger and harder items help retain smaller items

  
  

why are leaves not nutritional

• In the autumn, nutrients in leaves are translocated back to stems and trunks
  of trees

• Shed leaves are mostly cellulose and lignin, and toxic phenolic compounds
  (e.g., tannins)

• Insects do not like to graze on shed leaves

how do fungi make leaves nutritional

• Spores latch on to leaves and have enzymes that digest lignin and cellulose

• Leaf becomes a partly digested carbohydrate with protein from the fungi

creation of fine particulate organic matter

• once leaves are conditioned by fungi, shredders tear the leaf apart to get to the mycelium

• then the leaf is recolonized by bacteria and fungi for further conditioning

what happens to fine particulate organic matter after further conditioning

• filter feeders create more

• or, deposit feeders consumer it

final destination of leaf matter

• Eventually all leaf matter is converted to animal tissue and CO2

• The greater the diversity present in the stream, the more efficiently
  leaves are processed (each species has its own specializations)