Experiments 3

SimMantids

SimMantids

• did not want excessive oscillations

• short handling time and low search efficiency

• no outbreaks, no cycling

Yellowstone - community dynamics

• Attention and scrutiny of policies has led to extensive research about community dynamics and how management decisions affect them

• Elk

• Aspen

• Wolves

• Lake Trout

• Beaver

• Lodgepole Pine

• 1988 fires

  • ⅓ of the park burned

  • Large fires had swept through the park before

• Fire disturbance regime

  • Frequent small fired and infrequent large fires

Self-thinning of lodgepole pine

• A decline in population density resulting from intraspecific competition

• As the density and biomass increases, competition increases

• Many young trees sprouted, but long term this density cannot be supported

Wolves in yellowstone

• Extirpated the wolves from the national parks and the lower 48 states

• Protection of elk and big game populations

• Elk population blew up, uhoh

• Tried to cull the elk but this was unpopular with people

• Wolf recovery plan

• People don't like this plan either

Aspen’s decline

• Quaking aspen is the only significant hardwood tree present in Yellowstone

• Fire suppression hypothesis

  • Frequent fires may produce ideal conditions for aspen regeneration. Fires create open, high-light areas. Combined with high moisture, aspen regeneration increases. 

• Elk herbivory hypothesis

  • Elk are eating the trees

• Elk browsing has a dramatic effect on the structure of aspen groves

Urchin and kelp

• Kelp form huge underwater forest

• Urchins feed on the kelp, reducing the kelp

• When adding a sea otter, urchins decrease and kelp increases

• Adding an orca decreases sea otter and kelp and increases urchins

Wolves were reintroduced into yellowstone in 1996

• Aspen populations increases

• Wolves did not effect elk numbers but did alter where elk foraged

Ecology of fear

• Wanted to see if they could alter foraging behavior by experimentally manipulating predation risk

• Differing amounts of cover

• Aviary using seed-eating gerbil and owls

• Were allowed to feed with or without owls present

• Ate more when protected by brush

Beavers in yellowstone

• Allogenic engineers

• Feeding on woody riparian species

• Beaver has disappeared from the landscape

• Hydrology has changed

• Water table dropped

• Hypothesized that the loss of heaver had caused water tables to drop to the point where willow could not recover, even if wolves caused a decline in elk browsing

  • Enclosures protected willow from elk browsing, 

  • located in areas where the water table has been raised using artificial dams

    • Simulated beavers

  • Excluding elk promoted willow growth but this was enhanced if dam were used to raise the water table

  • Restoration of willow communities would be facilitated by returning beaver to yellowstone

Bottom Up- Best evidence comes from the tree-link food chains of small, headwater streams

• Primary consumers do not feed directly on living plant tissue

• Feed on leaf litter and detritus, base of the food chain

• If community structure was under bottom-up control, reducing the supply of leaf litter would reduce the density of primary consumers and their predators 

  • Strung a net over the steam and caught the leaf litter

• The abundances of both predators and primary consumers increase as leaf litter increases, which suggests bottom-up forces drive community structure

Hurricane

• When resilience is medium, hurricanes do not cause any species to be lost from the community, but the relative abundance of the different species does change

• When resilience is low, hurricanes tend to change both species composition and relative abundances in the simulated dune

Lakes subjected to nutrient pollution - alternative state

• As nutrient addition increases, lakes pass a threshold nutrient level and switch from a clear-water state to a cloudy water state

• Abrupt switch

• Post switch, reducing nutrient input will not necessarily return the lake to a clear water state

• Drastic measures required to get it out of the alternate stable state

• Yellowstone Hypothesis - diversity leads to stability

• Grassland communities with more species would be better able to resist droughts

• Measured resistance using a similar approach to that used in sand dune simulations

• Measured relative abundance in a doubt year and a normal year

• More diverse communities were more resistant

Mosquitos in florida

• Asian tiger mosquito

• Dengue fever, West Nile

• Warm temp, high humidity, rainfall

• Experiment: how temperature and humidity affect egg survival

  • Placed in tubes with different temp and humidity

  • Relative humidity did not affect at 22 degrees

  • 24: higher RH, better

  • 26: higher RH, even better

  • Suggest that the fundamental niche of AT includes all combinations

  • Both species have similar fundamental niches

Gause Competitive Exclusion

• Two species with the same niche could not coexist

• One would outcompete the other and drive it to extinction

• One paramecium drove the other too extinction

• Both had the same niche, one won

• Competitive exclusion

Park Beetles

• Two species of beetles

• Grown on their own, each species was chilling

• Grown together, one drove the other to extinction

• Sometimes it switched due to bacteria, temperature, infanticide

Plankton paradox

plankton are not affected by competitive exclusion

Brown desert rodents

• Used fences to exclude rodents from patches of desert and insecticides to exclude ants from other patches

• Left some patches untouched,, excludes ants and mice from some patches

• When excluding mice, ant colonies increased a lot

• When excluding ants, rodents increases some

• Both excluded, 5x as many seeds, 2x dense

• Showed that species don’t have to be closely related to compete

Stephen Hubble’s “neutral” theory of biodiversity

• Species distributions change randomly over time

• Makes a useful null hypothesis

  • States that there is no effect of the treatment

Mosquito common garden experiment

• At the larval stage, the AT wins in all three evenings. The average larval mass of the YF is severely reduced when it competes with the AT

• Globally stable

• There are competitive advantages at different life stages between the two species

  • Egg mortality rate

    • Eggs of AT are more susceptible to desiccation

  • The relative strength of competition between two mosquito species can depend on whether a third mosquito species is present

Ecological impacts of parasitism

• Parasites rob hosts of resources, so survival or reproduction can be reduced

• Great Horned Owl

  • Parasite load

    • Measure of the severity of infection in a host organism

  • Black flies, inject a protozoan endoparasite

  • When hares are scarce the death rates of fledgling owls increase

Carl Huffaker mites - metapop

• Two mite species, one was a predator of the other

• When all oranges were easily accessible, the predator mites ate all the prey mites, one or both populations went extinct

• When isolating the oranges, he created refuges for prey and the population started cycling 

Topminnows - Parasites

• Parasitic worm

• Infected minnows develop black spots, reduces the expected lifespan and decreases their fitness

• Parasite laid can be determined by counting black spots

• Immune system defends against infection, sometimes infiltrated

  • Attack alleles and defense alleles

• What happened when mutations were allowed in the womb

  • Complete extinction

• What happened when mutations were allow din both host and parasite

  • Most fish died, but some escaped infection and rebounded, with the worm population eventually dying out

• The most abundant genotype in a pool had the most parasites

• Sexually reproducing fish have rarer genotypes