Lecture 12: Impacts on Food Webs

Question 1

What are three subtle impacts invasions can have on populations and food webs?

Answer 1

1. Character displacement
2. Diet shifts
3. Indirect effects in food webs

Question 2

What is an impact?

Answer 2

any measurable change to the environment caused by the invader. Often these changes include BOTH positive and negative in terms of their direction of effects on community members or environment

Question 3

What are the different levels impact can be measured at?

Answer 3

1. individual organisms (mortality, injury)
2. genetic
3. Population (abundance, age structure)
4. Community (diversity, food web structure)
5. Ecosystem (productivity, nutrient cycling, habitat quality)

Question 4

Impacts are the outcome of what?

Answer 4

• the interaction between an invader and it’s environment

- invaders, abundance, invaders ecological function and composition of the invaded community all play a role

Question 5

What is competitive exclusion?

Answer 5

no competing species can occupy the same niche in a stable environment

Question 6

What is limiting similarity ?

Answer 6

• coexistence can only occur if the species niches are differentiated

Question 7

Explain Character displacement

Answer 7

• an evolutionary effect of competition
  L> competition may exclude similar species from coexisting (limiting similarity) OR it may select for differences that permit coexistence (character displacement
• morphological divergence of sympatric species
  L> two species are less similar where they occur together than where they occur apart
  L> aka similar when apart, different together
• changes reflect shift in resource use
  L> usually foraging

Question 8

Explain the example of character displacement in mud snails

Answer 8

• co occurring snails choose different food particle sizes

- when allopatric (not together) they pick sizes that are similar

Question 9

Explain the example of character displacement with cisco in lake Michigan with the invasion of alewife

Answer 9

• Cisco managed to survive while all other planktivores wereoutcomepted by the alewife -
• did this by feeding on ethic prey, morphological change in gill rankers to change what food they could take on
• thicker and less numbers = bigger resources being used
• gill size and number is a stable trait in a taxon and can be used to define a ton SO when it changes this is a big deal
• *took 20 years

Question 10

Explain diet shifts in native species in the presence of an invader.

Answer 10

• occur when energy flow with a food web is altered as a result of invasion

Question 11

How do we detect diet shifts?

Answer 11

• gut content analysis
  L> traditional
• stable isotope analysis
  L> tells you if it’s carnivores, where its feed from BUT not exactly what
  L> based on the idea that certain isotopes like N is enriched when there is more animal tissue being consumed. This N ratio changes depending on how carnivores the sp is. It’s a measure of its trophic level. The same sp can vary in placement on the food web in different areas. SO N15/N14 tells you where they are on the food chain
  **N15 is enriched in animal tissue relative to the content of it’s prey
  L> Carbon tells you where they are feeding (C13/C12)

Question 12

Explain diet shifts in lake trout in the presence of bass.

Answer 12

• typically feed son minnows inshore bc offshore it’s only phytoplankton
• bass comes in and reduces minnows
• lake trout suffer via comp for them
• lake trout now mostly feed on zooplankton
• *proved with isotopes and gut content. Lower C ratio indicated feeding offshore
• Lower N indicating feeding lower on the food web

Question 13

Describe brook trout diet shift in the presence of white sucker.

Answer 13

• feeds on zooplankton offshore and inverts inshore 50/50
• white sucker feeds on benthic inverts
• white sucker outocmeptes brook trout so now their died is mostly zooplankton

Question 14

Explain the impact of Corbula amurensis in San Fran

Answer 14

• introduced filter feeder
• it’s really prolific and eats a lot of phytoplankton
• lots of clam biomass now and not much feeds on it
• mysis mercedis decreases due to this
• diet shifts were seen in striped bass, starry flounder etc all which prefer to eat mess

Question 15

Explain the impact of zebra mussels in Lake St. Clair.

Answer 15

• Walleye is the dominant fish
• phytoplankton are cleared out by zebra mussels
• change conditions of the water i.e. clarity
• the walleye is use to turbid water, rooted veg was spares before and started growing, fish that like living in those took over so the dom pisc now was small mouth bass
• zebra removed photo, so they compete with daphnia for zoo, SO everything in the chain goes hungry.

Question 16

Effects of Corbicula in the Potomac River?

Answer 16

• introduced invasive fw mussel, increases clarity of water
• veg beds formed
• pos effect on certain kinds of water fowl that do well with veg beds
• bass family does well too

Question 17

Explain energy flow in the black sea after mnemiopsis invasion

Answer 17

-before: 
phyto->zooplankton->anchovy->piscivores 
-After: 
phyto->Zoo+Anchovy->Mnemiopsis 
**comp

Question 18

Explain shifts in the population of Cerithidea after introduction of a mud snail (Ilyanassa)

Answer 18

• introduced mud snail fed on the eggs of the native mud snail

Question 19

Explain the situation with the food web of flathead lake Montana

Answer 19

• introduced kokanee salmon
• introduced opossum shrimp
• shrimps preferred food was zooplankton which was also the preferred prey of kokanee
• shrimp avoided being eaten by kokanee by staying at the bottom of the lake during the day and only coming up at night to feed. Collapse of kokanee fishery
• shrimp also provide food to young lake trout (another introduced sp) , which were in low numbers until the shrimp was introduced
• lake trout eventually fed on kokanee
• kokanee was fed on by bald eagles but once it disappeared, the eagles left the area. and bear population declined in the are auto

Question 20

Explain the effect of the introduction of Peakcock Bass to Gatun Lake

Answer 20

1. The introduction of Cichla may have resulted in lower biomass of phytoplankton in the lake due to it’s multi trophic level impact. Cichla was found to predate on adult Melaniris chagresi, to the point that there was reduction in the adult population by 90% in 1971 and 1972. The young of M. chagresi almost exclusively predate zooplankton, therefore, with Cichla predating M. chagresi, the predation pressure on the zooplankton is lifted and their populations can increase. With zooplankton populations increasing, it can be expected that phytoplankton populations will decrease because zooplankton predate on them.
2. M. chagresi is considered to be a key species in the food web of Gatun Lake because it is a primary prey source for three higher-trophic level species (Gobiomorus dormitor, Tarpon atlanticus and Chlidonias niger). In regions where Cichla was introduced and the M. chagresi populations are lower due to heavy predation by this introduced piscivore, these secondary consumers that depend on M. chagresi are absent and were only found in regions not invaded by Cichla.
   It is expected that there would be an increase in insect populations in regions where Cichla was introduced because it predates on smaller prey fishes, which primarily feed on insects. A dramatic increase in mosquito populations was noted in 1969, coinciding with when the Cichla population was peaking in the region. Alternatively, it could be suggested that variation in rainfall could alter the local mosquito population; however, the change in annual rainfall from 2238mm in 1968 to 2194mm in 1969 was a very slight change. Contrariwise, in 1970 the annual rainfall increased by 1.5x and coincided with mosquito levels to drop, suggesting that another factor aside from rainfall was an important factor in determining mosquito population levels

Question 21

Explain the genetic effects of introduced species

Answer 21

• different species int he same genera do not ineterbreeed bc of reproductive isolation
• but hybridization , interbreeding is not rare
• plants and birds for ex
• for genetic material to shift between species, hybrids must cross with a parent species which rarely happens bc hybrids are often inferior to their parents

Question 22

Explain reproductive interference

Answer 22

• even if no gene mixing occurs, the process of hybridization can be deleterious, through wasted reproductive effort.
  ex: brook trout hybridizes with native bull trout producing sterile hybrids

** interbreeding between native and exotic species creates sterile hybrids

Question 23

Explain Genetic swamping

Answer 23

• interbreeding between native end exotic species creates fertile hybrids
  **no pure native species after three generations
  Ex:
• mallard duck introduced for hunting and the following native ducks: the native american black duck, hawaiian and the native florida mottled duck

Question 24

Give examples of genetic swamping in fish

Answer 24

• cutthroat trout hybridizing with the introduced rainbow trout = extinction
• 65% of all Apache trout populations contain individuals with rainbow trout genes

Question 25

Give examples of genetic swamping in mammals

Answer 25

• Euro grey wolf hybridizing with domestic dogs
• Domestic dogs hybridizing with simian jackal, Ethiopia
• Wildcats in africa and europe hybridize with feral housecats

Question 26

Explain some other points about hybridization

Answer 26

• risk of extinction by hybridization is greater when endemic populations are reduced or fragmented (human activity)
• hybridization sometimes produces invasive forms
  L> ex: Africanized honey bee
  L> American cordgrass interbreeding with European