TRITROPHIC INTERACTIONS

READINGS: FREEMAN, 2005

Chapter 53

TRITROPHIC INTERACTIONS

• Eating (trophic) relationships often link several species in a community through herbivory, predation and/or parasitism.

• When the links go across three eating (trophic) levels, they are called tritrophic interactions.

• Species that play an special role in trophic relations are called “keystone species”.

A TRITROPHIC INTERACTION

• Hover flies sip nectar from flowers and in turn are eaten by a spiders.

• This is a version of: Predators eat herbivores, and herbivores eat plants.

ANOTHER TRITROPHIC INTERACTION (I)

• Daphnia are herbivores on a unicellular algae and the prey of damsel flies.

• A decrease in algae would soon result in a decrease in Daphnia and later a decrease in damsel flies.

TRITROPHIC INTERACTION (II)

• A sudden decrease in Daphnia would result in a decrease in damsel fly larvae and an increase in unicellular algae.

• A sudden increase in damsel fly larvae would result in a(n) _____ of Daphnia and a(n) _____ of unicellular algae.

FOOD CHAIN (I)

• A trophic interaction that links three or more levels is called a food chain.

• Where the trophic levels are dynamically linked, any change in abundance of one population within the chain can result in changes in abundance of the other populations.

FOOD CHAIN (II)

• All food chains begin with producers [green plants, green algae and blue-green algae (cyanobacteria)].

• Herbivores are known as primary consumers.

• Predators are known as secondary or higher level consumers.

PREDICTED DYNAMICS OF TRITROPHIC INTERACTIONS (I)

• Where eating relations exert control on the abundance of populations within a food chain, a change in the abundance of one population can have an influence on the other populations.

• A change in abundance of producers will first change the abundance of primary consumers and later the abundance of secondary consumers.

PREDICTED DYNAMICS OF TRITROPHIC INTERACTIONS (II)

• A change in abundance of secondary consumers will first change the abundance of primary consumers and later the abundance of producers.

• A change in abundance of primary consumers will change the abundance of one or both of the other immediate trophic levels and the timing of change is difficult to predict (depends on life history characteristics, etc.).

WILLOW-HARE-LYNX• Willow is a primary food of

artic hare in the winter when food is scarce.

• Although evidence suggests that lynx control hare cycles, the hare population is subject to both top down and bottom up control.

• The interaction of food and predation has a strong influence on hare abundance.

WOLF-MOOSE INTERACTION REVISITED (I)

• There is no strong evidence that wolves control moose populations on Isle Royal.

• There is good evidence that declining moose populations may result in wolf population increases.

WOLF-MOOSE INTERACTION REVISITED (II)

• Moose populations are known to fluctuate dramatically -- from a low of around 500 to a high of around 2,500 in the last nearly 50 years.

• Prior to the immigration of wolves, the moose population showed similar fluctuations.

• What is a primary cause of these fluctuations?

?

Moose-Balsam Fir Interaction• The island had a high density of

balsam fir (a common Christmas tree), estimated at 46% of overstory prior to moose immigration. Today, it is only 5%.

• Nearby islands, which have no moose, have balsam fir as a large component of their forests.

• Thus, the decline of balsam fir has been attributed to moose browsing.

• Although not optimum forage, it can be up to 59% of food for moose during the winter.

Balsam Fir-Moose-Wolf Interaction on Isle Royale (I)

• Current research is focused on the tritrophic interaction between wolf-moose-balsam fir.

• It attempts to use tree ring data to account for fluctuations in moose and wolf populations.

Balsam Fir-Moose-Wolf Interaction on Isle Royale (II)

• This research is being conducted using a bottom up and top down (trophic cascade) model.

• It also uses data on temperature and precipitation in the form of annual actual evapotranspiration (AET).

Balsam Fir-Moose-Wolf Interaction on Isle Royale (III)

• The bottom up hypothesis predicts that plant growth is determined by temperature and precipitation. This is a version of the primary production scenario.

• The top down hypothesis predicts that changes in one trophic level result in opposite changes in the level below it.

• For example, a decrease in moose abundance should produce increased plant growth if moose herbivory limits plant growth.

Balsam Fir-Moose-Wolf Interaction on Isle Royale (IV)

• Future observations on Isle Royale should provide a good opportunity to test these prediction. It will be sometime before answers are available.

• Given the fragile nature of the wolf population on the island, experimental studies are not being currently conducted.

EXPERIMENTAL STUDIES OF TRITROPHIC INTERACTIONS

• Experimental studies of tritrophic interactions in field settings are difficult, time consuming and expensive to conduct; but some notable ones have been carried out in more recent years.

• Field studies on insectivorous birds-plant eating insects-oak saplings.

• Artificial pond studies with newts-frog tadpoles-algae.

• Sea star-mussel,chiton,limpet,barnicle - algae manipulations in intertidal tide pools.

Effects of Predation by Birds on Herbivory by Forest Insects (I)

• An experimental study has focused on insectivorous birds-leaf eating insects-white oak saplings.

• A number of bird species and insectivorous insects were present during the study.

• One species of oak.

Effects of Predation by Birds on Herbivory by Forest Insects (I)

• Hypothesis: Birds reduce insect populations and their damage to leaves of oak saplings.

• Method: 3 sets of 30 white oak saplings were treated by:

* sprayed with insecticide. * doing nothing (control) * covering with white nylon mesh to

keep birds out, but not insects. Insect density and missing leaf area was recorded

for all three set.

Effects of Predation by Birds on Herbivory by Forest Insects (IIa)

• Results: Insect density in N per 10,000 cm2

0.25

6.45

10.75

0

2

4

6

8

10

12

Sprayed Control Birds Excluded

INSECT DENSITY

Data from ECOLOGY 75: 2007-2014 (1994).

Effects of Predation by Birds on Herbivory by Forest Insects (IIb)

• Results: Missing leaf area (%)

Data from ECOLOGY 75: 2007-2014 (1994).

0

5

10

15

20

25

30

35

40

Sprayed Control Birds Excluded

MISSING LEAF AREA

Effects of Predation by Birds on Herbivory by Forest Insects (III)

• Conclusion: White oaks are important trees in a number of deciduous forest communities. The growth rate of oak tree saplings is a function of leaf area. Thus, insectivorous birds increase growth rates of deciduous forest trees.

Note: One of the investigators teaches in Bio @ UIC..

Influence of Predation on Competition in an Amphibian Community (1)

• The red-spotted salamander is known to prey on a number of species of frog and toad tadpoles.

• Tadpoles, the larval stages of frog and toad species, have similar diets and thus show strong interspecific competition for food.

Influence of Predation on Competition in an Amphibian

Community (2)• Hypothesis: Increasing predator abundance

will benefit some competitors and harm others.

• Method: Six competing populations of tadpoles were subjected to different intensities of predation by a species of salamander. % survival was recorded at the end of the experiment.

Effect of Predation on Competition (3a)

•Results: % survival for 3 species

DENSITY OF SALAMANDERS

TADPOLE SPECIES 0 2 4 8

Spadefoot Toad 93% 58% 43% 18%

Spring Peeper 4% 28% 27% 30%

S. Leopard Frog 63% 58% 24% 12%

Declining survival as predator density increases

Increasing survival as predator density increases

Effect of Predation on Competition (3b)

•Results: % survival for the other 3 species

DENSITY OF SALAMANDERS

TADPOLE SPECIES 0 2 4 8

Southern Toad 38% 24% 5% 8%

Barking Tree Frog 2% 21% 12% 3%

Grey Tree Frog 48% 30% 12% 17%

Declining survival as predator density increases

Little consistent effect of predator density

Effect of Predation on Competition (3c)

• Summary of Results: In the absence of salamander predation, spadefoot toads were the most abundant species in the amphibian community. But as the number of salamanders increased, the most abundant competitor was the spring peeper. All but one of the other species decreased as predator density increased.

Effect of Predation on Competition (4)

• Conclusion: A predator population can change the relative abundance of prey populations by reducing the numbers of superior competitors. If a predator species is removed from a community, the entire community structure can be changed significantly.

AN EXPERIMENT THAT DEFINED KEYSTONE PREDTOR

• An experiment with an intertidal community defined the concept of keystone predator.

• It involved sea star-mussel,chiton,limpet,barnicle - algae interactions.

• See page 1230 and Figure 53.17 in Freeman (2005) for details.

OTHER STUDIES OF KEYSTONE PREDATORS

• Sea otters promote growth of kelp beds by preying on sea urchins.

• Lobsters keep sea urchins from overgrazing kelp beds.

Of Acorns, Mice, Moths, Deer, Ticks, Spirochetes -and Lyme Disease?(1)

• Gypsy moth outbreaks and Lyme disease pose major problems for people who live in deciduous forests of the Northeast and Great Lakes. They are part of a web of eating relations centered around acorns.

• Acorn production determines the abundance of white-footed mice and deer.

• Abundant mice suppress gypsy moth outbreaks, and mice and deer support tick populations that harbor the Lyme disease causing spirochete.

Of Acorns, Mice, Moths, Deer, Ticks, Spirochetes -and Lyme Disease? (2)

• Lyme disease (LD) is an infection caused by bacterium that is carried by deer ticks.

• An infected tick can transmit the spirochete to the humans and animals it bites.

• Untreated, the spirochete parasitizes tissues, and can cause a number of mild to severe symptoms.

Of Acorns, Mice, Moths, Deer, Ticks, Spirochetes -and Lyme Disease? (3)

• The gypsy moth was introduced into Massachutes around 100 years ago and has become the most destructive pest in deciduous forests.

• It has only one generation per year and hatches from eggs about the time that leaves are emerging.

• The larval (caterpillar) stage feeds on leaves and during a major outbreak can almost defoliate an entire forest. Adult moths do not feed.

Of Acorns, Mice, Moths, Deer, Ticks, Spirochetes -and Lyme Disease? (4)

• Red oaks produce large numbers of acorns every 2 to 5 years (masting).

• White-footed deer mice eat acorns and their populations boom after masting.

• These mice are important predators on gypsy moth pupa; so as their population increases, more moth pupae are eaten.

Of Acorns, Mice, Moths, Deer, Ticks, Spirochetes -and Lyme Disease? (5)

• White-tail deer seek acorns to eat, carrying adult ticks that mate.

• Adult deer ticks drop off the deer and overwinter in the leaf litter on the ground.

• The following spring, adult females lay eggs that hatch into larval ticks.

Of Acorns, Mice, Moths, Deer, Ticks, Spirochetes -and Lyme Disease? (6)

• Deer tick larvae infest mice which carry the Lyme disease spirochete.

• The spirochete is transmitted to the larval tick during a blood meal.

Of Acorns, Mice, Moths, Deer, Ticks, Spirochetes -and Lyme Disease? (7)

• Tick larvae molt into nymphs that overwinter on the forest floor.

• In the spring infected nymphs seek hosts such as deer and humans.

Of Acorns, Mice, Moths, Deer, Ticks, Spirochetes -and Lyme Disease? (8)

RED OAK HUMAN

GYPSY MOTH

LEAF ACORN DEER TICK

WHITE-FOOTED DEER MOUSE

LYME DISEASE SPIROCHETE

WHITE-TAIL DEER

Science 2-13-98, pp 1023-1026

Risk of Lyme Disease

TRITROPHIC INTERACTIONS

READINGS:

FREEMAN

Chapter 53