BIOL274 food webs

Predation

The consumption of one living organism by another

What are four different types of predations?

‘True’ predators, Grazers, Parasites, and Parasitoids

True predators

Kill prey straight after attacking it

Grazers

Remove only part of an individual, with rarely lethal effect. They attack a large number of individuals throughout their lifetime

Parasitism

Effect is rarely lethal or takes a long time to kill as only part of an individual is removed. Usually only one individual is attacked

Parasitoids

Intimately associated with one host, do not cause immediate death, but eventually death is inevitable

Ambush (‘sit and wait’) predators

Select profitable locations, waiting for prey to come along e.g. spiders

What determines encounter rate for ambush predators?

Location

Active predators

Move to search for prey, but still wait for certain conditions e.g. Wrybill bird waiting for correct tide to forage in mud

What determines encounter rate for active predators?

Predator movement/behaviour

What are the two different types of active predators?

Stalking and pursuit predators

What defines stalking predators? (with example)

Stalking predators such as cats spend more time and energy encountering prey

What defines pursuit predators? (with example)

Pursuit predators like lions and wolves spend more time and energy capturing and handling prey

The grazing mayfly moves over rocks to find algae. What kind of predator is it?

Active predator

Between true predators and grazers, which has a greater impact on their prey’s population dynamics?

True predators, since they kill their prey immediately

Why aren’t predators always fully aggregated around their prey?

1) To avoid competition with other predators; and

2) To avoid forming too large an aggregation to be at risk of predation themselves

Case study of predation risk influencing foraging activity

Tit birds restricting their activity when bank vole populations are low due to risk from Eurasian Pygmy Owls

Assumption guiding optimal foraging theory

Predators seek to maximise their net energy intake and thereby maximise their evolutionary fitness

Handling time

The time it takes to capture, take down and subdue prey items without consuming it

Based on the optimal foraging model, when should a predator specialise on one prey only?

If the average search time for that prey is relatively low

Should predators with short handling times be generalists or specialists?

Generalists

What is an issue with the optimal foraging model?

Animals are not ‘all-knowing’

Mayfly drift case study

Mayflies tend to be smaller when their predators (trout and stonefly) are present compared to when they are absent

Allometric scaling

Prey with larger body size have larger predators

Bergmann’s rule

For endotherms, body size for a species tends to increase with decreasing mean annual temperature, and vice versa

Example of Bergmann’s rule

Mean body mass of red-billed gulls has decreased from 1958-2004 in conjunction with rising mean ambient temperature

Caribou case study

Changing environmental cues bring earlier onset of plant growing season, creating a mismatch with caribou breeding season

Case study of the consequences of absence of coevolution

‘Tibbles the cat’ - cats causing extinction of Stephens Island Wren in 1894

Prey isocline

Determines the number of predators where there is no change in the prey population, setting dN/dt = 0

Case study of predator-prey cycles

• Lynx, snowshoe hares, and woody browse

• Rabbits and mustelids in the Wairarapa

T/F: Tendency for lagging cycles as described by Lotka-Volterra is quite rare in nature

True

Which of these predator-prey curves is most common in nature and why?

Type 2 - because predators are constrained by their unchangeable handling time

How does dispersal stabilise predator-prey population dynamics?

Leads to metapopulations, meaning that predators have less pervasive influences

Three factors leading to variable predatory-prey population dynamics

• Metapopulations

• Predators targeting demographically unimportant individuals

• Co-evolution

HSS theory

Takes a top-down view, stating that predators reducing herbivore populations allows plants to dominate and produce green habitats. Plants more dominant in odd-numbered food chains.

Bottom-up view

Abundance of energy at lower trophic levels determines those that can live above

Example of bottom-up trophic cascade

Lake Washington daphnia control

Example of top-down trophic cascade

Wolves in Yellowstone

What is a limitation of the HSS theory?

In many places plants have developed anti-herbivory adaptations and compensations for damage e.g. kawakawa being estringent

What are some plant compensations for damage?

Reduced death rate of plant parts, self-thinning, changed allocation of photosynthate

What chemical defences does rimu exhibit?

24% of their weight is tannins and >12% of their weight is terpenes, making them very estringent

What is an example of a plant physical defence?

Matagouri and harakeke being spiky

What is the tradeoff of chemical defence in plants?

More energy invested in chemical defence = slower growth rate

Deer impact on vegetation in NZ

Little diversity in lower canopy with lots of shrub removal, and inedible crown ferns now make up a high proportion of forest composition

Intraguild predation

One species predates upon (kills and eats) another species which competes for similar resources

Why are trophic cascades likely greater in aquatic than terrestrial systems?

Because phyto/zooplankton don’t have defence mechanisms the way terrestrial plants do

Keystone species

Species that function in a unique and significant manner, with an outsized ecological impact compared to their numeric abundance e.g. beavers and cassowaries

Why are cassowaries regarded as a keystone species?

Because many plant species rely on them for seed dispersal and germination. Cassowaries are the only species large enough to swallow some species’ seeds, and others rely on the cassowary’s digestive process to germinate.

Evenness

The abundances of different species in a system in relation to one another

Why was the intermediate disturbance hypothesis observed with Littorina snail predation in closed tide pools but not on exposed platforms?

On exposed platforms species richness peaks at lower snail density because the harsher environment is subject to greater environmental stressors, thereby alleviating competition

If a preferred prey is competitively dominant, when does prey species richness peak? (think Littorina snails in tide pool)

At intermediate predation densities

What effect does predation intensity have on prey richness if there is no competitive dominance among prey?

Increasing predation intensity reduces prey richness

Predator-mediated coexistence

A keystone predator prevents its preferred prey from outcompeting every other species and achieving complete system domination

Example of predator-mediated coexistence

Pisaster starfish preferentially feeding on competitively dominant Mytilus mussel

Main difference between community dominant and keystone species

Dominants are more numerically abundant

Direct interactions

When two species interact e.g. predation, mutualism, or interference competition

Interference competition

Where one species out-competes another, restricting its access to food

Indirect interaction

Where one species affects another via their effect on a third species. May arise throughout an entire community because of a single direct interaction between two component species

Exploitation competition

Consumer 1 decreases consumer 2 by eating their shared resource e.g. wasps competing with geckos for honeydew

Apparent competition

Two different prey species indirectly effect one another by their impact on the population size of their shared predator

Example of apparent competition

In fertilised plot, grass aphid showed up in greater numbers, therefore bringing more predatory ladybugs, which then decreased the population of nettle aphids. Negative interaction between grass aphids and nettle aphids

Interaction modification

Where one species modifies the effect of another, influencing a third interaction

Example of interaction modification

Trout feeding on stoneflies, having an indirect positive effect on mayflies

Conservation example of complex indirect interactions

Stoats and rats negatively indirectly affect mistletoe by decreasing pollinator bird populations

Example of indirect commensalism

When salamander was removed from a pond, Daphnia pulex out-competed Daphnia rosea so it went extinct, and the Phantom Midge larva now had nothing to eat and also went extinct. Beneficial to midge, neutral to salamander

Network theory

Number of species, links, and maximum number of links in a food web.

Connectance

Proportion of possible links that are realised

Linkage density

Average number of links for each species in the web

How can omnivory be thought of?

A species’ ability to adapt to the loss of a species that they rely on

Compartmentalisation

Tendency for species to interact more strongly and more frequently among themselves than with other species in community. Often organised around body size

When is a community said to have stability?

When it remains in or returns to the original structure and function after some disturbance

Disturbance

Any relatively discrete event that disrupts community structure and function

Variability

How much the community changes in time or space

Resistance

A community’s ability to not change from disturbances

Resilience

The time it takes for a community to return to its normal state after a disturbance

Findings of McIntosh’s stream stability experiment

Resistance to disturbance consistently increased with stream size, but resilience was significantly higher in smaller streams for some response variables. However, smaller streams were overall more variable

Alternative stable state

The state which a community can be put into following a sufficiently large disturbance

Hysteresis

The energy/resources required to return the system to its natural state

Causes of Lake Ellesmere’s alternative stable state

Big storm ripped up the macrophytes (water plants) which would have shielded from waves. Waves could now disturb more, suspending the sediments, leading to a constant turbid state

Factors improving food web stability

Species richness, presence of highly connected species, and compartmentalisation

Energetic hypothesis

Length of food chain is limited by basal energy supply, and most chains are short because energy is lost between trophic levels

Dynamic stability hypothesis

Food chains will be shorter in more variable environments, as long food chains take longer to recover

Ecosystem size hypothesis

Larger systems should have longer food chains because of their greater habitat heterogeneity and ability to support compartmentalized food webs

Limitation of ecosystem size hypothesis

It has gone largely untested in wetland and stream ecosystems

Findings in Lake Cayuga, upstate NY

Consumers at higher trophic levels had higher concentrations of heavier nitrogen isotopes. Stable isotopes were used to calculate the maximum food chain length in each ecosystem

What does the instability of long food chains mean for top predators?

Fluctuations cause extinctions of top predators

Despite its high productivity, why does Lake Ellesmere support a relatively short food chain?

Its inhabitants have invested lots of energy in defence mechanisms, so there is little energy available - so its tuna are relatively small

Redundancy

When different species share the same traits

Interaction facilitation

Where species within and between the groups are contributing to the success of the others

Complementarity effect

Species have different niches, so use resources in a different way. More resources are used, so ecosystems has a higher ‘function’

Sampling effect

Finding an ecosystem that is species-rich has a higher probability of containing species with high ‘functioning’. Difficult to distinguish from complementarity

Redundancy effect

Once species richness reaches some threshold, additional species are redundant

Facilitation effect

Species have a positive effect on other species functioning

Case study of high redundancy: Amazon headwaters in Venezuela

Areas of river without a certain fish species had very high sedimentation, as they are critical in cycling carbon with their migration

Case study of why ecosystem functioning is important

Nutrient enrichment leading to algal blooms in Mississippi and Missouri rivers, creating anoxic conditions

‘Chicken or egg’ question around diversity/function

Does diversity cause function, or function cause diversity?

Lake Victoria nile perch case study - why such a severe extinction?

Nile Perch is a large, omnivorous generalist predator. They hadn’t co-evolved with the local cichlids, leading to more than half of the 200 local species going extinct

Big South Cape island invasion case study

1963 rat invasion led to extinction of Stewart Island snipe, Stead’s bush wren, and the greater short-tailed bat

Traits displaying lack of native bird co-evolution with predators

Nesting on ground, and competing with one another for resources

Example of how invaders impact ecosystem functions and services

Rats kill Titi, who line their burrows with leaf litter. Therefore leaf litter is not cleared away and accumulates on the forest floor