Trophic Cascades

Question 1

Food-web consequences of depletions

Trophic cascades

Food webs as maps of predator control

Prevalence of trophic cascades

What is a food web?

Answer 1

Shows the links between consumers and their diets.

Maps of relationships between consumers and their diet, predators and their prey.

(1) What eats what (consumer-diet, predator-prey)
(2) Maybe what controls what
(3) Potential trophic cascades

Trophic cascades: linked interactions involving ≥ 3 trophic levels. Can be top-down or bottom-up.

However, ‘control’ depends on the strength of predator-prey interaction in relation to other factors.

The extent to which a species being removed is going to have an impact on others is really dependant on the interactions between species.

Question 2

What is a trophic cascade?

Answer 2

Three trophic levels which are connected and you demonstrate that there is a knock-on effect at one end of the chain then that would be a trophic cascade.

Cascades bottom-up and top-down.

Question 3

Give two examples of kelp based trophic cascades.

Answer 3

Temperate reef (Nova Scotia)

Breen PA, Mann KH 1976 Marine Biology 34: 137-142

Fishing pressure on Lobster and Wolfish - less urchin predation - Kelp habitat to urchin barren.

Temperate reef (S California)

Tegner MJ, Levin LA 1983 Journal of Experimental Marine Biology & Ecology 73: 125-150

Fishing pressure on Sheephead (Wrasse), Spiny Lobster and hunting Sea Otters reduces predation on urchins - kelp habitat or urchin barren.

Temperate reef

(Alaska/Aleutian Is)

Estes JA et al. 1998 Science 282: 472-6

Killer Whales were feeding on Seals and Sea Lion, however, with the Sea Otter recovery, the Killer whales are switching attention to predate on the Otters, causing the same cascade.

Temperate reefs (S Africa)

Barkai A, McQuaid C 1988 Science 242: 62-64

Fishing/ Perturbation (consider human exploitation as a trophic level for it to be a cascade) reducing rock lobster population. This causes less rock lobsters and more Whelks, reducing settling mussels and replacing them with macroalgae.

Question 4

Case study - southern ocean

Southern Ocean and NorthWest Atlantic Continental shelf, more complex case studies.

Answer 4

Baum JK, Worm B 2009 Journal of Animal Ecology 78: 699-714

Southern Ocean

Seasonal decreases in Apex predators such as Minke Whale, Adélie penguins (also feeding on Krill), Killer Whales (also feeding on Antarctic toothfish) and seals increase Antarctic Silverfish numbers, and causing a trophic cascade.

Fishery depletion of cod increases numbers of Snow Crab, Small Pelagic Fishes and Northern Shrimp, increasing large herbivorous zooplankton and decreasing phytoplankton abundances. (recap not that clear - could look at paper)

Question 5

Trophic cascades - great shark

Answer 5

Eastern seaboard of the US: large shark declines and mesopredator elasmobranch increases, 1970s-00s.

Declines of between 0.0 and -0.3 across all the sharks. They feed on

Elasmobranch mesopredators – skates and rays, smaller sharks, fine-tooth shark which are mostly prey for the shark species.

Baum JK, Worm B 2009 Journal of Animal Ecology 78: 699-714; Myers RA et al. 2007 Science 315: 1846-1850

Trophic cascade theory predicts there should have been declines in mesopredator prey, and this occurred in bay scallop fishery.

Bay Scallop is an important fishery, which has seen a decline.

Question 6

What type of data is collected on food webs and why?

Answer 6

Observational data – looking at large scale changes in the system and trying to sensibly infer what the evidence is for the changes that we are seeing.

• Lots of data is historical or comes from fisheries studies which are themselves very often inadequate. May know the catch but not be able to calculate any catch per unit effort. To infer from catches what the abundances are doing you want to know what the fishing effort involved is.
• • Critical experiments small scale
• • Different factors maybe at different scales
• • Scaling effects ~impossible to test
• • Large scale effects inferred from historical data, circumstantial evidence (observational not experimental data)

Question 7

Why may food webs not map predator control?

Answer 7

The biological part of ecosystems is complicated.

Each of the individual populations is incredibly complicated, as they consist of different life stages, it is subject to a whole range of different factors and process such as reproduction and starvation.

Food webs may not map predator control, is they do not drive prey abundances, and other factors such as recruitment are strong.

Question 8

How may the strength of food web connectivity be measured?

Answer 8

Link J 2002 Marine Ecology Progress Series 230: 1-9

Lots of species within system - how strong are the potential links between species. The equation of connectivity is saying the number of links that we find vs the total number of possible links

Food web connectivity, C

L = number of interactions - number of interactions which are detected, through direct observation, analysis of stomach content etc = number of species

L/S = links per species

CÍS = index of stability?

S and L may increase with greater study and L/S and C may change: increase? Thus best to focus on well-studied ecosystems

Question 9

Implications of food webs?

Direct interactions

Answer 9

Direct interactions

predation

interference competition

recruitment effects

shelter/habitat provision

feeding inhibition

Question 10

Implications of food webs?

Indirect interactions

Answer 10

Indirect interactions

• keystone predation
• trophic cascade
• exploitation competition
• apparent competition
• indirect mutualism
• indirect commensalism
• habitat facilitation

Question 11

What does the connectivity equation show when applied to a food web?

Answer 11

Link J 2002 Marine Ecology Progress Series 230: 1-9

One of the very best marine large-scale studies

North-East continental shelf, USA

• Fisheries
• Long-term ecological monitoring, research etc

Shows a highly connected food web.

A higher level of connectivity than other studies.

Question 12

What does the connectivity calculation by link show about the ecosystem and its complexity for the NE shelf ecosystem

Answer 12

Link infers that in that system there is extremely high connectivity, meaning the whole network is going to be quite diffuse on average. Things are going to have relatively low interaction strengths between them because there are lot of species that are generalists in it. Lots of species that are omnivorous that have a broad diet or switch their diet over time. Ontogenetic changes in those populations which make these changes more complex still. Even in its complexity, the model emits the fact that each species occurs at different life stages which are doing quite different things. If you included the life stages in there the connectivity would be even greater than acknowledged.

Question 13

What’s the balance of importance between top-down and bottom-up forces?

Answer 13

Are bottom-up forces still important?

• Prevalent paradigm in biological oceanography?
• Strong evidence e.g. of upwelling systems (plankton → seabirds bottom-up cascade)

How are top-down and bottom-up forces combined?

• Upwelling systems?
• Top-down control top predators → sardine/anchovy → zooplankton inferred from large scale fisheries etc data
• Bottom-up control phytoplankton → zooplankton sardine/anchovy → seabirds etc established from wildlife, oceanographic etc data

Sardines/anchovies as ‘wasp-waist’ species

Top-down effects also occurring in pelagic environments

These habitats are also very spatially complex.

Cury et al, wasp waist – the effect of top-down and bottom up acting as opposing forces in the food waste. Sardines being in the middle.

‘Wasp-waist’ control of marine ecosystems is driven by a combination of top-down and bottom-up forcing by a few abundant short-lived species occupying intermediate trophic levels that form a narrow ‘waist’ through which energy flow from low to high trophic levels is controlled.

Question 14

How common are cascades?

Answer 14

Contrasts in the literature

Pace et al. 1999 Trends in Ecology & Evolution 14 (2): 483-488 - says that they are everywhere

Pinnegar JK et al. 2000. Environmental Conservation 27: 179-200 - contrasts by pointing out that only 39 ‘well-documented’ cascades (small and large scale)

• 28 in N Atlantic/W coast of N America
• All: rocky intertidal (17), algal bed (14), coral reef (4), with very few on soft sed (2), however, one of the problems here is probably the intensity of study and the accessibility.
• ~all shallow, accessible to study, hard substratum
• The intensity of study? Intertidal and NE Atlantic indicate a low prevalence of cascades?

Question 15

What does a Nick paper contrasting the idea from Link, 2002 that the level of connectivity is high when you are looking at a well-studied ecosystem.

Answer 15

When you look at reef study’s Gastropods and urchins crop up time and again- seem to be pivotal. Predators such as Lobster and birds occur time and again.

Pivotal species are often quite well protected from predators.

The argument that – the level of connectivity is high when you are looking at a well-studied ecosystem – tends to break down a little bit. Very few predators can deal with spines and thick shells, - greater likelihood of there being top-down control.

Suggests we will go on to find cascades in less assessable environments

• Certainly (constrained by the small scale of measurement, lack of controlled experimentation etc)
• Sediment systems (but less amenable to study), remote methods - baited cameras.
• Parasites and pathogens as ‘predators’

Question 16

the study

Nick paper

which cascades vary in their strength

Answer 16

Borer ET et al. 2005 Ecology 86: 528-537

Plant log ratio – a measure of the existence of the cascade.

You have a predator which predates on a bunch of grazing species, feeding on plant material. The predator has, through its predation has a positive indirect effect on the plants.

Here, we tested various hypotheses about why trophic cascades occur and what determines their magnitude using data from 114 studies that measured the indirect trophic effects of predators on plant community biomass in seven aquatic and terrestrial ecosystems. Using meta‐analysis, we examined the relationship between the indirect effect of predator manipulation on plants and 18 biological and methodological factors quantified from these studies.

Question 17

Nick paper

which cascades vary in their strength

Various possible reasons for variable cascade strength including:

*

Answer 17

From Nick

(Borer et al, 2005)

1. Spatial heterogeneity
2. Diffuse food web structure
3. Low resource availability - affected by herbivore consumption rates
   
   1. Herbivore and predator consumption rates 31% of variation amongst 114 studies
4. In all systems, the strongest cascades occurred within association with invertebrate herbivores and endothermic vertebrate predators.

• Spatial heterogeneity (Heterogeneity Hypothesis): refugia for herbivores reduce search efficiency of predators
• Food web structure diffuse (Community Regulation Hypothesis): competition, omnivory etc weakening predator-prey interaction strength. Related to example from link paper. Diffuse food web structure means the stong lines of controlll are much less liekly.
• Low resource availability and quality (Resource Availability Hypothesis): reducing consumption rates of herbivores
• Short study duration (Duration Hypothesis): allowing insufficient time for plants to regenerate or be replaced by inedible plants low predator or herbivore efficiency (Efficiency Hypothesis): e.g. weak functional response or high metabolic cost.
  
  • Plant log-ratio: + = strong cascade (plant biomass greater with than without predator)
  • Conclude: possible positive relationship, so study duration a factor but largely due to marine benthos
  • Study duration/plant generation time
  • Conclude: study duration relative to plant generation time not obviously a factor
• Combinations

From Abstract

We found, in contrast to predictions, that high system productivity and low species diversity do not consistently generate larger trophic cascades. A combination of herbivore and predator metabolic factors and predator taxonomy (vertebrate vs. invertebrate) explained 31% of the variation in cascade strength among all 114 studies. Within systems, 18% of the variation in cascade strength was explained with similar predator and herbivore characteristics.

Within and across all systems, the strongest cascades occurred in association with invertebrate herbivores and endothermic vertebrate predators.

These associations may result from a combination of true biological differences among species with different physiological requirements and bias among organisms studied in different systems.

Thus, although cascade strength can be described by biological characteristics of predators and herbivores, future research on indirect trophic effects must further examine biological and methodological differences among studies and systems.

Question 18

Conclusions

Answer 18

Trophic cascades a useful concept and important paradigm but many processes (e.g. recruitment) disrupt them

And basis for identifying and predicting them is weak; they are poorly known

Food webs provide a basis for mapping predator control but high connectivity in marine systems evidently makes for low interaction strength as whole, although some specific links may be strong and provide a focus for trophic cascade effects

Question 19

Trophic cascades in benthic marine ecosystems

Answer 19

Trophic cascades in benthic marine ecosystems: lessons for fisheries and protected-area management – Pinnegar et al., 2000

Background:

• Review 39 documented cascades
• Over 21 locations
• All but 2 are on hard substrata (e.g. kelp, coral reef)
• 2 questions asked:
  
  • in which marine systems/habitats do we find the strongest evidence for trophic cascade effects, and what are the main characteristics of the organisms involved?
  • what factors affect the occurrence of these cascades and are there ways in which their importance might be predicted when systems are selectively changed by development or management
• Review of literature – confined to benthic cascades of 3 or more trophic levels – whereby interactions are based on only predation interactions

Question 20

Trophic cascades in benthic marine ecosystems:

Study

Results

Impact on ecosystem

Answer 20

Trophic cascades in benthic marine ecosystems: lessons for fisheries and protected-area management – Pinnegar et al., 2000

Results:

• More cascade effects are likely to be found in soft-substratum systems – crucial for many large-scale fisheries
• existing evidence is that cascade effects are to be expected when hard-substratum systems are subject to artisanal resource exploitation
• Caribbean reefs and the expansion of coralline barrens in the Mediterranean rocky-sublittoral will not be readily reversed in MPAs – due to other than predation- based cascades contributed to the cascade in the first place
• More cascades will be found in soft substrates – need to utilise less conspicuous organism such as polychaetes and crustaceans

Question 21

Trophic cascades in benthic marine ecosystems:

Study

Results

Impact on policy

Answer 21

Pinnegar et al., 2000

Found that information from trophic cascades is not enough to inform policy’s on fisheries for a variety of reasons

• Information suggests evidence for trophic cascades at small scale – premature to assume that they occur at large scales if they were looked for
• Nearly all evidence (Apart from 2) come from hard substrates which are unsuitable for trawls and only acceptable for recreational angling and
• We can thus expect cascade effects of exploitation in shallow subtidal hard-bottom systems including coral reefs, kelp forests and other macroalgal beds, where the exploitation is intensive. This may have profound effects on the immediate ecosystems and on occasion on adjacent ecosystems e.g. O’leary and McClanahan, 2010 and McClanahan and Muthiga (1988) – bioerosion by urchins due to fishing

Question 22

Trophic cascades result in large-scale coralline algae

study

Results

Answer 22

Trophic cascades result in large-scale coralline algae loss through differential grazer effects- O’LEARY, J.K. and MCCLANAHAN, T.R., 2010.

(O’leary and Mcclanahan, 2010)

CCA have been most widely studied in Caribbean and temperate reefs, where cover increases with increasing grazer biomass due to removal of competitive fleshy algae.

However, each of these systems has one dominant grazer type, herbivorous fishes or sea urchins, which may not be functionally equivalent.

Where fishes and sea urchins co‐occur, fishing can result in a phase shift in the grazing community with subsequent effects on CCA and other substrata.

Kenyan reefs have herbivorous fishes and sea urchins, providing an opportunity to determine the relative impacts of each grazer type and evaluate potential human‐induced trophic cascades.

In the Indian ocean

Results:

• CCA cover was highest in long-term fisheries closures and lowest in fished reefs throughout the study
• 2003 estimated effect of grazers was a difference of 14.4% CCA cover between closed and fished reefs
• removal of predatory fishes, such as triggerfishes and wrasses, results in a proliferation of sea urchins
• Herbivorous fishes positive influence CCA cover and growth by removing shading macroalgae
• Abundant sea urchins would reduce CCA growth through bioerosion
• Fish and sea urchin grazers indirectly benefit CCA by removing fleshy algae but have direct negative effects on CCA through bioerosion
• Found that effect of ENSO was lower than the effect of sea urchin grazing on CCA
• Fishing indirectly reduces CCA

We hypothesized that fish benefit CCA, abundant sea urchins erode CCA, and that fishing indirectly reduces CCA cover by removing sea urchin predators. We used closures and fished reefs as a large‐scale, long‐term natural experiment to assess how fishing and resultant changes in communities affect CCA abundance. We used a short‐term caging experiment to directly test the effects of grazing on CCA. CCA cover declined with increasing fish and sea urchin abundance, but the negative impact of sea urchin grazing was much stronger than that of fishes. Abundant sea urchins reduced the CCA growth rate to almost zero and prevented CCA accumulation