How are Tropical Marine Ecosystems Changing?

Question 1

The theory

As you change the number of organisms in an ecosystem, you expect the number of other things to change.

Insurance hypothesis: inherent ‘goodness’ of biodiversity is debated. (having lots of species is inherently a good thing)

Give some bits of evidence supporting this idea.

Answer 1

• Intuitive: complex ecosystems offer more options when conditions change. e.g. prey switching in food webs reduces depletion/extinction of a species
• Modelling:
  
  • Early work suggested that complex ecosystems seem to be quite fragile. e.g. long food-chains (without omnivory). Specialist feeding habitats may be more vulnerable to change.
  • Now recognise larger amounts of omvivory, where organisms are reliant on a number of food sources often from number of different trophic levels, help to stabilise population in the face of changes to environment.
• Experimental: complex ecosystems may or may not benefit from more species. e.g. productivity of grasslands

Question 2

What is the diversity - stability hypothesis?

Answer 2

Diversity-stability hypothesis: as species diversity declines, so does productivity and ability to recover from disturbance (a form of resilience), ie. change occurs in a linear fashion as species are lost.

The idea is that you get a linear function, the level of function declines proportionally to the amount of species being removed.

We are saying that all species are equal in having an important function, every species that is being removed is likely to have an effect.

Question 3

What is the Rivet hypothesis

Answer 3

Rivet hypothesis: threshold reached in species loss, before which loss has no effect but after which the impact is structurally catastrophic), ie. change occurs in a non-linear fashion as species are lost

Another idea, analogy based on wing of an aeroplane, individual plated riveted together. The idea is with structure that is retains its integrity if a few rivets are removed, at a certain point the structure is so weakened it totally breaks down.

You meet a inflection point beyond which something drastic happens.

Question 4

Tony paper - a paper trying to give sense to what happens to ecosystem function when biodiversity changes.

Answer 4

More recent paper; Cardinale BJ et al. 2012 Nature 486: 59-66

Authors try to give us a synthesis of the sense of what happens to ecosystem function when biodiversity changes.

Biodiversity loss affects ecosystems function by e.g.

1. Reduced efficiency of many ecological processes e.g. biomass production, nutrient recycling
2. Increased function stability e.g. biomass over time through various mechanisms including compensatory dynamics where e.g. competing species replace each other

They suggest a pattern like this, initially when you remove species you get a relatively gradual change in the function, but becomes more dramatic.

Some systems will be more resilient than others.

• Resilience - capacity to recover from disturbance
  
  • Dynamic robustness - resilient over broad range
  • Dynamic fragility - resilient over narrow range
• Resistance - capacity to avoid disturbance (concept alongside resilience, avoid disturbance in the first place)
  
  • Global stability - resistant to large perturbations
  • Local stability - resistant to small perturbations

Number of components in ecosystem, strength of linkages and number of links contribute to resilience, resistance.

Another similar paper - Scheffer et al 2001

Question 5

Tony papers - large scale coral - algal phase shift.

Answer 5

Gardner TA et al. 2003 Science 301: 958-960

• Fishing removes grazers, leading to algal overgrowth
• Nutrients contribute to algal overgrowth and coral loss
• 90-99% mortality of urchins in 1982-83 led to algal overgrowth

Long-term decline in coral suggests factors other than sea urchin mass mortality

Schutte VGW et al. 2010 Marine Ecology Progress Series 402: 115-122

• Later data than Gardner et al study, regional effects tested
• Most coral loss by ~1981, related to Acropora white band disease
• Greatest macroalgal increase related to sea-urchin die-off
• Trends different among regions (especially Florida Keys)

Question 6

Summary What does this theory predict?

Answer 6

Theory predicts that;

1. as species are lost ecosystem structure and function will change sooner or later
2. changes in state may occur quite abruptly and be difficult to reverse. Coral reefs provide striking examples of how marine ecosystems may change state and what causes the phase shift. The best data come from the Caribbean but the trajectory and most likely causes are debated.

Major changes such as of fishing likely pre-date the present day by a long way, but even ‘well-managed’ areas like the GBR are less than pristine, and there are ‘novel’ anthropogenic impacts to deal with.

Question 7

What threats are facing coral reefs?

Answer 7

Degradation of coral reefs

The last 50 years have seen what appear to be unprecedented large-scale events including coral and sea-urchin diseases, invasive species (e.g. lionfish), natural disasters (e.g. larger and more frequent cyclones), Saharan dust-storms (including iron, with is limiting nutrient and spores, coral and urchin disease events could have been caused by dust) nutrient inputs

To which are now added at least climate-related coral bleaching and ocean acidification

Question 8

WIDER READING: degradation of carribean coral reefs

Answer 8

Hughes 1994- Catastrophes, Phase Shifts, and Large-Scale Degradation of a Caribbean Coral Reef

• Increase in human population in Jamaica since 1970
• 50% coral cover in 1970, to 5% today
• 90% fleshy macroalgae cover today (note age of paper)
• Overfishing oringally led to increase in Echinoidea species, when there was a mass mortality event of these Echinoidea species between 1982-1984, there effect on the herbivory of algae was reduced. The low abundance of Echinoidea and the continued low numbers of herbivorous fish through predation combined with hurricane events, triggered an algal bloom in 1983.

Question 9

WIDER READING: sea urchins anfd juvinoile corals

Answer 9

Edmunds and Carpenter 2001- Recovery of Diadema antillarum (sea urchin species) reduces macroalgal cover and increases abundance of juvenile corals on a Caribbean reef

• Relates to previous paper by Hughes (1994).
• Some shallow reefs have been observed to have again, high density of urchins, indicating recovery
• 17 juvenile species of coral were recorded, due to reduction in macroalgae, through increased urchin density

Question 10

WIDER READING: coral - possible reversal?

Answer 10

Carpenter and Edmunds 2006- Local and regional scale recovery of Diadema promotes recruitment of scleractinian corals

• populations of Diadema now occur over a multi-kilometre-wide scale at six locations
• We conclude that population recovery of Diadema is occurring at both local and regional scales, and that grazing by this echinoid is creating conditions favouring the recruitment of corals.
• Starting to see possible reversal back to a coral dominated benthic community.

The phase change from coral to macroalgal dominance on many Caribbean reefs was exacerbated by the mortality of the echinoid Diadema antillarum in 1983–1984, and until recently, this sea urchin has remained rare on reefs throughout the western Atlantic. By the late 1990s, Diadema started to reappear in large numbers on some Jamaican reefs, and by 2000, the high densities were correlated with significantly greater abundances of juvenile corals. Here, we show that dense populations of Diadema now occur over a multi‐kilometre‐wide scale at six locations scattered along a 4100 km arc across the entire Caribbean. In all cases, these dense populations are found in shallow water (< 6 m depth) on outer reef communities and are associated with reduced macroalgal cover and enhanced coral recruitment. We conclude that population recovery of Diademais occurring at both local and regional scales, and that grazing by this echinoid is creating conditions favouring the recruitment of corals.