How is tropical biodiversity changing? Flashcards
What is a trophic cascade?
Trophic cascade – 3 or more trophic levels connected by effects - creates a trophic cascade.
Consequences of species losses - what are the different types of controll?
Top down (predation) or bottom up control (effects of nutrients)
Top down, predators would be expected to be inversely related to the prey.
Bottom up, relationship between successive interactions in the food web would be positive
Tony paper: fishes and unfished regions GBR
Graham et al. 2003 Environmental Conservation 30: 200-208
- Working on the GBR, looking at two regions (Palm islands and Whitsunday islands) and their different zones
- Blue - fished in some ways
- Green - legally protected from fishing activities
- Used dive data along a transect tape
- Predator and prey surveyed, with data on habitat complexity
- Coral trout important predators on the GBR
- Evidence of top-down effects on community structure
- Number of individuals vs biomass: more individuals but half the biomass in fished zones
- Collected predator data separately from prey data
- In terms of biomass in the green zones there is a lot more fish, as larger fish are targeted in the blue zones
- In terms of numbers there are more fish in the blue zone, as there are lots of fish, they are just not reaching the same size.
- Coral trout important predators on the GBR
- Evidence of top-down effects on community structure
- In the sites that are protected from fishing, where the biomass off coral trout is significantly greater you get much lower of biomass and abundances of prey species.
- This can be taken as evidence that when this predator is removed from the system, it has an effect on the number of prey - top down.
Tony paper: prey effects crown of thorns starfish / wrasse / triggerfish ect
Dulvy NK et al. 2004 Ecology Letters 7: 410-416
- Crown of thorns starfish, feeds on coral, fed upon by large wrasse, triggerfish etc
- At low levels of human impact (and fishing) there are large numbers of crown of thorns starfish as fishing removed predatory wrasses and triggerfish.
- At the highest human population the COTS is at GBR outbreak threshold. (spatial data)
- This is striking as they are remote low population areas
- Three trips were made
- The abundance of coral declining was replaced by turf algae
- Maroi wrasse has tough lips that can cope with COTS and Sea Urchins. Removing them can cause a trophic cascade within the system, causing a phase shift between coral and algae.
- Social data suggested the wrasse had gone locally extinct in some areas.
Why is top-down controll not the only factor?
- Many populations controlled by other sources of mortality (disease, parasites + fishing)
- Many populations controlled by recruitment (ie. larval survivorship of potential starvation, predation, dispersal)
- Scientific framework can be shaky, such as using large-scale correlations and time series where interpretation potentially explained by other factors (e.g. bottom-up controls, recruitment variability)
Summary
Substantial declines in abundances of large predators are clear in temporal (e.g. epipelagic) and spatial (e.g. coral reef) data.
Possible consequences of this include trophic cascades and related effects (e.g. loss of coral cover), where there is top-down (ie. predator-driven) control of prey species.
Effects of predators on prey are indicated by work on MPAs (e.g. GBR) and fishing pressure gradients (e.g. Fiji) but populations are likely often controlled by larval recruitment, rather than by predation
These trends are for whole groups but there is growing recognition of vulnerability of particular species
WIDER READING: Global Trajectories Decline in Coral Reefs
Pandolfi et al 2003- Global Trajectories of the Long-Term Decline of Coral Reef Ecosystems
- Compiled records summarising magnitude of change in 7 main guilds across coral reefs
- Large animals declined before small animals and architectural species
- Atlantic reefs declined before reefs in the Red Sea and Australia, but the trajectories of decline were markedly similar worldwide.
- Loss of species seen across all reefs with time
- All reefs were substantially degraded long before outbreaks of coral disease and bleaching.
- Reefs will not survive without immediate protection from human exploitation over large spatial scales
WIDER READING: triggerfish east coast of Africa
Mclanahan 2016- extent and variation of a coral reef trophic cascade
- Reduction of Balistapus undulates (Orange-lined triggerfish) has an effect on sea urchin abundance on the East African coast
- Density of Orange lined triggerfish was related to both urchin abundance and associated with higher cover of calcifying algae
- By-catch of this orange lined trigger fish drives the cascade
- Determination of the exact urchin species depended on geographic location/water depth and predation pressure
WIDER READING Coral reef cascades
Dulvy et al 2004- Coral reef cascades and the indirect effects of predator removal by exploitation
- Study on 16 Fijian islands.
- Removal of predators leads to an increase in crown of thorns starfish
- COTS triggers a phase shift, as they eat coral algal % cover increases
- The COTS therefore induces a change in benthic community structure
- This system is controlled via bottom up control through starfish recruitment. But also controlled top-down by predators, dependant on density/fishing pressure.
WIDER READING: COTS
Cowan et al 2017-Predators of the COTS and their role in preventing outbreaks
- Only 18 species that predate on adult COTS
- Juveniles, larval stages also have predators, total predatros at all life stages= 80
- Multiple species having an effect at multiple life historys responsible for control. No single species with disproportionate effect on COTS abundance.
- Unlikely that predatory release in and of itself could account for initial onset of CoTS outbreaks.
- In conclusion, reducing anthropogenic stressors that reduce the abundance and/or diversity of potential predatory will need to be used in conjunction with other management strategies to prevent, or reduce the occurrence, of CoTS outbreaks.
