Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Island restoration wikipedia , lookup
Biodiversity action plan wikipedia , lookup
Habitat conservation wikipedia , lookup
Biological Dynamics of Forest Fragments Project wikipedia , lookup
Reconciliation ecology wikipedia , lookup
Human impact on the nitrogen cycle wikipedia , lookup
Overexploitation wikipedia , lookup
Latitudinal gradients in species diversity wikipedia , lookup
Comparative ecosystem dynamics Kenneth T. Frank, Department of Fisheries and Oceans, Bedford Institute of Oceanography, Dartmouth, Nova Scotia, Canada • Outline: • Structure of marine ecosystems (collaboration with B. Petrie, N. L. Shackell, W.C. Leggett, J. Choi, J.A.D. Fisher) • Focus on the eastern Scotian Shelf (NW Atlantic) – Cascading trophic interactions • Comparative analyses: Is there a general pattern of response to over-exploitation? • Summary • Structure of marine ecosystems # Marine food chains normally operate this way – bottom-up TL 4 # Positive correlation between any trophic level in food chain # TL 3 # TL 2 Trophic Level 1 Time Southeast Scotland Frederiksen et al. 2006 Faroe Shelf Steingrund and Gaard 2005 Tuna Squid Sardine Shrimp Catch of four main fisheries in Gulf of California Under bottom-up control assume that: • Variability at base of food chain determines productivity at higher trophic levels • Exploitation of top predators has no impact on lower trophic levels in food chain • Over-fishing effects are reversible • So, in terms of EBM, why worry about anything else but the target species • In early 1990s groundfish stocks in the Northwest Atlantic, particularly cod, were over-fished and on the verge of collapse NW Atlantic Cod stocks that collapsed in the 1990s Location % Decline in Stock Size Year of Moratorium on Fishing Labrador and Northeast Newfoundland 99.8 1992 Southern Grand Bank 96.4 1993 95.9 1993 Southern Gulf of St. Lawrence 85.4 1993 Eastern Scotian Shelf 95.8 1993 Northern Gulf of St. Lawrence Despite long-term fishing moratoria nearly all have failed to recover # If top predators depleted then marine food chains may operate differently TL 4 # Negative correlation between adjacent trophic levels in food chain # TL 3 # TL 2 Trophic Level 1 Time • Large, complex systems considered immune to cascading trophic interactions • An exception to the rule - the eastern Scotian Shelf ~108,000 km2 Data rich area: Scientific surveys – 30+ years Four trophic levels Loss of entire functional group 161 cm 112 cm 120 cm 60 cm 76 cm 121 cm 73 cm 142 cm 108 cm Response of major forage fish species (planktivores) - Release from predation - Similar increases in macroinvertebrates Other geographic areas responded the same way TL4 TL3 TL1 TL2 • Large, complex systems not immune to cascading trophic interactions • Growing number of examples in marine ecosystems Under bottom-up control assume that: Exploitation of top predators has no impact on lower trophic levels in food chain Over-fishing effects are reversible Why worry about anything else but the target species Not true Not so simple Lots to worry about Lack of recovery despite no fishing, revealed weakness of knowing how ecosystems respond to exploitation Not all fisheries collapse when heavily exploited nor does the structure of all systems undergo transformation Comparative approach – 47 areas in the Northern Hemisphere (literature review) Published studies during past 10 years (a few earlier) Time series of single (dominant) species or functional groups TL1 TL2 TL3 TL 1 TL2 6 TL3 2 1 TL4 7 6 25 Surveys (local and large scale), landings data -- Positive correlations among TLs -- Negative correlations among TLs Contemporary diagnosis of 47 systems 45 42 47 44 42 40 43 41 - Strong latitudinal gradient in structure 46 Water temperatures exhibit strong latitudinal gradients < 5o C > 6o C Fish diversity exhibits strong latitudinal gradients Western Atlantic Number of fish species Eastern Coastal Shelf Abyssal Latitude (deg) Macpherson 2002 • In North Atlantic, pattern of trophic forcing related to temperature and species richness western areas eastern areas All areas heavily exploited yet species rich and warmer water areas remain bottom-up Potential mechanisms -from Frank et al 2006 TREE • Temperature influences growth rate, age at maturity, etc.; targeted species inhabiting warmer waters more resilient to exploitation effects (Myers et al. 1997) • Species richness acts through compensatory effects where the probability that a depleted species will be replaced by another is higher (e.g.Shackell and Frank 2007) • Trophic cascades can be eliminated or reduced by compensatory mechanism preventing the effects to propagate down the food web • Suggests warmer water, species rich systems can withstand higher levels of exploitation without changing trophic structure • Current research attempting to define exploitation thresholds to avoid undesirable states – linking exploitation rates, species richness, and temperature across many different systems Can withstand increasingly high exploitation until top-down Rapid change in system with modest increase in exploitation 2oC increase in temperature corresponds to 10% higher exploitation threshold Summary • Trophic structure of warmer water, species rich systems appears resilient to exploitation effects; conversely, structure of colder water systems much less resilient – Expect fishing effects to be reversible on target species in warmer water systems, but concern for rapid re-direction of fishing effort on compensating species – Fishing effects not easily reversed in colder water systems and large marine ecosystems are not immune to trophic cascades • Whether or not cod will recover remains unknown – going on 15 years since exploitation eliminated/greatly reduced • Could be very difficult to return to former state simply by wait and see approach • May be in permanently altered state • Should other types of intervention be considered? • Reintroductions as in terrestrial systems • Culling/removal of forage fishes (since no interest in a commercial harvest) or competitors (such as seals) • Some elements of fishing industry satisfied with current situation • Thank you for your attention