Lecture 6: Overexploitation Flashcards
What is ‘Overexploitation’ (aka ‘overharvesting’)?
- Harvesting a renewable resource faster than it can be produced.
- Of threatened or near threatened species on IUCN Redlist, 72% (6,241) are being overexploited for commerce (trade), recreation or subsistence.
- Biggest threat after habitat loss/degradation.
Overexploitation
- Can rapidly drive even super-abundant species to extinction e.g. passenger pigeon, (they was cheap source of meat, from billions to extinction).
- Rates been increasing: high human population, increased demand, advanced technologies for harvesting (harvest faster).
Harvesting can be sustainable
(A) Logistic population growth: growth starts off slow (not many individuals reproducing), then speeds up (more reproducing individuals), then slows down again as get close to carrying capacity (K), (carrying capacity = max number can have in an ecosystem).
(B) Maximum Sustainable Yield (MSY) Model: growth starts off slow, then increases, then slows as get close to K. In absence of exploitation, population growth is highest at half the K. The MSY depends on r (rate of population growth) and K. These vary depending on characteristics of species (reproductive rate etc).
Harvesting can be sustainable
Fixed harvest (quotas)
- If quota set too high - population driven to extinction (not sustainable).
- If set to MSY harvesting will be sustainable if initial population is above MSY, but will lead to declines if population dips below MSY.
- Low quotas are safer.
- Requires accurate estimates of population size, K and r.
Harvesting can be sustainable
Proportional harvests (dependent on population size) are more sensible.
- Harvest a certain proportion of the popualtion and then if a population size decreases (e.g. from disease etc) won’t end up harvesting more than should.
- Only have over-harvesting if have very high exploitation.
- Vey difficult to implement as require constant, accurate population monitoring.
Harvesting can be sustainable
- MSY models (fixed harvests & proportional harvests) are over-simplistic.
- r and K may vary e.g. depending on population age structure (lots of juveniles = low reproduction & environmental variation. (Also how long to reach maturity).
- More complex models exist but difficult to get accurate data to parameterise them. Also challenging to implement them, especially when moving across international boundaries.
- Illegal harvesting & other sources of mortality must be taken into account (e.g. disease, predation etc).
Types of overexploitation: Commercial
- Unsustainable harvesting driven by a legal or illegal market.
- e.g. fishing - about 33% (possibly up to 66%) are over-harvested.
- Overfishing seems to be increasing.
- Can lead to huge population crashes & collapse of the industry.
- Some popualtions managed well.
Types of overexploitation: Commercial
- E.g. Newfoundland Cod - population hunted unsustainably and population crashed by 99%. Haven’t recovered.
- Overfishing doesn’t often lead to extinction due to high reproductive rates of marine fish. May not recover even when harvesting has stopped.
Types of overexploitation: Commercial
Quotas can help but have problems:
- By- catch. Between 25-75% of the harvest (non-target species & excess catch) is dumped back into sea (dead or dying). Particular problem for bottom dredging.
- Some solutions - e.g. improvements to fishing equipment. Public awareness campaigns help people choose their sources of seafood wisely.
Types of overexploitation: Commercial
E.g. whaling.
- Hunted for meat & fats (palm oil).
- Technologies used for hunting improved so more hunted. Total catch has increased overtime.
- Larger species targeted first, e.g. Blue whales & population crashed. Then smaller species targeted.
- In 1985 International Whaling Commission (IWC) banned commercial whaling to allow stocks to recover.
- Some species recovered well. E.g. humpback whales were reduced from >100,000 to ~10,000: a 90% reduction. Populations now at >60,000 & increasing.
- Some haven’t. E.g. blue whale previous population: 200,000-300,000, currently ~2000-5000 despite being protected since 1966. (Slow reproductive rates & climate change)
Types of overexploitation: Commercial
- Even after populations recovered, have lower genetic diversity.
- Many pinnipeds (seals) were hunted almost to extinction for fur & blubber. Population sizes of many have recovered.
- Stoffel et al (2018) investigated current genetic diversity in 30 pinniped species. 11 species show genetic signs of a severe bottlenecks, effective population sizes declining to <500, sometimes <50.
- Current genetic diversity was lower for species that: went through bottlenecks, have small current population sizes, breed on land (easily accessible by human hunters).
- Bottleneck size not related to IUCN status, indicating population recovery in many cases (although 3 of the 4 most bottlenecked species are currently endangered).
Types of overexploitation: Commercial
Main solutions:
Certification schemes
- E.g. Marine Stewardship Council or Forestry Stewardship Council.
- Indicates the fishery meets certain standards related to sustainability.
- Rely on consumer awareness and buy-in.
International agreements
- Convention of International Trade of Endangered Species (CITES) protects 30,000 plant species & 5,800 animal species, only a small number of countries not signed up. Appendix I: Species highly endangered (3% of species) (e.g. African rhino) trade permitted in exceptional circumstances. Appendix II: Trade strictly controlled (most species). Appendix III: Request for assistance in controlling trade of species protected in a specific country.
Types of overexploitation: Commercial
- Illegal shipments commonly run by organised crime syndicates.
- Often seized at airports/ports.
- Most trade (or seizures) is of mammals.
- 76% of species are of CITES Appendix I or II species.
- Trade networks can be complex.
- Current global resources allocated to combating transnational wildlife crime are in the tens of millions of U.S. $ globally.
Types of overexploitation: Subsistence (personal/family consumption)
- Many people rely on wild plants, animals & fungi as food source.
- (Before farming/agriculture) ~10,000 years ago, all food came from wild.
- Now: > 1 billion people rely on wild plants, > 1/5th protein comes from wildlife in 62 developing countries, almost 20% protein intake comes from wild fish for >3 billion people.
- Some harvesting not sustainable due to increased human population sizes & advances in modern harvesting technologies.
Types of overexploitation: Subsistence
- Some locations particularly vulnerable: high human density, new road/transport links enable access, increasing commercial trade.
- Difficult to regulate, but considerations must be made to people who rely on subsistence hunting for food (not everyone has reliable food source e.g. supermarket).
