February 28 2014 I. Defining Overharvesting/ Overexploitation 1. O
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Conservation Ecology – Overharvesting Timothy Bonebrake – February 28 2014 I. Defining Overharvesting/ Overexploitation 1. Overharvesting is the “take” of organisms in a population that exceeds natural replacement rates. 2. Broadly refers to resource extraction of many sorts including hunting, fishing, logging and the collection of wild material for medicine, pets, etc. 3. Maximum sustainable yield (MSY) is the maximum number of organisms that can be sustainably removed from the population without dooming the population to extinction. 4. Some prefer overexploitation and feel that overharvesting refers only to organisms taken which humans have grown (but mostly they are used interchangeably – and that’s how I use them!). II. Overharvesting through human history 1. Overharvesting is not a new phenomenon to humans! 2. Prehistoric extinctions are believed to have been caused by humans. a. During the Quaternary many large mammals went extinct – in large part (probably) due to human hunting (but also climate change). b. It took less than 100 years to wipe out nine species of moa in New Zealnd through hunting and, subsequently, the large eagle that relied on moas went extinct by 1400. 3. Recent historic extinctions (especially on islands) are known to have been caused by overharvesting. a. Tortoises were exploited heavily by sailors and travelers as were dodos and other island fauna. b. Bison, once very common in the tens of millions on the North American plains was hunted to near extinction in the 19th century. III. Current overharvesting threats to biodiversity: tropical forests 1. Timber extraction from tropical forests accounts for 25% of wood production globally and catalyzes landscape transition to agriculture. 2. Selective logging can be widespread in spatial extent and not only threaten the target species (e.g. mahogany) but can cause habitat degradation. 3. Non-timber forest products are resources extracted from forests that do not involve tree harvesting. a. If done properly this could be an argument for NOT undertaking deforestation (i.e. an ecosystem service). b. If done in excess it can lead to endangerment (e.g. the Brazil nut, rosewood tree). 4. Hunting of vertebrates (bushmeat) and accidental mortality by humans threatens 19% of all tropical forest vertebrate species (for which we have data on decline cause). a. Species hunted for a variety of purposes including food, trophies, pets and medicine. b. Hunting can be for subsistence purposes, especially as human populations increase – but this can also be destructive especially in combination with other threats (e.g. habitat destruction – think tapirs!). c. Consequences of bushmeat also include disease contraction. d. Example: The golden coin turtle in southern China (and Hong Kong) is critically endangered due to harvesting for food and medicine. e. Bushmeat and hunting products can be highly lucrative on the international market. China (via Hong Kong) has high demand and therefore drives much of the international wildlife trade. f. Example: In Ghana it was found that when fish stocks were down bushmeat consumption went up, in other words, it is not only the local demands that drive hunting but global factors as well. III. Current overharvesting threats to biodiversity: aquatic ecosystems 1. Fish provide nearly half the world population with 20% of per capita protein intake (everybody eats fish!). 2. Fish stocks are declining globally and nearly 25% of fisheries worldwide have collapsed. 3. Fishing down the food chain has also become prevalent – large predatory fish become scarce and fishermen then must rely on smaller pelagic fish. 4. Incidental overharvesting (bycatch, blast fishing) can imperil many species as well. III. Cascade effects of overharvesting 1. The “empty forest” syndrome can have big ecosystem consequences. a. The loss of beavers, due to hunting for pelts, in North America had big hydrologic impacts – beavers are “ecosystem engineers”. b. Loss of large mammals can have downstream effects through “trophic cascades” (remember the wolves of Yellowstone?). 2. The “empty sea” can also occur as best exemplified by the loss of sea otters in Alaska after extensive trapping which caused decreases in kelp abundance due to the release of sea urchins. 3. So what are the consequences of shark fin soup consumption then? .