There’s enough on this planet for everyone’s needs, but not enough for everyone’s greed...

There’s enough on this planet for everyone’s needs, but not enough for

everyone’s greedGandhi

Overexploitation

• Exploitation involves living off the land or sea, using plant and animal products for food, medicine, shelter, fiber, and other products

• It is the second most important threat to the survival of birds, plants, and mammals

• It is also the 3rd most important threat to freshwater fish extinctions

History of Overexploitation

• Humans have always been ‘extractive’ by nature and ‘primitive’ societies still largely follow this MO

• While much of this appears sustainable, archaeological and paleontological evidence suggests premodern people have been driving species to extinction for 1,000’s of years


• Human colonization onto uninhabited islands or continents has typically been followed by mass extinction events of large-bodied vertebrates (e.g. Europe, parts of Asia, N and S Am, Madagascar and many Pacific islands

• We have awesome potential to kill (e.g. Passenger Pigeon, bison, and the Pau-Brasil tree)


• Exploitation of plants and animals is a large interest not just in developing countries, but everywhere

• In the US, it is estimated hunting generates 700K jobs and has an economic impact of $61B (1%)


• Impacts of Exploitation• Most activities are directed at a

single target species

Exploitation of Target Species

• Tropical Terrestrial Ecosystems• Timber extraction is a major threat• Approximately 5.8M ha of tropical

forests logged each year (25% world)• Single species approaches (e.g.

mahogany) are difficult to compare to other methods, but it is better than clear cutting


• A discrepancy between high inflation and slow growth rate of tree value creates the situation where cutting all trees irrespective of age is the best economic plan


• Subsistence Hunting has been around for more than 100K yrs, but consumption has greatly increased

• Impact varies tremendously across the tropics

• E.g. Sarawak (25K ton), Amazon (74-181K ton), C Af (1-3.7M ton)

• Much of this is too high (e.g. C Af is 6x higher than sustainable rates)


• Hunting rates are unsustainably high across the Congo (solid) and Amazon (open).

Extraction rate

SustainableRate (20%)


• Subsistence game hunting affects the structure of tropical forest mammal assemblages, as well as other groups through potential cascading effects


• Nontimber forest products (NTFP)• E.g. fruits, nuts, oil seeds, latexes, resins,

gums, medicinal plants, spices, dyes, fibers, and may others

• The ethnobotanical studies have catalogued the wide variety of plants used by different groups (e.g. India, 6Kof 16K angiosperms used for traditional medicines, 79% of trees in 1ha of Amazon utilized)


• The traditional view of NTFP is usually assumed to be sustainable and is viewed as a promising alternative to exploitive practices and/or landclearing

• Extractive reserves are one of the of the fastest growing categories of protected areas in tropical forests

• The sustainability of these areas is not fully understood at this time


• However, in tropical rural areas, the combined value of consumption and sale of forest goods may not make them sustainable over longer time periods

• A boom in homeopathic remedies has resulted in >150 sp of European plants becoming endangered and extirpation of many local populations


• Another potential problem is the impact of harvesting on demographic processes and structure

• Overharvested populations will succumb to demographic collapse


• Forestry• Only 22% of the world’s original

forest remain in large, relatively natural ecosystems

• Most are either boreal (48%) or tropical (44%), with only a fraction of temperate forests remaining (3%)

• Many are currently threatened from increased pressure from logging


• When commercial forestry expands into previously remote, roadless areas, it typically results in high levels of fragmentation of remaining stands

• A study of postlogging silviculture on wildlife suggests loss of structure (i.e. snags, woody debris) is particularly important to many species


• In Fennoscandia, 50% of the red-listed species are threatened because of forestry

• In WA, actively managed forests could support 100% of biodiversity whereas timber management on a 50-yr rotation at the landscape level could support a maximum of 87%

• Why? Largely structural simplification


• Hunting of large mammals, small game, and waterfowl are also major target species in temperate regions

• Actively managing to increase game animals has proved wildly successful for many species (e.g. deer 500K to 30M)

• In TX populations are >3M with 0.5M harvested annually


• Waterfowl provide another resource to manage at high levels

• In 2001, approximately 19.4M birds taken involving 1.6M hunters

• While this may provide a skewed view of resources, their habitats frequently aid other species of concern (true of waterfowl, probably not for deer)

• Alternative source ($) for landowners


• Aquatic systems, especially marine fisheries, have been well monitored

• Since the 1990’s global catches have leveled off for the first time in history, despite better technology being constantly developed and utilized


• Trends have leveled off (and fallen) with aquaculture increasing in importance


• Wild stocks continue to decline due to poor management

• E.g. A recent survey of 232 stocks showed 83% over the past 25yrs are on the decline

• E.g. Canadian cod 99.9% since 1960’s• By the way, are fish farms a good

alternative to wild caught? Are they more productive?


• There are many species that are targets in the oceans

• There are several traits that many vulnerable species in common (easy to capture and biologically least productive)

• E.g. occupy shallow waters, form dense shoals in predictable locations, commercially valuable


• Chinese bahaba is a fish that meets this criteria (frequent byproduct of fishing)

• Its bladder is popular for medicinal properties (7x gold)

• Relatively late maturity


• Some of these same traits apply to other groups that make species vulnerable

• E.g. abalone occur in shallow waters and species have been harvested from most valuable to least…densities have changed from 1-5K/acre to <1/acre


• Many freshwater taxa are subject to exploitation for food and other reasons

• In 2000, estimated 8.8M tons of inland fish caught (22.4M ton aquaculture)

• Economics: in US, estimated 35M spent $38B spent fishing/yr

• Europe, estimate 21.3M anglers


• Populations of 4/7 sp of salmon (Onchorhyncus) listed as endangered

• Problem?: • Problem: this may be the most visible

fish species in the world• While wild populations are

supplemented with hatchery fish, they quickly show divergence (genetically, phenotypically) from wild populations

Overfishing and dams


• Other species are also exploited• During 19th century, M’s of freshwater

mussels were collected• However, despite brief respite,

currently M’s of kg are exported to Asia

• Currently, pollution, dams, dredging, siltation and invasives

• In US, currently 72% (of 297) considered endangered with 27 extinct

Impacts of Exploitation on Non-Target Species

• There are direct effects of exploitation, but there are also indirect effects

• In addition, there may be damage to the environment, causing changes in ‘biological relationships’ or in landscapes or ecosystem functions


• Even logging directed at a single species can ‘puncture’ a hole in the forest

• Even highly selective logging operations with modest levels of incidental damage to nontarget trees can generate enough structural disturbance by increasing desiccation and dry fuel loads

• Furthermore, mechanized logging itself triggers surface fires


• Can you envision a scenario when loss of large-bodied frugivores may impact forest dynamics?

• Seedling recruitment is not clear under different levels of hunting pressure… however, in some heavily hunted forest dispersal can decrease by 60%

90% in some forests


• In temperate regions, loss of large mammals can have deleterious effects

• Beavers are ecosystem engineers can dramatically alter the landscape

• Recently, they are making a comeback• Loss of Grizzly bears had cascading

effects…increasing elk pop(s) and moose, decreasing riparian habitat, reducing songbird populations


• When predators are excluded, some herbivore populations can get extremely abundant, altering plant communities (loss of 95% of a rare orchid in WV due to excessive deer browsing)


• Marine fisheries are estimated to have a global by-catch of roughly 27M tons/yr

• How? Trawl fisheries, drift nets and gill nets

• Shrimp trawlers (with relatively small mesh size) are estimate to discard 5kg of by-catch for 1kg of shrimp


• Is this just impacting fish?• 19/21 Albatrosses (attempting to eat

baited fish)• Can they really be that impactful?• Each boat may set 1,000’s of hooks/day

with a total of >250M hooks/yr• It is estimated that 20K sea turtles are

caught in the Mediterranean/yr on hooks• Up to 120K sea snakes taken in Aust/yr


• We were talking about the impact of exploitation on non-target species (e.g. by-catch, albatrosses)


• Unfortunately there are many freshwater examples of problems

• No individuals caught since 2001

• May reach 300 kg• Only spawn in

specific rapids


• Indirect freshwater effects• Salmon bring a huge nutrient load

into freshwater systems as they return to spawn and die (8x inc in macroinverts)

• Early evidence for a correlation between size of salmon runs from 1924 to 1994 and tree-ring growth rates

Biological Theory to Sustainable Exploitation

• Density dependent factors impacting the natural rate of increase


• Why should we think populations to be able to withstand the elevated mortality that occurs with most forms of exploitation?

• There may be compensatory birth and growth rates to somewhat offset elevated death rates


• If a large number of individuals are going to die from one time to the next, is ‘thinning’ the equivalent?

• An easy way to approach the question of elevated mortality is to start with the logistic model, which gives the number of individuals at time t as:

Nt = Nmax / 1 + ((Nmax/N0 ) – 1)e-rt


• Where Nmax is the carrying capacity (which is not stable)

• Sometimes biomass is substituted for population size (B for N). Why?

Nt = Nmax / 1 + ((Nmax/N0 ) – 1)e-rt


• Why should we think populations to be able to withstand the elevated mortality that occurs with most forms of exploitation?

• There may be compensatory birth and growth rates to somewhat offset elevated death rates


• Classic logistic population growth to a maximum pop(n) size


• The growth of this classic population is

• What is the relationship N & Nmax ?

• Consider if the pop(n) is being exploited at a steady rate, then its rate of change per unit time (dN/dt) will be the difference between its surplus production and the yield (Y)

g(N) = rN (1 – N/Nmax)

dN/dt = rN (1 – N/Nmax) - Y


• Sustainable yield occurs at 50% of maximum pop(n) size…why?


• Stabilities of exploitation• In theory, we have discovered where

the MSY occurs• Problem?


• In terms of simple population dynamics, it is the ‘surplus’ that can be removed in any given year

• Depending upon population size, that will vary greatly


• MSY occurs at intermediate pop(n) size


• Stability of Exploitation• There is a theoretical number of

individuals taken…any problems?• Lots…

– People rarely do as they are told– Conditions do not always favor optimal

growth


• Constant Quotas• Constant quotas are determined when

the number of individuals removed is independent of the population size (another way to think…subtract Y from the surplus yield, rather than making Y proportion to N)

• This has been used to set many fishery quotas (e.g. Peruvian anchovies)

• Is this a good approach?


• Consider 3 levels of constant quota harvest (no relationship to N)

High Quota …will always crashMSY Quota …even when above NMSY, pop(n) will declineFinally, even Low Quota will decline if initial pop(n) is low


• Proportional exploitation (constant effort)• This approach ties ‘takes’ to pop(n) size• As long as the exploitation rate is below

the intrinsic rate of natural increase, r, then all equilibria are stable

• Has been shown to be important in tracking actual ‘take’ rates and the importance of monitoring pop(n) sizes


• Consider the size of the yield (Y), the exploitation rate (E) and the population size.

Y = EN (proportional exploitation)

Substituting… rN (1 – N/Nmax) = ENAnd as long as E<r then

N = Nmax (1- E/r)

g(N) = rN (1 – N/Nmax)


• As long as E < r, all equilibria are stable


• How do the two approaches compare?

• Examine data from Marten harvests

• Both approaches do relatively well at low rates


• Threshold Exploitation involves the use of pop(n) size thresholds to determine not only the rate of exploitation but also whether exploitation should take place at all

• Since all pop(s) fluctuate, it is this excess that will be harvested

• If a surplus does not exist, no take will be made (i.e. very safe approach)

Comparison of Approaches to Sustainable Yields

• There are many approaches to determining what approach should be established for sustainable yields

• Various strengths and weaknesses (think of a demographic model with all the parameters)


• Surplus Production models are also called surplus yield models or “production” models

• Frequently used in fisheries• If you know how yields have

responded to different levels of exploitation effort over time, then you could estimate the dome-shaped yield curve (thus not requiring new data)


• There are many pitfalls with this approach including it treats all years as replicates– Consider Cod

• The largest fisheries collapse ever (Peruvian anchovy) in the early 1970’s when it adopted a surplus production model that suggested levels were safe


• Yield-per-recruit models were developed originally as part of the “dynamic pool” concept in the landmark fisheries book written by Beverton and Hold (1957)

• The dynamic pool attempts to keep track of separate processes that add to a population (e.g. recruitment and growth) or subtract from it (e.g. natural and catch mortality)


• Imagine a species that becomes more valuable as it ages

• If exploitation is weak, most of the fish or trees will be large when taken

• If you wait too long, natural death occurs

• This model searches for the level of mortality that maximizes the yield under this tradeoff between numbers and value


• Once the level of mortality has been found that maximizes the yield per recruit, we can calculate the total yield that will be obtained from a given level of mortality, if we know how many recruits are coming each year

• This approach is used in many countries• Does require info about how the value (often

size) of individuals increases with age, as well as an estimate of natural mortality rates


• For some species ($ or conservation) there is enough data for full-blown population models

• These models combine data on all demographic parameters (survival, age at maturity, age-specific mortality)


• Assessment of population growth rates is a common practice to deduce whether populations are declining as a result of direct exploitation or exploitation of their habitats

• Another use is to diagnose the reasons for population declines and to suggest the likelihood of success of various management strategies (e.g. loggerhead turtles, eggs vs. older adults)


• Projected annual strandings for Kemp’s ridley seas turtles under levels of compliance by shrimpers (current to full compliance)


• Adjustments based on recent results• If we can monitor either the

population itself or the numbers of individuals taken, we can adjust the quotas each yr based upon new information

• This is currently the approach for waterfowl and mammal hunting in the US (read Box 8.1 on martens)


• Demographic rule of thumb• Parameter-dependent models are useless

for the vast majority of the world’s exploited species…why?

• One of the best-known rules of thumb models to determine if exploitation of tropical mammals can be sustainable

• It assumes maximum potential production (Pmax) occurs at about 60% of K

Pmax = (0.6D x λmax) – 0.6D


• Of course there are many potential problems with this simplistic model

• There are problems with cull rates compared to mortality rates…why?

• Are density estimates accurate? Why?

• Estimating λmax is not easy and variable


• There are spatial and temporal problems of establishing accurate production numbers

Overexploitation

There’s enough on this planet for everyone’s needs, but not enough for everyone’s greed...

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