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Here is a term paper on the ‘Impacts of Climate Change on Aquatic Biodiversity’ especially written for school and college students.
Climate change is considered to be one of the principal threats to biodiversity and to the structure and functioning of ecosystems. Although the causes and likely impacts are subject to debate, the scientific consensus is that climate change is a hard reality. Average global temperatures have increased by ~0.6°C over the past century. During this period, both marine and freshwater systems have warmed.
Over geological time, climate has varied, influencing the distribution and suitability of habitats, which in turn, have influenced the distribution and dispersal of species. It is therefore, realistic to expect that further climate change will have a strong controlling effect on habitats, communities, species and individual organisms in the future.
Term Paper # 1. Climate Change and Lakes:
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Lake primary productivity is closely linked to mean air temperature and the length of the growing season and fish production is positively correlated with mean annual air temperature. In the North American Great Lakes, Meisner et al (1987) suggested that an increase in mean air temperature by two per cent could lead to an increase in fisheries yield of ca. 25 per cent.
An increase in potential fish production following climate change may be limited by a greater probability of hypolimnetic oxygen depletion in productive lakes. The timing and intensity of lake stratification is likely to change, with implications for lake fishes, their parasites and their prey. Recent modelling studies suggest that the negative effects of nutrient enrichment on lake algal dynamics may become increasingly problematic as temperatures increase.
Fishes found in shallow habitats or habitats with restricted water exchange, e.g. shallow lakes and ponds, will be affected by increased water temperatures following climate warming and in extreme cases, loss of habitat, or death if these systems dry out Some systems may become ephemeral following future climate change and become fish-free or only partly utilised by fish. Increased lake levels following winter precipitation will improve access to additional spawning or feeding habitats for some species e.g. pike (Esox lucius).
Term Paper # 2. Climate Change and Rivers:
Apart from increases in water temperature, climate change is likely to impact riverine systems following shifts in precipitation patterns (e.g. increases in winter, reductions in summer, including an increased probability of extreme events. Residence times, import and export of organic matter, dilution of pollutants, primary production and dissolved oxygen concentrations are all likely to be altered.
Riverine fishes display a complex array of environmental requirements and major changes in seasonal flow patterns are likely to have significant consequences. Migratory species have evolved to utilise predictable floods for migrations and changes in the frequency or intensity of floods may impact the ability of adult fishes to successfully reach spawning areas.
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The character of running waters depend fundamentally on changing patterns of rainfall or melting snow, while floods or droughts can have major effects on the ecology of these lotic ecosystems. Their small size means limited capacity to absorb atmospheric heat, so that stream temperatures often correspond closely to changing air temperature. Warming effects could be of major significance in such habitats where most animal species are cold-blooded invertebrates, fishes or amphibians.
Climate change scenarios predict significant increases in extreme precipitation events, where flood intolerant species or sensitive life stages, e.g. eggs or larvae, could become displaced or killed. However, in some river systems fishes have proved to be remarkably resilient to flooding and increased winter flooding may prove beneficial to certain species, providing additional feeding or spawning opportunities.
If hydrological regimes shift, e.g. reduced surface or groundwater flows during periods of drought, hydrological marginal habitats such as floodplains or wetlands may become disconnected from the main river channel, with subsequent impacts on habitat availability for fish and their production and diversity. Reductions in stream flow during warm periods may lead to increased stream temperatures, decreased concentrations of dissolved oxygen and reduced dilution of pollutants.
Term Paper # 3. Climate Change and Estuaries:
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Estuaries represent the interface between marine, freshwater and terrestrial environments and are extremely complex ecosystems where salinity, temperature and oxygen fluctuate according to tidal amplitude and season. Estuarine communities are often structured according to salinity resistance and are well adapted to fluctuations in salinity, temperature and oxygen.
Estuarine habitats are likely to experience very different hydrological regimes under future climate conditions and the effects of climate change will potentially be complex. Decreased summer precipitation will affect freshwater inputs, which will increase residence times and the time taken to flush nutrients and pollutants from the system and lead to increased intrusion by saline waters.
Predicted increase in sea levels will also lead to shifts in salinity profiles, which are likely to result in changes in estuarine fish community structure towards salinity-tolerant estuarine fishes or those typical of fully marine habitats. Although there is considerable variation in nutrient load between regions, some estuaries in Britain and Ireland have undergone eutrophication. The risk and frequency of estuarine algal blooms may increase in nutrient-rich estuaries following climate change.
Reduced freshwater inputs during hot dry summer months could increase residence times and reduce the dilution of dissolved nutrients. This, combined with increased summer temperatures might lead to increased phytoplankton production and the risk of low oxygen conditions. Predicted increase in winter precipitation will result in greater run-off to surface waters, including estuarine waters.
Increased freshwater discharge during winter will result in an overall decrease in salinity and shifts in estuarine salinity gradients. Residence time will fall and nutrients, contaminants, and organic material will be transferred to coastal waters more rapidly, potentially reducing the productivity of estuarine habitats.
Term Paper # 4. Climate Change and Marine and Coastal Habitats:
The impacts of climate change on marine ecosystems extend beyond increased water temperature and include changes in oceanic circulation, sea level rise, increasing frequency of storm surges, changes in chemistry including acidification and nutrient availability. The likely ecological consequences of these changes to marine ecosystems are understandably diverse, but include changes in the phenology of species that form the base of marine food webs, e.g. phytoplankton and zooplankton, with clear implications for fishes and other taxa.
If changes in the biotic (e.g. seasonal availability of food) and abiotic (e.g. water temperature, salinity, circulation) environments of marine fishes are significant, it is likely that interactions between individuals and species will be modified, impacting population and community dynamics and leading to shifts in the structure of marine fish assemblages. Harley et al. (2006) suggested that changes in the chemistry of marine waters may be more important than changes in temperature.
For instance, the oceans have absorbed large volumes of CO2 which has led to significant acidification of seawaters. If global emissions of CO2 continue, it is feared that the average pH of the oceans could foil by 0.5 pH units (equivalent to a threefold increase in H+ ions) by 2100. Although the impact of such acidification is likely to be less extreme in the temperate seas than in tropical or southern seas, it has clear potential to impact ecologically important calciferous organisms, such as molluscs, cold-water corals, echinoderms, foraminifera and coccolithophores.
Increased concentrations of dissolved CO2 also have the potential to impact the physiology and reproductive success of water breathing organisms including larger invertebrates and fishes. Increases in sea level due to thermal expansion of seawater and the melting of polar ice may reduce the area of inter-tidal habitats as coastal waters encroach especially if coastal defences are present.
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Sea surface warming has been associated with increased and decreased phytoplankton abundance in cooler and warmer regions, respectively. Through bottom-up effects, this impact can propagate up food webs through herbivorous and carnivorous zooplankton. That this will have an effect on higher trophic levels seems inevitable and it is likely that fish and other top predators will have to adapt to changing spatial distribution of primary and secondary productivity within marine pelagic ecosystems following climate change.
Results from continuous plankton recorder surveys have shown that south of 59°N in the northeast Atlantic, (e.g. in the seas around Britain and Ireland) phytoplankton has shown a significant response to climate change, with increased abundance and a marked extension of the growing season. The timing (phenology) of major oceanic trophic events such as spring blooms, seasonal peaks in zooplankton abundance and the timing of hatching of fish eggs can be of central importance to fish stocks.
Variation in pelagic food webs can be driven by fluctuations in plankton production and effects of climate change on plankton dynamics are transmitted to upper trophic levels (e.g. fishes). Temperate marine environments may be particularly vulnerable to changes in phenology because the level of response to climate change may vary across functional groups and trophic levels. This is important because recruitment success of higher trophic levels is highly dependent on synchronisation with pulsed planktonic production.
The copepod Calanus finmarchicus is of key trophic importance in the northeast Atlantic, but is in pronounced decline. It is being gradually replaced by its warm- temperate congener C. helgolandicus with some negative impacts on fish recruitment in species including cod. In the North Sea, C. finmarchicus has shown a rapid and almost complete collapse, with a twenty-fold decrease recorded in the northern North Sea between 1958 and 1998 and an increasing overall prevalence of temperate Atlantic and neritic (shallow-water) taxa.
Atlantic inflow into the North Sea is increasingly thought to be the main regulator of long-term abundance of C. finmarchicus in the North Sea. Unlike temperate Atlantic taxa such as C. helgolandicus, C. finmarchicus cannot overwinter in large numbers in the North Sea because it is too shallow and cold and must therefore, migrate to deeper over-wintering areas (e.g. the Faroe-Shetland Channel).
Rising temperatures would result in increased winter survival of temperate and neritic species in the North Sea. However, two ecological features clearly differentiate these two species of calanoid copepod: their temperature preferences and over wintering strategy.
In the north Atlantic phytoplankton and zooplankton species plus communities have been associated with Northern Hemisphere Temperature (NHT) trends and variations in the North Atlantic Oscillation (NAO) index. These have included changes in species distribution and abundance, the occurrence of sub-tropical species in temperate waters, changes in overall phytoplankton biomass and season length, changes in the North Sea ecosystem, community shifts, phenological changes and changes in species interaction.
Over the last decade numerous other investigations have established links between the NAO and the biology of the north Atlantic including the benthos, fish, seabirds and whales. While the NAO index integrates variability in many hydro-climatic parameters, it is assumed that the increase in temperature over tike last decade has had a primary role in influencing the ecology of the north Atlantic.
Indirectly the progressive freshening of the Labrador Sea region, attributed to climate warming and the increase in freshwater input to the ocean from melting ice, has resulted in increasing abundance, blooms and shifts in seasonal cycles of dino-flagellates due to the increased stability of the water-column.
Sex determination in an atherinid fish, the Atlantic silverside (Menidia menidia), is under the control of both genotype and temperature during a specific period of larval development. The sex ratios of the progeny of different females are variable and differ in their responsiveness to temperature. This demonstrates that sex ratio in fishes that normally have separate sexes can be influenced by the environment.
Like the North Atlantic, many long-term biological investigations in the Pacific have established links between changes in the biology and regional climate oscillations such as the El Nino Southern Oscillation (ENSO) and the Pacific Decadal Oscillation (PDO). In the case of the Pacific these biological changes are most strongly associated with El Nino events which can cause rapid and sometimes dramatic responses to the short-term Sea Surface Temperature (SST) changes accompanying El Niño events.
Changes in mesozooplankton abundance have also been related to large-scale climate influences in the Californian upwelling system. In the Southern Ocean the long-term decline in krill stock has been linked to changes in the extent of winter ice, which has been related to warming temperatures.
Term Paper # 5. Climate Change and Sea Grass Habitat:
There are about 60 species of sea grass or eel-grass, the only known flowering plant to live permanently submerged in coastal waters in the world, that grow from the mid-intertidal to 50-60m depth. They inhabit all types of substrate but the most extensive sea-grass beds are on soft sediments like sand and mud.
The roots and rhizomes of sea grass stabilise large areas of shallow water sediment and these meadows are highly productive and important nursery grounds for many species including commercially exploited fish and crustaceans. These grasses are the main food for many threatened species like manatees, dugongs and green sea turtles, all threatened species as well as of great public interest. The extent of meadows has been greatly reduced due to such human activities.
Climate change is likely to have significant consequences, not only due to changing temperature but also to changing sea levels with effects on coastal hydrography and increasing frequency of damaging storms. While the changes may make new areas available for colonisation by sea grasses and improve conditions for them in others, in the short-term, the overall effects of climate change are likely to be negative.
Kelp forests are marine ecosystems dominated by large brown algae (Phaeophyta), which are among the most biologically productive habitats in the marine environment are thickest in summer, declining during winter storms and high surf, though some plants may survive for up to three years. These forests provide a habitat utilized by many species, including marine mammals, fish, other algae and a vast numbers of invertebrates.
Many of the species characteristic of the forests is highly adapted to the conditions provided by the kelps, the keystone species of these ecosystems. Climate change may influence kelp forests through the direct effects of temperature, moving the boundaries of the areas where they have the potential to survive. In addition, nutrient inputs necessary for the kelp to thrive may be altered due to changes in the geographical or seasonal distributions of rainfall and up welling.
In kelp forests of southern California, the proportion of cold water fish such as the green spotted rockfish, have fallen and warm water species like the garibaldi have increased since the 1970s. An increase in the numbers of sea urchins has also contributed to title decline. These are normally passive grazers of algae but graze the kelp when the waters become warmer, as in El Nino events.
In the Northern Hemisphere former kelp forests have become ‘sea urchin barrens’ due to over fishing, removing the predators that would normally control the urchins. Disruption of ecosystems due to climate change may reinforce the imbalances that have resulted in the overgrazing of the kelps.
Term Paper # 6. Climate Change and Intertidal Organisms:
The main impact of fluctuating climate on intertidal ecosystems would be through changes in sea level and temperature. There is considerable evidence over past epochs of changes in sea level; deposits of shells of intertidal molluscs and barnacles are found above and below present sea levels, including the well-known submerged forests and raised beaches. Owing to the high heat capacity of the ocean, sea temperature is a more conservative parameter, so that at low tide intertidal organisms are exposed to the greater fluctuations of air temperature.
Hence, observed changes in intertidal biodiversity can show east-west trends in species range and abundance as well as north-south trends. For example, during rising temperatures in the last two decades, recognised southern species of barnacles and molluscs have moved eastward along the English Channel and from north to south down the east coast of Scotland.
Barnacles, which have planktonic larvae, are known as good indicators of climatic variation as once they have settled they are immobile. Rapid sea level change resulting from the melting of ice caps as well as thermal expansion of seawater may significantly alter habitat distributions due to topographical factors, the changed shapes of the coastline and depth distributions near to the shore, changing the hydrography.
Term Paper # 7. Climate Change and Shifts in the Distribution of Marine Fishes:
There have been numerous historical accounts of apparent distribution shifts of fish in response to climate change, of which the herring/pilchard shift is a particularly clear illustration. It appears that the recent warming of the north Atlantic is responsible for a shift in distribution of some species. Counts of novel immigrant or vagrant species have been positively correlated to the increased water temperatures in the north Atlantic over the last 40 years.
Perry et al (2005) demonstrated that many exploited and unexploited North Sea fishes have apparently demonstrated a marked response to recent increases in sea temperature: nearly two- thirds of species (21/36) shifted mean latitude and/or depth over a 25 year period. Approximately half of the species with a latitudinal boundary of distribution (northerly or southerly) in the North Sea showed a northerly shift in their boundary.
The most significant shift was demonstrated by the blue whiting, whose southern limit moved northwards ca. 820 km in only 25 years. They speculated that if temperatures continue to increase in the North Sea, blue whiting and redfish will probably be lost from the North Sea and bib will extend their range to encompass the whole region.
They also highlighted that species with “faster” life histories, e.g. those with significantly smaller body sizes, faster maturation and small sizes at maturity, tended to shift their distribution and that it was those species that responded most strongly to climate change.
Term Paper # 8. Climate Change and Coral Reefs:
Corals most often exist as colonial organisms composed of thousands of individuals, called polyps. All species of coral secrete calcium carbonate (CaCO3), and the majority of coral species form reef structures over time. Coral reefs harbour more than 25 per cent of all known fish and provide our oceans with the highest biodiversity of any marine ecosystem. Coral reefs are an ecosystem that have the second biggest biodiversity in the world and have been described as the rainforests of the ocean.
For years they have been threatened by man, physical destruction from boats, fishing and pollution. Now their biggest threat may be from climate change. 15 per cent of the world’s reefs have already been lost and 30 per cent may the in the next 30 years. Surface warming and acidification of the oceans adversely affect the health of coral reefs. Surface warming increases the likelihood of coral bleaching (stress- induced expulsion of unicellular algae resulting in the loss of coral color) and, if conditions are warm for long, can cause reef mortality.
Ocean warming is directly reducing coral cover through coral bleaching. Reef- building corals harbours zooxanthellae that live symbiotically within their tissue. Zooxanthellae provide their coral host with food and oxygen and in return, the zooxanthellae receive nutrients, carbon dioxide, and an enemy-free shelter. This symbiotic relationship evolved tens of millions of years ago and has been critical to the success and evolutionary radiation of corals and to the development of reef ecosystems.
When summer time water temperatures are just a degree or two warmer than usual for a few weeks, this critical yet delicate symbiotic relationship breaks down and the zooxanthellae are expelled, often leading to the coral’s death. The greater the magnitude or duration of the warming leads the greater the mortality and effect on coral populations. The phenomenon is called “coral bleaching” because the coral animal appears to turn white after the zooxanthellae loss.
This is because without their zooxanthellae symbionts, which contain various photosynthetic pigments, corals are nearly transparent and the white, external calcium carbonate skeleton that the coral polyps live on becomes plainly visible. It is also stated that corals could become rare on tropical and sub-tropical reefs by 2050 due to the combined effects of increasing dissolved carbon dioxide (CO2) and increasing frequency of bleaching events. Since the impacts of increased CO2 are greater at higher latitudes, cold-water corals are likely to show large reductions in geographical range this century.
The increase in ocean temperature is variable and quite subtle: on the order of 1°C over the last several decades. But even such modest changes have caused mass coral mortality events around the world during some of the especially warm summers we have all experienced over the last ten years. In 1998, when an intense El Nino greatly warmed much of the western Pacific and Indian Oceans, coral bleaching was widespread, causing mass coral mortality in many countries.
For example, in Palau, more than 90 per cent of the corals on some reefs bleached and at least 50 per cent perished. Even some isolated reefs were impacted. In the Maldives, in the East Indian Ocean, bleaching caused coral cover to plummet to only about 5 per cent.
Undersea surveys made by the scientists of National Coral Reef Research Institute of ZSI, Port Blair during 8th to 15th May 2010 indicated that most of the coral colonies in several islands of Andaman and Nicobar Islands were bleached. The main causative factor for the bleaching event is, the temperature in the Andaman Sea stood at 31-32°C for a long period this year, making the sea warmer than the previous two years.
It is believed that the warmer-than-usual sea temperature is a consequence of the late onset of the monsoon over the Bay of Bengal and Andaman Sea. Such bleaching phenomenon is also reported in Gulf of Thailand, Malaysia and Burma during May 2010.
Ocean warming can also indirectly kill corals by magnifying the effects of infectious diseases, which are one of the primary causes of coral loss, particularly in the Caribbean. The number, prevalence and impacts of diseases of corals and many other types of marine animals have been increasing over the last 20-30 years. The severity of marine diseases could increase with temperature for several reasons.
Because elevated water temperature causes corals physiological stress, it can also compromise their immune system, potentially making them more susceptible to infections. Additionally, increased temperature could also benefit bacterial and fungal pathogens, making them more fit and/or virulent. A recent study found that anomalously high ocean temperatures greatly increased the severity of the coral disease white syndrome on the Great Barrier Reef. Disease outbreaks only occurred on reefs with high coral cover after especially warm years. The disease was largely absent on cooler reefs.
The temperature increases required to trigger a white syndrome outbreaks were relatively modest as most disease outbreaks occurred on reefs where the temperature was only 1-2°C warmer than usual. Other evidence also points to temperature as an important driver of coral epizootics. For example, some coral diseases such as black band disease become more prevalent or spread faster in the summer.
However, not all coral epizootics are caused by anomalously high temperature. Some major outbreaks have occurred during relatively cool periods or years, such as white band disease, which decimated the then-dominant branching corals Acropora palmata and Acropora cervicornis in the Caribbean in the 1980s.
Term Paper # 9. Climate Change and Mangroves:
Mangroves are intertidal communities of primarily tree species of the tropics and sub-tropics. These forests protect coasts against storms, tides and cyclones. Mangroves are important in relation to animal and plant productivity, as nutrient sinks, for substrate stabilisation and as a source of wood products. Warming may promote expansion of mangroves to higher latitudes but they are susceptible to frost and drought.
Many mangrove forests are excessively exploited, thereby reducing resilience in the face of sea-level rises. With a 1m sea level rise in Cuba, more than 300,000 hectares of mangroves would be threatened. The most luxuriant growth of mangroves with the greatest diversity is in the Indo-Pacific region. The Sundarbans, a “World Heritage Site” in India and Bangladesh, account as the largest single mangrove unit globally. The area under Sundarbans was large in the past, but has been reduced to the present level due to degradation that took place during last two hundred years.
Logging operation, aquaculture, reclamation of swamps, paddy cultivation on the east coast of India and salt production on the west coast are the main reasons for degradation, resulting into shrinking of tidal forests throughout the Indian coast. Climatic factors like temperature fluctuation, humidity, precipitation, number of rainy days, regular wind flow, radiation and fresh water flow in the region act as the most significant factors for development and succession of mangroves.
Mangroves in tropical region are extremely sensitive to global warming and depend upon the rate of sea level rise relative to growth rates and sediments supply, space for and obstacle to horizontal migration, changes in climate-ocean environment. Sea level rise will affect mangroves by eliminating or modifying their present habitats and creating new tidally inundated areas to which some mangrove species may shift. Extent of high tidal mudflats constitutes major share of the tidal mudflats, especially in Gujarat.
This will provide great potential to the mangroves of the region for adjustment and adaptation against sea level rise. Climate unsuitability is another factor responsible for mangrove’s change and disappearance. Hardy species like Avicennia sp. recolonised the area fast in the area like the Gulf of Kutch, but this is not true in case of the species of genus Rhizophora, Ceriops, Sonneratia and Aegiceras in the Gulf of Kutch and other regions and Heretia sp. in Sundarbans as they were unable to recover from the effect within short period of time.
Global warming and sea level rise would bring changes in most of the region, resulting in alternation in mangrove setting. In deltaic mangroves, there is consistent seaward movement of the mangroves but this may take a reverse change due to rise in sea level. Topography of deltaic mangroves, especially in Sundarbans is such that landward migration of mangroves, coupled with accretion of sediment might more or less keeps pace with rising sea level.
It is also expected that average global rainfall will increase with marked regional variations. If this happens, climate change is likely to lead to an increase in species migration pole wards. This may result into better environment for mangroves in semi-arid region like Gulf of Kutch. Many species are sensitive to fast changes, especially to anthropogenic disturbance and sea level rise.
If pace of sea level is high, these species may not be successful to compete and may loose in favour of hardy and great coloniser, especially Avicennia marina, A. alba, Acanthus ilicifolius and Suaeda sp. in semi-arid in Gujarat and A. officinalis and other species in the moist region. It is expected that species diversity may suffer in some areas, especially in Andaman and Nicobar Islands.
Most predictions suggest that future rises in relative sea level will be of the order of 100-200cm/100 years. If this projection becomes reality, mangroves of the world may suffer serious loss and majority of the species may fail to adapt new environment.
The extensive mangrove systems of the Sundarbans in the Bay of Bengal are examples of river-dominated systems where relative sea level may raise less owing to the influx of large amount of silt. There is little information on this aspect of mangrove ecology, but evidence suggests that mangroves can cope with accretion rates of the order of 10cm/100 years, albeit with some change in community structure and species composition.
Term Paper # 10. Remarks on the Impacts of Climate Change on Aquatic Biodiversity:
There is a growing scientific consensus that human activities have modified the composition of the atmosphere and that these changes have and will continue to, cause significant shifts in the climate including changes in air and water temperatures, precipitation, solar radiation and wind speed. As might be expected, human-forced climate change has and will continue to affect as humans rely heavily on aquatic systems for many goods and services, e.g. food production, recreation, nutrient recycling and gas regulation.
Impacts of climate change on aquatic systems and their inhabitants are therefore likely to have widespread implications for future human populations around the globe. Existing international agreements and legislation, e.g. the Ramsar convention (UNESCO, 1971) and the International Convention on Biological Diversity (UNCED, 1992) provide a clear obligation for Government and managers to respond to this challenge. In order for Government to react, they rely on scientists to provide them and other interest groups with reliable information regarding the responses of natural systems to climate change.
Harley et al. (2006) noted that ecologists face immense challenges if they are to predict how natural systems will respond to environmental conditions that have no parallel in recent time. The studies on long-term monitoring will provide baseline data against which climate change can be quantified. These data are essential for modelling responses to climate change at the individual, population or community level. However, to date, little work has examined how interspecific interactions might influence the response of aquatic taxa to climate change and there is a pressing need for such studies.
India being largest developing country with nearly two-thirds of the population depending directly on the climate sensitive sectors such as aquatic, agriculture and forest resources. The projected climate change under various scenario is likely to have implications on food production, water supply, biodiversity and livelihoods. Thus, India has a significant stake in scientific advancement as well as an international understanding to promote mitigation and adaptation. This requires improved scientific understanding, capacity building, networking and broad consultation processes.