Major Threats

Threats to freshwater systems and species are numerous, overlapping, and operate over a range of scales. The embeddedness of freshwaters within the larger landscape, coupled with the fact that human communities require freshwater resources to survive, means that few freshwaters around the world remain pristine. Most freshwaters are subject to multiple anthropogenic stresses, and this multiplicity can complicate the identification of threat pathways and appropriate conservation levers. Threats can be variously classified, but here we recognize habitat degradation, water pollution, flow modification, species invasion, overexploitation, and climate change as major, often overlapping categories. These threats can be further described in terms of their origins (Table 1).

Habitat degradation encompasses habitat alteration, outright habitat destruction, and loss of access due to fragmentation, all of which are described briefly here. Virtually any modification to natural land cover within a catchment has the potential to alter downstream freshwater habitats, including floodplains. Land cover conversion for agriculture, urbanization, forestry, road-building, or other activities can result in changes in flow, sediment regimes, riparian and aquatic vegetation, water chemistry, and other parameters that together define freshwater habitats (Figure 1). Direct modifications like streambank mining may also make freshwaters inhospitable for some native species

Table 1 Major threats to freshwater species and habitats

Major threats to

Description

Origin

freshwater ecosystems

Local

Catchment

Extra-catchment

Habitat degradation

Degradation and loss

X

X

Fragmentation by dams and inhospitable habitat

X

segments

Flow modification

Alteration by dams

X

X

Alteration by land-use change

X

Alteration by water abstraction

X

X

Overexploitation

Commercial, subsistence, recreational, poaching

X

X

Water pollution

Agricultural runoff (nutrients, sediments, pesticides)

X

Toxic chemicals including metals, organic compounds,

X

X

endocrine disruptors

Acidification due to atmospheric deposition and mining

X

Species invasion

Altered species interactions and habitat conditions

X

X

resulting from accidental and purposeful introductions

Climate change

Results in changes to hydrologic cycle and adjacent

X

vegetation, affects species ranges and system

productivity

Note that, in nearly all cases where both local and catchment origins are listed, local stresses are transferred downstream to become catchment Impacts elsewhere. Introduced species originate outside a catchment but introductions occur at individual locations and can spread both up- and downstream. Modified with permission from Abell R, Allan JD, and Lehner B (2007) Unlocking the potential of protected areas for freshwaters. Biological Conservation 134: 48-63, with permission from Elsevier; major categories from Dudgeon D, Arthington AH, Gessner MO, Kawabata Z, Knowler DJ, L^veeque C, Naiman RJ, Prieur-Richard A, Soto D, Stiassny MLJ, and Sullivan CA (2006) Freshwater biodiversity: Importance, threats, status and conservation challenges. Biological Reviews 81: 163-182.

Natural stressors/geographic setting (climate, geology, latitude, etc.)

Stream I channel - modification

::::

Urbanization/

residential development

Forest practices

-z-z

Agriculture

Mir

ing

-----

Recreation and mgmt

Atmospheric deposition

-

Human activities 1

; Dams channelization diversions levees revetments

::::

Increasing population roads construction point sources wastewater pets

:::::

Fragmentation fertilizers pesticides roads monoculture compaction sedimentation

::::

Fertilizers livestock pesticides habitat alt. irrigation compaction animal waste

:::::

Habitat alt. toxic waste oil gravel extraction heavy metals liming

::::

Roads construction habitat alt. boating fishing fish intro., poisoning

NOx SOx air toxics liming

-

Figure 1 Threats and threat pathways in freshwater ecosystems. Modified from Bryce SA, Larsen DP, Hughes RM, and Kaufmann PR (1999) Assessing relative risks to aquatic ecosystems: A mid-Appalachian case study. Journal of the American Water Resources Association 35: 23-36, with permission from Blackwell.

Figure 1 Threats and threat pathways in freshwater ecosystems. Modified from Bryce SA, Larsen DP, Hughes RM, and Kaufmann PR (1999) Assessing relative risks to aquatic ecosystems: A mid-Appalachian case study. Journal of the American Water Resources Association 35: 23-36, with permission from Blackwell.

without destroying habitats entirely. Habitat loss can take a variety of forms, such as through wetland draining, dewatering of a river system, disconnecting a river from its floodplain, or conversion of lotic to lentic habitat through reservoir construction. Freshwater species may lose access to habitat when their dispersal or migratory routes are impeded, either by constructed barriers like dams or virtual barriers like highly degraded, and therefore impassable, river reaches. Assessments of the extent of habitat alteration, loss, and fragmentation are notoriously difficult to undertake at broad scales (Table 2).

Water pollution is related to habitat degradation and is typically classified as either point or nonpoint source. Point source pollution can be traced to an identifiable, single source like a pipe draining directly into a freshwater. Nonpoint source pollution, like runoff containing fertilizers from agricultural activities or oil from urban centers, comes from multiple diffuse sources and can be far more difficult to mitigate. Acid deposition and other toxic substances transported by air from outside a drainage basin are a special kind of nonpoint source pollution. Many pollutants are chemicals, such as pesticides and endocrine disruptors, but sediments, nutrients, and other 'natural' materials can also act as pollutants when present at abnormal levels. Even temperature can serve as a pollutant, such as when discharge from a power plant is hotter than normal river water or that from a deep reservoir is colder.

Like habitat degradation, flow modification can also result from either landscape activities or direct modifications to freshwaters, and often both simultaneously. Any landscape activity that alters infiltration and associated runoff, or even precipitation in the case of broad-scale climatic impacts, can change a freshwater system's hydrograph or hydroperiod, in the case of flowing and still waters, respectively. The effect of urbanization on reducing infiltration opportunities is well-documented, and soil compaction from activities like forestry can have similar consequences. River impoundments designed for hydropower generation, irrigation, flood control, navigation, or other uses generally alter the timing and volume of flows, as well as sediment and thermal regimes. Water withdrawals as well as returns and interbasin transfers alter flow regimes as well, even if the total volume of water over time may be relatively unchanged. In general, any modification to the natural flow regime, defined by flow magnitude, timing, duration, frequency, and rate of change, has the potential to affect native species adapted to it.

Overexploitation and species invasion can both affect species populations and communities directly. Overexploitation, or the unsustainable removal of individual animals or plants for commercial or other purposes has primarily affected some species of larger fish, some reptiles, as well as mussels and other large macroinvertebrates. Overexploitation has only rarely been implicated as the single cause in the extinction of individual species, but it has likely been a contributing factor in the decline of many. Species invasion, through accidental or intentional introductions of non-native species, including through the opening up of previously inaccessible habitats, has had severe consequences for freshwater species in some instances. Impacts can include direct competition with or predation on native species, hybridization, habitat modification, and the introduction of disease and parasites. Species living in closed habitats like lakes appear to be particularly vulnerable to impacts from species invasion.

Table 2 Alteration of freshwater systems worldwide

Alteration

Pre-1900

1900

1950-1960

1985

1996-1998

Waterways altered for navigation (km)

3125

8750

-

>500000

-

Canals (km)

8750

21 250

-

63125

-

Large reservoirs®

Number

41

581

1105

2768

2836

Volume (km3)

14

533

1686

5879

6385

Large dams (>15 m high)

-

-

5749

-

41 413

Installed hydrocapacity (MW)

-

-

<290000

542000

~660 000

Hydrocapacity under construction (MW)

-

-

-

-

~126000

Water withdrawals (km3/year)

-

578

1984

~3200

~3800

Wetlands drainage (km2)b

-

-

-

160000

-

aLarge reservoirs are those with a total volume of 0.1 km3 or more. This is only a subset of the world's reservoirs.

^Includes available information for drainage of natural bogs and low-lying grasslands as well as disposal of excess water from irrigated fields. There is no comprehensive data for wetland loss for the world.

Reproduced with permission from Revenga C, Brunner J, Henninger N, Kassem K, and Payne R (2000) Pilot analysis of global ecosystems: Freshwater systems. Washington, DC: World Resources Institute.

aLarge reservoirs are those with a total volume of 0.1 km3 or more. This is only a subset of the world's reservoirs.

^Includes available information for drainage of natural bogs and low-lying grasslands as well as disposal of excess water from irrigated fields. There is no comprehensive data for wetland loss for the world.

Reproduced with permission from Revenga C, Brunner J, Henninger N, Kassem K, and Payne R (2000) Pilot analysis of global ecosystems: Freshwater systems. Washington, DC: World Resources Institute.

Climate change is a final major category of threats to freshwaters, overlapping with habitat degradation, flow modification, and species invasion. Changes in global surface temperature and precipitation patterns will translate to changes in water temperature, water quantity, and water quality in the world's rivers, lakes, and other wetlands. Freshwater biodiversity will be affected indirectly through habitat alteration, and directly where species' life histories are tightly adapted to particular temperature or flow regimes. Dispersal opportunities to more hospitable habitats may be highly limited, especially in systems already fragmented or otherwise modified.

Project Earth Conservation

Project Earth Conservation

Get All The Support And Guidance You Need To Be A Success At Helping Save The Earth. This Book Is One Of The Most Valuable Resources In The World When It Comes To How To Recycle to Create a Better Future for Our Children.

Get My Free Ebook


Post a comment