Agriculture, Ecosystems and Environment, 42 (1992) 165-176 165 Elsevier Science Publishers B.V., Amsterdam

Wetland and Aquatic Habitats

MILDRED E. MATHIAS 1 and PETER MOYLE 2

t Department of Biology, University of California, Los Angeles, CA 90024 (U.S.A.) 2 Department of Wildlife and Fisheries Biology, University of California, Davis, CA 95616 (U.S.A.)

ABSTRACT

Mathias, M.E. and Moyle, P., 1992. Wetland and aquatic habitats. Agric. Ecosystems Environ., 42: 165-176.

Riparian wetland areas often represent critical corridors for animal and plant dispersion in wildland watersheds and downstream river systems. It is essential that integrated management of riparian wetland areas be developed to reverse the loss of biological diversity. Agricultural and urban uses, and related water developments, have led to a marked decline of stream-side wetland habitats. Six major ways are discussed in which conventional agriculture alters wetlands and aquatic habitats: wetland drainage, water diversions, stream channelization, bank stabilization, grazing, and the release of agricultural pollutants. This article discusses these practices and suggests ways biological diversity can be protected, or even enhanced. In addition, aquaculture is discussed as a new force which affects the diversity of aquatic organisms. Aquaculture methods range in intensity of management from low to high. The higher the intensity the potentially more disruptive practices can be to surrounding aquatic systems. Management for biological diversity as well as for food production should be encouraged.

INTRODUCTION

In many parts of the world, wetland and aquatic habitats have been significantly altered, directly or indirectly, by agriculture (Table 1). Between 1955 and 1975 in the United States, for example, almost 80% of the wetlands were converted to agricultural use (United States Office of Technology Assessment, 1983).

TABLE 1

Summary of conversion of wetlands to other uses in the United States 1

Use Area (ha) Freshwater Wetlands

Agriculture 4,745,000 Open water 485,000 Urban 375,000 Other uses 335,000

© 1992 Elsevier Science Publishers B.V. All rights reserved 0167-8809/92/$05.00

166

Total conversions 5,940,000

Saltwater Wetlands Agriculture 3,600 Open Water 108,500 Urban 43,300 Other uses 38,800

Total conversions 194,000

rl'aken from Tiner (1984)

Agricultural practices typically affect wetlands directly on-site. But effects on aquatic systems are usually indirect and may result from distant run-off containing silt, residues from fertilizers and pesticides, or irrigation water from dams constructed many kilometers upstream.

It is estimated that approximately 35% of the rare and endangered animal species are in some way dependent on wetlands, since they are essential nesting, feeding, resting, and wintering habitats for large numbers of bird species. In addition, coastal wetlands are important spawning and nursery areas for over one-half of the commercial marine fishes (Office of Technology Assessment, 1987).

In the dry southwestern United States, for example, a high percentage of rare and endangered species are aquatic. Because these aquatic habitats are often "islands" in the larger "dry" landscape, their isolation has led to accelerated speciation and other genetic responses expected for small, local populations. In California, 65% of 113 native fish taxa are either extinct, officially listed as in danger of extinction, or need special management to keep them from becoming endangered in the near future (Moyle et al., 1989). There is also evidence that a similar status is descriptive of 44% and 57% of the fish species in two river systems in the midwestern United States (Karr et al., 1985).

Farms and ranches often border on, or contain, natural bodies of water including marshes, potholes, vernal pools, lakes, streams, and estuaries. Pond and marsh drainage, water diversion, use of fertilizers and pesticides, and grazing by livestock, are common agricultural practices which modify natural water habitats. In addition,the rural landscapes are often altered to create farm ponds, reservoirs, and irrigation canals. Although farm ponds and reservoirs may provide additional aquatic habitat for many organisms, and the pond borders may provide new areas for wetland vegetation, the biological diversity in these new aquatic habitats is typically lower than that usually found in natural systems.

Riparian wetland areas often represent critical corridors for animal and plant dispersion in wildland watersheds and downstream river systems. Thus,

167

it is essential that integrated management of these areas be developed if loss of biological diversity is to be reversed. For example, some streams support more life because their flows are augmented by irrigation water during times of low natural flow. Wetlands may be created and maintained using agricultural run-off. Selective removal of trees by logging may increase the amount of sunlight reaching a stream and result in increased production of fish. Managing watersheds in their entirety could allow vegetation manage- ment programs, such as logging practices and rangeland prescribed burning programs, to link parcels of forest and range with a network of riparian buffer strips to create a lattice of habitats available to a variety of animal and plant species.

Stream-side settings often have productive soils because of sediment deposited during flooding. Such riparian habitats originally provided a broad spectrum of resources for wildlife. For example, episodic reproduction of stream-side plants creates a patchy mosaic of roosting or nesting sites, variation in trophic resources, thermal or escape cover, and important corridors for migration. Many of these rich alluvial soils have been developed for agricultural production. However, some areas retain natural vegetation harvested only by grazing animals.

Agricultural and urban uses, and related water developments, have led to a marked decline of these stream-side habitats. There are six major ways in which conventional agriculture alters wetlands and aquatic habitats: wetland drainage, water diversions, stream channelization, bank stabilization, grazing, and release of pollutants. This paper discusses these practices and suggests ways biological diversity can be protected, or even enhanced. In addition, aquaculture is discussed as a new force which affects the diversity of aquatic organisms.

WETLAND DRAINAGE

For centuries wetlands have been drained to claim additional land for farming and urban development and to reduce populations of mosquitoes and other pests. There are various drainage methods, including water-table manipulation of essentially flat landscapes, tile drains in humid areas to reduce water-logged conditions, and surface drainage of small areas called potholes. In the last 20 years, these conversions have caused major declines in populations of plants and animals that are dependent on wetlands.

In response to reduced habitat, a system of wetland refuges has been created in North America by groups, both public and private, whose primary objective is the protection of migratory waterfowl. In addition, these

168

organizations have worked with farmers to develop various ways of managing precious wetlands, especially in the drier regions of major flyways and areas where many species of waterfowl breed. Unfortunately, these efforts have been only partially successful because tax laws in most countries generally favor conversion of wetlands to agricultural and other uses.

For instance, in the central valley of the State of California, wintering waterfowl typically "roost" in state and federal refuges, but forage in surrounding agricultural lands. Rice paddies are important feeding areas for ducks and geese but new, more efficient, agricultural practices (laser leveling of fields, planting of short stature rice varieties, burning of fields after harvest, etc.) are leaving less "spilled" rice available for waterfowl.

Altering some of these agricultural practices could tip the balance in favor of waterfowl provided there are economic incentives for farmers to do so. For example, tall-stature rice promotes a higher diversity of aquatic life in rice paddies than does short-stature rice because it creates more open water by shading out aquatic weeds. Use of short-stature rice tends to result in an increase in the use of herbicides to control weeds. Tall- stature rice also provides more food for waterfowl because it cannot be harvested as efficiently as the short stature varieties. This means, of course, that tall-stature rice yields fewer pounds per acre than short-stature rice. Since current production of rice exceeds demand, the creative use of subsidies to United States rice farmers could encourage implementation of agricultural practices favorable to wildlife, provide income from waterfowl hunting, and insure the long-term health of the rice industry.

WATER DIVERSION

In drier areas of the world, major, sometimes long-distance diversions of water have taken place in order to provide water for agricultural use, achieve flood control, and generate hydroelectric power. The biological effects of these diversions have been significant. Dam construction world-wide was moderate through about 1900 with 10 or 20 new projects initiated each year (Pitts, 1984). The rate of construction accelerated through about 1940 with around 100 projects each year from 1920 to 1940. After 1945, there was almost an exponential increase in new projects; there were over 200 dams constructed in North America alone each year between 1962 and 1968. Some have suggested that if all countries were considered, as many as 700 dams were constructed each year during the 1970's but that the rate has declined in recent years.

169

The effect of dams and water diversions on hydrophilic organisms is particularly severe in arid areas where water is in heavy demand for irrigated agriculture. In such locations, many species of aquatic organisms are endangered, and nesting and wintering areas for migratory waterfowl are becoming increasingly scarce. An example of the dilemma that can result is the case in the United States of the Truckee River in the State of Nevada. Water is diverted for agriculture by the Newlands Irrigation District. This has caused the loss of water for the StiUwater National Wildlife Refuge, a major stopover for migratory waterfowl, and reduced river flows essential for an endangered fish, the cui-ui sucker (Chasmistes cujus), to complete its spawning run out of Pyramid Lake. Under present irrigation practices, it does not appear that agriculture, waterfowl, and fish can coexist.

An example of compatible agricultural operations and wildlife protection is the recent agreement of The Nature Conservancy and the American Farmland Trust to purchase the UX Ranch in the State of Nevada. The ranch operation was in financial difficulty and required an immediate infusion of capital. The two organizations purchased the property as a means of assisting the owners, with the intent that the owners will repur- chase the property over a period of time. In return for the financial assistance, the owners have granted the two organizations permanent conservation easements on the marsh land (Anon, 1988).

The effects of water diversions are many and often complex. At their most extreme, diversions dry up watercourses completely, eliminating aquatic and riparian communities. More often, they alter flow regimes, often by diminishing flows at times of the year when stream flows are normally low. This results not only in less aquatic habitat but also in major changes in water quality increases in temperature, decreases in oxygen, and reduced water velocities. The net effect can be a major shift in aquatic species to ones that are considerably less desirable or that form a very different ecosystem.

For instance, coldwater streams that favor trout and salmon may be converted to warmwater streams favoring nongame species, including introduced species. For some streams, the diversion of limited water from their lower reaches during drought years may leave insufficient water for the passage of anadromous fishes such as salmon to reproduce, even though upstream habitats may be undisturbed.

Most irrigation diversion systems include a dam and a water-storage reservoir. These new water systems are rarely a substitute for the natural waterways they replaced. Most stream organisms do not adapt well in environments of non-flowing water, especially when water levels are unstable. With most reservoirs, the amount of water in the reservoir is determined by

170

external water demands, not by the needs of aquatic organisms so water flows and lake levels are unstable with regard to aquatic organisms. In addition, reservoirs are often sites into which lake-adapted fishes have been introduced to provide improved recreational fishing. This creates further difficulties (competition, predation, etc.) for native organisms that may colonize the reservoirs.

Water for irrigation can also be pumped from underground aquifers slow, hidden rivers which contain a great deal of fairly ancient water. Aquifers may contain endemic organisms which are highly adapted for cave life, or they may be the origin of spring sources of water which contain endemic organisms. Pumping in excess of groundwater recharge can result in the loss of aquifer-dependent organisms.

For example, Devil's Hole is a small, spring-fed water hole near Death Valley, California, that is maintained by a large aquifer. The development of a nearby irrigated agriculture system using well water resulted in a drop of water level at Devil's Hole. This threatened a species of pupfish (Cyprinodon diabolis), as well as other endemic organisms, with extinction. The pumping was stopped, and the critical habitat preserved, but only after a major legal battle (Ono et al., 1983).

The key to reducing the impact of irrigation diversions is the develop- ment of a system to include the aesthetic, service, and economic value of instream water as factors in management decisions. The high aesthetic value of streams and rivers has long been recognized; they are like magnets for people, especially in water-short regions where streams and their riparian corridors provide cool relief from xeric landscapes. This high aesthetic value also extends to abundant, accessible, and often endemic flora and fauna, and is enhanced by the scientific value of each system and its inhabitants, especially rare and unusual species.

Flowing water often serves a number of functions including flushing off pollutants, providing passage for anadromous fishes, providing a source of water and habitat for wildlife, and moderating the local climate. The service value of aquatic systems often blends with their direct economic value in the production of adult fish for on-site fishing or production of young fish of migratory species like salmon. Additional economic value derives from the fact that riparian areas are important centers of wildlife, which in turn attract people for viewing, hunting, fishing, picnicking, and general relaxation.

Nonetheless, the non-commercial value of water is rarely more than a small fraction of its commercial value. Hence, the use of water for instream purposes must be justified largely on a non-economic basis. Possible areas of research in this regard include study of the efficiency of irrigation systems in different soil conditions, examination of flow regimes that favor patterns

171

of fish migration and the needs of aquatic and riparian species, assessment of the long-term values of fisheries that depend on streams, and the social benefits of recreation and relaxation provided by natural riparian systems.

CHANNELIZATION

In many regions, agricultural activity and urban development are inhibited by the inconveniences and dangers brought on by streams and rivers which meander through the country-side. The main purposes of channelization are: flood control; expeditious movement of water through farmland; and reduction of meandering in order to stabilize field boundaries. Channelization has been practiced for the past hundred years and, by now, most of the streams in many areas are at least partially channelized.

Studies comparing unchannelized and channelized sections of the same stream demonstrate a much higher diversity of all types of organisms, from aquatic invertebrates to fish to riparian vegetation, in the unchannelized sections. Some species that have not been displaced are confined to "islands" of non-channelized sections of the stream and may face local extinction because of random demographic and stochastic events.

Recognizing the value of unchannelized stream sections for protecting biological diversity and supporting water quality, is the first step towards stream protection and restoration. In some areas, channelized sections can be restored if the old stream bed is still present. If the flood control values of the channelized sections are high or restoration is not possible, the addition of artificial structures within the channel may provide some habitat complexity and increase in diversity.

With proper management of agricultural lands within a watershed, channelization is often not necessary. It may even be detrimental in the long run. As Karr and Schlosser (1978) state: for long-term benefit to society, the best management option for improved water resources is to continue high-yield agriculture with best management practices on the land surface and more natural channel management.

BANK STABILIZATION

Bank stabilization is another form of stream channelization. From an agricultural perspective, its purposes are to keep a river within a channel so that the rich soils of the flood plain can be farmed without periodic flooding, and to maintain stable property lines land owners on one side of a river

172

occasionally lose land to bank erosion while landowners on the other side of the river gain an equivalent amount of land through deposition. Bank stabilization is also employed to reduce the silt load in a stream through eliminating erosion. This is accomplished with mixed success through placing large boulders, old car bodies, and other large objects in the stream where erosion is a problem.

Another common practice is to construct new, higher and presumably permanent stream banks referred to as levees. These structures are often built in conjunction with the construction of flood control dams upstream to stabilize stream flows. The result is a loss of natural stream habitat and riparian woodlands, areas which are normally rich in plant and animal life. Bank stabilization is difficult to achieve and requires constant maintenance. Consequently, public funds are often the source of original construction as well as maintenance on the assumption that the common good is being served. Agencies and organizations are reevaluating such practices since increased population pressures and reduced biological diversity is tending to redefine the common good.

Obviously some flood control and bank stabilization will always be necessary to protect towns, bridges, and other structures. However, such methods may not be cost-effective for farmland. Alternatives to bank stabilization include the creation of meanderbelts and the use of flood bypasses. Meanderbelt areas with a high risk of flooding annually could be converted back to riparian forest, whereas areas with low risk of flooding should be cultivated. In a meanderbelt, stream banks are not protected by artificial means. Instead, the river is allowed to move through a landscape naturally. Most erosion and deposition will take place in the wild riparian areas, a process that will be slowed as protected riparian vegetation expands. An example of a detailed meanderbelt proposal is one developed for the Sacramento River in northern California by the Sacramento River Preserva- tion Trust (Kraemer, 1984). Meanderbelts on lowland rivers provide outstanding seral and mature mosaics of habitat and effective wildlife corridors if adequate buffer zones border them in agricultural land areas.

One of the most successful flood control measures for the Sacramento River in California has been the construction of several flood bypasses essentially wide, fiat, leveed channels that allow flood waters to bypass towns and other flood prone areas. During most years, flooding does not occur or is of short duration. This permits the bypass lands to be intensively cultivated with a low risk of crop loss. In many areas, the water channels that border the bypasses support riparian habitat on each side of the bypass. Bypasses, areas prone to frequent flooding, can be used solely for natural

173

habitat providing an opportunity for restoration of floodplain forests, a greatly diminished habitat throughout the world.

Agricultural and urban uses, and related water developments, have led to a marked decline of these stream-side habitats with serious implications for wildlife. The Willow Flycatcher's decline, for example, has been attributed to physical disturbance during the nesting season. Typically, these birds build their nests at the ends of willow branches at just the right height for the average cow to spill the eggs or young while browsing past the nest. Simply postponing the start of grazing until after the nesting season may be a sufficient strategy to protect this species. The decline of Swainson's Hawk has also been related to loss of large oak trees, required for nesting, and their dependence on agricultural residues for feed. Re- creating this habitat would require aggressive intervention, including planting and irrigating large numbers of new plants plus modifications in timing of agricultural activities.

LIVESTOCK GRAZING

Livestock, especially in arid areas, are attracted to water courses because of lush, palatable vegetation and available shade. Over long periods of time, heavy use of riparian areas by livestock results in elimination of riparian plant communities, collapse of stream banks, and pollution of the water through animal wastes and silt. In the western United States, a high percentage of the streams flowing through public and private grazing lands have been severely altered by grazing, resulting in a reduction of species richness in both aquatic and riparian habitats. Platts (1981) states that "...the best opportunity for increasing resident salmonid fish populations in the West is to improve those riparian/stream habitats adversely modified by improper livestock grazing in the past" (p. 62).

The solutions to the grazing problem lie mainly in better management of stream-bank zones. In some situations, complete elimination of grazers may be necessary. In most cases, rotational grazing schemes would allow multiple use of the areas without habitat destruction. On public lands, there are many situations where the economic value of stream fisheries substantially outweighs the value of the livestock. However, it is difficult to justify the elimination of livestock grazing since the economic return from recreational use of the land does not flow to the farmer.

AGRICULTURAL POLLUTANTS

174

"Non-point source" pollution from farmland has become a major cause of degradation of aquatic habitats (Karr and Schlosser, 1978). The offending pollutants include: pesticides, herbicides, and other agricultural chemicals; animal wastes; silt from erosion; and salts and heavy metals leached from the soil. Waterways in which such pollution is heavy have low biological diversity and tend to be dominated by a few tolerant organisms.

Often, the effects of pollution are reversible. In New York, for example, the sand darter (Ammocrypta pellucida) has recolonized streams from which it was once extinct. The solution was reduction of erosion from farm and forest lands so streams could recreate the beds of clean sand it required for successful reproduction (Daniels, personal communication 1988). In Malaysia, streams flowing through rubber plantations are poor in fish life because of pollution from chemicals (Furtado, personal communication 1988), while in Sri Lanka, where the plantations are less intensively managed, the fish life in the streams is surprisingly similar to that found in nearby protected rainforest (Moyle, unpublished data).

Special types of mixed agricultural systems, commonly known as chinampas, were developed by Central American Indians in wet lowlands and in marshy areas of higher valleys. Chinampas employ a mixture of shallow ponds and agricultural plots. The ponds are sources of aquatic protein, primarily fish, and also of green manure which is periodically dredged up and deposited on the crops. These systems are known to have been highly productive and to have persisted over many centuries although they have almost completely died out.

Recently, some attempts have been made to re-establish chinampas to determine their viability under current economic and social conditions. As with agroforestry systems, attention is directed primarily toward the productivity of target organisms and the need for labor and financial inputs. The influence of chinampas on viability of non-target populations is unknown. The extreme scarcity of functioning chinampa systems severely limits current research possibilities. Small, isolated chinampas probably provide little useful information on how non-target species might fare in regions having chinampas as a dominant form of land use. The research questions pertinent to chinampas as a conservation tool are similar to those for mixed crop systems except that the presence of aquatic species is an important component of chinampas that is absent from most mixed systems (Gomez-Pompa and Jimenez-Osornio, 1990).

AQUACULTURE

175

Aquaculture is a branch of agriculture devoted to the culture of aquatic plants and animals. It is practiced in fresh water and in estuarine and marine environments, with the cultivated organisms confined in a variety of enclosures. Most aquacultural systems produce only one species of organism and are, thus, monocultures. However, polyculture has been widely used in the Orient for at least 25 centuries. As is typical for monoculture produc- tion, competitors, predators, and diseases are controlled.

Aquacultural systems range in intensity of management. At the low end of the spectrum, juvenile animals are collected from the wild and maintained in ponds or enclosures where they depend only on food available from the natural food chain within the enclosure. Farming of oysters, dams, mussels, scallops and some seaweed farming fall into this category. Low intensity farms often occupy large areas, up to more than 1,000 hectares in the case of some shrimp farms.

In medium intensity aquacultural systems, natural food supplies are supplemented, and wastes from these added foods may function as fertilizers that increase natural food production within the enclosure. These systems typically provide supplemental aeration and water movement to maintain adequate levels of oxygen. Rates of water exchange, aeration, and food supplementation can increase production to very high levels in some shrimp and salmon systems using floating pens. However, these systems are vulnerable to collapse if the supporting flows are disrupted for even a short period of time.

High intensity systems are similar to those of medium intensity, except that the target organisms are completely dependent on the feeding and water system for life support. Whereas low intensity aquacultural systems affect other organisms primarily through cooption of habitats, high intensity systems are troublesome primarily because of the large amount of food and waste that flows through the system. There can be additional problems from the use of high levels of antibiotics to prevent major disease outbreaks among animals maintained at high densities. High intensity systems are being expanded to occupy large areas, particularly in the case of marine culture in netpens and with land-based raceway systems.

Relatively little attention has been given to the conservation implications of aquaculture, in part because its spread in North America and Europe has been recent and there are as yet only small areas devoted to it. However, the clear economic benefits of aquaculture are stimulating a rapid expansion of the industry as well as creating increasing recognition of its potential for environmental effects.

In Louisiana, crayfish (Procambarus spp.) have been raised in conjunction with rice in a system that successfully minimizes effects on biological

176

diversity. After a rice crop is harvested, a given field is kept flooded so that crayfish (either already present or introduced) can graze on the rice. In six to nine months, the crayfish are harvested and the field is converted to pasture until it is time to plant rice again. The resulting two year rotation through rice, crayfish, and pasture, is not as common as intensive crayfish culture, but should be encouraged because of its positive environmental features. Fewer pesticides are used during rice culture because of potential damage to the crayfish crop. The continuously flooded fields attract waterfowl and many other aquatic organisms. In fact, Bardach et al. (1972) list the abundance of other organisms, especially predators on crayfish, as a major problem associated with such operations.

REFERENCES

Anonynaous, 1988. Wildlife and farmland groups join to save ranch and wetland. Garden, 12 (1): 24. Bardach, J.E., Ryther, J.H. and W.O. McLarney, 1972. Aquaculture: The Farming and Husbandry of

Freshwater and Marine Organisms. Wiley Interscience, NY, 868 pp. Brown, D.E., 1985. Arizona Wetlands and Waterfowl. University of Arizona Press, Tucson, AZ, 169 pp. Gomez-Pompa, A. and Jimenez-Osornio, JJ., 1990. Some Reflections on Intensive Traditional Agriculture.

University Press of America (in press). Karr, J.IL and Sehlosser, l.J., 1978. Water resources and the land-water interface. Science, 201: 22%234. Karr, J.R., Toth, L.A. and Dudley, D.R., 1985. Fish communities of midwestern rivers: A history of

degradation. Bioscience, 35: 90-95. Kraemer, T.J., 1984. Sacramento River environment: A management plan. In: ILE. Warner and K.M.

Hendrix (Editors), California Riparian Systems. University of California Press, Berkeley, CA. Meffe, G.K. and Vrijenhoek, ILC., 1988. Conservation genetics in the management of desert fishes.

Conserv. Biol., 2 (2): 157-169. Moyle P.B., Williams, J.D. and Wikramanayake, E., 1989. Fish species of special concern of California.

California Department of Fish and Game, Sacramento, CA, 222 pp. National Wetlands Policy Forum, 1988. Protecting America's Wetlands: An Action Agenda. The

Conservation Foundation, Washington, DC. Ono, ILD., Williams, J.D. and Wagner, A., 1983. Vanishing Fishes of North America. Stonewall Press,

Washington, DC. Platts, W.S., 1981. Protection and enhancement of Pacific salmonids on ranges grazed by livestock: An

overview. In: T.J. Hassler (Editor), Propagation, Enhancement, and Rehabilitation of Anadromous Fish Populations and Habitat. Humboldt State University, Arcata, CA.

Tinct, ILW. Jr., 1984. Wetlands of the United States: Current status and recent trends. National Wetlands Inventory, Fish and Wildlife Service, U.S. Department of the Interior.

U.S. Congress, Office of Technology Assessment, 1983. Wetlands. U.S. Government Printing Office, Washington, DC.

U.S. Congress, Office of Technology Assesment, 1987. Technologies to maintain biological diversity. U.S. Government Printing Office, Washington, DC.