D 4.2.1.3 Algal blooms

In the aquatic environment, population explosions occur mainly in planktic microalgae (algal blooms).

However, population explosions are being observed increasingly in soil macroalgae. Algal blooms have been reported for many hundreds of years and are a natural manifestation of biotic variability in ecosys-tems. However, there is scarcely any doubt that toxic algal blooms and blooms causing severe ecological damage have been occurring increasingly over the past decades, both in freshwater ecosystems and in coastal waters and marginal seas (Smayda, 1990; An-derson, 1995). Many algal blooms are caused by for-merly non-native species (Bederman, 1990).

It is estimated that there are 4,000–5,000 plankton species. Of these, some 300 species are known that are capable of population explosion. Only about 60–80 species, some 2% of floral-forming algae, are considered to be harmful, for instance because they form toxins or lead after their death to oxygen defi-ciency and thus to the death of fish (Smayda, 1997).

Motile species (flagellates) dominate the harmful al-gae, accounting for 90%. The majority of the other Production [mill. t]

Crop Attainable Actual Unprotected

Maize 729 449 295

Rice 1,047 509 184

Wheat 831 548 400

Potatoes 464 273 123

Cassava 623 157 21

Groundnuts 87 23 5

Sorghum 184 58 9

Attainable world production of cereals and other crops compared with actual yields and the estimated

unprotected yields (i.e. if no crop protection measures were implemented).

Source: Gregory et al., 1998

Crop Actual losses [%] Potential losses [%]

Pests Diseases Weeds Pests Diseases Weeds

Maize 15 11 13 19 12 29

Rice 21 15 16 29 20 34

Wheat 9 12 12 11 17 24

Potatoes 16 16 9 26 24 23

Cassava 13 12 10 50 50 70

Groundnuts 13 12 10 30 50 75

Sorghum 13 12 10 30 50 80

Table D 4.2-2

Actual and potential (in the absence of control measures) losses caused by pests, diseases and weeds to the world's harvests.

Source: Gregory et al., 1998

98 D Risk potentials of global change

species are cyanobacteria. Manifestations of damage vary depending upon the species, effect and biomass reached by the algae (Table D 4.2-3).

In many cases, eutrophication and changed nutri-ent composition are implicated in the emergence of harmful algal blooms (Paerl, 1997; Burkholder and Glasgow, 1997). For instance, in the catchment area of Tolo Harbor, Hong Kong, the rise in nutrient inputs from anthropogenic sources between 1976 and 1986 correlates closely with the rise in the incidence of red tides (water discoloration caused by algal blooms;

Lam and Ho, 1989). In Japan, long-term studies have shown a steady rise in the incidence of red tides from 44 in 1965 to more than 300 in 1975 (Murakawa, 1987). After nutrient reductions were implemented, the number of red-tide events has now dropped by half.

In many instances, hazards to human life have been averted by cost-intensive food monitoring. Al-gal blooms cause the greatest economic damage in aquacultures, coastal fisheries and drinking water supply. In the mussel cultures on Seto Island, Japan, the loss over an 18-year period has been estimated at more than US-$ 100 million (Smayda, 1997). In New York Bay, losses in the scallop fishery have come to about US-$ 2 million annually (Kahn and Rockel, 1988). Experts of the ECOHAB (Ecology and Oceanography of Harmful Algal Blooms) program in the USA describe the economic effect as ‘signifi-cant, but hard to quantify overall’. The financial costs of individual algal bloom events permit an order-of-magnitude estimate of total damage, but there are no

national, not to mention global, assessments of total costs.

D 4.2.1.4

Invasion by alien species

In the following, ‘invasion by alien species’ refers to the deliberate or accidental anthropogenic introduc-tion, establishment and spread of species outside of their original territory. Throughout the world, these processes have changed terrestrial biota and coastal waters and rank beside land-use change and popula-tion over-exploitapopula-tion as one of the prime causes of the loss of biological diversity (Heywood and Wat-son, 1995; Sandlund et al., 1996). Widely known ex-amples of devastating invasions include wasps and the opossum in New Zealand, rabbits in Australia, Mediterranean weeds in North America and the dis-semination of algae from the Pacific to the Mediter-ranean.

The risk potentials attaching to invasion by alien species generally also involve the population explo-sion of these species. This differs from cyclic popula-tion outbursts of native species in two aspects. These aspects are of major significance to risk evaluation:

1. In its present, essentially human-caused extent, in-vasion by alien species is novel and is associated with far greater uncertainties than the natural spread of species.

2. Alien species are often not subject to effective control by opponents (competitors, parasites, predators, pathogens), so that the persistency and

Algae Active agents or effect Damage

Algae of various groups, e.g. Oxygen deficiency, disturbed Water discoloration (red tides), Noctiluca (causes marine phosphorescence), food webs, toxins fish kill, death of invertebrates,

Chrysochromulina bloom in Europe in 1988 destabilization of the ecosystem

Diatoms of the genus Chaetoceros Mechanical impairment of Fish kill, loss of all branchia (gills) etc. mussels of a year Dinoflagellate Gambierdiscus toxicus Ciguatoxin Ciguatera: fish poisoning,

particularly through consumption of predatory fish

Diatoms of the genus Domic acid ASP (Amnesic Shellfish

Pseudo-Nitzschia Poisoning) caused in humans by

the consumption of mussels, also in piscivorous seabirds Dinoflagellate Pfiesteria piscida Largely unknown Lesion in fish, leading to fish kill;

neurotoxic to exposed humans Cyanobacteria, e.g. Anabaena Hepatotoxins and others Liver damage and death in humans

and livestock Table D 4.2-3

Examples of harmful algal blooms.

Source: Expanded and adapted from Horner et al., 1997

magnitude of damage can be far higher than for mass outbursts of native species. If at all, effective control is often only possible by means of biologi-cal pest control methods (e.g. prickly pear in Aus-tralia by Cactoblastus, thistles in Canada by Rhin-iozyllus conicus; Box D 4.2-2).

The spread of alien species

It is only through human agency that the spread of alien species has reached a level at which it becomes a serious threat to native communities and ecosys-tems. The human-induced spread of species can fol-low several pathways:

• Species are introduced accidentally through trade (wool, timber, cereals), are transported adhering to vehicles, and are imported as domestic animals and fishes for aquaculture etc. In the marine envi-ronment, exotic aquatic organisms are mainly dis-seminated through the ballast water of ships and organisms growing on ship hulls. Through inten-sive air traffic, the worldwide spread of pathogens, in particular, is a growing problem.

• Species can be imported for a given purpose but then escape, for instance from botanical or zoo-logical gardens (e.g. vine louse, raccoon, Caulerpa alga), aquaculture and scientific institutions (e.g.

Varroa mite).

• Species are deliberately released to the wild, above all agricultural crop species, silviculturally utilized tree species and grazing animals.

Today, the greater part of global human food supplies are produced from introduced species that originally had a very limited range (e.g. maize, potato, rice;

Hoyt, 1992). In no region have these plants estab-lished themselves in the wild and introgressed with the natural vegetation. Only the weeds that were in-troduced unintentionally in the process have estab-lished themselves in other flora, in some cases with considerable adverse effects (Mooney and Drake, 1986). A different assessment must be made of the worldwide spread of grazing animals (cattle, sheep, goats, horses, camels). These have not only caused considerable damage to native vegetation (horses in North America, cattle in Australia, goats on ocean is-lands), but have moreover drawn in their wake the establishment of European pasture plants (e.g. Festu-ca pratensis, Trifolium subterranaeum, Bromus ssp.).

In conjunction with grazing pressure, the pasture plants have competed with the natural vegetation and have partially usurped it (e.g. Bouteloua Steppe in North America). European Mediterranean weeds have displayed particular competitive vigor, and have completely changed the vegetation of the arid regions of the Earth.

Ecological impacts of invasion by alien species

The consequences of invasion by alien species in nat-ural or near-natnat-ural ecosystems can vary greatly from region to region. In some regions, for example, it can enrich the natural flora and fauna (e.g. in Germany).

As a rule, however, it leads to great diversity in en-demic species, and with it valuable genetic resources, being displaced by a small number of species distrib-uted worldwide.

The manifold ecological impacts of alien species have been well documented by numerous authors (e.g. Vitousek, 1986; Drake et al., 1989; D’Antonio and Vitousek, 1992; Sandlund et al., 1996; see also Box D 4.2-1). Possible primary consequences in-clude:

• Damage to human health (e.g.Asian tiger mosqui-to as a vecmosqui-tor of dengue and yellow fever),

• Crop losses and failures (e.g. European starlings on the American continent),

• Altered geochemical cycling (e.g. the crab Mysis relicta modifies the surrounding terrestrial ecosys-tem in the lakes of Montana),

• Modification of entire landscapes (e.g. woody vine Cryptostegia grandiflora from Madagascar in Aus-tralia),

• Displacement or extermination of elements of the native flora and fauna (e.g. Purple Loosestrife Lythrum salicaria of European origin in the USA),

• Clogging of pipes and waterways (e.g. zebra mus-sel in North America),

• ‘Extinction’ (loss of oxygen supply) of lakes and ponds (e.g. water hyacinth in African wetlands),

• Elevated risk of fire (e.g. Asiatic cogon grass and Brazil peppertree in Florida).

Numerous further secondary effects occur:

• Habitat degradation (e.g. African grasses in for-mer rainforest areas of Brazil),

• Dissemination of further exotic species by an al-ready established invasive species (e.g. the Indian mynah, a bird species, promotes the spread of gua-va seeds on Hawaii),

• Consequential damage caused by pesticides used to control alien organisms (e.g. in the USA the control of Dutch elm disease with DDT poisoned numerous song-birds),

• Hybridization (crossbreeding) with native organ-isms (e.g. North American grass in England).

It can generally be assumed that invasions by alien microorganisms and animals will lead to a greater ex-tent of damage, ubiquity and persistency than inva-sion by alien plants. Microorganisms, including fungi, have been spread worldwide, generally unintention-ally, and their establishment has in some cases led to very considerable ecological and economic damage.

100 D Risk potentials of global change

Examples of this are the potato famine in Ireland (1845–1851) caused by the potato fungus Phytoph-thora infestans and the elm and chestnut diseases in central Europe and North America (caused by Cera-tocystis ulmi and Cryphonectria parasitica). The high mobility and higher reproductive potential of mi-croorganisms play an important role here. However, in a broader perspective it must be noted that species extinction caused by the establishment of alien species has as yet 'only' been observed on islands or in aquatic ecosystems. A worldwide extinction of a terrestrial, continental species caused by alien species has not yet been reported anywhere (Hey-wood and Watson, 1995). However, it is probable that, depending upon the species, ecosystem and en-vironmental conditions in question, the establish-ment of alien species will cause damage through local pressures, population losses and the associated loss of genetic diversity.

To eradicate alien plant and animal species that have firmly established themselves in native commu-nities is not possible or only at high cost (Box D

4.2-1).The regeneration of damaged ecosystems can take decades. It is most probably impossible to control damage caused by alien microorganisms. Losses of endemic species on islands or in aquatic ecosystems represent an irreversible damage.

Among the German public, the risks associated with invasions are not generally a politically relevant issue. Alien species are only perceived by the public as being problematic if damage becomes extremely large or very plain (e.g. Dutch elm disease). By con-trast, the international global change research com-munity devotes great attention to this issue.

Economic impacts of invasion by alien species The economic damage resulting from an invasion in agriculture, forestry or fisheries can be estimated in terms of harvest failure, compensation for income lost through harvest loss or the costs incurred for restoration or damage limitation. By contrast, an eco-nomic valuation of a possible loss of biodiversity pre-sents major methodological difficulties (Hampicke, 1991; WBGU, 1994, 1997a).

Box D 4.2-1

Case study: the golden apple snail in Asia Situation

The golden apple snail originates in the Paraná swamp re-gions of Paraguay. In 1982, it was introduced officially under a government program to the Philippines as a foodstuff and to raise income in rural regions. In the late 1980s, it was also introduced in China, Indonesia, Malaysia, South Korea, Tai-wan, Thailand, Vietnam and the Pacific island states. The hope that the snail would be suitable as a protein source for domestic consumption and as a commodity for export to Asia and Europe soon proved to be misplaced. Due to their poor taste, the market price of the snails remained low, and even farmers with low incomes refused to eat them. Howev-er, the snails then spread in the rice paddies, causing consid-erable crop losses, rice saplings being a preferred feed.

The Consequences on the Philippines

On Luzon, the largest island of the Philippines, the snail in-flicted a harvest loss of more than 25% upon more than half of all farmers in 1990–1991; every tenth farmer even suffered a total failure. Moreover, the snail has secondary effects

upon human health, as it is the intermediate host for a lung-worm that causes meningitis in humans. Attempts are now under way to contain the plague by means of organized col-lection campaigns, keeping ducks, improving the manage-ment of water levels and applying snail poisons. These poi-sons, however, are extremely toxic to fish and pose further health hazards to the farmers.

What was the mistake?

If market analyses had already been carried out prior to in-troduction, these would have shown that the golden apple snail was neither suited to boost exports nor to supplement the food supply. Since the beginning of rice cultivation in Surinam, the snail was already known there as a prime pest in rice fields. Ecological characteristics would have identi-fied the snail as a potential invader, but in the Philippines there is no statutory requirement to carry out an evaluation of exotic organisms prior to introduction.

The losses of US-$ 28–45 million correspond to 25–40% of the annual costs of rice imports to the Philippines (Table D 4.2-4). This sum would have permitted the establishment of a functioning quarantine program for all new agricultural in-troductions in the Philippines.

Type of cost Cost estimate

[mill. US-$]

Harvest loss with replanting 12.5–17.8

and control

Cost of replanting and 2.8–10.3

control

Control with molluscicides 12.5–17.2

and collection by hand

Total costs to the farmers 27.8–45.3

Harvest loss without control 48.0

and replanting

Table D 4.2-4

Estimation of the economic damage inflicted upon the rice farming sector in the Philippines.

Source: Naylor, 1996

A complete economic analysis of the damage caused by established alien species has only been car-ried out in a few cases. Nonetheless, these examples (Boxes D 4.2-1 and D 4.2-2) illustrate that the losses and costs of control can be considerable and unpre-dictable. The issue arises as to the point in time at which control makes the most economic sense. In many cases prevention would appear to be the most effective measure (e.g. exchanging ballast water at high sea, inspecting timber imports). Control mea-sures that are only undertaken when massive ecolog-ical and economic damage have occurred can be very lengthy and costly (e.g. rabbits in Australia; Box D 4.2-2).

Ecological susceptibility and risk management

Since its inception, applied ecology has discussed the question of whether alien species will be able to es-tablish themselves successfully or not, but generaliz-able predictions continue to be impossible (Mooney and Drake, 1986; Heywood and Watson, 1995). In

view of the extensive trade in plant and animal prod-ucts and the great number of organisms that are dis-persed by international ship and air traffic, the risk of a successful and harmful establishment of alien species would generally appear to be low. In addition to the characteristics of the alien species (such as ge-netic variability, reproductive potential), the success of invasion depends upon the characteristics of the native flora and fauna. In Central Europe, for in-stance, the risk of invasion by alien species is far low-er than that of invasion by European species in othlow-er regions of the world (Mooney and Drake, 1986;

Niemelä and Mattson, 1996). European weeds adapt-ed to an agricultural cropping regime have been more successful in all parts of the world than the cor-responding species from other regions have been in Europe. Many more seeds of Australian flora were brought to Europe with wool and cereals than were transported in the opposite direction, but no Aus-tralian plant has run to seed in Europe. In other re-gions, however, Australian species have become weeds (e.g. Melaleuca in the USA, Hakea and Acacia Box D 4.2-2

Case study: biological control of European rabbits in Australia

The establishment of the European rabbit in Australia is a classic case of the unpredictable and costly consequences of introducing alien species (Williams, 1998b).The rabbits were introduced from England in 1859 for hunting purposes. Af-ter 50 years, they had already colonized most of the Aus-tralian continent. The main cause for this rapid spread was the absence of natural enemies such as weasels and foxes, which regulate rabbit populations in Europe. The rabbit population explosion led to severe degradation of the ter-restrial environment (e.g. vegetation cover destruction and soil erosion) and the endangerment and extinction of native plant and animal species.

Control strategies and their effects

After all attempts had failed to control the rabbit population outburst by means of chemical or mechanical measures (e.g.

poison, traps, fencing and intensive hunting), an attempt at biological pest control was made in 1871 with the introduc-tion of the European red fox. It soon became clear that the fox not only killed rabbits, but also native species which, due to their special characteristics (predominance of marsupials, which generally cannot withstand the competitive pressure of mammals when in direct competition), reacted particular-ly sensitiveparticular-ly. They recovered much more slowparticular-ly from deci-mation by the fox than the highly fertile rabbits.The fox thus threatened the native fauna without controlling the rabbits effectively and durably.

Rabbit control by means of the pathogen of myxomato-sis was much more successful. This is a virus disease that has been used since 1950 specifically to control rabbits.The mos-quito-borne virus spread rapidly in more humid regions, where it killed about 90% of the rabbit population. Howev-er, in drier seasons and regions the effect was found to be far

weaker. Although rabbits resistant to myxomatosis have in the meantime been reported, in the temperate regions of Australia the virus still provides an effective form of biolog-ical control.

An even more efficient control of the rabbits was achieved by means of the rabbit calicivirus, which causes a form of hemorraghic fever known in Asia, Europe and Mex-ico. Despite extensive precautions, the virus escaped from an experimental island in 1995 and entered the south of Aus-tralia. Within 1 year, the pathogen had already spread across the entire range of the rabbits, in which it caused mass mor-tality. The native flora has recovered appreciably since then.

Fears that the native fauna might be damaged by the virus have not been confirmed. In the meantime, a preparation with the virus has been officially approved and is being used

Fears that the native fauna might be damaged by the virus have not been confirmed. In the meantime, a preparation with the virus has been officially approved and is being used

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