Dealing with invasions

It is not straightforward to visualize the multidimensional niche of a species when more than three dimensions are involved (see Chapter 2). However, a mathematical technique called ordination (discussed more fully in Section 16.3.2) allows us to simultaneously analyze and display species and multiple environmental variables on the same graph, the two dimensions of which combine the most important of the niche dimensions. Species with similar niches appear close together on the graph. Influential environmental factors appear as arrows indicating their direction of increase within the two dimensions of the graph. Marchetti and Moyle (2001) used an ordination method called canonical correspondence analysis to describe how a suite of fish species - 11 native and 14 invaders -are related to environmental factors at multiple sites in a regulated stream in California (Figure 7.3). It is clear that the native and invasive species occupy different parts of the niche space: most

Figure 7.3 Plot of results of canonical correspondence analysis (first two CCA axes) showing native species of fish (•), introduced invader species (A) and five influential environmental variables (arrows represent the correlation of the physical variables with the canonical axes). (After Marchetti & Moyle, 2001.)

Figure 7.3 Plot of results of canonical correspondence analysis (first two CCA axes) showing native species of fish (•), introduced invader species (A) and five influential environmental variables (arrows represent the correlation of the physical variables with the canonical axes). (After Marchetti & Moyle, 2001.)

of the native species occurred in places associated with higher mean discharge (m3 s-1), good canopy cover (higher levels of percent shade), lower concentrations of plant nutrients (lower conductivity, |S), cooler temperatures (°C) and less pool habitat in the stream (i.e. greater percent of fast flowing, shallow riffle habitat). This combination of variables reflects the natural condition of the stream.

The pattern for introduced species was generally the opposite: invaders were favored by the present combination of conditions where water regulation had reduced discharge and increased the representation of slower flowing pool habitat, riparian vegetation had been removed leading to higher stream temperatures, and nutrient concentrations had been increased by agricultural and domestic runoff. Marchetti and Moyle (2001) concluded that restoration of more natural flow regimes is needed to limit the advance of invaders and halt the continued downward decline of native fish in this part of the western USA. It should not be imagined, however, that invaders inevitably do less well in 'natural' flow regimes. Invasive brown trout (Salmo trutta) in New Zealand streams seem to do better in the face of high discharge events than some native galaxiid fish (Townsend, 2003).

Of the invader taxa responsible for economic losses, fish are a relatively insignificant component. Table 7.2 breaks down the tens of thousands of exotic invaders in the USA into a variety of taxonomic groups. Among these, the yellow star thistle (Centaurea solstitalis) is a crop weed that now dominates more than 4 million ha in California, resulting in the total loss of once productive grassland. Rats are estimated to destroy US$19 billion of stored grains nationwide per year, as well as causing fires (by gnawing electric wires), polluting foodstuffs, spreading diseases and preying on native species. The red fire ant (Solenopsis invicta) kills poultry, lizards, snakes and ground-nesting birds; in Texas alone, its estimated damage to livestock, wildlife and public health is put at about $300 million per year, and a further $200 million is spent on control. Large populations of the zebra mussel (Dreissena polymorpha) threaten native mussels and other fauna, not only by reducing food and oxygen availability but by physically smothering them. The mussels also invade and clog water intake pipes, and millions of dollars need to be spent clearing them from water filtration and hydroelectric generating plants. Overall, pests of crop plants, including weeds, insects and pathogens, engender the biggest economic costs. However, imported human disease organisms, particularly HIV and influenza viruses, cost $7.5 billion to treat and result in 40,000 deaths per year. (See Pimentel et al., 2000, for further details and references.)

The alien plants of the British Isles illustrate a number of points about invaders and the niches they fill a technique for displaying species niches...

... shows why native fish are replaced by invaders a diversity of invaders and their economic costs species niches and the prediction of invasion success

Table 7.2 Estimated annual costs (billions of US$) associated with invaders in the United States. Taxonomic groups are ordered in terms of the total costs associated with them. (After Pimentel et al., 2000.)

Type of organism Number of invaders Major culprits Loss and damage Control costs Total costs

Microbes (pathogens)

Mammals

Plants

Arthropods

Birds

Molluscs

Fishes

Reptiles, amphibians

> 20,000

Crop pathogens

32.1

9.1

41.2

20

Rats and cats

37.2

NA

37.2

5,000

Crop weeds

24.4

9.7

34.1

4,500

Crop pests

17.6

2.4

20.0

97

Pigeons

1.9

NA

1.9

88

Asian clams, Zebra mussels

1.2

0.1

1.3

138

Grass carp, etc.

1.0

NA

1.0

53

Brown tree snake

0.001

0.005

0.006

NA, not available.

(Godfray & Crawley, 1998). Species whose niches encompass areas where people live and work are more likely to be transported to new regions, where they will tend to be deposited in habitats like those where they originated. Thus more invaders are found in disturbed habitats close to transport centers and fewer are found in remote mountain areas (Figure 7.4a). Moreover, more invaders arrive from nearby locations (e.g. Europe) or from remote locations whose climate (and therefore the invader's niche) matches that found in Britain (Figure 7.4b). Note the small number of alien plants from tropical environments; these species usually lack the frost-hardiness required to survive the British winter. Shea and Chesson (2002) use the phrase niche opportunity to describe the potential provided in a given region for invaders to succeed - in terms of a high availability of resources and appropriate physico-chemical conditions (coupled with a lack or scarcity of natural enemies). They note that human activities often disrupt conditions

Figure 7.4 The alien flora of the British Isles: (a) according to community type (note the large number of aliens in open, disturbed habitats close to human settlements) and (b) by geographic origin (reflecting proximity, trade and climatic similarity). (After Godfray & Crawley, 1998.)

Waste ground Hedges and shrub Arable and gardens Rocks and walls Woodland Coasts Streamsides Marsh and fen Grass Heath Mountains

Proportion of alien species in total flora

Europe North America Mediterranean Asia

South America China

Turkey and Middle East South Africa New Zealand Japan Australia Central America Atlantic Islands Tropics India

Number of alien species

Number of alien species in ways that provide niche opportunities for invaders - river regulation is a case in point. Not all invaders cause obvious ecological harm or economic loss; indeed some ecologists distinguish exotic species that establish without significant consequences from those they consider 'truly invasive' - whose populations expand 'explosively' in their new environment, with significant impacts for indigenous species. Managers need to differentiate among potential new invaders both according to their likelihood of establishing should they arrive in a new region (largely dependent on their niche requirements) and in relation to the probability of having dramatic consequences in the receiving community (dealt with in Chapter 22). Management strategies to get rid of invading pests usually require an understanding of the dynamics of interacting populations and are covered in Chapter 17.

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