Predicting the spread of diseases and other invaders in a changing world

We are only at an early stage of projected trends in global climate change but already there is evidence of responses by the flora and fauna. Thus, shoot production and flowering of a variety of plants is happening earlier, many birds, butterflies and amphibians are breeding earlier, and shifts in species' ranges have been detected both polewards and towards higher altitudes (Walther et al., 2002; Parmesan & Yohe, 2003). We can expect much more dramatic changes in the potential ranges of both native and invasive species in the coming century.

climate change models predict a shifting geographic template of abiotic factors...

... which will be reflected in new patterns of invader risk

Figure 7.29 The optimal management scenario trajectories for different starting configurations of the southern emu-wren metapopulation. Each circle represents one action. Concentric circles show the repeated execution of a strategy before the next strategy is implemented. Note that each trajectory ends with a do-nothing action when the metapopulation state is such that lack of management action does not produce a probability of extinction that is significantly worse than any active strategy. (After Westphal et al., 2003.)

(a) Only largest patch occupied

Baseline

(b) Only two smallest patches occupied

(c) All patches occupied

(d) Only two largest patches occupied "C5 ^ —► C E2~

Strategies:

E5, enlarge the largest patch E2, enlarge most connected

(smaller) patch C2, corridor from most connected patch to neighbors C5, corridor from largest patch to neighbors E7, create new patch and subsequently enlarge DN, do nothing, not significantly worse than any strategy the case of mosquitoes and dengue fever

Dengue fever is a potentially fatal species currently in New Zealand appears to be capable of carry-

viral disease currently limited to trop- ing the disease. Worldwide, the two most important vectors are ical and subtropical countries where its Aedes aegypti and A. albopictus. Both have been intercepted at New mosquito vectors occur. No mosquito Zealand's borders and the latter, which is tolerant of somewhat colder conditions, has recently invaded Italy and North America. If a vector mosquito population becomes established, it needs only a single virus-carrying human traveler to trigger an outbreak of the disease. de Wet et al. (2001) used knowledge of the fundamental niches of the two mosquito species in their natural ranges (in terms of temperature and precipitation), coupled with climate change scenarios, to predict areas of high risk of invasion of the vector and establishment of the disease. Under present climatic conditions, A. aegypti is unlikely to be able to establish anywhere in New Zealand whereas A. albopictus could invade the northern part of North Island (Figure 7.30a). Under a climate change scenario at the more extreme end of what has been predicted, most of North Island and some of South Island would be at risk of invasion by A. albopictus. Under the same scenario, the greater Auckland area in the north of North Island, where a large proportion of the human population lives, would become susceptible to invasion by the more efficient virus vector A. aegypti (Figure 7.30b). Vigilant border surveillance is vital, with most emphasis on northern ports of entry, in particular Auckland (with 75% of air passenger arrivals, 74% of bulk shipping cargo and 50% of the imported tyres that provide a prime transport route for mosquito larvae) (Hearnden et al., 1999).

Spiny acacia (Acacia nilotic subspecies indica) is a woody legume whose native range encompasses parts of Africa and extends as far east as India. It has invaded many parts of the world, including Australia where it was originally introduced for shade, fodder and ornamental purposes. It has spread widely and is now considered a noxious weed because it reduces pasture production and impedes stock mustering and access to water. On the basis of conditions in its natural range, Kriticos et al. (2003) first determined the species'

Figure 7.30 Dengue fever risk maps for: (a) Aedes albopictus for (i) present climatic conditions and (ii) for a high-range climate change scenario for 2100, and (b) A. aegypti for (i) a high-range climate change scenario for (i) 2050 and (ii) 2100. (After de Wet et al., 2001.)

the case of invasive acacias

Figure 7.30 Dengue fever risk maps for: (a) Aedes albopictus for (i) present climatic conditions and (ii) for a high-range climate change scenario for 2100, and (b) A. aegypti for (i) a high-range climate change scenario for (i) 2050 and (ii) 2100. (After de Wet et al., 2001.)

fundamental niche in terms of lower and upper tolerance limits and optima for temperature and moisture, and the thresholds for cold stress, heat stress, dry stress and wet stress (water-logging). They then modeled the invasive potential of spiny acacia under two climate change scenarios. Both assumed a 2°C temperature rise, coupled with either a 10% increase or 10% decrease in rainfall, because there is considerable uncertainty about the effects of global change on precipitation in Australia (Figure 7.31). The actual current distribution of spiny acacia is widespread within the range indicated by the model, but it has not yet spread to all predicted areas. When climate change is taken into account, its eventual invaded range should be much greater, particularly because the plant is expected to become more efficient in its use of water as a result of a fertilization effect of increased atmospheric carbon dioxide. Thus elevated atmospheric concentration can have both indirect effects, via climate change, and direct effects on the performance and distribution of plants (Volk et al., 2000). Further spread of this species should be containable because trees can be physically removed and the spread of seeds (in stock feces) can be prevented as long as animals are not moved indiscriminately. A crucial component in containing the invasion will be raising public awareness of the weed and how to control it (Kriticos et al., 2003).

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