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arrival and spread of invaders

4.4 Predicting the

arrival and spread of invaders

4.4.1 The Great Lakes – a great place for invaders

4.4.1 The Great Lakes – a great place for invaders

DISPERSAL, MIGR ATION AND MANAGEMENT CHAPTER 4 93 3 million liters of Caspian ballast water, containing a diversity of plants and animals (even the cholera bacterium Vibrio cholerae has been found in ballast water). One management solution is to make it compulsory (rather than voluntary) to dump ballast water in the open ocean – this is now the case for the Great Lakes. Other possible methods involve fi lter systems when loading ballast water, and on-board treatment by ultraviolet irradiation or waste heat from the ship’s engines.

The most damaging invaders are not simply those that arrive in a new part of the world – the subsequent pattern and speed of their spread also matters. Zebra mussels (Dreissena polymorpha) have had a devastating effect (Section 1.2.5) since arriving in North America via the Caspian Sea/Great Lakes trade route. The expansion of their range occurred quickly through all commercially navigable waters, but over- land dispersal into inland lakes, mainly attached to recreational boats, has been much slower (Kraft & Johnson, 2000). Geographers have developed a method to predict human dispersal patterns based on distance to and attractiveness of destina- tion points, and Bossenbroek et al. (2001) adopted their modeling approach to predict the spread of zebra mussels through the inland lakes of Illinois, Indiana, Michigan and Wisconsin (364 counties in all). The model, which we need not con- sider in detail, has three steps dealing respectively with the probability of a boat traveling to a zebra mussel source and inadvertently picking up mussels, the prob- ability of the same boat making a subsequent outing to an uncolonized lake, and the probability of zebra mussels becoming established in the uncolonized lake.

Figure 4.7a shows the number of lakes colonized by zebra mussels in each county as predicted by the model. This can be a fraction of one, because of the probabilistic

(a) (b) Infected lakes 0 – 0.25 0.25 – 0.5 0.5 –1 1 – 3 > 4 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 3 3 3 2 4 4 5 7 100 km 100 km N N

Fig. 4.7 (a) The predicted distribution (based on 2000 iterations of a computer model of dispersal) of inland lakes

colonized by zebra mussels in 364 counties in the USA; the large lake in the middle is Lake Michigan, one of the Great Lakes of North America. (b) The actual distribution of colonized lakes as of 1997. The numbers refer to the number of lakes in each county predicted to be invaded (a) or actually invaded (b). (After Bossenbroek et al., 2001.)

94 PART 1 ECOLOGICAL APPLICATIONS AT THE LE VEL OF INDIVIDUAL ORGANISMS

nature of the predictions, while the maximum number colonized per county ranged up to four or more. The predictions proved to be highly correlated with the pattern of colonization that actually occurred until 1997, indicating that the model is real- istic. Note that parts of central Wisconsin and western Michigan were expected to be colonized, but no mussel colonies were documented. Bossenbroek’s team suggest that invasion may be imminent in these locations, which should therefore be the focus of biosecurity efforts and education campaigns.

Each new lake that is colonized by zebra mussels represents a separate invasion from a neighboring lake, analogous to the original invasion of North America from Eurasia but on a smaller scale. Muirhead and MacIsaac (2005) were interested to know whether all lakes in the landscape possess equal potency as sources for inva- sion of other lakes. Or are some created more equal than others in their capacity to infect? Lakes in a landscape have the characteristics of a network, not unlike the human brain or the internet. The question is: are some lakes likely to serve as inva- sion hubs, and others as dead ends?

The spiny waterfl ea, Bythotrephes longimanus, was discovered in Lake Ontario in 1982, from where it dispersed among the interconnected Great Lakes as well as to adjacent inland lakes. The waterfl ea produces highly resistant resting eggs that become entangled in fi shing gear, resist the desiccation associated with road-travel of boats between lakes and subsequently detach and hatch in new lakes (MacIsaac et al., 2004). Muirhead and MacIsaac surveyed recreational boaters at boat ramps and marinas to determine which invaded and uninvaded lakes were visited by boaters departing from particular invaded lakes. It turns out that most invaded lakes connect to only a few uninvaded counterparts (0–5), while some lakes connect to many others (16–25) and may act as invasion hubs (Figure 4.8).

4.4.2 Lakes as infectious agents 4.4.2 Lakes as infectious agents 50 40 30 20 10 0 50 40 30 20 10 0 0–5 ₆–10 11 –15 16 –20 21 –25 26 –30 31 –35 3 6–4 0 4 1–4 5 46 –50 To invaded lakes To uninvaded lakes Frequency

Number of outbound connections

Frequency

Fig. 4.8 Frequency

histogram of the number of links between invaded source lakes and invaded and uninvaded destination lakes from recreational boater surveys in 2003. It is the links to uninvaded lakes that really matter in terms of invasive spread through the landscape. (After Muirhead & MacIsaac, 2005.)

DISPERSAL, MIGR ATION AND MANAGEMENT CHAPTER 4 95 Lake Muskoka was the fi rst inland lake to be invaded by the waterfl ea as a result of regular boat traffi c from Lake Huron, one of the Great Lakes (Figure 4.9). It quickly developed into a regional hub for invader spread for two reasons. First, all of its outbound traffi c to inland lakes was to uninvaded lakes and, second, the total amount of traffi c leaving was high (1452 people towed boats on trailers from the lake each year). As lakes become invaded, the number of uninvaded lakes decreases while the vulnerability of remaining uninvaded lakes changes. Thus, Lake Muskoka is unlikely to continue to serve as a hub because most of its outbound traffi c is now to other lakes that have already been invaded. On the other hand, the heavy out- bound traffi c from Lakes Simcoe (3774 annual movements) and Kashagawigamog (1840 movements) is predominantly to uninvaded lakes. These two lakes are devel- oping into the most important invasion hubs (Figure 4.9). Lake Panache, which is located in northern Ontario away from the current cluster of invaded lakes, supports a large recreational fi shery and Muirhead and MacIsaac’s study suggests it may well be an important hub of the future. Thousands of lakes are still uninvaded and man- agers need to know how the network functions so they can focus effort in the most effective way to educate and/or regulate boaters.

You may be wondering whether a tiny waterfl ea is worth all the bother. Certainly, it does not cause such economic or ecological harm as the zebra mussel. Its impor- tance here lies in an ability to act as a general indicator of the means and pattern of spread of aquatic organisms from lake to lake.

Lakes in a landscape are obvious examples of ‘discontinuous’ habitat. It is because assorted lakes are differentially linked into the network that invasion hubs can be identifi ed for special management treatment. Other patchily arranged habitats – such as oceanic islands, mountain ranges and forest remnants – can be expected to 4.4.3 Invasion hubs

or diffusive spread? 4.4.3 Invasion hubs or diffusive spread?

Fig. 4.9 Diagram of network traffi c from previously to recently invaded lakes based on records of fi rst reporting. Stages in

the development of the ever-increasing invasion network are indicated by the year in which lakes were invaded (from left to right). Shaded boxes represent existing or developing network hubs. Thousands of lakes (not shown) are so far uninvaded. (Lake Kashagawigamog is abbreviated as Kash.) (After Muirhead & MacIsaac, 2005.)

Joseph Go Home

Mary

Huron Muskoka Peninsula

Fairy Vernon Rosseau Canning Soyers Temagami Simcoe Kash Wood Pigeon Manitou Lake of Bays Harp Whitefish Sugar Loon Barnard Panache Black Donald Nipissing Ahmic George Skeleton 1989 1990 1991 1992 1993 1995 1996 1997 1998 1999

96 PART 1 ECOLOGICAL APPLICATIONS AT THE LE VEL OF INDIVIDUAL ORGANISMS

function in a similar way, unless the organisms’ powers of dispersal are so strong that the habitats are functionally continuous. Thus, the timing and pattern of inva- sive spread depends on both the geography of the discontinuous habitat and an organism’s dispersal potential.

Then again there are habitats that are ‘continuous’, including large expanses of grassland (prairies, steppes, pampas, agricultural land) or forest (rainforest, boreal forest, plantations). In these cases the spread of invaders can be envisaged as a progressive wave of diffusion across the landscape, at a rate that depends on the species’ intrinsic rate of population growth and its ‘coeffi cient of diffusion’. A high coeffi cient of diffusion may be associated with the ability to hitch a ride with humans or other animals or on the wind, or the capacity to walk or fl y quickly through the landscape.

The red fi re ant (Solenopsis invicta) has spread rapidly through much of the south- ern USA with dramatic economic consequences (Section 1.2.5). The species, which originated in Argentina, occurs in two distinct social forms. The single-queened (monogyne) form and the multiple-queened (polygyne) form differ in patterns of reproduction and modes of dispersal. Queens from monogyne colonies take part in mating fl ights and create new colonies, whereas queens from polygyne colonies are adopted into already existing nests after mating. As a result, the monogyne popula- tions spread a thousand times faster than their polygyne counterparts (Holway & Suarez, 1999). The ability of managers to predict potentially problematic invaders and devise strategies to counter their spread depends on a thorough understanding of dispersal behavior.

We now have some power to predict the species that pose the greatest risk as invad- ers, and know the major invasion routes; national biosecurity strategies are based on this information. However, as Perrings et al. (2005) point out, protecting national borders is diffi cult because those whose actions result in invasions usually bear no legal responsibility and do not have to pay the costs associated with invaders. In economic terms, invasion costs are an ‘externality’ of global trade – an unintended side effect whose cost is not refl ected in the market price of the goods that pose the risk. Invasions are a form of biological pollution; the same externality problem occurs if the costs of chemical pollution are borne by society in general rather than by those who pollute. It is now more generally accepted that polluters should pay for the damage they cause – the ‘polluter pays principle’. This is an example where what was an externality (cost of pollution) becomes internalized (pollution costs refl ected in the market price of the industrial products). Can the same thing be done for invasion costs?

You saw in Section 3.3.3 that the probability of a successful invasion by a parrot species was positively correlated with the development of the international parrot market. There are many other examples among both animals and plants. What is needed, according to Perrings’ team, is a measure that confronts exporters with the costs of their actions – the introduction of invasion risk-related tariffs. Import tariffs can be expected to reduce export activities that are particularly risky, which is desirable, but such tariffs could disproportionately hurt the economies of poor countries. Thus, the team suggests that tariffs should be coupled with international fi nancial support for low-income countries that adopt biosecurity-enhancing measures and, thus, confer a worldwide benefi t.

4.4.4 How to manage invasions under globalization 4.4.4 How to manage invasions under globalization

DISPERSAL, MIGR ATION AND MANAGEMENT CHAPTER 4 97 Human industry and resource exploitation inevitably have consequences for natural ecosystems. Given the undeniable need to exploit forest for timber (Sections 4.5.1, 4.5.2), to generate energy (Section 4.5.3), and to grow agricultural crops (Section 4.5.4), the aim of resource managers is twofold. First, to designate areas where this might be done with minimal consequences for the natural world and, second, to ensure that production landscapes are managed in a sustainable manner. I will come to the question of the zoning of production and conservation across landscapes and waterscapes in Chapter 10, because this is the domain of community and ecosystem ecology. Here, I will confront the question of sustaining individual species in exploited landscapes, with continuing emphasis on the relevance of species mobility for conservation.

The fl ying squirrel Pteromys volans in Finland has declined dramatically because of habitat loss, habitat fragmentation and reduced habitat connectivity caused by intensive forestry. This nocturnal squirrel favors spruce-dominated forest (Picea abies) with a marked component of deciduous trees such as aspen (Populus tremula), birch (Betula spp.) and alder (Alnus spp.). Aspens are probably the most important of these, providing both food for the squirrels and shelter in the form of woodpecker cavities. Areas of natural forest, critical to the squirrels, are now separated by clear- cut and regenerating areas.

The core breeding habitat of a fl ying squirrel only occupies a few hectares, but individuals, particularly males, move to and from this core for temporary stays in a much larger ‘dispersal’ area (1–3 km2

), and juveniles permanently disperse away from their parents within this range. Reunanen et al. (2000) compared the landscape structure around known fl ying squirrel home ranges (63 sites) with randomly chosen areas (96 sites) to determine the forest patterns that the squirrels need. They fi rst established that the landscape could be divided into optimal breeding habitat (mixed spruce-deciduous forests), dispersal habitat (pine (Pinus sylvestris) and young forests) and unsuitable habitat (young sapling stands and open habitats). Figure 4.10 shows the spatial arrangement of breeding and dispersal habitat for a typical fl ying squirrel site and a random forest site. Overall, fl ying squirrel land- scapes contain three times more suitable breeding habitat and 23% more dispersal habitat than random landscapes. But, most notably, squirrel dispersal habitat is much better connected (fewer fragments per unit area) than random landscapes.

Reunanen and his colleagues recommend that forest managers should restore and maintain a deciduous mix of trees for optimal breeding habitat. And of particular signifi cance in the context of dispersal behavior, they need to ensure good physical connectivity between optimal breeding and dispersal habitat.

In stark contrast to the fl ying squirrels, bats can sometimes be favored by certain forestry practices! Bats vary in wing shape and echolocation calls in a way that infl uences their operation in different habitats. For example, those with long thin wings, which make them less maneuverable, or with calls of relatively low fre- quency, fi nd it diffi cult to negotiate cluttered habitats. As vegetation regenerates after logging, stem density and vegetation clutter (structural complexity) increase. With this in mind, Law and Chidel (2002) investigated the effects of logging (15 years later) in a Eucalyptus forest in New South Wales, Australia. Prior to logging, the tall, wet forest was dominated by two tree species, Sydney blue gum (Eucalyptus saligna) 4.5 Species mobility and management of production landscapes 4.5 Species mobility and management of production landscapes 4.5.1 Squirrels – axeman spare that tree

4.5.1 Squirrels – axeman spare that tree

4.5.2 Bats – axeman cut that track 4.5.2 Bats – axeman cut that track

98 PART 1 ECOLOGICAL APPLICATIONS AT THE LE VEL OF INDIVIDUAL ORGANISMS

and silvertop stringybark (Eucalyptus laevopinea). By recording bats using ultrasonic detectors, the researchers were able to document the number of passes made by a variety of species that could be separately identifi ed from their calls. They compared bat activity both in logged and unlogged forest patches, and in different locations in each case – along forest tracks, off track in mid-forest and along unlogged ‘ripar- ian’ margins beside forest streams.

Surprisingly there was no statistically signifi cant difference in bat activity between unlogged and logged locations. What really mattered was the presence of forest tracks as dispersal pathways for feeding activity. In both logged and unlogged situ- ations, much higher activity was recorded on forest tracks (183 and 196 bat move- ments per night in logged and unlogged forest respectively) than off track (5 and 36 movements per night). The stream riparian areas, which like forest tracks provide linear pathways for the bats, showed intermediate activity (55 and 26 movements per night). Bat species richness was actually highest on forest tracks in the logged areas.

The least maneuverable bats would be predicted to do well in the uncluttered fl ight paths provided by tracks. This was indeed the case (Figure 4.11a–d), but in addition the highly maneuverable Rhinolophus megaphyllus made use of the pathways (Figure 4.11e), perhaps because insect prey density was higher there, or because of enhanced hunting success or simply as a ‘commuter’ route.

An index of clutter was calculated for every recording site to integrate percentage vegetation cover in each of four strata – groundcover, shrub, understorey trees (including Eucalyptus regrowth in the logged sites) and large eucalypts emerging into the upper canopy. Overall bat activity declined signifi cantly with the increasing

Fig. 4.10 The spatial

arrangement of patches (dark) of breeding habitat (left hand panels) and breeding plus dispersal habitat (right hand panels) in a typical landscape patch containing fl ying squirrels (top panels) and a random forest location (bottom panels). The fl ying squirrel patch contains 4% breeding habitat and 52.4% breeding plus dispersal habitat, compared with 1.5% and 41.5% for the random landscape. Dispersal habitat in the squirrel landscape is much more highly connected than in the random landscape. (After Reunanen et al., 2000.)

1000 m

Random 1000 m 1000 m

DISPERSAL, MIGR ATION AND MANAGEMENT CHAPTER 4 99

clutter provided by regrowth in logged forest and the amount of understorey euca- lypts in unlogged forest (Figure 4.12). This indicates that low bat activity away from tracks and stream corridors was related to clutter.

While the opening up of forest tracks has clear benefi ts as feeding dispersal routes for many species of bats, and the bat community recovers well within 15 years of logging, it would be unsafe to conclude that logging is a blessing in disguise. Bird

Fig. 4.11 Mean counts per night (+ standard error) of fi ve bat species recorded at sites off track, along tracks and along

stream riparian corridors. Histograms that do not share the same letter are signifi cantly different. (a)–(d) Species considered to be clutter-sensitive because of their morphology and behavior; (e) a clutter-tolerant species. (After Law & Chidel, 2002.)

3

2

1

0

–20 0 20 40 60 80 100 120 140 160 Index of forest clutter

Log number of bat passes per night

Fig. 4.12 Relationship

between total number of recorded bat movements and an index of forest clutter for all logged and unlogged sites combined. (After Law & Chidel, 2002.)

(e) Rhinolophus megaphyllus

B B

A A

Log number of passes per night

2.0

1.5

1.0

0.5

0

Off-track On-track Riparian B B (d) Vespadelus pumilus A AB 2.0 1.5 1.0 0.5 0

Off-track On-track Riparian A A B B 2.0 1.5 1.0 0.5 0

(a) Chalinolobus morio (b) Vespadelus darlingtoni (c) Falsistrellus tasmaniensis

Off-track On-track Riparian B B A A _AB AB AB B A A AB AB

Log number of passes per night

2.0

1.5

1.0

0.5

0

Off-track On-track Riparian

2.0

1.5

1.0

0.5

0

Off-track On-track Riparian B B A A B B Logged Unlogged

100 PART 1 ECOLOGICAL APPLICATIONS AT THE LE VEL OF INDIVIDUAL ORGANISMS

diversity, for example, takes 30–50 years to recover after logging (Williams et al., 2001). And even for the bats, essential maternity roosts occur in the hollows of large mature trees that would take more than 100 years to grow. Thus, sustainable forestry practice in Australia requires that fi ve such habitat trees are retained per hectare. The principles of sustainable harvesting will be considered in more detail in Chapter 7.

For many species, conservation involves leaving important structures in place, such as trees of particular species (fl ying squirrels, Section 4.5.1) or age (bats, Section 4.5.2). With wind farms, on the other hand, it is similarly large structures that are the problem.

Wind farms can be built on land or at sea, each posing its own threats to migrat- ing and dispersing birds. On land, soaring birds such as falcons and vultures are at particular risk of colliding with the turbines (up to 100 m above the ground), par- ticularly because the engineers often select their locations for the same wind-related reasons that birds select their routes. Many wind farms are also planned for marine settings – in Europe, for example, more than a hundred applications have been submitted (Garthe & Huppop, 2004). Each may consist of as many as 1000 turbines,