Dispersal is a vital process for species persistence and it is one of the most important, yet least understood, features of ecology, population biology, and evolution (ref). There are both benefits and costs associated with dispersal. Dispersive individuals will experience reduced competition from conspecifics, however this comes at a number of costs, including; unfamiliarity of a new habitat, intolerance of new microhabitat or microclimatic conditions, and the energetic cost of dispersal, which can incur trade-offs in life-history traits such as fecundity (Easteal and Floyd, 1986, Green, 1962; Roff, 1977; Inglesfield & Begon, 1983, Endler, 1977, 1979). CONS AND PROS OF DISPERSAL
Dispersal dynamics during range expansion is a topic of interest for many areas of science, especially invasive species biology (Sax et al., 2005). Range expansion is a complex process. Range expansion dynamics of a species is dependent on the interplay between ecology and evolution, and changing physical geography and climate (Clobert et al., 2001). These factors result in accelerating and decelerating range expansions in both space and time (Andow et al. 1990; Shigesada et al. 1995; Silva et al. 2002; Liebhold and Tobin 2006). Determining and understanding all the processes that contribute to the rate in which a species spread through a new environment is important for a number of reasons: 1) it gives insight into the future of species under predicted climate change conditions; 2) it helps in the development of management and control efforts for invasive species; and 3) it provides insight into historical biogeographic processes (ch 15 invasive sp - hastings, skellam, …). WHAT EFFECTS RANGE EXPANSION AND WHY RANGE EXPANSION IS IMPORTANT. MAKE A CLEAR DEFINITION BETWEEN RANGE EXPANSION AND DISPERSION
Modern invasions provide perfect opportunities for biologist to study range expansion in the wild. The invasive cane toad, Bufo marinus, is currently expanding its range in Australia at an exceptional rate and has been the subject in a number of recent range expansion studies. There are many advantages of studying the Australian cane toad invasion. Firstly, we know where, when and how many individuals were introduced to Australia (see Easteal, 1981). This enables the direct measurement of rate processes such as genetic change and spatial spread, as opposed to native species who have long established boundaries (inlude chapterSax et al., 2005). Secondly, the spread of cane toads has been closely monitored since their introduction ~75 years ago (eg. Easteal, 1981; Freeland and Martin, 1985; Sabath et al., 1981; Urban et al., 2007). This is of great value because large data sets that document invasions over a period of time long enough to allow for accurate estimates of speed and acceleration are rare (Urban et al., 2008). Thirdly, cane toads are still advancing their range (Llewelyn et al., 2010). This means ecological, evolutionary and landscape variables can be explicitly linked with observed invasion rates. INTRODUCE CANE TOADS. WHY THEY ARE A GOOD STUDY SPECIES FOR RANGE EXPANSION.
The cane toad invasion in Australia has received a lot of attention in attempts to gain further understanding of the mechanisms that promote the dispersal process. Factors that are well established as influential in determining rates of cane toad dispersal include climatic variation and landscape structure. Less attention, however, has been given to evolutionary processes that may affect dispersal rates. This is in despite of the fact that rapid evolutionary changes are common during invasions (Whitney and Gabler, 2008) and the apparent importance of such processes in our understanding of invasive biology (Lee, 2002). Here I aim to identify and discuss? factors that control the rate and pattern of spread of cane toads in Australia. I also endeavour to establish any gaps in our current understanding of the processes that influence range expansion of this invasive species. WHAT I WILL BE TALKING ABOUUUUTT.
I need to set boundaries
The cane toad (Bufo marinus) in Australia
Since its introduction into Australia's northeast coast in 1935, the cane toad has spread across large areas (see figure 1.). As the most successfully introduced amphibian species, the cane toad has one of the most extensive, documented histories of introduction of any vertebrate (reviewed in Easteal, 1981; Estoup et al., 2004). The geographic range of the cane toad in Australia is still expanding in eastern and northern Australia (refs). There are currently three main expansion areas, one in Western Australia, one in Northern Territory, and one in New South Wales, . The expansion front in the western expansion area (WEA) in Western Australia is traveling at the fastest rate and is moving west at approximately 50km/yr (double check -Kearney et al 2010). The expansion front in the central expansion area (CEA) in Northern Territory was initially part of the western movement but has branched off and is now penetrating into semi-arid Australia rarely exceeded 20 km/yr (Urban et al., 2008). The third expansion front is located on the east coast in New South Wales (the eastern expansion area, EEA) and has reached as far south as 31.8°S (ref). This southeastern expansion front advanced at average rate of 1.5km/yr in the ca 1.1 km/yr) of this southern front (Seabrook 1991, Estoup et al. 2004).past/between 19xx and 200x, about 16 times slower than the northern expansion (Estoup et al., 2004)
During the initial phase of colonization, toads were dispersing at an estimated 10-15km/year (Urban et al., 2008). Three decades later range expansion through the Gulf of Carpentaria in northern Australia occurred at approximately 30km/year. Between 2000-2006 invasion speed accelerated to a maximum rate of 60 km/year as toads approached the western part of the Northern Territory (Urban et al., 2008). Following that, cane toad dispersal rate decelerated to ~45km/yr as toads invaded the Kimberley region, north Western Australia (Urban et al., 2007).
High levels of phenotypic variation allow organisms to use a wide variety of habitats (Thomas et al., 2001). This is the case with cane toads which occurs in every vegetation zone in Australia, except high altitude closed forests and arid districts of the far west (ref). This ability to colonize diverse habitats has enabled the species to inhabit a large area of Australia and they have now spread across more than 1.3 million square kilometers (Urban et al., 2007).
Aim to investigate factors that promote and limit toad dispersal
ECOLOGICAL FACTORS THAT INFLUENCE THE RATE OF RANGE EXPANSION IN CANE TOAD
Life History
Life history traits are commonly studied to determine the 'invasiveness', and consequent range expansion abilities, of a species (book ref). Invasive species across all taxa possess traits that enable them to colonise and move through novel environments more readily than other, non-invasive species. The most common traits that enhance range shift potential include large clutch sizes (r-strategists), opportunistic feeders, habitat generalist and large temperature tolerance (refs).
Much of the cane toad's success as an invader is explained by life history traits.
Van Bocxlaer et al. (2010) demonstrated that cane toads possess all of the life history traits that have lead to the large range distributions of Bufonidae (true toads) worldwide. They did this using statistical methods to phylogenetically reconstructing an optimal range-expansion phenotype in the true toads. These traits include four adult traits (1-4) plus three reproductive and developmental traits (5-7) as follows: 1) Terrestrial or semi-terrestrial adult niche - which reduces the dependence on water; 2) Parotoid glands - toxicity to deter predators and rehydration abilities; 3) Inguinal fat bodies - energy reserves; 4) Large body size - high water storage ability in bladder; 5) Opportunistic oviposition site - ability to use all kind of water bodies for egg laying; 6) Large clutch sizes; and 7) Exotrophous larva (food from environment) - less parental investment (see table 1.).
It's broad diet means that food availability appears not to impede toad distribution (Zug and Zug, 1979).
The relatively large clutch sizes of cane toads (7,500-30,000 eggs/female) compared to other amphibians (most are around 100? Ref?) and a survival rate of approximately 30-70% (Schwarzkopf and Alford, 2002) enables newly colonized toad populations to rapidly increase (Easteal, 1981; Estoup et al., 2004), presumably promoting earlier dispersal due to conspecific competition (Lever, 2001). HOW TOADS ARE GOOD COLONISERS
The large size of adult cane toads (sul is between 90-140mm, ref) facilitates an increased reproductive potential in females by allowing them to carry more eggs (Zug and Zug, 1979). The large relative bladder size of Bufo species enables optimal water retention (Bentley, 1966). Presumably, large body size also minimizes water loss due to a smaller surface-to-volume ratio (as seen in other large animals ref).
The combination of large body size and the presence of large parotoid glands decrease the probability of predator attacks, increasing longevity once adulthood is reached (Zug and Zug, 1979). There are two advantages of possessing parotoids glands; they secrete toxins when the animal is under predation stress (ref) and they have large granular alveoli which facilitate the retention of large quantities of water during the dry season via a highly hydrophilous glycosaminoglycans secretion (Toledo and Jared, 1993).
These life history traits have enabled cane toads to successfully invade over 50 countries worldwide (Lever, 2001), however, they do not directly affect the rate of spread of cane toads in Australia. They do so indirectly when acted upon by phenotypic or genetic adaptation. This is because life history traits are influenced by abiotic variables through space and time. These abiotic factors are discussed below.
LANDSCAPE HETEROGENEITY
Landscape heterogeneity incorporates factors that change through space and time such as spatial pattern 1) climate, 2) landscape structure, 3) habitat connectivity, 4) barriers. These factors often cause strongly differing expansion rates to occur through space (ref). Differing expansion rates can occur simply because species-speciï¬c movement behavior and/or ï¬tness traits change under varying environmental and landscape conditions (With, 2002). In the case of Australian cane toads, landscape heterogeneity has proven to be a powerful influence on the species range expansion dynamics, causing acceleration and deceleration in vanguard populations (ref). For example, the rate of spread is about 16 times higher in the northern expansion area than in the eastern expansion area (Estoup et al., 2004). The major factors of landscape heterogeneity are discussed below.
Spatial pattern
Patches, corridors, ecotones, patch boundaries, and the like gives ecological landscapes a spatial pattern and texture (Clobert et al., 2001). Habitat patches differ in their internal structure and may vary in their resistance to movements, and dispersing individuals may therefore travel at different rates and over different distances depending on which patch type they encounter. The route of movement and how individuals move through such a complex mosaic is important and unlikely to be linear or random (ref) (figure or dif pathways?).
Do certain habitat types create different resistance for an expanding population of cane toads? The answer here is most certainly yes. Toads move faster in regions characterized by open habitats of low, constant elevation and high road density (Brown et al., 2006; Phillips et al., 2007; Schwarzkopf and Alford, 2002; Seabrook and Dettmann, 1996; Urban et al., 2008). Cane toads have been reported to actively select relatively open habitats rather than densely vegetated areas, both within the native range in South and Central America (Zug and Zug, 1979) and in Australia (Seabrook and Dettmann, 1996). This may be a behavior employed to save energy. For example, Brown et al. (2006) demonstrated that toads moved more rapidly on roads (compared to densely vegetated habitat) on the second jump that they took, but not the first, suggesting that exhaustion is an inhibiting factor to dispersal through dense vegetation.
Corridors or landscape connectivity may further facilitate movement, increasing dispersal distances and producing an overall directionality to movements (Haddad, 1999). Invasive species often use natural corridors to enhance their dispersal (With, 2002). Radio-tracking cane toad movements has revealed that toads use roads as dispersal corridors (Brown et al., 2006; Seabrook and Dettmann, 1996). These studies have shown that toads may prefer to travel along roads when the surrounding habitat consists of dense habitat. However, both of these studies failed to produce any estimates on the different dispersal rates occurring between toads that selectively travel down roads and those that do not. While it is presumed that the difference in dispersal rates will be dependent on the surrounding vegetation of these roads, there is no empirical evidence to support such claims. Therefore, the notion that roads act as a natural dispersal corridor that promotes range expansion remains unclear.
Perhaps another type of natural corridor that could be used by toads is waterways, such as drainage systems, rivers and creeks. Freeland and Martin (1985) report that toads disperse quicker in catchment areas and along drainage systems than the average estimates of dispersal rates. Despite the fact that cane toads are heavily reliant on water for survival (Lever, 2001; Zug and Zug, 1979) the obvious potential for waterways to aid in toad dispersal is yet to be investigated. Future telemetry experiments, similar to those mentioned above (need more detail?), that investigate waterways as potential dispersal corridors would provide more information on the movement patterns of cane toads.
Temporal patterns
Landscapes do not only change through space, they are also temporally dynamic due to climatic variation and interactions with other species (With, 2002).
Temperature
Environmental temperatures heavily influence body temperatures of ectotherms, and body temperature in turn profoundly affects physiological processes (Hutchison and Maness, 1979). With this considered it is not surprising that the dispersal ability of cane toads is strongly influenced by air and water temperatures. For example, activity and dispersal in cane toads have been found to increase with increasing temperature. Easteal and Floyd (1986) and Easteal et al. (1985) revealed that toads in Australia spread through lower latitudes at an uniformly increasing rate between 1949 and 1974. In a more extensive study, Urban et al. (2008) confirmed this using toad dispersal data to map expansion rates from 1935 to 2007. This has been further established by a number of field tests (Brown et al., 2006; Phillips et al., 2007; Schwarzkopf and Alford, 2002).
Cane toads in Australia now inhabit regions where the minimum monthly temperature drops below 5.0°C and the maximum monthly temperatures reaches beyond 37.0°C (Urban et al., 2007). It appears that toads have the ability to survive extremely high temperatures, suggesting that maximum temperature will not restrict the future spread of cane toads through northern Australia.
Although not a constraint in the northern toad expansion, temperature is the major limitation in the EEA and largely explains the slow expansion rates previously observed along the east coast. The cold temperatures typical of the southeast coast of Australia restrict adult locomotor ability (Kearney?). In addition, adequate breeding grounds are less abundant in the south because toads prefer to breed in water bodies that are at high temperatures (Evans et al., 1996).
Water
Cane toad activity is not only constrained by extreme maximum and minimum temperatures, it is also heavily depend on precipitation and evaporation (Sutherst et al., 1995; Zug and Zug, 1979). Anurans have permeable skin and most rely on moist environments to stay hydrated (ref). Cane toads are no exception and rely heavily on regular access to water. Therefore, water availability becomes a critical factor in determining dispersal rates of the species. Using modeling systems based on high-resolution cane toad data and presence/absence surveys Easteal et al. (1985) and Urban et al. (2008) demonstrated that toads increase their dispersal activity with increasing humidity and abundant water bodies. Field observations (using radio-telemetry) also confirmed this, showing that toads prefer to move on wet and humid nights (Phillips et al., 2007; Schwarzkopf and Alford, 2002).
In stark contrast, Freeland and Martin (1985) found that the distance moved each year by the frontal population was not related to rainfall. It is unlikely this discrepancy is due to experimental procedures because Freeland and Martin (1985) used the same technique as Easteal et al. (1985). The most likely explanation for this inconsistency is that Freeland and Martin's (1985) study did not consider interactions between rainfall and other factors that are likely to be acting on toad dispersal, such as temperature. Failure to include interactions into a model can lead to vast differences in explanation power. For example, (Urban et al. 2008) found that 73.2% of the toad invasion range can be explained by the interaction between spatial heterogeneity and environmental factors; however, when these factors were measured separately environment and space explained only 0.1% and 26.3% of the variation, respectively.
In northern Australia, advancement in the WEA and the CEA is heavily restricted by annual rain regimes patterns. The dry and wet seasons that define the tropics heavily influences toad dispersal. Toads can endure extremely high temperatures in tropical areas, however, this is only possible if there is enough accessible water to keep them from desiccating. Therefore, toad movement is largely, if not entirely (e.g. in semi-arid country), limited to wet season months.
- length and intensity of the wet/dry season will largely impact yearly dispersal rates, where more rain = more dispersal opportunities
EVOLUTIONARY FACTORS THAT INFLUENCE THE RATE OF RANGE EXPANSION IN CANE TOAD
Adaptation following invasion events
Typically, explanations of rapid range expansions are ecological (Simmons and Thomas, 2004), while evolutionary genetics have been largely unexplored (Lee, 2002). However, when a species enters a novel environment it is likely to be presented with different selection forces (Sax et al., 2005). Selection can act on dispersal capacity or physiological tolerance in response to environmental gradients, such as temperature (ref). Changes in morphology (refs), physiology (refs), phenology (refs), or plasticity (refs) are common responses to selection pressures exerted by new habitats. Consequently, rapid evolution of invasive species is not uncommon (Whitney and Gabler, 2008). These evolutionary responses can alter rate of spread, as well as ecological processes of an animal (Sax et al., 2005).
Historically, the varying rates of cane toad range advance across Australia has been explained by climatic variables and geographical factors (Estoup et al., 2004). However, in the areas of accelerated spread environmental variation does not adequately explain the high rates of range advance (Urban et al. 2008). It is only until recently (last 10 years) that adaptive evolution has been seriously considered as an influence on expansion rates of toads. Such studies, however, have concentrated on the WAE. Therefore, the following discussion on adaptive evolution focuses solely on changes found in expansion front that is spreading west across northern Australia.
Selection on dispersal/Spatial selection
The most obvious traits that one would expect to influence the expansion rate of an invasive species are traits that are directly related to dispersal ability. Such traits are selected for in vanguard populations by a process called spatial selection. Therefore, it is one of the most, if not the most important selective forces that has potential to influence expansion rates via an evolutionary pathway. Spatial selection is a process that generates differences in dispersal ability through space and time. For example, toads at the front of the invasion have higher dispersal abilities than toads from the core population. This occurs in expanding populations because individuals are separated by their dispersal ability. Toads that have a higher ability to disperse will form the very frontline of the invading population at any point in time (eg. Ref, Phillips et al 2006). These frontline individuals will breed with each other (Easteal and Floyd, 1986; Endler, 1977). If there are any heritable traits that are related to dispersal ability, then these traits will be passed on to the offspring, and the offspring of the toads on the front will have relatively higher dispersal ability (compared to the offspring of individuals from populations behind the frontline). If this were to occur every generation, the process of spatial assortment will continually select for increased dispersal in the vanguard population. Contributing to this runaway evolutionary effect are density effects (Phillips et al., 2010). Individuals that are at the forefront of an invasion will experience low-density environments (i.e., fewer conspecifics in the area) and, consequently, leave more offspring. Therefore, the evolution of increased dispersal ability is driven by the interaction of density effects and spatial selection. Any adaptations for rapid dispersal that occur will accumulate in the invasion front and, as a result, the vanguard population itself will increase its dispersal speed through time (Alford et al., 2009).
It is understandable that the majority of adaptive evolution studies on cane toads have focused solely on spatial selection and its influence on dispersal ability. Spatial selection has been used to explain a number of dispersal related adaptations found in cane toads. For example, shifts in behaviour (frequency and distance of movement, straightness of displacement) where demonstrated by Alford et al. (2009) and Phillips et al. (2008), changes in life history trait (growth rate) has been demonstrated by Phillips et al. (2009), while changes in both morphology (leg length; Phillips et al., 2006)) and in locomotor ability (endurance; Llewelyn et al., 2010) have also been shown.
Alford et al.'s (2009) study best demonstrates spatial selection theory by establishing a pattern in an observed shift in behavioral traits. Three populations of cane toads were radio-tracked and displacement rate (m/day) and mean distance moved per move were recorded. One population was collected from a location where toads had been established for ~50 years. The remaining two populations were both frontline populations, one from 1992 and the other from 2005. Behavioural shifts, including frequency and distance of movement, and straightness of displacement, was found to be highest at the 2005 frontline, lowest in the oldest, most established population, and in between in the 1992 frontline. This is the only study that demonstrates a change between frontline populations where dispersal ability is shown to be selected upwards during invasion (of suitable habitat). (explain better, advantage of using 2 frontlines…)
Repeatability
Phillips et al 2008
- 4 pops from different stages of invasion front, used radiotelemetry and released from same location (eliminates environmental variability)
- complements Alford et al 2009 - found frontal toads move more often, move further with each move, and follow straighter paths
Difference between testing frontline toads like Alford and 4 pops from the same year (phillips)?
Proof that its genetically based otherwise older pops will have the same dispersal ability??
Phillips et al 2009
- same 4 pops as Phillips et al 2008
- found that (juvenile) growth rate increased with distance from introduction point, suggesting consequence of r-selection during range expansion (that is, increased growth rate = increased pop growth, upwards selection for dispersal ability)
- caveats include: increased growth rate may not lead to increased pop growth if trade-offs are present (eg. Fecundity or survival); increased growth rate may be result of unknown environmental variation; increased growth rate may be result of correlated selection on other aspects of phenotype (eg. Body size) instead of r-selection.
Llewelyn et al 2010.
- 2 pops, one from 1yr behind frontline and one from 70 yr old pop (Townsville).
- tested at same time in same location under same conditions
- speed and endurance measured on runway
- higher endurance but not speed at invasion front
- contradicts Phillips et al 2006
Phillips et al 2006
- same place, different time
- other studies are all different places, same time - effects?
- found toads that reached a location first had relatively longer legs
- also found that toad with relatively longer legs moved fasted over a short distance and further in 24hrs
Need to link this to the reason for my relative leg length experiments.
I will test difference in RLL in space not time. Will be more similar to Llewelyn et al 2010 who didn't find any difference in leg length (or speed). That may be because RLL changes because of phenotypic plasticity and Llewelyn didn't test the absolute frontline, instead they tested 1yr behind it.
Some studies only used 2 pops so no pattern was established.
The strength of these studies is that their results have been demonstrated under laboratory conditions and observed in the field.
Which traits are heritable?
One of the most interesting thing about cane toads and dispersal is their drive to disperse, even into suboptimal conditions such as semi-arid environments. So where does this apparent drive come from? Is behavior the driver or the consequence of the increase in these dispersal ability traits. That is, do toads with greater endurance and longer legs have a more mobile behavior because they have evolved traits during the invasion process that enable them to move for longer periods of time? Or have toads evolved an inherent drive to disperse which, through increased activity, produce divergences in the leg length and endurance? For example, frontline toads that have greater endurance might simply reflect a developmentally plastic response to sustained high levels of activity (Llewelyn et al., 2010). I have no idea what to suggest for future here!
The major limitation of this area of study is that not one of the above studies determined the underlying mechanisms to these trait changes. Are the toads adapting towards a better dispersal genotype? Or does it merely represent plasticity associated with encountering new (toad-sparse) environments?
Genetic adaptation likely because otherwise all pops behind frontline should be more similar i.e. no pattern of increasing traits. However, toads have already demonstrated high levels of phenotypic variation so it is likely due to both.
A very recent study has made the first step to addressed this issue by looking at hereditary in dispersal ability (Phillips et al., 2010). The results of the study suggest that there is slight, but discernable, heritability occurring in dispersal ability, suggesting a genetic basis to the increasing dispersal abilities of the vanguard toads. This study empirically tests evolution on accelerating range fronts and is one among a very small group of studies that have done the same in other species (Cwynar and MacDonald, 1987; Hughes et al., 2007; Simmons and Thomas, 2004). More importantly, Phillips et al. (2010) are potentially the first to quantify, albeit tentatively, the heritable basis of changes in dispersal rates resulting from range shift. More empirical tests are needed in this area of study, not just on cane toads but on other species that have advancing populations, to help us better understand the potential importance of the spatial selection process.
Llewelyn et al.'s (2010) experiment could have looked more closely at the different contribution between adaptive plasticity and evolution by conducting a split-clutch, translocation design experiment. This would involve the collection of eggs (after controlling for one sire) from each population and raising half of each clutch in each environment from which they came.
Needs further explanation or should I just site a paper that explains the experiment?
Also, reciprocal transplants (mentioned above) and careful correlations between ï¬tness and phenotype (e.g. Arnold & Wade 1984) can be used to confirm that environmental variation alone does not account for varying rates of spread.
Example of models helping understand…
Suthurst et al 1995 modelled predicted range of toads using temperature, rainfall and humidity range from native distribution of the toad in Central America, Mexico, and southern Texas as reference. The accuracy of this model is reduced under the following situations: unique conditions (not found in native range), adaptation, phenotypic plasticity or random sampling of naïve genetic variation (thomas et al 2001, lee 2002, more refs in urban et al if needed). (urban et al 2007). Consequently, this model largely underestimated potential range. Did not accounted for this dynamic interaction between organism and environment (kearney).
THE MODELING PERSPECTIVE
Modeling species current and future dispersal helps us to understand the response of species to their environment and to predict their dispersal rate and range. The practice of modeling dispersal uses tools from mathematics and statistics, data management and geographic spatial analysis (Niche modeling book). This approach to studying species range dynamics demonstrate how complex a phenomenon dispersal actually is.
The difficulty in these models is determining which factors to include. Using the most influential factors will accurate model. Events are stochastic eg climate
need to determine how fast they will get to x AND where their range will stop. Both of these remain unexplained to some degree.
Prediction:
A number of studies have attempted to model the toad invasion in the EEA and/or in the NEA. The first was done by… Some have used such models to predict time of arrival and have all largely underestimated the ability of the toads to increase spread. Others have used models to quantify and qualify the different factors involved in range expansion. Such theoretical models are very complex and the understanding of the advantages and disadvantages of the various models that have been used over the past ?? years are beyond the scope of this review. I will, however, point out the importance of these theoretical models….(can quantify the 'comparative' level of impact of each process, ) Empirical data to complement such models is relatively scarce and is certainly an area that needs attention in the future.
Generally, models of range expansion assume constant rate (Hastings et al., 2005). However, due to the many factors that influence toad dispersal rates that I have discussed throughout this review, it is clear that the expanding range of cane toads in Australia does not advance at a constant rate. Complex models have been produced in attempts to explain this (Kearney et al 2008, all the rest). These models can be used to gain understanding in dispersal and interactions between factors that may be hard to do empirically. The complexity of these models demonstrates the complexity of the phenomenon. Outside the scope of this review however I think it is important to note because ultimately all the studies done on individual factors dont tell us anything about expansion rates unless they are incorporated into models and maps - important tool for prediction.
The large number of potential factors of dispersal and the fact that many of them have a random element (eg. Human-assisted dispersal, adaptive evolution) means that the accuracy of these models
The difficulty of modeling lies in quantifying interactions between all the factors. The high number of variables (factors of dispersal), many of which require a random element incorporated in the model (e.g. niche evolution), means that a model that were to include all factors would be ineffective in its outcome. Presumably, that is why the prior models focus on one set of factors, such as climate or physiology, and their interactions within each set. Therefore, it seems clear that although we have a good understanding of the importance of each factor and their potential effects on varying rates of range expansion, we haven't developed a thorough understanding of how they do this, especially how the factors impact on each other throughout the range expansion process.
Conclusion (already at max word limit for this section)
There has been a large focus on the invasion biology of cane toads, with more recent attention focusing on range expansion. In this review I have established a wide range of factors that contribute to the range expansion dynamics of this invasive species. There is a strong appreciation of the importance of environmental variables and their effects on cane toad dispersal, however, it is now clear that these factors alone cannot explain the acceleration and deceleration of range expansion that has been observed over invasion history of the cane toad in Australia.
In attempts to further understand the range expansion dynamics of cane toads, recent literature has addressed evolutionary processes that influence toad expansion rates, using spatial selection theory to explain shifts in behavioural, morphological and physiological traits. Although these studies demonstrate the importance of spatial selection process, they fail to determine what is driving the observed changes in dispersal related traits. They could be behavioural
And how much is due to adaptive plasticity and how much is due to adaptive evolution. genetic experiments such as reciprocal translocatioin can be used to determine dif in plastic and genetic responses. It is evident that more studies, which incorporate hereditary experiments (e.g. Phillips et al. 2010) are required in this area.
The relatively short period over which these behavioural, morphological and physiological shifts have emerged (~50 generations, ref) testiï¬es to the intense selective pressure exerted on vanguard populations of range-shifting species (ref). Not only is this process important for our understanding of the rates of invasion by non-native species, but it also has implications for the rate of range-shift in native taxa affected by climate change (Pearson 2006).
Modeling future range advance is an extremely complex process. While both ecological and evolutionary mechanisms likely have interacted to accelerate (and decelerate) invasion speeds, the complexity of such interactions, coupled with a limited understanding of the ways in which many factors (specifically evolutionary factors) affect spread, limits our ability to predict future range advance. We still need to enhance our understanding of the ways each of these processes impact on each other before we can combine them together in as attempt to explain spread.
Future study is necessary in all areas:
The capacity for adaptation needs to be more thoroughly explored/evolutionary potential - different potential for both south and northern invasion fronts, homing/how do they find water in desert?, hereditary reliance?
Dispersal corridors - waterways
Future attempts to predict range dynamics for invasive species should consider heterogeneity in both the environmental factors that determine dispersal rates and the mobility of invasive populations because dispersal-relevant traits can evolve in exotic habitats. - this comment comes from my stuff on predictive models which I have not sent you.
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