No description available.

No description available.

No description available.

No description available.

Turbidity has generally been viewed as having detrimental effects on fish; yet, many turbid habitats in the world are also abundant with fish. This phenomenon is often explained as fish enjoying reduced predation pressure in turbid habitats. This represents a trade-off situation where fish should select clear or turbid habitats that provide maximum net benefits. Because turbidity reduces light penetration, both predator and prey visual ranges are reduced, rendering both less efficient foragers. For this reason, I suspected that the benefits of a turbid environment would be greatest in the presence of predators and hypothesized that when predation risk is high, prey should prefer turbid water. Laboratory experiments showed that regardless of predation risk, fathead minnows (Pimephales promelas) preferred feeding in a turbid habitat. The presence of a predator, yellow perch (Perca flavescens) or black bullhead (Ameiurus melas), caused minnows to reduce feeding. There was on interaction between water clarity and predation risk, water clarity and predation risk, thus, appeared to affect the minnows’ habitat selection independently. The predator’s effect on the prey was the same whether in turbid or clear water. Using the prey distributions established in the lab experiment, key parameters and assumptions were identified for a computer model which simulated both prey and predator responses to turbid water and their interactions. The model predicted that prey would always prefer the turbid habitat when one was available. Predators generally used both clear and turbid habitats. Only when its foraging efficiency was reduced significantly did the predator show strong avoidance of turbid water. As the number of predators increased, predators used both clear and turbid habitat more evenly. Turbid environments seem to provide important habitats for small and juvenile fish. It may benefit small fish by reducing predator efficiency or reduce prey energy expenditure. / October 2006

Turbidity has generally been viewed as having detrimental effects on fish; yet, many turbid habitats in the world are also abundant with fish. This phenomenon is often explained as fish enjoying reduced predation pressure in turbid habitats. This represents a trade-off situation where fish should select clear or turbid habitats that provide maximum net benefits. Because turbidity reduces light penetration, both predator and prey visual ranges are reduced, rendering both less efficient foragers. For this reason, I suspected that the benefits of a turbid environment would be greatest in the presence of predators and hypothesized that when predation risk is high, prey should prefer turbid water. Laboratory experiments showed that regardless of predation risk, fathead minnows (Pimephales promelas) preferred feeding in a turbid habitat. The presence of a predator, yellow perch (Perca flavescens) or black bullhead (Ameiurus melas), caused minnows to reduce feeding. There was on interaction between water clarity and predation risk, water clarity and predation risk, thus, appeared to affect the minnows’ habitat selection independently. The predator’s effect on the prey was the same whether in turbid or clear water. Using the prey distributions established in the lab experiment, key parameters and assumptions were identified for a computer model which simulated both prey and predator responses to turbid water and their interactions. The model predicted that prey would always prefer the turbid habitat when one was available. Predators generally used both clear and turbid habitats. Only when its foraging efficiency was reduced significantly did the predator show strong avoidance of turbid water. As the number of predators increased, predators used both clear and turbid habitat more evenly. Turbid environments seem to provide important habitats for small and juvenile fish. It may benefit small fish by reducing predator efficiency or reduce prey energy expenditure.

Turbidity has generally been viewed as having detrimental effects on fish; yet, many turbid habitats in the world are also abundant with fish. This phenomenon is often explained as fish enjoying reduced predation pressure in turbid habitats. This represents a trade-off situation where fish should select clear or turbid habitats that provide maximum net benefits. Because turbidity reduces light penetration, both predator and prey visual ranges are reduced, rendering both less efficient foragers. For this reason, I suspected that the benefits of a turbid environment would be greatest in the presence of predators and hypothesized that when predation risk is high, prey should prefer turbid water. Laboratory experiments showed that regardless of predation risk, fathead minnows (Pimephales promelas) preferred feeding in a turbid habitat. The presence of a predator, yellow perch (Perca flavescens) or black bullhead (Ameiurus melas), caused minnows to reduce feeding. There was on interaction between water clarity and predation risk, water clarity and predation risk, thus, appeared to affect the minnows’ habitat selection independently. The predator’s effect on the prey was the same whether in turbid or clear water. Using the prey distributions established in the lab experiment, key parameters and assumptions were identified for a computer model which simulated both prey and predator responses to turbid water and their interactions. The model predicted that prey would always prefer the turbid habitat when one was available. Predators generally used both clear and turbid habitats. Only when its foraging efficiency was reduced significantly did the predator show strong avoidance of turbid water. As the number of predators increased, predators used both clear and turbid habitat more evenly. Turbid environments seem to provide important habitats for small and juvenile fish. It may benefit small fish by reducing predator efficiency or reduce prey energy expenditure.

No description available.

The systems of interest in this study are the spread of epidemics and invasions from a small propagule introduced into an arena that was initially devoid of the given species or stage of illness. In reaction-diffusion models, populations are continuous. Populations at low densities have the same growth functions as populations at high densities. In nature, such low densities would signify extinction of a population or of a disease. This property can be removed from reaction-diffusion models by small changes in the formulation so that small populations become extinct. This can be achieved by the use of a threshold density or an Allee effect, so there is negative growth at low densities. Both these alterations were made to the Fisher model, a predator-prey model and a two stage and a three stage epidemic model. A semi-numerical method, termed the Shooting method, was developed to predict the shapes and velocities of these wave fronts. This was found to correctly predict the velocity, the peak density of the invading stage or species and the width of the wave front. It was found that in oscillatory cases of the multi species models, a high threshold can remove the wave train or wake which would normally follow the wave front, so the wave becomes a soliton. The next step is to investigate probable causes of persistence behind the initial wavefront. To do this, discrete time and space versions of the models were formulated so that experiments investigating persistence can be carried out in a two dimensional arena with less computational effort. The formulations were chosen so that at reasonable time and space steps the discrete models show no behaviour different to that of the reaction diffusion model, and so that the Shooting method could also be used to make predictions about these wavefronts. Three mechanisms of persistence are investigated; environmental heterogeneity, long range dispersal and self organised patterns.

Evolution can occur rapidly, along timescales that are traditionally regarded as 'ecological'. Despite growing acceptance among biologists of rapid evolution, a strong paradigm of contemporary evolution is still absent in many sub-disciplines. Here I apply a contemporary evolution viewpoint to conservation biology. Specifically, I examine the impact of cane toads (Bufo marinus) on Australian snakes. Toads were introduced into Australia in 1935, have spread rapidly and represent a novel, extremely toxic prey item to na�ve Australian predators (including snakes). Based on dietary preferences and geographic distributions I find that 49 species of Australian snake are potentially at risk from the invasion of the toad. Furthermore, examination of physiological resistance to toad toxin in 10 of these �at risk� species strongly suggests that most species of Australian snake are poorly equipped to deal with a likely dose of toad toxin. Even species that are highly resistant to toad toxin (such as the keelback, Tropidonophis mairii) face indirect fitness costs associated with consuming toads. Within a population of snakes however, the impact of toads is unlikely to be random. For example, the examination of several component allometries describing the interaction between snakes and toads revealed that, within a species, smaller snakes are more likely to ingest a fatal dose of toad toxin than are larger snakes. Further consideration of the interaction between snakes and toads suggests that toads will not only be exerting differential impact on snakes based upon morphology, but also exert non-random selection on prey preference and resistance to toad toxin in snake populations. To examine the possibility of a morphological response by snakes to toads, I examined changes in the body size and relative head size of four species of snake as a consequence of time since exposure to toads. Two of the species (green treesnakes and red-bellied blacksnakes) are predicted to face strong impacts from toads. These two species showed an increase in mean body size and a decrease in relative head size as a consequence of time since exposure to toads; both changes in an adaptive direction. In contrast, the other two species (keelbacks and swampsnakes) are predicted to face much lower impact from toads, and these two species showed little or no evidence of morphological change associated with time since exposure to toads. These results indicate an adaptive change in morphology at a rate that is proportional to the predicted level of impact for each species, strongly suggesting an evolved response. Red-bellied blacksnakes (a toad-vulnerable species) were further assessed for evolved responses in prey preference and toxin resistance. Comparisons between toad-exposed and toad-na�ve populations of blacksnakes revealed that snakes from toad-exposed populations exhibited slightly higher resistance to toad toxin and a much-reduced tendency to eat toads, when compared with toad-na�ve snakes. Na�ve snakes exhibited no tendency to learn avoidance of toxic prey, nor were they able to acquire resistance to toxin as a result of several sub-lethal doses, suggesting that the observed differences between populations is evolved rather than acquired. Together, these results strongly suggest that blacksnakes are exhibiting an evolved shift in prey preference and toxin resistance as a consequence of exposure to toads. Thus, it appears that snakes are exhibiting adaptation at multiple traits in response to exposure to toads. Given the high likelihood that these adaptive shifts have an evolved basis, it appears that the impact of toads will decrease with time in many snake populations. But what about toads? Because the outcome of the interaction between a toad and a snake is also mediated by the body size and relative toxicity of toads, it is important to understand how these traits vary in space and time. Exploratory analysis revealed that toads exhibit a decrease in body size and a decrease in relative toxicity as a consequence of time since colonisation, indicating that their impact on native predators decreases with time. Additionally, there appears to be meaningful spatial variation in toad relative toxicity, indicating that some populations of native predators are facing higher impact from toads than others. Overall, these results clearly indicate the importance of assessing the potential for rapid evolutionary response in impacted systems. Doing so may provide evidence that some species are in less trouble than originally thought. Additionally, and as more data accumulate, it may be possible to characterise certain categories of environmental impact by their potential for eliciting adaptive response from �impacted� species. This approach has strong implications for the way conservation priorities are set and the way in which conservation dependent populations are managed.