in fish communities
Peter Eklöv Umeå 1995
Department of Animal Ecology Umeå University
S-901 87 Umeå
Akademisk avhandling
som med vederbörligt tillstånd av rektorsämbetet vid Umeå universitet för erhållande av filosofie doktorsexamen i ekologisk zoologi kommer att
offentligen försvaras den 3:e februari 1995, kl 0900 i hörsal E, Humanisthuset.
Examinator: Prof. Christian Otto, Umeå
Opponent: Prof. Gary G. Mittelbach, Hickory Comers, USA
ISBN 91-7174984-5 Cover by Görel Marklund
S-901 87 Um eå, Sw eden Date Of issu e February 1995 Author Peter Eklöv
. Effects of behavioural flexibility and habitat complexity on predator-prey ' e interactions in fish communities
A b s tra c t
This thesis treats mechanisms important for predator-prey interactions in fish communities. Especially, the effects of behavioural flexibility and structural complexity for predator efficiency and antipredator responses of prey have been treated in a number of experiments using piscivorous perch (Perea fluviatilis) and pike (Esox lucius) as predators and juvenile perch and roach (Rutilus rutilus) as prey. These experiments have included pool, enclosure and whole lake studies at various temporal scales, ranging from instantaneous measurements to samplings over years.
In the absence of prey refuge, piscivorous perch were favoured by being in a group compared to when single. In contrast, pike were less efficient and grew less when five compared to when they were single. When they were five, the larger pike had a higher growth rate compared to the smaller individuals due to interactions between individuals.
In the presence of prey refuge, both piscivorous perch and pike decreased their foraging efficiency compared to prey refuge absent and perch switched to a sit-and-wait foraging mode in which they were less efficient and had a lower growth rate compared to pike. Pike also caused a stronger antipredator response of perch prey compared to piscivorous perch.
Piscivorous perch-induced habitat restrictions in juvenile perch and roach did not affect growth rate of juvenile perch whereas growth rate of juvenile roach was negatively affected. The higher growth of juvenile perch compared to roach was related to that juvenile perch increased exploitation of structure associated prey in refuges. At the same time, the structural complexity formed an almost complete refuge for juvenile roach whereas juvenile perch showed a significant mortality in all predator treatments. Juvenile roach both formed tight schools and used the refuge to escape predation from piscivorous perch whereas juvenile perch avoided piscivorous predation by using the refuge only. Juvenile roach had a higher capacity to avoid piscivorous perch compared to juvenile perch but juvenile perch responded to predators by a more flexible refuge use than juvenile roach.
Juvenile perch showed a flexible predator inspection behaviour and always stayed in the predator-free part of the pool in the presence of piscivorous perch. In contrast, roach increased switch frequency between the two parts of the pool and stayed in the vegetation structure even when piscivorous perch were located in this part. The difference in juvenile perch and roach behaviours can be related to that they use different cues in the environment when assessing predation risk.
The field study suggested that the quantity and quality of habitat structural complexity are o f major importance for the distribution patterns of perch and pike. The spatial distributions of the fish suggested a strong behavioural interaction between predator and prey. A comparison between the different studies in this thesis suggests that the scale of the study (temporal and spatial) is important when evaluating the effects of individual behaviour on predator-prey interactions in a population and community perspective.____________________________
Key words
predator-prey interactions, behavioural flexibility, habitat complexity, foraging mode, sit-and-wait, group foraging, Perea flu v ia tilis, Esox lucius, Rutilus rutilus, size-structured interactions, species-specific antipredator behaviour, proximate cues, habitat distribution, temporal patterns
Language English •SBN 91-7174984-5 Number of p ag es ____________ I_______________________ ^_______ 134________
Signature y Date 1994-12-16
List of papers ...4
Introduction ... 5
Species descriptions ... 6
M ethods ... 6
The foraging mode and the efficiency of piscivorous predators...7
The importance of mutualistic and competitive interactions... 7
The importance of prey refuges ... 8
The role of prey refuges and antipredator capacities for predator-prey*prey interactions 9
Effects of developmental constraints for antipredator response to predatory c u e s ... 10
Predator-prey interactions in structurally complex environments: spatial and temporal p atterns ... 11
Concluding rem arks ... 12
R eferences ... 14
A cknow ledgm ents ... 19
Appendices: Papers I-VI
- LIST OF PAPERS -
This thesis is a summary and discussion of the following papers which will be referred to in the text by their Roman numerals.
I Eklöv, P. 1992. Group foraging versus solitary foraging efficiency in piscivorous predators: the perch, Perea fluviatilis, and pike, Esox lucius, patterns. Animal Behaviour 44:313-326.
II Eklöv, P. and Diehl, S. 1994. Piscivore efficiency and refuging prey: the importance of predator search mode. Oecologia 98:344-353
III Persson, L. and Eklöv, P. 1995. Prey refuges affecting interactions between piscivorous perch and juvenile perch and roach. Ecology, in press.
IV Eklöv, P. and Persson, L. 1995. Species-specific antipredator capacities and prey refuges: interactions between piscivorous perch (Perea fluviatilis) and juvenile perch and roach (Rutilus rutilus). Submitted manuscript.
V Eklöv, P. and Persson, L.1995. Predation risk and antipredator response: proximate cues for refuging juvenile fish. Submitted manuscript.
VI Eklöv, P. 1995. Effects of habitat complexity and prey abundance on the spatial distribution of piscivorous perch (Perea fluviatilis) and pike (Esox lucius) in a temporally varying environment. Manuscript.
Papers I, II and III are reproduced with the permission from the publishers.
Effects of behavioural flexibility and habitat complexity on predator-prey interactions in fish communities
Introduction
The outcomes of species interactions has successfully been used to predict community
dynamics and have been a major topic in ecology during the last decades (Paine 1966, Murdoch and Oaten 1975, Hassel 1978, Werner et al. 1983, Sih 1987, Persson et al. 1995). Species interactions involve various aspects of predator-prey interactions in which particular behaviours of organisms determine the nature and magnitude of the interaction (see Werner 1992). This imply that the behavioural repertoire of individual organisms should be critical for the outcomes of predator-prey interactions. Therefore, particular behaviours of individuals also most likely have further consequences at the population and community levels. Despite this, the majority of / theory concerned with species interactions have essentially ignored behaviour and very few studies have addressed the linkage between adaptive behaviour and population dynamics (see Werner 1992). Nevertheless, there is a growing interest in mechanistic approaches in ecological studies in which behavioural results are employed to generate explicit mechanisms for species interaction theory (see reviews in Persson and Diehl 1990, Werner 1992, Persson et al. 1995).
Flexible behaviour and habitat complexity can be suggested to have strong effects on predator- prey interactions because the two components affect the encounter rate between predators and prey (Jeffries and Lawton 1984, Werner 1991, Werner and Anholt 1993). Flexible behaviour of individuals may affect encounter rates between predators and prey by choice of activity level and habitat (see Persson et al. 1995). Habitat complexity may affect the encounter rate between predators and prey by that the structure affect the visual contact between predators and prey.
In my thesis, I have used the mechanistic approach to study the effects of behavioural flexibility on predator-prey interactions in relation to habitat heterogeneity in fish communities.
Because of the different approaches and scales (spatial and temporal) used, my thesis touches ecological questions relevant to the individual, population and community levels. I have focused on three major questions: 1) how do interactions between individual predators affect their foraging efficiencies? 2) what factors determine the efficiencies of the antipredator behaviours of prey species? 3) how do different types and densities of structural complexity affect the outcomes of predator-prey interactions?
Species descriptions
In my studies I have used three of the most common fish species in Scandinavian lake ecosystems: perch {Perea fluviatilis), pike (Esox lucius) and roach (.Rutilus rutilus) (Svärdsson 1976). Roach is an omnivorous species that feeds mainly on zooplankton as a juvenile but shifts to omnivory and feeds on a mixture of zooplankton, macroinvertebrates,
detritus and plant material later in life (Prejs 1984, Johansson and Persson 1986). Perch is a carnivorous species that undergoes three major ontogenetic shifts with respect to diet and habitat (Persson 1988). As juveniles, they first feed on zooplankton, mainly in the pelagic zone and with an increase in size they shift to benthic feeding mainly in the littoral zone (Persson 1986). The final and adult stage is dominated by piscivorous feeding in both the littoral and pelagic zones. The length of the zooplanktivorous stage is negatively correlated with the density of roach (Persson and Greenberg 1990) and in lakes with high densities of roach, perch shift to macroinvertebrates at a small size and age (Persson 1983, 1986). Juvenile perch that shift to feeding on macroinvertebrates at an early stage show retarded growth and very few individuals turn to piscivory (Persson 1986). Piscivorous perch are active social predators that benefit by group foraging (I) and their efficiency depends on the degree of structural complexity (E). Pike have a very short zooplanktivorous stage and become piscivorous at very small sizes (Giles et al. 1986). As a piscivore, pike use a sit-and-wait strategy and stay mainly in the littoral zones of lakes (Diana et al. 1977, Diana 1980). Pike consume a large variety of prey, dependent largely on prey availability (Neuman 1968, Mann 1982). They are highly cannibalistic and have been found to consume conspecifics up to about 50% of their own size under natural conditions (Craig and Kipling 1983).
Methods
I performed three pond enclosure experiments (papers ITV), one pool experiment (paper V) and a field study (paper VI). The first two pond experiments were conducted to test the effects of predator numbers (paper I) and the presence of prey refuges (paper II) on the foraging efficiencies of piscivorous perch and pike. In the first experiment, I constructed large plastic enclosures (72 m^) in which I manipulated piscivorous perch and pike numbers, and I cleared all natural vegetation providing an open water environment. In the second experiment, which was conducted the following year together with Sebastian Diehl, we used the same enclosures and allowed submerged vegetation to grow in one third of the enclosure. Piscivorous perch or pike were manipulated as present or absent and were restricted to the open water part of the enclosures by a dividing net, while two size classes of juvenile perch could move freely between the two habitats. In both experiments, I quantified the behaviours of predators and
prey from a mobile tower which reached about 6 m above the water surface, and which was placed at the edge of the pond.
Papers El and IV are from an experiment I conducted together with Lennart Persson.
We constructed plastic enclosures similar in size to those of papers I and II (70m^) and
experimentally analyzed the effects of structural complexity and the impact of piscivorous perch on survival, diet, growth and behaviour of juvenile perch and roach. The structural complexity consisted of artificial vegetation which was either absent, formed a partial refuge for the prey or formed a complete refuge for the prey. The behaviours of predators and prey were quantified using the same method as in papers I and II.
The experiment described in paper V was also conducted together with Lennart Persson. In an indoor wading pool (radius 135 cm), we subjected juvenile perch and roach to different types of structure (artificial vegetation and PVC pipes) and predators (piscivorous perch and pike) to test the precision of the antipredator response of the two prey species. The behaviours and mortalities of the prey were quantified.
Paper VI is a field study in which I have used gill net fishing, electrofishing and underwater observation to quantify fish abundances in a small low productive lake. Fish abundances were quantified in relation to habitat availability, vegetation density and resource abundance. The movement patterns of piscivorous perch and pike were quantified by individual marks and recaptures.
The foraging mode and efficiency of piscivorous predators
The importance o f mutualistic and competitive interactions
Predators typically use different foraging modes and have generally been categorized as either sit-and-wait or actively searching foragers (Pianka 1966, Schoener 1971, Huey and Pianka
1981). The search mode of an individual predator has often been related to its response to the behaviour and density of its prey (Huey and Pianka 1981, Gendron and Staddon 1983, Sih
1984). What has been less treated is the search mode of predators in relation to gain and costs in mutualistic or competitive interactions between individual predators, which occurs in group foraging and territoriality. Group foraging and territoriality are two major aspects of an
individual's potential foraging repertoire which can increase its energy intake rate and reduce its risk of predation (Hixon 1980, Schoener 1983, Magurran et al. 1985, Clark and Mangel
1986). Furthermore, mutualistic interactions between group foraging top predators may increase predator population density (Wollkind 1976). In contrast, territorial behaviour have been found to be of decisive importance for limiting population densities because increased population densities will increase intraspecific interactions among predators and thereby reduce
their foraging efficiency on available prey (Wollkind 1976, Patterson 1980, Grant and Kramer 1990). Efficiencies of predators are dependent on their abilities to respond behaviourally to differences in prey availability by changing travel speed or foraging mode (Gendron and Staddon 1983, East and Magnan 1991).
The importance of prey refuges
In heterogeneous environments, prey refuges in the form of complex physical stmcture have been shown to decrease encounter rates between predators and prey and cause behaviourally flexible predators to switch from an active to a more stationary search mode (Savino and Stein
1982, 1989, Anderson 1984). A predator's potential capture rate outside refuges is dependent on the balance between its own capture ability and the prey's ability to escape into the refuge.
Therefore, it is important for a predator which forages close to prey refuges to minimize the duration of the interaction with the prey, and catch it before it can manoeuvre and reach shelter (Isaacs 1975). In this context, ambush foraging might be a profitable strategy because many ambush predators have high strike efficiencies (Webb 1984, Eklöv and Hamrin 1989). In contrast, actively searching predators are often characterized by low strike efficiencies, and instead are often dependent on group foraging for prey capture (Schaller 1972, Nursall 1973, D-
The major results that emerged from experiments I and II concerned predator efficiency and growth in relation to predator activity levels which were a function of the absence and presence of prey refuges. In the absence of a prey refuge, perch were more efficient and had higher individual growth rates when five individuals were actively foraging in a group than to when they were single (I). In contrast, pike had lower efficiencies and grew less when they were five than when they were single. When there were five, the larger individuals had higher growth rates compared to the smaller individuals due to interactions between individuals. In the presence of a prey refuge, both predators decreased their foraging efficiencies compared to the absence of a prey refuge and perch switched to a sit-and-wait foraging mode in which they were less efficient and had lower growth rates compared to pike (II). Thus, when prey refuges are present, perch are able to switch foraging mode but at a cost of lower foraging efficiencies compared to pike. The higher impact of pike on prey also caused a stronger antipredator behavioural response of 0+ and 1+ perch. The prey used a higher proportion of the refuge in the presence of pike and stayed at a longer distance from pike than from piscivorous perch.
The results suggest that perch should be more efficient than pike in open water environments such as in the pelagic zone of lakes, whereas pike should be favoured over piscivorous perch in environments which provide refuges, such as the littoral zone of lakes.
The role of prey refuges and antipredator capacities for predator- prey*prey interactions
The impact of the simultaneous presence of predation and competition on prey communities is closely linked to the size of the individual organism (Ebenman and Persson 1988). As a result, interactions between species cannot be generally classified as competition or predator-prey, because the primary interaction between individuals of different species may shift over ontogeny as a result of an increase in individual size (Werner 1988, Wilbur 1988, Polis 1991).
Conflicting size-specific selection pressures over ontogeny often have different effects on different species leading to asymmetries in competitive and predator-prey interactions (Werner and Gilliam 1984, Werner 1988, Persson 1988). Habitat complexity is likely to affect such asymmetric interactions due to species/size-specific competitive abilities in different habitats and because habitat structural complexity may act differently as a prey refuge for different species.
In addition to a reduce predation risk by moving into protected habitats, prey can evolve antipredator traits which reduce the probability of a successful attack once encountered by the predator (Hileman and Brodie 1994). These species-specific antipredator capacities have been separated into pre-attack (immobility, crypsis and schooling) and post-attack capacities (spines, shells, distastefulness) (Sih 1987b).
Previous experiments at smaller spatial scales (aquaria, small enclosures in lakes) have shown that juvenile perch forage with higher efficiency than juvenile roach on prey associated with structurally complex habitats and juvenile perch are also less affected in their foraging performance by the presence of physically complex structures than roach (Persson 1991,
1993). At the same time, pool experiments demonstrated that juveniles of perch and roach respond to piscivorous predation in species-specific ways by differing in their ability to use qualitatively different prey refuges (Christensen and Persson 1993, V).
The results from experiments IE and IV showed that predator-induced habitat restriction in juvenile perch and roach did not affect growth rate of perch whereas growth rate of juvenile roach was negatively affected. Thus, compared to juvenile roach, juvenile perch may
compensate more for lost foraging opportunity in the open water via increased exploitation of structure associated prey in refuges. As a result, the competitive interaction between juvenile perch and roach may alter via this predator-induced habitat shift. At the same time, the structural complexity formed an almost complete refuge for juvenile roach whereas juvenile perch showed a significant mortality in all predator treatments. The higher antipredator capacity of roach compared to perch explained the differential survival of the two prey species. Roach used alternative antipredator strategies either by forming dense schools or moving into the prey refuge, whereas juvenile perch only moved into the refuge in the presence of piscivorous
perch. On the other hand, juvenile perch used the different parts of the prey refuge in a more flexible way depending on the presence of predators and refuge type whereas juvenile roach used the different parts of the prey refuge in fixed proportions over all refuge treatments. These results from the perch-roach system suggest that the presence of structurally complex prey refuges may have profound effects on the foraging and antipredator capacities of juvenile perch and roach in the presence of piscivorous perch and this can be related to the two species distributions in lakes with different degrees of structural complexity.
Effects of developmental constraints for prey antipredator response to predatory cues
When an animal moves in an environment, it uses different cues to facilitate its behavioural decisions (e.g. to maximize food intake, to minimize predation risk etc.). These cues, termed proximate factors or “rules of thumb” have been a focus in behavioural studies aimed at predicting the cost and benefits of animal decisions (Stephens & Krebs 1986; Krebs &
Kacelnik 1992). The proximate cues may differ from the ultimate factors which cause the resulting behavioural patterns of the organism (Sih 1986). For example, when a prey escapes predation by moving into a vegetation refuge habitat, the proximate cue for that behaviour may be the structure of the habitat, whereas the ultimate reason is to reduce predation risk. The precision of the response of the organism depends on how well the behavioural repertoire can handle the different cues present in the environment (Drickamer & Vessey 1985).
The ability to make precise assessments of predation risk with a subsequent change in behaviour will also depend on the flexibility of prey behavioural responses to varying predation risks (Sih 1992). A high behavioural flexibility may allow the prey to assess predation risk more efficiently and switch between safe and unsafe habitats (Sih 1992). In this context, an important factor for prey survival is the difference between the assessed predation risk and the ability to make a precise response to minimize that predation risk.
In the pool experiment (V), juvenile perch showed a flexible predator inspection behaviour and always stayed in the predator-free part of the pool in the presence of piscivorous perch. In contrast, roach increased switch frequency between the two parts of the pool and stayed in the vegetation structure even when piscivorous perch were located in this part. Roach swam faster than juvenile perch in the presence of piscivorous perch whereas juvenile perch swam faster than roach in the presence of pike. Juvenile perch used both vegetation and pipe structure whereas roach only used vegetation structure as a refuge. These results suggest that juvenile perch displayed a more flexible behaviour than roach which simply moved into
vegetation under the threat of predation irrespective of predator location. The difference in
juvenile perch and roach behaviours can be related to the different selection regimes that the two species are exposed to over their ontogeny (IV). Animals which change habitat over ontogeny are often subjected to different and conflicting selection pressures on important behavioural and morphological traits at different stages of their development (Werner and Gilliam 1984; Werner 1988). The shifts in prey types used by perch involve a three-fold increase in prey length over ontogeny. Perch are morphologically and behaviourally preadapted to cope with a number of resources types during their ontogeny. For example, they have a body morphology and a behaviour which is usually associated with benthivorous feeding such as a relatively low cruising speed and high manoeuvrability, a relatively deep body, laterally inserted pectoral fins and enlarged dorsal and anal fins. At the same time, perch have morphological and behavioural traits typical for piscivorous foraging such as a relatively large gape size, a thick caudal
peduncle and an attacking feeding mode (Webb 1984). Roach do not show the same increase in prey size with an increase in body size over the life period (Persson 1988). As a consequence, they can be suggested to experience less conflicting selection pressures on foraging
performance over ontogeny than perch. Roach also have a relatively small gape size, high schooling tendencies and a higher swimming speed than perch, which is congruent with a planktivore feeding mode (Persson 1986). Thus, the cues used by juvenile perch and roach when assessing predation risk can be suggested to be results of their different life history patterns. The higher swimming capacity can be hypothesized to allow roach to use a relatively simple rule when encountering predators. Under natural field conditions this simple rule used by roach when exposed to predators, is generally adaptive and results in low predation
mortality compared to that of juvenile perch (IV). If roach have the capacity to escape predators by rapidly fleeing into the vegetation they will not, in contrast to perch, have to evolve more sophisticated antipredator behaviours. In contrast, the behavioural and morphological characteristics of perch can be suggested to be in conflict with the ability to outmanoeuvre predator attacks, and to use antipredator behaviours such as schooling (Magurran 1990; Webb and de Buffrénil 1990). Thus, differences in how prey asses and react to predation risk in relation to their life histories are important to consider when developing decision rules in the form of ”rules of thumb” for prey antipredator behaviour.
Predator-prey interactions in structurally complex environments:
spatial and temporal patterns
Spatial heterogeneity in the form of structural complexity provides high environmental
dimensionality and has been found to affect strongly the behaviours and foraging efficiencies of predators and prey (Savino and Stein 1982, 1989, Whitehead and Walde 1992, II, IH). Both
the quality and quantity of structural complexity have been shown to affect the interaction between predators and prey as a result of species-specific differences in predator efficiency and prey antipredator response (Savino and Stein 1989, Angermeier 1992,1, DI, IV, V). Different types of predators have been found to react in different ways to varying degrees of structural complexity as a result of interactions between structural complexity and foraging mode, and the decrease of encounter frequency with prey that accompanies an increase in structural
complexity (Savino and Stein 1982, 1989, H).
In lakes, habitat heterogeneity is most commonly present in the form of littoral zone vegetation or a depth gradient diversity, both of which may have profound effects on species abundance and diversity (Werner et al. 1977, Tonn and Magnusson 1982, Eadie and Keast
1984, Benson and Magnusson 1992). Greater diversity of habitats within a lake appears to create an opportunity for a more spatially heterogeneous fish community (Benson and Magnusson 1992) and different forms of structural complexity may provide shelter for fish to escape piscivorous predators (Werner and Hall 1988, Mittelbach 1988).
The purpose of paper VI was to explore the temporal variation in habitat distribution of fish. Potential fish species interactions were related to different densities of structural
complexity in a lake. The study involved quantitative estimates of different habitat types, estimates of macroinvertebrate prey availability and distribution and movement patterns of the fish. Predictions concerning the spatial distributions of fish were based on the results of my previous experimental studies (paper I-V). These studies showed that prey refuges are species- specific suggesting that the survival of prey depends on the availability of different types of refuges in the environment (IV, V). Alternatively, prey may use schooling as an antipredator strategy (V). The activity level of piscivorous perch and pike were predicted to be low since both predators have been found to use a sit-and-wait strategy in the presence of prey refuges (II). The spatial distribution of pike was predicted to be size dependent because of interactions between individual predators of different sizes (I, II). For example, large-sized piscivorous pike were found to interact with smaller pike and stayed closer to the prey than did smaller pike (I, II).
My study showed that the rapid decrease in abundance of 81-110 mm perch in the littoral zone in spring was a result of predation mortality and/or movement of these perch out to the pelagic zone. Perch of size 80-110 mm, that were found in the littoral zone in July, were highly associated with vegetated habitats. This size class of perch can be suggested to be highly vulnerable to piscivorous predation and the fact that perch and pike >160 mm stayed close to high density patches of perch of size 80-110 mm suggest that these perch were subjected to a high predation risk.
Both the biomass and diversity of macroinvertebrates increased monotonically with
vegetation density whereas perch abundance was maximum at an intermediate vegetation density. The preferred vegetation density of perch can be explained by foraging advantages and as an antipredator strategy. Pike size was inversely related to vegetation density and the largest pike individuals selected the tree structure habitat suggesting that the smallest pike (<160 mm) were subjected to a high predation risk from the larger pike. In addition, pike <160 mm avoided high density patches of perch of size 81-110 mm indicating that there were size- specific interactions between pike individuals.
Perch group size decreased with increasing vegetation density and the different size classes of perch occurred at different numbers in different group sizes. Perch <80 mm always occurred in groups larger than two individuals and never occurred in the same groups as perch of size >160 mm. In contrast, perch >160 mm preferred to stay in smaller groups and mainly remained solitary or in pairs. Large-sized perch showed no tendencies for homing behaviour and moved actively around the whole lake whereas pike showed a homing behaviour staying in the same zone or moving to the neighbour zone.
The difference in activity patterns of large-sized perch and pike may have consequences for their potential effects on prey populations at different time scales since different activity levels suggest different abilities to track variations in prey fish abundances. Furthermore, the different abilities to affect the habitat distributions and mortalities of different size ranges of prey suggest that piscivorous perch and pike would have the potential to affect their prey populations differently. The comparison between enclosure/pond studies (paper I-V) and the lake study suggests that the scale (both temporal and spatial) of the study is important when evaluating predator-prey interactions.
Concluding remarks
My thesis demonstrates the significance of the behavioural flexibility for predators" foraging efficiency and for the antipredator responses of prey. Depending on their ability to use different foraging strategies, predators may differ in their behavioural responses to changes in prey availability and structural complexity. Particularily, the presence of different forms and densities of structural complexity have the potential to interact with the behaviours of predators and prey, altering the outcomes of the predator-prey interactions. Refuge seeking and schooling behaviour were the two main antipredator strategies displayed by the prey fish in my studies.
Here, the ultimate responses of the prey to proximate cues given in the environment differed between the prey species suggesting that the prey display different behavioural flexibilties to the presence of predators. Such differences in behavioural flexibility in prey also have the potential to affect the interactions between prey species. Exploring the implications of flexible behaviour
on predator-prey dynamics would be a possible way to link behavioural ecology and population ecology, and would therefore be a fruitful path to increase our knowledge about the dynamics of ecological communities. In this context, a combination of different temporal and spatial scales in studies would improve our possibilities to understand the individual behaviours of organisms in a population and community perspective.
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Acknowledgements
This thesis has its origins in the Finnish archipelago, at my grand parents home where I as kid spent many summers angling perch. After all I think my mother is partly responsible for my interest for things going on below the water surface. By, her good hand with everything that has to do with the sea, I guess somehow made me grow my interest for the aquatic
environment. I tried to exercise my interest for fishing at home, at the driest place in Sweden the Alvar on Öland, where I as thirteen years old shot a 3 kilo female pike at the spawning grounds with my brothers airgun. Later on I have become more sophisticated when catching fish. Nowadays I use electrofishing.
With no doubt, Lennart Persson has been the most scientific influential person for me during my PhD studies. Thank you Lennart for your excellent supervision! Not only for your constructive critisizms on manuscripts but also for your scientific sharpness, your untiring optimism and capacity to solve scientific problems in less than five minutes which I had thought of for two weeks. Thanks Lennart for laying the “smörgårdsbord” of science.
Before I started my PhD studies I met Sebastian Diehl at the Limnology Department in Lund where we both started on our PhD studies and since then, we have shared many experiences together. Thank you Seb for many good times at the experimental ponds, for the science discussions, the Wednesday cooking and your friendship!
Many other persons have contributed to this thesis. Stellan F. Hamrin made me start working on fish ecology and we spent many good times together at Lake Bolmen and Lake Ringsjön. At that time, I was involved in several of Stellans projects together with Marie Svensson and Marie Eriksson. Thank you Maries" for all the fun we had in the field with everything from pulling gill nets to discussing what is up and down of the trawl. During the time in Lund, I was inspired to the aquatic research by discussions with many persons.
Especially, I enjoyed the time together with the other graduate students and the dissections of scientific literature in the fish ecology research group. Thank you all for a very pleasant time!
One of the persons involved in the fish ecology research group was Lars Johansson. Lars taught me a lot about ups and downs in fish ecology research and also how to enjoy the good things in life. Thank you Lars for many fascinating discussions and good laughs over a beer at
“Storkällaren” in the good company of happy friends. In Lund I also met Eva-Maria Diehl for the first time and since then, I had the pleasure to work with her and share many good times both in Lund and in Umeå. Thank you Eva-Maria for your ever lasing happiness and good friendship.
Like many others from southern Sweden I was burden with the prejudices that people in the northern part of Sweden are very silent and unsocial. To my pleasure, I immediately
recognized that it in fact was the opposite. The department has the real spirit that just makes you happy and in contrast to my prejudices the social climate is unique because of our frequent social activities and because of the staff: Göran Andersson, Gunnar Borgström, Lena
Burström, Stig-Ola Ivarsson and Görel Marklund. Thank you for your support and friendliness in everything from technical assistance, tagging fish, drawing figures and guiding the paths through the catacombs of administration. Another important social event occurs every monday when we come together at the floor hockey tournament, a good way to keep you young an fit (at least psychically).
Many colleagues at the department have made my time in Umeå very pleasant.
Especially, I want to thank Göran Amqvist for your ever lasting optimism and friendliness, Bent Christensen for many discussions about everything in life, science, how to do a jibe or which windsurfing sail to use, Göran Englund for always finding the right statistical tests, Frank Johansson for your frank realism and for your diving company and Kjell Leonardsson for teaching me many secrets of Excel. During my time at the department I shared the room with Tarja Oksanen, Göran Sjöberg and Åsa Eriksson. I hope that the frequent changes does not mean that I have been unendurable.
I have also shared good company with the people in our research group. Our frequent scientific discussions and many hours of good company in the field have formed a real “group spirit”. Of those I have not mentioned before are Eva Wahlström for introducing the woman spirit into the male dominated group, Per Byström for your northern Swedish mentality although you are from Kramfors and Joakim Ohlsson for contributing with new, fresh power to the group.
My experiments could not have been done without the assistance from many persons. I thank Jens Andersson, Christina Gustavsson, Camilla Johansson, Jan Johansson, Jesper Leijerstam, Robert Lothian and Gustav Samelius for enduring my commands at the electrofishing in Lake Degersjön or for many hours of observations with gloves and down jackets during those warm summers of Västerbotten. I also thank Barbara Giles for the
comments on the summary.
I would also like to thank Sven Nordqvist for allowing me to use his drawings in my thesis. With your help I think even my sons would appreciate the content of my thesis.
Finally, my deepest love to Aina, Simon and Isak. You are the spring of happiness and the real meaning of life.
This thesis was financially supported by grants from the Fisheries Research Board of Sweden, the Swedish Council for Forestry and Agricultural Research to Lennart Persson, The Royal Swedish Academy of Science (Hierta Retzius Stipendiefond), J. C. Kempes Minnesfond and Stiftelsen Seth M. Kempes Minne.
