1. What are Allee effects? 2. Mechanisms for Allee effects 3. Population dynamics: modelling demographic Allee effects 4. Genetics and evolution 5. Conservation and management 6. Conclusions and perspectives

Allee Effects in Ecology and Conservation

Landscape mosaics and ecological theory. Origin of landscape pattern. Response of individuals and populations to landscape pattern. Landscape pattern, population dynamics and population genetics. Effects of landscape pattern on species interations. Implications for conservation. Summary: Ecology of mosaic landscapes: Consolidation, extension and application. Index.

Mosaic landscapes and ecological processes

Species usually have to use indirect cues when assessing habitat quality. This means that it is possible for humans to alter habitats in a way that causes a discrepancy between the cues and the true quality of different habitats. This phenomenon is called an ‘ecological trap’. Here we show that the trap may lead to a behaviourally mediated Allee effect, where population growth is reduced because of non-ideal choices of individuals. The reduction is greatest at low densities because more individuals can choose their preferred habitat when competition for breeding sites is reduced. An ecological trap may lead to multiple equilibria in population dynamics and cause deterministic extinction in habitats that are capable of sustaining a viable population. We also study the efficiency of three mechanisms that may rescue a population from this extinction trap: natural selection acting on habitat preferences and two forms of phenotypic plasticity, experience-based learning and a philopatric preference for the natal habitat. Selection is most efficient in short-lived species with large heritable variation in habitat preferences, whereas in long-lived species, plastic traits outperform genetically determined preferences. The simple philopatric strategy generally produces the most favourable outcome. It hardly differs from the optimal strategy that assumes perfect and immediate knowledge of habitat change, and is very robust to non-ideal variation in the strength of habitat preferences. We conclude that conservation biologists need to ensure that cues for habitat choice correlate with habitat quality.

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Ecological traps in changing environments: Ecological and evolutionary consequences of a behaviourally mediated Allee effect

All landscapes are mosaics of habitat patches of different types (Forman and Godron, 1981). Therefore, there will always be edges between habitat patches in a landscape. These habitat edges may be associated with a higher diversity of plants and animals, traditionally called an edge effect (Odum, 1971). However, the current use of the edge-effect concept summarizes a diverse group of phenomena that occur in habitat edges (Harris, 1988; Yahner, 1988; Angelstam, 1992). For example, interaction between different organisms living in different habitat patches in the landscape mosaic may sometimes cause effects related to habitat edges (Janzen, 1986; Wilcove, McLellan and Dobson, 1986). Predator—prey interaction might be related to habitat edges. However, the relationship between predation rate and habitat edge depends on the specific habitat use and density of the potential nest predators, as well as the configuration of habitat patches in the landscape. Predators might increase the predation rate in habitat edges for several different reasons. First, the habitat edges might have a higher density of prey and the predators might view the edges as good foraging patches (Gates and Gysel, 1978). Secondly, the predators might use the habitat edges as traveling lines (Bider, 1968; Chapter 4) and therefore spend more time in the habitat edge than in other parts of the habitat patch.

Effects of landscape composition on predation rates at habitat edges

One of the rare examples of a single major gene underlying a naturally occurring behavioral polymorphism is the foraging locus of Drosophila melanogaster. Larvae with the rover allele, forR, have significantly longer foraging path lengths on a yeast paste than do those homozygous for the sitter allele, fors. These variants do not differ in general activity in the absence of food. The evolutionary significance of this polymorphism is not as yet understood. Here we examine the effect of high and low animal rearing densities on the larval foraging path-length phenotype and show that density-dependent natural selection produces changes in this trait. In three unrelated base populations the long path (rover) phenotype was selected for under high-density rearing conditions, whereas the short path (sitter) phenotype was selected for under low-density conditions. Genetic crosses suggested that these changes resulted from alterations in the frequency of the fors allele in the low-density-selected lines. Further experiments showed that density-dependent selection during the larval stage rather than the adult stage of development was sufficient to explain these results. Density-dependent mechanisms may be sufficient to maintain variation in rover and sitter behavior in laboratory populations.

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Evolution of foraging behavior in Drosophila by density-dependent selection.

Nest site choice and reproductive success of lapwings was studied during 1988-90 at a farmland site dominated by cereal crops in central Sweden. The most important factor causing breeding failures was farming operations, which accounted for 85% of all nest losses. Difference in hatching success between habitats; unsown tillage (9% hatching success), sown tillage (78%), fallow fields (31%) and grassland (67% hatching success) was an effect of different farming practices

Karl Gunnar Kallebrink

Papers

Hatching Success of Lapwings on Farmland: Differences between Habitats and Colonies of Different Sizes