CHAPTER 4 Factors affecting Distribution and Abundance 2
CHAPTER 4 Factors affecting Distribution and Abundance : 2) Habitat Selection 1
Learning Outcome 1. Define habitat 2. Acquire knowledge on selecting habitat based on behavior mechanism 3. Describe the hierarchical decision making 4. Explain the habitat choice in bird (Cody, 1985) 5. Explain the Theory of Habitat Selection 2
Habitat selection and behaviour • Even though some organisms can occupy certain habitats they choose not to do • their distribution is thus limited by their behavior • E. g. : Anopheles culicifacies 3
Definition of Habitat • any part of the biosphere where a particular species can live either temporarily or permanently 4
Two (2) Approaches to study Habitat Selection • Proximal approach– behavioral mechanism of how animals choose habitat • Ultimate approach – evolutionary approach – adaptive reasons for habitat choice and evolutionary significance of the behavior – conferring survival value and high fitness - e. g. : aphid 5
• Habitat selection between animals (behavioral mechanisms - choice) and • plants (dispersal affected by chemical and physical factors – no choice) are different 6
Mechanisms of Habitat Selection -Environmental gradient -Chemical -Behavior -Body Size 7
1. Simple Habitat Selection – Based on Environmental Gradient • Habitat selection for the isopod, Porcellio scaber is dependent on moisture gradients (dry-wet) • It moves randomly and as it finds moist air it slows down and comes to rest 8
2 a. Simple Habitat Selection – Based on Chemical cues • Some use chemical cues to select habitats – corn borer females, oviposit on corn because attracted by volatile odors (chemicals) from corn plants 9
2 b. Simple Habitat Selection – Based on Chemical Cues • Leaf beetle (Family: Coleoptera) select their habitat on cucurbitans (squash, melons, cucumber) due to the chemical cucurbitacins. • 1500 sp. of Coleoptera also do the same 10
3. Habitat Selection Based on Behavior – Dancing oviposit Mosquito • Some organisms do not select a habitat even though it is suitable for its developing stages • In India Anopheles culicifacies (a malaria vector) is not found in rice fields after the plant grows to a height of 12 inches (20 cm). • These rice field supports other Anopheles sp. • Russel & Rao (1942) could not find eggs of the mosquito but when they transplanted eggs to older rice fields, the eggs survived and produced normal adults 11
• The absence the mosquito is due to the selection of oviposition by females. • Mechanical obstruction by the rice stalks was the limiting factor – the female does a dance before ovipositing and the rice plants prevents this action - barrier • In this way it restricts the species to a habitat range less than that which it could occupy 12
Habitat selection in relation to Body Size • • i. ii. • In desert area - larger species of rodents (Dipodomys merriami) prefer open habitats while smaller species (Chaetodipus penicillatus) prefer covered/sheltered habitats Experimental results (Thompson 1982) show habitat types to body size: Larger species in open habitats have enlarged hind legs – speedy locomotion and well developed hearing – reduce risk of predation Smaller species live in sheltered habitats to hide -reduces predation Habitat selection as a result of body size to reduce predation Dipodomys merriami Chaetodipus penicillatus 13
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Hierarchical decision making • Habitat selection operates at the level of the individual animal. • Decisions are made in hierarchical manner from a large scale to a microhabitat scale – eg. for a migratory bird. • Example- Hierarchical decision process by Mexican bird 15
Hierarchical decision making west south woodland shrub which of tree types? 16
Conceptual model of the variables that influences habitat choice. 17
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Hunting behavior Song dialect Habitat Choice in Birds (Cody, 1985) Idiosyncratic behavior 19
1. Birds of prey - habitat selection involves Perch sites • 3 species of hawks in which their habitat choice depends on their foraging behavior – all feed on the same kind of food i. Swainson’s hawk – open areas with less trees –are the best at soaring and hunt from the air much more than percheswings suited for soaring and hunt from air ii. Ferruginous hawk– open areas with less trees – wings intermediate for soaring Ferruginous hawk iii. Red tailed hawk – select many perch trees or cliffs – wings less suited for soaring - so, it will sit on perches and look for prey • In all 3 cases, habitat selection is tied to their hunting behaviour/methods 20 The red tailed hawk
2. Pipit – Idiosyncratic Behavior • Pipit: i. Tree pipit (Anthus trivialis) – found only in areas that has trees ii. Meadow /ground pipit (A. pratensis) – in treeless areas • For the tree pipit the tree is important for landing after it sings it song while flying, but for the meadow pipit it finishes its song on the ground • Both have similar requirements – both feed the same type of organisms on • Thus the tree pipit is excluded from the habitat of meadow pipit because it needs a perch to land after singing. In this way their behaviour limits their range. 21
3. Song Dialects • Song birds have many dialects that distinguish subpopulations and also influence habitat selection. • Habitat selection in birds is partly a genetic trait but can be modified by learning and experience • Song birds (sparrows) in California are subdivided into 4 dialects – their habitat is homogeneous yet they have distinct boundaries based on the dialect types • Dialects are learned and culturally transmitted from parent to offspring – birds mate with birds of same dialect – thus socially determined dialect boundaries also are genetic boundaries restricting gene flow 22
shrub/wetland Yellowthroat forest opening Hooded Warbler Deciduous/conifer trees Ovenbird 23
Evolution of Habitat Preference 24
Habitat recognition may be exacting (Making great demands on one's skill, attention, or other resources) : • salmon returning to spawn in the same stream in which they hatched due to olfactory cues (sense of smell). 25
Habitat recognition may be imprecise : (Lacking exactness and accuracy of expression or detail) • aphids landing on plants – taste the chemical suitability of the plant to feed - if suitable, it feeds - if not found, they fly away and find suitable plants 26
• Generally organisms that have specific habitat predictability (salmon) are exacting in their habitat selection • While those faced with habitat unpredictability (aphids) must adopt more flexible habitat selection behavior 27
Theory of Habitat Selection (Fretwell, 1972) 28
Theory of Habitat Selection (Fretwell, 1972) 1. Habitat is assumed to have a suitability for any species. 2. Suitability = fitness = assume females produce more offspring. 3. Suitability is not constant but affected by: (i) food supply, shelter and predators (ii) function of density of other individuals and such that overcrowding reduces suitability 29
Predictions of the Model • Habitat type: good, intermediate and poor • When sp. density is high - good, intermediate and poor habitats would have almost equal suitability (but different densities of sp. ) • Individuals would be crowded in the good habitats and at low density in the poor habitat 30
Ideal Free Distribution (IFD) • The term "ideal" means that animals are "ideal" in their assessment of patch quality; they know how profitable each available patch is. • The term "free" means that animals are capable of moving unhindered from one patch to another. 31
Ideal Free Distribution (IFD) • An ecological term that describes the way in which animals distribute themselves between several patches of resources. • The theory states that individual animals will aggregate in various patches proportionately to the amount of resources available in each. • The IFD theory predicts the distribution of animals that compete for resources that are distributed in patches (Fretwell, 1972 ; Fretwell and Lucas, 1970 ). • Example: if patch A contains twice as much food as patch B, there will be twice as many individuals foraging in patch A as in patch B. 32
Evidence for the Model. Fretwell 1972, 33
• Studies on yellow legged gulls – on Medes Island in Spain (Bosch and Sol, 1998) – nest in shrubby and grassy meadows • Gulls prefer shrubby areas and occupy them first. Shrubby habitats are good for nesting, where it provides some protection (hiding places) from predators and also shade to reduces heat stress • Gulls in the shrubby habitats laid 2. 8 eggs on average while those from grassy areas laid 2. 9 eggs • Fitness/suitability is nearly equal in both habitats – as predicted by Fretwell (1972). Both site can favour the gulls to produce more offsprings 34
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