Animal Foraging, Habitat Selection, and Territoriality
Course Announcements
Upcoming Lecture (Tomorrow): Cooperation and Altruism.
Data Analysis Lab (Tomorrow Afternoon): Crucial for analyzing assignment data. Groups will be segregated by R proficiency: beginners (extra help), comfortable, and independent (minimal help). A laptop with R and RStudio downloaded (link on Moodle from a few weeks back) is required for at least one group member. No need to update existing installations.
Test 2 (Next Week): Scheduled for Friday, covering all material after Test 1 until Test 2. A tutorial will be held on Thursday to provide more information.
Habitat Selection and Use in Animals
Learning Objectives for Today
Understand the main factors determining habitat selection and use in animals.
Distinguish between home range and territory.
Explain how habitat quality, diet, and energetics predict territoriality and optimal territory size.
Link habitat selection and territoriality mechanisms to mating systems and cooperative behavior.
How Animals Find Suitable Habitats: Dispersal Mechanisms
1. Regular Passive Dispersal
Definition: Animals do not expend energy or actively choose their destination; movement is passive.
Mechanisms: Wind, sea currents, river currents.
Benefits: No energy expenditure.
Costs/Risks: Getting lost (no control over direction), being carried to unsuitable locations if wind/currents change, only beneficial for r-strategists (species with high reproduction rates who can afford to lose many offspring).
Examples:
Ballooning Spiders: Common in areas like Hamilton, where young spiders release silk and are carried by the wind.
Insects and Plankton: Dispersed by water currents.
2. Accidental Dispersal
Definition: Accidental movement facilitated by external factors.
Mechanisms: Severe storms (e.g., birds/bats carried from Australia to New Zealand).
Outcome: Often results in single, rare sightings of individuals in new areas (e.g., a blue-footed booby in New Zealand). Colonization is rare as individuals may not find mates.
3. Active Choice Dispersal
Definition: Animals actively choose the direction and destination of their movement, expending energy in the process.
Common in: Most vertebrates (birds, mammals, fish) and large insects.
Mechanism: Walking, flying, swimming.
Costs: Energy expenditure.
Benefits: Animals choose locations based on resource availability, maximizing fitness.
Timing: Often occurs when offspring fledge and disperse from areas with high competition to new locations.
Habitat Selection: The Ideal Free Distribution (IFD) Model
Purpose: A model to predict how animals choose habitats to maximize their survival and reproduction (fitness) when they are free to move between habitats.
Assumptions of the IFD Model:
Fitness Maximization: Individuals always strive to maximize fitness (food, energy, reproductive success, survival) when selecting a habitat.
Resource Heterogeneity: Different habitats possess varying amounts of resources.
Density-Dependent Fitness: Individual fitness decreases as the density of other individuals in a habitat increases due to competition (negative density dependence).
Equal Competitive Ability: All individuals have the same ability to compete for resources, which is generally not true in nature but simplifies the model.
Perfect Knowledge: Individuals are able to assess and compare the fitness payoffs of different habitats accurately.
Cost-Free Movement: Individuals can move between habitats without any costs.
Predicted Outcome (using a monkey example selecting between three habitats: A (rich), B (medium), C (poor)):
Low Population Density: Individuals will primarily choose habitat A (highest resources) for high fitness.
Medium Population Density: As density in habitat A increases, competition rises, reducing individual fitness. Individuals will start spilling over into habitat B, where individual fitness might now be comparable or higher due to less competition, even with fewer resources.
High Population Density: With intense competition in habitats A and B, individuals will also populate habitat C. Despite its low resources, the very low population density in C means that the actual resources per individual become comparable to those in more resource-rich but highly populated habitats.
Real-World Evidence: Northern Pike Habitat Selection Study
Setup: Studied Northern Pike over 40+ years in a lake with a deep, resource-rich north part (Habitat A) and a shallow, resource-poorer south part (Habitat B). Fish could move between sections.
Findings: Habitat A was preferred at lower densities. As density increased in the north, pike dispersed more to the south, matching the IFD model's prediction of density-dependent habitat use.
Deviations from the IFD Model
1. Allee Effects
Definition: At very low population densities, individual fitness can decrease because individuals struggle to find mates or resources, leading to potential population crashes.
Implication for IFD: In such cases, a higher population density could lead to higher fitness, contradicting the IFD assumption of purely negative density dependence.
2. Use of Social Information
IFD Assumption Challenged: The IFD assumes individuals know the fitness payoffs of different habitats, but young or naive individuals often lack this knowledge.
Mechanism: Naive individuals use cues from other individuals to assess habitat quality.
Example: American Redstart Study
Setup: Compared bird distribution in normal environments vs. areas with playback of redstart feeding songs (simulating high density).
Findings: New adult redstarts increased significantly in the playback area, interpreting the sounds as a signal of high food availability, even if it wasn't true. Yearlings and returning adults did not show this effect. This shows that higher density habitats can be attractive to inexperienced individuals.
Heritability of Habitat Choice
Key Questions: Is habitat choice heritable? What are the species-level consequences of habitat choice?
Wecker's (1964) Deer Mice Study:
Species: Peromyscus maniculatus, found in North America in woodlands and grasslands.
Context: Anthropogenic disturbances are increasing grassland areas.
Experimental Design: 132 deer mice from different populations (grassland-born, woodland-born, varied experience) were placed in 30-meter pens with five compartments spanning both woodland and grassland habitats, connected by runways.
Aims: Understand if habitat choice was active, hereditary, or learned.
Findings: Habitat preference was dependent on both heredity (where parents came from) and learning (individual experience). Rearing woodland-born mice in grassland did not immediately override their natal site preference, but this preference could be re-learned over multiple generations. This indicates a complex interplay between genetic predisposition and environmental learning.
Home Range vs. Territory
Home Range
Definition: An area of repeated, non-defended use by an animal.
Characteristics:
Undefended.
Not used equally; some areas are used more intensively.
Can overlap with other individuals' home ranges.
Example: Puma vs. Snow Leopard Home Ranges
Puma: Males have very large home ranges, often overlapping with females but also maintaining distinct areas. Winter home ranges are smaller due to clumped resources; male ranges expand significantly during breeding season (summer) to find mates and exploit increased resources.
Snow Leopard: Males and females show more home range overlap. This is hypothesized to be due to their rarity and sparse distribution, making shared areas beneficial for finding mates. They tighten home ranges during the winter breeding season to increase encounter rates.
Territory
Definition: A actively defended core area, usually within an animal's home range.
Characteristics:
Actively guarded by an individual or group.
Not all animals are territorial.
Size depends on the costs and benefits of defense versus exclusion.
Economics of Territory Defense
Costs of Defense: Vigilance, fighting, time expenditure (reducing time for foraging, mating).
Benefits of Defense: Access to resources (food, mates, hiding spots, roosting sites).
Defensibility of Resources: Resources must be defensible (e.g., fixed food sources like fruit trees, not highly mobile prey like scattered mice) for territoriality to be worthwhile.
Graphical Representation (Costs vs. Benefits with increasing Territory Area):
Costs: Increase with territory area, often exponentially (e.g., C = ae^{bA} where A is area, a, b are constants), as more area requires more energy and time to defend.
Benefits:
Rich Territory: Benefits increase rapidly with area but then plateau (e.g., B{rich} = k1(1-e^{-c_1A})). A small rich territory yields high benefits quickly. Beyond a certain point, additional area provides diminishing returns (e.g., more food than can be consumed).
Poor Territory: Benefits increase at a slower rate and plateau later than rich territories (e.g., B{poor} = k2(1-e^{-c2A}) where k2 < k1 and c2 < c_1 or reaches plateau later). A larger poor territory might be required to achieve similar benefits to a small rich one.
Optimal Territory Size: Occurs where the difference between benefits and costs is maximized, or where marginal benefits equal marginal costs. It's not worth defending if costs exceed benefits.
Example: Golden-winged Sunbirds (Nectar Feeders)
Context: Defend territories based on nectar load of flowers. Defense reduces foraging time by allowing access to high-reward flowers within a smaller area.
**Thresholds for Territoriality (based on caloric net gain):
Lower Threshold: If flowers contain 2 to 3 microliters of nectar, it's worthwhile to defend a territory. The benefits of guaranteed food outweigh defense costs.
Upper Threshold: Defense becomes uneconomical if:
Too many intruders: The cost of fighting off numerous competitors outweighs the benefits.
Resource too abundant: If nectar is super-abundant (e.g., 4 to 6 microliters per flower), the owner cannot utilize all benefits, and the minimal reduction in foraging time doesn't justify defense. It's more efficient to roam and forage freely.
Territory Size Variation: Territory sizes varied immensely (up to 300 times between smallest and largest) but consistently contained an average of 1,600 flowers. This implies that sunbirds adjust territory size to maintain a consistent amount of resources. Sparse flowers lead to larger territories, clumped flowers to smaller ones.
Body Condition and Territoriality
Hypothesis: Territory defense is energetically costly, so only individuals in good body condition can afford it.
Example: Ruby Spot Damselflies
Observation: Only males with high fat reserves successfully hold territories.
Experiment: When males were forced to defend against continuous intruders, their fat reserves significantly decreased, demonstrating the high energetic cost of defense.
Conclusion: Only high-quality individuals (with sufficient energy stores) can afford the costs of territoriality.
Interplay of Territoriality and Mating Systems
Connection to Previous Lectures: Habitat selection and territoriality significantly influence mating systems.
Evenly Distributed Resources (High Monogamy Potential):
Resource Characteristics: Resources are found in similar amounts across the area, allowing females to choose small, specific patches.
Female Behavior: Females become territorial against other females to monopolize these small patches, as it's not efficient to move around extensively.
Male Behavior: Males, unable to defend vast territories that encompass multiple female patches, will often defend one female's small territory, leading to monogamy.
Patchy Distributed Resources (High Polygyny Potential):
Resource Characteristics: Resources are abundant in specific patches but sparse elsewhere, with large gaps between patches.
Female Behavior: Females are not territorial and will clump together in resource-rich patches, often sharing a territory, as moving to find similar resources is costly.
Male Behavior: Males can defend these large, resource-rich patches which attract multiple females, leading to polygyny.
Example: Field Voles
General Principle: Food is a key resource for females, and females are a key resource for males. The distribution and renewability of food determine territorial behavior and mating systems.
Species Feeding on Seeds and Fruit:
Food Characteristics: Patchy distribution, slow renewal.
Female Behavior: Evenly spaced and highly territorial (monogamous tendency).
Male Behavior: Non-territorial, with wide ranges (seeking out these spaced females).
Species Feeding on Grass:
Food Characteristics: Widespread distribution, rapid renewal.
Female Behavior: More clumped, less territorial (as food is abundant everywhere).
Male Behavior: Territorial, defending areas with clumped females (polygynous tendency).
Summary of Key Drivers of Territoriality
Food Abundance: Defending territory is only worthwhile at intermediate levels.
Too low: Not enough food to sustain the individual, not worth defending.
Too high: Owner can't use all the benefits, and too many intruders make defense too costly.
Food Distribution:
Defensible if patchy (e.g., fruit trees).
Not defensible if too widespread (e.g., grass).
Food Renewability:
More defensible if it rapidly renews and is predictable.
Less defensible if patchy and renewal is slow/unpredictable.
Mates: The distribution of females in space and time causes significant variation in male territoriality (e.g., favoring monogamy with evenly spaced females, or polygyny with clumped females).