Exam #2 F_W 2900.
Predator-Prey Dynamics and Management
Theory of Predator-Prey relationships
Lotka-Volterra models: Form the basis of contemporary predator-prey dynamics models
The approach makes serval simplifying assumptions:
Prey population experiences exponential or logistic growth
Mortalities are due to the overall predation rate, which is influenced by the functional and numerical response of the predator
H= density of prey population
P= density of predator population
Start with simple exponential population growth of a prey population H
H/t=rH
Incorporate predation through the response term ; the predation rate per individual predator per unit of time.
=-ln(1-Hkilled/H)/t
is a measure of the rate at which individual predators capture prey as a function of prey abundance
A large value indicates individual predators have a large effect on the prey population.
H/t=rH-HP
The H is the change in the density of the prey population.
rH is the intrinsic rate of growth of the prey population
aHP is the loss of prey due to predation (product of predation rate and density of predators and prey)
How about predators? In the absence of prey, a predator population (P) will decrease according to its intrinsic density-dependent mortality rate.
P/t=-mpP ( intrinsic density-dependent mortality rate)
The population can grow when prey is available based on the conversion rate , the efficiency at which predators use their food to reproduce.
=P/PHt
is larger when a single prey item has a larger effect on the predator population growth
P/t=HP-mpP
This is a model describing the effect of prey density on the growth rate of Predator population P
Equilibrium for prey and predator populations estimated as the point when the population growth rates = 0
Prey population= P*- r/a
P*= number of predators that will keep the prey pop growth rate at 0
Predator population: H*=mp/
H*= number of prey necessary to maintain the predator pop growth rate at 0
We don’t often see the dramatic prediction from this model in nature. Why?
Simplistic- doesn’t account for other resources that may limit prey or predator populations.
Models can be expanded by incorporating other aspects of population control.
A simple example is incorporating density-dependence of habitat carrying capacity to the prey species model.
H/t=rH((KH-H)/KH)-NP
The inclusion of density dependence results in a downward slope of the prey isocline
The intersection with the vertical predator isocline results in a stable equilibrium point.
The ultimate result is dampened population oscillation over time
Gets smaller
Will eventually each equilibrium (sorta of still small cycles)
Small cycles but barely there
This happens a lot more in nature
Can change if something happens in the environment
Predator Response to Changes in Density of Prey Populations
Functional Response: Changes in the number of prey consumed by a predator as prey density changes
Numerical Response: Changes in the density of a predator population as the prey density changes
These responses can be incorporated into the Lotka-Volterra models to further make them mor realistic
Three types of Functional response curves
Type 1: the number of prey items consumed by a predator increases linearly as prey numbers increase up to a threshold
A very simple model that does not account for handling time and assumes predators are constantly searching for prey
Doesn’t account for a lot
Type 2: The number of prey items consumed reaches an asymptote at a certain density, which accounts for handling time and the fact that predators are not constantly searching for prey
More biologic realistic
Still reaches the asymptote at certain density.
Type 3: low predation rates occur when a prey density is low but increase exponentially when the population grows beyond a certain threshold and eventually reaches an asymptote
Characteristic of a generalist predator keying in on a prey item as it becomes more abundant
Implication of Predator Functional Responses to Prey Populations
Type 1- each predator consumes a constant proportion of the prey population, regardless of prey density
Type 2- The proportion of prey population consumed decreases as predator satiation sets an upper limit on prey
Type 3- the proportion of prey population consumed decreases at low and high prey densities largest % of the population is captured at intermediate densities
Numerical Response:
There are two mechanisms that cause numerical responses:
Demographic Response: increasing prey leads to increases in predator reproduction or survival, leading to an increase in population
Aggregational Response: There is an increase in predator population due to immigration to an area with increasing prey density
Interesting aspects of predator behavior that influence dynamics
Generalist Predators: capture a wide variety of prey
Able to switch between prey sources as they become less available
Population can remain stable in the face of environmental changes because they can switch to different resources
A prey species population can experience increased predation from a generalist predator species if another prey species decreases in availability
Turns it attention to other available prey items
A prey species population can experience increased predation levels from a generalist predator if another prey species increases in availability, thus drawing in more predators.
For example raccoons and black bears
They will eat just about anything.
Specialist Predators: Unlikely to persist without their key prey species
North American black-footed ferret, which preys almost exclusively on prairie dogs
True specialists are rare, and most predators can target another prey when the preferred prey is absent or other prey becomes available
Prey Vulnerabilty
Age: very young or old individuals may be more vulnerable to predation
Physical condition: injured, malnourished or diseased individuals may be more vulnerable to predation
Enviorment condition: can increase encounters with predators
Ex: only one water hole during dry season
Activity: prey may be more vulnerability to predation when engaging in certain behavirors( eating, mating, sleeping)
Group size: in large size predators may be less successful
Group defense
Increase vigilance
Wildlife management: humans can decrease or indirectly increase vulnerability through management actions
Feeding the wild elk over winter
All grouped together in one area easy for the wolves to come in and killl
Ecological traps: that can increase vulnerable to preadtation
Predation Managment
An aspect of wildlife management that generates a lot of controversy
Predicting the results of predator control (effectiveness, unintended consequences) is difficult
The public often has strong feelings about predators, and especially strong feelings about lethal control of predators
When is predator control justified and scientifically defensible?
It is established that predators kill substantial numbers of the species of interest that would usually otherwise surviver if the predator was removed or controlled
It is established that reduced predation can facilitate reliably higher harvest rates or population sizes of the species of interest
It is established that given reduced predation, habitats can sustain more of, but also be protected from, an increased abundance of the species of interest
Establish that sustainable populations of the predator species will persist in and out of control areas
Predator control
Predator control can be useful when prey populations are below carrying capacity, but is unlikely to have much effect when prey populations are at or near carrying capacity.
Once at carrying capacity, it will max out anyway
Limiting factors (environmental) (food, shelter, home base)
There might be a short term effect but nothing long term
Predation has little effects on carrying capacity when it is high
Methods of Predator Control
Lethal Methods
Shooting, Poisoning, trapping
Leathal control of predators is the most controversial form of predator control
Non lethal methods
Translocation- capture and removal of individual predators to new areas. Mixed bag of effectiveness
Will translocation animals return to the original location or settle in the new location?
What effect will translocation have on the local area they are introduced to?
Is it an effective or efficient long-term solution?
Exclusion- normally includes fencing netting, and setting of other exclusion devices that limits or prevents predators from accessing prey
Even non-lethal control requires research and is not without controversy
Assume Preadtor Control is determined to likely be useful, which method should we use?
IT DEPENDS
Studies generally reported limited or uncertain effectiveness - Lethal Methods.
Studies generally reported strong evidence of effectiveness- Non-lethal methods.
Trophic Cascades
Trophic levels - functional classification of organisms in an ecosystem according to feeding relationships
Trophic cascade: a cascade of ecological consequences initiated by a change in upper levels of a food chain that work their way down the lower trophic levels.
Example: the purple sea star is a predator of mussels
When the sea star is present, there is a wide diversity of species present
But when the sea star was removed, it became a monoculture of mussels
This allowed the mussels population to grow unchecked
The Loss of a top predator drastically changed the community composition of the ecosystem
Preadtors can influence the effect of prey on an ecosystem through two potential mechanisms
Reducing numbers through actual predation
Forcing prey to adjust their behaviores to aviod predation
Ex: limiting where, when, how often , and on what they forage on
Behavioral effects can be just as important as actually reducing prey numbers through predation
Animal Behavior
We have already talked about
Habitat selection
Immigration and emigration
Generalist vs. specialists foraging
Functional responses to changes in prey density
Changes in habitat use and foraging by prey in presence of predators (trophic cascades)
Fitness is the appropriate lens to view animal behavior through
Fitness- Propensity to contribute offspring to the next generation
Direct component measured as lifetime reproductive success
Indirect component from helping parents/ siblings/ offspring produce offspring (“kin selection”)
Direct and indirect components make up an inclusive view of an individual's fitness (important for social animals)
Space Use
Space can be considered along a continuum from nomadic to territorial, based on spatial fidelity.
An area of space that an animal uses
Spatial fidelity- refers to the propensity of individual to consistently revisit and re-use areas
high= territoriality
low= nomadism
Home Range- the area regularly traversed by an individual during its normal activities of meeting its daily resource needs.
Arise through spatial fidelity
Home ranges are not defended and can overlap
Many animals do maintain a nearly exclusive Core Area within their home range
Territoriality: occurs when individuals or groups defend and area of use against other members of the same species
Continuum of Space Use
Nomadic- open ocean animals (follow the prey) this rearley happens
Homerange
Home range with exclusive core areas
Territories
Costs and Benefits associated with the development of areas of exclusive use
Benefits
Exclusive access to resources
Food
Limited nutrients
Dens/ nests
Mates
Space
Costs
Lost time (patrolling and defending territory)
Lost opportunities elsewhere
Energetic costs (associated with patrolling and defending territory)
Risk of territory loss
Where is the optimal size?
Where the space between costs and benefits is the largest
Determinants of home range size
The upper limit of range size is set by energetics and body mass
Larger animals can maintain larger ranges
Larger animals need larger ranges to procide necessary resources
Carnivores generally have larger ranges than herbivores of the same body size
Wide variation in range size is often observed among individuals of the same species- Why?
Resource availability
Landscape configuration
Interspecific competition
Sex
For example Raccoons in ND had larger home range where the resources were scarce and spread out
While raccoon home ranges in Louisiana bottmland forest where smaller, where resources are abundant and close together
Why is understanding space use important to wildlife management?
Direct connection between space use and habitat selection
Space use behavior can and will influence carrying capacity( limiting factor?)
Knowledge of space use can be used to make more accurate estimates of population size in an area.
Territoriality
This can lead to a uniform distribution of individuals on the landscape
Can limit population size below theoretical carrying capacity
Can have essential implications to the effectiveness of translocations
Dispersal and Migration
Dispersal→ one way movement of an animal from it’s current range to another, where a new home range is established
Individual behavior
Often occurs once in an animal’s life
Is often permanent
Most often seen in young animals leaving their natural range (due to not enough resources on hand)
Natal Dispersal→ dispersal of an animal from location or population of birth to a new location or population where it settles and reproduces
Adult Dispersal: Dispersal of a reproductively active member of a population to a new location or population where it settles and reproduces
Variability in Dispersal distances within a Population
Most don’t go far (short distance)
But a small percentage of the population will disperse very far
A sex bias is observed in many species
Mammals: males often disperse further than females
Birds: females often disperse further than males
Males attract females to his territory
Dispersal 3-stages process
Emigration: When an animal leaves its current range/ population or patch
Transfer: the process of moving through the landscape
Immigration: when an animal settles into a new range/population and patch
Dispersal is the connective tissue that holds metapopulation together
Thus, dispersal has a large impact on population dynamics and maintaining species population over a large spatial scale
Helps to facilitate genetic diversity
Range expansion and allows species to colonize new habitats as they become available
Why do animals disperse? - Ultimate causes
Avoid inbreeding
Reduce competition
Especially kin competition- you don’t want to compete with your genes!
Escape unfavorable environmental conditions.
Migration: Movements between distant regions used for different life history purposes
Although it is an individual behavior, similar movements of individuals often manifest as a Population-level behavior
Can occur repeatedly during an animal’s life
Varieteis of Migration
To- and- fro migration: animals migrate between distinct bredding and non-bredding regions (what you tradionally think about with birds)
Classic one we think of
Nomadism: doesn;t follow any regualr pattern or route but links temporay breeding sites located where conditions are ephemerally favorable
example : seen in insectas and bird species that breed in semi-arid regions and deserts
Lifetime migrations: migration that ecompasses an entire life cycle
Example: pacific salmon
Irruptions: Ocaaional and irregular movements of a large portion of a population into a region well beyond its normal breeding or non-breeding range
Example: snowy owl
Can’t predit when it will happen
Thinked to be tied with populaiton density
Partial Migration: some members of a population may engage in migratory behavor while other members remain resident
Moose and red tail hawks
Why is migration?
Migration is an adaption driven by ephemeral availabilty and changing location of resources:
When changes in habitiat quaility in differnet regions occur asynchronously, migration allow a sucession of temporary resouces to be exploited as they arise
Challenges of managing and conserving migratory species
Multiple juridictions
Everyone needs to be involed in the migration journey
Migrants often have different habitat requriments for each season and the migratory journey itself
Climate change
Effects of habitats
Effects on migration timing
Mating Systems!!!!!
Mating systems are tightly integrated with the social systems of many species
Understanding mating systems is important from a management perspective because of the direct impact on reproduction
Sexual selection acts on traits that humans put values on
Antlers on deers
Monogamy
The paring of a single male and female
~90% of avian species
<5% of mammal species
Life long monogamy
The pair stays bonded for live
Serial monogamy
Pair up for a season
Polygamy
One sex with multiple partners
Polyandry: 1 female with numerous males
Polygyny: 1 male mates with numerous females
More common in nature
Polygamy can be further categorized based on resource defense
Mate defense polygamy: the sex that has multiple mates defends the mates from potential competitors
Resource defense polygamy: the sex that has multiple mates defends a resource that attracts mates from potential competitors
Male elephant seal control and defiant favorable section of beach
Male red-winged blackbirds defend mating territories
Lek-based polygamy: one sex gathers to display to the other
Males display to the females normally
Prairie chickens
Promiscuity
Males and females have multiple mates and do not maintain any exclusive relationships
Very common among small and mid-size mammals
Mating system -> sexual selection -> sexual dimorphism
Sexual dimorphism: differences between the sexes as expressed in one or more traits
Often brought about by the Intrasexual competition
One sex chooses which members of the opposite sex to mate with
Lead to competition for mates in the other sex
Competition between males for females' attention drives the evolution of traits that serve to enhance attractiveness to females, but not much else
Example in turkeys males will
Display structures
Large snoods
beards
Matting displays
Giobbling
Strutting
Implication of sexual Dimorphosim for Management (why we care?)
Humans often value traits that arsie through sexual selction
Game species (trophy hunting)
Sexually dimophic traits make it easy to distinguish sex in the field, which can be useful
Able to tell the sex easy and not look to much at them
It allows sex-based haverest regulation to be implanted
Doe season, spring wild trukey (males
It aids population survey if it is easy to visually distinusgh sex (sex ratio)
Fundament Units of group orgiation
Birds
Males are fundamental units of organization in most birds
Males attract female to territors needed fo reproduction
Distructio of males on the landscape is ofthen determined by the selection of resources that enable successful reproduction
The distrubion of females is then dependent on the distrubution of the males
Mammals
Females are the fundamental unti of organization in most mammals
Females are the only sex able to nurse the young
It is the opposite of the birds
Social System
The increased levels of spatial overlap mean more shared responsibilities
Most wildlife species are not highly social
Most animals are solitary or temporarily interact to mate
Although solitary individuals can overlap in space and time
Many species will form Aggations at some point during the year
Aggregation are characterized by significant spital overlap of individuals but no shared duties
Often associated with life history events and migration, or when resources are concentrated in a limited area
The incipient unit of social organization is often a Pair (dyad) of individuals
The monogamous species this might be a male-female pair
Communal Groups: shared space and duties, often in the care and provisioning of young
Eusociality: extreme division of labor and reproductive cates (ants and bees)
Only found in one vertebrate species
Naked molerat
Generalized cost and benefits of living in groups
Costs
Increased competition
Spread of parasites and disease
Increased visbility to predators
Increase reproductive suppression
Increased risk of injury during combat
Benefits
Reduced per-capita predation risk
Increased vigilance
Group defense
Increased foraging success through group foraginign
Thermodynamic advantages of huddling
Information transfer about resources
Increased inclusive fitness
Increased Inclusive Fitness: Kin selection and Hamilton's rule
C<rB r= level or relatedness B= inclusive fitness benefits c= direct fitness costs.
Cooperation should be favored when the adjusted benefits (B) exceed the costs (c)
Kin selection is considered a primary factor in the evolution of the social system and cooperative breeding.