Ecology week 4

How do predators respond to changes in prey density

Increasing prey density causes an increase in the number of prey consumed per predator.

Functional response of the predator

In response to an increased density of prey predators eat more prey. This is known as a functional response, however the shape of the response graphically differs depending on the attack rate and handling time of the predator. Assumes one prey type at a time.

Type one functional response

f(N) = aN. This means that the number of prey eaten per capita depends on the attack rate and the number of prey available. Type one response is linear with the number prey consumed directly proportional to the number of prey available, leveling off at a high enough N prey. This response is typically seen in filter feeders and eventually levels off as rakers or filters get clogged, limiting their intake.

attack rate

rate of encounter, attack and capture per capita prey

Searching time

Time spent looking for prey, predators spend most time doing this at low prey density

Handling time

Time taken to consume prey. As prey density increases the highest proportion of time is spent handling.

Maximum consumption rate

at high prey density, the predator is constantly handling the as many prey as possible. This is maximum consumption rate and it is impossible to consume prey any faster.

Type two functional response

At a low density the predator feeds directly proportional to density dependence. As N increases, handling time takes up the majority of time, reducing the attack rate dependent on the predators efficiency and eventually levelling off. Number of prey eaten at each density is dependent on the predators attack rate and handling time

Type three functional response

In an environment with refuges for prey, the functional response has a sigmoid curve shape. At low density consumption is not directly proportional with density as a large proportion of prey can access refuge. At medium density, the majority of prey cannot access refuge and consumption is proportional with density depending on attack rate, at high density consumption is limited by handling time and begins to decrease before levelling off.

Prey size as a refuge.

Small predators cannot feed on large prey - large size acts as a refuge. Large predators preferentially feed on larger prey due to gaining the most energy from it - small size acts as a refuge. Selection for these depending on predator size.

Evolution of defense mechanisms in prey

Defense mechanisms are selected for as they reduce predation. These may be physical (morphological) defenses, chemical defenses, warning colourations or camouflage.

Camouflage

Prevents prey being spotted by predators, decreasing attack rate.

Batesian mimicry

Where a species which does not have a harmful (e.g. toxic) defense mechanism mimics the distinct markings of one which does. This decreases predator attack rate as predators will associate these markings with a negative outcome. e.g. poisonous coral snakes mimicked by other snakes.

Constitutive defenses

Fixed phenotype, not altered by external factors,

Predation risk

The risk for a single prey within a population of getting eaten. May be influenced by traveling in groups, with a decreased probability as group size /prey density increases.

Predation risk curves

Type 1: Prey density increases, predation rate remains the same until maximum capacity is reached, at which point prey density increases but attack rate stays the same. individual risk per prey declines

Type 2: Attack rate remains proportional to population density until the point where handling time becomes a limiting factor.. Attack rate decrease plus prey increase causes decreased predation risk with increasing density.

Type 3: Attack rate increases as refuges become unavailable causing overall increased prey risk, then declines as handling becomes a limiting factor

Change in functional response based on environmental factors

Environmental changes can shift a type two to a type 3 response, for example, bream behaved as type two foragers at high light levels due to visual foraging, however, lower light levels shifted them to type three responses, potentially due to darkness making it harder to spot low levels of predators.

Impact of conspecifics on functional response

as predator density increases, predation rate declined due to interference competition , therefore influencing the functional response.

Prey switching/optimal foraging

Predators switch their prey as one species becomes lower in abundance, reducing searching time. This can promote coexistence of prey species and stabilise predator prey dynamics by allowing another species to recover.

Experimental proof of prey switching

proportion of each species eaten changed as their relative density changes, the most abundant prey were preferentially consumed.

ontogenic diet shift

Species shifts their diet through their ontogenetic development. may be based on size, amount eaten at different phases.

Components of predation

FUnctional response, numerical response - more prey = more predators, developmental response -e.g. ontogenic diet shift, aggregative response - predators move to area where prey species are more abundant

Numerical response

Density of predators increases with density of prey with an increased rate of predator reproduction. This occurs until prey population begins to decline due to increased predation. When prey density declines past a point there is no longer enough prey to sustain predator population and they also decline.

Generalist vs specialist predators

Generalists and omnivores have more stable population dynamics as they are better able to adapt to changing conditions, with ontogenic diet shift to a more abundant prey option. This allows depleted populations to recover.

Implications of predator prey dynamics

Managing populations of game species to prevent situations such as overfishing - sustainable yield, controlling populations of agriculturak pests through biological control,

Functional classes of predator

Prey population growth

change over time = Exponential growth rate minus predator functional response

dN/dt = rN - aNP

the larger A is, the more the prey is affected by each predator,

predator population growth

Change over time = functional response - exponential decline (predator population)

dP/dt = c(conversion efficiency)*aNP - qP (death rate)

Predator prey dynamics

Couple the differential equations of predator and prey growth to analyse how they influence each other over time, with an aim to solve the equation for population size at which growth = 0 (aka at equilibrium)

Predator prey isocline - for prey

Zero isocline for prey is the point at which prey and predator density makes population growth = 0, keeping prey population in check.

0 = rN - aNP , P=r/a

Faster r (reproduction) in prey means more predators needed to keep population in check, higher a (attack rate) = fewer predators.

Predator prey isocline for predators

The point at which the growth of predator population is controlled due to the number of prey. greater q = more prey needed to prevent decline, greater a + c = fewer prey needed.

Population cycle isocline

P and N population sizes fluctuate in regular cycles at equilibrium. However the outcome may change depending on initial abundance of each population. If predator and prey populations sit precicely at the isocline intersection they will not change, and if the starting point is too extreme one population will crash. Model assumes no immigration, age, genetic structure. Prey population only limited by predation, predator is a specialist ( no prey switching), individual predators can consume an infinite number of prey, predator prey encounters are random, homogenous environment.

Non linear isocline for prey

Prey isocline is hump shaped, assuming a maximum and minumum sustainable population due to intraspecific competition

Laboratory studies of predator prey dynamics

Attempts to model predator prey dynamics in microcosms such as observations of interactions between paramecium and didinum resulted in repeated population crashes. Not reflective of observations in nature. Increasing complexity and adding various factors found in nature allowed us to determine factors which stabilise predator prey systems such as inclusion of refuges and by extension a type 3 functional response allowing prey recovery, immigration from outside the system, optimal foraging/diet switching, evolutionary changes in predators and prey.

Habitat heterogenity

The variation in habitat structure and resource availability in an ecosystem that influences species diversity and interactions. It can lead to different niches and promote coexistence among species. THis was demonstrated experimentally, with mites living on oranges. In a homogenous system prey were easy to find with nowhere to escape and populations crashed, however by increasing the landscape allowing for dispersal, and various microhabitats increasing complexity, predator populations stabilized as heterogenity increased.

Evolution and lotka volterra

Phase shift (where the function is moved horizontally) is observed in natural systems and does not match mathematical predictions. This is explained by evolutionary dynamics, which influence the predator and prey interactions over time, leading to changes in population dynamics

Lynx hare cycle

Observed lynx populations peaked every ten years due to variation in reproduction and survival. A large scale manipulation excluded specific predators showing that hare populations were strongly influenced by an interaction between predation and food shortage. Chronic stress from exposure to predators may reduce reproduction, leading to fluctuations in both lynx and hare populations.