Behavioral Ecology I - Foraging

Foraging refers to a species eating habits - what to eat, when to eat, where to eat, and when to stop feeding in a certain area. Foraging is molded by natural selection. Essentially, energy gained by feeding affects reproduction and without food there is no survival.

Energy consumed either gets absorbed or is voided as feces. From the energy absorbed, some of it is lost through respiration, digestion, tissue maintenance and movement. The energy left is used for growth. The energy is divided into energy used for somatic growth and energy used for reproductive output. (There are allocation tradeoff, but more on that later.)

Consumer types

There are several consumer types. Although they can be grouped into specialists and generalists, these are not discrete categories, but rather a continuum.

• Monophagous: 1 prey type
• Oligophagous: few prey types
• Polyphagous: many prey types

However, for comparison purposes, we will classify consumer types into two broad categories.

1. Specialists: Feed on one/few type(s) of prey.
• Advantages include being adapted to one single prey, which means they can overcome prey defenses. There is less competition.
• Disadvantages include the risk of prey extinction, making them vulnerable. There may also be nutrition problems due to unmixed diet. In areas of low prey population density, there is an increased search time.
2. Generalists: Feed on multiple types of prey.
• Advantages include low search time and diffused effect of prey toxins.
• Disadvantages include a high or strong interspecific competition. Generalists are also vulnerable to prey defense due to lack of local adaptation.

Optimal Foraging Theory

Optimal Patch Use Model

This model describes the ideal pattern a species should follow to obtain the maximum energy gain. This model takes into account certain assumptions:

1. Food is found in discrete patches
2. No energy is gained when traveling from patch to patch
3. Consumers can assess food's energy value in a patch

The model is represented as an energy gain curve. There's an optimal time to remain in a given patch and an optimum energy gain described by the tangent (with the steepest slope) to the energy gain curve.

Marginal value theorem: Optimum time to reside is defined by the rate of energy gain at the time of leaving the patch.

[Foxglove flower example]

Optimal Diet Model

Maximum foraging is calculated by E/T where E is the energy content and T is the total time spent searching and handling prey. 

T = s + h (searching + handling)

Organisms can behave as time minimizers or energy maximizers.

1. Time minimizers
• Minimize time to gain specific amount of energy
• Mouse - high risk of predation
• Antelopes - males minimize foraging to defend females against other males.
• Snails feeding on barnacles - eat small barnacles quickly, whereas eating large barnacles takes longer
• Prefer small barnacles due to less risk of predation
• Large barnacles expose snails to predators
2. Energy maximizers
• Focus on increasing prey profitability
• Bison, deer, penguins, birds, sunfish

Prey profitability

The profitability of a specific prey can be obtained by dividing its energy content by its handling time (E/h). Handling time refers to the time it takes a predator to attack, kill, and consume a prey once it has encountered it. This is assuming a simple system where there are only two prey types.

Rules of thumb:

(assuming prey type 1 is more profitable than prey type 2)

• If a predator encounters prey type 1 eat it on the spot. Always eat.
• If predator encounters prey type 2, eat if the gain from eating it exceeds the gain from rejecting it and searching for prey type 1.
• if E2/h2 > E1/(s1+h1) where s1 is the additional search time.

Other important notes:

1. A predator will specialize on a prey type only if its search time is low
2. A predator will switch from a specialist to a generalist as average search time for prey 1 increases

Simplifying assumptions of OFT Models:

1. A predator can sense the energetic value of a prey
2. Foraging behaviors are heritable
3. The model only considers 2 prey types
4. Energy content is the only influence on prey choice. (Does not consider other factors such as salt, H2O, etc.)

It is a simplistic theory, but there is a great amount of evidence that supports it.

Types of Energy Resources

There are many types of energy available for human use, but man tends to overuse some of them which leads to environmental problems.

1. Fossil Fuels: by far the most exploited energy resource but not for much longer. (Coal, Oil, Natural gas)
2. Nuclear Energy: highly productive, but comes with hazardous issues.
3. Hydroelectric Energy: harnesses the kinetic energy of flowing water. Sounds great but what happens to the river? And what if there is no river nearby?
4. Solar energy: just like plants have been doing for millions of years! Renewable because the sun will be around for a few billion years more! But if solar energy is so great, why are we not all using it?
5. Wind: Works great! As long as there is wind. Kind of noisy though.
6. Geothermal: harnessing the thermal energy beneath the Earth’s crust! Nees to find an opening though.
7. Tidal: using the moon’s gravitational energy that the oceans convert to kinetic energy.
8. Hydrogen Fuel Cells

Energy Budget of Food Production

Farming's Energy Budget

When farming and producing food, one needs to examine the inputs, outputs, flows and storages in order to determine the energy budget.

e.g. fuels, chemicals, labor, materials, transports, wastes, energetic content of products.

Also consider that the marketable product which is produced is only a % of the total output, which may not be marketable.

For every watt of input I get ___ watts of product (dairying, cereals, etc.)

Comparing Aquatic vs. Terrestrial Food Production

Terrestrial:

• Food harvested from 1st or 2nd trophic levels thus more energetically efficient.
• Solar energy capture (GPP) more efficient.
• Energy transfers between trophic levels not as efficient.

Aquatic:

• Most food is harvested from higher trophic levels, thus not as efficient
• Solar capture (GPP) not as efficient due to absorption of sunlight by water
• Energy transfers between trophic levels fairly efficient.

Quick notes on Sustainable Yield

Sustainable Yield (SY) refers to the increase in natural capital. It is the natural income that can be exploited each year without depleting the original stock or affecting its potential for replenishment.

If you see it as a business, you could consider sustainable yield as the 'retained profit' of a company. It is the amount that one has left over and can use to invest in other aspects of the business. That money can be used to expand and grow or to improve the business in any other way. It is the same with Sustainable Yield, that increase in natural capital is what is available for use and would not affect the environment. It is like the money a company can use to make itself better without going into debt.

* MSY means the maximum sustainable yield, and it is the one that is of interest commercially speaking.

Some important aspects to consider when calculating sustainable yield are:

• carrying capacity
• population size
• total biomass or energy at a given time
• Rates of change of population, biomass, and energy.

However, there is a convenient formula for calculating Sustainable Yield:

SY= Annual Growth and Recruitment - Annual Death and Emigration

Basically, what this calculates is how many organisms are there at a given point in time. It considers new individuals that came in, and individuals that died or left.

Sustainable Yield can also be calculated by

SY= (Total Biomass or energy at a Time T)+1 - (Total Biomass or energy at a Time T)

This is useful when calculating the changes in SY over a period of time. It would be used when comparing biomass in 2009 and biomass in 2010.


Here is an example on how sustainable yield is important for economic systems. It is a report on the commercial value of estuaries in Australia.
http://www.ozcoasts.org.au/indicators/econ_value_commercial_fisheries.jsp

Also, here is an ecological assessment of rivers and estuaries (also in Australia). This is the type of information that is useful when evaluating an ecosystem and its natural resources.
http://www.anra.gov.au/topics/coasts/pubs/estuary_assessment/est_ass_int_wpdd.html

Chesapeake Bay

Chesapeake Bay is an estuary located in the west coast of the United States. It receives water from over 100,000 streams and rivers and empties in the Atlantic Ocean. It is home to around 350 species of fish, dozens of species of shellfish and crabs, 16 species of underwater grasses. Waterfowl, ospreys, and shorebirds also inhabit nearby areas.

All of these species interact in this ecosystem, and form a very complex food web (made up of many food chains). As explained previously, the food chain begins with a producer, which in this case is the phytoplankton, which is then eaten by a number of first consumers, which are then eaten by other predators.

The main purpose of the food chain is to see how energy is transferred from one organism to another. However, pyramids are ways to obtain more data. They represent graphically how energy, matter, and population are distributed in a system. Two of them are the pyramid of biomass and the pyramid of productivity.

The pyramid of biomass shows the amount of biomass at each trophic level, which is calculated by multiplying the mass of an organism times the amount of organisms. The dry weight of an individual is roughly the same as the energy it contains.

The pyramid of productivity shows the flow of energy over time. Since energy is lost as it moves from 1 trophic level to another, the upper levels are always smaller than the lower ones. Only 10% of the energy in one trophic level is passed on to the next one.