A quick course in


by Jason Buchheim
Director, Odyssey Expeditions



Marine organisms face a variety of challenges in their quest for life and reproduction. They must obtain food for growth and survival, avoid being food for other organisms, cope with the physical environment, and have an effective strategy for bringing forth reproducing offspring.  

Foraging Ecology, the quest for food. 

First we will look at Foraging Ecology, the quest for food. Obtaining food is necessary for an organisms survival, growth and offspring reproduction. Without food, almost nothing else about an organisms relation to the world will matter for long. 

I. Obtaining food, necessary for: -survival -growth -offspring reproduction 

There are many different strategies for obtaining the necessary food. Organisms generally follow two basic strategies, being either a generalist or a specialist. A specialist will prefer only a single or few food types and a generalist will gladly eat many different food types. Both of the strategies have their advantages and disadvantages. A specialist may concentrate on a very nutritious food type; when it finds it favored food, it will be able to obtain most of its nutritional requirements, but it may have a hard time finding its favored food. A generalist may be surrounded by many different edible food, but like spinach, it may not be that good for the organisms, or it may take a long time to process the food, such as eating barnacles with their hard shells. 
Strategies for obtaining food
-Hunters, feed on larger, rarer organisms 
-Browsers, feed on smaller, commoner organisms
Diet width and composition varied 
-Monophagous, single food type SPECIALIST 
-oligophagous, few food types SPECIALIST
-polyphagous, many food types GENERALIST

Different diet widths of five sea star species
Seastar species #species of prey  
Percentage compositions of prey Strategy
MEDIASTER  35   38,11,4,3, others <3  GENERALIST
CROSSASTER  26  47,13,11,5, others <3  GENERALIST
SOLASTER  23,15,13,10, others <10  GENERALIST
**Within one group of organisms (seastars), there can be species that are either generalists or specialists. 
There are three possible reasons why should there be so much diet variation. 
-Optimal foraging 
An organisms may have a preference for a certain food type, and will choose it over other food types given equal access to choices 

Preference: Indication of foods chosen by a predator given equal access to choices Average population preference study in food choices of a sea urchin algal abundance avoidance preference Field Diet **Equal Access to different algae, the sea urchin has preferred and avoided food types 

Diet preference in the gastropod Nucella over 4 weeks  Barnacel Food Type
Barnacle food typesB-Balanus T-Tetraclita M-Mytilus C-Cthalamus 
**This study shows the gastropod Nucella can have strong or weak preferences
Switching: Will an organism switch preference with abundance characteristics? It may be to the advantage of an organisms with a preferred food type to switch to another food type if it becomes very abundant, but some specialists will not switch even when surrounded by lots of great food. 

Food preference in a gastropod with varying food abundances. Nucella % mussels eaten NO SWITCHING % mussels offered Acanthina barnacles food preference mussels SWITCHING relative food abundance *Strong food preferences may prevent food switching regardless of abundance. 

Optimal Foraging Theory: 

Problem, under a given set of circumstances, how should a predator forage? How should a predator respond to variation in environment, such as patchiness of food supply. In such a case, what path should the predator follow. 

To solve this problem, ecologists have come up with a model known as the Optimal Foraging Theory, in this model the predator will completely avoid the unprofitable, and completely pursue the profitable. 

The model has four assumptions: 1. Foraging behavior is variable, and heritable. 2. Possible responses to prey are constrained 3. Most efficient foragers will be favored by natural selection 4. Efficiency determined by maximizing energy gained in a set amount of time 

Two types of consumers are defined by their Energy/Time ratio (E/T). 

    1. Energy Maximizer: Fixed time to forage -maximize energy gained in that time 

    2. Time Minimizer: Fixed energy goal -minimize time needed to obtain particular energy amount

As a starfish can only feed at high tide on the intertidal zone, it is an energy maximizer. 

The foragers face their first dilemma when they must decide on the quality of the food item it will eat.  

Specialist: maximal efficiency by eating only "best" prey, but high time cost searching. 
Generalist: low time cost, but lower efficiency because it takes profitable and unprofitable prey. 
Search Time VS. Time Spent Pursuing, Capturing, and Consuming Prey searching time pursuit, capture, consumption time Time A B CPrey Species D E  

Optimal prey is the one where searching time and handling time is minimized 

**assume energy is the same, cost is time consumed  

Energy Gained VS Time Handling for five prey species E A D  

Energy B Gained C  

Time required to obtain and handle A--Best prey, most energy gained per unit of timeE--Worst prey, least energy gained per unit of time  

Predictions from Model 1. Highest rank prey should always be eaten. 2. Lower rank prey should be pursued and eaten only if this increases net energy gain. Gains>Costs 3. Exception to 2, -Take lower rank prey if recognition time is low low rank prey may be eaten if frequently encountered. 4. Predators should be more selective when prey are abundant and less selective when prey are scarce. Search Time VS Handling Time of Scarce and Abundant Prey scarce prey handling time Time abundant prey A B CPrey Species D E Optimal shifts to the left when a prey species become abundant**assume energy is the same, cost is time consumed  

Energy Gained VS Time Obtaining + Handling 5 prey species E D A Energy C Gained B Time required to obtain and handle prey Optimal prey shifts to the right with increasing abundance 5. Inclusion of lower ranked prey is independent of its own abundance and dependant on high ranked prey abundance. 

Crab Caranus mearas eating mussel mytilus edulus Optimal size prey?energy obtained/ time spent handlingpreyUnlimited food#eaten/crab/dayLimited Food#eaten/crab/day 1 2 3 4mussel size 5 6cm 1 2 3 4mussel size 5 6cm 1 2 3 4mussel size 5 6cm  

** This study shows that the snail will eat the best prey first, and save the lousy prey for last. Inclusion of the lower ranked prey is independent of its abundance and dependant on the abundance of the highest ranked prey.  


The foragers second dilemma: How long should a forager stay within a patch? If it stays too long it will be spending too much time for the amount of energy it is accumulating. 

Rate of energy extraction Time 

Cumulative rate of energy extracted energy extracted predicted time spent in patch t t t t p o TIME assumption- patches all same size, equally spaced 

Patch Quality Cumulative average energy high extracted low TIME 

**A predator should leave quicker in a low quality patch than in a higher quality patch 

Patch Quality Cumulative energy extracted t t t1 t2 

**Time spent in a patch increases if travel time is long. 

Reproduction Ecology 

Reproduction is one of the primary goals of any organisms. In order to keep a species from becoming extinct, its members must reproduce at least enough offspring to replace themselves. Organisms invest a considerable amount of time and energy into reproducing. The eggs and sperm they produce are costly cells to make, and their only use is for reproduction. Some organisms also invest a great deal of energy in mate selection and nest preparation. In the case of the higher organisms, parental care may take place, and the organisms will be investing its resources into its offspring for even years to come. If an organism did not have to invest all of this energy into producing and nurturing offspring, they would have more energy available for investing into more complex organs allowing them to possibly be better competitors in its environment. But reproduction is an absolute necessity for organisms that are not immortal, and I do not know any that are. Without reproduction, the species would be gone after just one generation. Reproduction is not necessarily the most important thing to a particular creature, it may be happy to just spend its time swimming and eating, but it is the primary goal of the genes within the creature. The genes (the instructions for creating an organism) are the only level that evolution takes place. Good instructions for creating an organism that is good at reproducing get passed on to further generations. If the instructions were poor, the organism will not reproduce, and the instructions will be lost. Only the good instructions get passed on. Natural selection is very much at work at filtering out successful instructional strategies. One could say that an organism is only the means to continue the flow of genes. That is what natural selection has programmed the genes to do; to creature organisms that deal with the environment well and reproduce. A gene that programmed its organism to be poor at dealing with the environment would probably not be passed on to the next generation. Only those genes that create organisms that are successful in the environment get passed on. This filtering process (natural selection) soon leaves only genes that are good at producing fit organisms. If the environment stayed constant, then evolution would have finished millions of years ago, but the environment is not constant. The genes mutate every once in a few million reproductions. Most of these mutations will be deleterious to the organisms that is created, but some may actually benefit the organisms. Natural selection will work at filtering out the good from the bad mutations. If the mutation is bad, the gene will probably not be passed on (such as a mutation that hindered an organisms ability to eat, the organisms starves to death and does not reproduce). A few of the mutations may actually help the created organisms (such as a gene that allowed for better sight, the organisms may be able to see food that others were missing, or to avoid predation, giving the organisms an advantage). A good mutation would quickly be passed on throughout the species population, and this is evolution! Evolution selects for the best adaptive traits to insure that each female replaces herself within her lifetime, ideally, she should replace herself and then add some. 

Evolution selects for the best adaptive traits to insure that each female replaces herself within her lifetime, ideally, she should replace herself and then add some. 

Problems in the evolution of life history.  

    1. What is the best size or age to begin reproducing? 

    2. How many times should an individual reproduce? 

    3. How many eggs should there be per clutch? 

    4. How large should the eggs be? 

    5. When in the year should reproduction occur? 

    6. How to locate a mate? 

    7. How can young locate an appropriate habitat?

ULTIMATE CRITERION: produce maximum number of reproducing offspring. 

Factors influencing the production of maximum number of reproducing offspring. 

I. Biology of Individual 

  • Size of adult 
  • Principle of allocation 
  • Egg size
II. Environmental Conditions  
  • Biotic environment, coping with the other organisms in the environment -competing with other organisms for food or shelter space, etc. 
  • Physical environment, coping with environmental stresses -dealing with hurricanes, toxins, sand, silt, temperature, etc.
Modes of Development, two strategies 

1. Planktotrophy: very small and numerous eggs with little yolk. Eggs are of low cost to make, so many can be made. The larvae must feed in plankton column after hatching. 

2. Lecithotrophy: relatively large, few, yolky, and costly eggs. Some nursed. Larvae are non-feeding, simple in form. Found in plankton, demersal, or benthic environments.  

Modes of Development
Factor  Planktotrophs  Lecithotrophs 
Cost to adult   low  +   high -
individualCare of young no   +-  high -+
Fitness of juveniles  low -   high +
Survival of young  low -  high +
Starvation  high -  low +
Predation  high -  low +
Access to adult habitat  low -  high +
Dispersal  high +-  low -+
Factors associated with alternative modes. 
Two Seastars with different Modes of Development Size at reproduction  Egg size  Egg number  Larval survival 
Pisaster  Large 20-90mg (Small)  Millions Low
Leptasterias Small 2g (Large) 100's to 1000's High 
The adults compete for food. Pisaster is the better competitor-Low interspecific competition allows for leptasterias to grow large-High interspecific competition hinders leptasterias growth.. 
**By having different modes of reproduction for two similar species in the same environment, they occupy different niches even though they compete for resources. This increases local diversity.

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