Spartine alterniflora, or cordgrass, is the dominate grass in many salt marshes.

Question

Spartine alterniflora, or cordgrass, is the dominate grass in many salt marshes. In zones with high nitrogen it occures in a "tall form" and is very productive . In zones with less nitrogen it iccures I a less productive."short form". Snail called periwinkles (littoraria irrota) feed on cord grass, and in turn are fed on by numarous predators such as crabs and turtles.The data on the next page (under the heading CORDGRASS)come from series of experements from the system. Refer to the study data to give a sign to boxes 1, 2,4,and 5 in the diagram. example which data support your choice for each box. Is there a trophic cascade in each environment? is it stronger in one environmet then the other/ Give asign to boxes 3 and 6, and explain your conclusion. Is the growth of cordgrass controlled mostly by top-down forces, bottom-up forces,or a combination of both? Explain ,reffering to the data and/or the diagram for evidance to support your conclusion. Fishing by humans depletes population of blue crabs:What effect would you expect this to have on cordgrass? Effects of snail on cordgrass: Cages wear established with low ,medium(natural)and hhigh snail densities and maintained for 8monthes. Figure 1A shows effect of snail density and marsh zone (low N=short zone)on grazing intensity on live cordgrass. radulations are marks left by snail in the leaves. Figure 1B shows effects of snail density and zone on aboveground sparine(cordgrass)biomass after 8 monthes. n=8 per treatment, and probability values given for two-way ANOVAs testing main and interactive effects. Z = zone, G = grazing intensity(snaildensity).

Explanation / Answer

1.a. Box 1 and 2 exhibit -- Strong negative effect

        Box 4 and 5 exhibit - Weak negative effect

The relationship depicted by the direction of arrow in boxes 1 and 4 represents a secondary consumer consuming a primary consumer which cause decline in population of the primary consumer and nutrient flow to the secondary consumer from primary consumer.

The relationship depicted by the direction of arrow in boxes 2 and 5 represents a primary consumer consuming a producer which cause decline in population of the producer and nutrient flow to the primary consumer from producer.

With reference to tabulated data the effect of predators on predators feeding on snails consuming high Nitrogen containing tall form of cordgrass (Box 1) is more than that on predators feeding on snails consuming low Nitrogen containing short form of cordgrass (Box 4). Hence box 1 represents strong negative effect whereas box 4 represents weak negative effect.

With reference to tabulated data the effect of predators on uncaged snails feeding on high Nitrogen containing tall form of cordgrass (Box 2) is more than that on uncaged snails feeding on low Nitrogen containing short form of cordgrass (Box 5). Hence box 2 represents strong negative effect whereas box 5 represents weak negative effect.

b. With reference to the tabulated data the trophic cascade exhibit stronger effect on the consumers directly or indirectly feeding on high Nitrogen containing tall form of cordgrass than consumers directly or indirectly feeding on low Nitrogen containing short form of cordgrass.

Box 3 exhibit -- Strong positive effect

Box 6 exhibit -- Weak positive effect

The secondary consumers like crabs and turtles are indirectly creating positive effect on the producers by feeding on snails. From the tabulated data we can infer that the predatory effects of secondary consumers feeding on snails consuming tall cordgrass in higher than on snails consuming short cordgrass. Hence box 3 represents strong positive effect whereas box 6 represents weak positive effect on producers.

c. The growth of cordgrass is controlled by combination of both top-down and bottom-up forces like nitrogen content and effect of snails. From the diagram it is evident that higher the availability of nitrogen content in the soil taller is the height of cordgrass grown. Taller the height of cordgrass grown there are higher probability of being consumed by snails on basis of tabulated data.

d. Depletion in the population of blue crabs due to fishing result in gradual and significant increase in the population of snails due to absence or limited population of predatory crabs. With the unlimited increase in population of snails the demand for producer or cordgrass also increases to feed the entire growing population. This would create a stress on the population of producers and might also gradually result in extinction of all the organisms involved in the food chain.

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