Decomposition and Recycling in Aquatic Ecosystems
Introduction Biogeochemical cycles are important to the sustainability of all life. Chemical elements necessary for the growth and reproduction of all organisms have a limited quantity on earth at any one time, other than the occasional meteor that brings with it new matter. It is therefore important that the recycling of these chemical elements is efficient. Autotrophs are the basis of almost all ecosystems. The rate that autotrophs produce and transfer energy is vital to the capacity of organisms that can inhabit these ecosystems.
To understand the rates in which certain species’ leaves decay and release the energy stored within them can demonstrate how quickly the energy becomes available to organisms in higher trophic levels. Between plant species the rate of decomposition varies according to leaf structure, chemical or physical defence and whether the species is allochthonous or autochthonous. Native species adapt to the predators that feed upon them and the same applies for the predators, adapting to the plants defences. Allochthonous species have not evolved with the native predators in its introduced environment.
In some cases this can result in the plant being more susceptible to predation. The aim of this experiment was to investigate and compare rates of decomposition of different species of leaf material and examine if there is a difference in tissue loss between the same species immersed in different wetland types. By examining the rate of decomposition of three different leaf types we can determine the rate that energy becomes available to the inhabitants of the ecosystem. It is hypothesized that allochthonous Salix species will not have defence against native predators and will produce the highest rate of tissue loss.
Eucalyptus will have the second highest rate of tissue loss due to its autochthony; while it has strong structural and chemical defences, predators have been able to adapt and successfully predate the Eucalyptus leaves. Phragmite will have the slowest rate of breakdown due to its structural defence. Methods Materials * 10 Salix leaves (dried) * 10 Eucalyptus leaves (dried) * 1 Phragmite leaf (dried) * 1 x 25cm mesh bag * Waterproof name tags * 1 m plastic string * 1 medium sized rock (ballast) * 2m fishing line * 3 paper bags * 4 paper clips Procedure 1.
Leaves of different species were weighed in mg and recorded, then placed into a mesh bag in three separate areas of bag, separated by plastic string weaved through mesh. Rock was placed into middle segment to weigh bag down over the three areas. 2. Bag was securely tied off and fishing line attached and tied to jetty in the Jock Marshall Reserve (JMR) Lake, where bag was submerged undisturbed for 56 days. 3. After 56 days bag was retrieved from wetland and leaves were carefully rinsed and placed into 3 separate paper bags to be dried for one week. 4. Dried leaves were retrieved and weighed.
The total tissue loss from initial weight before submersion until now was recorded. Class inputs data and a class mean and standard deviation of tissue loss/ day was gathered for both wetlands for each species Results Class input data into excel and a class mean and standard deviation was gathered from both wetlands for each species. Excel was used to produce the mean and standard deviation for all data, then graphed for ease of interpretation. Figure [ 1 ] Standard Deviation | Eucalyptus| Phragmite| Salix| JMR| 1. 21| 3. 12| 2. 81| Science Lake| 2. 85| 2. 99| 1. 30|
Phragmite had the lowest mean tissue loss of the three species and little difference in tissue loss between wetlands with a . 64mg difference. Eucalyptus had the second lowest with a total mean tissue loss of 3. 67mg, differing between wetlands by 2. 4mg. Salix had the lowest difference in mean loss between wetlands of . 04mg, the lowest mean tissue loss of the three species between wetlands. Discussion The aim of this experiment was to investigate and compare rates of decomposition of autochthonous and allochthonous leaf material immersed in different wetland types.
The results indicate the native species being more resistant to decomposition than the introduced Salix. The results did not vary greatly between wetlands for the species. This large difference in mean tissue loss for Salix was possibly related to its susceptibility to native predators. The structural weaknesses of Salix leave it susceptible to predation to a higher degree than Eucalyptus and Phragmite. Salix has a very thin cuticle and minimal schlerenchyma, which means that it has little support. The palisade close to the epidermal layer contains high degrees of protein, making them very attractive to herbivores.
Eucalyptus had a lower rate of tissue loss than the Salix. The structural differences between the Eucalyptus and the Salix mean that it is better defended against herbivory. The thicker cuticle and thicker epidermal layer slows the rate at which invertebrates can decompose the leaves. Cellulose covers the cell walls, due to its slow breakdown herbivores are less attracted to a leaf that burns more calories trying to eat it than it gets from eating it. Phragmite is heavily defended. Vascular bundles are all accompanied by schlerenchyma, a lot of structural support.
Bulloform cells have thinner vertical walls than horizontal and rolling of the leaf results. This protects the soft tissue area and protects it from herbivory to a great extent. The veins of this grass species run down the leaf with support from gurders, these are very rigid columns of tissue that are difficult for invertebrates to access the nourishing part of the plant. Eucalyptus has approximately eight times higher Lignin than Salix. Eucalyptus is a much less attractive food source than Salix having a leaf that is eight times tougher than Salix with a quarter of the protein.
Salix also has almost double the carbohydrate level than Eucalyptus. As an introduced species it is not well adapted to the predation tactics of native predators. Salix had approximately two times the loss than Phragmite. Eucalyptus has not only structural defences but a high level of chemical defence as well. Eucalyptus has approximately 4 times the amount of polyphenols than Salix and is more chemically protected than Phragmite. Phragmite has the most well adapted structural defence against decomposition and this is observed in the results.
The results suggest there is a great difference between species as to their rate of decomposition, especially according to the species origin. There are generally small differences between wetlands, differing on invertebrate abundance, temperature and Ph. Introduced species appear to have the greatest tissue loss and higher rate at which energy is processed. Ecosystems with introduced autotrophs as the primary producers will have a greater energy availability and a higher variety of organisms as a result.