This study will compare the colonisation and breakdown rates of different leaf types (including one artificial) in a river. The aim of this study is to test if leaf packs are more important to stream invertebrates as a food source than as a habitat. Although studies similar to this one have been carried out (Rowe & Richardson, 2001; Dobson & Hildrew, 1992) non have taken place in the river Sett which is in excellent condition due to its location in the High Peak National park in Derbyshire and flows from its source through a moor. Also few studies have combined the use of different types of real leaves as well as plastic leaves.
Method
Forty eight leaf bags with a 10mm mesh were produced. Sixteen of the bags contained dried, conditioned Alder leaves. Sixteen contained dried, conditioned Oak leaves. The other sixteen were filled with artificial leaves made of black plastic bags cut into rough leaf shapes. Approximately 5g of material was put into each of the real leaf bags, the exact weight was recorded. The plastic leaves were much lighter so only 3g (approximately) were put in each bag in order to maintain a similar volume of material in each bag, exact weights were recorded. Each bag was labelled to enable weight comparisons upon removal. The leaf bags were pegged out in the middle of the river Sett in the peak district. They were distributed randomly among the riffles and glides. After 1 week in the river half of each type of leaf bag was collected. After a further week the rest were collected. 3 kick samples were also taken in the river. In the laboratory the remaining leaf litter was dried, weighed and the invertebrates were identified and counted.
Species were allocated into functional feeding groups. Data was Log transformed to closer fit a normal distribution. It was then analysed by using standard descriptive statistics and 2-way ANOVA, post hoc test were run using Tukey’s method.
Results
The community is dominated by Nemouridae stonefly, along with Black fly (Simuliidae) and Baetidae mayfly they make up almost 80% of the individuals found. The remainder of the community is made up of 18 other taxa.
It was shown that the substrate type in the bag significantly affected the total numbers of invertebrates found therein (Table 1) (Figure 1). Post Hoc tests showed that the two real leaf, substrate types were not significantly different in terms of total invertebrate numbers, but they both contained significantly more individuals than the plastic substrate (Table1).
Table 1 ANOVA statistics showing differences in invertebrate nubers (total and for each functional feeding group) between leaf type and week. Results of Tukey post-hoc tests are shown where ANOVA showed a significant effect of leaf type.
When the organisms were separated into functional feeding groups it was found that shredders occurred in significantly higher numbers in the real leaf, substrate bags than in those filled with plastic leaves (Figure 2). There was no significant difference in the number of individuals found in each leaf type among the other three feeding guilds found (predators, grazers and filter feeders) (Table 1). They showed no preference for Alder, Oak or plastic.
Figure 1 Mean number of invertebrates (total) found in each substrate type over both weeks. Error bars ± 1 standard error.
Figure 2 Mean number of shredders found in each substrate type over both weeks. Error bars ± 1 standard error.
A significant difference was found between the breakdown rates of the different substrate types. Alder broke down the fastest, then Oak and the plastic leaves gained weight. (Table 2)
Table 2 ANOVA table showing differance in % leaf mass loss between leaf type and week. Results of Tukey post-hoc tests are shown.
It was also shown that the proportions of grazers was significantly higher within the stream as a whole (Kick samples) than in the leaf bags. Also filter feeders made up a significantly larger proportion of the leaf bag fauna than in the rest of the stream. The proportions of shredders and predators were statistically similar in the leaf bags to the rest of the stream. (Table 3)
Table 3 Multivariate ANOVA table showing differences in proportions of functional feeding groups collected by the two sample methods (leaf bags & kick net).
Discussion
We set out to test if leaf packs are more important to stream invertebrates as a food source than as a habitat. There was no significant difference in numbers of invertebrates found between oak and alder, as such we can now compare between real leaves and plastic. The results show that more invertebrates were found among real leaves than in the plastic leaves. This can be interpreted in different ways. One could infer from this that the invertebrates are attracted to the real leaves because they offer a food source which the plastic leaves do not. With this line of reasoning we could answer our aim and say that food source is more important than habitat. However it is also possible that the plastic leaves are not a suitable habitat, perhaps the plastic is too smooth for the invertebrates to attach themselves to. This raises the question of if the invertebrates are attracted to the real leaves as or food source or if they are being repelled from the plastic because it is poor habitat.
The numbers of invertebrates from different functional feeding groups found in plastic or real leaves shows that leaf type only had a significant effect on shredder species. Predators, grazers and filter feeders were found equally in both types of leaf bag. This demonstrates that the plastic leaves are indeed a suitable habitat for invertebrates. If the plastic is a suitable habitat then invertebrates are not avoiding the plastic they just prefer real leaves. However only the species which are eating the actual leaves, the shredders, showed this preference. The other feeding groups must be able to find enough food in the plastic leaves. Filter feeders do not use the leaf packs as food, only as a substrate from which to filter FPOM coming down stream. Predators also use the leaf packs only as a substrate within which to hunt for other invertebrates to eat, so it may not matter to them on what they are hunting as long as there is sufficient prey. Grazers eat the biofilm which grows on the substrate, we know that biofilm can grow on the plastic leaves as they increased in weight during the time in the river indicating microbial and fungal colonisation. The preference for real leaves when looking at total number of invertebrates can be explained by the fact that shredder species made up 52% of the fauna collected.
In relation to the aim we can say that for shredder species a leaf pack is more important as a food resource than as a habitat. However, for the other types of invertebrates using leaf packs we can only say that leaves are more important as a habitat than for shredders. We cannot prove that leaf packs are more important as a habitat than as food for non-shredders, we can only infer this by what we already know about the way they feed. Shredders make up the majority of the fauna collected, not only in the leaf bags (where you would expect to find a lot of them) but also in the kick samples of the stream bed, where the proportion of shredders found was statistically similar to that found in the leaf bags. Therefore as shredder are in the majority and perform a crucial role in the ecosystem it would be fair to say that leaf packs are more important to stream invertebrates as food than as a habitat.
References
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Dobson, M. & Hildrew, A.G. (1992) A test of resource limitation among shredding detritivores in low order streams in southern England. Journal of Animal Ecology, 61, 69-77.
Resh, V.H. and Solem, J.O. (1996) Phylogenetic relationships and evolutionary adaptions of aquatic insects. Pages 98-107 in Merritt, R.W. and Cummins, K.W. (Eds.) An introduction to the Aquatic Insects of North America, 3rd ed. Kendall/Hunt, Dubuque, IA.
Rowe, L. & Richardson, J.S. (2001) Community responses to experimental food depletion: resource tracking by stream invertebrates. Oceologia, 129, 473-480.
Word Count: 1,557 (Excluding references and figure legends)