In this experiment we will study the effect of herbivory in local gastropods on differing algal species, furthermore the effects this has on percentage cover of the algae.
Seasonal proportions of algae to barnacles varied on season
winter more barnacles, summer more algae,
Green most palatable, not much eats encrusting algae,
expected results
Aims
In our experiment we are testing two main hypotheses:
- Herbivores decrease algal abundance, and;
- The Strength of this effect is a function of whether there is some algae already in a plot.
Methods
Twenty 0.35 x 0.35m square plots will be marked out on the shore with bolts in opposite corners and numbered 1-20.
Herbivorous gastropods will be excluded from some plots by removing all the herbivores from selected plots and painting a border of antifouling paint which contains copper around these plots. Gastropods are deterred from crossing such barriers. Half painted plots will have half of each edge of the plot painted.
Existing algae will be scraped off appropriate plots with paint scrapers, metal brushes, and a blowtorch.
Data will be collected every week for ten weeks including week one, whereby algae is counted as percentage cover, and herbivorous gastropods will be counted in all plots.
Results
Results for: Green algae (significant)
Two-way ANOVA: week6 versus Herbivores, Scraping
Source DF SS MS F P
Herbivores 1 25921 25921.0 105.87 0.000 (significant)
Scraping 1 25 25.0 0.10 0.755
Interaction 1 0 0.0 0.00 1.000
Error 12 938 244.8
Total 15 2888
S = 15.65 R-Sq = 89.83% R-Sq (adj) = 87.29%
There is a significant difference in the abundance of green algae between herbivore treatments but no significant difference between scraping treatments. There is no significant interaction between the two.
It can be seen that those quadrats incorporating a paint treatment effectively excluded herbivores (i.e limpets and gastropods) so as to enable the increase of green algae colonising the rocky shore.
Results for: Bare rock (significant)
Two-way ANOVA: week6 versus Herbivores, Scraping
Source DF SS MS F P
Herbivores 1 12769 12769.0 55.16 0.000 (significant)
Scraping 1 100 100.0 0.43 0.523
Interaction 1 81 81.0 0.35 0.565
Error 12 2778 231.5
Total 15 15728
Results for: Limpets (significant)
Two-way ANOVA: week6 versus Herbivores, Scraping
Source DF SS MS F P
Herbivores 1 1425.06 1425.06 70.74 0.000 (significant)
Scraping 1 1.56 1.56 0.08 0.785
Interaction 1 14.06 14.06 0.70 0.420
Error 12 241.75 20.15
Total 15 1682.44
S = 4.488 R-Sq = 85.63% R-Sq (adj) = 82.04%
There is a significant difference in the occurrence of limpets between herbivore treatments but no significant difference between scraping treatments. There is no significant interaction between the two.
The graph indicates that the copper based paint treatment worked effectively to exclude limpets from the herbivore absent (HA) quadrats as opposed to quadrats that were not painted, herbivore present (HP).
There is no significant difference in the abundance of encrusting algae between herbivore treatments and no significant difference between scraping treatments. There is no significant interaction between the two. Furthermore;
There is no significant difference in the abundance of gastropods between herbivore treatments and no significant difference between scraping treatments. There is no significant interaction between the two.
Discussion;
The hypothesis that herbivores limit the abundance of algae on mid-levels on a rocky shore has been proven to be correct (Underwood and Jernakoff, 1981). More specifically, this experiment shows that limpets control the levels of green algae found on a rocky shore. Although there were other species of grazing gastropods present, including Austrocochlea, Bembicium, Nerita and Nodolittorina and another form of algae (encrusting), any interactions between them were insignificant. This is as opposed to plots that were not painted and which showed no significant increase in algal growth
The scraping treatments did not increase the rate of recruitment by algae. This is because algae settled on the substrata similarly whether or not it had been scraped. Thus the hypothesis that the presence of algae predetermines the likelihood of further settlement can be rejected.
In regards to the half painted plots, they were carried out so as to determine whether or not the actual paint had any effects on the experiment other than that of excluding herbivores. The results showed that there was no difference between the control and half painted plots which indicates that the paint had no detrimental effects to the experiment such as diffusing into the water and killing algae.
Barnacles played a very minor role in this experiment, and only achieved minor significant results. This is quite probably due to the barnacles taking up space needed for algal growth. Barnacles do not eat the green algae once it is established on a rock surface, because they are filter feeders; they only feed at high tides.
This experiment can further incorporate a multitude of angles that may have been left unanswered and/or need further discussion/analysis.
Is recruitment greater than consumption? In favourable conditions or circumstances our results may be flawed, by the amount of algae re-growing after consumption masking the initial consumption, to either leave more algae, the same amount, or less algae. Thus the rate of growth of algae is important to consider (Paine, 1974).
Which algae is most palatable? Coupled with the effects of growth rates, a species that is more palatable will give disproportional results by comparison to a species that is not consumed.
Do algae have defence mechanisms? The presence of secondary metabolites in red algae give it a defence advantage over other algae thus creating bias.
Do the thallus spread when consumed, yet leave same S.A? In a particular clump of algae is the thallus somewhat supports other thallus in the structure, therefore if only partially consumed the network structure holding the algae together will naturally spread, so although there will be less biomass our method of measurement (surface area) will be the same even though there has been herbivory present (adapted from; Gibbons, et al. 1986).
Bias, in intraspecific competition? If certain species that would otherwise contributed to a measurable amount of herbivory or whom would consume an algal species that none others do are being controlled by a predatory species, this will bring bias to our results, furthermore the stage this predator prey interaction is at, (Gibbons, et al . 1991)with regard to ‘time lags’ will also vary our result, thus it is important in the long term to repeat this experiment and maybe more results will be significant.
Were previous year’s experiments in the same area? Assuming we are not the first group to do this experiment in this area, is this area in a natural state to begin with? If not than our results are not representative.
Was the wave action and weather conditions representative? Physical factor also play a large role in the presence of any intertidal organisms (Creese, 1988). The surfing lull over the winter 04’ has been described by some as ‘uncanny’ it is the wave action that acts as transport for new algal ‘colonists’ thus, if there is no waves there is no colonists. This effects recruitment of algae.
Seasonal effect and proportion of algae growth. This experiment may in fact have been conducted at a time that is non representative of the community’s natural state for reasons in species life cycles and seasonal proportion (adapted from; Williams,G.A.et al ).
Human error can also be mentioned as a possible reason to the lack of statistical significance in the results, eg; was scraped areas scraped correctly? Were data figures counted correctly? Were species identified correctly? Was randomly selected areas random?
Maybe herbivory is not from Gastropods? We have not included the herbivory from crabs this is a somewhat substantial proportion I would have thought, thus we may be attributing consumption to the wrong species.
The results of this experiment show that herbivorous grazing limpets have a significant effect on the growth of green algae on a rocky shore in the mid-high region. Limpets dominate over other forms of gastropods due to their more prostrate morphology, resisting wave action and desiccation. Removing algae from the stratum prior to sampling showed very little effect on algal growth, and the paint used to mark out plot boundaries showed none.
References.
Carefoot, T., 1977, Pacific Seashores: Seattle, University of Washington.
Creese, R.G (1998) Ecology of Mollusan grazers and their interactions with marine algae in north western New Zealand. N.Z.J. Marine Freashwater Research 22:
427-444.
Dayton, P. K., 1975, Experimental evaluation of ecological dominance in a rocky intertidal algal community: Ecological Monographs, v. 45, p. 137-159.
Gibbons, M.J. & Griffiths, C.L. 1986. A comparison of macrofaunal and meiofaunal distribution and standing stock across a rocky shore, with an estimate of their productivities. Marine Biology 93: 181-188.
Gibbons, M.J., Pillar, S.C. & Stuart, V. 1991. Selective carnivory of Euphausia lucens in the southern Benguela. Continental Shelf Research 11: 625-640.
Little, C. and J.A. Kitching. 1996. The Biology of Rocky Shores. Oxford, Oxford University Press.
Paine, R. T., 1974, Intertidal community structure. Experimental studies on the relationship between a dominant competitor and its principal predator: Oecologia, v. 15, p. 93-120.
Underwood, A.J. & Jernakoff, P. (1981). Interactions between algae and grazing gastrods on the structure of a low-shore intertidal algal community. Oecologia 48: 221-233.
Williams,G.A. Davies,M.S. Nagarkar,S. (2000). Primary succession on a seasonal tropical rocky shore: the relative roles of spatial heterogeneity and herbivory. Marine Ecology Progress Series. MEPS Vol. 203.
Bibliography;
Dayton, P. K., 1975, Experimental evaluation of ecological dominance in a rocky intertidal algal community: Ecological Monographs, v. 45, p. 137-159.
Underwood, A. J. (1980) The effects of grazing by gastropods and physical factors on the upper limits of distribution of intertidal macroalgae. Oecologia 46: 201-213.