Ecological Patterns and Processes in Mangrove Ecosystems Ii Problem Set
During our Field Trip to the Cairns Airport Mangrove Boardwalk on 7th August, 2013, I observed a distinct distribution pattern in the locations of the Orange-Clawed Fiddler Crabs (Uca coarctata). Upon further investigation, I noticed that the larger males of the species (easily identifiable as those with one large claw) colonised the open areas located beside the Middle Creek viewing platforms, that were in full sun for a larger part of the day, and offered no protection from predators.
Conversely, the smaller females colonised the shadier and more forested area on the banks of the small stream, located between the two platforms, approximately 50 and 90 paces either side respectively, along the boardwalk. I was intrigued as to why this would be so. 2. FUNCTIONAL HYPOTHESIS The male Orange-Clawed Fiddler Crab (Uca coarctata) avoids shaded and vegetated habitats that hinder visibility. Inhabiting open and less structured mud-flat areas enable the male Fiddler Crab better visibility for predator detection, and improved scope for the use of courtship signals.
Male Fiddler Crabs rely upon their large claws for the provision of visual aids in many aspects of their day to day existence. They use their claws in territorial defence in order to ward off antagonistic males. Their complex signals also aid females in detection of, and their approach to, males of the same species. Wandering burrow-mating females may visit up to 100 males before making a choice of mate. Interestingly, those males who lose their claws, will regenerate another claw that requires less energy to function, thus making it more effective at signalling, and possibly leading to more successful meetings with females.
As the male Fiddler Crab is quite dependent upon visual cues, it needs to create its home in an open, flat environment with few structures able to obstruct his view of others, and their view of him. Competition and predation by other species of crabs may be higher in more vegetated areas of the mangrove, making it much more dangerous for conspicuously waving males to inhabit these areas. Whereas the female Fiddler Crabs, who do not wave and do not draw predators attention to themselves , may be better equipped to live in a more complex environment, with its higher concentration of competitors and predators. /2 3. HYPOTHESIS CITATION REFERENCE Orange-Clawed Fiddler Crab eyes are raised high above their bodies like periscopes, enabling them to have an excellent visual range with full panoramic field of view without the need for eye movements (Zeil & Hemmi, 2006). As a result of this, potential predators are seen as being above their horizon, and other crabs below their horizon (Layne et al. , 1997; Layne, 1998). The male Uca uses complex ‘waving’ patterns in order to attract females during courtship (Crane, 1975).
The waving of their claws occurs as a break in their visual horizon (Christy, 1995; Land & Layne, 1995a), in the area between prey flying above, and other crabs below – be they potential mates or rival. This waving behaviour may make them more obvious to potential predators. Osborne and Smith (1990) hypothesised that Uca species only colonise clearings or open canopy forest habitats. As their visual resolution is poor, the open mudflat habitat provides the crab with a calm visual environment, as opposed to the highly vegetated regions of the mangrove, with its complicated motion patterns (Zeil & Hemmi, 2006).
Uca may also avoid shadier areas because this change in light levels might reduce the effectiveness of their waving display. (Crane, 1975) proposes that darker forests may not be conducive to visual displays. Teal (1958) and Hyatt (1975) found that spectral sensitivity varies between Uca species depending on shade levels in their habitats. If crabs have ‘bright-light adapted’ eyes, then crabs that often use visual signals would move away from those areas where their vision has been reduced due to structural elements or shade (Nobbs, 2003), in order to maximise the viability of their waving displays. /3 REFERENCE LIST Christy J. H. (1995) Mimicry, mate choice and the sensory trap hypothesis. Am Nat. 146, 171-181. Crane J. (1975) Fiddler crabs of the world. Ocypodidae: Genus Uca. Princeton Univ. Press, New Jersey. Hyatt G. W. (1975) Physiological and behavioural evidence for colour discrimination by fiddler crabs (Brachyura. Ocypodidae, genus Uca). In: Vernberg, F. J. (Ed. ), Physiological Ecology of Estuarine Organisms. University of South Carolina Press, Columbia, South Carolina, 333-365. Land M. F. & Layne J. E. (1995a) The visual control of behaviour in fiddler crabs I.
Resolution, thresholds and the role of the horizon. J. Comp. Physiol A. 177, 81-90. Layne J. , Land M. F. & Zeil J. (1997) Fiddler crabs use the visual horizon to distinguish predators from conspecifics: a review of the evidence. J. Mar. Bio. 77, 43-54. Layne J. E. (1998) Retinal location is the key to identifying predators in fiddler crabs (Uca pugilator). J. Exp Biol. 201, 2253-2261. Nobbs M. (2003) Effects of vegetation differ among three species of fiddler crabs (Uca spp. ). Journal of Experimental Marine Biology and Ecology 284, 41-50.
Osborne K. & Smith T. J. (1990) Differential predation on mangrove propagules in open and closed canopy forest habitats. Vegetation 89, 1-6. Teal J. M. (1958) Distribution of fiddler crabs in Georgia salt marshes. Ecology 39, 185-193. Zeil J. & Hemmi N. M. (2006) The visual ecology of fiddler crabs. Aust. J. Comp. Physiol. 192, 1-25. …/4 5. POSSIBLE FACTORS THAT MAY EXCLUDE MY HYPOTHESIS In the preceding pages, I have formulated my hypothesis that I believe explains the biological interaction that I have observed.
I have presented documented, published and peer reviewed evidence to support my hypothesis, based upon scientific field experiments. These experiments alone will not definitively prove the validity of my hypothesis, due to a number of other factors not taken into consideration. These may include, but are not limited to: high annual rainfall, mean annual temperature variations, mean annual relative humidity, soil structure, competition, bacteria, salinity levels, pollutants, periods of inundation, time of year or season, time of day that data is taken, and unforseen predation. Any one of these factors ould be contributing to the interaction that I have observed, and can only be determined through further research and experimentation. …/5 6. POSSIBLE HYPOTHESIS TEST EXPERIMENT In the preceding pages, I have formulated my hypothesis that I believe explains the biological interaction that I have observed in male Fiddler Crabs (Uca coarctata). In order to definitively test that my hypothesis is correct, I would conduct a long term experiment on the crabs that reside on the flat, sunny and open areas beside Middle Creek at two locations – The Wandering Creek viewing platform; and The Tides Highway viewing platform.
Six areas of equal size would be selected, two areas would be provided ‘shade & vegetative structures’, another two areas would be provided ‘shade’ with no structures, whilst the remaining two areas would be left undisturbed. Shade coverings would be constructed, using the darkest/thickest grade of shade-cloth attached to sturdy lattice trelliswork, and then suspended above the soil in each of the applicable ‘shade’ areas. For the remaining two shade areas requiring ‘structure’, lengths of slim doweling would be suspended from the outer edges of trellis in an attempt to simulate above-ground structure.
The ends of the suspended doweling would end above the soil so that they might move with the wind. Once the structures were completed, the numbers of male Uca Coarctata in each area would be recorded by an observer who had remained still for 10 minutes. These recordings would be completed on a set day each month for a period of 18 months. At the end of this period, the location of male Uca crabs and their burrows should confirm or refute my hypothesis that male Uca Coarctata avoid shaded and vegetated areas that hinder visibility. PRACTICAL 3
ECOLOGICAL PATTERNS AND PROCESSES IN MANGROVE ECOSYSTEMS II PROBLEM SET 1. PHYSIOCHEMICAL INTERACTION During our Field Trip to the Cairns Airport Mangrove Boardwalk on 7th August, 2013, I observed a distinct zonational pattern within the Mangrove tree locations. The Smooth Fruited Yellow Mangrove (Rhizophoracea Ceriops australis) was most dominant in the salt marsh and the higher intertidal areas that are quite obviously dryer, with higher salinity levels due to a greater amount of evaporation, and less periods of saltwater inundation which leaves them exposed for a lot longer.
This species dominance continued, and was only sparsely interspersed by the Grey Mangrove (Avicenniaceae Avicennia marina), until the arrival of the first Red Mangrove (Rhizophoracea Rhizophora stylosa) approximately 150-180m along the Boardwalk. 2. FUNCTIONAL HYPOTHESIS The Smooth Fruited Yellow Mangrove (Rhizophoracea Ceriops australis) adapts its physiology in order to become desiccation tolerant, enabling it to withstand longer periods between inundations, thereby asserting its dominance in the higher intertidal areas and salt marsh zones of the mangrove.
Physiological adaptation of mangrove tree species, to physiochemical gradients across the intertidal zone, (including hydroperiod and soil salinity), offer clear explanations for zonation patterns in mangroves. For species to function in harsher physiochemical gradients, physiological adaptation is required in order to survive and thrive, and possibly dominate these areas over other less adapted species.
Rather than the shorter periods of inundations and the rapid rate of salinity change as experienced by species residing in the lower intertidal areas, those species residing in the higher intertidal areas and salt marsh margins have adapted to longer periods between inundations (longer constancy levels of dryness), exposure to a higher evaporation rate (leading to higher and more constant levels of salinity in the soil), and longer periods of dehydration and dessication. …/2 3. HYPOTHESIS CITATION REFERENCE
Zonation is a result of each species having its own ‘optima’ along the physiochemical gradient, which controls where that species occurs within the mangrove environment. When physiochemical conditions vary, the distribution of species in particular areas of the gradient is determined by physiological specialisation (Ball 1988; NcKee 1993, 1995). Salinity and frequency of tidal inundation influence the pattern of plant species distributions within tropical tidal forests (Macnae, 1968, 1969; Clarke & Hannon, 1970; Clough, 1984).
Various species change their physiological makeup, in order to adapt to a physiochemical niche in that environment. That change can lead to the domination of species in particular areas. In locations where periods of inundation had the lowest duration, Rhizophoracea Ceriops australis was found to dominate (Crase, Liedloff, Vesk, Burgman, & Wintle, 2013). In recent experiments conducted by Crase, Liedloff, Vesk, Burgman, & Wintle (2013), it was observed that the hydroperiod had the most influence over species dominance.
Soil salinity was observed to be of secondary influence, with the salinity of inundating water being the least influential variable. When one considers the spectre of rising of sea levels due to global warming, and the ensuing changes to the hydroperiod, we must consider the resultant spatial restructuring of species dominance in mangrove communities, and the effects on coastal erosion, mangrove rehabilitation, fisheries, and future coastal development planning and infrastructure. (Crase et al. , 2013).
REFERENCE LIST Ball M. C. (1988) Ecophysiology of mangroves. Trees 3, 129-142. Clarke L. D. & Hannon N. J. (1970) The mangrove swamp and salt marsh communities of the Sydney district. III. Plant growth in relation to salinity and waterlogging. J. Ecol 58, 351-369. Clough B. F. (1984) Growth and salt balance of the mangroves Avicennia marina (Forsk. ) Vierh. and Rhizophora stylosa Griff. In relation to salinity. Aust. J. Plant Physiol. 11, 419-430. Crase B. , Liedloff A. , Vesk P. , Burgman M. A. amp; Wintle, B. A. (2013) Hydroperiod is the main driver of the spatial pattern of dominance in mangrove communities. Global Ecology and Biogeography, (Global Ecol. Biogeogr. ) 22, 806-817. Macnae W. (1968) A general account of the fauna and flora of the mangrove swamps in the Indo-West-Pacific region. Adv. Mar. Biol. 6, 73-270. Macnae W. (1969) Zonation within mangroves associated with estuaries in North Queensland. Estuaries (ed. by G. E. Lauff) AAAS, Washington, D. C. pp. 432-441. McKee K. L. 1993) Soil physiochemical patterns and mangrove species distribution: reciprocal effects? Journal of Ecology 81, 477-487. McKee K. L. (1995) Seedling recruitment patterns in a Belizean mangrove forest: effects of establishmentability and physico-chemical factors. Oecologia 101, 448-460. …/3 4. 5. In the preceding pages, I have formulated my hypothesis that I believe explains the physiochemical interaction that I have observed. I have presented documented, published and peer reviewed evidence to support my hypothesis, based upon scientific field experiments.
These experiments alone will not definitively prove the validity of my hypothesis, due to a number of other factors not taken into consideration. These may include, but are not limited to: high annual rainfall, mean annual temperature variations, mean annual relative humidity, predation by other species, soil structure, competition, growth of bacteria, pollutants and elevation. Any one of these factors could be contributing to the interaction that I have observed, and can only be determined through further research and experimentation.
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