Intro
The rocky shore is an extremely variable environment, and organisms living in this community have developed adaptations that allow them to tolerate these conditions and create ecological niches for their populations. In this investigative report I will be discussing the way environmental and biological factors influence the distribution pattern and various adaptations of the organisms living in the intertidal zone. I will focus my discussion around two species – Ulva Lactuca and c.Ornata – and will elaborate on how their respective ecological niches, key adaptations and biological interactions help them to survive and reproduce in the changeable environment of the rocky shore.
Zonation & how environmental factors contribute
Organisms living in the intertidal community are alternately submerged in water and exposed to thermal extremes during the period of low tide, causing wide variations in oxygen availability, temperature, salinity and exposure. As well as dealing with these environmental changes, organisms must cope with subjection to strong selective pressures such as predation and competition for space. These factors combine to result in a spatially condensed community and a distribution pattern that we can identify as zonation, shown in the kite diagram as distinct horizontal bands of organisms that occur across the rock from the low tide zone to the splash zone. This zonation results from the changes of environmental factors along the gradient of low-high tide, allowing species with certain adaptations to inhabit certain zones depending on their tolerance range to particular abiotic factors. For example, organisms tolerant of high sun and wind exposure (e.g periwinkles and barnacles) are well adapted for life in the splash zone, whereas the majority of kelp and seaweed do not cope well with exposure and are better suited to the low-tide zone where they are fully immersed in the water. As abiotic conditions exceed the tolerance limits of a species, its zone of distribution finishes, allowing another species that is better adapted to those new conditions to form another zone. Interspecific competition generally occurs at the boundaries between zones, where the ecological niches of two species collide. In this instance one of the species will inevitably be eliminated as, according to Gause’s Law of competitive exclusion, two species that compete for the exact same resources cannot stably coexist.
Sea Lettuce
Sea lettuce – or Ulva Lactuca – is a green, sheet-like algae that ranges in size around 15cm. It grows in the low/mid tide zone, where it is adapted to tolerate tidal changes (the midlittoral zone is exposed to air during low tide) and variations in temperature and light intensity. For example, the fronds of sea lettuce have adapted to be extremely thin, essentially forming a two-layered sheet of photosynthetic tissue that can efficiently present bigger surfaces to the sun’s rays, maximising the photosynthetic potential of the sunlight found in shallow intertidal water. Physiologically, these thin blades (containing air spaces for buoyancy) also allow for easy absorption of nutrients and diffusion of gases from the water, as substances diffuse faster over short distances. Sea lettuce has a wide tolerance range of nutrient availability. In fact, it can flourish in moderate nutrient pollution (caused by sewage disposal, for example), as even small increases of nitrate/ammonium and phosphate levels in the water can support a huge increase in population growth in a phenomenon known as an algal bloom, which reduces dissolved O2 concentration in the water and poses a threat to species with little tolerance for low oxygen levels. As a result, sea lettuce density is used as an indicator to monitor pollution trends. Sea lettuce has a high rate of reproduction due to the massive amounts of mobile gametes and spores produced in its multi-stage reproductive process, which allows it to quickly colonise areas that have been cleared of other organisms following a disturbance (primary succession). Another adaptation that gives sea lettuce an advantage in the mid/low tide zone is the strong holdfast at the algae’s base, which allows it to cling to rocks and other substrates without being ripped away by wave action. By developing adaptations to accommodate for life in the low/mid tidal zone, Ulva lactuca facilitates its ability to carry out the processes of photosynthesis, growth and reproduction – the O2 and chemical energy formed by sea lettuce during the photosynthetic process causes sea lettuce (along with other producers) to play a critical role in maintaining both the water’s oxygen and carbon dioxide concentrations, and the trophic structure of intertidal communities as a whole. Sea lettuce is an important food source for herbivorous marine grazers e.g molluscs, sea urchins and crustaceans; the dense clumps also provide micro-habitats for small fish / snails. As shown in the kite diagram, other species of seaweed are found in the same low/mid tide zone, which indicates they are likely to be competing with sea lettuce for space on the rocks. Large numbers of neptune’s necklace were found 8m from the MHWM, where no sea lettuce was present. The maximum amount of sea lettuce was found at 9-10m from the MHWM, where numbers of neptune’s necklace were much lower, and none were found at 10m. This data strongly suggests competition between the species, with neptune’s necklace limiting the lower distribution of sea lettuce. We can observe that the realised niche of the sea lettuce is smaller than its fundamental niche as a result of this interspecific competition.