Environmental Factors Impacting Marine Ecosystems

Introduction

In marine habitats, a large number of organisms continue to exist in spite there being factors that could hinder their survival. In order to increase the chances of fertilization or survival of an offspring, marine invertebrates adapt to various reproductive strategies that are behavioral, structural and functional. Examples include: Asexual and/or sexual reproduction, larval dispersal, speciation among others. From an ecological point of view, the earliest stages of development in marine invertebrates advances in one of three directions which give rise to either planktotrophic larvae, pelagic lecithotrophic larvae or non-pelagic lecithotrophic larvae (Thorson, 1946). Whatever course of development is taken strives to ensure a decrease in larval mortality and increase in adult adaptability

On the other hand, environmental factors, both biotic and abiotic, affect marine ecosystems causing changes that are either beneficial or detrimental to marine invertebrates. In any case, these factors are crucial in impacting distribution and abundance of marine invertebrates (Munro, 1954)

This study main aim is to find out the influence of reproductive strategies on distribution and abundance of marine invertebrates balanced against influence of environmental factors on the same. It will seek answers to the following: Factors affecting distribution and abundance of a marine organisms, how reproductive strategies influence their dispersal and recruitment and how they are adapted to maintenance of populations in particular environments. Lastly, the influence that larval supply has on population or community structure and how relatively important it is to other factors. Inferences will be drawn from previous scholarly research relevant to the topic

Factors Affecting Distribution and Abundance of Marine Invertebrates

Factors are classified into abiotic and biotic factors. Abiotic factors comprise of the non- living components classified into physical and chemical factors. Examples include; Temperature, physical gradient, PH, Pressure, salinity, dissolved oxygen among others. On the other hand, biotic factors are the living components of the marine ecosystem which are classified into producers, consumers and decomposers. Factors such as predation fall under category of the biotic factors (Byrne & Pzeslawski, 2013).

Abiotic factors such as salinity, dissolved oxygen and temperature have a significant impact on the distribution and abundance of marine invertebrates (Hatum et al.,1991). Climatic change effects are divided into two: indirect and direct effects. Indirect effects impact community structures while direct effects impact the individual species, their growth, productivity, metabolism and survival rates by interfering with properties in the water encompassing them. These properties include: Salinity, temperature, chemical composition, stratification and mixing. Such associations influence predation, competition and community structures (Thieltges et al., 2008).

Benthic marine invertebrates and their planktonic stages of development live in an ecosystem where the levels of stress are constantly aggravated by global climatic changes (Adams, 2005).

Amongst the abiotic factors that directly influence marine invertebrates, temperature is crucial. It propels biological processes and sustains population growth, ecosystem processes and life history traits. The scope of the effects extends from those affecting bacteria metabolisms directly to those affecting development of invertebrates (Thieltges, 2008) and larvae increasing or decreasing temperatures also impact populations indirectly by influencing their supply of food (Byrne and Pzeslawski, 2013)

Coral reefs usually provide an area for mating/spawning, food and cover for marine creatures. A significant number of the marine species that live on coral reefs are negatively affected when acidification of water and increase in temperature damage the reefs. Due to this loss, some of the species can relocate to other areas that are rocky while those species that have been fully adapted to live in the reefs diminish. It is believed that if conditions persist in deterioration, there will be huge decrease in diversity of the invertebrate species in these environments (Byrne and Pzeslawski, 2013).

Another factor that has great influence on marine life is salinity (concentration of salts in water). Salinity can be naturally occurring, for example sea waters or estuaries where fresh water mixes with salty water or attributed by unnatural influences like climate change, flow of water and altered levels of water in marine ecosystems. Very high or very low than normal levels of salinity affect marine organism performance by changing rates of metabolism, reducing growth and survival and altering osmolyte concentration of body fluids (Beadle, 1931).

Dissolved oxygen concentration is also a factor affecting distribution and abundance of marine organisms. The levels of dissolved oxygen are continuously affected by photosynthesis, diffusions and aeration, decomposition and respiration, pressure changes, salinity and changing temperatures. The tolerance of dissolved oxygen by marine invertebrates differs from species to species and also at different stages of life (Munro, et al., 1954). For instance, the marine lamellibranch mollusc Mya arenaria is a facultative anaerobe. Whereas pure oxygen is harmful to fish, invertebrate Metazoan seem to thrive in (Munro et al., 1954).

Oceanic PH levels are usually altered by increasing or decreasing atmospheric carbon dioxide. Benthic marine invertebrates experience continuous fluctuations in stressors including PH and temperatures. Several studies have proven that some intertidal invertebrates are living near their physiological tolerance limits and drastic changes in the environment might push them to extremes of lethal levels (Tomanek 2008; Somera 2010). As the temperatures in the ocean increase, planktonic and early juvenile stages may be the weak link for success of marine invertebrates in the future since they are the stages very sensitive to stressors (Pechenik 1987).

According to recent studies on oceanic acidification, it has been found that there are negative effects on the survival and growth due to reducing PH levels. This may lead to reduction in recruitment by disrupting cues emerging from the substrate and used in the excerption of settling sites (e.g., by coral larvae). There is also evidence of the impacts of reduced PH in field studies where disintegration of the shells of live pteropods proceeded from acidification (Bednarsek et al. 2012)

Environmental gradients such as predation and competition have correlated effects and they constantly influence the distribution of organisms in marine ecosystems. The exposure of species to high predation leads to reduction in their physical activities and even changes in physical appearance such as shorter bodies. High competition levels lead to slowed growth and development which affected activity levels and physical appearance. It led to gradual growth and development and elongated bodies. For both gradients, the results were constant rather than threshold responses. To show their connection, studies done on marine invertebrates revealed that when the risk of predation was low, responses to competition increased provided competition and predator induced traits were in the same direction whereas, when the competition and predator induced traits were in opposing directions competition was larger under increased predation risks (Relyea, 2004)

The Influence of Reproductive Strategies on Dispersal and Recruitment and how they are Adapted to Maintenance of Populations in Particular Environments

The ecosystem has reproductive patterns that play and important part in regulating the distribution, structure and function of all marine species at different scales. The reproductive strategies of marine invertebrates vary extensively and even though many of them are able to reproduce asexually, they have sexual reproductive strategies that are diverse. Essential factors that contribute to the resilience of the ecosystem are strategies for dispersal ability which are elected as an adaptation to stability of the habitat and spatial heterogeneity. The ability to disperse is crucial for the structuring of small-scale populations and their connectivity amongst isolated populations. A large number of marine invertebrates have benthic adults that are sedentary while their larvae disperse extensively. This ensures that the input of new recruits is not connected to the local reproductive output (Roughgarden et al. 1988).

Simple marine organisms like corals are normally sessile (not free moving; attached permanently). This presents uncertainty during fertilization to the species that reproduce sexually. To increase their chances of fertilization, they release gametes in huge numbers and the tiny mobile sperms are able to find female gametes because they are bigger. Hermaphroditism and parasitism are strategies of reproduction often used by sessile and slow moving organisms. For instance, barnacles are hermaphrodites that utilize their elongated sexual organs for the transfer of male gametes. Complex organisms that are mobile have reproductive strategies that are diverse. These animals use sounds, pheromones, competitive courtship and visual clues among others to attract female partners. Some of them transform from male to female as they advance in their stages of development. In pursuance of a greater chance of species survival, many marine invertebrates produce immense amounts of tiny eggs into the environment that hatch quickly while other species are able to sustain their population by reproducing more frequently in their lifetime (Jenkins et al.2008).

The modes of development of marine invertebrates may be classified either according to feeding mode, type of development, location of development or to length of the planktonic period; long, short or lack of planktonic period (Eckter, 2003).

There are three phases in the colonization of marine habitats. First one is development whereby dispersal is included as a planktonic form, the second is testing to know whether a habitat is suitable or not and lastly settlement. For the later phase, invertebrates that are sessile attach themselves to substratum and undergo metamorphosis. During this period the organism is not likely to be detected by predators because of its small size, cryptic habitat among others. The ‘survival phase’ (fourth phase) kicks in and can last from days to months until this organism is preyed on. Settlement is the number of organisms that past the third phase, comprises of larval stages only, while recruitment is the number of organisms that pass through the fourth phase and it comprises both larval and juvenile stages (Keough & Jownes, 1982)

Supply Side Ecology; This is a term used to describe the consequences of variations in recruitment (Underwood and Fairweather, 1989). This is the inrush of new individuals into already existing population. It highly applies to open communities as opposed to closed communities. This is because closed communities totally sustain themselves when it comes to new members. To some extent, most of the marine ecosystems are open systems. Therefore, the pressure applied by the supply of new individuals to communities and populations complements the influence of local processes like competition and predation among marine invertebrates. A perfect example of supply side ecology is the settling of larvae on rocks (Rodger, 1986). Though there is massive input of new members, climatic factors highly affect the supply. According to (Menge et al., 2009) processes of supply side ecology in marine invertebrates are highly susceptible to continuing climatic variations.

Dynamics of adult population are driven by the survival of larval stages. According to research, marine invertebrates whose larval planktonic duration is long go through variations in their adult populations as opposed to species that have short or no planktonic period. In addition to this, recruitment of organisms is affected by duration of planktonic periods. Recruitment is highly variable in species with long periods- with instances of extensive recruitment in some years to no recruitment at all in other years. On the other hand, species with short or no planktonic period experience little variation in recruitment. The main reasons for fluctuations in species with long planktonic period are high fertility and variations in planktonic mortality rates.

When the amplitude of density fluctuations drop it causes a reduction in temporal variability. This is known as stabilization. Dispersal can stabilize patches which are dispersed by scattering off springs over heterogenous habitats (Todd, 1998)

Research done on Pachythyone rubra (sea cucumber) that gives birth to young ones with no planktonic periods revealed elevated spatial variability which was caused by spatially varying predation and abiotic factors and the absence of dispersal allow these patterns to persist (Eckert, 1999)

Stressors from the environment have been depicted to be the leading cause of tremendous fluctuations, exemplified by recolonization and extinction of invertebrates in marine ecosystems (Powlik and Lewis, 1996)

The stages of development in marine invertebrates are dependent on several selection pressures so as to perfectly fit in their environment. Selection should favor a pattern of reproduction that has the greatest efficiency- one that results in a large number of off springs with the least amount of energy used. This is because for reproduction to occur, it requires a high amount of the species energy budget. There are two classifications of selection pressures on reproductive patterns. The first one is those that impact production and survival of marine larvae, the second one is those that impact survival of adults right from metamorphosis to reproductive maturity. High mortality rates are heavily experienced during the early stages of development. According to earlier studies, about 99% of the planktotrophic species larvae die even before metamorphosis. Therefore, the pressures influencing mortality of larvae play a very critical role gradual change of general reproduction patterns (Vance, R. 1973).

Differences in dispersal of larvae attribute to dissimilarities in the three methods of reproduction in that survival of adults in non-pelagic forms is dependent on sources of mortality at the site of parental population, on the other hand, survival of adults in pelagic forms do not depend on sources of mortality at parental site or even for metamorphosized organisms (McEdward, 1997).

Conclusion

Marine invertebrates have adapted well into their ecosystem because of different attributes, one of them being the reproductive strategies developed overtime; From diverse reproductive modes of asexual and sexual reproduction, to different stages of development each with unique adaptations, recruitment, settlement and dispersal. Although the highest mortality occurs in their planktonic stage due to widespread dispersal rate, recruitment of new off springs can outdo the mortality rates that occur in a season. Their numbers seem to have significantly increased and populations maintained from extinction due to the inundation of new members, settlement of juveniles and their survival into adulthood.

However, these reproductive strategies are being antagonized by many environmental factors such as physical gradient, salinity, PH, temperature, dissolved oxygen concentration, predation, competition among others which have been influencing the population of species in a negative aspect especially because a majority of the factors are an aftermath of pollution by human beings and climatic changes such as global warming.

Global warming has caused tremendous changes in water bodies leading to disruption of marine population and systems. Human pollution on water bodies; oceans and lakes among, has led to water acidification, suffocation of organisms by plastic materials, leaching of elements into the organisms and loss of marine habitats. This is forcing many organisms to constantly adapt- by either recolonizing, moving to more suitable locations or, worse case scenario, undergoing extinction. Some species have managed to thrive in extremes conditions though their limits are being fully stretched. Unfortunately, others undergo extinction in the process of trying to adapt to rapid changes in their surroundings.

In conclusion, it can be argued that environmental factors influence distribution and abundance of marine invertebrates to a greater extent compared to reproductive strategies that influence the same.

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