Effect of taphonomy and methodological decisions on inferences of spatio-temporal distribution of molluscan assemblages and its paleobiological implications.

Abstract

Marine biodiversity changes through time and space. Identifying the drivers of such change is becoming especially important in the context of recent anthropogenic biodiversity loss. Shallow marine molluscan assemblages have long been recognized as good indicators of overall marine biodiversity and the health of the ecosystem at a regional scale. Their long geologic span and abundance in the fossil record also make useful diversity indicators of past ecosystems. Their complex ecosystem and durable shells enable their fossil record to be a reliable indicator of ecological interactions including predation and competition. A comparison of live assemblage (LA) and time-averaged death assemblage (DA) also provides important ecological insight into the changes in the molluscan community through time. Before one can use these signals for inferring spatio-temporal patterns from molluscan fossil assemblage, however, it is important to recognize that various taphonomic and methodological artifacts can potentially affect the accurate ecological signal. In this thesis, I tried to assess the influence of taphonomy and methodological decisions (such as sampling protocol, analytical method, and data categorization) on ecological inferences using time-averaged molluscan death assemblages. Using statistical modeling, I also proposed ways to recognize such influences and account for them. The first research problem explored how the degree of spatial live-dead similarity of an assemblage (spatial fidelity) is affected by the degree of post-mortem transportation in a tropical marine setting with a high sedimentation rate and high frequency of storms. Shells can be transported both within and out-of-habitat depending on the energy conditions of the surrounding habitats. Largescale mixing is more common in siliciclastic settings with a narrow shelf, high sedimentation rate, and those that are frequented by episodically high-energy events. By studying the live-dead (L-D) fidelity and modeling size-frequency distribution of the molluscan fauna from Chandipur-on-sea on the east coast of India, I attempted to evaluate the contribution of ‘‘out-of-habitat’’ versus ‘‘within-habitat’’ mixing in developing the molluscan death assemblages. The tropical cyclones originating above 15°N cause a high degree of lateral transport explained by the high compositional similarity of species within this latitudinal extent. The results indicate that those death assemblages are not likely to be a product of within-habitat mixing and they probably received considerable input via regional transport, facilitated by frequent tropical cyclones. The second research problem of the thesis explored how the spatial diversity of an assemblage is affected by the scale of the study and the choice of diversity index. Beta diversity or between-habitat diversity can be driven by various factors such as environmental, historical processes, and biotic interactions. The factors determining variability in composition at a small spatial scale are different from the determinant processes at larger scales. I tried to assess how the spatial scale of sampling can influence the nature of beta diversity of molluscs at a regional scale using LA and DA from the west coast of India. I developed a statistical approach to test if the observed variation in beta diversity is explained by the unequal spatial scale (grain size) of sampling. A realistic null model was developed by generating a beta diversity pattern with progressively increasing spatial scale using the observed data of DA and LA over 14 latitude bins. Our observed beta diversity pattern is significantly different from the null model pattern, implying that the unequal grid size of sampling does not explain the spatial variation in beta diversity in this region. Our results also demonstrate that the choice of the beta diversity index and the design of the null model can significantly influence the inference of spatial patterns of diversity. By choosing a combination of the robust models and indices (thereby reducing the effect of methodological artifacts), we could identify the responsible oceanographic variables shaping the regional beta diversity along the west coast of India. This study provides an approach for evaluating the effect of variable sampling scales on comparing regional beta diversity. It emphasizes the importance of understanding the role of sampling and spatial standardization while inferring about processes driving diversity changes. The third research problem of the thesis evaluated how the sampling intensity and evenness of an assemblage can alter ecological inferences regarding biotic interactions such as predation. Predation is an important evolutionary driver and predation estimates play an important role in understanding its role in shaping the molluscan ecosystem through time. The reliability of the inferences is dependent on the assumption that it is not influenced by other processes or methodological artifacts. Using a resampling technique, I evaluated the effect of evenness and sampling intensity of a community on the inferred predation estimates in molluscs. Theoretically simulated model communities representing different levels of evenness, predation intensity, and predatory behavior (selective, non-selective) were resampled without replacement. The variation in the inferred predation intensity and the number of prey species was noted. The results demonstrate that communities with highly selective predation are sensitive to evenness and sampling intensity. For non-selective predation, sampling intensity heavily influences communities with low evenness and low predation intensity. I also proposed a standardization protocol and validated it using predation data from four Plio-Pleistocene molluscan assemblages. Our approach highlights the importance of these methodological choices in influencing the predation estimates of fossil and recent assemblages. The findings reported in my thesis highlight the influence of factors such as taphonomy and sampling on the ecological inferences of molluscan assemblages. It also provides critical insights into how such influences can be recognized in recent and fossil assemblages. This will enable future researchers to standardize the data collected from spatio-temporally separated molluscan assemblages before using them for evaluating important ecological hypotheses.