By Dulcinea Groff
Feces of prehistoric organisms remaining in the sediment records harbor information that can lead to a picturesque reconstruction of an ecosystem from long ago. It is quite remarkable how many examples of fecal proxies exist and provide more information than just an indication of the presence or absence of an animal. In the early 1800’s, an eccentric paleontologist named William Buckland was the first to describe coprolites or fossilized feces. When feces become fossilized the organic components are replaced with minerals and any clue as to what the organism ate is replaced. Therefore, coprolites may not be very useful in understanding the ecology of past environments and organisms. Instead, other things associated with feces become proxies in paleoecological studies.
Fungal spores that decompose and live in the dung of past herbivorous organisms are very resistant to decomposition and become deposited in the stratified sediments of nearby lakes. Dung fungal spores are indicators of the presence of herbivore feces but also have the potential to be an indicator of the type of herbivore (species-specific) and the climatic conditions (moisture and temperature) (Krug et al. 2004). At the ecosystem level, the presence of dung fungal spores indicates the presence of herbivore grazing as the force driving the structure of vegetation, as opposed to climate or fire (Baker et al. 2013).
Dung beetle fossils are another proxy associated with feces that represent very distinct ecosystems. The dung beetle is specific to herbivore dung and the abundance of beetle fossils or subfossils represents the openness of an ecosystem’s forest canopy at a particular point in time. For example, a recent study illustrated that before humans were prevalent in Europe during the Last Interglacial cycle (132,000 – 110,000 yr BP) the vegetative structure of the temperate forests were semi-open. In comparison, during the period after the megafaunal collapse in the early Holocene (10,000 – 5,000 yr BP) the canopy structure was closed (Sandom et al. 2014).
Sediments within proximity to past seabird colonies contain proxies associated with seabird guano for past seabird population- and trophic-dynamics, as well as climate. Seabird guano is known to have high levels of elements like cadmium, zinc, iron, lead, and magnesium (Liu et al. 2013) due to biomagnification, the accumulation of these elements through the food chain. These biological elements (bioelements) found in seabird guano are a proxy used in paleoecology to estimate past seabird population dynamics, such as presence/absence and abundance. Another successful type of proxy used to reconstruct past seabird populations come from biomarkers found in sediments influenced by guano. Fluctuations in biomarkers such as cholesterol and cholestanol, two common sterols found in penguin guano from the past 8,500 years BP reflect variation in penguin population sizes in Antarctica (Huang et al. 2010).. Alkanol is a biomagnified sterol found in avian feces and originates from phytoplankton, which can be used as an indirect way to estimate marine productivity because phytols are used as proxies for vegetation
Assemblages of seabird bioelements found in guano remain in the sediments in high concentrations and fluctuate through time. These fluctuations are in response to changes in climate that indirectly influence seabird population dynamics by changes lower in the trophic food chain (Sun et al. 2013). For example, when less sea ice forms during climatic warm periods, fewer phytoplankton exist, because sea ice is important habitat for phytoplankton and marine algae at the base of the food web. Past climatic events have lead to variation in sea ice and the subsequent marine resources available to seabirds in high latitude environments like Antarctica, the Arctic, and Greenland (Huang et al. 2014). Declines in krill feeding on primary producers associated with sea ice temporally coincide with declines in avian bioelements and biomarkers found in sediments near seabird colonies in Antarctica dating back to 3000 y BP. Examples of seabird guano studies have not only taken place in environments where decomposition is generally slow, such as in the Arctic and Antarctic regions. Lower latitude studies of seabird guano in the South China Sea also used avian bioelements and demonstrated similar results in reconstructing long-term seabird population dynamics dating back to 1350 BP (Liu et al. 2006a). The seabird guano that is deposited in their terrestrial habitat is very influential in shaping the characteristics of the vegetation and soil, and even the way the terrestrial habitat functions.
In addition to the elemental analyses of seabird guano, the source of the seabird diet can also be reconstructed using guano. Stable isotopes are useful in deciphering the trophic level of a seabird colony based on the ratio of carbon and nitrogen found in seabird guano (Liu et al. 2006). For example, the guano of a piscivorous seabird the Arctic tern (Sterna paradise)will have a different C/N signature or greater accumulation of bioelements (Michelutti et al. 2010) when compared to the guano of a molluscivorous seabird such as the common eider (Somateria mollissima).
When fecal proxies are used in combination with other proxies for vegetation or climate a story begins to form. Organic and inorganic proxies associated with feces deliver valuable information about the function of an ecosystem with respect to the organisms present, trophic-dynamics of producers and consumers, and climate.
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