Paleoecology

Ecology

First up, some definitions:

• Ecology: the study of the relations and interactions of organisms to each other and to their natural environment. (NOTE: ecology does not mean the same thing as "environmental activism"!!)
• Paleoecology: not surprisingly, the study of the relations and interactions of fossil organisms to each other and to their natural paleoenivornments.
• Autecology: the study of an individual species with relation to its environment. In general, we deal with issues of paleo-autecology in other lectures (the diversity, behavior, biomechanics, etc. lectures). One very important aspect of autecology is the niche: the particular way of life of that species.
• Synecology, also called community ecology: the study of species of organisms interacting in an environment. We are going to focus on paleo-synecology in this lecture.
• Trophic: "feeding". Most important aspects of this in ecology would be:
  • Trophic Relationships: Who eats whom. These mark out relationships in a food chain of organisms that eat the organisms below them. However, since in nearly every case any species eats more than one type of food, trophic relationships are better displayed as a food web, comprised of many interlocking food chains.
  • Trophic Level: The position an organism occupies of a food chain. Producers do not consume other animals: in terrestrial ecosystems, these are plants (which "eat" sunlight). Decomposers break down the bodies of any kind of organisms. Consumers are those that eat other organisms: 1st order consumers are herbivores (plant-eaters); 2nd order consumers eat 1st order consumers; 3rd order consumers eat 2nd and 1st order consumers; and so on, up to the apex predator (consumers that eat lower order consumers but are not typically eaten by any species but themselves).

How does energy go from the non-living environment into the living world? One major way is photosynthesis. Here is the process greatly simplified:

And here is the photosynthesis equation, simplified:
6CO₂ + 6H₂O + Sunlight yields C₆H₁₂O₆ + 6O₂

Or, in other words, photosynthesizers (plants and algae) take in water and carbon dioxide and using sunlight convert these into sugars and waste oxygen gas. Photosynthesizers are the major form of autotroph: organisms that don't eat other organisms.

In contrast, most other life forms (animals, fungi, most "protists", etc.) are heterotrophs: we get nutrients from digesting other life forms, primarily via the aerobic respiration equation:

C₆H₁₂O₆ + 6O₂ yields 6CO₂ + 6H₂O + Energy

(That is, food (glucose) plus oxygen yields waste carbon dioxide and waste water, plus energy).

In this reaction, we take in sugars and oxygen and release waste water and carbon dioxide.

So plants, algae, and other photosynthesizes are basically made up of air (and a little bit of other matter drawn from soil and/or water), while animals and other heterotrophs are made of food (ultimately plants) and air. This most important aspect of ecology is thus trophic relationships: who eats whom. These mark out relationships in a food chain of organisms that eat the organisms below them. However, since in nearly every case any species eats more than one type of food, trophic relationships are better displayed as a food web, comprised of many interlocking food chains.

The trophic level is the position an organism occupies of a food chain. Producers do not consume other animals: in terrestrial ecosystems, these are plants (which "eat" sunlight). Decomposers break down the bodies of any kind of organisms. Consumers are those that eat other organisms: 1st order consumers are herbivores (plant-eaters); 2nd order consumers eat 1st order consumers; 3rd order consumers eat 2nd and 1st order consumers; and so on, up to the apex predator (consumers that eat lower order consumers but are not typically eaten by any species but themselves).

As you move up a food chain, there is less and less energy available. Plants receive much (but by no means all) of the energy of incoming sunlight; some of that energy becomes stored as their tissues, but the rest of it is used for them to run their bodies. Herbivores consume plants, and gain some (but not all) of that stored energy, some of which they used to build their own tissues but the majority being used for their own operation (running, breathing, pumping blood, etc., etc., etc.). Predators eat the herbivores, gaining some (but not all) the stored energy in the tissues of the herbivores, using some to build their own bodies, but the rest for their operations. Thus, the amount of energy available at each trophic level becomes less and less: this is described as the energy pyramid.

Investigation of individuals, populations, and communities of ancient organisms and their interactions with and dynamic responses to changing environments. On the one hand, the data is hampered by the various taphonomic filters and our inability to directly observe interactions. On the other, the temporal scope of paleoecology greatly exceeds "neo-ecology" and consequently may be better able to capture the "big picture".

Niche Theory

As defined by British ecologist C.S. Elton, a species' niche is "its place in the biotic environment, its relations to food and enemies". This zoo-centric concept was expanded by G. Evelyn Hutchinson to represent an n-dimensional hypervolume where the dimensions are environmental conditions and resources that define the requirements of the species to practice its way of life (i.e., to persist and propagate).

We can now recognize several aspects of niche theory:

• Fundamental Niche: Subset of environmental space in n dimensions consisting of the suite of combinations of variables that permit survival & reproduction of individuals
• Realized Environmental Space: The expression of these environmental variables that exist in nature at any particular time
• Potential Niche: The intersection of the fundamental niche and realized environmental space: consequently, the possible niche space that exists that the organism could occupy
• Realized Niche: The portion of the potential niche that the organism actually occupies at any given time
  • For instance, an organism might find the conditions in the South American Atacama Desert and the African Namib Desert equally within its potential niche. However, if the species arose in Africa it is unlikely to have travelled all the way to South America, so the Atacama is outside its realized niche

Niches are subject to change. Some niche variables (such as bedrock) don't move in ecological time scales (but of course do move on geologic time scales!) Others (e.g., distribution of sunlight over the year) move following various cycles. Still others (e.g., climate; presence/absence of other taxa) may move following far more complex patterns. As environmental variables shift in real space, organisms may move with them (the actual individuals for motile species; the propagules for plants and sessile animals), so long as the new region is within the potential niche and dispersion is possible from one space to another. (If these conditions are not met, it is likely that the population goes extinct!)

Communities: Are They Real>

A long, on-going debate in ecology is whether the communities we see in nature are real entities as such:

• Hypothesis 1: YES! Under this model, they represent broad multi-specific assemblages that have evolved numerous co-evolutionary adaptations so that they operate well together and less well when separated
• Hypothesis 2: NO! In this model, they are epiphenomena, simply the result of the partial overlap of habitat preferences of different taxa. (After all, you are going to wind up with organisms in the same place and time no matter what.)

Paleontology's contribution is to see if there are patterns of assemblages in the past that share the same taxa (or their immediate ancestors) as the present, or if the associations change over time; to examine if there were communities that were widespread in time and space in the past that are no longer extant; and to examine the long-term trends in ecology (macroecology).

One major test is for autochthony. Are the fossils in life position? Articulated? Are there low levels of breakage and abrasion? Are they poorly sorted in terms of size? Are there low levels of shared orientations (and thus little sign of current, and thus possible transport)? If so, than the animals were almost certainly living next to each other prior to burial.

(It is worth noting that some allochthony can be good!! For instance, we would like to know the nekton and plankton in a region, since they do interact with the benthos. And since most terrestrial sedimentary deposits are aquatic (lacustrine or fluvial), we would like to have some transport of the land animals and plants into these water bodies to know they were in the region.)

The best test is to find multiple stratigraphic and/or geographic occurrences of the same assemblage: more commonality indicates a better chance it is not just a random taphonomic assemblage.

Paleoecology shows that some communities do persist for long periods of time, both extant styles or extinct ones (like the mammoth steppe of the Pleistocene Arctic). On the flip side, there are non-analogue biomes: combinations of taxa (some extant) that are found together in the past which are never found together today.

Assessing Trophic Relationships

One of the most important aspects of ecology (and paleoecology) is determining who ate whom: that is, reconstructing the food web. Sometimes there is direct trace fossil evidence: for example, this humerus of the Late Cretaceous Asian duckbilled dinosaur Saurolophus: the arrows point to bite marks generated by a tyrannosaurid (almost certainly Tarbosaurus, which is known from the same fauna).

However, sometimes we do not have such direct evidence. In these cases, we can make relatively reasonable assessments. For example, it is unlikely that small carnivores habitually killed the largest herbivores in their environment (while the latter were adults, at least: babies wouldbe fair game), and it is unlikely that there were species that were not eaten by SOMETHING.

In the case of the herbivores, however, it becomes somewhat more difficult. For one thing, it is often the case that the conditions that are best to preserve plant fossils (especially of leaves and fruits) are not good for preserving bone, and vice versa. So we often lack good knowledge of the flora that co-existed with any fauna. That said, we have the digested plant remains in coprolites as positive evidence that herbivores ate particular plant species (although referring a coprolite to a particular herbivore species can be difficult.)

Paleoclimate

Fossils represent one of our major clues to restoring paleoclimate. One primary technique is the modern analog approach. Some aspect of the community (species composition, for instance) is compared from a modern environment (whose parameters are known) to a fossil one.

One classic example is leaf margin analysis. It is observed in modern floras that proportion of entire (that is, smooth) leaves in a flora has a rather strikingly accurate correlation with mean annual temperature. So looking at fossil leaf populations, the ratio of entire vs. jagged leaves in the fossil horizon is used to predict what the ancient mean annual temperature was at the time of deposition.

Similarly, the distribution of some species (for instance, some foraminiferans) are proxies of ancient water temperature. Looking at the the changing distribution of fossils of these species through time and space allows paleoclimatologists to track changes in ancient temperatures. One particular paleothermometer is Neogloboquadrina pachyderma, whose direction of coiling changes at 8°C. Tracking changes in coiling direction down stratigraphic section allows the recognition of when the local temperature passed this treshold.

Isotopic Paleontology

The hard parts of organisms retain stable isotopic ratios of the time of formation so long as diagenesis does not otherwise alter these. Some of the more commonly utilized isotopic elemental systems in paleontology are C, O, N, P.

These have been used to reconstruct:

• Climate, normally temperature and/or evaporation (often ¹⁸O)
• Other habitat issues (for instance, upland vs. lowland ¹⁸O isotope signatures are different)
• Diet:
  • ¹³C fractionation differs for different types of plant biologies and also for marine vs. continental
  • ¹⁸O fractionation varies for terrestrial vs. aquatic
  • ¹⁵N fractionation varies by trophic level (increases for higher-order consumers)
• Physiology (body temperature for endotherms ("warm-blooded" animals) will be environmental independent; the ancestral ectothermic ("cold-blooded") condition matches local environmental temperature

To Syllabus.

Last modified: 7 December 2020