Today's post will cover a very important concept in ecology - trophic interactions.
By 'trophic interactions', I am referring to what may be colloquially called the 'food chain'. However, the latter term is a lot less descriptive and often makes people think of an almost linear dynamic, with one species eating another species eating another species up to the top species in whatever ecosystem that is. The problem is, as with many things in science, the real story is far more complex than what the common person might think. In any given ecosystem, species that eat one species may have multiple predators, and even a top predator may have creatures that prey on its young. Additionally, there are scavengers, which do not fit well into a simple linear model.
Trophic interaction is referring to any interaction that involves one organism getting energy from another. This ranges from Tyrannosaurus rex preying on a Triceratops, to Triceratops grazing on ferns, to scavengers that consume a dead Ankylosaurus and even the bacteria, fungi, etc. that work to decay the body and recycle the elements and biomass back into the ecosystem.
One way to visualize this is what is known as a 'food web'. A food web looks at an ecosystem and tries to visually depict all the trophic interactions between the species present. These are extremely useful, as it can reveal the roles that a species plays in that environment, and it can simply help understand the dynamics of the system in question. I actually created a rough draft of a food web for the Hell Creek ecosystem, in order to better understand the interactions between the species present, which will also allow us to create a more accurate simulation than we might be able to otherwise:
Note: Light blue arrows represent fresh water 'food chain', dark blue represent marine 'food chain', green represents terrestrial 'food chain'. Red arrows represent connections of relative uncertainty. Orange represents energy flow only from juveniles/hatchlings of the species the arrow is coming from.
As can be seen simply from looking at the web, there is an incredibly complex flow of energy in the ecosystem. Certain species/groups of species [not all species are represented by their own label, as all species within that category would have identical arrows and it would take up far more space] provide resources for a wide variety of other species. For example, it can be seen that freshwater pelecypods are very important in the freshwater part of the ecosystem - considering this label includes multiple different species, the cumulative abundance would likely be high enough to sustain the creatures that feed upon them - it may also be that these species avoided competition by specifically targeting a particular pelecypod species, though if so it would be very hard to prove with any great certainty.
For the terrestrial part of the food chain, terrestrial invertebrates serve as a key food source - considering how large of a category that is, it is hardly surprising. However, there are no body fossils of such invertebrates from Hell Creek [though it's possible that I simply have not heard of them] - the only evidence I am aware of is an ichnotaxa ('trace fossil') of a leaf beetle, and marks on fossil leaves that appear to be made by insects. This can be explained by the fact that invertebrates, especially those that are soft-bodied, are far less likely to fossilize than an animal with solid bones. Despite this, the abundance of species with dentition implying a diet primarily or at least partially composed of insects and other invertebrates is strong support for a diverse and abundant population of such creatures.
The last thing I will cover will be some of the terms that are used to describe the type of role a species plays in the trophic interactions or energy flow in the ecosystem. They are the following:
Primary producers: A primary producer is an autotroph that gets its energy from the sun (or, in some cases, chemical energy from sources such as hydrothermal vents). This may also include 'seston', which is small particles of organic matter that are the remains of dead organisms - this may also be called 'marine snow'. Examples of primary producers include plants, phytoplankton, and algae.
Primary consumers: These are heterotrophs that get their energy by eating primary producers. Examples of primary consumers include Triceratops and the freshwater pelecypods.
Secondary consumers: As one might infer, secondary consumers are the animals that eat the primary consumers. For example, Tyrannosaurus rex would be a secondary consumer, as it eats Triceratops, and the extremely common bowfin-like fish Kindleia fragosa eats freshwater pelecypods and would also fit in this category.
Tertiary consumers: These creatures eat secondary consumers. Note that not all ecosystems include animals in this category, and that the categories of consumers are not mutually exclusive. For example, Tyrannosaurus rex is both a secondary and tertiary consumer, as it eats Chirostenotes and Triceratops.
Decomposers: This is an extremely important role in any ecosystem - it includes the scavengers that prey on carcasses, the bacteria that work to decay leaf litter and dead creatures (in addition to scavengers), and more. In doing so, these creatures take the elements in the bodies of those dead creatures and recycle it back into the food web. Without decomposers, nutrients would remain locked up in bodies of dead creatures, and the ecosystem would slowly run out of nutrients and would die off.
In terms of relative abundance in a terrestrial ecosystem, this value decreases from top to bottom in the above list (with the exception of decomposers). This is because at higher levels in the chain, there is less biomass available to use - it is being taken up by the organisms lower in that chain. Additionally, there can only be so many consumers - if there are too many, they could eat themselves to extinction.
I hope this post has been informative, and has given you a bit more knowledge about how energy and nutrients move through an ecosystem. It's not just a dino-eat-dino world out there!
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