Trophic and community structure
Are they real? We are tempted to think of communities of organisms - either taxa or guilds of organisms that frequently co-occur and interact in the same environment - as parts of a coherent, machine-like mechanism (E.G.: African ratel and honey-guide). Maybe, or maybe they are present together coincidentally (Norway rat and black rat snake). With living ecosystems, we can at least chart the regular flow of energy through an environment as it passes through trophic levels from producers to primary consumers, secondary consumers, etc. E.G.: The trophic structure of the Galápagos Islands:
If we are uncertain about living communities, fossil communities are real trouble for three reasons:
>And yet, we can identify some communities with reasonable confidence and even begin to reconstruct their trophic structure when there are multiple stratigraphic and/or geographic recurrence of an association. Some analogous communities have formed multiple times in Earth history, with similar
community structure but different taxonomic composition: reefs, for example.
- Fossilization biases: All of the biases that filter the preservation of information about a particular creature are multiplied when we consider their effect on communities. Indeed, we can only start to be comfortable with the possibility that a fossil fauna represents a community of living organisms if:
- Fossils in normal life position
- Fossils in articulation
- Low levels of breakage and abrasion
- Poorly sorted in terms of size of fossils
- Low levels of orientation due to currents
- Time averaging: Communities change radically over ecological time, typically measured in tens, hundreds, or thousands of years. E.G: the classic floral succession that progresses from disturbed ground to climax forest. The rock units in which fossils are preserve often concentrate organismal remains across larger time intervals, i.e. over evolutionary time typically measured in millions of years. Thus, the fossil faunas represent homogenized time averages of successions of ancient communities.
- Uncertain trophic structure: Of course, we usually don't know who actually ate whom unless we are lucky enough to identify an organism's meals in:
or to find predation marks on a prey item's body.
Biological ghosts: Indeed, some extant communities only make sense if we consider their extinct members:
- Australian predators: Many of the large herbivorous mammals of Australia show evidence of adaptations for escaping large predators including:
- Rapid escape
- Nocturnal activity
- Cryptic coloration
But in the modern fauna, there is no large native predator from which they are escaping. This is in contrast to nearby New Zealand, which has been home to many large and rather helpless land animals (many of which are/were flightless birds).
The Neogene fossil record, however, contains several predators that might well have preyed on them, including:
The "Tasmanian wolf" Thylacinus cynocephalus, extinguished in Australia roughly 3000 years ago but survived in Tasmania into the 1930s.
The "marsupial lion" Thylacoleo carnifex. This creature and its relatives had:
- Stabbing incisors & large carnassials
- Long legs and fingers with opposable thumbs
- Weight = 110-140 kg
The giant rat kangaroo Propleopus (discussed in previous lecture.)
Any of several semi-terrestrial crocodiles, such as Quinkana fortirostrum
The giant monitor lizard Megalania prisca
- North American enigma: Why is North America home to one of the fastest-running of all ungulates, the pronghorn? (Still image) In speed, this creature greatly outclasses any living native predator in speed. The likely answer:
Miracinonyx, a Pleistocene North American cheetah.
- Biological ghosts can be prey organisms, also. Consider the komodo dragon, Varanus komodoensis:
This is the largest living lizard, length = 3 m, weight = 200 kg. Capable of killing prey 10 times its own weight using both brute strength, a highly septic bite, and the ability to track prey as is dies of infection. It is most successful on small islands of SW Pacific where it feeds on pigs, goats, deer, horses & water buffalo. In this case, it seems greatly to outclass its potential prey, except when we consider that its range was formerly inhabited by island dwarf elephants.