Comparison of early land-based ecosystem
Aim
The aim of this exercise is to see how drawing food webs can help us understand
fossil ecosystems
Introduction
In the talk we discussed the early evolution of land-based ecosystems, which rapidly
diversified between 300-400 million years. In this activity you will study two fossil
sites, one in the Orkneys of Scotland about 380 million year old, and a second at
Nyrany in the Czech Republic about 300 million years old. Both sites contain fossil
animals and plants that lived in and around freshwater lakes.
In the table, you are given information about what organisms were present at each site
(P stands for Present, A stands for Absent). You are also told what each organism ate.
Draw food webs for both ecosystems with arrows showing the flow of energy from
one organism to another. In other words, if a fish eats algae draw an arrow from the
algae pointing to the fish.
Clue: Start by writing the names of the primary producers at the bottom of the page,
above them list the primary consumers, and place the predators at the top. You could
also separate those animals that lived in the lake from those that lived on the shore.
Unfamiliar words: A detritivore is an animal that lives off dead and decaying organic
matter like a millipede, a polychaete is a kind of worm, and crustaceans include
animals such as crabs and lobsters.
Organism
Orkney
Nyrany What did they eat?
Land plants
Algae
Millipedes
Tetrapods
P
P
A
A
P
P
P
P
Fish
P
P
Crustaceans
Spiders
Aquatic snails
Polychaetes
Springtails
Dragonflies
P
A
P
P
P
A
P
P
P
P
P
P
Cockroaches
A
P
Primary producers
Primary producers
Terrestrial detritivores
Predators (fish, crustaceans, snails,
polychaete worms, millipedes, spiders,
cockroaches, dragonflies)
Predators (crustaceans, fish, polychaetes,
snails, dragonflies), grazer (algae) or
aquatic detritivore
Grazers (algae)
Predators (millipedes, springtails)
Predators (polychaetes), Grazers (algae)
Aquatic infaunal detritivores
Terrestrial detritivores
Terrestrial detritivores, Predators
(millipedes, springtails)
Terrestrial detritivores (plants)
Questions
1. Which of the two ecosystems is the most complex? What does this tell you
about the evolution of land-based ecosystems, 300-400 million years ago?
2. Are there any aspects that might expect to see in a modern food chain that are
missing in one or both of your food webs?
Answers
1. How complex and inter-connected are the lake and land ecosystems at each
site? What does this tell you about the evolution of land-based ecosystems,
300-400 million years ago?
The later ecosystem is much more complex and diverse with many more
connections between the land and the lake. In fact there in no energy flow at
all from the lake onto land in the earlier ecosystem. Amphibians evolved
shortly after the formation of the first site and subsequently played a major
role in linking together the food webs of the two environments. This analysis
shows us that land-based ecosystems went through a period of rapid evolution
from 400-300 million years ago.
2. Are there any aspects that you might expect to see in a modern food chain that
are missing in one or both of your food webs?
Even in the later food web based on the Nyrany fossil site there are no
herbivores. Primary consumers mostly fed off dead and decay plant matter
rather than living plants (i.e. they were detritivores) This is actually a slight
simplification of reality as some springtails, millipedes, cockroaches, and
dragonflies may have supplemented their diet from living plants. Nevertheless
there is remarkably little evidence of widespread herbivory (plant-eating) until
slightly later in the Earth’s History.