BIOSCI 109: Lecture 9 - Ecosystems 1: Biotic Networks

Food Chains

Energy links between different organisms in an ecosystem based on feeding habits. It shows just one flow of energy.

Food Webs

Food webs are interconnected and have multiple food chains that are linked together. This linkage shows how species of the same ecosystem can be associated with each other even without directly interacting with each other.

Abiotic Factors on Food Webs

Abiotic factors also play a huge role in the energy transfers between organisms in food webs.

Light pollution is one example of one that isn't talked about often. It has many affects such as:

• Changing the photosynthetic & reproductive patterns of plants, having implications on herbivores.
• Making it easier for predator to spot prey
• Disrupting nutrient cycling, circadian rhythms, and other reproductive cycles

Mutualism

An interaction between two different species where both groups benefit. Mutualism tends to occur with many different organisms to build mutualistic networks.

For example, one bee species may be associated with polinating several different flower/plant species.

Ecosystem Disturbances

Any disturbance to one part of an ecosystem is bound to affect another part because of how complicated and interconnected ecological networks are.

Individuals in an ecosystem can have all sorts of interactions with each other and these interactions may indirectly affect others too. So the same concept applies to disturbances.

Trophic Cascades

Describes the indirect and unexpected effects that occur when a disturbance happens.

You can think an ecosystem as a deck with many poles on it. Some poles may be stronger and holding up the deck more, while others not as much. But, knocking over one pole may cause other poles in the surroundings to lose stability as well.

Keystone Species

Are specific species of an ecosysten that has a disproportionately amount of importance to an ecosystem and its processes compared to its biomass.

The removal of these species typically means a wide array of trophic cascades that'd affect more than one species.

They tend to be:

• A key food fource
• A top predator
• A key herbivore or detritivore
• A seed dispenser/pollinator

Keystones Species Examples

• Wolves are the key stone species for the Yellowstone ecosystem. They help keep elk populations in check so that they don't overgraze and decimate various primary producers of the ecosystem. This supports the ecosystem more and also opened up niches for beavers too.

• Otters are keystone species in kelp forest ecosystems as they help keep sea urchin populations in check, ensuring there's enough kelp for primary production, habitat, and so on.

• The kererū are a keystone species because they're incredibly important and the only ones that can disperse the seeds of three different large-seeded tree species. Areas with no/low kererū sees no recruitments of these trees, having implications on primary productive, habitat available, and so on.

• Scale insects are important for accessing and distributing sugars from trees for other flora and fauna to use, supporting a wide range of other species.

• Seabirds help connect the nutrients from the marine ecosystem to the terrestrial one by spreading their nutrient-rich guano around. Many different plants rely on this guano to grow and thrive. They're seen on a lot of different terrestrial habitats, even far away from the shore. They have major implications on habitats, food availabilities, and other species that rely on the nutrients too.

Invasive Species & Ecosystems

They're kind of the opposite to keystone species, instead of benefiting the ecosystem as a whole, they tend to cause major disturbances, some direct & indirect.

The indirect effects they cause can also be called meltdowns depending on the severity of them.

Meltdowns

A term used for the extreme trophic cascades caused by the introduction of invasive species.

An example of this is on Christmas Island

• The introduction of yellow crazy ants led to a dramatic decrease of crab populations there. This meant that Giant African Snails were able to freely invade the island and many different plants also lost an important disperser for their seeds. The lack of dispersal allowed invasive plants to take over.

Pest Management & Trophic Cascades

When removing just one or a few pests, we also have to take into consideration the effects it could have on other species too, whether native or other pests. Once they've estbalished themselves in an ecosystem, the same concepts of a trophic cascade can occur.

Typically in places with more than one pest, the removal of one pest species just means that the other pest species gets to thrive and succeed in the ecosystem, which doesn't really solve the issue.

So, we'd have to identify the sort of direct and indirect interactions (competition, seed dispersal, etc) that pest may have with others (competitor release).

Pest Management & Trophic Cascades Examples

• Pigs and goats on the islands of Papua New Guinea, while invasive, helped to graze on weeds. The removal of them led to weeds overtaking some ecosystems.

• Hauturu's breeding success was affected by invasive rats, cats, as well as Cook's petrel. The removal of cats, which in theory should've helped increase breeding rates, made breeding rates plumet as now rats could completely dominate that niche. The removal of both pests allowed their breeding rates to succeed and skyrocket.