Trophic cascades, over-exploitation, population regulation Flashcards
What is a trophic cascade?
Powerful indirect interactions that can control entire ecosystems. They occur when predators limit the density and/or behaviour of their prey and thereby enhance survival of the next trophic level.
What is the top-down control theory?
The world is green because predators control herbivore abundance which allows accumulation of plant biomass.
What is the bottom-up control theory?
Plants may not be edible, chemical defences control herbivore populations, predators compete for scarce herbivores.
What is an example of a marine trophic cascade?
(Estes, 1995). When sea otters were hunted to near extinction in the 20th century, sea urchin populations increased to high densities, removing kelp from kelp beds. When there are otters, the urchin populations are kept in check and healthy kelp beds are maintained.
What is an example of a terrestrial trophic cascade?
The loss of aspen from US mountain forests has been correlatively linked to the extirpation of grey wolves from Yellowstone National Par, and the subsequent increase in browsing pressure on aspen suckers by elk.
How do predators influence plants?
1) by altering the density of herbivores
2) by altering the foraging behaviour of herbivores.
The case for behaviourally mediated trophic cascades
Suggestions were made when discussing the reintroduction of wolves into Yellowstone that the wolves would alter elk foraging behaviour, and create a ‘landscape of fear’ - elk would avoid aspen in risky areas i.e. open vulnerable sites. Wolf reintroduction was followed by a decline in elk, although some of this may b attributed to a drought and increased hunting. the behaviour change hypothesis runs contrary to knowledge that active predators rarely produce constant predation risk cues and so should have weakest fear mediated effects.
What is happening to fish stocks?
They are decreasing in abundance, maturity, size and weight. Most fish stocks are fully or over-exploited - in 2006 87% of fish stocks were exploited, over-exploited or collapsed. Farmed fish is having an increasing contribution to fish stocks. For northeast Atlantic cod, average size has decreased by over 10cm between 1930 and the 2000s.
How does age affect reproduction in fish?
Older fish are the best spawners. Red Snapper can live up to 54 years but today few are older than 10 - large consequence for number of eggs produced.
What is the theory of shifting baselines?
Each generation takes a new degraded baseline as the normal. Smaller populations seem normal, aims to restore systems don’t return to the original baseline. EXAMPLE: The sailfin grouper - Darwin described it as common but it is now on the IUCN Red list but the Galapagos is still described as a pristine environment.
What happened on Easter Island?
A remote island, located more than 4,000km from the coast of South America. Was colonised in 400-600CE by the Polynesians. Had a forest of large palms and other trees and craters held drinking water. Humans spread across the island, occupying nearly all the available space., The forest was destroyed for plants and agricultural crops - resources were over-exploited. (or it could have been slave traffic or diseases from rats).
What is the carrying capacity?
The maximum population an environment can support. When carrying capacity is exceeded the environment is degraded and the final carrying capacity may end up being lower than the original. Species loss can lead to trophic cascades which decrease the carrying capacity of the environment.
What are some of the effects of climate change?
Climate change is now considered a major driving force of species declines and extinctions. It has been shown to affect the phenology of many organisms, but interestingly these shifts are often unequal across trophic levels, causing a mismatch between the phenology of organisms and their food. e.g. insectivorous birds that have advanced their breeding phenology less than the phenology of their food supply. e.g. the timing of blooms of zooplankton and phytoplankton have changed at different rates. e.g. caterpillar peak abundance is earlier in the season, pied flycatcher populations declined more strongly in areas in which caterpillar phenology had changed the most - long-distance migratory birds are not tracking the peak food abundances.
What is the maximum sustainable yield?
The number (or weight) of a species that can be removed from the stock of animals without impacting the long-term stability of the population.
Population Density
In a ‘closed’ population, population density is controlled by the birth and death rates as there is no immigration or emigration.
