(kim) 18. Trophic cascades 2: Small mammal dynamics in Beech Forests Flashcards
Analyse this chart
Year 0 (=yr 4)
- Normal summer disperal of stoats
- Normal autumn peak of mice
Year 1 - Seedfall
Flowering > Delayed implantation prevents response
(Productivity of stoats fixed but mice do respond, they just go on breeding for longer/winter breeding which impacts the following spring)
Year 2 Post - Seedfall
- Mouse population in Spring now very high
- By the next breeding season of stoats there is an abundance of food which means the mothers are very healthy and she is able to feed all the young very well meaning they all survive and young stoats come out of the nest in the Autumn just as the mice are starting to dissapear.
Year 3 - Crash
-Mice are already going down before young stoats come out so the stoat population plummits as there is not ebough resources to sustain the population and with the added pressure of trapping the stoat population falls even quicker (solid black line vs dashed black line)
Year 4 - Recovery (=Yr 0)
Mice breed normally, stoats out of sync and it takes Year 4 for the breeding patterns to be in syn again.
What is the reason for population increases in rodents?
-Significant correlation between what and what?
-Significant correlation between seed production and rodent numbers
Increase driven by extended breeding and improved juvenile recruitment, not larger litters
More of the younger survive due to higher availability of resources
Is seedfall the only explanation for rodent population increase?
(Is correlation causeation?)
No.
The male flowers of the beech trees fall down into the litter when they are spent (pollen exhausten) and create a food source for litter dwelling invertebrates/insects.
Mouse gut analyses shows many caterpillars - so their response to “seedfall” could be influenced by insects
What is the response of mice to seedfall?
Mice keep breeding through winter after seedfall, adding many new young in August (A), when normally none
What is a stoats preffered diet in NZ?
Mice
Define
‘Functional Response/FR’
Predicts that more of a given prey eaten per predator as density of prey increases
“You get more of a given food item the more there is of it available to be eaten”
“More mice are eaten per stoat”
When mice are abundant do stoats decrease their predation on birds?
NO.
Stoat guts contain about the same % birds at all normal densities of mice
Bird predation may decrease when mice population is extremely high
But the event of the above is rare
Define
‘Numerical Response’
The change in predator density as a function of change in prey density.
Explain the breeding process and reproductive system of stoats known as ‘delayed implantation’
Eggs are fertalised and it reamins in the uterus of the adult female after mating only floating free in the uterus and maintained by low levels of ovarian hormones for 9 months.
They mate in November usually, and then the zygote develops for a couple of weeks and after that they just float but remain actively maintained by low level hormones.
They eventually emplant in the following spring, in the following spring when th inrease in the day length (which the stoats percieve with their eyes) releases another hormone from the brain it sends a signal to the ovaries to increase high levels of progesterone which makes the embryo implant.
3-4 weeks of gestation follows and then the young are born.
Which means the young born this year were fertalised from ova/egg shed last year.
What is the ‘silver spoon’ prediction?
Stoats for example, that are born in the seedyear, have a higher mortality rate than stotes born outside of seed year.
This is because there is such an abundance of Stoats once they leave the nest that their is not enough resources to sustain them all and so more of them die.
‘Mean lifespan lower, because mortality rate after independence higher
What is the Nematode P____e and what does it affect?
The Nematode Parasite damages brains (specifically stotes)
Lives in nasal sinuses immediately at the back of the sinuses immeidately infront of the brain.
If they get blocked often and there are large numbers of them, they wiggle around and desolve the bone to expand the area where they live, eventually starts making holes in the chamber, intensely irritating.
Stote cannot get rid of them.
Stotes inherit the parasite from mice (not in NZ though)
Extra mice reduce juvenile majority of stoats, why?
Many stoats are born = large cohort, distinguishable for years
After 3-6 months of age, survival is poor as competition for food gets extra tough when mice decline
Many juvenile stoats die at or before birth during bad beech mast years, why is this not necessarily a bad thing for the species?
-Smaller cohorts have better survival rates after independance as there is not as much competion for prey amonst the stoats as their numbers are down.
In extra bad crash years, many adult females fail to breed at all
