Species diversity, trophic structure, disturbance & succession Flashcards
species diversity
Species Richness & Relative Abundance
benefits of biodiversity
- stability (better able to withstand disturbances & invasive species)
- higher productivity
- more ‘ecological space’ (i.e. niches) occupied
how do we study food webs
-
identify what animals eat
-> direct observation
-> regurgitates
-> pellets
-> gut contents
-> stable isotopes: N15 has ↑ retention in animal tissues than N14 -> N15 accumulates at ↑ trophic levels -
mathematical modelling
-> enables us to examine no. of ecological scenarios (e.g. what happens if diff sp are removed / added to community)
-> often requires advanced maths & high levels of computing power to model whole communities
why are food chains so short (despite theie complexity)?
2 main hypotheses:
- energetic hypothesis
-> Only ~ 10% of energy stored in organic matter of each trophic level is converted into organic matter at next trophic level
-
dynamic stability hypothesis
-> long food chains are less stable than short
-> pop fluctuations at lower trophic levels are magnified at higher trophic levels
energetic hypothesis
prediction: food chains should be longer in habitats of higher primary productivity
- amount of energy at bottom of food chain is greater
- more energy available to be transferred to higher trophic levels
dynamic stability hypothesis
prediction: food chains should be shorter in unpredictable env
- env fluctuations ↓ pop sizes at low trophic levels
- as a result high trophic levels become extinct
Other factors limiting the length of food chains
SIZE MATTERS
- animals at higher trophic levels usually larger than animals at lower trophic levels
- size of animal and its feeding mechanism limit size of food it consumes
- generally large carnivores cannot survive by feeding on small animals
- high energetic cost of finding / catching / consuming ‘small’ organisms
- upper size limit
EXCEPTIONS: some animals can feed on v large no. of smaller animals -> eg. Baleen whales, Basking sharks
- specialist suspension feeders
- huge creatures can survive by eating only tiny krill & other small organisms
- specialised feeding mechanisms enables baleen whales to feed on huge quantities of krill
stability
tendency of a community to maintain relatively constant composition and no. of sp in the face of disturbance
responses to disturbance
-
fragility
-> when it remains unchanged by minor disturbance but is dramatically altered by major disturbances -
robustness
-> when it remains largely unchanged even during major disturbances -
resilience
-> rapidly return to former structure following disturbance -
resistance
-> undergo relatively little change in face of disturbance
+ve impacts of disturbance
- create opportunities for some sp -> eg. less competitive sp
- enhances habitat patchines
-> helps maintain diversity
-> abiotic heterogeneity
state and explain the ‘intermediate disturbance hypothesis’
‘Moderate levels of disturbance create conditions that foster high diversity’
↑ disturbance => ↓ diversity
- stress exceeding tolerance of many sp
- high frequency disturbance prevents slow colonisers/growers from establishing
↓ disturbance => ↓ diversity
- competitively dom sp exclude other sp
intermed. disturbance => ↑ diversity
- open up habitats for occupation by less competitive sp
- but… conditions not so severe as to exceed env tolerances / prevent recovery of sensitive community members
1º succession
(on expposed rock)
can occur on any type of bare land, inc sand dunes at edge of ocean & on exposed rock
- Seeds & spores carried by wind land on exposed rock & begin to grow
- 1st sp to colonise new land (mosses & lichens) = pioneer species
- pioneer sp can germinate easily & withstand harsh conditions eg. low nutrient & water availability
- pioneer sp die & decompose -> dead organic matter forms soil
- seeds of small plants & grasses land on this soil & grow
- plants at this early stage of succession are adapted to survive in shallow, nutrient-poor soils
- roots of these small plants form network that helps hold soil in place & prevent it from being washed away
- small plants die & decompose => soil becomes deeper & more nutrient-rich
- larger plants, shrubs & trees can now begin to grow -> require more water, which can be stored in deeper soils
- final sp to colonise new land become dom sp of now complex ecosystem
- final community formed = climax community
colonisation related to 3 key processes
-
tolerance
-> early arrivals may neither facilitate nor inhibit arrival of later sp -
facilitation
-> early arrivals may facilitate arrival of others (e.g. via fertilising soil) -
inhibition
-> early sp may inhibit arrival of others so successful colonisation occurs in spite of early arrivals
2° succession
disturbance => clears most vegetation BUT soils remain intact (e.g. Yellowstone fires)
- area will often return to its former state
- areas cleared for agriculture -> if left can return to prev climax community
-> eg. rainforests of c. America : 2º forest regrown after logging
Retreat of glaciers in Glacier Bay (SE Alaska)
info
retreating for the last 300 years
moraines (material left by moving glacier) can be dated via:
- age of trees, direct observation
- studying communities at diff distances from mouth of bay – oldest closest to mouth & newest furthest away –
-> can examine succession going back in time - space-for-time substitution/chronosequence
