EEMB 159 Midterm 2

density dependent control: disease (and examples)

disease is an important regulator of population size

1960s: rinderpest in even toed ungulates (widespread desiccation of cattle and herbivores)

1994: canine distemper in lions (crashed the population)

community structure in the African savanna

African savanna has a high abundance and diversity of large herbivorous mammals

• high abundance and shared space / resources suggest intra- and interspecific competition should be high BUT its not

Lamprey 1963: how does the large variety of herbivores in the African savanna co-exist on a shared resource base? (giraffe, impala, buffalo)

Tanzania: identical habitat preferences but little competition for food

1. vertical segregation (giraffe primarily vegetation tops, buffalo from the mid range, impala from the ground)

2. lateral microhabitat separation (impala used vegetation edges (ecotones) between grasslands and woodlands)

3. seasonal shift / increase resources (seasonal movement of all three species reduced foraging on limited resources)

Jarmen 1974: resource partitioning between small and large herbivores

1. small herbivores: seek out isolated high quality food patches, widely spaced and eaten whole, increases spatial separation from other small animals, tend to be solitary or live in pairs

2. large herbivores: utilize abundant poorer quality food patches, do not eat the entire plant before moving on, decreases spatial separation and allows herd formation

how is competitive exclusion prevented in the African savanna? (the four main reasons based on previous experiments)

competitive exclusion is prevented via vertical segregation of foraging, lateral microhabitat separation, seasonal shifts, and food preference

do predators play a significant role in regulating the population size of their prey species in tropical savannas? (two possibilities)

1. may be the case if predator density is high in relation to prey density (ex. managed game parks, Kruger national park)

2. may NOT be the case in large open habitats where predator density is low relative to prey density (ex. Serengeti national park)

ecosystem dynamics in tropical savanas

savanna ecosystems are dynamic, and small perturbations may bring about marked changes in populations of dominant species (changes in one species can cascade throughout)

• perturbations may be abiotic or biotic or a combination of both

savanna ecosystem dynamics: cascading effects of removing rinderpest disease (3 different cascades)

1. wildebeest increase (high reproductive rates) → buffalo decrease (slower, outcompeted)

2. wildebeest increase → decreases plains (dominant stock grass) → herbs (i.e. forbes) increase (no longer having to compete) → gazelle / impala increase (more herbs) → cheetah increase (more prey)

3. wildebeest increase → decrease stock grass in woodlands → fires decrease (no fuel) → small trees increase (not getting burned) → giraffe increase (more food) → large trees decrease (a bottleneck)

why did woodlands in the northern Serengeti recover in the mid 1980’s to early 1990’s but declined in the Masai Mara to the north?

the density of elephants

• Serengeti population declined by 81% from 1970 to 1986 bc of poaching and migration to safety in Masai Mara

• elephants have large impacts on woody vegetation via consumption and trampling

savanna ecosystem dynamics: cascading effects of elephants (2 different cascades in Masai Mara)

1. elephants increased → sapling decrease → woodlands decrease

2. elephants increased → woody thickets decreased (passageways) → herbivores decrease (less protection against predation) → grass standing stock increased → fire increased (more fuel) → trees/shrubs decreased → thickets decreased

grassland stable state is maintained by herbivores

generalization of ecosystem dynamics in African savannas (3 possibilities)

• herbivores degrade woodland and fire maintains grasslands

• fire alone is both the cause of woodland loss and agent of maintenance

• fire and herbivores are equal in degradation and maintenance

definition of a tropical rainforest

a non-seasonal forest dominated by broad leaf evergreen trees, where rainfall is abundant and relatively constant (hot/wet)

general characteristics of tropical rainforests

• >2000 mm/yr of rainfall, average humidity 80%, no or very short dry season

• rain usually falls in the middle of the day (the most sun exposure → lifts up the most air)

• mean annual temperature range 25°C to 35°C (small temperature variation)

• the most variation in temperature from day to night (up to 10°C) compared to seasonal variation (not very much)

• high biodiversity: world’s most species rich terrestrial community

• dominated by woody plants (grasses virtually absent and trees from a continuous canopy) (variation based on climate, precipitation, soils, altitude)

• physiognomy: structure and appearance are similar but species composition differs between S. America, Africa, SE Asia (no one species dominates)

biogeography of tropical rainforests

Neotropical rainforests (50% of world’s rainforest), African rainforest, SE Asia (& pacific islands) (30% of world’s tropical forests but fragmented), small patches in various places (via human destruction)

what is the vegetation structure of tropical rainforests? what are the three layers of trees you would find?

tropical rainforests may be stratified with two or three tree layers, a shrub and a herb layer

1. emergent layer (A-story) (tallest trees, discontinuous layer, sympodial crowns to gather as much light as possible)

2. canopy layer (B-story) (the roof of the rainforest, continuous layer, rounded crowns to deal with competition)

3. understory layer (C-story) (dense where there are breaks in the upper canopy, patches, discontinuous layer, monopodial crowns (maximize surface area for solar irradiation)