Food webs and patterns in species richness

Question 1

Pyramid of numbers

Answer 1

Pyramid of numbers = less consumers than producers, energy is lost between trophic levels

Question 2

Food webs

Answer 2

More complex with numerous interactions
More realistic
Allow species to feed across multiple food chains

Question 3

Indirect effects of food webs

Answer 3

Removal of one species may cascade another trophic level
Predator removal = increase in prey density
Competitor removal = increase in competitors
Sometimes there are unexpected indirect effects. Removal of one species may increase density of one competitor but decrease the density of another. Removal of predator may decrease the abundance of prey species through competitive release – especially evident during management (biological pest control or eradication of invasive species)

Question 4

Trophic cascades

Answer 4

When a predator reduces the abundance of its prey it cascades down to the trophic level below
Prey’s own resources increase in abundance
In a 3-level chain top predators increases plant density, in a 4-level chain a top predator may reduce the abundance of an intermediate predator, increasing herbivores density and thus decreasing plant density
A loss of large predators and herbivores high in the food chain has cascading events in marine, terrestrial and freshwater ecosystems throughout the world

Question 5

4 levels of trophic cascades

Answer 5

Abundances of top carnivores and herbivores are positively correlated, as are those of primary carnivores and plants

Question 6

Top down and bottom up control

Answer 6

Green plants dominate because predators keep herbivores in check = top down
Green plants dominate because they have evolved defences to keep herbivores I check = bottom up
Depends on ecosystem productivity; desert vs rainforest

Question 7

Keystone species

Answer 7

Some species are more intimately woven into the fabric of the food web than others
Strong interactors are those whose removal would produce a significant effect on at least one other species
Keystone species – some strong interactors would lead (through their removal) to significant changes spreading throughout the food web = affects biodiversity more greatly, plays a critical role in maintaining the structures of an ecological community and whose impact on the community is greater than would be expected based on its relative abundance or total biomass, not always predators, can occur at any trophic level throughout the web, sometimes it is not their trophic role rather that they act as ecological engineers, disproportionate impact, potential to exert powerful effects

Question 8

Omnivory

Answer 8

Omnivores feed from more than one trophic level

Question 9

Patterns in species richness

Answer 9

Species richness – number of species in a defined spatial unit
Biodiversity is variation in life at all levels of organisation from genes to ecosystems
Species richness is the most common currency of biodiversity
Factors affecting species richness:
- Geographic = latitude, altitude, depth
- Factors correlated with latitude = climatic variability, productivity, age, harshness
- Factors independent of latitude = isolation, disturbance, heterogeneity
- Biotic factors = predation, parasitism, competition, succession
Regional species richness (γ) is partitioned between: within-community (α) and between-community (β) richness. In saturated communities, species richness is high and niche spaces are fully utilised
Productivity and resource richness vary – for plants, productivity depends on which conditions or resources are limiting to growth, productivity of the environment for animals follows to a similar trend, productivity can lead to an increase in species richness where it goes along with a broader range of resources or niches
Warmer environments allow species with narrower niches to persist, therefore such environments support a higher species richness
Spatial heterogeneity increases the resource spectrum - heterogenous (variable) environments allow co-existence of competing species (niche partitioning), provides a greater variety of microhabitats, microclimates, refuges from predators etc
Environmental harshness reduces species richness – those dominated by an extreme abiotic condition (acidic soils, alkaline lakes, polluted rivers), some species are adapted to these extreme environments, generally lower pH associated with a lower species richness, acidic versus calcareous grassland
Temporal variation in conditions and resources may be predictable (or not) and may operate on timescales from minutes to millennia – climatic variation in a seasonal temperate environment; different species suited to conditions at different times (temporal niche differentiation), different annual plants germinate, grow, flower and set seed at different times during the season, phytoplankton and zooplankton succession in seasonal lakes
Environmental age: evolutionary time – communities may differ in species richness because some are closer to equilibrium than others, others are still undergoing succession, the tropics may be so species rich because they have been relatively undisturbed for millennia, temperate areas are still recovering from an ice age, Arctic is species poor and Antarctic is species rich (latter affected by last ice age)
Habitat area – larger islands contain more species, species relationships are consistent ecological patterns, one reason larger areas or islands tend to contain more types of habitat, larger islands are also bigger targets (increasing rates of new species arrival) and can hold larger populations of more species (decreasing rate of species extinction), on balance they receive more species while losing fewer = number of species on an island is determined by a balance between immigration and extinction, dynamic balance, continual species turnover, rate of immigration also depends on isolation (e.g distance from mainland) = theory of island biogeography
Hotspots of species richness – 44% of plant species, 35% of vertebrates (no fish) are endemic to 25 hotspots, occupying a small proportion of the earth, knowledge of spatial distribution of species richness is a prerequisite for determining conservation priorities, both globally and locally, 34 terrestrial and 10 marine yellow hotspots of biodiversity have been identified

Question 10

Biodiversity

Answer 10

Biodiversity is crucial for the functioning of ecosystems
Oxygen, food, fresh water, fertile soil, medicines, shelter, protection from storms and floods, stable climate and recreation all have their source in nature and healthy ecosystems