BIOL3701 Lecture Notes - Lecture 12: Heterotroph, Spruce Budworm, Habitat Fragmentation
26 Jun 2018
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Ecological Interactions
Need to conserve processes AS WELL as patterns
oAll organisms are linked to others
E.g. as predators, prey, pollinators, parasites etc.
Measuring Networks
The frequency distribution of the number of connections per node
oThe – Degree Distribution
If all nodes are equally important, or equiprobably connected We have a
random or homogenous network
oBut the internet is heterogeneous – some nodes have many
connections, most have few
Heterogeneous networks are most resistant to a random loss of nodes than
homogeneous networks
In random networks:
oThe degree distribution follows an exponential distribution:
The probability of having more than the average number of links
drops quickly
Skewed or heterogeneous networks:
oMay arise from a preferential attachment mechanism in which links
tend to be formed with nodes that already have links
Such a model generates a power-law degree distribution
Real Mutualistic Networks
Jordano et al. (2003)
oSurveyed plant-pollinator and plant-frugivore networks
oFound about 66% exhibited a ‘truncated power law’ degree
distribution
Truncated power law (TPL) Two power functions separated by
a cut-off (truncated) size
Modes
oNetworks may be one-mode
All nodes are of the same kind
oTwo-mode
Nodes belong to one of two classes
Connections are formed between nodes of one class and nodes
of the other
Plant-animal Networks
find more resources at oneclass.com
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Nested networks
Another property of networks is the
extent to which they are nested
Real plant-animal networks tend to be
highly nested
Specialists interact with species that are subsets of
the species with which generalists interact
oGeneralist thrive in variety of conditions
and use variety of resources i.e. heterotroph
oSpecialist thrive in a narrow range of
conditions
A core of generalists interact with each other
oAnd specialists tend to interact with the
most generalist species
Mutualistic networks appear to be more nested
than analogous two-mode predator-prey or plant-
herbivore networks
Mutualistic Networks
Built upon weak and asymmetric links
Degree distribution and nestedness are measures that are based on
qualitative data
oi.e. whether a link is present or absent
Frequency distribution of dependence values is skewed
Most pair-wise dependencies are weak a few are quite strong
oThe relationships are asymmetrical
oE.g. a plant depends heavily on a particular pollinator, but the pollinator
does not depend on the plant – visits others as well
Modularity
The properties of asymmetry and nestedness give rise to modularity or
compartmentalization in networks
Form building blocks of network assembly
Such patterns seem to be very general and appear in mutualistic networks
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oFrom many different habitats and ecosystems
Implications for Biodiversity Conservation
Heterogeneity and nestedness confer robustness to species loss and habitat
fragmentation
oMaybe only if species loss is random
Robustness of the network:
oHeavily dependent on the few highly connected species that glue the
network together
Important phylogenetic signal in the number of interactions per species – for
many networks
oRelated species play a similar role
oNetworks may indicate how phylogenetic information is lost through
coextinction cascades
Conservation of Beneficial Interactions
Benefit of ecosystem services such as pest control, pollination and
decomposition
In the past:
oData collected has focused on simple food chain or links I larger
communities
E.g. crop – herbivore – parasitoid
Thus missing other interactions that might determine the effect
of beneficial insects
Limits our ability to follow the impacts of management actions
Host-parasitoid interactions
oFew webs have been published
Predation is more difficult to assess and quantify
oAlthough new technology (Kasper) use molecular techniques to
examine range of prey
Other interactions:
oDecomposition – dung beetles, maggots
Undesirable Interactions and their Control
Introduction of invasive species
oE.g. Argentine ant has disrupted seed dispersal in South Africa and
Australia
Classical biological control involves the deliberate release of invasive
organisms
Many species around the world have been introduced for pollination
Lopezaraiza-Mikel, Hayes et al. (2007) The impact of alien plant on native
plant-pollinator network
find more resources at oneclass.com
find more resources at oneclass.com
Document Summary
Need to conserve processes as well as patterns: all organisms are linked to others. E. g. as predators, prey, pollinators, parasites etc. The frequency distribution of the number of connections per node: the degree distribution. If all nodes are equally important, or equiprobably connected (cid:0) we have a random or homogenous network: but the internet is heterogeneous some nodes have many connections, most have few. Heterogeneous networks are most resistant to a random loss of nodes than homogeneous networks. In random networks: the degree distribution follows an exponential distribution: The probability of having more than the average number of links. Skewed or heterogeneous networks: drops quickly: may arise from a preferential attachment mechanism (cid:0) in which links tend to be formed with nodes that already have links. Such a model generates a power-law degree distribution. Jordano et al. (2003: surveyed plant-pollinator and plant-frugivore networks, found (cid:0) about 66% exhibited a truncated power law" degree distribution.
