CH. 10 Flashcards
species in a simplistic view is how many species at a time
2-3
Food webs
A diagram that attempts to construct the interconnectedness of relationships in nature.
Usually focuses on predator/prey relationships, but have also been built for parasite host relationships (although less studied – particularly for relationships where parasites might damage, but not kill hosts).
Web connectedness
Consumers towards the top of the food web tend to be identified based on species, those at the bottom tend to be aggregates of species.
Trophic Levels within the Food Web
You can identify the base of the food chain by the direction of the arrow; those at the base of the food chain are providing a resource for other species, without pulling from other species.
Does culling a predator increase the abundance of its prey?
odzis found that seals have a direct negative effect on hake, but they also eat hake predators and competitors.
Concluded the effect of the seals on hake is not obvious, but that it is unlikely that culling seals would benefit the hake fishery because of the multitude of indirect pathways between seals and hake.
One reason the base tends to be aggregated species groups is because
species richness is greater at the base, and these species tend to be small in size, difficult to identify and have feeding relationships that are hard to quantify.
Energy Flow Webs
Measures the amount of energy (usually in terms of biomass) through the food web.
involves a lot more effort (have to collect biomass and measure) than just observing who eats whom; as a result not many high quality ones have been constructed.
Energy flow has been found to be a ______________ predictor of the strength of relationships.
poor
Functional Food Webs
An interaction web shows the strength of the interactions between species within a community.
Generally constructed by removing species from the community and observing the responses of the remaining species.
One study removed different species from an algal/grazer intertidal community on the coast of Washington (state). This study concluded that there were a “few strong interactions embedded in a majority of negligible effects” (Paine, 1992).
Paine (1969) coined the term keystone species to indicate a species whose effect on the community is
disproportionately large relative to their abundance.
Keystone species
have a large impact relative to their biomass
Dominant species
who constitute a large fraction of a community’s biomass and whose impacts are large, but not disproportionate to their abundances.
Identification of Keystone Species
Easy after they have been removed from an environment, harder a priori.
High feeding rate
Preference for consuming prey that are competitive dominants
Many times whether a species acts as a keystone species depends on context of the interaction.
Beckerman et al. (2006) and Petchey et al. (2008) modeling handling time as
an increasing function of the ratio of prey to predator size.
They correctly predicted up to 65% of the trophic links in four real-world food webs (they had assumed that predators prefer to eat the most energetically rewarding prey).
These studies make an important link between foraging theory and species interactions.
Body size has long been recognized in structuring food web interactions.
Many ecological features scale with body size. Some examples:
Metabolic rate
Movement speed
Rate of encounter
Handling time
Feeding rate
Wootton and Emmerson (2005) showed
the per capita interaction strength was positively related to the ratio of prey weight to predator weight in the collection of four food web studies.
Emmerson and Raffaelli (2004)
predator/prey size ratio was correlated with the strength of trophic interactions in experimental food webs, but the form of this relationship varied with predator species.
Brose et al. (2008) showed a
unimodal relationship between predation rate and the predator/prey body mass ratio in a lab study on predatory ground-dwelling beetles and spiders.
Elton’s intuition was correct:
Body size relationships play a major role in determining the pattern and strength of trophic interactions within food webs.
Four important types of Indirect Effects:
Exploitative competition
Apparent competition
Trophic cascades
Keystone predation
see models of the types of indirect effects
indirect effects take __________ to reach outcomes than direct effects.
longer
indirect - fix card
As a result of the pathway affecting one species on another via an intermediate, we can assume that indirect effects take longer to reach outcomes than direct effects.
This results in two expectations:
1. indirect effects will take longer to develop than direct effects
2. indirect effects will be weaker than direct effects (due to attenuation of effect size with each trophic link)
Support for these expectations?
Schoener (1993) reviewed the literature and concluded that:
Direct effects were usually stronger than indirect effects
Short-chain indirect effects were stronger than long-chain indirect effects
Menge (1995)
Importance of direct and indirect effects in marine intertidal food webs
Examined the results of perturbation experiments in 23 marine rocky intertidal habitats around the globe.
Concluded that direct and indirect effects were comparable in magnitude in the communities studied.
Two important types of ecological niches
Mutualistic Networks
Parasites and parasitoids
Montoya et al. (2009) evaluated the relative importance of direct and indirect effects in nine well-studied empirical food webs.
Estimated per capita interaction strengths between all species in the nine food webs .
Also found that direct and indirect effects played similarly important roles in determine a response to perturbation, the combined effects of indirect interactions could often counteract the direct effects.
They found that for a 40% of predator-prey links, predators had a net positive effect on the abundance of their prey due to the predominance of indirect effects.
Indirect Effects Conclusions
fundamentally important in food webs
May often lead to counterintuitive results (recall also the Cape Fur Seal study)
mutualistic Networks
example
plant/pollinator interactions
- see slides
Parasite networks have largely been ignored, despite the fact they were discussed in great detail by Charles Elton (1927).
Possible reasons:
Parasites are usually small and cryptic, difficulty to identify.
Hard to quantify by standard ecological methods.
A few recent examples have found that when included parasites make up about 75% of the links in food webs; the total biomass of parasites in food webs may exceed that of top predators; including parasites in food webs increases food chain length and food web connectedness.
Challenges of Including Parasites in Food Webs:
- Presence of complex life cycles/multiple hosts make it difficult to position the parasite within a food web
- Parasites feed on, but rarely kill hosts. Similar to herbivores? But, how to measure energy transfer?
- Parasites can affect the behaviors of their hosts, often making them more susceptible to predators (which influences host mortality rate and rate of energy transfer between trophic levels).
Interactions among the species in a community can be diagrammed as
ecological networks
culling a. predator …
may not increase abundances of its prey due to complex species interactions
Energy flow webs measure
the amount of energy (biomass) moving between species in a food web, but are poor predictors of strength of interactions between species or impact of removing species
keystone species,
have a disproportionate impact on the environment when considering their abundance within the environment.
studies have found that body size
plays a significant role in determining strength of species interactions
Parasitic relationships are particularly difficult to quantify due to
Multiple host life cycles
indirect effects occur when
one species affects another through an intermediate species. Have been found to have a fundamental impact on species networks
One overall conclusion that has been shown with food webs is that more diversity leads to
more weak interactions and greater modularity and this results in greater stability of those ecosystems.
