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
