Networks and Webs Connected- ness web Who eats whom Algal groups Energy flow web Biomass consumption Functional web Effects based on species removal

Transcription

1 Networks and Webs Connectedness web Who eats whom Algal groups Energy flow web Biomass consumption Functional web Effects based on species removal

2 Connectedness Webs Presence of interactions, no strengths Predators consume multiple prey Top of web more specified than bottom (bottom tends to be broad functional groups) Benguela marine food web, South Africa. Q: Would harvesting Cape fur seals benefit hake fishery? (Yodzis 2001)

3 Connectedness Webs Benguela food web: Seals eat hake, but also eat hake predators and competitors Combined impact of direct and indirect effects not clear Yodzis estimated over 28 million possible interactions between seals and hake in this web

4 Connectedness Webs Green webs (basal resources from primary producers) vs Brown webs (basal resources from detritus/microbes) Food webs may have both sources of energy Green energy may turn over quickly (fast portion; brown energy may turn over slowly Mixture of these may contribute to foodweb stability

5 Connectedness Webs Subweb: Microbial loops Ducklow (1986) only 2% of the C taken up by bacteria ended up in higher pools 14 C-glucose -> bacteria -> protozoans/larger zooplankton Often a small amount of material from the microbial loop is used by higher trophic levels (<20%) Bacteria compete with algae for inorganic nutrients Bacteria mediate chemical reactions and decomposition of organic material

6 Connectedness Webs Topology of webs: patterns of connectance, nestedness, modularity Connectance: proportion of possible links actually present Nestedness: specialists (spp with few links) mostly interact with generalists (spp with many links) Modularity: groups of spp mostly interact with themselves

7 Energy Flow Webs Describes energy or biomass moving between species in food web Assumes relationship between importance of links and amount of energy flow True for some community properties such as bioaccumulation Generally a poor predicator of interactions and effect of species removals. Consider microbial loop, which may be most important for remineralization of inorganic nutrients

8 Energy Flow Webs Teal, JM. 1962, Energy flow in the salt marsh ecosystem of Georgia. Ecology 43: Light energy not captured Energy lost in respiration

9 Charles Elton (1927) Energy Flow Webs predators larger than prey; parasites smaller than hosts biomechanical constraints Pyramid of numbers (Eltonian pyramid) Inverted biomass (gm -2 ) pyramid possible with primary producers with high turn-over rate (gm -2 yr -1 ) though production pyramid cannot be inverted because of thermodynamic Raymond Lindeman (1942) introduced ecological efficiency no energy transfer perfectly efficient, generally 5-15% only. Proposed productivity may limit food chain length.

10 Functional Webs Function or Interaction Web shows the strength of interactions between species not all species interactions are equally important to community dynamics. Paine (1980) proposed measuring interaction strengths by experimentally removing species and documenting the response of remaining community members Yodzis (1988) questioned value of short-term manipulations (pulse experiment) in understanding equilibrium dynamics (need press experiment) Ecology 69: However, recent work suggests even inaccurate estimates of interaction strength greatly improve predictions of community response to perturbation (like climate change, etc) Novak et al Ecology 92:

11 Functional Webs Experimental manipulations Size-structured exclusion cages Control Exclosure Dorn et al. 2006

12 A Functional Webs Trophic Cascade and size-structured Dry season effects in field cages B Wet season Large Fishes Large Fishes -1.87* -2.13* Vertebrates and Macroinvertebrates Vertebrates and Macroinvertebrates Microinvertebrates Microinvertebrates * -2.26* -3.36* Floating Mat Epiphytic Algae Floating Mat Epiphytic Algae Chick et al. 2008

13 Functional Webs

14 Functional Webs Food webs contain a few strong interactions and many weak ones; frequency distribution of interaction effects non-normal A preponderance of weak interactions may stabilize food webs.

15 Functional Webs For example: Piscivorous bass control the abundance of small fishes in lakes Creates a trophic cascade that can affect water quality

16 Functional Webs Keystone species - A species whose effect on the community is disproportionately large relative to their abundance Pisaster starfish increase species diversity on Pacific intertidal by consuming competitive dominant Sea otters reduce sea urchin abundance, releasing kelp and associated fauna