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Species 1 Competition Predation Herbivory Parasitism Mutualism Commensalism + + Species 2 + + + + 0 How do we compare communities in space and time?
Objectives 1. Be able to describe and compare communities using food webs and diversity measurements 2. Understand and give examples of species that have a strong inﬂuence on communities 3. Describe the concepts of top-down and bottom-up control of food webs, and give examples of each 4. Predict the outcome of disturbance on communities Thursday, November 12, 2009 1 Thursday, November 12, 2009 2 How do we describe communities? Species Diversity • We can describe communities with models, both word and numerical • Species diversity combines: - Species Diversity - Food Webs - Species richness (number of species) - Relative abundance (evenness) Thursday, November 12, 2009 3 Thursday, November 12, 2009 4 CQ Species Diversity Food Webs • Which forest community is more diverse?
A. Community 1 B. Community 2 • • Depict trophic structure (feeding relationships) Can be very complex - Simplify by removing weak relationships Food web of ﬁsh in Venezuelan stream Thursday, November 12, 2009 5 Thursday, November 12, 2009 6 Trophic Relationships Trophic Relationships • Autotrophs (primary producers) synthesize their own food • Heterotrophs obtain nutrition from eating other organisms - Photosynthetic (energy from the sun) - Chemosynthetic (energy from inorganic substances) - Primary consumers - Secondary consumers - Tertiary consumers - Decomposers Thursday, November 12, 2009 7 Thursday, November 12, 2009 8 Food Webs Food Webs • Not all interactions in a food web are of equal importance • Dominant species: Most abundant or have the highest biomass - Dominant species - Keystone species - Foundation species Caribbean marine food web Thickness of line = interaction strength
Thursday, November 12, 2009 9 Thursday, November 12, 2009 Redwoods
10 CQ Food Webs Keystone Species
Why does the number of species in the community decrease when Pisaster is removed?
A. Pisaster is a superior competitor B. Pisaster predation kept superior competitors in check C. Competitive exclusion D. Resource partitioning • • Keystone species: Large inﬂuence, often low abundance Example: Pisaster sea stars on the Paciﬁc coast Thursday, November 12, 2009 11 Thursday, November 12, 2009 12 Food Webs Community Control • Foundation species: Inﬂuence community by changing physical environment • Communities can be thought to fall under 2 broad categories of control Quaternary consumers - Bottom-Up -- Nutrients - Top-Down -- Predators Carnivore Tertiary consumers Carnivore Secondary consumers Carnivore Primary consumers Herbivore Beavers
Plant A terrestrial food chain Primary producers Thursday, November 12, 2009 13 Thursday, November 12, 2009 14 Community Control Community Control • • Bottom-Up Control Nutrients determine community structure • • Top-Down Control: - Predators control community
structure With your neighbor : Predict what will happen to this community when otters are removed. Thursday, November 12, 2009 15 Thursday, November 12, 2009 16 Otters & Kelp Forests Kelp Forest
Thursday, November 12, 2009 Urchin Barren Otters & Kelp Forests
Effects of top-down control depends on food chain length the population size and status in the Aleutian Islands are animals are commonly seen. getic requirements of free whales and the caloric value o estimate that a single killer w sume 1825 otters per year an otter population decline co REPORTS caused by as few as 3.7 wha Strikingly rapid changes in few were found. Marked increases in sea otters disappearing? starvation, because sea urchins are the prinsigns of disease, none were fo Why are ecosystem have accompanie urchin biomass during the population decline cipal prey of sea otters in the Aleutian Islands vated contaminant concentrati population declines (Fig. 1). at Adak (Fig. 1) are further evidence against (19 Although we looked specifically for reported Aleutian Isla - Bottom-Up).Explanation: Otter prey have decreased at in the Island were n otters Adak subsequent analyses from 39 s (ﬁshing pressure) density, the kelp forests were Aleutian archipelago have sho randomly selected sites (4). O are restricted a few - Top-Down Explanation: Otter predators have increased at towithin small are numerous Adak the wides 1991, wh is inconsistent (killer whales) sites were randomly chosen f in otter numbers. ment of plant tissue loss to The collective evidence th Using similar procedures at t conclude that increased killer 1997, caused the otter kel tion haswe resurveyed thedecli peated the measurements of the population size and status o to herbivory. Islands 10-y in the AleutianOver the are un urchin size and density inc animals are commonly seen. F duce stocks eight-fold free-r an and abandoned this region because of severely depletedrequirements ofincreas geticWhaling Com- quences of altered catch restrictions imposed by the International whales while and the caloricand their pre whales kelp density declined value of mission and moved south into the central North Pacific (Fig. 1D) Response of Killer a to exploit smaller Bryde’s whales (Balaenoptera brydei) 12 (Fig. 1). The W and factor that estimateof a single killer whw female sperm whales. Before commercial kelp tissue lossimportant food and The vast majority of whales were removed from rich summer sume 1825 ottersto herbivory per year reso feeding grounds in a small portion of the northern North Pacific Ocean, just to 1.1% per day in declineas gray 1991 cou otter population migratory route 47 Ocean and Bering Sea. In waters within 370 km (200 nautical (29 miles) of the Aleutian Islands and north coastal Gulf of Alaska Observations w 1997 (Fig.1). cultural matrilines m alone, a minimum of 62,858 whales and ancaused 1.8 million estimated by as few as 3.7 whale ecotypes (32). Three 18 Fig. 1. (17) Changes Thursday, November 12, 2009 over time at several islands in theof whale biomass were taken between 1949 and 1969. Asrapid changes ch A in sea otter abundance Aleutian archipelago of Strikingly eastern North Pacifi 1997 revealed similar in tonnes a measure of (D) kelp of change and concurrent changes in (B) sea urchin biomass, (C) grazing intensity, and the magnitude densityin whale abundance in this other marine mamm Amchitka, and Kagalaska Is accompanied region over this time, only 156 whales wereecosystem after harvested there have offshores, whose die measured from kelp forests at Adak Island. Error bars in (B) and (C) indicate 1 SE. The proposed 1969. Altogether, at least a half million great whales whales and sea otter Killer were specific ecotypes I mechanisms of change are portrayed in the marginal cartoons—the one removed from the North Pacific Ocean andpopulation declines (Fig. 1).are on the left shows how the southern Bering Sea Southern Ocean, o ited at west-central were cet during this period. By the mid-1970s, all otters stocks in the kelp forest ecosystem was organized before the sea otter’s decline and the one onOcean right shows great whalethe Adak Island on large ne the were severely diminished. Although some mostly Aleutian North Pacific much of the past forests were half and pinnipe how this ecosystem changed with the addition of killer whales as an apex predator. Heavy arrows density, the kelp summercentury, species have exhibited remarkable recoveries (e.g., gray whale the austral winter (3 and humpback biomass (1990s and represent strong trophic interactions; light arrows represent weak interactions. whale), the combined currentmillennia selected Thus, (4).isOn before. sites Pacific fee the North it randomly early 2000s) is estimated to be only 14% of preexploitation because killer whale levels (B.P., unpublished data). numerous at Adak inavailability, the prey 1991, whe The extreme, rapid, concentrated reduction of whale biomass increased sites were randomly someconsumpt chosen fo from the northern North Pacific Ocean must have profoundly least of the wh influenced the workings of the ecosystem byFig. 2.population altering Population trends and ment otters in Clam Lagoonto d ofextending tissue loss (so plant The sequential h level interaction strengths of two general kinds: those sea human depletion of downward in the food web from the greatUsingto their preyprocedures at the whales similar Why did killer whales suddenly extending upward eat food web from theKuluk Bayexpectation. Pin begin to in the otters? great this (open circl adjacent and those whales to their predators. Our focus is on1997, we A) The Bering Sea, and Gulf the potential conseAlaska. ( resurveyed shortlypopu rate the kelp of after 1970s, peated calculated as the slope of of pl - Loss of other prey species (marine mammals, whales?) the measurements jhuman cessation of the regime shift. the natural log of the number o to herbivory. Over thethen Harbo 10-yea versus year, for seals and fur sea lio Kuluk bet urchin size and seals and Bayincr density seals reasons, 1997 was – 0.345 (SE such as the h 0.058) duce an eight-foldof increase capturing a icantly differentease size0 (R2 from and less smaller while kelpClam Lagoon, the rat 0.027). In density declined b We surmise that a factor of 12 (Fig. 1). The(SEo this same period atively 0.006 av was rare, some further expanded kelp tissue loss to herbivory th i is not signiﬁcantly differentprofi calorically least fro P .867; statistical power beg 1.1%0per day inAleutian Islands to 1991 to 47.5 declines, and by the 0.9]. (Fig.1). ObservationsKul rates in m 1997 The measuredorder of magnitu an Lagoon revealed similarthrough low density chan Fig. 1. (A) Changes in sea otter abundance over time at several islands in the Aleutian archipelago of 1997 differed signiﬁcantly ( P 0.001). (B) urchins to overgraze Survival rates and concurrent changes in (B) sea urchin biomass, (C) grazing intensity, and (D) kelp density Amchitka, and signiﬁcantly Isl Kagalaska be analyses of otters differed sequent to the Aleut measured from kelp forests at Adak Island. Error bars in (B) and (C) indicate 1 SE. The proposed limited Killer whales –1) and Kuluk B and sea otters goon (0.88 year mechanisms of change are portrayed in the marginal cartoons—the one on the left shows how the 2 ited the 3.52, 1 df, P declines of fur seals oa 1 west-central Aleutian 0.001). kelp forest ecosystem was organized before the sea otter’s decline and the one on the right shows Initial Tugidak Island (in the Kodiak much of the past half century, an how this ecosystem changed with the addition of killer whales as an apex predator. Heavy arrows in substantial part by excessi in 1956 –1968 represent strong trophic interactions; light arrows represent weak interactions. millennia before. these harvests,and ofis of b Thus, it harbor ne numbers Otters & Kelp Forests • Otters & Kelp Forests • j 3 trophic levels
Thursday, November 12, 2009 4 trophic levels
19 474 16 OCTOBER 1998 VOL 282 SCIENCE www.sciencemag.org
Thursday, November 12, 2009
Fig. 2. The sequential collapse of marine mammals in the North Paciﬁc Ocean and southern Bering Sea, all shown as proportions of annual maxima. Great whales: International Whaling Commission reported landings (in biomass) within 370 km of the Aleutian archipelago and coast of the western Gulf of Alaska. Harbor seals: counts and modeled estimate (1972) of Tugidak Island (36). Fur seals: average pup production on St. Paul and St. George islands, of juveniles and adults. It is began well in advance of th altering many facets of mar
k Although Fig. 2. Populationthe timingthesemagnitude o trends data are fro and and su the harbor seal population a sea otters in Clam 20ﬁrst counted in 1974, Lagoon (soli when 1978 to 1995 (L. adjacent Kuluk Bay quantiﬁed, Jemison, pe (open have occurr circle not observations). Alaska. (A) The rate of popula A corollary is that the li calculated as the slope of theincre
l Disturbance Disturbance • Any event that damages or disrupts community structure and function • • Intermediate Disturbance Hypothesis - Can increase or decrease species diversity - Moderate disturbance generates maximum species diversity
Do we observe this pattern in all communities? • • • Intensity of disturbance Frequency of disturbance Natural or Human-related - Some communities depend on disturbance (ex. lodgepole pine & ﬁre) Thursday, November 12, 2009 21 Thursday, November 12, 2009 22 Disturbance: Chaparral Disturbance: Chaparral • Much of the plant community in San Diego is chaparral • Fire in Southern California - Mediterranean climate - Dominated by oak & other droughtresistant shrubs - 1892: Fire suppression began - Humans have increased the number of ﬁres - Has suppression resulted in more intense
ﬁres (fuel build-up)? - Fire-adapted • No ! Large intense ﬁres occurred historically (drought, Santa Ana winds) Thursday, November 12, 2009 23 Thursday, November 12, 2009 24 CQ Disturbance: Chaparral Disturbance: Chaparral • What frequency of forest ﬁres do you predict will result in the highest diversity of species in the chaparral?
A. Low (ﬁres every 100 years) B. Medium (ﬁres every 30 - 50 years) C. High (ﬁres every 5 years) • With your neighbor : Make a list of disturbances that affect chaparral communities Thursday, November 12, 2009 25 Thursday, November 12, 2009 26 Disturbance: Chaparral Succession • How long does each disturbance type impact community dynamics?
Fire Human activity Invasive species Few years to 100 years • • What happens after a disturbance? Succession is the sequence of community changes after a disturbance Cold Snap Drought Days, weeks, few years... Long-term climate change Really long time (1000+ years) - Primary succession: No soil exists when succession begins - Secondary succession: Soil remains after a disturbance
(only vegetation removed) 2 to 5 years 30 years
Thursday, November 12, 2009 27 Thursday, November 12, 2009 28 Succession Succession • • Pioneer species: initial species to colonize a habitat following a disturbance Common traits • Models of succession - Facilitation: Make the environment more favorable - Inhibition: Inhibit establishment of later species - Tolerance: Have no effect on later species
Inhibition (shading, herbivory) - Grow quickly - Produce many offspring - Disperse over large distances - Often small in size Facilitation (Nitrogen ﬁxation)
Thursday, November 12, 2009 29 Thursday, November 12, 2009 30 Succession Anza-Borrego Desert • What will the coastal chaparral community look like in 50 years? • • - It depends on disturbance Assignment: Build a food web for a community in the Anza-Borrego desert Due in class Thursday (Nov 12) Chaparral Grassland Chaparral Thursday, November 12, 2009 31 Thursday, November 12, 2009 32 Patterns of Species Diversity Patterns of Species Diversity • • Species diversity varies on a global scale Correlates with latitude and size (of land mass) • • Latitudinal gradient in biodiversity Why are there more species in the tropics than the poles? - Evolutionary History - Climate Tropical rainforest Boreal forest Thursday, November 12, 2009 33 Thursday, November 12, 2009 34 Patterns of Species Diversity • Species-area curve - More diverse habitats - Less extinction & higher speciation Thursday, November 12, 2009 35 ...
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This note was uploaded on 12/02/2009 for the course BILD BILD 3 taught by Professor Woodruff during the Fall '08 term at UCSD.
- Fall '08