POPULATIONS

Population size - N = # of individuals in an area

• time = t

• density = # of individuals / unit space

• dN/dt = (b-d)N or rN, since b-d = r

• Nt = N0e(b-d)t

= N0ert

• see figs. (exponential increase)

• Logistic equation:

• dN/dt = rmaxN[(K-N)/K]

• Reproduction in marine systems is often confined to discrete periods, followed by recruitment of large numbers of larvae that settle and die-off throughout the year (Fig 3.2)

Depressions of population growth are usually due to factors that limit carrying capacity:

1) available food

2) space

3) density-dependent mortality

4) disease and parasitism

5) cannibalism

• Marine benthic algae and invertebrates often export all their reproductive products in the form of pelagic gametes or larvae

• Thus, local changes in population size can be due to immigration of settling; emigration may also occur with crowding

• So: dN/dt = LN [(K-n)/K] + i - e

Age Structure and Population Growth:

Simplest case - population begins with large #’s of individuals of zero age

• However, usually overlapping generations

Life Table:

1) lx = # of animals alive at beginning of interval x

2) dx = # dying during interval x

3) Lx = avg. # of individuals alive during interval x (lx + lx + 1)

4) ex = life expectancy from time period x of an individual of age x (= sum of Lx from time x to the end divided by lx)

5) mx = # of young produced by a female of age x to (x + 1)

•Thus, the change in population size from the beginning to the end of the cohort, Ro, is the sum of the probability of death at time x times the fecundity per individual, mx, or:

Ro =

•Table 3-1

• lx or dx - allows for construction of survivorship curve

• Another way to follow cohort is to measure diminution of numbers with time (Fig. 5)

• Can also estimate mortality by estimating dx - collecting accumulations of dead animals

•(need exoskeleton - Fig. 3-6)

• r and K selected spp.

• K efficiency and reproductive capacity

• Can we observe a gradient in rmax among a spectrum of adaptive types?

• Allan , 1976 showed life history of major zooplankton (rotifers, cladocerans, copepods) which conforms to gradient from unpredictable, often food-rich small lakes and inshore waters of large lakes, to more predictable and food-limiting waters of lakes and open ocean (Fig. 3-7)

• Rotifers and Cladocorans (more unpredictable) have higher rmax than Copepods

• Be careful about body size (inverse correlation of r with body size)

• Total energy fraction allocated to reproduction (reprod. effort) related to energy used for non-reproductive functions - might reflect adaptation to disturbed environments (Gadgil and Solbrig, 1972)

• Hydrobia neglecta -occurs in temporary ponds that often dry up

• Hydrobia ulvae - more permanent habitats

• Hydrobia ventrosa - devotes more energy to reproduction

• H. neglecta is a brooder and produces fewer eggs per female than H. ulvae Why????

Limiting Resources:

• Interactions:

• interference competition - Balanus balanoides - individuals overgrow others

• exploitation competition - uptake of nutrients by diatoms

• Resource renewal -

• siphon-nipping - Tellina – siphons can grow back

• predation from plaice (flatfish) – Pleuronectes platessa

• Niche-Width and Resource Spectra:

• Pynopodia helianthoides (sun sea star- feeds on almost any invertebrate

• Conus - very selective - fish, polychaetes, etc. some deadly to humans

Optimal Foraging Theory:

• Quality - Vadas, 1977 - urchins prefer seaweeds (relatively easy to assimilate) – calories used as index

• Prey Size - Carcinus maenus (green crab)- handling time reached max. at intermediate shell length

• Food Density - search time - as food density dec., an aminals optimal diet should consist of a broad range of diets

• Redshanks (shorebirds) prefer the amphipod Corophium volutator (amphipod) to the more food-rich polychaete worms (ease of spotting fast-moving amphipods)

Interspecies interactions

• Evidence for Competition:

• Theoretically, extensive niche overlap of 2 spp. should result in competitive displacement

• Hutchinson, 1961 - Paradox of the Plankton - many plankton with same nutrient requirements

EVIDENCE : - Experimental Manipulations

• Connell, 1975 - Remove competition and then observe ecological expansion of competing species

• Connell, 1961 - pioneering study on Calif. coast

• Balanus balanoides vs. Chthamalus stellatus - transplants and removals

• Chthamalus survived successfully in the intertidal zone occupied by Balanus

•Pisaster ochraceus - removal from rocky intertidal - body wt. of Leptasterias hexactis inc. (Menge, 1972)

EVIDENCE: Laboratory Experiments

• Fenchel, 1975 noted that sympatric populations of the mud snails H. ulvae and H. ventrosa always differed in size, whereas allopatric populations showed no size difference

• Character displacement - body size and mouth size correlated

Displacements in Nature:

Macoma balthica - restricted to intertidal muddy sediments in open habitats of normal salinity. However, in brackish areas (Chesapeake Bay and Baltic Sea) - much broader range

• (Kohn, 1971, 1966) Conus californicus - single spp. feeds on wide variety of foods –highly specific radula

• Conus in Hawaii - 30 spp. - narrow range of food types

Contiguity of Niche Space:

• Neohaustorius schmitze - upper intertidal beaches, SE U.S.

• Haustorius spp. - longer maxillae - can filter larger particles

•Thus, niche division is by particle-size selection

•Styles of Competition:

• We assume that competitive exclusion is hierarchical

• A(superior)B(superior)C

• (NOT non-hierarchical, or a network, triangular)

Predation and Community Structure:

• Predation - consumption of one living organism by another

• Littorina littorea

Stabilizing Forces:

• sub-Arctic planktonic environment - single common copepod Calanus finnmarchicus feeds on relatively few diatom spp. in homogeneous water column

• sea stars can devastate lower intertidal zones preventing the establishment of invertebrate populations

• Asterias rubens causes such intense predation that the lower rocky intertidal of N.E. England is almost free of potential prey

Refuge - M. calif. and above foraging range of

P. ochraceus

Prey Switches -

• Thais lapillus shifts its prey preference between barnacles and mussels - depending on which is more abundant

• This prevents overexploitation

Evolutionary Shifts:

• If sufficient genetic variance exists, a prey may be able to elude the predator permanently through evolutionary change

• Brown seaweed Desmanestia produces sulfuric acid which discourages grazing. The black tunicate Ascidia nigra also produces acid as an effective deterrent to predators

Disturbance - Local eradication of one or more spp. as a result of severe modification of the structural environment

• Can be anthropogenic - dredge spoil dumping

• Also natural - each year adult Limulus polyphemus migrate inshore and devastate local mud-flat invertebrates

•Hurricanes

• Competitive dominant may be eliminated - lost to fugitive species - can colonize new habitats

Intermediate Disturbance - Predation Hypothesis

• Paine and Vadas, 1969

• Intermediate levels of disturbance and predation tend to conserve maximum # of spp.

•Draw graphs

Succession - orderly sequence of spp. observed in a habitat following a large disturbance that eliminates the local biota

Connell and Slatyer, 1977 - 3 major processes:

1) some spp. alter habitat in such a way as to facilitate the entry of others

2) Some spp. might modify the habitat so that it becomes less suitable for earlier spp. in the successional sequence

3) Early occupants monopolize the habitat at the expense of all other spp.. As long as early occupants survive, they suppress colonization of all other spp.

• Little & Little, 1980 - Algal succession - San Clemente Island, Ca.

• Ulva californica - early colonizer

• Pelvetia fastigata - late colonizer

• Algal production - decrease from early to late

• Late succession spp. allocated more energy for structural components - firm attachment tended to resist urchin population - also against wave-shearing