ECOLOGY Primary Production and Energy Flow How do I become more productive?

Ecosystem Ecology The term “ecosystem” first proposed by Arthur Tansley (1935):  Though the organisms may claim our primary interest,…we cannot separate them from their special environment, with which they form one system. It is the [eco]systems so formed which, from the point of view of the ecologist, are the basic units of nature on the face of the earth.” Ecosystem Ecology: The study of energy, water and nutrient flows (flux) in ecosystems:  Biotic and abiotic processes  Fundamental areas of interest include primary production, energy flow and nutrient cycling

Definitions Population: a collection of individuals of the same species Community: an assemblage of populations in an area or habitat Ecosystem: a biological community plus all of the abiotic factors influencing that community

Definitions Production: creation of new organic matter Primary Production (PP): fixation of energy by autotrophs  PP measured in various ways:  usu. rate of CO 2 uptake (g/m 2 yr 1 )  Or -- biomass or O 2 produced Gross Primary Production (GPP): total amount of energy fixed (or CO 2 taken up) by all autotrophs in an ecosystem. Net Primary Production (NPP): amount of energy left over after autotrophs have met their own energetic needs (respiration and, ie, the amount of energy available to consumers). NPP = GPP – R PP

Trophic Dynamics Lindeman (1942): “The Trophic Dynamic Aspect of Ecology”  Trophic dynamics: transfer of energy from one part of an ecosystem to another  First suggested grouping organisms within an ecosystem into trophic levels. The number of trophic levels in an ecosystem is limited by energy losses with each transfer or conversion of energy between trophic levels

Trophic Dynamics  Each T.L. feeds on T.L. immediately below.  As energy is transferred from one T.L to another, energy is degraded/lost due to:  Limited assimilation  Consumer respiration  Heat production  Energy quantity decreases with each successive trophic level

A Simplified Food Web Primary Producers (Plants) Primary Consumers (Herbivores) Secondary Consumers (Predators)

A Simplified Food Web Primary Producers (Plants) Primary Consumers (Herbivores) Secondary Consumers (Predators) PAR

A Simplified Food Web Primary Producers (Plants) Primary Consumers (Herbivores) Secondary Consumers (Predators) reflected PAR absorbed by Chl a

A Simplified Food Web Primary Producers (Plants) Primary Consumers (Herbivores) Secondary Consumers (Predators) reflected PAR absorbed by Chl a photorespiration heat production

A Simplified Food Web Primary Producers (Plants) Primary Consumers (Herbivores) Secondary Consumers (Predators) reflected PAR absorbed by Chl a photorespiration heat production

A Simplified Food Web Primary Producers (Plants) Primary Consumers (Herbivores) Secondary Consumers (Predators) reflected PAR absorbed by Chl a photorespiration heat production respiration heat production limited assimilation

A Simplified Food Web Primary Producers (Plants) Primary Consumers (Herbivores) Secondary Consumers (Predators) reflected PAR absorbed by Chl a photorespiration heat production respiration heat production limited assimilation

Hence the Trophic Pyramid Biomass and productivity generally decrease with trophic level

Energy Flow In A Temperate Deciduous Forest Gosz et al. (1978): study of solar energy flux at Hubbard Brook experimental forest:  Of the total energy input via solar radiation:  15% reflected  41% converted to heat.  42% absorbed during evapotranspiration.  2.2% fixed by plants as GPP  1.2% used in plant respiration.  1% left for NPP

Energy Flow In A Temperate Deciduous Forest Gosz et al. 1978: < 1% of total energy input converted to NPP Of the NPP available to consumers (herbivores), 96% lost as consumer respiration Insufficient energy left to support a viable population at a 3 rd trophic level.

Factors Controlling Terrestrial PP PP variable but most strongly correlated with temperature and moisture.  Highest PP under warm, moist conditions An indicator of PP is actual evapotranspiration (AET):  Total amount of water that evaporates and transpires off a landscape during the course of a year ( mm H 2 O/yr )  affected by temp and precipitation  ecosystems with high AET tend to be warm, receive large amounts of precipitation  ecosystems with low AET tend to be cold, receive little precipitation or both  eg hot deserts and cold tundra have low AET

Among different types of ecosystems, AET is correlated with NPP Rosenzweig (1968) estimated influence of moisture and temperature on rates of PP by plotting the relationship between annual NPP and AET.

Within similar ecosystems, temps tend to be similar, so moisture (precipitation) tends to be the controlling factor eg Sala et al. (1988) study of 9,498 sites in the grasslands of central North America: Factors Controlling Terrestrial PP

Patterns of soil fertility also explain significant variation in NPP within terrestrial ecosystems (assuming equal temp. and moisture)  Liebig (1840): concept of nutrient limitation Liebig’s Law of the Minimum NB: Liebig’s law somewhat simplistic: 2 or more factors may be simultaneously limiting in many systems  Nutrient availability controls patterns of PP in agricultural ecosystems... Factors Controlling Terrestrial PP

...and in natural ecosystems: Shaver and Chapin (1986):  Added commercial fertilizer (N+P+K) to several tundra systems in Alaska  NPP 23 – 30% higher on fertilized plots. Factors Controlling Terrestrial PP

Effects of nutrient additions depend on prior nutrient availability  PP responds to additions of limiting nutrients eg Bowman et al. (1993):  Experimental fertilization of wet and dry alpine meadows, Niwot Ridge, CO.  Wet meadows had higher initial [N], [P]  4 treatments:  N; P; N+P; control Factors Controlling Terrestrial PP

Bowman et al. (1993): Results more dramatic in dry meadow (lower initial [N], [P])  N, N+P both produced signif.  biomass In contrast, weaker response in wet meadow  only N+P had an effect sugg. N-limitation in dry meadow; co-limitation in wet meadow Light might also limit NPP in wet meadow;  biomass might produce enough shading to inhibit growth response to nutrient additions. * * * * = statistically significant response Factors Controlling Terrestrial PP

Phytoplankton are the dominant primary producers in aquatic ecosystems Aquatic NPP generally limited by nutrient availability  temperatures generally less variable in the ocean than on land Several studies have found proportional relationship between [P] and phytoplankton biomass, chlorophyll a and NPP in lakes. Factors Controlling Aquatic PP

Observational studies in Japan, North America: eg: Hogetsu and Ichimura 1954 Ichimura 1956 Sakamoto 1966 Dillon and Rigler 1974 Smith 1979

Lake Fertilization Experiments eg. studies at Experimental Lakes Area, Ontario (Mills and Schindler 1987, Findlay and Kasian 1987): Lake 226 divided by vinyl curtain  Each basin = 8 ha, 500,000 m 3 One side fertilized with P:

Marine NPP is also controlled by nutrient availability Highest rates of NPP in areas with greatest nutrient availability:  continental margins (runoff from land, bioturbation of bottom sediments)  areas of upwelling (deep nutrient-laden waters rise to euphotic zone) Open ocean tends to be nutrient poor (relies on vertical mixing for nutrients)

Unlike lakes, marine NPP appears to be limited primarily by [N]: Granéli et al. (1990) nutrient enrichment experiments, Baltic Sea:  Fertilized flasks containing indigenous phytoplankton spp.  N, P, control treatments  N treatments led to increased chlorophyll concentrations.

Temperature and precipitation (terrestrial) and nutrient availability (aquatic) explain most of the variation seen in NPP, but not all:  Residual Variation: proportion of variation not explained by the independent variable.  Dillon and Rigler (1974) suggested environmental factors besides nutrient availability significantly influence phytoplankton biomass. Factors Controlling NPP

Summary Terrestrial Primary Production is generally limited by temperature and moisture. Aquatic Primary Production is generally limited by nutrient availability.  usually P in freshwater ecosystems  N in marine ecosystems Energy losses limit the number of trophic levels found in ecosystems.