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That is, each 1-mg/L increase in phosphorus produces a 0.20-mg/L increase in chlorophyll a.

Example 14.2 Peak summer chlorophyll contents in lakes, Ca, have been empirically related to phosphorus concentration at spring turnover, Pw, as in equation (15.1): Ca = 0.37 P^91 (both concentrations in mg/L). According to this relation, what is the sensitivity of chlorophyll to phosphorus at a concentration of 15 mg P/L?

Solution First, using equation (15.1), the expected concentration of chlorophyll a in the lake is

Using equation (14.10), the sensitivity would be expressed as dCa

This indicates that a 1-p.g/L increase in phosphorus would be expected to produce a 0.264-mg/L increase in chlorophyll a.

14.3.2 Resources and Environmental Conditions

The chemical resources include oxygen, CO2 and water as well as other macronutrients and micronutrients. Other resources are light and habitat. The latter includes substrate, shade or sun, shelter, territory and space. Resources can also interact with environmental conditions. Some of the most important environmental conditions are temperature, humidity, pH, soil, salinity, fire, wind, and photoperiod.

The nutrients most often limiting are nitrogen and phosphorus. The ratio N/P in most biomass is about 16 : 1. In aquatic systems it is about 28 : 1. If phosphorus is supplemented, cyanobacter will fix nitrogen to bring the ratio back to nominal values. Therefore, phosphorus is the usual limiting nutrient. Cyanobacter are not abundant in marine environments. Thus, nitrogen may become limiting in saline systems. In the open ocean the micronutrient iron has been shown to be a limiting nutrient. Other micronutrient limitations may be significant in algal succession in lakes (see below).

The highest temperature that any organisms are known to tolerate is microorganisms at 88°C. Among animals, some fish and insects can go up to 50°C. Some organisms are known to perform better under temperature variations than at any particular constant temperature. Plants that use the C4 photosynthesis outproduce the C3 plants at high temperatures and light intensities. They also conserve water better. Conversely, the C3 plants are more competitive under cooler, low-light conditions. The fraction of wild grasses that use the C4 pathway varies along the Atlantic coast of North America, starting from 80% in Florida, 48% in the Carolinas, 34% in the mid-Atlantic region, 23% in New England, to 12% in the northern Maritime Provinces of Canada.

Water is, of course, one of the key determinants of the type of ecosystem that will exist in a terrestrial environment. A rough classification based on amount of precipitation can be made as follows:

Humidity affects water balance and cooling in plants and animals. As humidity falls below saturation, water is lost by evapotranspiration. Evapotranspiration is needed by plants to help them transport nutrients from the roots to the leaves. Too-low humidity can cause plant stomates to close to limit water loss. This prevents oxygen from diffusing out and CO2 in, favoring photorespiration in C3 plants. High humidity, on the other hand, both limits nutrient transport and encourages growth of plant diseases.

Many terrestrial ecosystems are adapted to periodic fires. Foresters have learned that too aggressive suppression of fires has resulted in shifts in population as less fire-resistant

0 to 25 cm/yr 25 to 75 cm/yr 75 to 125 cm/yr 125 cm/yr or more

Desert

Grassland or open woodland Dry forest Rain forest species outcompete the adapted natives. Wind can be a factor in dispersal of seeds, pollen, and insects. As a climatic factor it can affect survival of a species in harsh conditions. Pressure may be a factor only in the ocean, where it can reach 1000 atm. Pressure of that magnitude can affect the conformation and function of enzymes; organisms adapted to the deep ocean have evolved unique enzymes for that condition.

Photoperiod has a significant effect on the behavior of plants and animals. The best known example is that the flowering of many plants is controlled by the length of the day. Many physiological responses of animals are also tied to day length. Birds molt, migrate, and breed in response to day length. Insect eggs are stimulated to go into a resting stage by long days, so they will not hatch until the following spring, even if temperature and other factors are favorable.

14.3.3 Tolerated Range of Factors

Each species can tolerate a range within each factor. More precisely, their favorable range of factors can be defined as a region of the vector space defined by all the factors. Determining this space is very difficult. Laboratory tests can be misleading. The marine hydra Cordylophora caspa was found in the laboratory to have an optimal growth rate at 16 parts per thousand salinity. However, the species is never found at that salinity in nature, only at much lower levels. Evidently, some other factor in nature is limiting its distribution to only a part of the salinity range that it tolerates. Field and laboratory experiments are both needed to understand a species' range.

Rainfall (inches) Relative Humidity (%)

Figure 14.12 Temperature-moisture climograph. (a) The successful introduction of the Hungarian partridge to Montana, the unsuccessful introduction to Missouri, compared to average conditions in its native breeding range in Europe. (b) Conditions in Tel Aviv, Israel showing conditions favoring an outbreak of the Mediterranean fruit fly in 1927. (Redrawn from Odum, 1983; original from Twomey, 1936.)

An example of the combined effect of variables can be seen in Figure 14.12. The plot shows a temperature-moisture climographs, a phase-plane plot of temperature and rainfall. The numbers around each polygon indicate month of the year. The plot shows three climographs. One represents average conditions in the region of Europe where the Hungarian partridge naturally breeds. The other two are for the states of Montana and Missouri, where attempts were made to introduce the partridge. The introduction was successful in Montana but failed in Missouri.

The natural range of an organism can be examined to determine which levels of which factors are associated with the boundaries of the range. However, it is thought that different factors may operate within a range. In fact, it has been demonstrated that individuals near the edge of a geographic range may differ genetically from those in the center. The genetic variation may be due to natural selection, which in this context is called factor compensation. Thus, organisms living in the warmer region of a species range might not be able to tolerate the colder climate that other more adapted individuals of the same species can. The locally adapted populations are called ecotypes. For example, the jellyfish Aurelia aurita has a southern population that cannot swim outside the temperature range 11 to 36°C. A northern population is sharply limited to between 0 and 28°C. Although they are the same species, subpopulations have evolved to flourish over different temperature ranges.

A species is often limited by geographic factors such as mountain ranges or oceans, which in turn limit their dispersal. If transplanted outside their normal range, they reproduce and spread. This is seen most clearly in the many cases where humans introduced foreign species to an area, sometimes deliberately and sometimes inadvertently. Often, these species become nuisances and displace native species. Examples include the gypsy fly, the chestnut blight, and the starling in the United States, and the rabbit and the cane toad in Australia.

14.3.4 Species Interactions

Finally, a major factor that can limit the spread of a species is their interactions with other species. Within the ranges favorable to an organism, it is the interactions with other species that seem to dominate its distribution and abundance. Species interactions can be classified as beneficial, detrimental, or neutral in their effects on each of the interacting species. Note that a species that benefits from another may also become dependent on it. Thus, its range may be limited by the range of the species it depends on. A variety of major interaction types and subtypes have been identified (Table 14.5). We will give examples of several.

Competition arises when two species attempt to exploit the same resources. When the negative effect is due to the fact that one species' use of the resource reduces its availability to the other, it is called resource competition. In interference competition, one species has a more direct effect on the other's ability to compete for a resource. It may do so by chasing it away or by producing a chemical that inhibits competing species. Chemical inhibition of competitors is also called allelopathy. For example, sage plants (Salvia leucophylla) produce gaseous terpenes, which adsorb onto the soil near the plant and inhibit germination of seedlings. Some species of sponge produce a toxin that acts on other sponge species.

TABLE 14.5 Types of Two-Population Interactions"

Interaction Type Species 1 Species 2

TABLE 14.5 Types of Two-Population Interactions"

Interaction Type Species 1 Species 2

Neutralism

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