(i) Preamble

We have been discussing the factors that might influence the geographic distribution of single species (e.g. fundamental niche, competition, historical factors, etc).

We already appreciate, however, that species never occur alone, but instead tend to coexist with other species in what are known as communities. To what extent are such co-occurrences of species in communities the result of the interdependence of species versus their co-distribution being essentially independent (i.e. interspecific interactions do not determine the extent and kinds of species that co-exist in communities)?

Stated another way, are species within communities co-dependent and co-adapted entities (to each other and to shared abiotic conditions) such that are geographically distributed as discrete units? The alternative is that communities are simply a random collection of species that are co-distributed more by historical accident than any deterministic factor.

The first hint that species may exist as co-dependent units was based on observations of plants that tended to occur together in distinct climatic zones (based largely on temperature), e.g. the "floristic belts" of von Humboldt and "life zones" of Merriman, both terms from the late 1800’s. Today, other abiotic conditions such as precipitation, humidity, soil characteristics, in addition to temperature, and the microbial and animal life in considered in definitions of so-called bioclimatic zones, ecoregions, and biomes, i.e. regions defined on the basis of distinct abiotic and biotic characteristics involving climatic and soil conditions and distinct (from other biomes) assemblages of plant and animal species.

For example, does it seem reasonable that the highly coincident distribution of 12 species of very distantly-related taxa shown below could occur randomly? Such patterns stimulated biogeographers to explore factors that might result in the nonrandom association of species with particular abiotic conditions (e.g. biomes).

Coincident distribution of 12 species (7 spruces, 2 birds, and 3 voles) across northern North America. The taxa shown are "typical" of northern coniferous forests. They tend to spread southward only in higher elevation areas (i.e. Rocky Mountains) suggesting the importance of temperature in the distribution of all species (or some other factor closely related to temperature).

 The exact number and composition of biomes are not universally agreed upon, it’s more the concept that the earth’s biota can be grouped into distinct assemblages that is the key point. These assemblages ultimately depend on various abiotic and biotic conditions as well as the overall level of productivity of a particular region.

Productivity is essentially the rate at which energy from the sun is fixed into organic material via photosynthesis such that it can be utilized by non-photosynthetic life forms. The more energy that is available to transfer among trophic levels will have a profound impact on the structure and complexity of life forms that can be sustained in particular geographic regions (and, hence, the nature of particular biomes).

For instance, shown below is a typical energy flow diagram for a geographic area. The area of each level in the diagram represents any of energy level, productivity, biomass, or abundance. Because energy can neither be created or destroyed, but only transferred from one form to another and when this occurs, some of the usable energy is lost (i.e. during metabolism, up to 90% of the energy is lost to the production of heat), as one moves up the flow diagram, the available energy is reduced. This typically results in lower biomass, abundance, etc at successively higher trophic levels.

As such, it is clear that the complexity and structure of a community in any particular regions (i.e. nature of the biome) will be ultimately dependent on the available energy input at the base of the system. The input energy comes from sunlight, which interacts with other factors such as temperature to influence productivity. Regional climatic conditions, therefore, have a critical role to play in regional productivity and hence the nature of biomes across the globe. Such variation in productivity is one of the explanations proposed for latituidinal gradients in species diversity (there is a general increase in species diversity in a variety of taxa as one moves from the poles to the equator). We’ll see the connection more explicitly later in this section.

(ii) Distributions of communities in space and time

Before we discuss the kinds of biomes and their characteristics we should explore the natures of community structure in space and time.

There have been several models proposed to explain how communities, as collections of species, are distributed in space. The basic difference amongst these models is the extent to which species comprising a community change in abundance coincidentally and the rate of change (do they suddenly "drop out" and replaced just as suddenly by another species or is the change from one species to another gradual?). The more coincident the changes in individual species’ abundance are in space the more "tightly linked" or coupled is the community. If species in a community are tightly linked, then this implies that biotic interactions among species must be very important in determining their distribution (and hence the nature of the community or biome) OR the species are responding in a similar way to limiting abiotic factors (e.g. temperature). Either way, the community assemblage is not determined randomly.

When discussing the idea and nature of biomes, we should also recognize that communities and biomes and their distributions change over both short and long term evolutionary scales.

The key idea here is that of "succession", that is the progressive change in community structure, composition, and (perhaps) function with time (from a few years, decades to across millions of years). Primary succession takes places when a community is rebuilt "from scratch", i.e. from a place devoid of life and the soil on which it depends. Imagine a volcano or glacier that destroys all leaving just bare rock. Primary succession begins from here. Secondary succession occurs when the soil is left after a disturbance (a flood or fire) and the community rebuild from this point on.

In either case, the point is that succession represents what often appears as an "orderly" change in the species composition and community character over time from "pioneering" species which immediately modify the environment (just by their presence) and eventually give way to better adapted species which ultimately give way to a more stable "climax" community. Such succession is largely determined by climate and local soil conditions and the nature of the community that is nearby and provides the source of immigrant species. Of course, chance colonizations, species interactions, and the particular dispersal capabilities of species in nearby communities also influence the successional changes that occur.

The examples shown in the figures below indicate the successional changes to plant communities over relatively short time frames (300 years) to postglacial changes that occur in the distribution of oak species over the last 18,000 years. In the former example, community character is summarized as "foliage height": it changes from essentially less than 10 m with a predominance of grasses to over 50 m. Clearly, the kinds of plant species and the animals that utilize such habitats change across this time as well as the soil characteristics and even microclimates because of such succession. The oak figure makes the same point, but over a much longer time frame and over a much larger geographic scale.

Fig. (above) Successional changes in a rainforest habitat over 300 years.

Fig. (below) Changes in spatial distribution of major biomes types in eastern North America the maximum of the last glaciation (18,000 ybp) to 200 ybp.

Such changes in biomes have obvious implications for changes in distribution of individual species (in this case oaks) and the animals that depend on them.

Fig. Changes in the extent of coverage by oak species (Quercus) from 18,000 ybp (top left) to 500 ybp (bottom, right). Records are from pollen deposits in lake sediments. Darker areas represent more intense oak pollen deposits.

We can even consider evolutionary changes over tens to hundreds of millions of years in the composition of faunas to be a kind of "succession". Consider the major changes in the abundance and composition of major animal taxa that have been recorded in the marine fossil record (see below). The change in faunal composition from the "Cambrian" to "Paleozoic" to "Modern" faunas in the sea has been staggering.

The bottom line of the above is that as we now explore the kinds and nature of different biomes, we must keep in mind that they are really "snapshots" of diversity and its distribution in space. The only thing certain about them is that they are imperfect in their absolute definition, the boundaries between any two biomes are "fuzzy", and they do and will change in character and geographic distribution over time!