The role of animals in successions

The structure of communities and the successions within them have most often been treated as essentially botanical matters. There are obvious reasons for this. Plants commonly provide most of the biomass and the physical structure of communities; moreover, plants do not hide or run away and this makes it rather easy to assemble species lists, determine abundances and detect change. The massive contribution that plants make to r and K species and succession necromass and the late successional role of trees

Table 16.3 Physiological characteristics of early and late successional plants. (After Bazzaz, 1979.)

Attribute

Early successional plants

Late successional plants

Seed dispersal in time Well dispersed Seed germination: enhanced by light Yes fluctuating temperatures Yes high NO- Yes inhibited by far-red light Yes high CO2 concentration Yes

Light saturation intensity High

Light compensation point High

Efficiency at low light Low

Photosynthetic rates High

Respiration rates High

Transpiration rates High

Stomatal and mesophyll resistances Low

Resistance to water transport Low

Recovery from resource limitation Fast

Resource acquisition rates Fast

Poorly dispersed

No No No

High

High

High

Slow

Slow?

determining the character of a community is not just a measure of their role as the primary producers, it is also a result of their slowness to decompose. The plant population not only contributes biomass to the community, but is also a major contributor of necromass. Thus, unless microbial and detritivore activity is fast, dead plant material accumulates as leaf litter or as peat. Moreover, the dominance of trees in so many communities comes about because they accumulate dead material; the greater part of a tree's trunk and branches is dead. The tendency in many habitats for shrubs and trees to succeed herbaceous vegetation comes largely from their ability to hold leaf canopies (and root systems) on an extending skeleton of predominantly dead support tissue (the heart wood).

Animal bodies decompose much more quickly, but there are situations where animal remains, like those of plants, can determine the structure and succession of a community. This happens when the animal skeleton resists decomposition, as is the case in the accumulation of calcified skeletons during the growth of corals. A coral reef, like a forest or a peat bog, gains its structure, and drives its successions, by accumulating its dead past. Reef-forming corals, like forest trees, gain their dominance in their respective communities by holding their assimilating parts progressively higher on predominantly dead support. In both cases, the organisms have an almost overwhelming effect on the abiotic environment, and they 'control' the lives of other organisms within it. The coral reef community (dominated by an animal, albeit one with a plant symbiont) is as structured, diverse and dynamic as a tropical rainforest.

The fact that plants dominate most of the structure and succession of communities does not mean that animals always follow the communities that plants dictate. This will often be the case, of course, because the plants provide the starting point for all food webs and determine much of the character of the physical environment in which animals live. But it is also sometimes the animals that determine the nature of the plant community. We have already seen how seed-eating insects and rodents can slow successions in old fields and sand dunes by causing a higher seed mortalilty of later successional species. A particularly dramatic example of a role for animals, and on a much larger scale, comes from the savanna at Ndara in Kenya. The vegetation in savannas is often held in check by grazers. The experimental exclusion of elephants from a plot of savanna led to a more than threefold increase in the density of trees over a 10-year period (work by Oweyegha-Afundaduula, reported in Deshmukh, 1986).

More often though, animals are passive followers of successions amongst the plants. This is certainly the case for passerine bird species in an old-field succession (Figure 16.15). Arbuscular mycorrhizal fungi (see Section 13.8.2), which show a clear sequence of species replacement in the soils associated with an old-field succession (Johnson et al., 1991), may also be passive followers of the plants. But this does not mean that the birds, which eat seeds, or the fungi, which affect plant growth and survival, do not influence the succession in its course. They probably do.

animals are often affected by, but may also affect, successions

Figure 16.15 Top: bird species distributions along a plant succession gradient in the Piedmont region of Georgia, USA. Differential shading indicates relative abundance of the birds. (After Johnston & Odum, 1956; from Gathreaux, 1978.) Bottom: distributions of vesicular-arbuscular mycorrhizae in the soils associated with an old-field succession in Minnesota. Differential shading indicates relative abundance of spores of species in the genera Scutellospora, Glomus and Acaulospora. (After Johnson et al., 1991).

Prairie warbler

Field sparrow

Grasshopper sparrow

Wood thrush

Hooded warbler

Summer tanager

Cardinal

Grasshopper sparrow

Community type

Bare field

Grassland

Grass-shrub

Forest

Scutellospora spp.

Glomus spp.

Acaulospora elegans

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