Demographic Factors

Demographic limits to plant distribution include those factors that adversely affect recruitment or increase mortality. Demography therefore involves many factors that are not the property of species or populations but are instead a function of habitat and location (Antonovics et al., 2001). Thus geographic barriers to dispersal as well as the provision of microsite space within a habitat both come under this heading. The principles of Island Biogeography (MacArthur & Wilson, 1967) relate species numbers on islands to an equilibrium that becomes established between the rates

Fig. 1.13 Distribution of five lineages of deciduous oak (A-E) based on a wide-ranging study of cpDNA from over 11000 trees. The spatial patterns reflect regions of glacial refugia and subsequent migrations. The distribution of lineage (B) indicates the distinct nature of these oceanic marginal populations. (Reproduced with permission from Petit et al., 2002.)

Fig. 1.13 Distribution of five lineages of deciduous oak (A-E) based on a wide-ranging study of cpDNA from over 11000 trees. The spatial patterns reflect regions of glacial refugia and subsequent migrations. The distribution of lineage (B) indicates the distinct nature of these oceanic marginal populations. (Reproduced with permission from Petit et al., 2002.)

Fig. 1.14 Geographic differentiation of the distribution of cpDNA haplotypes for British and Irish oaks after applying a Kriging average where green are areas with no overall dominance, and white, orange and yellow represent areas of dominance for the three most dominant haplotypes. (Reproduced with permission from Lowe et al., 2005.)

Fig. 1.14 Geographic differentiation of the distribution of cpDNA haplotypes for British and Irish oaks after applying a Kriging average where green are areas with no overall dominance, and white, orange and yellow represent areas of dominance for the three most dominant haplotypes. (Reproduced with permission from Lowe et al., 2005.)

at which species colonize an island as compared with the rates at which they become extinct. For any particular island, species richness depends on the area, topography, number of habitats on the island, as well as accessibility to a source of colonists and the species richness of that source. These principles clearly illustrate the necessity in demography to consider the nature of the habitat and how it modifies recruitment and mortality rates, particularly at population margins where opportunities for recruitment and the hazards of extinction are different from core locations. Patchiness that occurs in marginal situations can frequently be found to accelerate population decline and decrease species richness (Eriksson & Ehrlen, 2001). However, the effect is not consistent as there are numerous instances where local species richness can be increased as fragmentation can aid seedling recruitment.

The soil seed bank is also a space with a demographic dimension. The number of embryo plants that lie dormant in the soil until there is an opportunity for them to germinate adds both numbers and variation to nascent populations. Disturbance can also be included under the heading of demography as it affects both recruitment and mortality. Ruderal plants (Latin rudera, broken rocks) are defined as plants that can withstand frequent and severe physical disturbance (Table 1.1). The disturbance factor can be discussed as a negative phenomenon, as are all processes that destroy plant biomass (Grime, 2001). Nevertheless, from a demographic point of view, disturbance also has a positive aspect in providing fresh space for colonization, as does rejuvenation of communities in aiding diversity through limiting the extent to which any one species can permanently dominate a habitat. At the subspecies level, metapopulation development on a larger scale is facilitated in sites that are frequently disturbed and where extinctions create opportunities for new migrants (Hanski, 1999).

Other biological factors influencing recruitment and mortality are those characteristics which are often referred to as life history traits or life history strategies (for definitions see Table 1.1). These include size, growth pattern, resource storage, as well as reproductive strategies such as whether a plant is an annual or a perennial, male, female or hermaphrodite, a rapid reproducer with many seeds, or a slow reproducer with few seeds. Theoretically, species are expected to select for an optimal set of characteristics that will produce the highest population growth rate in a particular environment (Sibly & Antonovics, 1992). The search for such optimal sets of characteristics can be studied experimentally under controlled conditions as well as with numerical models, but in the field where environments fluctuate from one year to another the attainment of an optimal solution is elusive and will probably always remain so.

1.5.1 Limits for reproduction

Compared with animals, plants are endowed with a broad and versatile range of reproductive mechanisms. Consequently, there is a continuing debate about the nature of plant species and whether they differ from animal species as independent lineages or 'units of evolution', or are merely arbitrary constructs of the human mind, which result from the never-ending activities of over enthusiastic taxonomists who insist on giving specific names to the numerous subspecies of dandelions, blackberries and oaks. An analysis of the phenetic and crossing relationships in over 400 genera of plants and animals has indicated that although discrete phenotypic clusters exist in most genera (>80%), the correspondence of taxonomic species to these clusters is poor (<60%). The lack of congruence as perceived by botanists, it is argued, may be caused by polyploidy, asexual reproduction and over differentiation by tax-onomists, but not by contemporary hybridization (Rieseberg et al., 2006). This same study pointed out that crossability data indicated that 70% of taxonomic species and 75% of phenotypic clusters in plants correspond to reproductively independent lineages (as measured by post-mating isolation), and thus represent biologically real entities. It can be argued that, contrary to conventional wisdom, when a wide spectrum of plant species is considered and not just the celebrated horror stories of plants such as dandelions, plants are more likely than animal species to represent reproductively independent lineages (Rieseberg et al., 2006).

Despite the findings of the above study, fertile hybrids between species are common and chromosomal barriers to fertility where they arise can be removed as a result of polyploidy. Consequently, it can be argued that plants differ from animals in that there is much greater gene flow between species. Plants also differ from animals in being sedentary and are therefore strongly selected for particular habitats and as a result ecotypic variation contributes to genetic diversity. Polymorphisms and polyploidy, together with the genetic memory provided by the seed bank, are all powerful means for augmenting genetic variation and provide the flowering plants with a facility for rapid adaptation to change that is rarely found in higher animals. As is argued throughout this book, plants from marginal habitats, are seldom lacking in genetic diversity and in some cases are even more diverse in peripheral than core habitats, with the former frequently being colonized by hybrid species (for references see Chapters 2 and 4).

Was this article helpful?

0 0

Post a comment