This hypothesis states that the relationship between species richness of birds and foliage height diversity is positively linear in both temperate and tropical forests (see Karr and Roth, 1971).
Total species richness is generally divided into (a) within-habitat, and (b) between-habitat components.
The importance of habitat heterogeneity in population dynamics is also emphasized by Lomnicki (1980) who takes cognizance of the variation in individual resource intake within a single species population. According to Lomnicki, stability is impossible in a population with equal resource partitioning among individuals, while unequal resource partitioning allows highly stable populations.
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Another result of unequal resource partitioning and patchy habitats is the regulation of population density by emigration. Emigration promotes lower population density, higher food density and greater stability of the remaining population (left behind). However, the regulation of population density by emigration is not an adaptation of a population to assure its stability but rather is a byproduct of the maximization of individual fitness in patchy environments (Lomnicki, 1980). The population stability is chiefly due to the fact that natality is counterbalanced by migrant mortality. Lomnicki further believes that the reproductive rates in patchy environments are linearly proportional to random mortality in local habitats.
Slatkin (1974) reviewed the effects of habitat heterogeneity in multi- species interactions and the consequences of heterogeneity on competing species. Most natural habitats are patchy and heterogeneous in space and time and the evidence is mounting that habitat heterogeneity plays a critical role in species abundance and stability (Hansson, 1979; Pianka, 1978). The habitat patches may be identical or dissimilar.
In the former cases the species can still differ according to their dispersal potential and competitive ability. Spatial heterogeneity in the environment constitutes a strong stabilizing influence that counters the destabilization resulting from competition.
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In a more recent study, Hassell (1980) has examined the dynamic effects of spatial variability in environment as applicable to (1) a single- species system, (2) a parasitoid-host system, and (3) a disease-host system. All these systems were examined with due specification of the patchy habitats between which the animals were distributed. In all three cases, the same conclusion emerged, viz., the more contagious the distribution of the animals per patch, the greater is the population stability. In the first and third systems, this arises because increased clumping accentuates the density- dependent mortality within a patch.
On the other hand, in the two species parasitoid-host system results from differential susceptibility of the host individuals to parasitism. The same conclusion, viz., that increased spatial heterogeneity leads to greater stability in population densities, has been reached by Stenseth (1980) who has shown that an increased number of species exists in a patchy habitat as compared to a homogeneous habitat.
Stenseth further concludes that;
(1) the rate of reproduction generally increases faster in a homogeneous habitat than in heterogeneous;
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(2) in patchy habitats the sex ratio at birth tends to be female-biased; and
(3) larger-sized clutches are produced in highly unstable environments.