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.W. Beck J. Exp. Mar. Biol. Ecol. 249 2000 29 –49 Keywords
: Fractal dimension; Gastropod; Habitat structure; Habitat complexity; Structural components; Rocky intertidal
1. Introduction
Habitat structure may affect the diversity and abundance of species in many systems Grinnell, 1917; Gause, 1934; Huffaker, 1958; Connell, 1961; Kohn and Leviten, 1976;
Connor and McCoy, 1979; Menge and Lubchenco, 1981; Werner et al., 1983; Leber, 1985; Underwood and Chapman, 1989. Studies of the effects of habitat structure are
nearly as common as those of competition and predation, but there are few reviews of habitat structure, and it is rarely discussed in textbooks McCoy and Bell, 1991. This
lacuna occurs because there has been little consistency in the 1 definition or 2 measurement of habitat structure between different studies or habitats. These problems
make it difficult to compare methods and results between studies, which constrains the development of a broader understanding of the effects of habitat structure on the
diversity and abundance of species.
To address the first problem, McCoy and Bell 1991 offered a definition of habitat structure, which can help guide research and enhance interpretability of results from
different studies and habitats. Their definition of habitat structure proposes that it is composed of at least two major factors, complexity and heterogeneity, and their effects
and measurement are scale-dependent McCoy and Bell, 1991; Beck, 1998; Downes et al., 1998. Complexity encompasses variation in habitat structure attributable to the
absolute abundance of individual structural components. Structural components are distinct physical elements of the habitat, e.g. rocks, trees, pits, and pneumatophores
McCoy and Bell, 1991; Downes et al., 1998. Heterogeneity encompasses variation in habitat structure attributable to variation in the relative abundance of different structural
components McCoy and Bell, 1991; Beck, 1998; Downes et al., 1998. The basis of an effect of heterogeneity is that the effects of different structural components are
independent of their complexity.
McCoy and Bell 1991 helped clarify the difference between complexity and structural components, but these two elements of habitat structure are often confounded.
Some studies suggested that particular structural components affected the density and diversity of animals but did not account for differences in complexity e.g. Inglis, 1994;
Dittel et al., 1996; Primavera, 1997. For example, Miller and Carefoot 1989 compared the effects of two different structural components, rocky pits and adult barnacles, on the
density of juvenile barnacles. They concluded that adult barnacles had a greater effect on the density of juvenile barnacles than pits but did not consider that these results might be
largely explained by the greater complexity e.g. SA of their treatments with adult barnacles.
Other studies suggested that complexity affected the density and diversity of animals, but their manipulations of complexity were confounded with differences in structural
components e.g. Russ, 1980; Coull and Wells, 1983; Gilinsky, 1984; Leber, 1985; Bell et al., 1987; Pennings, 1990; Diehl, 1993. In these studies, complexity was manipulated
M .W. Beck J. Exp. Mar. Biol. Ecol. 249 2000 29 –49
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in each treatment by changing either the type of structural component e.g. one species of algae vs. another or the number of different structural components e.g. one vs. two
types of vegetation. These latter studies confounded complexity, structural components, and heterogeneity. When studies of habitat structure are confounded, it is not possible to
identify the important elements of habitat structure or the mechanistic basis for their effects on species.
Habitat complexity and structural components have been experimentally separated in just a few studies McGuinness, 1984; Dean and Connell, 1987; Jenkins and Sutherland,
1997. Stoner and Lewis 1985 showed that the total abundance of epifaunal crustaceans was affected primarily by the surface area of vegetation. It mattered little
whether this complexity was created by seagrass or algal components. A few species, however, had greater densities in plots with algae than in plots with similar complexities
of seagrass i.e. there were effects of structural components over and above the effects of complexity.
The lack of distinction between complexity and structural components is tied to the second major problem that limits our understanding of habitat structure; there is little
consistency in the measurement of complexity between different studies and habitats McCoy and Bell, 1991; Beck, 1998. This problem arises because most indices of
complexity are based on specific structural components e.g. counts of number of trees or pits, which severely limits their use and interpretation. A number of indices of
complexity, e.g. D and SA, can be used more broadly, because their measure of complexity is independent of specific structural components. At present, there is little
justification for the use of one index over another but see Carleton and Sammarco, 1987; Underwood and Chapman, 1989; McCormick, 1994; Beck, 1998, and experi-
ments are necessary to determine which index best represents the elements of complexity that affect species e.g. Jacobi and Langevin, 1996.
In previous work, I showed that the effects of complexity on gastropods could be measured and compared in rocky intertidal and mangrove habitats Beck, 1998. These
habitats had very different components pits and pneumatophores, respectively and complexities as measured by several indices of complexity Fig. 1: D, vector dispersion
2
VD, chain-and-tape Chain, and consecutive substratum height difference odh . In
the rocky intertidal habitat, the density of gastropods was significantly and positively correlated with most indices of complexity, and D had the highest correlations with
density. From these observations, three hypotheses of the effects of habitat structure on the
density and richness of gastropods in this rocky intertidal habitat were formulated. 1 The complexity of habitats positively affects the density and richness of gastropods. 2
D represents elements of complexity that affect the density and richness of gastropods better than other indices. 3 The effects of specific structural components on the density
and richness of gastropods are independent of the effects of complexity. I manipulated complexity and structural components to directly test predictions from these hypotheses.
These manipulations test whether complexity, structural components, or both affect the density and richness of gastropods, and they help identify the appropriate indices of
complexity. If the elements of habitat structure can be clearly defined and measured, it will make information from different studies and habitats more commensurable and help
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.W. Beck J. Exp. Mar. Biol. Ecol. 249 2000 29 –49
Fig. 1. Measurement of the indices of structural complexity. a Transect showing profile of habitat profile
2
can be obtained from a real habitat or an experimental mimic. b Illustration of the calculation of VD, odh
n
and Chain at the 5-mm interval. VD is a measure of the variance in u ; VD 5 n 2 o a c
Y
n 2 1 , n is the
s h
s d
jd
s d
1
number of separate triangles along the transect. c Illustration of the calculation of D by the dividers method. The points are the apparent length of the transect measured by ‘dividers’ of increasingly greater intervals. The
equation for the line and the calculation of D are shown.
reduce the confusion that hinders our understanding of the effects of habitat structure on community structure.
2. Methods
