Co-dominant Species Research Articles

The Ecology of a Semi-Evergreen Forest Community in Sri Lanka

Semi-evergreen forests cover the dry zone plains of Sri Lanka and constitute four-fifths of the island's vegetation. In a sample area of 3 kM2, in the Polonnaruwa Sanctuary, 63 tree species were found; 46 were characteristic of other dry zone forests, and 17 occurred only under special edaphic and biotic conditions. As in other dry zone forests, Drypetes sepiara (Euphorbiaceae) prevailed with a relative density of 21.3 percent. The Shannon index of diversity was 4.23 bits per individual, of which 79.4 percent was attributable to evenness; most species had few individuals. Dominance was shared between species typical of the subcanopy and canopy. Measures of diversity and of dominance between species placed the semi-evergreen community in Polonnaruwa as a type intermediate between tropical rainforest and deciduous monsoon forest. Measures of diameters (DBH) and estimates of the height of trees indicated that all species with typically very large trees had few individuals that were distributed more or less evenly through all the size classes. Typically smaller species had large numerical representation. These facts are discussed in light of dominance relationships and regenerative patterns in the community, and are related to possible evolutionary trends. The distribution of most species was clumped, but that of certain rare species was random. Clumping at 2,500 m2 plot size usually meant clumping at smaller-sized plots. Clumping on a large scale reflected local differences in species dominance. Five shrub associations were distinguished by density and constitute dominant and co-dominant species. Differences were related to the amount of light penetrating through the tree canopy and to edaphic factors. Glycosmis pentaphylla (Rutaceae) was the dominant shrub species in the climax association which flourished under a fairly closed tree canopy and lacked an herbaceous layer. ALTHOUGH EVERGREEN RAINFORESTS and monsoon forests of tropical Asia have received considerable attention from eaologists (e.g., Richards 1962, Ashton 1964, Ogawa et al. 1961, 1965), the semi-evergreen forests have been less intensely studied. In Sri Lanka the forests have been surveyed primarily from an economic perspective (e.g., Andrews 1961). This paper aims first, quantitatively to describe some structural features of a community of semi-evergreen forest in Sri Lanka; second, to examine the dominance relationships between species in this community. These relationships are briefly related to possible successional patterns in the dry zone of Sri Lanka, and to possible evolutionary trends among plants in general. The semi-evergreen forest has been described by Walter (1971: 209) as one in which the upper tree layer is defoliated for some of the year [during the droughti,' while the lower tree layer retains its foliage. According to certain structural features, annual rainfall and drought duration, it is a type intermediate between tropical evergreen rainforest and deciduous monsoon forest. The tropical island of Sri Lanka has a gradation of forests from lowland and montane rainforests to lowland scrub forests and savanna (fig. 1). The majority of the forests may be roughly classed as semievergreen, however. These cover the most extensive physiographic feature of Sri Lanka, the lowland plains in the dry zone. Several dassificatory schemes have been proposed for the vegetation of Sri Lanka (e.g., Koelmeyer 1958, Gaussen et al. 1964, Fernando 1968). Mueller-Dombois (1968) evaluates these schemes and offers an up-to-date reclassification according to long-term climatic information. The community of forest described herein is located within the Polonnaruwa Sanctuary in the northwestern dry zone at 070 56' N and 81? 00' E. Polonnaruwa was the capital of Sri Lanka for varying periods between the 8th and 13th centuries. It was completely abandoned by the end of the 13th century, and natural vegetation reclaimed the land for the next 700 years. The area was resettled only at the beginning of the 20th century. Most of the natural vegetation has been maintained within the Sanctuary, although the shrub layer has been removed in areas immediately surrounding archaeological sites. The cutting of trees is prohibited, but poaching, particularly of young trees of construction-pole size, is not completely prevented. Parts of the study area are completely devoid of young trees and shrubs. These areas have been largely omitted in the present sample. The study site encloses approximately 3 km2, is bounded by water and cultivation, and is continuous with more extensive expanses of forest by a narrow

Experimental Species Removal: Demographic Responses by Sigmodon hispidus and Reithrodontomys fulvescens

Experimental species removal was conducted for three years to assess the demographic consequences of interspecific interactions between cotton rats ( Sigmodon hispidus ) and fulvous harvest mice ( Reithrodontomys fulvescens ). Six 1.6-hectare (ha) plots were monitored monthly by live-trapping; two control, two Sigmodon -only, and two Reithrodontomys -only plots were maintained by appropriate species removal. Frequent trapping and habitat discontinuities between plots minimized the effect of interplot movement and maintained the experimental perturbation. Seasonal vegetation analyses were conducted on each field. Density, percent reproduction, and expectation of further life but not survival were significantly lower for both Sigmodon and Reithrodontomys on experimental plots (codominant species removed). Lower density for both species and lower reproduction for Reithrodontomys were significantly correlated to lower abundance of preferred habitat on these plots. Lower expectation of further life reflected increased mobility ( AvD p ) of Sigmodon on experimental plots, thereby contributing to lower densities and decreased reproduction through lower mate contact. Cotton rat mean individual body weight was significantly lower and male harvest mouse mean body weight significantly higher on experimental plots; lower cotton rat body weight reflected immigration onto experimental plots in response to a density gradient. Sigmodon sex ratio was skewed toward males on control plots, whereas Reithrodontomys sex ratio was skewed toward males on both plots; differential movement or trappability by either sex may be responsible for this result. The significantly lower proportion of adults and young adults on Sigmodon experimental plots was due to emigration; there was no between-plot age class difference for Reithrodontomys . Most demographic differences between control and experimental plots are explained by differential movement, survival, or abundance of preferred habitat. However, increased mean body weight of male Reithrodontomys and earlier appearance of Reithrodontomys juveniles on experimental plots remain unexplained. These results suggest interspecific interactions and/or behavioral interactions may be of secondary importance in determining the demographic structure of rodents in this system.

Thecamoeba in succession series of some peat-bog vegetations

The Thecamoeba belong to the protozoan class Rhizopoda. The thecae, or shells, are built up of rather resistant material, usually pseudochitin or silica. This characteristic faciliates identification in preserved and even dried or subfossil material. They occur in very wet (submerged) to moist habitats. In the succession series of the vegetation in pools and ditches we can find different conditions, and many factors will change in the surrounding water during the succession, e. g. relative humidity, influence of rainwater on trophic level and pH value. Particularly the oligotrophic peat-moor vegetations in Germany have been a subject of study of thecamoebian ecology (HARNISCH, 1927; JUNG, 1936). HARNISCH was the first to describe several associations of Thecamoeba, that showed a distinct preference for specified vegetation types. The mesotrophic and eutrophic wet surroundings have scarcely been investigated (DE GRAAFF, (1956). There is still much to be learned about the ecology of Thecamoeba in the vegetation of the peat-bogs in the lower part of the Netherlands. As an example one succession series of the vegetation in such a peat-bog shall be discussed. The succession starts in open water about 1,5 to 3 metres deep. The first plants found belong to the Potametalia (WESTHOF and DEN HELDT, 1969). In this association organic material accumulates on the bottom. In this way the bottom is built up. When this mud is at about 50 centimetres below surface the plants belonging to the Scirpo-Phragmitetum (WESTHOF and DEN HELDT, 1969) can be found. These plants form a strong mat of roots that “floats” on the underlying mud and dead plant material. This mat ultimately reaches the surface of the water thus making the settlement of other plants possible. These plants belong to the association Thelypterido-Phragmitetum (WESTHOF and DEN HELDT, 1969). Further accumulation of organic matter and the growth ofSphagnum leads to the last association in which samples were taken: The Pallavicinio-Sphagnetum (WESTHOF and DEN HELDT, 1969). Two territories were visited, both Dutch nature reserves in the northwestern part of the province of Overijsel: “e Weerribben” and “de Lindevallei”. In the Weerribben two types of plant succession series were sampled, one beginning in open eutrophic water and the other in more isolated parts beginning in mesotrophic water. The first we callPhragmites australis series and is discussed above, the secondCarex series (named after the dominating plant species or genus). In the Lindevallei one series was sampled, resembling thePhragmites australis series of the Weerribben which also shall be calledPhragmites australis series. Samples were taken by pressing liquid out of mosses. These mosses were arranged in a microzonation along with the zonation of higher plants, going from submerged to moist vegetation. At The similarity between the drier stages of our peat-bogs and those of Jung's peat-moors, can be explained as due to the increasing influence of rainwater in the bogs. Peat-moors are almost entirely drained of rainwater. There is a strong similarity in all series, in relation to humidity. Differences between the series may be found after more detailed analysis of data, because this survey refers only to dominant and codominant species.

Species Removal Studies. I. Dispersal Strategies of Sympatric Sigmodon hispidus and Reithrodontomys fulvescens Populations

Sigmodon hispidus and Reithrodontomys fulvescens are the codominant rodent species of the Texas coastal prairie. Eight isolated 1.6 hectare replicated plots were used to study dispersal strategies of these rodents. Each replicate was live-trapped for nineteen months. For two control replicates all captured individuals were identified, sexed, weighed, marked, and released. In two perturbed replicates all Reithrodontomys were removed, in two perturbed replicates all Sigmodon were removed, and in two other perturbed replicates both species were removed. Monthly dispersal of individuals into plots where that species was previously removed was measured. The number of dispersers of both sexes of Sigmodon and Reithrodontomys were positively correlated with monthly changes in control (resident) density. Sex ratio comparisons between dispersers and residents reflected nonsignificant differences for both species. The rank-order of weight (age) classes of dispersers was positively correlated with the classes found in control plots for female Sigmodon and male Reithrodontomys entering both types of perturbed conditions. Pre-emption by Reithrodontomys appeared to inhibit the immigration of the larger (adult) classes of Sigmodon males. Higher proportions of Reithrodontomys females, compared to males, invaded both types of perturbed plots. Resident populations of both species appeared to inhibit conspecific immigration. The rank-order templated dispersal strategy found during this study has the advantage of introducing a stabilized age structure into newly acquired habitat space.

Fungal and Insect Parasites Contributing to Niche Differentiation in Mixed Species Stands of Eucalypt Saplings

Parasitic damage to the leading shoot of young regrowth eucalypts was found to vary considerably between trees and between different stands but it averaged greater than 20 % 'effective leaf area loss' overall. Many fungal parasites and some of the phytophagous insects responsible for this damage exhibited host specificity or host preference towards trees of a particular subgenus of Eucalyptus. These findings are discussed in relation to the hypothesis that parasites play an important role in the maintenance of stable associations between codominant species of Eucalyptus.

Plant Competition Mediated by Host-Specific Parasites-a Simple Model

It is suggested that competing co-dominant species in natural ecosystems may be maintained in equilibrium by an ecological, negative feedback system involving reciprocal, host-specific parasites. A simple model is derived to describe this relationship and a hypothetical example computed to illustrate its use. The model has important implications for the management of natural forests and for the practice of monoculture. These are briefly discussed.

Shrub Ecotypes in a Salt Desert

A SALINITY gradient extends from the highly saline beaches of the Great Salt Lake of Utah to the surrounding non-halomorphic soils, with a correlated vegetation gradient in which a succession of plant communities grows in concentric zones. The chief communities, in decreasing order of soil salinity, are dominated by (1) Salicornia–Allenrolfea; (2) Sarcobatus vermiculatus; (3) Atriplex–Eurotia; and (4) Artemisia tridentata1. Within these communities are perplexing local distributions of co-dominant species. Typical communities within the Atriplex–Eurotia zone include pure Atriplex confertifolia, Atriplex–Eurotia, pure Eurotia lanata, A. confertifolia Artemisia spinescens–Eurotia grass, and pure Atriplex nuttallii. Although these communities are separated by sharp ecotones, local soil variations are insufficient to explain the distribution of the different plant communities2.

Segregation and Symmetry in Two-Species Populations as Studied by Nearest- Neighbour Relationships

Much work has been done in recent years on the spatial patterns of natural populations of plants, and it has been customary to study one species only at a time. Suppose, however, an investigator were concerned with a population containing two co-dominant species. Then, besides determining the density and spatial pattern of each of the codominant species considered separately it would also be interesting to know how the individuals of one species were arranged with respect to the individuals of the other species. The two species could be described as 'unsegregated' if any individual plant of one of the species was as likely to be found growing close to members of the other species as to members of its own species. Conversely, the two species could be described as 'segregated' if they had a tendency, however slight, to occur in one-species clumps, so that any individual was more likely to be found near members of its own species than near members of the other. In studying the relationship between two species growing in the same area it has been usual to sample the area with quadrats and then determine whether the number of joint occurrences of the two species in the same quadrat was significantly greater (or less)) than chance expectation; if so, the two species were said to be positively (or negatively) associated. However, as Greig-Smith (1957, Ch. 4) has shown, the result obtained inevitably depends on the size of quadrat used. It is thus meaningless to assert that a pair of species is, say, positively associated, unless at the same time one specifies at what scale this positive association manifests itself. Nevertheless, the degree to which the individuals in a two-species population are, colloquially speaking, 'mingled together' is clearly an intrinsic property of the whole population, and one independent of scale. Furthermore, it should be possible to measure this property in such a way that the result obtained is independent of any arbitrary characteristic (such as quadrat size), of the sampling method. It is for this purpose that the concept of segregation, as described in the previous paragraph, is proposed. The smaller the degree of 'mingled-togetherness' of a pair of species, the greater will be their degree of segregation. A population of two species, A and B, may exhibit two entirely different types of pattern, as shown in Fig. 1. One type of pattern, the 'unsegregated' type, is shown in Figs. la and lb. In Fig. la the two species are present as two co-extensive random populations. In Fig. lb the population as a whole is clumped, but within every clump the individuals of species A and the individuals of species B are present in the same proportions and are arranged independently of each other. Figs. lc, 1d, le and If show examples of the second, or 'segregated', type of pattern.

Sugar Maple-Basswood Studies in the Forest-Prairie Transition of Central Missouri

Composition studies of sugar maple-basswood communities have been reported in Minnesota, (Daubenmire, 1936), Wisconsin, (Eggler, 1938), and in Iowa, (Aikman and Smelser, 1938). In Minnesota and Wisconsin the principal associate of the basswood, Tilia americana L., was the sugar maple, Acer saccharum Marsh. The black maple, A. nigrum Michx. f., in central Towa replaced the sugar maple as a codominant species. In a later study in central Iowa red oak, Quercus rubra L., was reported as a common species in mesic sites, (Kucera, 1950). Red oak ill Wisconsin was reported by Eggler as a third species of importance in the type. Red oak as well as American elm, Ultm?us americana L., and slippery elm, U. rubra MuhI., were ranked next in importance in the Minnesota stands. It is the purpose of this paper to present comparative data pertaining to hitherto-unreported maple-basswood examples in central Missouri. Steyermark (1940) reported sugar maple-white oak and sugar maple-bitternut associations in the Ozark region. These studies, however, generally excluded the forestprairie transition in central Missouri. Settler reports (Ind. World Comm. Adv., 1880) referred to the timber resources of Boone County in which sugar maple and basswood as well as other species were considered abundant.