Edward F. Connor

The assembly of species communities chance or competition

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INDIVIDUALS–AREA RELATIONSHIPS: THE RELATIONSHIP BETWEEN ANIMAL POPULATION DENSITY AND AREA

We compiled studies that report data on the relationship between animal population density and patch or island area for 287 individual species and 21 faunas. We tested the assumption of the equilibrium theory of island biogeography that population densities are independent of area by performing a meta-analysis using the linear correlation coefficient,r, as a measure of the effect of area on population density. We fit meta-analyses that used a random-effects model to these data to test for the effects of taxa, habitat, latitude, spatial scale, and overall population density. We also fit meta-analyses that used a fixed- effect model to the same data to estimate the repeatability of measurements of the correlation between population density and area within species. Contrary to the equilibrium theory of island biogeography, our results indicate that, on average, animal population densities are positively correlated with area, which suggests that density compensation may be uncommon. This result was found for individual species, but not for faunas. We found taxonomic differences in the correlation between population density and area, with insects and birds having on average large or moderately large positive correlations, respectively, and mammals having correlations near zero. Observations within individual species showed considerable repeatability. The observed overall positive cor- relation between the population density of individual animal species and area is best ex- plained in the context of the resource concentration hypothesis. Our results imply that the regional abundance and persistence of animal populations may depend strongly on the presence and continued persistence of a few large patches of suitable habitat, rather than on a regional network of small and large habitat patches.

COMPARATIVE SEED SHADOWS OF BIRD-, MONKEY-, AND WIND-DISPERSED TREES

Although spatial patterns of seed distribution are thought to vary greatly among plant species dispersed by different vectors, few studies have directly examined this assumption. We compared patterns of seed rain of nine species of trees disseminated by large birds, monkeys, and wind in a closed canopy forest in Cameroon. We used maximum-likelihood methods to fit seed rain data to four dispersal functions: inverse power, negative exponential, Gaussian, and Student t. We then tested for differences in dispersal characteristics (1) among individuals within species, and (2) among species dispersed by the same vector. In general, an inverse power function best described animal-dispersed species and the Gaussian and Student t functions best described wind-dispersed species. Animal-dispersed species had longer mean dispersal distances than wind-dispersed species, but lower fecundities. In addition to these distinct differences in average dispersal distance and functional form of the seed shadow between animal- and wind-dispersed species, seed shadows varied markedly within species and vector, with conspecifics and species within vector varying in their dispersal scale, fecundity, and clumping parameters. Dispersal vectors determine a significant amount of variation in seed distribution, but much variation remains to be explained. Finally, we demonstrate that most seeds, regardless of vector, fall directly under the parent canopy. Long-distance dispersal events (>60 m) account for a small proportion of the seed crop but may still be important in terms of the absolute numbers of dispersed seeds and effects on population and community dynamics.

Species Number and Compositional Similarity of the Galapagos Flora and Avifauna

The flora and avifauna of the Galapagos Islands are used to reexamine quantitative analyses of species numbers and compositional similarities. Conflicts in the results of previous analyses of the Galapagos flora are reconciled and are shown to be the result of using different species numbers and physiographic parameter measurements. The idea that Galapagos plant species numbers largely reflect conditions in the archipelago during the Pleistocene glaciations (i.e., that relaxation times are very long) is criticized, and the more parsimonious alternative, that they reflect recent conditions, is proposed. Generalizations about factors determining species numbers that are based on multiple regression and correlation are precarious. The number of botanical collecting trips to each of the Galapagos Islands is a better predictor of species numbers than are area, elevation, or isolation. Two null hypotheses concerning the determination of floral and avifaunal compositional similarities among the Galapagos Islands...

The evolution and adaptive significance of the leaf-mining habit

Feeding inside leaves by leaf-mining insects has been viewed as an adaptive innovation for consuming foliage. The leaf mine presumably provides its occupant shelter from the detrimental effects of the physical environment, protection from attack by natural enemies, and potentially a means of avoiding plant defenses concentrated in particular layers of leaf tissue. We examine several hypotheses concerning the adaptive significance of leaf mining using both the published literature and experiments. We also examine the notion that leaf mining is an innovation that has led to an adaptive radiation by examining the phylogeny and species richness of leaf-mining lineages and their sister groups. Both advantages and disadvantages accrue to insects that feed inside leaf mines in comparison to external-feeding folivores. The advantages of feeding within a leaf mine include: a lower incidence of disease infection, a microenvironment with lower evaporative demand and therefore protection from desiccation, protection from the direct and indirect effects of UV radiation by the leaf-mine epidermis, and avoidance of plant defenses resulting in higher feeding efficiencies. The disadvantages of feeding inside a leaf mine include: lower mobility resulting in higher mortality from parasites, higher mortality associated with premature leaf-abscission, and smaller average body size and lower fecundity. Leaf mining is certainly a viable means of feeding upon foliage, and in particular instances it might be the most successful means of feeding on leaves. However, the lower species richness of leaf-mining lineages in comparison to their external-feeding sister groups indicates that the evolution of leaf mining does not represent an innovation that has led to an adaptive radiation. Perhaps the evolution of leaf-feeding per se and the evolution of the ability to feed externally on leaves rather than concealed feeding modes such and mining and galling represent innovations that have resulted in adaptive radiations in the Insecta.

DIFFERENTIAL RESOURCE USE BY PRIMATES AND HORNBILLS: IMPLICATIONS FOR SEED DISPERSAL

Arboreal frugivores, such as primates and hornbills, are important seed dis- persers for many tropical plant species, yet the degree to which they use the same resources is unknown. If primates and hornbills consume the same fruit species, they may be redundant in their roles as seed dispersers, and the loss of one of these taxa may be compensated for by the other. To examine resource use by tropical frugivores, we quantified the feeding habits of two hornbill species, Ceratogymna atrata and C. cylindricus, and five primate species, Colobus guereza, Lophocebus albigena, Cercopithecus pogonias, C. cephus, and C. nictitans, in the lowland rainforest of south-central Cameroon. Based on over 2200 feeding observations recorded between January and December 1998, we characterized the diets and estimated dietary overlap among frugivore species. Previous studies have cal- culated dietary overlap by counting the number of diet species that two animals share, often leading to inflated estimates of overlap. Our method incorporated the proportional use of diet species and fruit availability into randomization procedures, allowing a clearer as- sessment of the actual degree of overlap. This added complexity of analysis revealed that, although the diets of a hornbill and a primate species may have as many as 36 plant species in common, actual dietary overlap is low. These results suggested that there are distinct hornbill and primate feeding assemblages, with primates consuming a greater diversity of plant species and higher levels of nonfruit items like leaves and seeds. Using Correspon- dence Analysis, we also identified two primate assemblages, separated largely by degree of frugivory and folivory. In addition, we found that hornbills feed at significantly higher strata in the forest canopy and eat fruits of different colors than primates. Averaged across the year, overlap between groups (hornbill-primate) was significantly lower than combined within-group overlap (primate-primate and hornbill-hornbill), showing that primates and hornbills have dissimilar diets and are not redundant as seed dispersers. In equatorial Africa, primate populations face greater declines than hornbill populations because of hunting. It is unlikely that seed dispersal by hornbills will compensate for the loss of primates in maintaining forest structure.

Early Leaf Abscission: A Neglected Source of Mortality for Folivores

We present evidence for a simple, yet heretofore overlooked, cause of mortality for folivorous insects, early leaf abscission. We discuss characteristics of both herbivores and host plants that are likely to influence folivore mortality via leaf abscission. Many plants have been shown to abscind leaves that are diseased or damaged (Jacobs 1962). The complete mechanism by which plants do so, however, is less clear. Apparently, leaf abscission involves interactions of many plant compounds including ethylene, auxins, and abscisic acid (Milborrow 1974). Whatever the physiological mechanism of leaf abscission, the consequence for a leaf-feeding insect is disjunction from the host plant. Separation from the plant can increase herbivore mortality in a number of ways. The excised leaf itself is probably a poorer nutritional source than an intact leaf (Haukioja and Niemeld 1977) especially in plants that can melanize, such as oaks. Since many herbivorous insects require more than one leaf for development, starvation may ensue unless the insect can relocate on the same or another suitable host plant. Herbivores detached from the host plant could be more susceptible to predation and fungal attack while on the ground. The size and density of the host plant could influence the ability of a detached folivore to relocate the host. Obviously, an insect displaced by leaf abscission is less likely to return to the foliage of a large, solitary oak than to any individual plant in a dense patch of herbs. From the plants perspective, leaf abscission can be hypothesized as a trade-off resulting in the conservation of resources. If a leaf is damaged sufficiently, water loss might become prohibitive or photosynthate loss might be greater than the photosynthetic capability of the undamaged portion; consequently, abscission should occur when losses exceed gains. Long-lived plants that produce many leaves over their lifespan could abscise damaged leaves with relative impunity, but the cost would be high for annual herbs with only a few leaves. For example, Orians and Solbrig (1977) have demonstrated that plants with ephemeral leaves must compensate for constructing short-lived leaves with higher ates of photosynthesis. Furthermore, if there were some damage threshold for abscising leaves, large leaves should be less likely to abscind than small leaves with the same absolute area damaged by herbivores. The effect of leaf abscission on mortality of folivores hould depend not only on plant characteristics, but also on the relative mobility of the folivore. Species whose feeding habits restrict hem to a single leaf or a few leaves, such as leaf miners or gall formers, should experience greater mortality as a result of leaf abscission than should species that can readily move to other leaves. Moreover, mortality as a result of early leaf abscission should be greater in the more sedentary developmental stages (egg, larva, pupa). Folivorous insects that can

Interspecific competition and species co-occurrence patterns on islands: null models and the evaluation of evidence

A tracking band-pass filter useful for locking onto, and passing, an input signal which changes rapidly in frequency. The filter has a frequency pass band controlled by a rectangular voltage waveform which is derived from the output signal passed by the filter. The filter output signal is translated from a sine wave to a pulse wave. The period between each two successive pulses is measured by counting the cycles of an oscillator during each respective period. The time-representing count is inverted to a frequency-representing count which controls the duty cycle of a rectangular wave used to make the filter track the changing frequency of the input signal.

Latitudinal Gradients in the Species Diversity of North American Mammals

Latitudinal gradients in species-diversity (=numbers of species) are well known. They usually consist of a fairly regular increase in the numbers of species of some higher taxon from the poles to the equator (equatorially-centered gradients, Stehli, 1968). However, latitudinal gradients are known in which maximum diversity does not occur at, or near, the equator (nonequatorially-centered gradients, Stehli, 1968). Various explanations for such gradients have been proposed (see Fischer, 1960; Pianka, 1966; Sanders, 1968, 1969), but none is very satisfactory. A few taxa, such as the fauna of sand beach (Dexter, 1972) and soft-bottom marine (Rosenburg, 1974) habitats, display no latitudinal gradient in species-diversity. Some instances can be attributed to the lack of an accompanying gradient in habitat diversity (Abele, 1974; Dexter, 1972), but others cannot. The analysis of the distribution of North American mammals by Wilson (1974) provides a recent example of the inadequacy of this explanation. Wilson divided the North American continent into 445 quadrats (Fig. 1; as did Simpson, 1964), tabulated the numbers of species of mammals within each quadrat, and plotted these numbers against latitude. Latitudinal trends were determined by fitting a regression line to the resulting cloud of points. Wilsons analysis showed an irregular increase in the numbers of species of quadrupedal mammals (excludes bats) per quadrat from the Arctic region to Panama. The numbers of species per quadrat in the tropics are nearly the same as in the temperate region. He dem-

Species–Area Relationships

A species–area relationship is simply the observation that the number of biological species found in a region is a positive function of the area in the region. Species–area relationships are depicted graphically as a bivariate plot of species richness on the ordinate and area on the abscissa, a species–area curve. Species–area relationships appear to be ubiquitous, which have been observed for a wide array of taxa ranging from diatoms to fish, insects, birds, vascular plants, and mammals and for geographical entities such as islands, political entities, woodland, grassland, and cropland habitat patches, lakes, river drainages, and artificial substrates from microscope slides to synthetic sponges and slates.