Density-area Relationship Research Articles

Modeling the density–area relationship in a dynamic landscape: an examination for the beetle Tetraopes tetraophthalmus and a generalized model

The relationship between local population density and habitat area is an important factor in spatial population ecology. I examined how density dependence in the growth of local insect populations and their host plant patches, combined with patch birth and death, and insect dispersal affect the density–area relationship. I constructed a simulation model to examine the relationship for an insect herbivore, Tetraopes tetraophthalmus, inhabiting patches of its host plant, Asclepias syriaca. Given the observed growth of insect populations, patch growth, dispersal of insects, and change in the number of patches within the landscape, the model predicts that T. tetraophthalmus density should decrease with increasing A. syriaca patch size. The model also predicts moderate amounts of temporal variation in the relationship. A more general insect herbivore-host plant model was also developed to extend the results. The general model shows that density dependence in patch and insect population growth rates have large effects on the density–area relationship. The density–area relationship was strongly affected by density dependence in insect growth. Increasing density dependence in insect growth caused insect density to decrease with increasing patch size. Temporal variation in the relationship was most strongly affected by density dependence in patch growth. Variation in the density–area relationship increased as density dependence in patch growth increased. The results of this study show that density–area relationships can be variable and are not necessarily a species-specific trait. Application of density–area relationships, especially in dynamic landscapes, need to be aware of and account for factors that affect the size and number of patches as well as growth and dispersal of the target populations.

The importance of the relationship between population density and habitat area

The relationship between population density and habitat size, the density-area relationship, has received considerable, but often sporadic and unfocused attention (Root 1973, Risch 1981, Kareiva 1983, Bach 1988, Matter 1997). Recent reviews of the topic have revealed that there is a great deal of variability in the relationship with species showing patterns of increasing, decreasing, and constant density with habitat size (Bowers and Matter 1997, Bender et al. 1998, Connor et al. 2000). Despite this considerable variability, the overall trend across species appears to be increasing density with habitat size, especially for insects and birds (Bender et al. 1998, Connor et al. 2000; but see Gaston et al. 1999). Mammals show fairly constant densities with habitat area with about as many positive as negative density-area relationships (Bowers and Matter 1997, Connor et al. 2000). A variety of different mechanisms have been offered to account for density-area relationships. Root (1973) explained the positive density-area relationship seen for herbivorous insects in the 'resource concentration hypothesis'. He proposed that density would increase with area via dispersal if emigration rates were greater for smaller, and immigration rates were greater for larger patches (Root 1973). Subsequently, predatory rates (Risch 1981), habitat quality (Hanski 1994, Matter 1997), and social interactions (Bowers and Matter 1997, Bowers and Dooley 1999) that vary with patch size have been proposed to account for both positive and negative density-area relationships. Bowers and Matter (1997) proposed that habitat selection at small spatial scales may produce negative density-area relationships while positive relationships may arise at larger scales through colonization-extinction dynamics. Density-area relationships may also arise from methodological problems such as the mis-estimation of patch size for edge and interior species (Bender et al. 1998) or increasing amounts of non-habitat with census area (Smallwood and Schonewald 1996, Gaston et al. 1999). Gaston et al. (1999) detail many of the problems that can occur when estimating density-area relationships; however, many of the statistical problems discussed can be avoided through the use of alternative methods. Gaston et al. (1999) point out that for log density-log area plots the minimum allowable value is 1/area, which may produce a non-zero expected slope as well as heteroscedasticity. These statistical difficulties can be overcome by using an area-sensitive Poisson model (Feller 1968, Connor et al. 1997, Matter 1997). Here the expected abundance of organisms at a site is simply the mean density across all sites multiplied by site area raised to a power. This model can be fit with and without the exponent using generalized linear models to determine if density varies with area. The model has the advantage of having the appropriate variance structure and allows for the inclusion of inhabitable sites with zero density (Matter 1997). Despite research describing patterns of density with habitat area and attempting to attribute mechanisms responsible for it, researchers have yet to appreciate the importance of density-area relationships in a wider context. Similar to patterns of aggregation, density-area patterns influence a variety of ecological processes. Here, I illustrate how the density-area relationship of individual species is of fundamental importance for spatial population and community ecology and is pertinent to conservation issues.

Population density and area: the role of within- and between-generation processes over time

The density–area relationship has received considerable attention recently. A limitation of empirical examinations of the density–area relationship is that they have been conducted over short time spans, focusing on within-generation mechanisms such as dispersal and ignoring potential between-generation effects. I used simulation modeling to investigate the effects of both within- and between-generation factors that vary with patch size on the density–area relationship. In general, between-generation factors had a greater effect on long-term density–area patterns than within-generation processes, but within-generation processes are important under certain conditions. Interactions between within- and between-generation processes produced several non-intuitive results. The results of this study demonstrate that to fully understand density–area patterns both within- and between-generation patterns must be investigated.

Density, Survey Area, and the Perfection (Or Otherwise) of Ecologists

Species are not all equally abundant. Attempts to understand why have frequently involved comparative studies of a range of species. with the aim of identifying associated variation in other traits. While a number of characteristics have been linked with rare-common differences among species. the most frequently cited association is with body size. Specifically, a negative interspecific relationship has been observed between body size and population density (both logarithmically transformed) in a number of animal assemblages. Several explanations for this pattern exist, although most studies consider only that based on the amount of energy available to species of different body sizes. Unfortunately, the interpretation of comparisons between population densities is complicated by a negative relationship between densities and the size of the study areas over which they have been calculated. Population studies carried out over small areas tend to generate high density estimates. while estimates from large areas tend to be low. This density-area relationship implies that interspecific density- body size relationships may be confounded by differences in survey area among studies from which density estimates are compiled. Two lines of evidence support this argument. First, when density estimates derive from different studies. the survey area of the study is a better predictor of the density of a species than is its body size. Second, density and body size seldom show a simple negative relationship when the densities of all species are estimated from a single survey area. The density-area relationship is a problem for comparative studies because it is unclear to what extent density differences among species are genuine and derive from biological differences. or are artefacts of differences in survey effort and methodology at different spatial scales.

Does variation in census area confound density comparisons?

1. Estimates of density are not always independent of the area over which populations are censused. Instead, lower densities tend to be recorded for species when they are censused over larger areas. This may have serious implications for both intra‐ and interspecific comparisons of density, with relevance to such issues as conservation prioritization and management strategies, because differences in density may simply result from differences in census areas. 2. Here, we use long‐term population census data for British birds in the period 1968–91, drawn from the Common Birds Census (CBC), to examine relationships between density and census area within species, and the consequences of these relationships for intraspecific and interspecific density comparisons. 3. Most British bird species exhibit statistically significant negative relationships between density and census area. We used these relationships to standardize mean density estimates for all species to a common census area. These area‐adjusted estimates were usually very similar to geometric mean density estimates calculated without reference to census area. 4. For a subset of species recorded from a large number (> 30) of census sites in each year in the period 1968–91, we used intraspecific density–area relationships for each year to standardize mean density estimates to a common census area in all years. Again, the area‐adjusted estimates for each year were usually very similar to the simple geometric mean density estimates calculated for the species in the same year. 5. These results are encouraging, but are certainly a consequence of the relatively limited range of census areas used here, and the fact that the mean census area varies little across species, or across years within species. Moreover, those species occupying few sites are the most likely to have area‐biased densities, but are the species for which area‐correction will be most difficult. 6. Overall, the results suggest that past analyses conducted using the CBC data are unlikely to have been seriously confounded by variation in census area. Nevertheless, they do highlight that the effects of census area will require consideration by anyone planning to measure or use densities for comparative purposes either within or among species.

Landscape Ecology of Mammals: Relationships between Density and Patch Size

A much discussed issue in landscape ecology is how processes that operate within spatially subdivided subpopulations scale-up to create a larger, landscape-level dynamic. A first step in answering this question is to ask to what degree subpopulations within a landscape vary in performance. Here we test the null hypothesis that densities of mammalian populations are constant over patches of varied size, i.e., that performance, as estimated via density, does not covary with patch area. Using a composite database from published studies, we found that densities of 20 of 32 species did not vary with patch area, while five showed increasing and seven decreasing density-area relationships. Studies reporting significant density-area relationships tended to include a greater number of patches of a greater range of sizes than those that reported no relationship, suggesting that statistical power may be an issue. Landscapes comprised of smaller, less-isolated patches tended to have negative density-area relationships and landscapes with larger, more isolated patches tended to have positive density-area relationships. Our results suggest that no consistent density-area relationship operates over all systems of patches. Instead, the patterns appear to be scale-dependent: frequent movement of individuals in the process of selecting habitats (patches) over smaller-scaled landscapes produced negative density-area relationships; movement of individuals among more isolated patches appeared to involve larger- and longer-scale population processes involving colonization and extinction and positive density-area relationships. Despite the fact that patches represent a central focus in landscape ecology, they appear to be a construct of human convenience rather than biological entities with a set number and kind of processes.

Density-area relationship of the pine moth (Dendrolimus spectabilis BUTLER) on pine trees on a roadside slope

Density-area relationship of the pine moth (<i>Dendrolimus spectabilis</i> BUTLER) on pine trees on a roadside slope

Mortality Rates of Insular Anolis Lizards: A Systematic Effect of Island Area?

Previous articleNext article No AccessNotes and CommentsMortality Rates of Insular Anolis Lizards: A Systematic Effect of Island Area?S. Joseph Wright, Robert Kimsey, and Claudia J. CampbellS. Joseph Wright Search for more articles by this author , Robert Kimsey Search for more articles by this author , and Claudia J. Campbell Search for more articles by this author PDFPDF PLUS Add to favoritesDownload CitationTrack CitationsPermissionsReprints Share onFacebookTwitterLinkedInRedditEmail SectionsMoreDetailsFiguresReferencesCited by The American Naturalist Volume 123, Number 1Jan., 1984 Published for The American Society of Naturalists Article DOIhttps://doi.org/10.1086/284192 Views: 7Total views on this site Citations: 7Citations are reported from Crossref Copyright 1983 The University of ChicagoPDF download Crossref reports the following articles citing this article:Yuval Itescu, Rachel Schwarz, Shai Meiri, Panayiotis Pafilis, Sonya Clegg Intraspecific competition, not predation, drives lizard tail loss on islands, Journal of Animal Ecology 86, no.11 (Oct 2016): 66–74.https://doi.org/10.1111/1365-2656.12591Héctor Hugo Siliceo-Cantero, Andrés García Differences in growth rate, body condition, habitat use and food availability between island and mainland lizard populations of Anolis nebulosus in Jalisco, Mexico, Journal of Tropical Ecology 30, no.55 (Jun 2014): 493–501.https://doi.org/10.1017/S0266467414000297Gabriel Pinto, D. Luke Mahler, Luke J Harmon, Jonathan B Losos Testing the island effect in adaptive radiation: rates and patterns of morphological diversification in Caribbean and mainland Anolis lizards, Proceedings of the Royal Society B: Biological Sciences 275, no.16521652 (Aug 2008): 2749–2757.https://doi.org/10.1098/rspb.2008.0686Lauren B. Buckley, Joan Roughgarden A hump-shaped density-area relationship for island lizards, Oikos 113, no.22 (Feb 2006): 243–250.https://doi.org/10.1111/j.2006.0030-1299.14401.xRobert B. Kimsey Host association and the capacity of sand flies as vectors of lizard malaria in Panama, International Journal for Parasitology 22, no.55 (Aug 1992): 657–664.https://doi.org/10.1016/0020-7519(92)90015-DRobin M. Andrews Demographic correlates of variable egg survival for a tropical lizard, Oecologia 76, no.33 (Aug 1988): 376–382.https://doi.org/10.1007/BF00377032 J. A. Stamps , and M. Buechner The Territorial Defense Hypothesis and the Ecology of Insular Vertebrates, The Quarterly Review of Biology 60, no.22 (Oct 2015): 155–181.https://doi.org/10.1086/414314

Effect of Harvest Management and Temperature on Forage Yield, Root Carbohydrates, Plant Density and Leaf Area Relationships in Alfalfa ( Medicago sativa L. cultivar ' Moapa ') 1

Stage of growth at harvest had a greater influence on forage production and plant density of alfalfa (Medicago sativa L.) than stubble height remaining after harvest when plants were grown under field conditions at Tucson, Arizona. Decline in forage yield and plant population was greatest during the period when night temperatures were highest. The percentages of A-HC (acid-hydrolyzable carbohydrates) in roots of alfalfa plants harvested at the 50% bud stage of growth declined to a lower level and remained less than when harvested at the 25% bloom stage. Generally, LAI (leaf area index) for plants harvested at the 25% bloom stage was higher than when harvested at the 50% bud stage. Plant density declined greatest during the second year and when plants were harvested at the 50% bud stage of growth.