Species Abundance Distribution Models Research Articles

Large biodiversity datasets conform to Benford's law: Implications for assessing sampling heterogeneity

Inadequate sampling can cause biased estimates of species diversity, as species occurrence generally follows a log-normal distribution with a long tail. Understanding this sampling bias is fundamental to inform biodiversity conservation actions. However, currently available tests to assess data quality, such as fitting species abundance distribution (SAD) models and rarefaction curves are computationally costly and can still lead to erroneous conclusions.We evaluated Benford's law (first digit distribution) as a complementary method to assess data heterogeneity and survey coverage in large biodiversity datasets, including eBird data for 157 countries and three non-avian GBIF datasets. We also tested conformity to Benford's law of four simulated communities with different SAD models and four corrupted datasets with log-normal SAD. Finally, we evaluated the effect of including rare species in three datasets on the conformity to Benford's law and also compared Benford fit to the results of traditional methods to estimate survey completeness in seven datasets.Species-rich datasets with a large number of observations tended to obtain a good fit. Benford conformity can be a simple and sensitive measure of sampling evenness, complementing traditional methods to assess quality data in large-scale studies. Benford's test can reflect species abundance heterogeneity, especially in log-normally distributed data, but was not ideal to evaluate surveys completeness, as its results diverged from those of traditional methods.As the contribution of citizen science continues to increase in biodiversity monitoring, this fast and efficient method can play a critical role to assess the quality of datasets.

How Do Biological and Functional Diversity Change in Invaded Tropical Marine Rocky Reef Communities?

Evidence so far shows that most alien species (AS) have negative impacts on native biodiversity and are changing biodiversity in almost all environments. Here, we study eight rocky shores at four sites containing reefs with invaded communities and other not-invaded (control) communities, to evaluate the effects of four marine invasive species on biological and functional diversity. We used the adjustment and selection approach of species abundance distribution models (SAD), taxonomic diversity indices and functional diversity indices based on hierarchical grouping matrices (FD—Functional Diversity). In addition to comparing invaded and not-invaded communities, we also performed the same analysis, but removed the invaders (AS removed) from the matrices. The geometric-series model was best adjusted to the majority of communities. The diversity indices suggest that the taxonomic diversity is lower in invaded communities, while the functional diversity indices suggest a change in the functional space of invaded and not-invaded communities, with a greater amount of functional space filled by species in the not-invaded communities. Taxonomic and functional diversity indices were successful in identifying processes that determine the biological diversity of invaded communities, as they seem to obey a pattern that reflects the reduced diversity of invaded communities.

Passive restoration in Araucaria Forest: useful ecological indicators in monitoring successional advancement in exotic tree plantation landscapes

Monitoring successional advancement is a complex field involving a constant search for applied ecological indicators which facilitate monitoring of secondary forests for both active and passive restoration. In this study, the authors investigate the successional advancement of floristics and tree structure within Araucaria Forest (AF) fragments under passive restoration in a context where exotic tree plantations (mainly Pinus L. genus) dominate the landscape. The ecological indicators used were floristic dissimilarity (β‐diversity inferences), indicator species, ecological groups of species, basal area, and species abundance distribution (SAD) models (α‐diversity inferences). A total of 182 tree species belonging to 91 genera and 43 botanical families were identified. A high β diversity was verified for which each site has indicator species (for the locations CD—Dicksonia sellowiana; CO—Cryptocarya aschersoniana; and PG—Pinus taeda), where pioneer species contributed to much of the abundance. Different SAD models are useful for describing passive restoration sites in exotic tree plantation landscapes, namely Lognormal, Mandelbrot, and Zipf. SAD models together with basal area, taxonomic group (e.g. Myrtaceae assemblage), and tree abundance in ecological groups are strategic ecological indicators for monitoring successional advancement in AF.

<p><strong>Diversity, abundance and distribution of grasshopper species (Orthoptera: Acrididea) in three different types of vegetation with different levels of anthropogenic disturbances in the Littoral Region of Cameroon</strong></p>

Grasshoppers have been identified as excellent monitors of landscape use. Despite their importance, their composition and distribution in the highly disturbed Littoral Cameroon is still unknown. The aim of this study was to determine the effect of human activities on diversity, abundance and distribution of grasshopper species in the Littoral region of Cameron. We investigated three types of vegetation differing remarkably on the level of anthropogenic impact (farmlands, fallows and forests), using sweep netting. The eight non-parametric estimators for specific richness, abundance, α and β diversity indices and species abundance distribution models, were used to compare the structure of communities among vegetation. Overall, 38 species belonging to three families and ten subfamilies were recorded. The Acrididae was the most diverse family. The species richness, abundance and diversity were higher in farmlands than in fallows and in forests. Five species occurred exclusively in farmlands, one in fallows and four in forests. Eyprepocnemis plorans, Coryphosima stenoptera, Serpusia opacula were overall the most abundant species respectively in cultivated farms, fallows and forests. Species abundance distribution fitted the Motomura model in all sites. Serpusia opacula is considered as a useful indicator since its presence and abundance significantly depend on the rate of forest naturalness. The farmlands were characterized by short vegetation while the fallows and forests were dominated by tall grasses and tall trees respectively. Anthropogenic disturbances promote the species richness, diversity and abundance of open meadow species, while it is detrimental to forest species which are sensitive, specialized and have limited dispersal abilities. Key words: Grasshopper, diversity, abundance, distribution, bioindicator

Species abundance distribution models of Toona ciliata communities in Hubei Province, China

The study of plant species abundance distribution (SAD) in natural communities is of considerable importance to understand the processes and ecological rules of community assembly. With the distribution of tree, shrub and herb layers of eight natural communities of Toona ciliata as research targets, three different ecological niche models were used: broken stick model, overlapping niche model and niche preemption model, as well as three statistical models: log-series distribution model, log-normal distribution model and Weibull distribution model, to fit SAD of the different vegetation layers based on data collected. Goodness-of-fit was compared with Chi square test, Kolmogorov–Smirnov (K–S) test and Akaike Information Criterion (AIC). The results show: (1) based on the criteria of the lowest AIC value, Chi square value and K–S value with no significant difference (p > 0.05) between theoretic and observed SADs. The suitability and goodness-of-fit of the broken stick model was the best of three ecological niche models. The log-series distribution model did not accept the fitted results of most vegetation layers and had the lowest goodness-of-fit. The Weibull distribution model had the best goodness-of-fit for SADs. Overall, the statistical SADs performed better than the ecological ones. (2) T. ciliata was the dominant species in all the communities; species richness and diversity of herbs were the highest of the vegetation layers, while the diversities of the tree layers were slightly higher than the shrub layers; there were fewer common species and more rare species in the eight communities. The herb layers had the highest community evenness, followed by the shrub and the tree layers. Due to the complexity and habitat diversity of the different T. ciliata communities, comprehensive analyses of a variety of SADs and tests for optimal models together with management, are practical steps to enhance understanding of ecological processes and mechanisms of T. ciliata communities, to detect disturbances, and to facilitate biodiversity and species conservation.

Ecological effects of fire severity and time since fire on the diversity partitioning, composition and niche apportionment models of post-fire understory vegetation in semi-arid oak forests of Western Iran

Monitoring and evaluating changes of the various characteristics of ecosystems after disturbances are essential to protect the services provided by each ecosystem. In the present study, different ecological characteristics including structure, composition and diversity at different post-fire times (one, five and ten years) in low and high fire severities were investigated in semi-arid oak (Quercus brantii L.) forests of western Iran. One hundred twenty-six 1.5 × 1.5 m plots were sampled in 14 patches (12 burned and 2 unburned). Alpha and beta diversity indices as well as species abundance distribution models of ecological niche apportionment were produced. The results showed that the distribution pattern of species, composition and diversity were influenced by the severity and time since fire. Based on Tokeshi's niche apportionment models, the low severity and short post-fire time generated a pattern with higher heterogeneity (MacArthur fraction; MF) than high severity and short post-fire time (dominance decay; DD). In contrast, long-term effects were less pronounced. Non-metric multidimensional scaling (NMDS) showed that there are different patterns of vegetation composition in low and high fire severities. The plant composition of the studied areas at low severity but with a longer time period since fire was more similar to the control area. In contrast, in the high fire severity, unburned and burned plots with different times since fire did not occupy overlapping areas in the ordination space. Severe fires also increased the alpha diversity at all spatial scales whereas the beta diversity only increased at the largest scale. Change of plant diversity pattern with time was more homogenous in low fire severity and more patchy in high fire severity. We concluded that the changes in the structure, composition and diversity of the post-fire vegetation were influenced both by fire severity and time since fire and that restoration actions to promote vegetation recovery should be adapted accordingly.

Niche and Neutrality Work Differently in Microbial Communities in Fluidic and Non-fluidic Ecosystems.

This data-intensive study investigated the delicate balance of niche and neutrality underlying microbial communities in freshwater ecosystems through comprehensive application of high-throughput sequencing, species abundance distribution (SAD), and the neutral community model (NCM), combined with species diversity and phylogenetic measures, which unite the traditional and microbial ecology. On the genus level, 45.10% and 41.18% of the water samples could be explained by the log-normal and Volkov model respectively, among which 31.37% could fit both models. Meanwhile, 55.56% of the sediment samples could be depicted by the log-normal model, and Volkov-fitted samples comprised only 13.33%. Besides, operational taxonomic units (OTUs) from water samples fit Sloan's neutral model significantly better than those in sediment. Therefore, it was concluded that deterministic processes played a great role in both water and sediment ecosystems, whereas neutrality was much more involved in water assemblages than in non-fluidic sediment ecosystems. Secondly, log-normal fitted samples had lower phylogenetic species variability (PSV) than Volkov-fitted ones, indicating that niche-based communities were more phylogenetically clustered than neutrally assembled counterparts. Additionally, further testing showed that the relative richness of rare species was vital to SAD modeling, either niche-based or neutral, and communities containing fewer rare species were more easily captured by theoretical SAD models.

Testing species abundance distribution models in tropical forest successions: Implications for fine-scale passive restoration

Tropical forests are recognized worldwide for providing ecosystem services that contribute greatly to biodiversity and ecosystem functioning. In this study, we hypothesize that tree species abundance distribution is determined by the commonness-rarity balance of species during tropical forest recovery through natural regeneration. We sampled 45 20 m × 50 m plots across three sites in secondary forests with different stand ages (5, 10, 15, and 20 years) after shifting cultivation, and an old-growth forest (>100 years old) located in the Northern Amazon forest, Venezuela. We explored the variations in woody species abundance distribution and compositional similarity using a chronosequence approach. There was a clear pattern of increasing species abundance in forest successions, from second-growth to old-growth forests. Species abundance in the old-growth forest was much higher than in the second-growth forest. The niche-preemption model showed a high goodness-of-fit to the 5-year old second-growth forest, while 10- and 15-year old second-growth forests and the old-growth forest showed a significant fit to the lognormal model. Along the species-rank axis, the most common species accounted for 54% of the total proportional abundance at the earliest successional stage (5-year old), whereas the relative proportion of the most common species remained relatively stable at 22%–25% in the following successional stages (i.e., old-growth forest). Species dominance was lower in old-growth forest than those in second-growth forest, where there were seven species with the highest importance values within the community. There were obvious differences in relation to the old-growth forest, where rare species were present. The results we present here are crucial for designing and implementing proper, goal-oriented forest restoration strategies, and should aid future research.

Comparing Allee effect-based and dispersal-based neutral models for species abundance distribution patterns

Dispersal is important for biodiversity maintenance in both neutral and niche theories. However, little is known about the potential role of Allee effect at the community level. In the present study, we developed neutral models for quantifying the separate and joint influences of the Allee effect and dispersal process, respectively, on species abundance distribution (SAD) patterns. Tree census data from Barro Colorado Island (BCI), Panama were used as the case to compare different neutral SAD models. Results showed that Allee effects were not detected in the BCI tree SAD curve. By contrast, the neutral models with the incorporation of dispersal process (including both immigration and emigration) can remarkably improve the fitting power of neutral models on the BCI tree SAD curve. Finally, even though the influence is not detectable, the Allee effect-based SAD models still might be alternative SAD models for model comparison and null hypothesis testing.

Impact of disturbance on species diversity and composition of Cyperus conglomeratus plant community in southern Jeddah, Saudi Arabia

The changes in species diversity and composition of Cyperus conglomeratus plant community caused by anthropogenic disturbance were estimated and analyzed in four selected sites at southern Jeddah in Saudi Arabia. The ecological disturbances in the study area have their major effect on species richness, where about 76% of the perennial species in the study area were disappeared as the result of the human impact on C. conglomeratus community in the most disturbed sites. Interspecific competition results from C. conglomeratus indicated that it is a competitor species led to exclusion of the other species from the highly disturbed sites which affected by the Industrial City in Jeddah. Therefore, resulting in a remarkable decrease in species diversity (Shannon’s index = 0.664) coinciding with maximum dominance of C. conglomeratus (Simpson’s index = 0.656). The general trend of plant species richness and diversity in moderately disturbed sites in the study area showed remarkable increase in both richness (S = 16 species) and diversity (Shannon’s index = 1.792), coinciding with minimum dominance (Simpson’s index = 0.253). The resulting graphs of species abundance distribution models representing the highly disturbed sites tend toward geometric series pattern of species abundances, where one (C. conglomeratus) species dominate the community with the remainder fairly uncommon, assuming competitive exclusion and resource exhaustion in such degraded and harsh ecosystems.

Plant diversity and community structure of Brazilian Páramos

In Eastern South America, high altitude grasslands represent a mountain system that has a high number of endemic species. However, studies on the ecology of plant communities in these environments remain scarce. We aimed to evaluate the patterns of biodiversity and structure of plant communities from rocky outcrops in high altitude grasslands of three areas at the Caparao National Park, southeastern Brazil, by sampling 300 randomly distributed plots. Then, we compared the floristic composition, relative abundance, and biological and vegetation spectra among areas. We classified species as endemic and non-endemic and verified the occurrence of endangered species. Species richness was evaluated by rarefaction analysis on the sampling units. The importance value and species abundance distribution (SAD) models were assessed. We also performed an indicator species analysis. We sampled 58 species belonging to 49 genera and 32 families. The number of species decreased with increasing altitude, with significant differences being observed among areas regarding richness, abundance, and cover. Of the total number of species, 10 are endemic to the Caparao National Park and 17 are listed on the Brazilian Red List of endangered species. The dominant families on all peaks were Asteraceae and Poaceae. The SAD models showed lognormal and geometric distributions, corroborating the fact that 10 species that were common to all three areas were also the most dominant ones in the communities and showed the highest importance values, which ranged between 35% and 60%. Indicator species analysis revealed that 28 species (48.27%) were indicators. Of these, 42.85% had maximum specificity, meaning that they occurred only in one area. Thus, the number of species per life form ratio was similar among areas, yet vegetation spectra differed, especially for hemicryptophytes. The altimetric difference among the areas showed to be a very important driver in the community assembly, influencing the evaluated variables, however, other drivers as soil depth, slope and water could also influence the community structure on a smaller and local spatial scale.

Examining the patterns and dynamics of species abundance distributions in succession of forest communities by model selection.

There are a few common species and many rare species in a biological community or a multi-species collection in given space and time. This hollow distribution curve is called species abundance distribution (SAD). Few studies have examined the patterns and dynamics of SADs during the succession of forest communities by model selection. This study explored whether the communities in different successional stages followed different SAD models and whether there existed a best SAD model to reveal their intrinsic quantitative features of structure and dynamics in succession. The abundance (the number of individuals) of each vascular plant was surveyed by quadrat sampling method from the tree, shrub and herb layers in two typical communities (i.e., the evergreen needle- and broad-leaved mixed forest and the monsoon evergreen broad-leaved forest) in southern subtropical Dinghushan Biosphere Reserve, South China. The sites of two forest communities in different successional stages are both 1 ha in area. We collected seven widely representative SAD models with obviously different function forms and transformed them into the same octave (log2) scale. These models are simultaneously confronted with eight datasets from four layers of two communities, and their goodness-of-fits to the data were evaluated by the chi-squared test, the adjusted coefficient of determination and the information criteria. The results indicated that: (1) the logCauchy model followed all the datasets and was the best among seven models; (2) the fitness of each model to the data was not directly related to the successional stage of forest community; (3) according to the SAD curves predicted by the best model (i.e., the logCauchy), the proportion of rare species decreased but that of common ones increased in the upper layers with succession, while the reverse was true in the lower layers; and (4) the difference of the SADs increased between the upper and the lower layers with succession. We concluded that the logCauchy model had the widest applicability in describing the SADs, and could best mirror the SAD patterns and dynamics of communities and their different layers in the succession of forests. The logCauchy-modeled SADs can quantitatively guide the construction of ecological forests and the restoration of degraded vegetation.

Effect of man-made disturbances on understory plant richness of oak forests in Iran

Abstract The purpose of this study was to investigate the impacts of disturbance degree on the species richness and species diversity of oak forests in the Kurdistan Province (Iran). The study ran on three treatments: undisturbed, moderately disturbed, and highly disturbed. For each treatment, there were selected three forest patches with similar physiographic conditions and on each patch there were specified three 400 m2 plots for recording floristic data. The species’ name and the abundance of herbaceous species were recorded on 5 micro plots sized 1.5 × 1.5 m for each sample. Menhinick, Margalef, the number of species indices and also rarefaction, jackknife methods and species abundance distribution models were used to estimate the herbaceous species richness for the three treatments. The results showed that all the richness indices had the highest values for the undisturbed forests and the lowest values for the highly disturbed ones. Based on all models, undisturbed forests were showed more diverse than highly disturbed area. Generally the negative impact of human activities (wood cutting and grazing) on the herbaceous species richness was found much stronger in the highly disturbed forests and in moderately disturbed forests compared the undisturbed ones. With increasing destruction intensity, the herbaceous species richness in the oak forests decreased dramatically.

A general framework for predicting delayed responses of ecological communities to habitat loss

Although biodiversity crisis at different spatial scales has been well recognised, the phenomena of extinction debt and immigration credit at a crossing-scale context are, at best, unclear. Based on two community patterns, regional species abundance distribution (SAD) and spatial abundance distribution (SAAD), Kitzes and Harte (2015) presented a macroecological framework for predicting post-disturbance delayed extinction patterns in the entire ecological community. In this study, we further expand this basic framework to predict diverse time-lagged effects of habitat destruction on local communities. Specifically, our generalisation of KH’s model could address the questions that could not be answered previously: (1) How many species are subjected to delayed extinction in a local community when habitat is destructed in other areas? (2) How do rare or endemic species contribute to extinction debt or immigration credit of the local community? (3) How will species differ between two local areas? From the demonstrations using two SAD models (single-parameter lognormal and logseries), the predicted patterns of the debt, credit, and change in the fraction of unique species can vary, but with consistencies and depending on several factors. The general framework deepens the understanding of the theoretical effects of habitat loss on community dynamic patterns in local samples.

Impact of land‐use change on flower‐visiting insect communities on an oceanic island

Abstract Land‐use change has profoundly impacted pollinator communities throughout the world. However, the processes through which it acts on pollinator diversity and composition are still poorly understood, especially in highly vulnerable island ecosystems. In this study, we investigated the distribution, abundance, richness and composition of flower‐visiting insects to assess their response to land‐use change in Terceira Island (Azores). Flower‐visiting insects were sampled over 2 years using a standardised protocol along 50 transects across five different habitats corresponding to a land‐use gradient. Insect species were classified as indigenous or exotics. We assessed changes across habitats using multiple diversity indices, species abundance distribution models (SAD) and species composition metrics (β‐diversity), along with plant species composition. We observed that indigenous flower‐visiting insects were dominant, both in abundance and species richness, across the entire land‐use gradient. Species diversity varied only slightly across the gradient. SADs were lognormal in all habitats, with very few truly common and rare flower‐visiting insects and a prevalence of species of intermediate abundance. Species replacement was significantly higher mainly between the two most contrasting habitats (i.e. natural forests and intensive pastures) but was significantly correlated with species replacement of host plant species across the gradient. Our results revealed that the Azorean flower‐visiting insect communities were highly simplified across the entire gradient with little difference between habitats. In the absence of strong exotic competitors, indigenous flower‐visiting insects expand their range and occupy new anthropogenic habitats, also facilitating the expansion of a large number of exotic plant species.

An extensive comparison of species-abundance distribution models.

A number of different models have been proposed as descriptions of the species-abundance distribution (SAD). Most evaluations of these models use only one or two models, focus on only a single ecosystem or taxonomic group, or fail to use appropriate statistical methods. We use likelihood and AIC to compare the fit of four of the most widely used models to data on over 16,000 communities from a diverse array of taxonomic groups and ecosystems. Across all datasets combined the log-series, Poisson lognormal, and negative binomial all yield similar overall fits to the data. Therefore, when correcting for differences in the number of parameters the log-series generally provides the best fit to data. Within individual datasets some other distributions performed nearly as well as the log-series even after correcting for the number of parameters. The Zipf distribution is generally a poor characterization of the SAD.

Analyzing fractal property of species abundance distribution and diversity indexes

Community diversity is usually characterized by numerical indexes; however it indeed depends on the species abundance distribution (SAD). Diversity indexes and SAD are based on the same information but treating as separate themes. Ranking species abundance from largest to smallest, the decreasing pattern can give the information about the SAD. Frontier proposed such SAD might be a fractal structure, and first applied the Zipf–Mandelbrot model to the SAD study. However, this model fails to include the Zipf model, and also fails to ensure an integer rank. In this study, a fractal model of SAD was reconstructed, and tested with 104 community samples from 8 taxonomic groups. The results show that there was a good fit of the presented model. Fractal parameter (p) determines the SAD of a community. The ecological significance of p relates to the “dominance” of a community. The correlation between p and classical diversity indexes show that Shannon index decreases and Simpson index increases as p increases. The main purpose of this paper is not to compare with other SADs models; it simply provides a new interpretation of SAD model construction, and preliminarily integrates diversity indexes and SAD model into a broader perspective of community diversity.

Using species abundance distribution models and diversity indices for biogeographical analyses

We examine whether Species Abundance Distribution models (SADs) and diversity indices can describe how species colonization status influences species community assembly on oceanic islands. Our hypothesis is that, because of the lack of source-sink dynamics at the archipelago scale, Single Island Endemics (SIEs), i.e. endemic species restricted to only one island, should be represented by few rare species and consequently have abundance patterns that differ from those of more widespread species. To test our hypothesis, we used arthropod data from the Azorean archipelago (North Atlantic). We divided the species into three colonization categories: SIEs, archipelagic endemics (AZEs, present in at least two islands) and native non-endemics (NATs). For each category, we modelled rank-abundance plots using both the geometric series and the Gambin model, a measure of distributional amplitude. We also calculated Shannon entropy and Buzas and Gibson's evenness. We show that the slopes of the regression lines modelling SADs were significantly higher for SIEs, which indicates a relative predominance of a few highly abundant species and a lack of rare species, which also depresses diversity indices. This may be a consequence of two factors: (i) some forest specialist SIEs may be at advantage over other, less adapted species; (ii) the entire populations of SIEs are by definition concentrated on a single island, without possibility for inter-island source-sink dynamics; hence all populations must have a minimum number of individuals to survive natural, often unpredictable, fluctuations. These findings are supported by higher values of the α parameter of the Gambin mode for SIEs. In contrast, AZEs and NATs had lower regression slopes, lower α but higher diversity indices, resulting from their widespread distribution over several islands. We conclude that these differences in the SAD models and diversity indices demonstrate that the study of these metrics is useful for biogeographical purposes.

Analysing and modelling the impact of habitat fragmentation on species diversity: a macroecological perspective

My research aimed to examine a variety of macroecological and biogeographical patterns using a large number of purely habitat island datasets (i.e. isolated patches of natural habitat set within in a matrix of human land uses) sourced from both the literature and my own sampling, with the objective of testing various macroecological and biogeographical patterns. These patterns can be grouped under four broad headings: 1) species–area relationships (SAR), 2) nestedness, 3) species abundance distributions (SADs) and 4) species incidence functions (function of area). Overall, I found that there were few hard macroecological generalities that hold in all cases across habitat island systems. This is because most habitat island systems are highly disturbed environments, with a variety of confounding variables and ‘undesirable’ species (e.g. species associated with human land uses) acting to modulate the patterns of interest. Nonetheless, some clear patterns did emerge. For example, the power model was by the far the best general SAR model for habitat islands. The slope of the island species–area relationship (ISAR) was related to the matrix type surrounding archipelagos, such that habitat island ISARs were shallower than true island ISARs. Significant compositional and functional nestedness was rare in habitat island datasets, although island area was seemingly responsible for what nestedness was observed. Species abundance distribution models were found to provide useful information for conservation in fragmented landscapes, but the presence of undesirable species substantially affected the shape of the SAD. In conclusion, I found that the application of theory derived from the study of true islands, to habitat island systems, is inappropriate as it fails to incorporate factors that are unique to habitat islands.&nbsp

Analysing and modelling the impact of habitat fragmentation on species diversity: a macroecological perspective

My research aimed to examine a variety of macroecological and biogeographical patterns using a large number of purely habitat island datasets (i.e. isolated patches of natural habitat set within in a matrix of human land uses) sourced from both the literature and my own sampling, with the objective of testing various macroecological and biogeographical patterns. These patterns can be grouped under four broad headings: 1) species–area relationships (SAR), 2) nestedness, 3) species abundance distributions (SADs) and 4) species incidence functions (function of area). Overall, I found that there were few hard macroecological generalities that hold in all cases across habitat island systems. This is because most habitat island systems are highly disturbed environments, with a variety of confounding variables and ‘undesirable’ species (e.g. species associated with human land uses) acting to modulate the patterns of interest. Nonetheless, some clear patterns did emerge. For example, the power model was by the far the best general SAR model for habitat islands. The slope of the island species–area relationship (ISAR) was related to the matrix type surrounding archipelagos, such that habitat island ISARs were shallower than true island ISARs. Significant compositional and functional nestedness was rare in habitat island datasets, although island area was seemingly responsible for what nestedness was observed. Species abundance distribution models were found to provide useful information for conservation in fragmented landscapes, but the presence of undesirable species substantially affected the shape of the SAD. In conclusion, I found that the application of theory derived from the study of true islands, to habitat island systems, is inappropriate as it fails to incorporate factors that are unique to habitat islands.