Oyomoare Osazuwa-Peters
October 12, 2011
Lost in Space (Graduate Seminar)
BIOTIC HOMOGENIZATION
Introduction
Biotic homogenization (BH) is the process of increasing spatial similarity of a particular
biological variable over time (Olden et al. 2004). Historically, initial recognition of the BH
concept has been attributed to Charles Elton in his treatise that described the collapse of
Wallace’s faunal realms due to global commerce (Olden & Poff 2004). The study of BH has
accelerated in the last two decades owing to the realization that BH is an important component of
the biodiversity crisis inextricably linked to anthropogenic factors, with the potential to result in
the loss of biological distinctness across landscapes on a global scale. The perceived severity of
the BH process and gravity of its predicted consequences is reflected in tags like ‘the
Homogecene era’, ‘Planet of weeds’ and ‘Geography of nowhere’.
Progress and synthesis in the sub-field in Ecology that studies BH has been limited because the
BH concept has historically been plagued by misconceptions, a lack of an operational definition
for the concept, and a lack of consensus on appropriate methods to quantify BH. For example,
some published reports equated the loss of species diversity in local communities to BH, others
assumed BH was the only consequence of human induced species invasions and extinctions, and
many ignored the temporal component required for establishing BH. Also, because BH is
sensitive to spatial scale, there seem to have been contradictory results for the same taxa when
BH is considered at the local, regional and global scale, thus requiring caution in the application
of results. However, significant contributions by researchers in the field have helped to realign
and focus studies in this area. It is now clear that BH is a multifaceted concept with genetic,
taxonomic and functional components that can either occur in concert or independently; although
most studies have focused on quantifying changes in taxonomic similarity across space and/or
time. Moreover, the outcome of species invasions and/or extinctions is not limited to BH, but
biotic differentiation can also result, depending on the identity of the species gained or lost from
the system. Further contributions from this field have shown that BH can only be meaningfully
quantified by estimating degree of change in similarities of communities across space and time.
Most studies have neglected the temporal component and many have focused on species
invasions as the causal factor for BH. It is only recently that a few studies have looked at
situations in which BH may result due to non-random extinctions that favor the spread of habitat
generalists over habitat specialists. Also, others have shown that BH is not exclusive to species
invasions and/or extinctions, but can result from changing patterns in the abundance of native
generalist species. Below is a description of 16 papers from this field. Overall, these papers
demonstrate that there is an alignment towards better understanding of BH, its ecological and
evolutionary consequences in the face of climate change, using a variety of approaches including
modeling , going beyond taxonomic homogenization to higher order ecological effects as food
webs, focusing on a wide diversity of taxa (birds, amphibians, plants, fish, insects, parasitoids)
and causal factors (e.g. diseases), and accounting for the temporal and spatial aspects of BH.
Oyomoare Osazuwa-Peters
October 12, 2011
Lost in Space (Graduate Seminar)
1. Olden, J.D. & Rooney, T.P. (2006). On defining and quantifying biotic homogenization.
Global Ecology and Biogeography, 15, 113-120.
This is a review of the literature on BH that aims to clarify misconceptions about the BH concept
and provide a working definition that encompasses the different biological levels that comprise
varying dimensions of BH. Further, these authors use the literature to highlight the strengths and
weaknesses of the various methods quantifying BH and proffer recommendations for progress in
BH research. They show that the main focus of previous research on BH has been on taxonomic
homogenization. Using simulations, they show that taxonomic homogenization is not necessarily
preceded by the combination of species invasions and extinctions, neither is taxonomic
homogenization equivalent to changes in species diversity alone. This work is important because
it synthesizes current knowledge on BH, identifies shortfalls in order to realign research, and
makes constructive and practical recommendations for moving the field forward.
2. Rooney, T.P., Olden, J.D., Leach, M.K. & Rogers, D.A. (2007). Biotic homogenization and
conservation prioritization. Biological Conservation, 134, 447-450.
These authors review the double edged potential of the BH concept to provide insight to
conservation strategies when applied sensibly, but lead to misleading conclusions when
inappropriately applied. They draw evidence from the literature and use deductive hypothetical
examples. This paper demonstrates that BH and biotic differentiation can both provide evidence
of conservation failures. Further, it emphasizes change in species composition across spatial and
temporal scales and not species richness as the key to detecting BH patterns. Most importantly,
the review highlights the need for researchers to choose the appropriate scale to study BH, for
their results to be relevant to conservation. This paper makes valuable contribution to correct
wrong and misleading applications of the BH concept to conservation issues.
3. Clavel, J., Julliard, R. & Devictor, V. (2011). Worldwide decline of specialist species: toward
a global functional homogenization? Frontiers in Ecology and the Environment, 9, 222-228.
This paper reviews the concept of species specialization against the framework of ecological and
evolutionary theories (e.g the ecological niche concept), in a bid to understand the functional
component of BH and its consequence for ecosystem functioning. They used a wide array of
published research that focused on measuring BH, past and recent records of the decline of
specialist species, and impact of generalist and specialist species on the ecosystem. This paper
mainly establishes the sensitivity of specialist species to past and current global change to the
benefit of generalist expansion. The paper emphasizes functional homogenization (FH) as the
most important component of BH because of its effect on ecosystem processes mediated through
species traits. Thus, the authors come up with a measurement of FH and recommend its use as a
bioindicator. The paper is a good review that clearly outlines underlying conceptual basis for
BH. It is a good synthesis of work done so far in the field and makes a practical contribution to
the progress of the field by generating a simple and mathematically sound index for measuring
Oyomoare Osazuwa-Peters
October 12, 2011
Lost in Space (Graduate Seminar)
FH. The weakness of the paper is the neglect of the temporal aspect of homogenization. True,
there’s currently a negative relationship between specialist abundance and habitat degradation,
but what was this relationship like in the past when? Therefore, a reference point in time should
have been incorporated.
4. Abadie, J., Machon, N., Muratet, A. & Porcher, E. (2011). Landscape disturbance causes
small‐scale functional homogenization, but limited taxonomic homogenization, in plant
communities. Journal of Ecology, 99, 1134-1142.
The authors investigated the strength of the homogenizing effect of spatial and temporal
components of landscape disturbance on taxonomic and functional diversity in plant
communities on relatively fine scales. Landscape disturbance was spatially quantified as edge
density, an indicator of habitat fragmentation, and was temporally quantified as habitat
conversion based on extent to which the habitat had been modified in 20 years. Taxonomic
diversity was quantified as, and  diversity while functional diversity was computed using a
community specialization measure. Their results provide evidence of FH based on the increase of
generalist species at the expense of specialist species. While there was no evidence for
taxonomic homogenization because  and  increased with fragmentation and habitat
conversion. This is a fine contribution to the field because the authors explicitly explore the issue
of scale using 8 circular 10 m2 plots nested within 50 1 km2 randomly selected squares within a
region in France. This is important because most other studies are conducted at a regional or
continental scale. The paper accounts for taxonomic and functional components of BH across
spatial and temporal scales. Further, this work demonstrates that functional and taxonomic
homogenizations are not always congruent, and the importance of drivers of BH such as invasion
changes with the scale at which it is considered.
5. Feurdean, A., Willis, K.J., Parr, C.L., Tanţău, I. & Fărcaş, S. (2010). Post‐glacial patterns in
vegetation dynamics in Romania: homogenization or differentiation? Journal of
Biogeography, 37, 2197-2208.
These authors go back 11000 years before present to explore vegetation similarity across regions
and elevations in Romania in search for distinct intervals of homogenization or differentiation
and to isolate potential causal factors. Data used were eight fossil pollen records consisting of
pollen counts, broken into 250 years intervals and representing a range of low to medium to high
elevation sites. Using measures of similarity as Bray-Curtis similarity analysis and Principal
Components Analysis, the paper shows that over time there were changes in similarity among
sites, with alternate periods of homogenization, differentiation and some periods of stability.
They report ten significant periods of differentiation, particularly the last 250 years. While there
were eight periods of significant homogenization, most recent of this was 1000 years BP. The
two elevation categories showed distinct patterns of similarity over time. This work is
controversially important because it demonstrates that BH can result primarily from natural
causes as biotic interactions, while biotic differentiation is an important consequence of
Oyomoare Osazuwa-Peters
October 12, 2011
Lost in Space (Graduate Seminar)
anthropogenic activities. The paper expressly establishes the fact that patterns obtained in
vegetation homogenization assessment studies depend on specific time period and the length of
time interval covered by the study. So that previous reports of BH in recent times can be
explained as representing single snapshots in time. The strength of this paper is in its explicit
accounting for the temporal component of BH or biotic differentiation, which is largely ignored
in much of the literature.
6. Shiganova, T. (2010). Biotic Homogenization of Inland Seas of the Ponto-Caspian. Annual
Review of Ecology, Evolution, and Systematics, 41, 103-125.
This is a review paper in which the author traces through time the origin, history and impact of
species invasions to three southern inland Eurasian seas (Black Sea, Sea of Azov, and Caspian
Sea) that comprise the Ponto-Caspian. Shiganova (2010) makes extensive use of published
reports to account for the composition, time, source and pathways of invasion and uses
comparative analytical methods to identify important timelines and major drivers. This paper
implicates international shipping as a major culprit, introducing non native species to the Black
Sea, which in turn acts as a donor to the other Eurasian seas. The consequence has been a decline
in native species and concomitant increases in non-native invaders of the Ponto-Caspian. Thus,
BH of the Ponto-Caspian is taking place because the same invaders, especially gelatinous
species, shape community structure and functioning of the different Eurasian Seas. This
contribution is valuable due to its extensiveness, the time frame it covers (from 1900s), and its
focus on low diversity but highly endemic aquatic systems. The main weakness of this paper is
that the comparative analysis is basically descriptive, and there no statistical tests of change in
community similarity across the Ponto-Caspian. This makes the conclusion of BH circumstantial
at best, and the review paper reads more like a narrative.
7. Laliberté, E. & Tylianakis, J.M. (2010). Deforestation homogenizes tropical parasitoid–host
networks. Ecology, 91, 1740-1747.
The main question in Laliberte and Tylianakis (2010) is whether parasitoid-host networks can
become homogenized with increasing habitat specialization. They used data on 48 parasitoid
host networks representing spatio-temporal replicates across a gradient of habitat simplification
defined by 5 distinct habitats (forests, abandoned agroforests, managed agroforests, pasture and
rice fields). This work demonstrates a significant homogenization of interaction composition
with deforestation, such that rice and pasture sites were more similar relative to forested habitats.
This homogenization was attributed to altered consumer foraging success because of the ease
with which parasitoids locate hosts in simpler deforested habitats. The strength of this work is
the combined spatio-temporal approach, and its focus on an unexplored but important aspect of
BH, namely host parasitoid networks. However, besides the inference made on mean body size,
there is little evidence for the explanation of reduced parasitoid success in complex habitats. This
work takes the study of BH beyond single trophic levels to food webs. It relates the study of BH
to conservation as it demonstrates an important role for tropical agroforestry systems in
inhibiting BH of interaction networks.
Oyomoare Osazuwa-Peters
October 12, 2011
Lost in Space (Graduate Seminar)
8. Smith, K.G., Lips, K.R. & Chase, J.M. (2009). Selecting for extinction: nonrandom diseaseassociated extinction homogenizes amphibian biotas.
These authors use a null model approach to examine the homogenizing effect that results from
the selectivity of extinctions associated with the spread of a novel fungal pathogen on amphibian
species. Data used comprised of variously compiled species list of amphibians in eight different
sites across Central America, before and after declines associated with epizootic diseases caused
by outbreaks of Batrachochytrium dendrobatidis (Bd), a pathogenic fungus. They found that Bd
associated extinctions were non-random and have resulted in decreased  diversity among sites,
and compositional homogenization of regional amphibian diversity. This taxonomic
homogenization has also corresponded to a phylogenetic and functional homogenization due to
the high extinction of endemic species. The focus of this paper is novel because it deals with
disease associated extinctions as a driver of BH at different levels of diversity, whereas most
studies focus on species invasions as drivers of BH. The paper describes statistical methods in a
clear replicable fashion and uses simple graphics that send powerful messages.
9. Ekroos, J., Heliöl, J. & Kuussaari, M. (2010). Homogenization of lepidopteran communities in
intensively cultivated agricultural landscapes. Journal of Applied Ecology, 47, 459-467.
The authors address the lack of empirical evidence for BH by comparing the response of
buttereflies and geometrid moths across 134 differently fragmented landscapes in terms of
taxonomic diversity and ecological traits of species. They found a negative relationship between
agricultural intensity and  and  diversity. Also, there was an increase in the proportion of
generalist species and the average mobility of butterfly communities with increasing arable
cover. Lastly there was a pattern of decreasing  diversity associated with greater percentage of
habitat generalists and mobility only in high agricultural intensity areas. The strength of this
work is in the spatial scale (paired plots of 0.25 km2) considered, being adequate to capture the
effect of landscape heterogeneity and the range of the taxa (butterflies) of interest. However, its
major weakness is a total neglect of time, which makes conclusions of BH across space
inappropriate since the data represents a single snapshot in time with no previous reference point.
Nevertheless, this work is relevant to understanding BH by showing that agricultural
intensification can lead to the loss of habitat specialists and poor dispersers, potentially
homogenizing communities across space.
10. Smart, S.M., Thompson, K., Marrs, R.H., Le Duc, M.G., Maskell, L.C. & Firbank, L.G.
(2006). Biotic homogenization and changes in species diversity across human-modified
ecosystems. Proceedings of the Royal Society B: Biological Sciences, 273, 2659 -2665.
This paper tests for BH in species composition and plant functional traits using a national
ecological surveillance data for Great Britain. The data covered 1572 fixed plots nested within
238 1 km2 randomly sampled squares and consisted of species richness data for two time points
(1978, 1998). Traits selected were related to either the regeneration phase (dispersal vectors, and
seed bank longevity) or establishment phase (specific leaf area, canopy height) of plant growth.
Oyomoare Osazuwa-Peters
October 12, 2011
Lost in Space (Graduate Seminar)
They used measures of similarity such as Jaccard coefficient for species composition and Gini
coefficient for traits to estimate change in similarity across space and time. Their results showed
a positive association between change in mean  diversity and species compositional similarity
across space or with time. However, they reported a reduction in trait variance as  diversity and
taxonomic similarity declined. The authors concluded that through years of human modification
of the British landscape, communities retained different historically determined residual
assemblages of species, but these converged on a smaller range of optimal trait syndromes. This
is an excellent example of explicitly considering spatial and temporal scales. The use of small
sampling plots nested within large squares and comparison between a 20 year intervals increased
their power to detect signals of taxonomic differentiation but functional homogenization.
11. Olden, J.D. & Poff, N.L. (2004). Ecological Processes Driving Biotic Homogenization:
Testing a Mechanistic Model Using Fish Faunas. Ecology, 85, 1867-1875.
This work set the pace in exploring the mechanisms underlying BH. They use empirical data to
validate the predictions of a previously developed conceptual model. The model predicts the
extent of homogenization or differentiation based on distinct mechanisms that capture
interactions between native, non-native species and the environment. They used data on
freshwater fish fauna at three spatial scales (continental, provinces and watersheds). Based on the
correspondence of some of the model’s predictions with empirical data, the authors show that
fish community homogenization across spatial scales could be explained by widespread
introductions of ubiquitous species coupled with either no extinctions or differential patterns of
native species extinctions. The approach used in this paper is novel for answering BH questions.
In the least, models can show us potentially what does not work in nature. Alternatively, it may
be an oversimplification of the nature of the system, as in this paper the authors have reduced the
problem of BH as due to single events of species introductions and extinctions. Also, the paper
varies the spatial scales considered, which is good because the identity of dominant mechanisms
may change with scale. This work goes beyond describing patterns resulting from BH, to
exploring underlying mechanisms.
12. Rooney, T.P., Wiegmann, S.M., Rogers, D.A. & Waller, D.M. (2004). Biotic Impoverishment
and Homogenization in Unfragmented Forest Understory Communities. Conservation
Biology, 18, 787-798.
Rooney et al. (2004) compared understorey herb communities in 62 upland forest stands between
1950 and 2000, with a goal to assessing shifts in taxonomic composition, extent and rates of BH,
and to identify possible mechanisms for observed changes. They found a slight decline in native
species density coupled with a drastic increase in exotic species presence. They also report a
spread of habitat generalists at the expense of specialists, resulting in an increase in taxonomic
homogenization among sites. They found that native species (particularly animal
pollinated/dispersed) declined the most in the absence of deer hunting. This paper is a
straightforward method of quantifying taxonomic homogenization by surveying communities
Oyomoare Osazuwa-Peters
October 12, 2011
Lost in Space (Graduate Seminar)
across multiple sites at two time points. The authors also consider spatial scale by assessing
change at local (1m2), community (20m2) and regional scales. The major weakness is not with
the paper itself but in the approach used, due to differences between sampling methods and
intensities at the two time points. This paper is a relevant contribution to the study of BH because
it demonstrates the usefulness of temporal comparisons and shows that protecting habitats by
maintaining them within parks is not sufficient to prevent BH.
13. Naaf, T. & Wulf, M. (2010). Habitat specialists and generalists drive homogenization and
differentiation of temperate forest plant communities at the regional scale. Biological
Conservation, 143, 848-855.
To assess the extent of change in  diversity among herb communities of ancient broadleaf
forests in Northwest Germany, the authors resampled 175 semi-permanent plots after twenty
years. They aimed to quantify change in  through time, using three different similarity indices.
They also relate estimates of  diversity change to changes in the numbers of habitat generalists
and specialists. This work showed that after twenty years, there is a tendency towards taxonomic
homogenization that is independent of exotic speices or habitat specialization. However, the
magnitude of change in  diversity varied depending on dissimilarity metric applied. Finally, the
study attributed the trend of taxonomic homogenization to shifting range of native species. This
is an important contribution because it underlies the point that choice of similarity index can
influence the magnitude of biological similarity estimated.
14. Qian, H. & Ricklefs, R.E. (2006). The role of exotic species in homogenizing the North
American flora. Ecology Letters, 9, 1293-1298.
This paper uses the BH concept to evaluate the influence of exotic species on floristic similarity
at the continental scale and to determine if  diversity of native and exotifcs are governed by
similar factors. Using databases and literature, the authors extracted data on the native and exotic
flora of America North of Mexico, climatic variables, and human population data. Their results
showed that exotics were more differentiated at close distances, but showed homogenizing
patterns with increasing distance due to being widespread. The number of exotic species was
correlated with human population but unrelated with climatic variables, implying that exotics
may not be affected by increasing climate change as expected for natives. This is a unique
application of the BH concept, by separating analysis of exotics and natives to isolate their roles
in increasing similarity of communities at the continental scale. The only problem is a total
neglect of time to tell us how these relationships have change through time.
15. Olden, J.D., LeRoy Poff, N., Douglas, M.R., Douglas, M.E. & Fausch, K.D. (2004).
Ecological and evolutionary consequences of biotic homogenization. Trends in Ecology &
Evolution, 19, 18-24.
Oyomoare Osazuwa-Peters
October 12, 2011
Lost in Space (Graduate Seminar)
This is a review article that explores current knowledge on the BH concept as a basis for making
predictions of ecological and evolutionary consequences. The data used are published reports in
the literature. This work emphasizes the importance of genetic and functional components of
BH, which have largely been under-studied relative to taxonomic homogenization. The authors
outline some of the ecological and evolutionary implications associated with BH, including
disruption of local adaptations linked to genetic homogenization, and decreased resilience of
homogenized systems due to environmental perturbations. They conceptualize a framework for
using a trait based approach to investigate the consequences of functional homogenization. This
paper is important because it takes research in BH further, by raising questions about the
consequences of BH to catalyze research that will be useful to conservation and management.
However, it seems contradictory that the authors call for BH studies at larger spatial scales, since
ecosystems are managed locally and the patterns resulting from BH may be compounded by
geographic differences at the global scale.
16. Devictor, V., Julliard, R., Clavel, J., Jiguet, F., Lee, A. & Couvet, D. (2008). Functional biotic
homogenization of bird communities in disturbed landscapes. Global Ecology and
Biogeography, 17, 252-261.
The goal of this paper was to assess the relationship between fragmentation/disturbance and
functional homogenization of bird communities at the country level in France. Functional
homogenization was estimated using a simple and novel index, the Community Specialization
Index (CSI), which is defined as the average specialization of species present in a community.
Data used were densities of birds from the French Breeding Bird Survey, as well as estimates of
landscape fragmentation and disturbance across France. There was a consistent negative
relationship between CSI and measures of environmental degradation across habitats and
biogeographic zones. The paper demonstrates the efficacy and robustness of CSI as an accurate
measure of homogenization. It is a relevant contribution because it measures functional
homogenization, which is scarcely considered in many BH studies. The limitation is the lack of a
temporal component to the study and the novel index has no standard values for undisturbed
habitats as a reference.
Conclusion
The collection of papers above represents only a tiny fraction of information on BH. However, it
is clear that BH is occurring at an accelerated rate that needs to be matched with innovative
research. So far research on BH has been skewed towards taxonomic homogenization, and
limited by the inadequate methods applied. Future efforts should focus on the prevalence of
neglected but important components of BH, underlying mechanisms and consequences on
relevant scales that will be useful for incorporation into conservation and management strategies.
There should also be emphasis on devising standard methods and indices for studying BH at
appropriate spatial and temporal scales.