Biotic homogenization is an emerging, yet pervasive, threat in the ongoing biodiversity crisis. Originally, ecologists defined biotic homogenization as the replacement of native species by exotics (McKinney and Lockwood 1999), but this phenomenon is now more broadly recognized as the process by which ecosystems lose their biological uniqueness (Olden and Rooney 2006). Specifically, ecologists now differentiate between taxonomic, functional, and genetic homogenization. Taxonomic homogenization, or the increased similarity of species over space and time, has occurred across numerous taxa, including plants (Rooney et al. 2004), insects (Dormann et al. 2007), fish (Rahel 2000), birds (Devictor et al. 2007), and mammals (Isaac et al. 2014). Rising community similarity can occur via multiple pathways. For example, communities may become more similar following species invasions due to increased species richness (i.e. changes in α diversity), or, conversely, communities may become homogenized following the loss or replacement of native species (i.e. changes in β diversity). While such taxonomic homogenization has received the lion’s share of the attention, functional homogenization, or the convergence of ecological niches and functional roles among community members, may be more consequential due to the constraints imposed on both biodiversity and ecosystem processes (Olden et al. 2004; Clavel et al. 2011). Indeed, functional homogenization is a near ubiquitous consequence of human agency and is driven by the loss of specialized species or functional groups and their subsequent replacement by generalist taxa (Fig. 1). The loss of rare or endangered species, though, is not a new phenomenon, and conservationists have been combating such processes through reintroductions for generations. However, such translocations and human-assisted dispersal events can lead to a third, genetic, form of homogenization, whereby native populations lose local adaptations or evolutionary potential through reductions in allelic diversity (Olden et al. 2004). This process, commonly referred to as outbreeding depression, has long been a concern among reintroduction biologists, but the increase in genetic homogenization due to invasive species, hybridization events, and increasing human-assisted dispersal (McDonald et al. 2008) is an emerging concern in conservation and evolutionary biology.
Figure 1. Regional species pools consist of generalists and specialists with specific functional traits. Anthropogenic gradients of disturbance filter species traits and ultimately determine the functionality of the community. Low disturbance ecosystems tend to result in communities of specialists with high complementarity and high ecosystem function, while highly disturbed systems favor generalist species and lead to communities homogenized both taxonomically and functionally. Adapted from Clavel et al. (2011).
What are the drivers of biotic homogenization?
All forms of biotic homogenization have broadly been tied to human activities, but the mechanisms driving these patterns are diverse. For example, landscape simplification can lead to resource declines while agricultural intensification precipitates the heavy use of pesticides, and both processes have generated taxonomic and functional homogenization in managed landscapes (Dormann et al. 2007; Gámez-Virués et al. 2015). Similarly, the loss of habitat and resources following urbanization can induce all three forms of biotic homogenization (Clavero and Brotons 2010; Luck and Smallbone 2011; Morelli et al. 2016), as can competition and hybridization with invasive species (Devictor et al. 2007; McDonald et al. 2008; Isaac et al. 2014). Even seemingly mundane practices like supplemental food via backyard bird feeders can favor exotic species and homogenize local communities (Galbraith et al. 2015), and the intentional management of overabundant herbivore populations can promote exotic, generalist flora through reductions in native species (Rooney et al. 2004). The mechanisms can also be cryptic. For example, introduced species regularly hybridize with endemics and can homogenize local gene pools (Dowling and Childs 1992; McDonald et al. 2008), and the construction of human infrastructure such as dams often unknowingly facilitates such processes. Moreover, looming ecological threats, like climate change, promote the expansion of generalist species and homogenize regional bird communities (Davey et al. 2012), while oceanic acidification has promoted similar changes in coral reef communities (Hughes et al. 2003; Burman et al. 2012). Ultimately, biotic homogenization at all levels appears to be an inescapable consequence of human agency.
Is biotic homogenization novel?
Clearly, the causes of biotic homogenization are numerous and the consequences great, but is this homogenization novel? As global species assemblages converge toward communities of generalists, it can be argued that homogenized ecosystems are the antithesis of novelty, with nearly identical communities replicated across the globe. Yet, relative to Pleistocene or even pre-Columbian baselines, simplified local communities composed of non-native species are almost certainly novel. Such mixing of biota, however, is not a new phenomenon, and novel communities have clearly arisen throughout earth’s history (Vermeij 1991; Williams and Jackson 2007). Nevertheless, the pace and trajectory of the current biotic homogenization crisis is unparalleled (Ricciardi 2007), and human agency has undoubtedly facilitated such changes. Moreover, drivers like rapid climatic warming, agricultural intensification, booming urban centers, uncontrollable invasive species, intensive overharvest of wildlife, and immeasurable human-derived food subsidies likely have few analogs in earth’s history. Consequently, it appears that the most novel components of biotic homogenization are the mechanisms driving this phenomenon rather than the process itself. In the end, the novel pressures exacted upon our ecosystems by human activities will likely continue to blend the planet’s biota (Fig. 2), with unknown consequences for future ecosystem stability and function.
Figure 2. The “Anthropogenic blender” homogenizing earth’s ecosystems, sensu (Olden 2006). Reproduced from http://depts.washington.edu/oldenlab/biotic-homogenization/.
~By Phil Manlick~
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