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You Can’t Evolve If You’re Extinct: Novel Ecosystems & the Forgotten Food Web

Jordan, Marilyn 9/13/2011

Novel ecosystems — new, historically unprecedented combinations of species caused by environmental change, human actions, introduction of new species and loss of native species — are now ubiquitous, and conservation ignores them at its peril. These ecosystems collectively cover close to 40% of the terrestrial ice-free globe as mapped by Ellis et al. (2010). Novel freshwater, estuarine and marine ecosystems also exist, but have not been similarly mapped.

Conservationists have often thought of novel ecosystems as degraded or worthless, although some now argue they may be potentially valuable habitats (Kareiva 2008; Ellis 2009). These ecosystems are certainly a varied lot, ranging from slightly altered to totally transformed. So how should we determine the value of a novel ecosystem? When can we simply rely on the resilience of nature to restore diversity, functionality and production of ecosystem goods and services, and when will active management be needed?

My answer: We should base our assessments on an understanding of both the benefits and deficiencies of novel ecosystems and their implications for genetic and species diversity, trophic linkages and ecosystem function. Let me illustrate this approach by focusing on what invasive plant species can mean for the diversity and nativity of producer species in terrestrial systems.

Extinctions and decreased diversity and abundance of resident species caused by intertrophic impacts of invasive predators and pathogens are well documented, whereas the effects of introduced plant species are less obvious and sometimes controversial (Davis 2003; Powell 2011; Gurevitch and Padilla 2004). Certainly, invasive plants can have many different types of impacts on plants, animals and ecosystem processes; some are positive and some negative, depending on context (Vila et al. 2011). However, invasives typically cause decreases in an ecosystem’s producer species diversity and biomass, resulting in reduced nutrient uptake (Cardinale et al. 2007 and 2011). These patterns hold true in both aquatic and terrestrial ecosystems, and among herbivores, detritivores and predators (Cardinale et al. 2006).

Changes in food webs and energy flow among all trophic levels as diversity decreases have rarely been considered in biodiversity-ecosystem function studies (Schlaepfer et al. 2011). In particular, altered insect abundance and diversity can have profound effects (Wilson 1987). Herbivorous insects are the largest taxon of primary consumers: they convert plant material of low caloric density into nutritious packages — high in fats and proteins — that are essential for the growth and reproduction of many species of animals, including carnivorous insects, birds, and mammals.

Plants produce an array of toxic defense chemicals that discourage herbivory. Most insect species are specialists and can consume only those plant species containing the specific class of defensive compounds to which that species has adapted. Thus, introduced nonnative plants can rarely be eaten by native insects unless the nonnative plant species is closely related to a local plant lineage and shares similar defenses. A smaller number of insect species are more generalized in their use of host plants, but even they can use only a small number of plant species (Burghardt et al. 2010; Tallamy et al. 2009; Fox and Morrow 1981). Other studies on many insect orders and arthropod classes in different habitats and locations, and in experimental systems, have also found large reductions in aerial arthropod species diversity and biomass as nonnative plants increased (Heleno et al 2008; Litt & Steidl 2010; Herrara & Dudley 2003; Haddad et al. 2001). Thus, a shift from native to nonnative plants can clearly result in bottom-up reductions of energy available to higher trophic levels in food webs. Those of us old enough to remember the high “bug splat” density on vehicle windshields ~40 years ago can confirm that insect abundance appears much reduced today, though increases in nonnative plants may not be the only cause.

Loss of specialist insects is but one example of a widespread “replacement” of specialist species by generalists in many taxa and in many contexts as a result of disturbance and global change (Clavel et al. 2011). This loss of functional diversity results in functional homogenization (FH) of natural communities. Consequences of FH are likely a loss of ecosystem resilience, stability and ecosystem services at a landscape scale, since homogenous communities are less variable in their responses to disturbance (Clavel et al. 2011).

There are examples of some native species of plants and animals at least partially adapting to the impacts of invasive species, and of some invasive species becoming less damaging over time. However, we probably can’t wait for evolutionary time and the resilience of nature to fully restore functional ecosystem diversity from the bottom up. For example, after >100 years in Florida, Melaleuca quinquenervia hosts only 8 species of herbivores, compared with 406 species in its homeland (Costello et al. 1995);
Phragmites australis after 300 years in North America hosts 5 species versus 170 at home (Tewksbury et al. 2002). As species become increasingly rare, dispersed or extirpated, we are losing the diverse genetic material needed for evolution and adaptation to change.
You can’t evolve if you are extinct.

So what do we do about novel ecosystems altered by introduced species? Preventing the introduction of new potentially invasive species is the single most important strategy — for once established, invasive plants, animals and pathogens are nearly impossible to eradicate. We must prioritize scarce resources and be strategic in deciding which invasive species should — and can — be managed. Management and restoration of novel ecosystems will also require a triage approach. Sometimes it will be important and possible to restore native species and communities, and sometimes (most times?) we will
accept novel ecosystems and work with them. Novel management strategies tailored to suit different ecoregions, microclimates, land uses and socioeconomic settings will be needed in order to maximize the conservation value and ecosystem services provided by
these altered ecosystems (Hobbs et al. 2009).

A “whole ecosystem” approach (Ward 2011) is likely to work well for the increasingly (sub)urbanized and expanding developed matrix in which many conservation lands are embedded, as well as for relatively intact natural areas that contain human-impacted inholdings. Successful management of such areas must include the needs and concerns of human residents. About one-half of invasive plant species were introduced as ornamentals, and desirable ornamental plants have characteristics that contribute to invasiveness (Drew et al. 2010). Thus, strategies directed towards the nursery industry and consumers are essential. Increasing the use of native plants (and discouraging the use of invasive plants) in residential and commercial landscaping could be widely beneficial in supporting native insects and other native species across whole ecosystems.

Above all, we need a deeper understanding of novel ecosystems in order to manage them flexibly, innovatively, in ways that will promote ecosystem services and resilience to change while still protecting our native evolutionary biological capital. Declarations that novel ecosystems are the future might be true, but the work of approaching them intelligently requires a full appreciation of their individual strengths and deficiencies.


 

References
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Image credit: Maigh/ Flickr