The community as an ecological unit, Barbour, Burk & Pitts 1987

This article provides an overview of types of plant communities and the process of succession in those communities.

In each type of habitat, certain species group together as a community. Fossil records indicate that some of these groups (or very closely related precursors) have lived together for thousands or even millions of years. During that time, it is possible that an intricate balance has been fashioned. Community members share incoming solar radiation, soil water, and nutrients to produce a constant biomass; they recycle nutrients from the soil to living tissue and back again; and they alternate with each other in time and space. Synecologists attempt to determine what is involved in this balance between all the species of a community and their environment [Barbour 1987: 155].

Community concepts and attributes

A plant community is an identifiable stand of plants growing together in a certain spot. Clusters of species, called associations, are often found growing together in several different places within a larger region. “An association is a particular type of community, which has been described sufficiently and repeatedly in several locations such that we can conclude that it has: (a) a relatively consistent floristic composition, (b) a uniform physiognomy [appearance], and (c) a distribution that is characteristic of a particular habitat” [Barbour 1987: 156].

There are opposing views about why particular plant species are often found growing together in a plant community. The continuum view posits that species distribution is driven individualistically by each species’ particular tolerance to various environmental conditions. By contrast, the association view suggests that a plant community is an integrated whole, whose component species are interdependent.

Whatever the reasons that particular species tend to grow together in stands, however, such stands “exhibit collective or emergent attributes beyond those of the individual populations” [Barbour 1987: 159]. Examples of such community attributes include its vertical structure, canopy cover, species composition and diversity, biomass, productivity, stability, and nutrient cycling, for example.

Succession

Ecological succession is an important concept that helps explain the particular assemblage of plants growing in a given location.

“Plant succession is a directional, cumulative change in the species that occupy a given area through time” [Barbour 1987: 230]. This does not refer to cyclical changes that occur over seasons, nor to changes occurring in response to climate shifts over extremely long time spans like thousands or millions of years. Rather, succession is when the composition of plants at a particular site changes over a period of decades to centuries.

Succession begins when pioneer species colonize bare ground. These first arrivals tend to be opportunists that grow fast, reproduce quickly, and do not live long. The early successional plants start to improve the habitat conditions for other, more competitive plants to then take over, displacing the pioneers. “One of the driving forces behind succession is the effect plants may have on their habitat. Plants cast shade, add to the litter, dampen temperature oscillations, and increase the humidity, and their roots change the soil structure and chemistry. … Both the environment and the community change, and this metamorphosis is due to the activities of the organisms themselves.” [Barbour 1987: 233]

Overtime, slower-growing, larger, longer-living plant species outcompete the earlier successional species, eventually forming a climax community, which is not subsequently replaced by any other community. “Succession often leads to communities with greater and greater complexity and biomass and to habitats that are progressively more and more mesic (moist)” [Barbour 1987: 233]. Such changes result in climax communities tending to be self-sustaining due to efficient nutrient cycling and internal moderation of external fluctuations in temperature and humidity.

The particular plant composition of a climax community depends on the regional climate, as well as local soil conditions and topography, meaning that several climax communities can exist in a given landscape.

Typically, many plant communities coexist in a complex mosaic pattern. That is, one climax community does not cover an entire region. … In [some] cases, the mosaic reflects topographic differences, such as south-facing versus north-facing slopes, basins with poor drainage and fine-textured soil versus upland slopes with good drainage and coarser soil, or different distances from a stress such as salt spray. In such cases, the communities within the mosaic do not bear a successional relationship to one another; they constitute a toposequence. Each community in a toposequence may, in fact, be a climax community [Barbour 1987: 238].

Understanding ecological succession can help us to predict the future vegetation of a site by observing its current vegetation. “It is often possible to estimate a community’s future composition by extrapolation from changes measured in a short time, by comparing other communities that have plants of different ages, or by noting differences between overstory plants and understory seedlings” [Barbour 1987: 231] In some cases, the understory seedlings will later become the canopy, provided the localized conditions support this succession.

Conceptualizing communities as natural entities: a philosophical argument with basic and applied implications, Steen et al. 2017

Ecological restoration aims to recreate lost or degraded ecological communities. However, “community” has been a difficult concept to define – should the definition stress dominant species, species interactions, or a subset of strongly interacting species? These authors propose defining community on the basis of co-evolutionary relationships among species.

We propose that an Evolutionary Community is conceptualized as a unique grouping of species, which occur in a given geographic area and are connected by interspecific and abiotic interactions that have evolved over time [Steen 2017: 1021].

By treating communities “as entities that have formed over evolutionary time; this [Evolutionary Community] concept allows for a philosophical platform to help us understand what many conservation and restoration efforts are trying to accomplish” [Steen 2017: 1031]. That is, it offers a way to conceptualize the end goal of a restoration project. A particular evolutionary community could be recreated by assembling the constituent species, resulting in the ecological interactions among the species resuming as before.

What processes cause a group of species to cohere into a community? We argue that the parts of Evolutionary Communities are bound together by interspecific interactions in a shared biotic and abiotic environment, which promote co-evolution and community structure and dynamics. For example, longleaf pine trees are conduits for lightning strikes that ignite a highly flammable understory, often including dropped longleaf pine needles. The resulting ground fires are necessary for reproduction of other species and maintain habitat suitable for others (e.g., gopher tortoises). Gopher tortoises, through the process of burrow creation, provide structure important to other species. The establishment of one or more of the species listed above facilitated the persistence of additional species [Steen 2017: 1025].

Likewise, the demise of one species will negatively affect, or even cause the demise of, other species that depend on it. Thus, the reason to preserve or recreate an integral community is to support the interdependent component species, each of which in turn support the community as a whole.

Non-native plants reduce abundance, richness, and host specialization in lepidopteran communities, Burghardt et al. 2010

This research evaluates the impact of the invasion of non-native plants in the distribution of lepidopteran (butterfly, skipper, and moth) communities. The authors assert that although the introduction of non-native plants has not resulted in a “global extinction”, they have had a considerable impact on how ecosystems functionthey often result in significant bottom-up reductions of energy available in local food webs.

The experiment established four gardens near mature woodlots containing most, if not all, of the native species planted within the treatment. The richness and abundance were then compared for lepidopteran communities found on native versus corresponding non-native congener[4] species of 13 woody plant genera. For example, the genus Acer (maple) was selected for this study because the native and non-native maples were widespread in that area. In separate plots, the researchers also compared native plants and unrelated (non-congeneric) non-native plants for lepidopteran richness and abundance.

The study found that lepidopterans suffer from the replacement of native plants by non-natives, especially when those non-natives are unrelated to any native plant species. The authors explain that “insect herbivores adapted to the chemical challenges [toxic plant defenses] of particular native hosts may be able to adopt a novel plant species as a host if its phytochemistry is sufficiently similar to the original hosts” [Burghardt 2010: 10]. Over the two-year study, lepidopteran abundance and richness were depressed both on congener and (unrelated) non-congener non-native plants, but especially on the latter.

The study found that lepidopterans suffer from the replacement of native plants by non-natives, especially when those non-natives are unrelated to any native plant species.

Specialist lepidopteran species, which require specific diet and habitat conditions to survive, fared worse on non-natives than did generalists, which can eat a variety of foods and survive in many different habitats. The authors note, for example, that “geographically novel congeners were acceptable hosts to less than half of the generalists and only one fourth of the specialists that we found on native congeners in 2009” [Burghardt 2010: 11]. Only 7% of specialist species used non-congener non-natives as hosts.

The authors argue that the loss of lepidopteran diversity and abundance due to the displacement of native plant species with non-natives can ripple up the food chain, reducing diversity at higher trophic levels. Reduced diversity leads to lower ecosystem productivity and stability, thus disrupting the whole system.

The authors argue that the loss of lepidopteran diversity and abundance due to the displacement of native plant species with non-natives can ripple up the food chain, reducing diversity at higher trophic levels. Reduced diversity leads to lower ecosystem productivity and stability, thus disrupting the whole system.

Because insect herbivores are near the hub of most terrestrial food webs, comprising essential food stuffs for an incredible diversity of insect predators and parasitoids, spiders, amphibians, lizards, rodents, bats, birds, and even higher predators such as foxes and bears, it is particularly important to understand changes wrought by non-native plants on this critical taxon [Burghardt 2010: 13].