1. Introduction
Insects, by their very nature, defy comprehensive conservation planning. Their vast diversity, measured by both numbers of species as well as life history traits, preclude detailed knowledge of the status and distribution of all but a few species. Because of this, almost all insect conservation efforts are focused at the species level. But serious insect conservation requires goals that are set at the faunal level and conservation success requires efforts that identify and conserve all species at all levels of biological organization. This seemingly daunting task is complicated in agricultural landscapes by high levels of habitat loss and fragmentation, resulting in population isolation. In many regions, once widespread insect communities are now functionally trapped on islands of suitable habitat and subject to a variety of stressors associated with isolation and small population sizes [
1]. The threats from habitat fragmentation relative to conservation are well documented and entire books have been devoted to the subject (e.g., [
2,
3]). Fragmentation exacerbates a variety of threats to populations [
4] and cumulatively, the ecosystems and the insect communities they support.
In highly fragmented agricultural landscapes, regional insect communities are dominated by “landscape-dwelling” (or remnant independent) species that have adapted well to the anthropogenic landscapes that dominate most agricultural areas [
1]. The members of these communities effectively utilize disturbed habitats composed of similarly tolerant plant species that dominate many agricultural and urban landscapes. The individual insect species composing the community are able to sustain populations across the human dominated landscape and for the most part, these individual species are not at risk for regional extirpation.
However, hidden away within these broader fragmented landscapes there are hundreds, perhaps thousands of insect species that are incapable of surviving within the anthropomorphic matrix [
1,
5,
6,
7,
8,
9]. These species are closely tied to native ecosystem remnants that have escaped agricultural, industrial or urban landuse conversion. Typically persisting as small populations within small and widely dispersed remnants of once widespread habitat types, “remnant-dependent” (hereafter referred to as r-d) insect species comprise the bulk of the extinction-prone biodiversity surviving within this highly fragmented landscapes [
1,
10]. These insects characteristically have very narrow life history requirements that limit their potential distribution in disturbed habitats. For example, monophagous insects restricted to wetland adapted hostplants are unlikely find suitable conditions within disturbed, converted, or invasive species dominated habitats. Panzer
et al. [
9] estimate that in the wetlands, grasslands and savannas of northeast Illinois (USA), roughly 15% of all insect species may require ecosystems remnants and estimate that the total number of conservative insect species for this region exceeds 2,000 species. In highly fragmented landscapes, these r-d species comprise the bulk of insects expected to be threatened by regional population declines that could result in regional species extirpations. This community would benefit from conservation strategies that implicitly incorporate their needs.
Clearly, it is impossible to develop individual strategies that simultaneously address the threats to thousands of regionally imperiled insect species. Conservation strategies that use surrogate conservation targets such as representative terrestrial community types [
11,
12,
13,
14] can efficiently conserve the persisting habitats of many r-d insects [
10,
15], but do little to address the unique threats to insect communities imposed by extreme landscape fragmentation and resulting population isolation. Artificial population fragmentation and reduced habitat size increases the rate of population extinctions, producing impoverished insect communities, two of the factors that put r-d insect communities at risk in highly fragmented landscapes. While traditional small preserves may initially conserve a subset of the regional r-d community, over time individual species are likely to fade as isolated populations become extinct more rapidly than new populations are founded [
16]. This problem is compounded in landscapes such is the Midwestern United States where over 99.9% of native grasslands have been converted to agriculture [
17]. In Indiana, a typical prairie remnant may be 15 ha or less in extent, separated by 10 km or more from the nearest similar remnant, and supports small, vulnerable populations of many r-d insect species.
While the real and theoretical impacts of habitat destruction relative to population dynamics in insects are well documented (e.g., [
4,
10,
18,
19]), very little research exists on the potential role habitat restoration and recreation could play in the conservation of r-d insect communities. When insect communities are considered within the context of habitat restoration, it is usually as an afterthought. Typically coarse ecological guilds or higher taxonomic groupings are compared between restorations and ecosystem remnants as a “measure of restoration success” [
20,
21,
22]. These assessments are almost always focused at taxonomic levels that provide little insight into the population level dynamics that underpin the conservation of intact communities. Rather, these types of assessments assume that restoring the functional aspect of insect communities is indicative of “restoration success”, which certainly has validity if restoring ecosystem functions and processes were among the initial objectives of the restoration. But these coarse assessments shed no light on the potential benefits to r-d species and communities or how habitat restoration may reduce threats to r-d insect communities.
Well designed and managed restorations can increase habitat for r-d insects [
23,
24,
25]. But while criteria for size and structural components of restoration design criteria abound for vertebrates, especially amphibians and birds (e.g., [
3,
26,
27]), r-d insects and communities are rarely incorporated into restoration design criteria. When insects are included in restoration planning it is limited to one or two endangered species [
28,
29,
30,
31,
32]. Such species focused efforts, as important as they are, are more related to species-level recovery and are not considered further herein.
The practice and theory of restoration ecology is well advanced, but biased towards restoring ecosystem functions and processes (e.g., [
33,
34]). Insect species are typically considered as members of ecological guilds, and once classified, individual species become “redundant”. Granted, increasing ecological redundancy is good [
33,
34,
35], but by and large, there is no appreciation for the conservation of declining r-d insect communities within the overarching framework of restoration. Insect community response is relegated to the “if you build it, they will come” model. Of course, many species of insects will be present on all restorations, but the species that respond readily to this restoration philosophy are primarily members of the landscape dwelling community that thrives in the surround anthropomorphic matrix. The r-d community, whose species would most benefit from restoration, are typically ignored. Here, I address how habitat restoration and habitat recreation (hereafter simply referred to as restoration) can be used as a generalized strategy to stabilize regional declines in r-d insect communities in highly fragmented landscapes. Because conservation and restoration require site-based actions, I explain how these generalized criteria were incorporated into two very different restorations in Northern Indiana, USA (
Figure 1).
Figure 1.
The locations of restorations at Kankakee Sands (1) and Houghton Lake (2). Both sites are located within the state of Indiana in North America.
Figure 1.
The locations of restorations at Kankakee Sands (1) and Houghton Lake (2). Both sites are located within the state of Indiana in North America.
2. Generalized Conservation Threats to Remnant-Dependent Insect Communities in Highly Fragmented Landscapes
Before any effective conservation strategy can be developed and implemented, it is imperative that it is placed within the context of threat reduction [
36]. Threats to regional insect communities are always site specific and effective conservation actions address site-specific threats. However, r-d insects in highly fragmented landscapes are exposed to several general threat types that can be addressed by habitat restoration. While developing and implementing conservation strategies on a species by species is not practical, it is possible to conceptualize restoration strategies that have broad benefits to r-d species, both known and unknown, in fragmented landscapes.
2.1. Small Population Size
Small populations are generally at a greater risk of extinction than large populations [
37]. In highly fragmented landscapes, small patches of remnant habitat may support populations that are below thresholds that protect from loss of genetic variability and related problems of inbreeding and genetic drift, population fluctuations due to variations in birth and death rate, environmental fluctuations due to variation in predation, competition, disease and food supply, and natural catastrophes that occur at irregular intervals.
Restoration can be used to increase habitat size and heterogeneity, increasing population size for individual species, reducing the threat from stochastic events. Restoration of complex habitat mosaics across environmental gradients can increase ecological resilience to environmental fluctuations and catastrophic events. In order to produce expanded habitat, restorations must include all (or most) requisite resources for remnant restricted insects, both known and unknown.
2.2. Population Isolation/Disrupted Population Dynamics
Fragmented landscapes force an artificial metapopulation structure on plants and animals that are restricted to ecosystem remnants. Despite most species having wings, many insects are not adept at navigating across converted landscapes (e.g., [
5,
38]) and for many species gene flow or recolonization of unoccupied habitat is unlikely. Metapopulation models predict that isolated populations are more likely to go extinct in the long run than populations that are slightly or well-connected [
4]. In intact landscapes, declining populations can be “rescued” by immigration from a nearby expanding population. In fragmented landscapes, the distance between fragments may prevent this from happening resulting in low levels of habitat occupancy [
4] and as a result, isolated ecosystem remnants generally support a subset of regional r-d insect communities.
Restoration can be used to expand occupied habitat to the point that connectivity is enhanced or reestablished between ecosystem remnants. Stepping stone models or traditional corridors [
29,
39] can be used to enhance connectivity, but restoration of contiguous habitat may be required to enhance connectivity for insects with limited dispersal abilities [
40]. Intuitively, restoration strategies that assume limited dispersal capabilities will accommodate more species than will stepping-stone models [
39]. To influence connectivity, restorations must include requisite resources for remnant restricted insects throughout the intervening expanse if artificial metapopulations are to be restored to patchy or contiguous population structures.
2.3. Inappropriately Scaled Disturbance Regimes/Ecological Processes
The impacts to ecological communities from traditional ecological disturbances, such as fire, severe storms events and flooding are exacerbated by fragmentation, increasing the potential for local population declines or extinction. Even at sites managed to maintain ecological integrity, managed disturbance regimes such as prescribed fire can be implemented at scales that negatively impact populations [
6,
10,
41]. Ironically, disturbance such as fire and seasonal flooding may be required to maintain suitable habitat for many r-d species, but these disturbances may negatively impact insect populations, increasing the probability of local population extinctions if there are no nearby source populations from which immigrants can recolonize recently disturbed habitats [
10,
41]. Similarly, ecological processes such as seasonal flooding or nutrient loadings may alter plant communities, resulting in declines of sensitive habitats [
42] and the r-d insects they support.
Restoration designs can be scaled to specifically accommodate ecological disturbance regimes. Restorations that increase connectivity across landscapes that are impacted asynchronously by disturbances are likely to enhance recolonization following local extinction events. Increasing habitat size allows disturbances to be managed to create refugia from negative impacts, reducing the probability that disturbances produce local extinctions. Likewise, restorations can be designed to influence site hydrology and nutrient cycling.
2.4. Future Predicted Climate Regimes
Future climates in many regions are expected to diverge from current climatic regimes [
43], increasing negative stress to isolated insect populations. Small ecosystem remnants that persist in many highly fragmented landscapes typically have reduced environmental complexity as well, and do not effectively buffer habitats and species against changing climate. Predicted changes in temperature and the seasonality of precipitation will likely have dramatic impacts to isolated ecosystem remnants and the insects that they support in many regions.
Restorations can be explicitly designed to improve local resilience to future predicted climatic regimes. For example, restorations can be used to restore connectivity between ecosystem remnants to provide access to nearby microhabitats and refugia that may buffer against climatic extremes. In many cases, restorations can be explicitly designed to increase local ecological heterogeneity in order to directly increase ecological resilience within and between restored habitats. It is also possible to design restorations to offset specific local threats from future predicted climatic regimes.
4. Conclusions
Everything we know about island biogeography and metapopulation dynamics points to the decline of r-d insect communities in highly fragmented landscapes. While some might argue that we can maintain at-risk species using intensive interventions such as facilitated dispersal [
56] or population augmentations, at best these actions can target just a few, well known or charismatic species. Without effective strategies, the vast majority of r-d insects will continue to decline into obscurity.
The two examples highlighted here demonstrate the applicability of restoration as a conservation tool for conserving r-d species in highly fragmented landscapes. While it is possible to envision generalized threats to r-d insect communities in fragmented landscapes, threat reduction strategies must be adjusted to address site specific conditions. Implementing generalized strategies without incorporating site-specific realities will create inefficiencies that waste resources. For example, planting high-diversity grasslands in the uplands surrounding Houghton Lake would have doubled initial restoration costs of those agricultural fields, and there is no reason to believe that r-d species will particularly benefit from a more diverse upland restoration. Likewise, restoring connectivity to nearby habitats, the “universal conservation solution” in fragmented systems, may not be feasible or even required to significantly reduce threats in many situations. Regardless, habitat restoration can be an effective strategy at isolated sites reducing other stressors caused by habitat loss/fragmentation, especially threats from small population size and inappropriately scaled ecological disturbances.
It is worth noting that ecological restoration is an expensive endeavor. Incorporating the needs of r-d insect communities significantly increases restoration cost. Restoring entire plant communities to increase habitat for known and unknown r-d insects may double or triple the cost for initial planting relative to traditional restorations which typically restore just 10%–20% of vascular plant diversity. Because local genotypes for the vast majority of species are not available commercially, the seed for the majority of plant species must be wild collected or grown in on-site nurseries.
Are r-d insect communities worth the added cost and complexity required to incorporate into appropriate restoration designs in highly fragmented landscapes? Insects comprise the vast majority of regional biodiversity, and in fragmented landscapes, imperiled insects likely outnumber all other imperiled plants and animals combined. Insect communities provide ecosystem services including nutrient cycling, pest control, pollination, and wildlife nutrition [
57]. It is time for entomologists to engage with restoration ecologists to ensure that our interests are met by the restorations of the future. Too often insect conservationists settle for the inexpensive solutions to threats, remaining too timid to seek real threat reduction strategies. Insects provide valuable ecological services [
57] that are worthy of the conservation investment required to maintain intact insect communities. Alternatively, we can simply accept the increasing impoverishment of insect communities in fragmented landscapes, and maintain our status quo as the observers of ecological decline.