INSECTS AS PALEOENVIRONMENTAL INDICATORS
Insects are generally adapted to highly specific environmental conditions (Triplehorn and Johnson, 2005). Consequently, environmentally sensitive insects can provide useful insights into local ecological conditions (Elias, 1994, 2010; Coope, 2004, 2009). While fossil insects have been well studied at numerous Quaternary and Tertiary sites the world over (Elias, 2010), they require further exploration at Rancho La Brea (RLB). Fortunately, the life-history and present day climate-restricted geographic distributions for all of the species selected for this study are well documented (Matthews, 1977; Ashworth, 1979; Elias, 1994; 2010; Coope, 2004; 2009). Predatory and scavenging species, in particular, which do not rely on a specific food source (e.g., obligate plant host) can migrate independent of flora into suitable habitat as environmental conditions change (Coope, 2009; Elias, 1994, 2010).
The opportunity to use insects as paleoenvironmental indicators (when confidently identified and with provenance established) exists because most Quaternary, and even many Tertiary, insect species are extant. It is surely plausible to assume that they would have occurred and thrived under the same conditions that they do today (Matthews, 1977; Ashworth, 1979; Elias, 1994; 2010; Coope, 2004; 2009). Further, ecological associations of insect species can be traced back for millions of years in some cases, strongly supporting the inference that the ecological requirements of insects identified from the Late Pleistocene in RLB have not changed up to the present (Elias, 1994, 2010; Personal comm., S. Elias, 2016). Notably, apart from two scarab beetles that may have specialized on the dung of large and now-extinct mammals (Miller et al., 1983), all RLB fossil insects have been identified as representatives of extant species.
Fossils of extant insect species whose temperature ranges are well documented can provide quantitative constraints on both summer and winter paleotemperatures (Elias, 1994, 2010; Coope, 2004, 2009). The use of insects as a paleoclimate proxy is an established technique (Coope, 2004, 2009; Elias, 2010; Brooks and Langdon, 2014). For example, it is possible to document rapid climate change using fossils of winged predaceous or scavenging insects, particularly ground beetles (Carabidae) and darkling beetles (Tenebrionidae) since many species in these families are able to disperse rapidly to more suitable territory in response to abrupt variations in environmental conditions (Coope, 2004, 2009; Elias, 1994, 2010). Multiple species of tenebrionids, and carabids in particular, have been used as paleoclimate indicators for many decades (Coope, 2004, 2009; Elias, 1994, 2010). They may respond to temperature fluctuations faster than plants and they have distinct advantages in contexts where pollen analysis and other paleobotanical techniques do not provide clear signals (Coope, 2009, Elias, 1994, 2010).
Thus, the specific ecological requirements of insects found at RLB should provide equally specific information about the paleoenvironment of the tar pits and Los Angeles Basin within radiocarbon time. It is the specificity of information that is crucial here: extant vertebrate species represented at RLB may also provide paleoenvironmental information, but it is necessarily of a far more general character because such species are highly vagile, often migratory, and potentially able to tolerate a wide range of climatic conditions (Stock and Harris, 1992). The species selected for study (as well as the majority of insect species identified from RLB) are restricted to certain habitats and climate ranges within their geographic distributions, which may extend past California, the Pacific Coast, and even into the American southwest.