508 Glossary Hill numbers A continuum of diversity measures that quantify diversity in units of equivalent numbers of equally abundant species [Developed by British ecologist Mark Hill.] Hinterland A region lying beyond an urban area, but often influenced by it Historic rainforest All land areas with a potential to support the tropical rainforest as determined by the climatic and physiographic conditions, whether they are currently forested or not This concept provides a baseline to calculate the rainforest loss in a country or region over a historic period Historic range of variability (HRV) A description of the full range of landscape characteristics that occurred in the past Also, historic range of variation Historical biogeography See BIOGEOGRAPHY Historical ecology The study of how human societies and ecosystems change and in turn transform one another through time in local and regional landscapes Data are drawn from geology, archeology, history, and various other sources Historical processes The evolutionary and ecological processes that occur in the recent or distant past including immigration, emigration, speciation, extinction, drift, and/or past selection by the environment Holarctic A biotic realm in the temperate, boreal, and arctic regions of the Northern Hemisphere with strong affinities among the regional floras and faunas Holistic management A philosophy and practice of livestock grazing management, used by a minority of ranchers in Africa, North America, and Australia, attempting to integrate economic, social, and ecological goals Holobiont A collective term for all the organisms engaged in a symbiotic relationship; e.g., the coral animal and the diverse assemblage of microbes that live associated with the coral Holocene The epoch in time extending from about 11,700 years ago, approximately when the last ice age ended, to the present day Holometabola Insects characterized by complete metamorphosis, a wingless larval stage, and an intermediate pupal stage Holomorphology The totality of characters that are indicative of phylogeny (molecular, behavioral, and so on) Holoparasite A parasitic plant that lacks chlorophyll and cannot photosynthesize Compare HEMIPARASITE Holoplankton See ZOOPLANKTON Homeostasis The ability of an organism or other biotic system to maintain a state of relative internal stability in the presence of external change Home range The area where an animal typically spends its time; the expanse traversed in meeting life requirements over a given period Homogeneous Made up of components that are of the same kind; having uniform parts or elements Compare HETEROGENEOUS Homogenization A blending of species throughout the globe, largely as a result of trade and transportation Homoiohydry The ability of an organism to maintain a constant internal water balance independent of fluctuating environmental conditions Contrasted with POIKILOHYDRY Homologous Denoting common ancestry; describing structures, processes, sequences, behaviors, and so on that are derived from a common ancestral structure Homology The existence of a similarity that is explained by common ancestry Homonymy The fact of the same scientific name being applied to two or more species Homoplastic Relating to or exhibiting homoplasy (see next) Homoplasy A character or character state acquired by parallel or convergent evolution that bears resemblance to a character in a different group Homozygosity In a diploid organism, the presence of the same alleles at a given locus (e.g., A1A1) Compare HETEROZYGOSITY Homozygous Possessing the same gene form (allele) on both chromosomes Horizon See SOIL HORIZON Horizontal transmission The transmission of pathogens within generations of hosts Host The individual that is parasitized by a parasite Host range The range of different organisms in which a virus (or a plasmid) is able to replicate successfully Hotspot An area with exceptionally high biodiversity, especially one threatened by habitat loss Called ‘‘hot’ because such areas are often selected as priority targets for conservation or management efforts A region that is unusually rich in a particular group of species (e.g., birds), compared with average sites in the same geographic region; contrasted with a COLD SPOT Also spelled hot spot HRV historic range of variability Human-altered Describing a habitat or ecosystem that has been significantly changed by humans; e.g., the conversion of natural forest into pasture or the reclamation of a wetland for agriculture Human-dominated Describing a habitat or ecosystem where human presence and activities strongly influence living and nonliving ecosystem components Human ecology A type of ecology focused on the species Homo sapiens (humans), including their relationship with their natural, manufactured, social, and cultural, and environments Can be considered a subset of SOCIAL ECOLOGY Humanism A philosophy focusing on the needs, interests, and values of humans Ecohumanism is the belief that this must include concern for the natural environment because of its value to humans Human well-being People’s condition and capacities allowing secure access to food, water, energy, and shelter of adequate quality to meet their needs, and provide good health as well as social connections Husbandry The practice of raising livestock and growing crops Hutchinsonian niche The set of environmental conditions, or opportunities, that will permit a species to exist indefinitely [Named for the British-American ecologist G Evelyn Hutchinson.] Hutchinsonian ratio The body size ratio of larger species over smaller species in a pair of species; according to this coexisting species are expected to have a greater body size ratio than would occur by chance, because competition prevents coexistence of species that are morphologically too similar Hybrid An individual that results from crossing between individuals from two or more populations, races, subspecies,